Observation of the onset of torsion-induced, mode-specific dissipative intramolecular vibrational redistribution (IVR)

Abstract

Evidence is found showing that coupling with vibration-torsion ("vibtor") levels of both in-plane and out-of-plane vibrations is instrumental in causing dissipative intramolecular vibrational redistribution (IVR). Both zero-electron-kinetic-energy (ZEKE) and two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy are employed to investigate a series of bands located ~ 1200 cm -1 above the S1  S0 origin in p-fluorotoluene (pFT). Transitions in this wavenumber region have been the focus of a number of studies probing intramolecular vibrational redistribution (IVR). By recording both ZEKE and 2D-LIF spectra, a prepared S1 population is projected onto both the ground state cation and ground state neutral energy states, respectively, giving added confidence to the assignments. The spectral region under discussion is dominated by a pair of fundamental bands, but for the first time, we present explicit evidence that this is complicated by contributions from a number of overtones and combinations, including vibtor levels. We deduce that very different extents of coupling are present across a 60 cm -1 window of the spectrum, even though the density of states (DOS) is similar; in particular, one of the fundamentals couples efficiently to the increasing bath of levels, while one does not. We explain this by the influence of serendipitous near-coincidences of same-symmetry levels.

Vibration-modified torsional potentials and vibration-torsion (“vibtor”) levels in the m-fluorotoluene cation

“…In previous work, it has been noted that vibrations that involve motions close to the coordinate that demonstrates high-amplitude motion [52][53][54] are expected to show increased coupling. However, we have noted in very recent work 18 The Journal of Chemical Physics that it is important to look at the whole motion of a vibration. The present work demonstrates that even then, there are subtleties as to which vibrations couple most efficiently.…”

“…In discussing the V 3 barriers in the S 0 state, it was the steric interaction between the methyl hydrogen atoms and the ortho hydrogen atoms that was key, and this was also the case for the V 6 barrier in molecules that belonged to the G 12 MSG, which has been discussed previously. 18,55,56 When V 3 was large, the barrier was discussed in terms of asymmetry in the two C-C bonds that neighbored the C-CH 3 bond, with a relationship being found between the difference in bond order of the two C-C bonds and the barrier height. Furthermore, it was found that for mFT + , the geometry was pseudo-cis in the cation, as we find-Fig.…”

“…The coupling of methyl torsion and vibrational motions has been the subject of a series of studies on toluene, [1][2][3][4][5][6][7] parafluorotoluene (pFT), [8][9][10][11][12][13][14][15][16][17][18] and para-xylene (pXyl) 15,19,20 using a combination of fluorescence and photoionization spectroscopies. These studies have shown that such coupling can be examined and understood in detail in the low-wavenumber region and is likely to be prevalent to higher wavenumbers, driving the transition to statistical ("dissipative") intramolecular vibrational redistribution (IVR).…”

Torsions, low-frequency vibrations, and vibration–torsion (“vibtor”) levels in the m-chlorotoluene cation

“…1,2 A very recent example highlights the role vibrational excitation has in light harvesting. 3 Understanding the processes occurring in complicated molecules is greatly aided by detailed studies on small molecules, and recent examples from our, the Reid and Lawrance groups have looked at toluene, 4,5,6 para-fluorotoluene (pFT) 7,8,9,10,11,12,13,14,15 and para-xylene (pXyl), 10,16,17 using a combination of fluorescence and photoionization spectroscopies. These studies have elucidated how vibration-vibration and vibration-torsion coupling can drive the transition to statistical ("dissipative") intramolecular vibrational redistribution (IVR), underpinning energy dispersal and photostability.…”

“…These studies have elucidated how vibration-vibration and vibration-torsion coupling can drive the transition to statistical ("dissipative") intramolecular vibrational redistribution (IVR), underpinning energy dispersal and photostability. 10,12 Timbers et al 18 have concluded that meta-fluorotoluene (mFT) undergoes IVR more than an order of magnitude faster than pFT, showing that the location of substituents is likely to be important in the coupling.…”