Simulation study of solvent shifts of vibrational overtone spectra

Abstract

Buckingham's theory of the solvent shift of vibrational spectral frequencies predicts that the shift of the v ¼ 0 ! n overtone transition is n times the shift of the fundamental v ¼ 0 ! 1. We test this prediction by molecular dynamics simulations using existing intermolecular potential models for liquid N 2 and dilute N 2 in liquid Ar, at standard state conditions. We extend Buckingham's theory by including additional intramolecular potential and perturbation terms which lead to solvent-induced anharmonicity, i.e. Oðn 2 Þ terms in the solvent shift. The simulations show that Buckingham's prediction is not accurate for N 2 at standard liquid state conditions. We find that at these conditions there is a significant positive Oðn 2 Þ contribution to the solvent shifts and that for n $ 20 the shifts change sign from red to blue. Simulation results and indirect evidence from shock wave experiments with liquid N 2 show that Buckingham's prediction is more accurate for high-pressure high-temperature conditions, where the shifts are blue and only slightly nonlinear in n.

A Simulation Study of the Fundamental Vibrational Shifts of HCl Diluted in Ar, Kr, and Xe: Anharmonic Corrections Effects

“…The study of the fundamental and overtones infrared and Raman vibrational solvent shifts gives relevant information about the molecular interaction depending of the intramolecular vibration [1][2][3][4][5][6][7]. In an earlier work, according to Buckingham, it was deduced that the diatomic solvent shift associated to the vibrational transition 0 → ν of a cubic anharmonic oscillator presents a linear dependence with the vibrational quantum number ν [1].…”

“…Like in the previous work [8] both the diatomic vibration and rotation will be treated with a quantum approach (small system S), while the translational degree of freedom, both of the diatomic and the solvent molecules, will be treated classically (the bath B). Unlike the previous study [8] where the vibration was modeled by means of a cubic anharmonic oscillator, in this work the vibration will be characterized by using a fifth-order Dunham anharmonic oscillator [6,7]. Fourth-and fifth-order terms of the intramolecular potential must be considered because they have relevant effects on the anharmonic solvent shift.…”

“…In the present work we will study the solvent shifts of HCl also in Ar and Kr and additionally in liquid Xe by considering the non-linear corrections to the solvent shift deduced by Alessi et al [6,7]. Like in the previous work [8] both the diatomic vibration and rotation will be treated with a quantum approach (small system S), while the translational degree of freedom, both of the diatomic and the solvent molecules, will be treated classically (the bath B).…”

“…This result, which is valid in principle for the fundamental band or lower overtones of several diatomics, would break down for the fundamental bands of some molecules and high overtones of much ones. Recently, Alessi et al [6,7] has deduced the non-linear (ν (ν + 1)) correction (anharmonic correction) to the linear Buckingham solvent shift for a diatomic molecule perturbed by an atomic or molecular solvent. Alessi et al [6,7] analyzed the solvent shift of N 2 diluted in liquid Ar and pure N 2 liquid, both at normal conditions, founding that the non-linear solvent correction, also called anharmonic shift [6,7], represents only the 1% of the fundamental shift of the isotropic Raman band.…”

“…Recently, Alessi et al [6,7] has deduced the non-linear (ν (ν + 1)) correction (anharmonic correction) to the linear Buckingham solvent shift for a diatomic molecule perturbed by an atomic or molecular solvent. Alessi et al [6,7] analyzed the solvent shift of N 2 diluted in liquid Ar and pure N 2 liquid, both at normal conditions, founding that the non-linear solvent correction, also called anharmonic shift [6,7], represents only the 1% of the fundamental shift of the isotropic Raman band. This result is due in part to the small anharmonicity parameter χ e of N 2 (6.0732 · 10 −3 ) [6,7]; however, because the HCl anharmonicity constant χ e is clearly much larger than the N 2 one, we expect a major contribution of non-linear solvent shift for the fundamental band of HCl diluted in Ar, Kr, and Xe.…”