Rotational dynamics of methyl groups in m-xylene

Abstract

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH 3 groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methylmethyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.

On the Dynamics of Xylene Isomers Excited in the Vacuum‐Ultraviolet (VUV) Region

“…The coupling between the methyl rotors has been discussed in numerous works on the IVR of xylenes. It has been found that, in general, substitution by methyl groups accelerates the IVR process because of the interaction of the methyl rotors with the ring and with each other . This suggestion is consistent with the observation in the EI spectra that R is lower for p ‐xylene, for which the methyl groups can be thought of as uncoupled.…”

“…The role of methyl substitution and internal rotation in the acceleration of intramolecular vibrational redistribution (IVR) has attracted the interest of researchers for many years . Hyperconjugation and van der Waals interactions between the methyl rotor and the ring are believed to induce coupling between several normal vibrational modes, which thus leads to faster IVR processes.…”

“…[24][25][26] The role of methyl substitution and internal rotation in the acceleration of intramolecular vibrational redistribution( IVR) has attractedt he interest of researchers for many years. [27][28][29][30][31] Hyperconjugation and van der Waals interactions between the We report on the dynamics of electronically excited o-, m-, and p-xylene on the femtosecond timescale by employing the vacuum-ultraviolet pump-IRp robe mass spectrometry technique. The molecules were excited by the fifth harmonic (l = 160 nm) of aT i:sapphirel aser at as uperposition of the S 3 valence with severalR ydberg states.…”

Quasielastic neutron scattering experiments including activation energies and mathematical modeling of methyl halide dynamics

“…Similar to discussions in Ref. 23, a more complex transition scheme combined with dispersion could result in less pronounced librational peaks, and a further suppression of intensity could occur via the above-mentioned damping. The coupling model also yields transitions in the range of 16-18 meV, and these additional features as well as the magnitude of the splitting would not contradict the experimental DOS ͑Fig.…”

Rotational dynamics and coupling of methyl group rotations in methyl fluoride studied by high resolution inelastic neutron scattering

“…In addition, a slight asymmetry of the peak can be seen although this certainly less pronounced as in a previous study of m-xylene. 17 In order to obtain a value of the split of the tunneling sublevels three Lorentzians, representing the AA → AE, AA → E a E b , and AA → E a E a transitions with relative intensities 2:1:1 should be used. Due to the slightly worse than expected instrumental resolution ͑see above͒ in combination with the expected small difference of only 0.02 eV between the AA → E a E b and AA → E a E a transitions we were not able to describe both transitions separately.…”