The effects of torsion–vibration coupling on rotational spectra: Toluene reinterpreted and refitted

Torsion-vibration coupling in S1 toluene: Implications for IVR, the torsional barrier height, and rotational constants

“…12,24 In the present case, the interaction between the close lying states m 6(±) and 20 1 m 3(∓) (see Fig. 6) leads to states with significantly mixed character and consequently large changes in the rotational contours for the associated bands.…”

“…12 Our recent reassessment of the rotational line positions in S 0 toluene shows how torsion-vibration coupling perturbs constants such as F and contributes to dF values. 24 The requirement for a large dF where the resonance involves almost coincident zero-order states. From top to bottom, the three panels show (a) the spectrum predicted using the rotor constants reported by Borst and Pratt 10 and the M 20 frequency determined here, (b) the spectrum predicted using our rotor constants with torsion-vibration coupling switched off, and (c) the spectrum predicted using our rotor constants with the torsion-vibration coupling turned on.…”

“…From the coupling matrix, it is straightforward to show that the effective barrier height, V Effective 6 , determined without accounting for torsion-vibration coupling is given approximately by Ref. 24,…”

“…24 From the formulae given in Ref. 24, the changes predicted for these constants in S 1 as a result of torsion-vibration coupling can be calculated.…”

“…The methyl rotor in toluene provides a benchmark for torsional motion involving a 6-fold barrier and consequently has been the subject of numerous studies. 10,12,[16][17][18][19][20][21][22][23][24] Spectral observations of transitions to various S 1 m states have been reported by a number of authors. (Effectively, m denotes the rotor angular momentum quantum number.)…”