Pure Rotational Spectroscopy of Asymmetric Tops in the Undergraduate Classroom or Laboratory

A. J. MineiCollege of Mount St. Vincent, Riverdale, NY

S. A. CookePurchase College SUNY, Purchase, NYPure Rotational Spectroscopy of Asymmetric Tops in the Undergraduate Classroom or Laboratory

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 distortion

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.

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.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 page

Probably everyone in this room has contributed to this field!

BackgroundIt is useful if the student is familiar with VSEPR theory and electronegativity.

Quantum numbers J, K-1, K+1

Rotational constants A, B, C.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)Patterns: a-type R-branch

Patterns: b-type R-branch, K-1 = 0,1

Patterns: b-type R-branch, K-1 = 2,3

Often, K-1 = 2 splittingPatterns: c-type Q-branch

These patterns are typically spaced by

SpecFitter A multiplatform Java program for the pedagogic fitting of spectra

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 trifluoroacetateLab activity Purchase College

9 Students at Purchase College performed the Assignment Lab successfully.

6 Biochemistry majors2 Chemistry majors1 Senior citizen

c-type spectrum

Therefore A = 1475 MHz

Alternative operation mode

Peak picking subroutineCant pgopher, JB95 and AABS do all this?

Yes.

Distinctions:

SpecFitter is:

Specifically aimed at learning how to fit pure rotational spectra

Potentially multiplatform (Windows, Mac, Linux)

Open source

A one window application, MouseEvent driven 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 Novicks High resolution spectroscopy class).

Total of 22 respondents for pre-activity questionnaire, 19 respondents for post-activity questionnaire.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

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.