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 beam Fourier transform microwave spectrometer. The rotational constants were 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‘ J K = 34.8064(34) kHz, δ‘ J = −0.010 48(24) kHz, and R‘ 6 = −0.000 40(18) kHz. The diagonal elements and one off-diagonal element of the quadrupole coupling tensor were determined to be χ aa = −0.113 10(48) MHz, χ bb = 1.941 36(87) MHz, χ cc = −1.828 26(45) MHz, and χ bc = ±4.8954(22) MHz. Using BSSE-corrected supermolecular Møller−Plesset (MP) perturbation 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 were determined. Argon adopts a position above the ring plane (Ar−ring distance R = 3.44 (exp), 3.53 (MP2, r e ), 3.55 Å (MP4, r e ) shifted 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.