Vibrational Dynamics of Terminal Acetylenes:  II. Pathway for Vibrational Relaxation in Gas and Solution

Abstract

The pathway for vibrational-energy flow following the excitation of the first excited state of the acetylenic C-H stretch is investigated for a series of 10 terminal acetylenes in room-temperature gases and dilute solutions using transient absorption picosecond infrared spectroscopy. The transient absorption infrared spectra are obtained at three different probe frequencies. These experiments separately detect the population of the excited C-H stretch state, the population of vibrational states with 2 quanta of acetylenic C-H bend excitation, and the population of all other vibrational states with C-H stretch absorption frequencies within the laser bandwidth (25 cm -1 ) of the C-H stretch fundamental frequency. These measurements show that the initial redistribution event for the isolated molecule involves population transfer to vibrational states with bend overtone excitation. The secondary intramolecular vibrational-energy redistribution (IVR) process, which involves population transfer to the remaining near-resonant vibrational states, occurs on a time scale that is about 5 times slower than the initial redistribution event. The same relaxation pathway is observed in dilute solution. The total relaxation rate in solution for the slower process can be quantitatively described using a simple model where IVR and solvent-induced vibrational-energy relaxation (VER) proceed independently. The main effects of the solvent are to increase the extent of population relaxation for the first stage of IVR and to cool vibrational excitation rapidly in the low-frequency acetylene wag normal-mode vibrations produced by the IVR dynamics.

On the origin of mode- and bond-selectivity in vibrationally mediated reactions on surfaces

“…Similarly, Pate and co-workers [55][56][57] showed that solvents ranging from non-polar CCl 4 to highly polar CCl 3 CN do not significantly alter the initial IVR decay rate of the acetylenic C-H stretch (v = 1). The time constants, shown in Fig.…”

Vibrational relaxation of CH3I in the gas phase and in solution

“…For example, an extensive treatment of the relaxation following excitation of the fundamental C-H stretch in a series of acetylenic molecules by Yoo et al [15][16][17] shows that some of the molecules have two intramolecular relaxation times in the gas phase although only the fast component survives in solution. They also determine that the fast relaxation comes from strong coupling between the initially excited C-H stretch and a sparse set of states containing two quanta of C-H bend and that the slow relaxation results from weak coupling of these states to the remaining states.…”

“…1 Although many studies examine vibrational relaxation in either the gas phase 2-5 or in solution, [6][7][8] only a few experiments directly compare the relaxation dynamics for the isolated and solvated molecule using the same technique. [9][10][11][12][13][14][15][16][17] In this paper we examine the influence that solvation has on IVR and IET by comparing the vibrational relaxation of CH 3 I in various solvents and in the isolated molecule.…”

“…Compari-son of the IVR rate in supercritical fluid solvents to the gas phase rate by Fayer and co-workers, [9][10][11][12] who examine W͑CO) 6 , and by von Benten et al, 13 who study benzene, reveals the extent of the solvent influence in these systems. In addition to the supercritical fluid experiments, Yoo et al [15][16][17] directly compare IVR in both the room temperature gas and in liquid solutions for various acetylenic molecules. Here we study the vibrational dynamics of CH 3 I in the gas phase and in solution.…”

Chemical dynamics of vibrationally excited molecules: Controlling reactions in gases and on surfaces

“…Studying the fate of an initially energized molecule requires time resolution on the order of the characteristic few hundred femtosecond encounter time, and modern ultrafast laser technology puts experiments in that regime. The most extensive studies of vibrational dynamics in liquids follow the historical pattern of first assessing energy transfer (25), and there are recent examples (54)(55)(56)(57)(58)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69) that particularly address the relaxation of COH stretching excitations. The general picture that emerges from these studies is that in relatively weakly interacting solvents, such as chloroform, the vibrational state structure and couplings of the molecule influence the energy flow within the initially excited molecule most strongly.…”