The low frequency density of states and vibrational population dynamics of polyatomic molecules in liquids

Abstract

Instantaneous normal mode calculations of the low frequency solvent modes of carbon tetrachloride ͑CCl 4 ͒ and chloroform ͑CHCl 3 ͒, and experiments on the vibrational population dynamics of the T 1u CO stretching mode ͑ϳ1980 cm Ϫ1 ͒ of tungsten hexacarbonyl in CCl 4 and CHCl 3 are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump-probe experiments measuring the vibrational lifetime of the T 1u mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl 4 , the vibrational lifetime decreases as the temperature is increased; however, in CHCl 3 , the vibrational lifetime actually becomes longer as the temperature is increased. The change in thermal occupation numbers of the modes in the solute/solvent systems cannot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the anharmonic coupling matrix elements are considered. The calculated differences in the temperature dependences of the densities of states appear too small to account for the observed difference in trends of the temperature dependent lifetimes. This suggests that the temperature dependence of the liquid density causes significant changes in the magnitude of the anharmonic coupling matrix elements responsible for vibrational relaxation.

Vibrational relaxation of a polyatomic solute in a polyatomic supercritical fluid near the critical point

“…[16][17][18] Analysis of experiments in liquids has shown that an increase in density causes the vibrational lifetime to become shorter. 17 High pressure experiments in liquids and solids have shown that vibrational line frequencies are also density dependent. 22 A red shift results from an increase in the attractive part of the intermolecular potential.…”

“…[16][17][18] The results on SCF systems presented below will be discussed in terms of relationships between the vibrational experimental observables and the local solvent density about a solute when the system is near the critical point. The use of transient vibrational spectroscopy, which interrogates the dynamics of a system on the ground state potential surface, avoids possible complications associated with changes in intermolecular interactions that occur upon excitation to excited electronic states.…”

Vibrational Lifetimes and Spectral Shifts in Supercritical Fluids as a Function of Density: Experiments and Theory

“…Instantaneous normal mode calculations in CCl 4 , CHCl 3 , and CS 2 show cutoffs in the DOS at ∼150, ∼180, and ∼200 cm -1 , respectively. 67,68 The higher frequency portions of the DOS are dominated by orientational modes. Since ethane, fluoroform, and CO 2 are much lighter than the liquids cited above, it is expected that their DOS will extend to substantially higher frequency.…”

Vibrational Dynamics of Carbon Monoxide at the Active Site of Myoglobin: Picosecond Infrared Free-Electron Laser Pump-Probe Experiments

“…32 Vibrational relaxation of a relatively small molecule in a solvent, e.g., CO vibrations of W(CO) 6 in CCl 4 or CHCl 3 , involves lower frequency vibrational modes of the molecule, lower frequency vibrational modes of the solvent, and modes of the solvent continuum (instantaneous normal modes). 16,17,23 However carbonyl VR in heme-CO is qualitatively different from the smaller molecule systems because heme is a large molecule; neither a protein nor a solvent is required for VR to occur. The model compound VR data in Figure 4 show that VR in hemes without proteins can be just as efficient as with proteins.…”

“…Studies of vibrationally excited CO have been used to investigate dynamics in many condensed-phase systems, including CO dissolved in monatomic and diatomic liquids, 20,21 crystalline CO, 22 organometallic complexes in solution containing CO bound to a single metal atom or a cluster of metal atoms, 17,[23][24][25] CO bound to clean metal surfaces, [26][27][28] CO bound to heme proteins, 12,13,15,29 and CO bound to model compounds. 15 Although the mechanical dynamics of such complicated systems ordinarily are quite difficult to treat theoretically, CO introduces several useful simplifications.…”