Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. III. Equilibrium simulations of vibrational energy relaxation of azulene in carbon dioxide

Abstract

The expressions for vibrational energy relaxation (VER) rates of polyatomic molecules in terms of equilibrium capacity time correlation functions (TCFs) derived in the first paper of this series [J. Chem. Phys. 110, 5273 (1999)] are used for the investigation of VER of azulene in carbon dioxide at low (3.2 MPa) and high (270 MPa) pressure. It is shown that for both cases the VER times evaluated on the basis of the same potential model via solute–solvent interaction capacity TCFs by means of equilibrium molecular dynamics (EMD) simulations satisfactorily agree with the nonequilibrium (NEMD) molecular dynamics [J. Chem. Phys. 110, 5286 (1999)] and experimental [J. Chem. Phys. 105, 3121 (1996)] results as well. Thus it follows that these methods can complement each other in characterizing VER from different points of view. Although more computational power and refined methods of dealing with simulated data are required for EMD simulations, they allow the use of powerful tools of equilibrium statistical mechanics for investigating the relaxation process. To this end, an analysis of VER mechanisms on the basis of normal mode and atomic representations is carried out. The influence of temperature and CO2 pressure on azulene normal mode spectra and solvent assisted intermode coupling in connection with the eigenvector structure is investigated in great detail. The normal mode capacity cross-correlation matrix reveals the significance of intermode coupling, which significantly contributes to intramolecular vibrational energy redistribution (IVR). As a new concept, partial normal mode relaxation rates are introduced. It is shown that these rates demonstrate similar properties as the energy exchange rates through particular normal modes in nonequilibrium simulations. Atomic spectra and friction coefficients are characterized by a complicated frequency dependence due to contributions from many normal modes. Atomic capacity TCFs and partial relaxation rates are analyzed and reveal a similar picture to that obtained from NEMD simulations. These results show that VER and IVR cannot be separated from each other and have to be considered as mutually connected processes.

Watching Photoinduced Chemistry and Molecular Energy Flow in Solution in Real Time

“…The theoretical background of classical MD calculations is provided elsewhere. [96,123,124] Direct non-equilibrium MD simulations have been found to reproduce important features of the energy gain and loss of FC-active CI 2 stretch modes observed in the experiments. A key finding of the studies of Kaeb et al is that most of the excess energy deposited by the IR pump is dissipated into the solvent through the lowest-frequency CÀH mode (CH 2 rocking vibration), while the lowest-frequency mode (CI 2 bending) is merely a spectator in the overall process of energy redistribution and relaxation and is not significantly populated at all on the timescale of the relaxation.…”

“…As a result of this balance, VET is mostly determined by x(w) at frequencies w >~100 cm À1 , that is, the range beyond the lowest-frequency modes describing complex collective motions of the liquid. [69,123,124] x(w) is defined in Equation (7) (C(t) is proportional to the force autocorrelation function).…”

“…[46,112] Beyond such an approach, flexible MD calculations should be able to treat IVR and VET in solution, even for polyatomic systems. Recently, VET of azulene in solution and supercritical fluids [122][123][124] and VET as well as IVR of diiodomethane in solution was investigated in full MD simulations. [96] The latter study has accompanied our recent experimental investigations of the intra-and intermolecular vibrational energy flow of CH 2 I 2 in solution.…”

Echtzeit‐Beobachtung photoinduzierter Chemie und molekularen Energietransfers in Lösung

“…Den Hauptbeitrag hierzu leistet ξ ( ω ) bei Frequenzen oberhalb des Energiebereiches der kollektiven Moden des Lösungsmittels, d. h. ω >100 cm −1 69. 123, 124 ξ ( ω ) ist dabei durch Gleichung (7) definiert. Hierbei ist C ( t ) proportional zur Kraftautokorrelationsfunktion, und Re steht für Realteil.…”

“…112 Darüber hinaus sollten MD‐Rechnungen jedoch auch für IVR‐ und VET‐Prozesse mehratomiger Moleküle in Lösung anwendbar sein. Kürzlich wurden vollständige MD‐Simulationen des VET von Azulen in überkritischen Fluiden und in Lösung122–124 sowie zu VET und IVR von Diiodmethan in Lösung beschrieben 96. Letztgenannte Studie begleitete unsere Experimente zum intra‐ und intermolekularen Schwingungsenergietransfer von CH 2 I 2 in Lösung.…”