MICROWAVE-SPECTRA, STRUCTURE, AND ELECTRIC-DIPOLE MOMENT OF THE AR-ISOCYANIC ACID VAN-DER-WAALS COMPLEX

The microwave spectra of Ar-HNCO and Ar-DNCO have been investigated with a pulsed-nozzle Fourier-transform microwave spectrometer in the region of 8 to 18 GHz. One state is observed for each isotopomer and the a- and b-type transitions are fitted to an asymmetric top Hamiltonian. The spectral constants obtained from this fit for Ar-HNCO are A = 11224.5667(9) MHz, B = 1937.6357 (5) MHz, C = 1638.6608 (2) MHz, Delta(J) = 14.689(10) kHz, Delta(JK) = 388.52(13) kHz, Delta(K) = -313.85(37) kHz, delta(J), = 2.471(2) kHz, chi(aa) = -0.946(3) MHz, and chi(bb) = 2.024(4) MHz. Stark effect measurements determine mu(a) = 1.3940(50) x 10(-30) C m [0.4179(15) D] and mu(b) = 5.1219(83) X 10(-30) C m [1.5355(25) D] for Ar-HNCO. The inertial defects, Delta in u Angstrom(2), are 2.5629 and 2.555 for Ar-HNCO and Ar-DCNO, respectively. A planar T-shaped structure is determined from the effective moments of inertia with an Ar to HNCO center-of-mass separation of 3.577 Angstrom. The moment of inertia data indicate that the oxygen is tilted toward the argon and the hydrogen is oriented syn with respect to argon. However, the small measured value of mu(a), in the complex compared to mu(b) in the monomer, HNCO, suggests that the hydrogen undergoes internal rotation approximately about the heavy atom axis of HNCO (a inertia axis of HNCO) which would mean the effective moments of inertia are averaged over this motion. Comparison of the quadrupole coupling constants in HNCO to Ar-HNCO also provides evidence in support of this internal motion. The uncertainties shown in parentheses are one standard deviation. (C) 1995 Academic Press, Inc.