The rotational spectrum of the oxazole-argon complex has been experimentally studied in the microwave region between 3 and 21 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. The rotational constants were found to be A = 5012.894 86 (14) MHz, B = 1398.428 151(32) MHz, and C = 1388.952 841(31) MHz. The centrifugal distortion constants are DJ' = 5.524 ll(28) kHz, DJK' = 37.1990-(30) kHz, DK' = -35.922(28) kHz, 6; = 0.026 26(21) kHz, and R'6 = -0.0oO 49(19) kHz. The diagonal elements of the nitrogen quadrupole coupling tensor were determined to be Xaa = 2.3032(6) MHz, Xbb = -4.0526(8) MHz, and xCc = 1.7494(4) MHz. With the help of supermolecular Mgller-Plesset perturbation theory at second (MP2) and fourth order (MP4(SDTQ)) using a (14slOp2dlf)[7~4p2dlfl basis set for argon, a 6-31G(+sd+sp) basis for oxazole, and basis set superposition corrections, stability (MP2, 316; MP4, 304 cm-I), equilibrium geometry, charge distribution, and other properties of the complex were determined. Argon adopts a position above the ring plane (Ar-ring distance: ro, 3.46; MP2, 3.64; MP4, 3.58 A) clearly shifted from the centrum of the ring in the direction of the oxygen atom. The complex is predominantly stabilized by dispersion interactions, while the position of the argon atom is determined by exchange repulsion forces that direct Ar toward the 0 atom. A new way of analyzing van der Waals complexes and predicting structuraland other complex properties is presented. Investigation of Ar-oxazole, Ar-benzene, and Ar-CO reveals that there is no charge transfer between the complex partners, contrary to previous claims made in the literature.

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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.