Y. Pak et al., COUPLED-CLUSTER SPECTROSCOPIC PROPERTIES AND ISOMERIZATION PATHWAY FOR THE CYANATE FULMINATE ISOMER PAIR, NCO-/CNO-/, The Journal of chemical physics, 106(12), 1997, pp. 5123-5132
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.