1 Signals from coupled protons that are close together (in - PowerPoint Presentation

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Signals from coupled protons that are close together (in Hz) can show distorted patterns.When ν >> J, the spectra is said to be first-order.Non-first-order spectra assume more complex shapes than Pascal’s triangle predicts and can only be analyzed with the help of computers.

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>>J

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>J

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≈J

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<<J

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J

Complications in 1H NMR:

Signal distortion in non-first-order spectraSlide2

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Top spectrum taken at 90 MHz Dn ≈ 0.3x90 = 2.7 Hz apart

Bottom spectrum taken at 500 MHz

Dn

≈ 0.3x500 = 15.0 Hz apartSlide3

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Complications: OH groups are typically singlets (broad)

Frequently protons attached to hydroxy- or amino- functional groups do not show coupling and can appear broad.Slide4

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For Hydroxyl protons, the lack of splitting is the result of “fast proton exchange.” Proton exchange may be slowed or stopped by removal of traces of water or acid or by cooling. This allows observation of coupling in accordance with the n+1 rule.Slide5

Non-first-order Spectra are common at lower Magnetic Fields

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NON-H Nuclei:The proton nucleus possesses a greater magnetic moment and magnetic resonance frequency than do other spin-active nuclei.Below is a sketch of resonant frequencies for some common nuclei.This is not a real spectrum, since the NMR instrument is tuned to acquire a signal from just one nucleus at a time.Slide7

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Fourier Transform NMR excites just one nucleus at a time (ie 1H nucleus @ 300,000,000 ± 3000 Hz)Slide8

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Carbon-13 Nuclear Magnetic Resonance13C resonate at lower E (they are less sensitive to H0) In a 7.05 T magnet where 1H’s resonate at 300 MHz, 13C’s resonate at just 75 MHz

13C NMR spectra have much lower S/N than 1

H NMR spectra 13C NMR spectra typically lack the coupling information found in

1H NMR spectra13C NMR spectra have a larger chemical shift

range than 1H NMR spectra

Is the range larger in Hz? Typical 13C spectra are analyzed for chemical shift and # signals only! (Integrals and Multiplicity are not reliable/ available)

Range in Hz:

(

13

C: 200 ppm = 200x75 = 1500 Hz);

1

H: 12 ppm = 12x300 = 360 Hz)Slide9

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13C signals are typically well resolved from one anotherThe chemical shifts of carbon atoms in 13C NMR depend on the same effects as the chemical shifts of protons in 1H NMR.

Chemical Shifts in

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C NMRSlide11

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Attached protons couple to 13C nuclei and complicate spectraSlide12

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Using this technique simplifies the spectra of bromoethane to two single lines.

The splitting is removed through an electronic method called “

broad-band decoupling

”

:

In broad-band decoupling a pulse is applied to the proton range causing rapid

- flips of hydrogen nuclei, and effectively averaging their local magnetic field contributions.Slide13

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Recall coupling is mutual. Why don’t we see proton signals split by their attached carbons? Carbon-carbon coupling is not visible in 1H NMR spectra due to the very low probability of two 13C nuclei being adjacent to each other in a single molecule (.0111 x .0111 ~ .0001).Slide14

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Decoupling enhances some 13C signals more than others, and so areas no longer correspond to the number of nuclei present.Slide15

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Why is the S/N so low for 13C NMR?Because of the low abundance of 13C It has a weaker inherent magnetic resonance (1/6000 as strong as 1H)Its lower E means fewer nuclei showing net absorption.

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Weak H

0

Strong H

0

When the energy difference is small, so is the population difference between the two spin states. The smaller the



E, the weaker the signal.Slide16

Diastereotopic Protons

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Diastereotopic Nuclei

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ProblemsSlide19

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ProblemsSlide20

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