p-fluorotoluene. I. Methyl (CH3 and CD3) internal rotation in the S1 and S states

Abstract

Acceleration of intramolecular vibrational redistribution by methyl internal rotation. II. A comparison of m fluorotoluene and pfluorotolueneAn analysis of the methyl rotation dynamics in the S 0 (X1 A 1) and T 1 (a3 A 2) states of thioacetone, (CH3)2CS and (CD3)2CS from pyrolysis jet spectra J. Chem. Phys. 95, 3137 (1991); 10.1063/1.460871Nonlinear dynamics of methyl rotation and intramolecular energy diffusion in pfluorotoluene Supersonic jet ScSo spectroscopy (resonance-enhanced multiphoton ionization, fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the SI and So internal rotation energy level structure of p-fluorotoluene with a CD 3 methyl rotor as well as to extend observations of the CH 3 rotor structure. The observed rotor energy levels 2';;;m.;;;8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V6 are evaluated. V6 may be obtained independently from B by observations of aE 3 , the observed splitting of the 3ai' and 3a!{ rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6 = -211E3 holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and SI-S0 fluorescence results are combined with the present work to produce a set of preferred values for these constants. The values in cm -I for the So state are B = 5.46 (2.82) and V6 = -4.77 ( -4.77) for CH 3 (CD 3 ) rotors. The SI values are B = 4.90 (2.54) and V6 = -33.0 ( -25.2). The 20% barrier height reduction occurring on transformation from a CH 3 to a CD 3 rotor is similar to that observed in other systems. Calculation implies that the staggered conformer is the minimum energy configuration for both electronic states. Many of the SI-SO rotor transitions are forbidden, and a discussion is given of induced intensity mechanisms that involve coupling of internal rotation to overall rotation or coupling of internal rotation to electronic motion. Substantial energy-level perturbations often occur for states with m;;.5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations. 6362

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.…”

“…Interestingly, Okuyama et al, 30 Zhao and Parmenter, 31 and Zhao 32 assigned a corresponding band in their LIF spectrum to the 1 7 transition. The dispersed fluorescence spectrum in ref.…”

Molecular symmetry group analysis of the low-wavenumber torsions and vibration-torsions in the S1 state and ground state cation of p-xylene: An investigation using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy

“…In previous work, our group has also studied the pFT molecule; 4,12,13 as has the group of Lawrance, 14,15 some of which was in collaboration with ourselves. In addition, we shall make passing comment on previous fluorescence work on pFT, 16,17,18 ZEKE spectroscopy on toluene by Lu et al 19 and ZEKE spectroscopy on pFT by Takazawa et al 20 and ourselves. 12,13 In the present work, we examine the S1  S0 electronic transition in p-xylene (pXyl), which has two methyl groups that are located on opposite sides of a phenyl ring, using REMPI and ZEKE spectroscopy.…”

Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene

“…But transitions to a 2 levels are always symmetry forbidden in a jet cooled condition and hence remain absent in most cases. However, in the case of p-fluorotoluene the band observed at 37 cm −1 in the excitation spectrum was assigned as a torsional 3a 2 transition, 17 as no band near this frequency region was found in the excitation spectrum of p-fluorostyrene 18 due to the absence of the methyl group in this molecule. Though this transition is forbidden, sometimes it may gain intensity due to the coupling of the internal rotation with the overall rotation or other low frequency modes present in the molecule.…”