INTERMOLECULAR FORCES IN VAN-DER-WAALS COMPLEXES BETWEEN ARGON AND AROMATIC-MOLECULES - ROTATIONAL SPECTRUM AND AB-INITIO INVESTIGATION OF ISOXAZOLE-ARGON

The rotational spectrum of the isoxazole-argon complex was studied in the microwave region between 3 and 25 GHz using a pulsed molecular bea m Fourier transform microwave spectrometer. The rotational constants w ere found to be A = 4974.2534(2) MHz, B = 1382.291 788(24) MHz, and C = 1371.647 236(36) MHz. The centrifugal distortion constants are D'(J) = 5.624 21(30) kHz, D'(K) = -27.8794(256) kHz, D'(JK) = 34.5064(34) k Hz, delta'(6) = -0.010 48(24) kHz, and R'(6) = -0.000 40(18) kHz. The diagonal elements and one off-diagonal element of the quadrupole coupl ing tensor were determined to be chi(aa) = -0.113 10(48) MHz, chi(bb) = 1.941 36(87) MHz, chi(cc) = -1.825 26(45) MHz, and chi(bc) = +/- 4.8 954(22) MHz. Using BSSE-corrected supermolecular Moller-Plesset (MP) p erturbation theory at second (MP2) and fourth order (MP4(SDTQ)) with a (14s10p2d1f)[7s4p2d1f] basis set for argon and a 6-31G(+sd+sp) basis for isoxazole, stability (MP2 308 cm(-1); MP4 283 cm(-1)), equilibrium geometry, charge distribution, and multipole moments of the complex w ere determined. Argon adopts a position above the ring plane (Ar-ring distance R = 3.44 (exp), 3.53 (MP2, r(e)), 3.55 Angstrom (MP4, r(e)) s hifted from the center of the ring toward the NO bond. The complex is predominantly stabilized by dispersion interactions while its geometry is more a result of exchange repulsion forces, which direct Ar toward the most electronegative atoms of the ring, namely O and N.