The Relative Influences of Ring Modes and Methyl Internal Rotation on the Cold Beam IVR of p‐Fluorotoluene

Abstract

Abstract. Room temperature (300 K) experiments have earlier established that the replacement of a fluorine atom in p-difluorobenzene (pDFB) with a methyl group to make p-fluorotoluene @FT) generates a qualitative difference in intramolecular vibrational redistribution (IVR) characteristics as seen in the S, states. Here we report S , S , fluorescence excitation and dispersed single vibronic level (SVL) fluorescence spectra that have been obtained for IVR comparisons in the cold (5 K) environment of supersonic expansions. As in the 300 K experiments, the cold beam S, vibrational energy threshold for IVR is substantially lower in pFT. The vibrational congestion in dispersed fluorescence that reveals extensive S, level interactions first appears after pumping an S, pFT level near 800 cm-'. In contrast, congestion in pDFB spectra is still absent from levels with twice this energy. Attention is directed to the relative S, ring level structures as a potential contributor to the distinctive IVR behaviors. Dispersed fluorescence from the S, zero point levels and fluorescence excitation spectra are combined with published information about S, fundamentals to show that the S, vibrational level structures of the two molecules are as closely related as those of an isotopomer pair. It is argued that the small differences in S, fundamentals cannot be a principal contribution to the qualitative IVR differences.

Methyl rotor dependent vibrational interactions in toluene

“…IV C the 11 1 region in S 0 where the separation of the m = 0 and 1 levels is larger than expected. This change in the m state energies for 11 1 is analogous to the different shifts seen for the m = 0 and 1 states in the 14 2 7 Our results are compared with theirs in Table IV. There is excellent agreement between the two sets of data, although there is a consistent trend for our values to be ∼2%-4% below those reported by Davies et al We are unsure of the reason for this difference.…”

“…This reveals that the coupling matrix elements are similar in the m = 0 and m = 1 cases but that the separation between the zero-order levels is different. We then compare our spectral results with the outcome of the analysis of the quantum beat data obtained by Davies et al 7 Following this we consider the separation of the m levels for 11 1 because it provides a simpler illustration of the effect responsible for the m-dependent shifts seen in the 14 2 on the generality of the effect and its consequences for timeresolved IVR experiments.…”

“…We first determine the "zero-order" states and their coupling strengths from the relative intensities and band separations (Tables III and IV In principle, the terms to be extracted from the analysis of the resonance are the energies of the three zero-order states, i.e., E(14 2 ), E(11 1 ), and E(19 1 The relative intensities depend on the oscillator strengths of both the excitation and emission steps. These can be calculated from the eigenvectors for the Fermi resonance states (see Table VI).…”

“…1 For example, there is an increase in IVR rate by two orders of magnitude in p-fluorotoluene (pFT) compared with p-difluorobenzene (pDFB) at similar vibrational energies. [1][2][3][4][5][6] In order to provide a detailed understanding of the mechanisms leading to this effect, Reid and co-workers have developed the technique of picosecond time resolved photoelectron spectroscopy as a probe of the behavior of relatively low-lying vibrational levels, with particular application to toluene and pFT. [7][8][9][10][11][12][13][14] The aim of the experiments has been to investigate couplings where there are comparatively few potential interactions, allowing the mechanism involved to be deduced.…”

Torsion and vibration-torsion levels of the S1 and ground cation electronic states of para-fluorotoluene

“…38 What is clear from the results here is that the vibration-torsional coupling is pervasive and involves different vibrations, with the symmetry of the vibration dictating which m levels can interact. Further, subsequent interactions between the vibtor levels and vibrational states can lead to eigenstates that are admixtures of these, and hence provide generalized routes for energy flow between torsional and vibrational motions and hence, as discussed by (amongst others) Parmenter et al 1,23,24,25,31,35 , Reid and Davies 28 and Gascooke et al, 14 accelerated IVR in methyl-group-containing molecules. We also deduce that the vibration-torsional coupling is subtly different for each of the vibrational levels (fundamentals, combinations and overtones), as expected.…”