NY S A Cooke Purchase College SUNY Purchase NY Pure Rotational Spectroscopy of Asymmetric Tops in the Undergraduate Classroom or Laboratory Context Atkins and De Paula Physical Chemistry ID: 641267
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A. J. MineiCollege of Mount St. Vincent, Riverdale, NYS. A. CookePurchase College SUNY, Purchase, NY
Pure Rotational Spectroscopy of Asymmetric Tops in the Undergraduate Classroom or LaboratorySlide2
Context:Atkins and De Paula “Physical Chemistry”Section 12-4: “The Rotational Energy Levels”(a) Spherical rotors
(b) Symmetric rotors(c) Linear rotors(d) Degeneracies and the Stark effect(e) Centrifugal distortionSlide3
Context:McQuarrie and Simon “Physical Chemistry: A Molecular Approach”
Section 13-8: “The Rotational Spectrum of a Polyatomic Molecule Depends on the Principal Moments of Inertia of the Molecule”“There are no simple expressions for the energy levels of an asymmetric top molecule. Generally, their rotational spectra are fairly complicated and
do not exhibit any simple pattern”Section 13-9: “…vibrational spectra of polyatomic molecules turn out to be easily understood in terms of the HO approximation. The key point is the introduction of normal coordinates, which we discuss in this section”. Slide4
Hypotheses:Pure rotational spectra from asymmetric tops do contain easily recognizable patterns. (Low frequency and low temperature helps!).These patterns are identifiable by undergraduate students.
A “top down” approach to teaching rotational spectroscopy of asymmetric tops can follow a traditional pedagogic method in chemistry for teaching NMR and IR spectroscopy.Slide5
Related workGordy and Cook, Chapter 7.5“Assignment and Analysis of Complex Rotational Spectra”, Z.Kisiel
, in: J.Demaison et al. (Eds.), Spectroscopy from Space, Kluwer Academic Publishers, Dordrecht, 2001, pp.91-106.Pollnow
and Hopfinger, J. Chem. Ed. 1968, p 528.Nick Walker et al. Rotational Spectroscopy – Wikipedia pageProbably everyone in this room has contributed to this field!Slide6
BackgroundIt is useful if the student is familiar with VSEPR theory and electronegativity.Quantum numbers J, K
-1, K+1Rotational constants
A, B, C.Slide7
Patterns:
a-type R-branch
Center of group = (B
+
C
) × (
J
+1)
Span of group = (
B
-
C
) × (
J
+1)
What about
A
?
“Asymmetry” =
Asym
Asym
= (center
K
-1= 0) – (left K-1 = 1)“Reduced” Asymmetry, = Asym / [(B-C)×J]
Separation between groups = (
B
+
C
)Slide8
Patterns:
a-type R-branch
Slide9
Patterns: b-type R-branch, K-1 = 0,1Slide10
Patterns: b-type R-branch, K-1 = 2,3
Often,
K
-1
= 2 splittingSlide11
Patterns: c-type Q-branch
These patterns are typically spaced by Slide12Slide13
SpecFitter
– A multiplatform Java program for the pedagogic fitting of spectraSlide14Slide15Slide16Slide17
a
-type helper app:
1 10000 A 1553.380(222)
2 20000 B 325.07774( 61)
3 30000 C 311.26018( 66)
4 200 -DeltaJ -0.01652( 87)E-03
5 1100 -DeltaJK -0.0609( 40)E-03
2,2,3,3-tetrafluoropropyl
trifluoroacetateSlide18
Lab activity – Purchase College
9 Students at Purchase College performed the “Assignment Lab” successfully.
6 Biochemistry majors
2
Chemistry majors
1 Senior citizen Slide19Slide20Slide21
c
-type spectrum
Slide22
Therefore
A
= 1475 MHzSlide23Slide24
Alternative operation modeSlide25Slide26Slide27
Peak picking subroutineSlide28
Can’t pgopher, JB95 and AABS do all this?Yes.Distinctions:
SpecFitter is:Specifically aimed at learning
how to fit pure rotational spectraPotentially multiplatform (Windows, Mac, Linux)Open sourceA “one window” application, “
MouseEvent
” driven Slide29
Class EvaluationsSample sizes6 students from Manhattanville College (concurrently in
PChem 2 – QM).4 students from College of Mount St. Vincent (concurrently in PChem 2 – QM).
12 students from Wesleyan University (concurrently in Stew Novick’s High resolution spectroscopy class).Total of 22 respondents for pre-activity questionnaire, 19 respondents for post-activity questionnaire.Slide30
Pre-activity questionnaireHave you previously assigned/analyzed a microwave spectrum?Yes – 9 No – 13
Post-activity questionnaireDo you feel more confident in analyzing microwave spectra? Yes – 17 No – 1 “Sorta” – 1
Are you more interested in microwave spectroscopy after the class than before?Yes – 16 No – 2 “Somewhat” – 1 Do you think the software presented is helpful in assigning asymmetric molecular microwave spectra?Yes – 19 No – 0 Slide31
Conclusion:Numerous undergraduate students have demonstrated that asymmetric molecular rotational spectra can be analyzed using a “top down” approach.Thanks:
Students at the four institutions for participating. The Southern New England Microwave Spectroscopy Consortium for encouraging conversations.