COUPLED-CLUSTER SPECTROSCOPIC PROPERTIES AND ISOMERIZATION PATHWAY FOR THE CYANATE FULMINATE ISOMER PAIR, NCO-/CNO-/

Three-dimensional near-equilibrium potential energy surfaces and dipol e moment functions have been calculated for the ground states of NCO- and CNO-, using the coupled cluster method with single and double subs titutions augmented by a perturbative estimate of triple excitations [ CCSD(T)] with a quadruple zeta basis set consisting of 150 contracted Gaussian type orbitals. The corresponding equilibrium bond distances a t their linear geometries are r(e)(NC) = 1.1934 Angstrom and r(e)(CO) = 1.2306 Angstrom for NCO-, and r(e)(CN) = 1.1866 Angstrom and r(e)(NO ) = 1.2741 Angstrom for CNO-. Full three-dimensional variational calcu lations have also been carried out using the CCSD(T) potential energy and dipole moment functions to determine the rotation-vibrational ener gy levels and dipole moment matrix elements for both NCO- and CNO-. Th e predicted band origin of the nu(3) band in the NCO- isomer (2114.4 c m(-1)) agrees well with the gas phase diode laser infrared result (212 4.4 cm(-1)). The variational analysis suggests possible revisions in t he assignment of the two experimentally observed hot bands which are a ffected by Fermi resonance. The calculated dipole moments of NCO- and CNO- in their ground vibrational states are 1.504 and 1.482 D, respect ively. The CCSD(T) method with a triple zeta basis set was employed to more broadly explore the isomerization path between the two isomers. In agreement with previous lower level calculations a broad and shallo w minimum corresponding to a cyclic oxazirine form was found. The imme diate vicinity of this local minimum in the potential energy surface a nd those of the two saddle points separating it from the linear isomer s were further refined using the same quadruple zeta basis used for th e two near-equilibrium regions. The equilibrium structures and energie s of the two neutral isomers, NCO, and CNO, were also calculated at th e same level of theory. For NCO, whose photoelectron spectrum has rece ntly been studied in detail, the predicted electron affinity and neutr al-ion bond distance changes agree well with the photoelectron results . The reported spectroscopic structure of NCO, however, is not support ed by the present CCSD(T) calculations. (C) 1997 American Institute of Physics.