Arnim Hellweg¹, Christof Hättig¹, Ilona Merke², Wolfgang Stahl^2
1 Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany
² RWTH Aachen, Institut für Physikalische Chemie, Landoltweg 2, D-52056 Aachen, Germany

J. Chem. Phys. 124, 204305 (2006).
(Received 13 March 2006; accepted 30 March 2006; published online 22 May 2006)

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. (c) 2006 American Institute of Physics.

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