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

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.