Microwave and theoretical investigation of the internal rotation in m-cresol

Abstract

The microwave spectrum of m-cresol (3-methylphenol) has been investigated using a molecular beam Fourier transform microwave spectrometer in the frequency range from 3 to 26.5 GHz. The rotation of the hydroxy group into two different unequal energetic minima leads to different spectra for the syn- and anticonformers. Because of a high potential barrier both conformers can be analyzed independently. The methyl group is undergoing an almost free internal rotation which is only hindered by small barriers and splits the vibrational ground state in two states of internal rotation denoted as A and E species. The spacing between the species is found to be up to 10 GHz. The potential for the internal rotation can be determined from the spectra and analyzed in terms of the Fourier components V3 and V6. For syn-m-cresol these parameters were determined as V3=673(3) GHz and V6=-335(24) GHz and for anti-m-cresol V3=95(5) GHz and V6=-416(46) GHz. The barriers to internal rotation were furthermore calculated with second-order Moller-Plesset perturbation theory and second-order coupled-cluster singles- and-doubles model (CC2) in the electronic ground state and with CC2 in the first excited state. The CC2 method is found to be an appropriate method to calculate potential barriers in electronic excited states of such compounds.

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

“…Another objective has been the study of the internal rotation of the hydroxyl group. This internal motion could not be investigated in the previous studies on o-and m-cresol 11,12 because in these cases it leads to a tilt of the molecular principal axis system and thus to two distinct conformers. The theoretical background of hindered rotations in C 2 symmetric potentials has been derived by Quade 19 and Bauder et al 20 The methods were employed, e.g., for phenol, 21 4-fluorophenol, 4-chlorophenol, and 4-bromophenol.…”

“…For low barriers the spectra can often not be reproduced within the accuracy of the experimental resolution. 12 Results of ab initio investigations of such cases show a dependency of the rotational constants on the torsional angle ͓A͑␣͒ = A 0 + A 1 cos 3␣ + A 2 cos 6␣ +¯͔ ͑see, e.g., Ref. 33͒.…”

“…10 The present publication concludes a series of combined microwave and quantum chemical studies on cresols. 11,12 p-cresol is currently under investigation in very different fields of chemistry: In atmospheric chemistry as a precursor for the formation of photo-oxidants and secondary organic aerosols, 13 in clinical chemistry for its influence on kidney failures, 14 and in food chemistry where p-cresol has been spotted to have an influence on the composition of the aroma of cheddar cheese. 15 For para substituted toluenes with a C 6 symmetry of the internal rotor ͑toluene, 16 p-fluorotoluene, 17 and p-chlorotoluene 18 ͒, very low barriers hindering the methyl torsion of less than 5 cm −1 were found.…”

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.…”