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The potential energy surfaces of ten electronic states of nitric oxide (NO) have been reexamined computationally, with state energies calculated using ab initio multireference methods. Our wave function expansions of 10×106 configurations improve upon the results of de Vivie and Peyerimhoff [J. Chem. Phys. 89, 3028 (1988)], who obtained excellent results from expansions of 16 000 configurations in 1988. We present results for the adiabatic properties re, Be, Te, and ωe, demonstrating standard errors of 0.012 Å, 0.026 cm-1, 620 cm-1, and 41 cm-1, respectively. Vertical excitation energies and oscillator strengths are also presented, as are potential energy surface curves, with special attention to the B/C avoided crossing. The technical issue of state-averaging effects is also discussed. [ABSTRACT FROM AUTHOR]

Characteristics of the ground electronic state of HNCO have been investigated theoretically in a series of eight ab initio analyses involving qualitative features of the electronic structure, the barrier to linearity, the NH(3Σ-)+CO fragmentation energy, the H–NCO bond dissociation energy, heats of formation of isomers of HNCO, fundamental vibrational frequencies and anharmonic force fields, the rovibrational spectrum of DNCO, and the precise Re structure of isocyanic acid. Sundry state-of-the-art electronic structure methods were employed in the study, including restricted and unrestricted Hartree–Fock (RHF and UHF), complete-active-space self-consistent-field (CASSCF), configuration interaction singles and doubles (CISD), Mo\ller–Plesset perturbation theory through fourth and occasionally fifth order (MP2–MP5), coupled-cluster singles and doubles (CCSD), and CCSD augmented by a perturbative contribution from connected triple excitations [CCSD(T)]. The one-particle basis sets ranged in quality from (9s5p1d/4s2p1d) to (13s8p3d2f/6s5p3d2f ) on the heavy atoms and from (4s1p/2s1p) to (6s2p1d/4s2p1d) on hydrogen. Several revisions of thermochemical data are proposed, in particular, a larger barrier to linearity of 5.7(3) kcal mol-1, an enhanced bond energy of 85.4(10) kcal mol-1 for D0(NH–CO), and more reliable relative energies for the isomers of HNCO, viz., γe(HOCN)=25.5(10), γe(HCNO)=70(2), and γe(HONC)=84.5(15) kcal mol-1. In addition, the experimental value D0(H–NCO)=113.0(2) kcal mol-1 is confirmed. These results lead to several new proposals for heats of formation (ΔH°f,0, kcal mol-1): HNCO(-26.1), HOCN(-0.7), HCNO(+43.0), HONC (+57.6), and NCO(+35.3). A complete quartic force field has been constructed for HNCO by combining RHF third- and fourth-derivative predictions with CCSD quadratic force constants subjected to the scaled quantum mechanical (SQM) optimization scheme.This force... [ABSTRACT FROM AUTHOR]

The vibrational frequencies of several linear polyynes HC[SUBN]H, HC[SUBN]I and IC[SUBN]I, for even-valued N up to 60, have been obtained using computational quantum chemistry. The bending normal modes have the appearance of classical transverse normal modes of a vibrating string fixed at both ends, for which the frequencies vary with the first power of the harmonic. Our calculations, however, reveal that in the limit of infinite chain length and infinite mass on the ends of the 'molecular string', the bending frequencies vary with the square of the harmonic, at all levels of theory. A derivation is presented to explain the discrepancy. [ABSTRACT FROM AUTHOR]