1 FALL 2017 CHEM 430 IR SPECTROSCOPY Long Lecture Play Dr. Justik - PowerPoint Presentation

1 FALL 2017 CHEM 430 IR SPECTROSCOPY Long Lecture Play Dr. Justik

Introduction The method provides a rapid and simple method for observing the functional group species present in an organic moleculeThe spectrum is a plot of the percentage of IR radiation that passes through the sample (% transmission) versus some function of the wavelength of the radiation related to covalent bonding CHEM 430 – IR Spectroscopy2

Introduction Instrumentation.Modern IR spectrometers are based on the Michelson interferometer Fourier transform infrared ( FT– IR) spectrometers: The absorption spectrum is obtained by means of Fourier transformation of an interferogram. Dispersive infrared spectrometers: Earlier instruments based on monochromators that disperse the radiation from an IR source into its component wavelengths - spectrum is obtained by measuring the amount of radiation absorbed by a sample as the wavelength is varied. Raman spectroscopy provides information complementary to that obtained from IR spectroscopy. CHEM 430 – IR Spectroscopy3

Vibrations of Molecules The IR Spectroscopic Process.The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms For a simple diatomic molecule, this model is easy to visualize: CHEM 430 – IR Spectroscopy 4

Vibrations of Molecules The IR Spectroscopic Process.There are two types of bond vibration:Stretch – Vibration or oscillation along the line of the bond Bend – Vibration or oscillation not along the line of the bond 5 5 H H C H H C scissor asymmetric H H C C H H C C H H C C H H C C symmetric rock twist wag in plane out of plane

Vibrations of Molecules The IR Spectroscopic Process.Each stretching and bending vibration occurs with a characteristic frequencyTypically, this frequency is on the order of 1.2 x 1014 Hz (120 trillion oscillations per sec. for the H2 vibration at ~4100 cm -1)The corresponding wavelengths are on the order of 2500-15,000 nm or 2.5 – 15 microns ( mm)When a molecule is bombarded with electromagnetic radiation (photons) that match the frequency of one of these vibrations (IR radiation), it is absorbed and the bonds begin to stretch and bend more strongly (emission and absorption) When this photon is absorbed the amplitude of the vibration is increased NOT the frequency CHEM 430 – IR Spectroscopy 6

Vibrations of Molecules The IR Spectroscopic Process.The result of the spectroscopic process is a spectrum of the various stretches and bends of the covalent bonds in an organic molecule CHEM 430 – IR Spectroscopy 7 7

Vibrations of Molecules The IR Spectroscopic Process.The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number of waves per centimeter in units of cm-1 (Remember E = ħ n or E = ħc/ l)This unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed – chemists like this, physicists hate it High frequencies and high wavenumbers equate higher energyis quicker to understand thanShort wavelengths equate higher energy CHEM 430 – IR Spectroscopy8 8

Vibrations of Molecules The IR Spectroscopic Process.This unit is used rather than frequency as the numbers are more “real” than the exponential units of frequencyIR spectra are observed for what is called the mid-infrared: 400-4000 cm-1The peaks are Gaussian distributions of the average energy of a transition CHEM 430 – IR Spectroscopy 9 9

Vibrations of Molecules The IR Spectroscopic Process.So how does the IR detect different bonds?The potential energy stretching or bending vibrations of covalent bonds follow the model of the classic harmonic oscillator (Hooke’s Law) CHEM 430 – IR Spectroscopy 10 10 Potential Energy (E) Interatomic Distance (y) Remember: E = ½ ky 2 where: y is spring displacement k is spring constant

Vibrations of Molecules The IR Spectroscopic Process.Aside: Physically here are the movements we are discussing:Stretching vibration: a typical C-C bond with a bond length of 154 pm, the displacement is averages 10 pm: Bending vibration: For C-C-C bond angle a change of 4° is typical, which corresponds to an average displacement of 10 pm. CHEM 430 – IR Spectroscopy 11 11 10 pm 154 pm 4 o 10 pm

Vibrations of Molecules The IR Spectroscopic Process.The energy levels for these vibrations are quantized as we are considering quantum mechanical particlesOnly discrete vibrational energy levels exist: Note there is no energy level below n = 0, at any temperature above absolute zero there is always the first vibrational energy level CHEM 430 – IR Spectroscopy 12 12 Potential Energy (E) Interatomic Distance (r) rotational transitions – (in microwave region) Vibrational transitions, n

Vibrations of Molecules The IR Spectroscopic Process.However, the application of the classical vibrational model fails apart for two reasons:As two nuclei approach one another through bond vibration, potential energy increases to infinity, as two positive centers begin to repel one anotherAt higher vibrational energy levels, the amplitude of displacement becomes so great, that the overlapping orbitals of the two atoms involved in the bond, no longer interact and the bond dissociates We say that the model is really one of an aharmonic oscillator, for which the simple harmonic oscillator model works well for low energy levels CHEM 430 – IR Spectroscopy 13 13

Vibrations of Molecules The IR Spectroscopic Process.Here is the derivation of Hooke’s Law we will apply for IR theory:Vibrational frequency given by: n : frequencyK: force constant – bond strength m: reduced mass = m1m2 /(m1+m2) Reduced mass is used, as each atom in the covalent bond oscillates about the center of the two masses CHEM 430 – IR Spectroscopy14 14

Vibrations of Molecules The IR Spectroscopic Process.What does this mean for the different covalent bonds in a molecule? Let’s consider reduced mass, m, first:The C-H and C-C single bonds differ by only 16 kcal/mole: 99 kcal · mol-1 vs. 83 kcal · mol -1 (similar K)Due to the reduced mass term, these two bonds of similar strength show up in very different regions of the IR spectrum: C─C 1200 cm-1 m = (12 x 12)/(12 + 12) = 6 (0.41) C─H 3000 cm-1 m = (1 x 12)/(1 + 12) = 0.92 (0.95) A smaller atom therefore gives rise to a higher wavenumber (and  n and E) CHEM 430 – IR Spectroscopy15 15

Vibrations of Molecules The IR Spectroscopic Process.What does this mean for the different covalent bonds in a molecule? When greater masses are added, the trend is similar (K’s here are different) C─I 500 cm -1 C─Br 600 cm-1 C─Cl 750 cm-1 C─O 1100 cm -1 C─C 1200 cm-1 C─H 3000 cm -1 A smaller atom therefore gives rise to a higher wavenumber ( and  n and E)and a larger atom gives rise to lower wavenumbers (and  n and E) CHEM 430 – IR Spectroscopy16 16

Vibrations of Molecules The IR Spectroscopic Process.What does this mean for the different covalent bonds in a molecule? Let’s consider bond strength, K:A C≡C bond is stronger than a C=C bond is stronger than a C-C bond wavenumber , cm-1 D HfFrom IR spectroscopy we find: C ≡C ~2100 200 C=C ~ 1650 146 C—C ~1200 83 Note the good correlation with the heats of formation for each bond!Stronger bonds give higher wavenumbers (and  higher n and E)CHEM 430 – IR Spectroscopy17 17

Vibrations of Molecules The IR Spectroscopic Process.The y-axis of the IR spectrum is in units of transmittance, T, which is the ratio of the amount of IR radiation transmitted by the sample (I) to the intensity of the incident beam (I0); % Transmittance is T x 100 T = I / I 0 %T = (I / I0) X 100 IR is different than other spectroscopic methods which plot the y-axis as units of absorbance (A). A = log(1/T) As opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional vs. concentration of other functionalities, it can be used vs. itself if standardized!!! CHEM 430 – IR Spectroscopy 18 18

Vibrations of Molecules The IR Spectroscopic Process.The intensity of an IR band is affected by two primary factors:Whether the vibration is one of stretching or bendingElectronegativity difference of the atoms involved in the bond:For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak. The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bending CHEM 430 – IR Spectroscopy 19 19

Vibrations of Molecules The IR Spectroscopic Process.It is important to make note of peak intensities to show the effect of these factors:Strong (s) – peak is tall, transmittance is lowMedium (m) – peak is mid-heightWeak (w) – peak is short, transmittance is high* Broad ( br) – if the Gaussian distribution is abnormally broad(* this is more for describing a bond that spans many energies )Exact transmittance values are rarely recorded CHEM 430 – IR Spectroscopy 20 20

21 II. Infrared Group Analysis A. General The primary use of the IR spectrometer is to detect functional groups Because the IR looks at the interaction of the EM spectrum with actual bonds, it provides a unique qualitative probe into the functionality of a molecule, as functional groups are merely different configurations of different types of bonds Since most “types” of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks) There are 4 principle regions: 4000 cm -1 2700 cm -1 2000 cm -1 1600 cm -1 400 cm -1 Bonds to H O-H single bond N-H single bond C-H single bond Triple bonds C≡C C≡N Double bonds C=O C=N C=C Single Bonds C-C C-N C-O Fingerprint Region

22 IR Spectroscopy I. Introduction The IR Spectrum The intensity of an IR band is affected by two primary factors: Whether the vibration is one of stretching or bending Electronegativity difference of the atoms involved in the bond: For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bendingIt is important to make note of peak intensities to show the effect of these factors:Strong (s) – peak is tall, transmittance is lowMedium (m) – peak is mid-heightWeak (w) – peak is short, transmittance is high* Broad (br) – if the Gaussian distribution is abnormally broad(*this is more for describing a bond that spans many energies) Exact transmittance values are rarely recorded

23 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies We have learned: That IR radiation can “couple” with the vibration of covalent bonds, where that particular vibration causes a change in dipole moment The IR spectrometer irradiates a sample with a continuum of IR radiation; those photons that can couple with the vibrating bond elevate it to the next higher vibrational energy level (increase in A) When the bond relaxes back to the n 0 state, a photon of the same n is emitted and detected by the spectrometer; the spectrometer “reports” this information as a spectral band centered at the n of the couplingThe position of the spectral band is dependent on bond strength and atomic sizeThe intensity of the peak results from the efficiency of the coupling; e.g. vibrations that have a large change in dipole moment create a larger electrical field with which a photon can couple more efficiently

24 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Remember, most interesting molecules are not diatomic, and mechanical or electronic factors in the rest of the structure may effect an IR band From a molecular point of view (discounting phase, temperature or other experimental effects) there are 10 factors that contribute to the position, intensity and appearance of IR bands Symmetry Mechanical Coupling Fermi ResonanceHydrogen BondingRing StrainElectronic EffectsConstitutional IsomerismStereoisomerismConformational IsomerismTautomerism (Dynamic Isomerism)

25 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Symmetry H 2 O For a particular vibration to be IR active there must be a change in dipole moment during the course of the particular vibration For example, the carbonyl vibration causes a large shift in dipole moment, and therefore an intense band on the IR spectrum For a symmetrical acetylene, it is clear that there is no permanent dipole at any point in the vibration of the CC bond. No IR band appears on the spectrum

26 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Symmetry H 2 O Most organic molecules are fortunately asymmetric, and bands are observed for most molecular vibration The symmetry problem occurs most often in small, simple symmetric and pseudo-symmetric alkenes and alkynes Since symmetry elements “cancel” the presence of bonds where no dipole is generated, the spectra are greatly simplified

27 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Symmetry H 2 O Symmetry also effects the strength of a particular band The symmetry problem occurs most often in small, simple symmetric and pseudo-symmetric alkenes and alkynes Since symmetry elements “cancel” the presence of bonds where no dipole is generated, the spectra are greatly simplified

28 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Mechanical Coupling In a multi-atomic molecule, no vibration occurs without affecting the adjoining bonds This induces mixing and redistribution of energy states, yielding new energy levels, one being higher and one lower in frequency Coupling parts must be approximate in E for maximum interaction to occur (i.e. C-C and C-N are similar, C-C and H-N are not) No interaction is observed if coupling parts are separated by more than two bonds Coupling requires that the vibration be of the same symmetry

29 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Mechanical Coupling For example, the calculated and observed n for most C=C bonds is around 1650 cm -1Butadiene (where the two C=C systems are separated by a dissimilar C-C bond) the bands are observed at 1640 cm -1 (slight reduction due to resonance, which we will discuss later)In allene however, mechanical coupling of the two C=C systems gives two IR bands – at 1960 and 1070 cm-1 due to mechanical couplingFor purposes of this course, when we discuss the group frequencies, we will point out when this occurs

30 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Fermi Resonance A Fermi Resonance is a special case of mechanical coupling It is often called an “accidental degeneracy” In understanding this, for many IR bands, there are “overtones” of the fundamental (the n ’s you are taught) at twice the wavenumberIn a good IR spectrum of a ketone (2-hexanone, here) you will see a C=O stretch at 1715 cm-1 and a small peak at 3430 cm-1 for the overtone overtone fundamental

31 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Fermi Resonance Ordinarily, most overtones are so weak as not to be observed But, if the overtone of a particular vibration coincides with the band from another vibration, they can couple and cause a shift in group frequency and introduce extra bands If you first looked at the IR (working “cold”) of benzoyl chloride, you may deduce that there were two dissimilar C=O bonds in the molecule

32 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Fermi Resonance In this spectrum, the out of plane bend of the aromatic C-H bonds occurs at 865 cm -1 ; the overtone of this band coincides with the fundamental of C=O at 1730 cm -1 The band is “split” by Fermi resonance (1760 and 1720 cm-1)

33 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Fermi Resonance Again, we will cover instances of this in the discussion of group frequencies, but this occurs often in IR of organics Most observed: Aldehydes – the overtone of the C-H deformation mode at 1400 cm -1 is always in Fermi resonance with the stretch of the same band at 2800 cm-1 The N-H stretching mode of –(C=O)-NH- in polyamides (peptides for the biologists and biochemists) appears as two bands at 3300 and 3205 cm-1 as this is in Fermi resonance with the N-H deformation at 1550 cm-1

34 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Hydrogen Bonding One of the most common effects in chemistry, and can change the shape and position of IR bands Internal (intramolecular) H-bonding with carbonyl compounds can serve to lower the absorption frequency 1680 cm -1 1724 cm -1

35 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Hydrogen Bonding Inter-molecular H-bonding serves to broaden IR bands due to the continuum of bond strengths that result from autoprotolysis Compare the two IR spectra of 1-propanol; the first is an IR of a neat liquid sample, the second is in the gas phase – note the shift and broadening of the –O-H stretching band Neat liquid Gas phase

36 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Hydrogen Bonding Some compound, in addition to intermolecular effects for the monomeric species can form dimers and oligomers which are also observed in neat liquid samples Carboxylic acids are the best illustrative example – the broadened O-H stretching band will be observed for the monomer, dimer and oligomer

37 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Ring Strain Certain functional group frequencies can be shifted if one of the atoms hybridization is affected by the constraints of bond angle in ring systems Consider the C=O band for the following cycloalkanones: 1815 1775 1750 1715 1705 cm -1 We will discuss the specific cases for these shifts during our coverage of group frequencies

38 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Electronic Effects - Inductive The presence of a halogen on the a -carbon of a ketone (or electron w/d groups) raises the observed frequency for the p-bond Due to electron w/d the carbon becomes more electron deficient and the p-bond compensates by tightening

39 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Electronic Effects - Resonance One of the most often observed effects Contribution of one of the less “good” resonance forms of an unsaturated system causes some loss of p-bond strenght which is seen as a drop in observed frequency

40 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Electronic Effects - Resonance In extended conjugated systems, some resonance contributors are “out-of-sync” and do not resonate with a group Example:

41 IR Spectroscopy I. Introduction The IR Spectrum – Factors that affect group frequencies Electronic Effects - Sterics Consider this example: In this case the presence of the methyl group “misaligns” the conjugated system, and resonance cannot occur as efficiently The effects of induction, resonance and sterics are very case-specific and can yield a great deal of information about the electronic structure of a molecule

42 IR Spectroscopy Group Frequencies and Analysis Introduction When approaching any IR spectrum be sure to use the larger-to-smaller region approach- do not immediately focus on any one single peak (even –OH or C=O) From the Hooke’s Law derivation we are using we find that the IR can be conveniently be divided into four major regions: Bonds to H Triple bonds Double bonds Single Bonds O-H N-H C-H C≡C C≡N C=O C=N C=C C-C C-N C-O C-X “Fingerprint Region” 4000 cm -1 2700 cm -1 2000 cm -1 1600 cm -1 400 cm -1

43 IR Spectroscopy Group Frequencies and Analysis Introduction If supporting information is available – molecular formula, chemical inferences – (i.e. this was the product of an oxidation reaction), assume this information is correct and the analysis of the IR should support it (later in your careers you can doubt information given to you) If a molecular formula is available, do an HDI! Many texts list various methods for approaching an IR spectrum; use the method that works best for you and stick to it. The most common mistakes in spectral analysis are those of “jumping the gun” to a conclusion (usually based on some small, insignificant peak) or taking a random haphazard approach to the spectrum (gee, here is an IR, oh, let’s start looking for phosphorus this time) Be methodical, develop a scheme and stick to it!

44 IR Spectroscopy Group Frequencies and Analysis Before we begin – Each functional group will be described as follows: Group General – What is most recognizable? What makes it different from similar groups? Group Frequencies (cm -1 ): Bondobservedn in cm-1 type of vibration Exceptions and things to watch Scale on bottom summarizes band positions and strengths Strong - Medium - Weak -

45 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkanes General – due to the small electronegativity difference between C and H, hydrocarbon bands are of medium intensity at best and give simple spectra Group Frequencies (cm -1 ): C-H3000-2800 Stretch Strained ring systems may have higher n -CH 2- ~1465 Methylene bend (scissor) -CH 3 ~1375 Methyl bend (sym) -(CH 2 ) 4 - ~720 Rocking motion 4 or more –CH 2 - (long chain band) C-C Not interpretively useful, small weak peaks

46 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkanes – Dodecane – C 12 H 26

47 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkanes – Cyclopentane – C 5 H 10

48 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkanes Additional – If the 1400-1350 region is free of interference, the presence of certain alkyl groups can be discerned: H H C C Methylene Methyl Scissor 1465 H H C C H Bend asymm 1450 H H C C H Bend symm 1375 usually overlap 1380 1370 gem -dimethyl 1370 1390 t -butyl

49 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkanes Additional – Example: Compare 2,2-dimethylpentane vs. 2-methylhexane: vs.

50 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes General – slightly more complex than alkanes; asymmetric C=C is observed as well as the sp 2 -C-H stretch. Still, bands are weak to medium in intensity Group Frequencies (cm -1):=C-H3095-3010 Stretch - Diagnostic for unsaturation- may be aromatic as well =C-H 1000-650 Out-of-plane (oop) bend - Can be used to determine degree of substitution C=C 1660-1600 Stretch - Can be reduced by resonance - Symmetrical C=C do not absorb - trans- weaker than cis-

51 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes – 1-octene – C 8 H 16 Note – you still have alkane present!

52 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes – trans -4-octene – C 8 H 16 Note – absence of C=C band, shouldering of C-H band

53 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes – cis-2-pentene – C 5 H 10 Note – shouldering of C-H band

54 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes – cyclopentene – C 5 H 8 Note – increased complexity due to ring vibrations

55 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes Substitution – The out of plane =C-H bend produces strong bands but interference can come from aromatic rings (similar oop) and C-Cl bonds (~700) monosubstituted cis -1,2 trans-1,2 1,1-disubstitued trisubstituted tetrasubstituted 1000 900 800 700 none, with weak C=C

56 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes Substitution – The monosubstitued band is very reliable; and the variance induced by electronic effects is observed monosubstituted (R-) monosubstitued w/lone pair group (ex. –Cl, -F, -OR) monosubstitued w/conj. group (ex. C=O, CN) The shifts are similar for 1,1-disubstitued systems 1000 900 800 700 overtone usually observed

57 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes Rings – Incorporation of a double bond endocyclic or exocyclic to a ring may shift the observed band Endocyclic: Ring strain shifts the C=C band to lower n (ex. cyclopropene) The adjacent C-C bond couples with the C=C system – if the resulting component vector is along the line of the C=C bond an increase in n occurs – this reaches a minima at 90o for cyclobutene (no net component along C=C bond) and rises again with cyclopropene 1646 1611 1566 1656 n C=C 1650

58 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes Rings – Endocyclic: If C=C at a ring fusion, absorption is reduced as if one further carbon was removed from the ring: The presence of additional alkyl groups on the ring dramatically raises n C=C n C=C 1611 n C=C 1656 n C=C 1566 1788 1883 1641 1675

59 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkenes Rings – Exocyclic: these C=C bonds give an increase in absorption n with decreasing ring size: As the angle between the two C-C bonds is reduced – more p character is required (sp = 180°, sp2 = 120°, sp3 = 109.5°, “sp>3” = <109° The p character of the double bond is reduced, but the stronger s bond is strengthened to a greater degree Think of the allene example (“2-membered ring”) as an extreme example n C=C 1940 1780 1678 1657 1651

60 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkynes General – can be symmetric, psuedo-symmetric or internal – greatly reducing the number of observed bands Group Frequencies (cm -1 ): C-H~3300 Stretch - Diagnostic for terminal alkyne C  C ~2150 Stretch - Can be reduced by resonance Symmetrical and psuedo-sym. C  C do not absorb  C-H 900-700 Bend (Text does not list) Possible not to observe any bands for the C  C system

61 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkynes – 1-hexyne – C 6 H 10 Nice terminal, asymmetric, well behaved alkyne

62 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkynes – 3-hexyne – C 6 H 10 A not-so-nice, internal, symmetrical alkyne

63 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Alkynes – 1-hexyne – C 6 H 10 Nice terminal, asymmetric, well behaved alkyne

64 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings General – not true alkenes; most of the small bands associated with them are not of diagnostic value; electronic effects of a single group on the ring can change the observed bands drastically Group Frequencies (cm -1 ): -C-H3050-3010 Stretch Also for alkenes C -H 900-690 Out of plane (oop) bend Can be used to determine substitution pattern 2000-1667 Overtone and combination bands If observed, similar too oop = C-H 1600-1400 Ring stretch – observed as two doublets (1600, 1580, 1500 & 1450) Greatly dependent on substituents

65 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings – toluene – C 7 H 8 Typical mono-substituted (EDG) ring

66 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings – o-xylene – C 8 H 10 Typical ortho-substituted (EDG) ring

67 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings – m -xylene – C 8 H 10Typical meta-substituted (EDG) ring

68 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings – p -xylene – C 8 H 10 Typical para-substituted (EDG) ring

69 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings – a -methylstyrene – C 9 H 10 Conjugated mono-substituted ring

70 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings Substitution – The aromatic out of plane =C-H bend produces strong bands but interference can come from alkenes (similar oop) and C-Cl (~700) Consider this region to only be reliable for alkyl-, alkoxy-, halo-, amino-, and acetyl substituted rings Interpretation is often unreliable for nitro-, carboxylic- and sulfonic groups The overtone of these bands is the dominant source of the combination and overtone bands observed at 2000-1667

71 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings Substitution – 900 800 700 600 mono ortho meta para 1,2,4 1,2,3 1,3,5

72 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Mononuclear aromatic rings Substitution – The aromatic combination and overtone bands are a set of weak absorptions that occur from 2000-1667. This is often obscured by C=O mono ortho meta para 1,2,4 1,2,3 1,3,5 The general shape of the pattern is used for determining substitution pattern; typically only a neat liquid sample gives an intense enough set of bands for analysis

73 IR Spectroscopy Group Frequencies and Analysis The Hydrocarbons Polynuclear and Hetero- aromatic rings General – All bands for these aromatic systems are similar to the mononuclear systems; shifts should be assumed, and analysis would be case-by-case

74 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Alcohols General – the best recognized group on carefully selected spectra, but H-bonding effects can drastically change the position, intensity and shape of the O-H band Group Frequencies (cm -1):O-H (free)3650-3600 Stretch Seen in dilute solution or gas phase spectra O-H (H-bond) 3400-3300 Stretch The “classic” H-bonded band, seen in addition to the free band in solution C-O-H 1440-1220 Bend Often obscured by -CH 3 bend C-O 1260-1000 Stretch Can be used to determine 1 o , 2 o , 3 o or phenolic structure

75 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Alcohols – 1-octanol Neat liquid sample gives classic spectrum

76 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Alcohols – 1-octanol Same sample in dilute CCl4 solution (solvent bands deleted for clarity)

77 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Phenols – p -cresolPresence of aromatic bands, sharper -OH

78 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Alcohols – Substitution – Using the position of the C-O stretching band, it is possible to suggest a 1 o, 2o, 3o or phenolic structure to the alcohol; but these should be considered as base values, that may be changed by the effects of conjugation or an adjacent ring system base value phenol 1220 tertiary 1150 secondary 1100 primary 1050 n C-O 1070 n C-O 1070 n C-O 1017 n C-O 1060 n C-O 1030

79 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Ethers General – like alkynes, the simplicity of the spectra may allow them to pass unnoticed – deduce from molecular formula if one should be present Group Frequencies (cm -1):C-O1300-1000 Stretch (asymm.) Absence of C=O and O-H will confirm it is not ester or alcohol Simple alkyl ethers usually one band at 1120, aryl alkyl ethers give two bands – 1250 & 1040

80 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Ethers – diispropyl ether Spectrum dominated by all other functionality

81 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Ethers – Additional Types Aryl and vinyl ethers – The effect of conjugation gives the C-O bond a small amount of double bond character, raising the observed n Furthermore, strongly asymmetric systems (aryl alkyl and vinyl alkyl ethers) may show an additional weak C-O band for the symmetric stretch at 1040 and 850 respectively

82 IR Spectroscopy Group Frequencies and Analysis sp 3 Oxygen – Alcohols, phenols and ethers Ethers – Additional Types Epoxides – Most important bands are the ring deformation bands at n asym 950-815 and nsym 880-750 Weaker “breathing mode” band is present at 1280-1230 Acetals and Ketals – Give four or five unresolved bands in the 1200-1020 region

83 IR Spectroscopy Group Frequencies and Analysis sp 3 Nitrogen – Amines Amines – Once presence is determined, the substitution at nitrogen is easy to determine; only the 3 ° amine may present a problem Group Frequencies (cm-1):N-H (-NH2) 3650-3600 (2 bands) 1640-1560 Stretch (sym. and asym.) Bend N-H (-NHR) 3400-3300 (1 band) 1500 Stretch Bend For alkyl amines, very weak – for aromatic 2 ° amines, stronger N-H ~800 Oop bend N-N 1350-1000 Stretch Remember 3 ° amines have no N-H bands

84 IR Spectroscopy Group Frequencies and Analysis sp 3 Nitrogen – Amines 1 ° Amine – tert-butylamineTwo band –NH2 peak appears as small “w”

85 IR Spectroscopy Group Frequencies and Analysis sp 3 Nitrogen – Amines 2 ° Amine – dibutylamineNote weakness of –NH- band (can be mistaken as C=O overtone, if carbonyl is present)

86 IR Spectroscopy Group Frequencies and Analysis sp 3 Nitrogen – Amines 3 ° Amine – tributylamineDifficult to discern from alkane – molecular formula for confirmation almost requisite

87 IR Spectroscopy Group Frequencies and Analysis sp 3 Nitrogen – Amines Ammonium Salts Almost certainly never encountered in neat samples, but an important component of amino acids and many pharmaceuticals Group Frequencies N-H 3300-2600 Stretch 1 ° salts are at the higher n end of this band, 3 ° salts at the lower end Additional band sometimes obs. at 2100 N-H 1600-1500 Bend 1 ° as two bands (sym. And asymm.), 2 ° at the upper end of this range, 3 ° absorbs weakly

88 IR Spectroscopy Group Frequencies and Analysis sp 3 Nitrogen – Amines Ammonium Salts – anilinium hydrochloride Spectrum is of a KBr disc sample:

89 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Along with alcohols, the most ubiquitous group on the IR spectrum. Although it is easy to determine if the C=O is present, deducing the exact functionality and factors that influence the position of the band provide the challenge Base C=O Frequencies (cm -1 ): C=O 1810 Stretch (sym.) Anhydride band 1 1800 Acid Chloride 1760 Anhydride band 2 1735 Ester 1725 Aldehyde 1715 Ketone 1710 Carboxylic Acid 1690 Amide

90 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – The carbonyl C=O frequency is very sensitive to the effects we went over previously – a quick recap Electronic Effects: Inductive vs. Resonance: On first inspection, the ester, amide and acid halide/anhydride all possess lone pairs of electrons that can resonate with the C=O (which should lower n) C N O F Cl S

91 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Electronic Effects: Inductive vs. Resonance: In the case of an oxygen or chlorine being adjacent to the carbonyl, each of these atoms resist the positive charge in the contributing resonance structure, and the inductive effect becomes a stronger factor C N O F Cl S This inductive effect draws in s electrons from the C=O, which strengthens the p bond – these carbonyls appear at higher n

92 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Electronic Effects: Inductive vs. Resonance: In the case of nitrogen, it is less electronegative than oxygen and has a greater acceptance of the positive charge in the contributing resonance structure, so the carbonyl is lowered in n C N O F Cl S The inductive effect of nitrogen compared to an sp 2 carbon is negligible by comparison

93 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Electronic Effects: Inductive vs. Resonance: Likewise in aldehydes and ketones there is the inductive donation of electrons to the s bond of the carbonyl which slightly weakens and reduces the n of the p bond (and explains the small difference between aldehydes and ketones) C N O F Cl S The inductive effect of nitrogen compared to an sp 2 carbon is negligible by comparison

94 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Electronic Effects: Inductive vs. Resonance: In addition, we discussed this effect in regards to a -halogenated carbonyls as one of the effects that can change group n C N O F Cl S The inductive effect of chlorine will draw s electrons through a -carbon, weakening the C=O s and strengthening the p

95 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Electronic Effects - Resonance: Not only is the C=O n lowered by the effects of conjugation, the peak may also be broadened or split by the contribution of the two electronic conformers The s-cis absorbs at higher n than the s-trans . Why?

96 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – Ring Strain Effects: C=O groups that can be incorporated into a ring are sensitive to this effect. As ring size decreases more p -character must be used to make the single bonds take on the smaller angle (re: sp >3 = <109 °). The p component of the C=O is weakened, but the s-bond strengthened, raising the overall n Cyclic ketones, esters (lactones), amides (lactams) and anhydrides exhibit this behavior To clear up confusion – there are two ways to strengthen the C=O Remove s bond character – p bond becomes more stronger (better overlap) – this is a result inductive w/d Remove p bond character – s bond becomes stronger – ring constraint

97 IR Spectroscopy Group Frequencies and Analysis Carbonyls General – H-bonding effects: C=O groups are reduced in n if some of the electron density is tapped off to form H-bonds: This effect can be inter- or intra-molecular:

98 IR Spectroscopy Group Frequencies and Analysis Carbonyls Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon Group Frequencies (cm -1 ): C=O 1715 Stretch (sym.) n Base , sensitive to change conj. w/C=C 1700-1675 n C=C reduced to 1644-1617 conj. w/Ph 1700-1680 n ring 1600-1450 C=O 1815-1705 Decreased ring size raises n 1300-1100 Bend

99 IR Spectroscopy Group Frequencies and Analysis Carbonyls Ketones – 2-hexanone Typical aliphatic ketone

100 IR Spectroscopy Group Frequencies and Analysis Carbonyls Ketones – 4-methylacetophenone Typical aromatic ketone,

101 IR Spectroscopy Group Frequencies and Analysis Carbonyls Ketones – Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon Group Frequencies (cm -1 ): C=O 1715 Stretch (sym.) n Base , sensitive to change conj. w/C=C 1700-1675 n C=C reduced to 1644-1617 conj. w/Ph 1700-1680 n ring 1600-1450 C=O 1815-1705 Decreased ring size raises n 1300-1100 Bend

102 IR Spectroscopy Group Frequencies and Analysis Carbonyls Aldehydes – Presence of the unique carbonyl C-H bond differentiates this group from ketones Group Frequencies (cm -1 ): C=O1725 Stretch (sym.) n Base , sensitive to change conj. w/C=C 1700-1680 n C=C reduced to 1640 conj. w/Ph 1700-1660 n ring 1600-1450 2820, 2720 Stretch Fermi doublet; Higher n band often obscured by sp 3 C-H

103 IR Spectroscopy Group Frequencies and Analysis Carbonyls Aldehydes – isovaleraldehyde Typical aliphatic aldehyde – note appearance of Fermi doublet and C=O overtone

104 IR Spectroscopy Group Frequencies and Analysis Carbonyls Aldehydes – anisaldehyde Typical aromatic aldehyde, note how C=O obscures the combination and overtone region – oop region would be used to determine substitution

105 IR Spectroscopy Group Frequencies and Analysis Carbonyls Carboxylic Acids – Various H-bonding effects lead to messy spectra, especially in the upper frequency ranges – be aware of the effects of monomeric, dimeric and oligomeric species on the spectrum Group Frequencies (cm -1 ): C=O1710 Stretch (sym.) n Base , sensitive to change; conjugation gives reduced n C-O 1320-1210 Stretch O-H 3400-2400 Stretch Overlaps C-H region in most cases; multiple “sub-peaks” can be seen for the dimeric and oligomeric species – simplified in non-polar solution or gas phase spectra

106 IR Spectroscopy Group Frequencies and Analysis Carbonyls Carboxylic Acids – propionic acid Aliphatic carboxylic acid – neat sample vs. CCl 4 solution (right)

107 IR Spectroscopy Group Frequencies and Analysis Carbonyls Carboxylic Acids – o -toluic acid Aromatic carboxylic acid, larger non-polar “end” of the molecule cuts down on the hydrogen bonding seen with the smaller, previous propionic acid

108 IR Spectroscopy Group Frequencies and Analysis Carbonyls Carboxylic Acids - Salts Salts are expressed as possesing one single and one double bond – the true picture is one that is isoelectronic with the nitro group, with two bonds to oxygen with a bond order of 1.5 Group Frequencies: 1600 1400 Stretch (asymm.) Stretch (sym.)

109 IR Spectroscopy Group Frequencies and Analysis Carbonyls Carboxylic Acids - Salts – ammonium benzoate Here is an example of ammonium and carboxylate moieties:

110 IR Spectroscopy Group Frequencies and Analysis Carbonyls Carboxylic Acids – Amino Acids – L -alanine Amino Acids combine the features of carboxylate and ammonium salts:

111 IR Spectroscopy Group Frequencies and Analysis Carbonyls Esters – Ester oxygen has an electron withdrawing effect that tends to draw in electrons within the C=O system, strengthening it compared to other carbonyls Group Frequencies (cm -1 ): C=O 1735 Stretch (sym.) n Base , sensitive to change conj. C=C 1735-1715 n C=C reduced to 1640-1625 w/Ph 1735-1715 n ring 1600-1450 conj. of sp 3 O 1765-1760 1850-1740 n C=O increases with smaller ring C-O 1300-1000 Stretch, 2 bands

112 IR Spectroscopy Group Frequencies and Analysis Carbonyls Esters – methyl butyrate Simple aliphatic ester

113 IR Spectroscopy Group Frequencies and Analysis Carbonyls Esters – methyl m- bromobenzoate Conjugation on the carbonyl end:

114 IR Spectroscopy Group Frequencies and Analysis Carbonyls Esters – phenyl acetate Conjugation on the sp 3 oxygen end:

115 IR Spectroscopy Group Frequencies and Analysis Carbonyls Amides – Amide nitrogen acts as a conjugating group with C=O, reducing double bond character; amide nitrogen appears similar to amime, including the effects of substitution Group Frequencies (cm -1 ): C=O 1685 Stretch (sym.) n Base , sensitive to change Can be as low as 1630 w/conj. N-H ~3300 Stretch Similar to amines, but typically more intense n C=O increases with smaller ring N-H 1640-1550 Bend N-H ~800 oop bend

116 IR Spectroscopy Group Frequencies and Analysis Carbonyls Amides – pivalamide Primary aliphatic amide

117 IR Spectroscopy Group Frequencies and Analysis Carbonyls Amides – 2- pyrrolidone Cyclic secondary amide - lactam

118 IR Spectroscopy Group Frequencies and Analysis Carbonyls Anhydrides – With acid halides, typically the highest n C=O; appears as two bands for the symmetric and asymmetric stretching modes Group Frequencies (cm -1 ): C=O 1830-1800 Stretch (asym.) n Base , sensitive to change conj. C=C 1778-1740 Stretch (sym.) Two bands of variable relative intensity n C=O increases with smaller ring C-O 1300-900 Stretch, multiple bands

119 IR Spectroscopy Group Frequencies and Analysis Carbonyls Anhydrides – iso- butyric anhydride Typical anhydride

120 IR Spectroscopy Group Frequencies and Analysis Carbonyls Acid Halides – Acid bromides and iodides are not often encountered; acid chlorides are the most prevalent (and useful) Group Frequencies (cm -1 ): C=O 1810-1775 Stretch (sym.) n Base , sensitive to change conj. w/Ph add. band Fermi resonance with combination and overtone region of aromatic ring C-Cl 730-550 Stretch If below 600, not observed using NaCl windows C-Br 650-510 Stretch Typically too low to obs. C-I 600-485 Stretch Typically too low to obs.

121 IR Spectroscopy Group Frequencies and Analysis Carbonyls Acid Halides – propionyl chloride Overtones of low n peaks can confuse some spectra

122 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Nitriles – The “other” triple bond group observed in IR, due to the higher dipole change during the stretching vibration, this band is more intense than C Cs. Group Frequencies (cm-1): C N 2250 Stretch (sym.) n sensitive to change from conjugation; usually stronger than C C

123 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Carbonyls Nitriles – benzonitrile

124 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Imines and Oximes – Often referred to as “derivatives” of carbonyl compounds, these groups are not often encountered in routine IR obs. Group Frequencies (cm-1): C =N (imine and oxime) 1685-1650 Stretch (sym.) n sensitive to change from conjugation; usually stronger than C C O-H (oxime) 3250-3150 Stretch H-bond effects N-O (oxime) 965-930 Stretch

125 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Imines and Oximes – acetone oxime

126 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Isocyanates and Isothiocyanates – Reactive groups, not often observed in routine qualitative IR Group Frequencies (cm -1):N= C=O ~2270 Stretch (sym.) broad n band – coupled vibration N=C=S ~2125 Stretch (1 or 2 bands) broad n band – coupled vibration

127 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Isocyanates and Isothiocyanates– tert -butyl isocyanate

128 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Nitro – Useful, easily incorporated group on aromatic rings, less often encountered on alkyl compounds Group Frequencies (cm -1): 1600-1530 1390-1300 Stretch (asymm.) Stretch (sym.) Aliphatic nitro 1550-1490 1355-1315 Stretch (asymm.) Stretch (sym.) Aromatic nitro

129 IR Spectroscopy Group Frequencies and Analysis sp 2 and sp Nitrogen compounds Nitro – o -nitrotoluene

130 IR Spectroscopy Group Frequencies and Analysis Sulfur Thiols and Sulfides – Sulfur, due to its large size shifts most observed IR bands to lower frequencies – often out of the observed region Group Frequencies (cm -1 ): S-H(thiol) 2550 Stretch (sym.) Unique region of IR spectrum C-S-C No useful information

131 IR Spectroscopy Group Frequencies and Analysis Sulfur Thiol (Mercaptan) – 1,2-ethanethiol

132 IR Spectroscopy Group Frequencies and Analysis Sulfur Sulfoxides and Sulfones – Oxidized sulfur, the S O bonds are useful for determining oxidation state, if the presence of sulfur is known Group Frequencies (cm -1 ):SO 1050 Stretch (sym.) O SO ~1375 ~1150 Stretch (asymm.) Stretch (sym.)

133 IR Spectroscopy Group Frequencies and Analysis Sulfur Sulfoxide (Mercaptan) – di- butyl sulfoxide Be wary of water in the spectrum of sulfoxides

134 IR Spectroscopy Group Frequencies and Analysis Sulfur Sulfone– di -butyl sulfone

135 IR Spectroscopy Group Frequencies and Analysis Sulfur Sulfonic Acids, Sulfonamides and Sulfonates – Sulfur equivalent of the carboxylic acid derivatives; the O or N groups act as we have observed Group Frequencies (cm -1 ): OSO ~1375 ~1150 Stretch (asymm.) Stretch (sym.) As for sulfones – groups bound to sulfur identify the group, just as with the carboxylic acid derivatives differ from ketones S-O (acid & sulfonate ) 1000-650 Stretch May appear as several bands O-H & N-H As for the carboxylic acid derivatives

136 IR Spectroscopy Group Frequencies and Analysis Sulfur Sulfonamides – p- toluenesulfonamide

137 IR Spectroscopy Group Frequencies and Analysis Phosphorus Phosphines – Phosphorus in its lowest oxidation state – many bands that overlap with other useful regions; exercise caution in interpretation using IR Group Frequencies (cm -1):P-H 2320-2270 990-885 Stretch (sym.) Bend PH 2 1090-1075 840-810 Bend, two bands P-CH 3 1450-1395 1350-1255 Bend, two bands P-CH 2 - 1440-1400 Bend

138 IR Spectroscopy Group Frequencies and Analysis Phosphorus Phosphines – tri- butylphosphine

139 IR Spectroscopy Group Frequencies and Analysis Phosphorus Phosphine Oxides – More common to observe these phosphorus compounds Group Frequencies (cm -1 ): Phosphate Esters, Acids and Amides – Often encountered in biological systemsGroup Frequencies (cm-1): P O 1210-1140 Stretch (sym.) P O 1300-1240 Stretch (sym.) R-O 1088-920 Stretch, 1 or 2 band P-O 845-725 Stretch

140 IR Spectroscopy Group Frequencies and Analysis Phosphorus Phosphine Oxides, Phosphate Esters – tri- butylphosphate

141 IR Spectroscopy Group Frequencies and Analysis Halogens Fluorides and Chlorides – Smaller halogen bonds to carbon in observed frequency range Group Frequencies (cm -1 ): Bromides and Iodides – Not often obs. Due to low n of C-X stretchGroup Frequencies (cm-1): C-F 1400-1000 Stretch (sym.) Monofluoroalkyl at lower n Polyfluoroalkyl at upper n Aryl fluorides up to 1450 C-Cl 785-540 Stretch Different conformers may give split peaks C-Br 650-510 Stretch Bend is obs. at ~1200 C-I 600-485 Stretch Bend is obs. at ~1150

142 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers All spectrometers consist of four basic parts that are coupled with all four parts of the spectroscopic process - irradiation , absorption-excitation , re-emission-relaxation and detection. Irradiation: Spectrometer needs to generate photons h n h n h n Detection- reemission : Spectrometer needs to detect the photons emitted by the sample and ascertain their energy Energy Absorption-Excitation: Spectrometer needs to contain the sample h n Relaxation rest state rest state excited state

143 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers Those four parts are: Source/Monochromator Sample cell Detector/Amplifier Output Dispersive IR spectrometers were the first IR instruments, however their simplicity and longevity allows them to continue in service – for most routine organic analyses their speed and resolution is adequateFor the most part, their design is austere and relies on simple mechanics and optics to generate a spectrum, very similar to simply rotating a glass prism to see different bands of visible light

144 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers Here is a general schematic:

145 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers Source is a heated nichrome wire which produces a broad band continuum of IR light (as heat) The beam is directed through both the sample and a reference cell A rapidly rotating sector (beam chopper) continuously switches between directing the two beams to a diffraction grating

146 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers The diffraction grating slowly rotates, such that only one narrow frequency band of IR light is at the proper angle to reach the detector A simple circuit compares the light from the sample and reference and sends the difference to a chart recorder

147 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers On the older instruments the motor in the chart recorder was synchronized (& calibrated) to the motor on the diffraction grating Because each spectrum is the result of the tabulation of the spectroscopic process at each frequency individually, it is said to record the spectrum in the frequency domain

148 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Dispersive IR Spectrometers Advantages – simple, easy to maintain – last the life of the source and moving parts Disadvantages – to cover the entire IR band of interest to chemists it is necessary to use two diffraction gratings At high q , the component frequencies are more spread out, so the resulting spectra appear to have various regions expanded or compressed The limit to resolution is 2-4 cm-1

149 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Fourier Transform IR Spectrometers FT-IR is the modern state of the art for IR spectroscopy The system is based on the Michelson interferometer Laser source IR light is separated by a beam splitter, one component going to a fixed mirror, the other to a moving one and are reflected back to the beam splitter The beam splitter recombines the two to a pattern of constructive and destructive interferences known as an interferogram – a complex signal, but contains all of the frequencies that make up the IR spectrum

150 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Fourier Transform IR Spectrometers The resulting signal is essentially a plot of intensity vs. time Such information if plotted would look like the following: This is meaningless to a chemist – we need this to be in the frequency domain rather than time….

151 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Fourier Transform IR Spectrometers By applying a mathematical transform on the signal – a Fourier transform – the resulting frequency domain spectrum can be observed FT-IRs give three theoretical advantages: Fellgett’s advantage – every point in the interferogram is information – all wavelenghts are represented Jacquinot’s advantage – the entire energy of the source is used – increasing signal-to-noise Conne’s advantage – frequency precision – Dispersive instruments can have errors in the ability to move slits and gratings reproducibly – FTIR is internally referenced from its own beam

152 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Dispersive and Fourier Transform Fourier Transform IR Spectrometers Justik’s advantage – does it give me what I need Single-beam instrument – collect a background (air has IR active molecules!) Fast – all frequencies are scanned simultaneously No referencing! Computer based – scaling and editing of the spectrum to squeeze out the most data; spectra are proportional (no stretching or squeezing of regions), comparison with spectral libraries Disadvantages – expenisve relative to dispersive instruments, and the components take more expertise and service calls to replace

153 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Sample size – typically the size of the beam – mm’s  mg’s Non-destructive – sample can be recovered with varying degrees of difficulty Liquid samples – the easiest IR spectra are those of “neat” liquid samples Solid samples are too dense for good IR spectra – inter-molecular coupling of vibrational states occurs and peaks are greatly broadened In the liquid state full 3-D motion is available, and these effects are averaged out and diminished The thickness of a sample can be decreased to reduce these effects further Thin film liquid samples are best!

154 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Liquid samples Sample cell cannot possess covalent bonds (SiO 2 , or glass is out) The most common cell is a pair of large transparent “windows” of inorganic salts Most common: NaCl – cheap, transparent from 650 – 4000 cm-1, but fragileLess common – AgCl, KBr, etc. – if you need transparency below 650, limit is practically 400

155 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Solution samples One way solids can be handled is as a solution Key is that the solvent picked will cover the least amount of the spectrum as possible, as it will also be present Common solvents typically are symmetrical, or have many halogenated bonds – low cm -1 : CCl 4, CHCl3, CH2Cl2, etc.The cell in this case is two NaCl (or other) windows with a spacer, the sample is loaded via a syringe into the cell:

156 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Solution samples A newer method involves the use of a polyethylene matrix, that will hold allow a solution sample to evaporate, leaving small portions of the sample embedded in the matrix The samples are “liquid-like” The only interference is that of hydrocarbon

157 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Solid Samples The most common treatment for solid samples is to “mull” them with thick mineral oil (high MW hydrocarbon) - Nujol® Just like with the polyethylene cards, the molecules of the sample are held in suspension within the oil matrix Again, the interference is that of hydrocarbon

158 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Solid Samples The connoisseurs method (with no organic interference) is to press the solid with KBr into a pellet Under high pressure the KBr liquefies and entraps individual molecules of the sample in the matrix These spectra are the only spectra of solids that are as interference free as liquids

159 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Differences in Spectral Appearence Compare the following three IR spectra of p -cresol Neat Sample

160 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Differences in Spectral Appearence Compare the following three IR spectra of p -cresol KBr Pellet

161 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Differences in Spectral Appearence Compare the following three IR spectra of p -cresol CCl 4 Solution

162 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Differences in Spectral Appearence Compare the following three IR spectra of m- nitroanisole Nujol Mull

163 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Differences in Spectral Appearence Compare the following three IR spectra of m- nitroanisole KBr Pellet

164 IR Spectroscopy Instrumentation and Experimental Aspects The IR Spectrometer – Experimental aspects Differences in Spectral Appearence Compare the following three IR spectra of m- nitroanisole CCl 4 Solution