A COUPLED-CLUSTER STUDY OF THE SPECTROSCOPIC PROPERTIES AND ELECTRIC-DIPOLE MOMENT FUNCTIONS OF NITROUS SULFIDE

Three-dimensional near-equilibrium potential energy surfaces and dipol e moment functions have been calculated for the ground state of nitrou s sulfide (NNS), using a large basis set and the coupled cluster metho d with single and double substitutions, augmented by a perturbative es timate of triple excitations [CCSD(T)]. The CCSD(T) equilibrium bond l engths with a correlation consistent polarized valence quadruple zeta (cc-pVQZ) basis set are r(e)(NN)=1.1284 Angstrom and R(e)(NS)=1.5904 A ngstrom, which have been corrected to 1.126 and 1.581 Angstrom, respec tively, based on the results of the corresponding calculations on the NN and NS diatomics. Rotational-vibrational energy levels and the corr esponding infrared intensities for NNS have been determined using vari ational methods with the CCSD(T)/cc-pVQZ potential energy and dipole m oment functions. The calculated band origins (cm(-1)) nu(1), nu(2), an d nu(3) and their intensities (km/mol) at the CCSD(T)/cc-pVQZ level ar e 740.7/38.6, 463.1/0.01, and 2061.4/385.8, respectively. A complete s et of second-order spectroscopic constants have been obtained from the ab initio potential energy surface using both the standard perturbati on theory formulas and the variationally determined rovibrational ener gies. Comparison of the theoretical vibration-rotation interaction con stants (alpha(i)) with those obtained from the published high resoluti on Fourier transform infrared (FTIR) spectra clearly demonstrate that the rotational quantum number (J) assignments must be revised in al th e observed hot bands. A new set of spectroscopic constants for NNS, de rived from a reanalysis of the published FTIR frequencies, is presente d. These are in excellent agreement with our CCSD(T) predictions. Valu es of the quadrupole coupling constants at each nucleus are predicted using multireference configuration interaction (MRCI) with the same cc -pVQZ basis. (C) 1996 American Institute of Physics.