On the internal rotations in p-cresol in its ground and first electronically excited states

Abstract

The overall rotation and internal rotation of p-cresol (4-methyl-phenol) has been studied by comparison of the microwave spectrum with accurate ab initio calculations using the principal axis method in the electronic ground state. Both internal rotations, the torsions of the methyl and the hydroxyl groups relative to the aromatic ring, have been investigated. The internal rotation of the hydroxyl group can be approximately described as the motion of a symmetrical rotor on an asymmetric frame. For the methyl group it has been found that the potential barrier hindering its internal rotation is very small with the first two nonvanishing Fourier coefficients of the potential V(3) and V(6) in the same order of magnitude. Different splittings of b-type transitions for the A and E species of the methyl torsion indicate a top-top interaction between both internal rotors through the benzene ring. An effective coupling potential for the top-top interaction could be estimated. The hindering barriers of the hydroxyl and methyl rotation have been calculated using second-order Moller-Plesset perturbation theory and the approximate coupled-cluster singles-and-doubles model (CC2) in the ground state and using CC2 and the algebraic diagrammatic construction through second order in the first electronically excited state. The results are in excellent agreement with the experimental values.

Conformational effects on the torsional barriers in m-methylanisole studied by microwave spectroscopy

“…The barriers decrease in m-isomers, because the substituents are further apart, which creates a symmetric local environment near the methyl group, even though the global frame of the molecule is still asymmetric. The p-isomers often possess the smallest barriers, 18,24,26 because the molecule is electronically and structurally symmetric, as already discussed in Ref. 18.…”

“…It is interesting to compare the respective torsional barriers when the ring methyl group is in m-position, because depending on the relative position of the substituents, considerable effects on the torsional barrier height can be observed. There are studies in the literature on the isomers of fluorotoluene, [19][20][21] cresol, [22][23][24] and methylbenzaldehyde, 25,26 stating very different barrier heights for the o-, m-, and p-isomers.…”

Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy

“…9 The V 3 /V 6 ratio as well as their signs are in agreement with results from theoretical calculations at the MP2/6-311++G(2d,p) level of theory; and fitting both V 3 and V 6 is required to obtain a reasonable fit. 9 In the investigation on p-tolualdehyde (5), Saal (1) toluene, 1 (2) p-fluorotoluene, 7 (3) p-chlorotoluene, 8 (4) p-cresol, 11 (5) p-tolualdehyde, 10 (6) p-toluic acid, 9 (7) 4-methylacetophenone (this work), and (8) p-methylanisole. 4 Lower trace: V 3 potential of the acetyl methyl group (red) in substituted acetophenones: (9) acetophenone, 12 (10) acetovanillone, 13 and (11) 6-hydroxy-3methoxyacetophenone.…”

Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol

“…They are commonly used as model systems for hydrodeoxygenation in catalytic chemistry for refining biofuels, [2][3][4][5][6] while the internal rotations of the functional groups are of great interest to many spectroscopists. [7][8][9][10][11][12][13] Their thermochemistry and kinetics have also been studied, e.g.…”

“…Methylphenols have been extensively studied using a large variety of spectroscopic techniques, including microwave absorption, 7,9 Laser Induced Fluorescence (LIF), 8,10,11,13,21 Resonance Enhanced Multiphoton Ionization (REMPI), 18,20,24 stimulated Raman-UV optical double resonance, 23 and Hole Burning spectroscopy. In addition to purely spectroscopic probes, these compounds have been ionized and detected using Time of Flight (TOF) 18,24 and Fourier-Transform Ion Cyclotron Resonance (FTICR) mass spectrometry.…”