Vibrational Coupling of Small Molecules with Potential Applications to Determination of Polymer Structures

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 23 (2018) pp 253-260

Vibrational Coupling of Small Molecules with Potential Applications to Determination of Polymer Structures

Allen Walker^†, Timothy A. Keiderling^‡ and Ahmed Lakhani^†,*

^†Department of Biophysical Chemistry & Mathematics, Calumet College of St. Joseph, 2400 New York Ave, Whiting, IN, 46394, alakhani@ccsj.edu; ^‡Department of Chemistry, University of Illinois at Chicago, M/C 111, 845 W. Taylor St., Chicago, IL, 60607
Received August 2, 2018. Accepted September 19, 2018.

Published: 23 November 2018

Abstract. Polymeric structures are often spectroscopically characterized using infrared (IR) methods that probe interactions between repeating units, much of which is due to through-space coupling of dipolar transitions. This results in characteristic intensity patterns in their IR spectra due to exciton splitting of the coupled vibrational transitions. In this paper, we present pedagogical examples appropriate for incorporation in the undergraduate curriculum that can be used to explain such a coupled oscillator interaction on a fundamental level through simple classically-based transition dipole coupling (TDC) calculations and which can be tested with student-accessible quantum mechanical (density functional theory, DFT-level) computations. With a carbon monoxide (C≡O) dimer model, variations of distance and angles are used to illustrate the breakdown of the TDC model for close-lying, non-bonded oscillators. Furthermore, frequency splittings and intensity distributions obtained from TDC predictions are compared to results from DFT calculations. The DFT method considers the effect of other electronic interactions, including through-bond coupling in bound systems, whereas the TDC model has only dipole coupling (through space). The TDC and DFT predictions come into agreement for larger separations. Extensions and limitations of this approach for interactions of more complex, realistic molecular oscillators are discussed, as are suggestions for student exercises.

Key Words: Chemistry by Invitation; physical chemistry;vibrational coupling; density functional theory calculations; infrared spectra

(*) Corresponding author. (E-mail: alakhani@ccsj.edu )

Article in PDF format (370 KB) HTML format

Supporting Materials:

A zip file is available online that contains three more tables. S1 shows the comparison of carbon monoxide dimer at a fixed distance of 3Å. S2 and S3 show the results for two parallel CH₂O molecules at various distance and angles. In addition, the file contains a detailed description of methods for the classical calculation that students might use. (169 KB)