A coupled cluster study of the spectroscopic properties and electric dipole moment functions of nitrous sulfide

3-D near-equilibrium potential energy surfaces and dipole moment functions have been calculated for the ground state of nitrous sulfide (NNS), using a large basis set and the coupled cluster method with single and double substitutions, augmented by a perturbative estimate of triple excitations [CCSD(T)]. The CCSD(T) equilibrium bond lengths with a correlation consistent polarized valence quadruple zeta (cc-pVQZ) basis set are re(NN)=1.1284 A and Re(NS)=1.5904 A, which have been corrected to 1.126 and 1.581 A, respectively, based on the results of the corresponding calculations on the NN and NS diatomics. Rotational-vibrational energy levels and the corresponding infrared intensities for NNS have been determined using variational methods with the CCSD(T)/cc-pVQZ potential energy and dipole moment functions. The calculated band origins (cm-1) ν1, ν2, and ν3 and their intensities (km/mol) at the CCSD(T)/cc-pVQZ level are 740.7/38.6, 463.1/0.01, and 2061.4/385.8, respectively. A complete set of second-order spectroscopic constants has been obtained from the ab initio potential energy surface using both the standard perturbation theory formulas and the variationally determined rovibrational energies. Comparison of the theoretical vibration-rotation interaction constants (αi) with those obtained from published high resolution Fourier transform infrared (FTIR) spectra clearly demonstrate that the rotational quantum number (J) assignments must be revised in all the observed hot bands. A new set of spectroscopic constants for NNS, derived from a reanalysis of the published FTIR frequencies, is presented. These are in excellent agreement with our CCSD(T) predictions. Values of the quadrupole coupling constants at each nucleus are predicted using multireference configuration interaction with the same cc-pVQZ basis. copyright 1996 American Institute of Physics