Simulation study of N2 overtone solvent shifts using improved potentials

• Published in: Molecular physics 2005-09, Vol.103 (17), p.2381-2396
• Main Authors: Alessi, Nick, Tolokh, Igor S., Goldman, Saul, Gray, C. G.
• Format: Article
• Language: English
• ISSN: 0026-8976; 1362-3028
• DOI: 10.1080/00268970500159406

The solvent shifts of vibrational overtone spectra are predicted by Buckingham's theory of solvent effects on Raman and infrared spectra to be linear with the overtone number. We test this prediction for liquid N 2 and dilute N 2 in liquid Ar by extending the theory and by implementing the predictions using molecular dynamics simulations. Changes are made to the representations of both the intermolecular and the intramolecular parts of the total Hamiltonian. The representation of the intermolecular part is extended by including two additional terms of a Taylor series expansion in the vibrational coordinate of the intermolecular energy. We find, however, that their inclusion does not contribute significantly to the predicted overtone shifts for the systems studied here. The intramolecular potential is represented by a Morse potential in contrast to Buckingham's harmonic oscillator plus a cubic perturbation term. In our calculation the only perturbation terms of the Morse oscillator Hamiltonian are four expansion terms of the intermolecular potential in terms of the vibrational coordinate. Accordingly, the basis functions used to calculate the perturbed energy levels are Morse eigenfunctions. The simulations show that Buckingham's prediction is rather accurate for liquid N 2 and dilute N 2 in liquid Ar, at standard state conditions, and for liquid N 2 at high-pressure high-temperature conditions. We find that for N 2 there is a small negative contribution due to quantum anharmonic corrections to Buckingham's harmonic solvent induced vibrational frequency shift. We also show how equivalent theoretical results are obtained when the intramolecular potential is represented as harmonic plus anharmonic terms up to the fifth power of the vibrational coordinate (i.e. a Dunham potential), instead of a Morse potential.