INTERMOLECULAR FORCES IN ARGON VAN-DER-WAALS COMPLEXES - ROTATIONAL SPECTRUM AND AB-INITIO INVESTIGATION OF AR-OXAZOLE

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 D-J' = 5.524 11(28) KHz, D-JK' = 37.1990-(30) kHz, D-K' = -35.922(28) kHz, delta(J)' = 0.026 26(21) kHz, and R'(6) = -0.000 49(19) kHz. The diagonal elements of the nitrogen quadrupole coupling tenser were determined to be chi(aa) = 2.3032(6) MHz, chi(bb) = -4.0526(8) MHz, and chi(cc) = 1.7494(4) MHz. With the help of supermolecular Moller-Plesset perturbation theory at second (MP2) and fourth order (MP4(SDTQ)) using a (14s10p2d1f)[7s4p2d1f] basis set for argon, a 6-31G(+sd+sp) basis for oxazole, and basis set superposition corrections, stability (MP2, 316; MP4, 304 cm(-1)), equilibrium geometry, charge distribution, and other properties of the complex were determined. Argon adopts a position above the ring plane (Ar-ring distance: r(0), 3.46; MP2, 3.64; MP4, 3.58 Angstrom) 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 O atom. A new way of analyzing van der Waals complexes and predicting structural and 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.