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 wi th a pulsed-nozzle Fourier-transform microwave spectrometer in the reg ion 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 = 112 24.5667(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 [1.53 55(25) D] for Ar-HNCO. The inertial defects, Delta in u Angstrom(2), a re 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 w ith an Ar to HNCO center-of-mass separation of 3.577 Angstrom. The mom ent of inertia data indicate that the oxygen is tilted toward the argo n 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 th e monomer, HNCO, suggests that the hydrogen undergoes internal rotatio n approximately about the heavy atom axis of HNCO (a inertia axis of H NCO) which would mean the effective moments of inertia are averaged ov er this motion. Comparison of the quadrupole coupling constants in HNC O 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.