A COUPLED-CLUSTER STUDY OF THE STRUCTURES, SPECTROSCOPIC PROPERTIES, AND ISOMERIZATION PATH OF NCS- AND CNS-

Three-dimensional near-equilibrium potential energy surfaces and dipol e moment functions have been calculated for the X (1) Sigma(+) ground states of NCS- and CNS-, using the coupled cluster method with single and double substitutions augmented by a perturbative estimate of tripl e excitations [CCSD(T)] with a set of 154 contracted Gaussian-type orb itals. The corresponding equilibrium bond lengths at their linear geom etries are r(e)(NC)=1.1788 Angstrom and r(e)(CS)=1.6737 Angstrom for N CS-, and r(e)(CN)=1.1805 Angstrom and r(e)(NS)=1.6874 Angstrom for CNS -. The predicted equilibrium rotational constants B-e of NCS- and CNS- are 5918.2 and 6282.7 MHz, respectively. The former agrees very well with the known experimental value (5919.0 MHz). Full three-dimensional variational calculations have also been carried out using the CCSD(T) potential energy and dipole moment functions to determine the rovibra tional energy levels and dipole moment matrix elements for both NCS- a nd CNS-. The corresponding fundamental band origins (cm(-1)) nu(1), nu (2), and nu(3) and their absolute intensities (km/mol) at the CCSD(T) level are 2060.9/306.1, 451.5/2.2, and 707.5/12.8, respectively, for N CS- and 2011.4/6.6, 343.7/2.3, and 624.9/0.2 for CNS-. The calculated nu(1) (CN stretching) value for NCS- is in very good agreement with th e experimental result, 2065.9 cm(-1). The calculated dipole moments of NCS- and CNS- in their ground vibrational states are 1.427 and 1.347 D, respectively The transition state geometry (saddle point) for the i somerization of NCS--->CNS- is predicted at the CCSD(T) level to be r( NC)=1.2044 Angstrom, R(CS)=1.9411 Angstrom and theta(angle NCS)=86.8 d egrees. Its calculated energy is 62.6 and 26.5 kcal/mol above the mini ma of NCS- and CNS-, respectively, including zero-point energy correct ions. The structure of the NCS radical was also optimized at the same level of theory, yielding ion to neutral bond length shifts in excelle nt agreement with those derived from recent photoelectron spectroscopy experiments. (C) 1995 American Institute of Physics.