Search

All rovibrational levels of HCN/HNC up to ∼16 000 cm-1, relative to the HCN minimum, for J=0, 1, 2, have been calculated accurately. All internal degrees of freedom are included in these calculations, performed on the realistic, empirical potential surface by Murrel et al. [J. Mol. Spectrosc. 93, 307 (1982)]. Body-fixed mass-scaled Jacobi coordinates are employed, together with the discrete variable representation of the large amplitude motion (LAM) angular coordinate, and a 2-D distributed Gaussian basis for the radial degrees of freedom. The successive diagonalization–truncation procedure results in a compact matrix representation of the full rovibrational Hamiltonian, allowing accurate and efficient determination of a large number (>350 for J=2, p=0 case) of highly excited LAM rovibrational states of HCN/HNC. This approach is suitable for a broad class of floppy, isomerizing triatomic molecules and van der Waals complexes. In addition to energy levels and wave functions, expectation values of Jacobi coordinates, 〈R〉, 〈r〉, and 〈θ〉, are calculated for most states. The majority of calculated J=1,2 levels lie above the top of the isomerization barrier, and are delocalized to a varying degree over both local minima. Rotation appears to lower the energy threshold for extensive delocalization; for the states with J=1, or 2, it is ∼460–480 cm-1 below that for J=0 states. Moreover, increasing rotational excitation affects significantly the degree of localization of a given state. [ABSTRACT FROM AUTHOR]

Results of the first accurate quantum calculation of the delocalized, large amplitude motion vibrational (J=0) levels of HCN/HNC, lying above the isomerization barrier, are presented. The recently developed DVR-DGB quantum method [Z. Bacˇic and J. C. Light, J. Chem. Phys. 85, 4594 (1986)] is employed in this work. A model, empirical surface by Murrell et al. is used. All modes are included; the energy level calculation does not involve any approximations. Over a hundred vibrational levels are calculated accurately for this model surface. A number of them lie above the isomerization barrier; some are extensively delocalized over both HCN and HNC minima. Analysis shows that for HCN/HNC the threshold for significant delocalization is determined by the height of the vibrationally adiabatic bending barrier. In addition, the nearest neighbor level spacing distribution is obtained and compared to that of LiCN/LiNC. Various computational aspects of the DVR-DGB approach, which is applicable to any triatomic molecule, are also discussed. The method is very suitable for efficient, accurate treatment of floppy molecules and molecules which can isomerize. The DVR-DGB (i.e., ray eigenvector) basis provides a rapidly convergent expansion for the delocalized (and localized) states. Consequently, a single diagonalization of the DVR-ray eigenvector Hamiltonian matrix, whose size is modest relative to the number of accurately determined energy levels, yields the energies of both localized and delocalized states. Accurate evaluation of the two-dimensional integrals in the potential matrix elements requires only 3–4 Gauss–Hermite quadrature points per dimension. [ABSTRACT FROM AUTHOR]

We present a fully quantal dynamical study of vibrational relaxation of CO in collisions with H2 treating H2 as a structureless projectile. The potential energy surface consists of a SCF part, including explicitly the variation with the CO bond distance, and a damped long range dispersion part. This potential model contains only two free parameters in the damping function which have been determined previously by fitting rotationally inelastic beam data for D2–CO. The dynamical calculations are performed within the infinite order sudden approximation (IOSA) and the coupled states approximation (CSA). The IOSA relaxation cross sections are generally a factor of ∼1.5 smaller than the CSA cross sections independent on the initial rotational state of CO for j≾–6. The relaxation rate constants are very sensitive to small changes of the interaction potential. After readjusting one of the damping parameters (within a range still acceptable with the beam data) the IOSA rate constants agree very well with the experimental ones for ortho H2. It is demonstrated that on the same level of approximation the present rate constants are about three times larger than those obtained from the potential energy surface of Poulsen. [ABSTRACT FROM AUTHOR]

Vibrational relaxation of CO (n1=1) in collision with ortho, para H2 is studied quantum mechanically, with the rotational degree of freedom of H2 included. This represents extension of our earlier study [Ref. 11, J. Chem. Phys. 81, 0000 (1984)], where H2 was treated as a structureless particle. The potential surface, described in detail in Ref. 11, consists of a SCF part, which includes explicitly the variation with the CO bond distance, and a damped long range dispersion contribution. The relaxation cross sections are calculated within the infinite order sudden approximation (IOSA) for CO rotation and within the coupled states approximation (CSA) for H2 rotation. Many of the trends clearly present in the cross sections can be understood in terms of the distorted wave approximation (DWA). The calculated relaxation rates in para H2 agree well (within a factor of 2) with the experimental results. However, judging from a limited number of calculated cross sections for relaxation in ortho H2, the ortho H2 relaxation rates would be comparable in magnitude to the para rates, in disagreement with experiment. Extensive comparison is made with the work of Poulsen and Billing (Ref. 12) and a number of significant, even qualitative differences regarding magnitude of some simultaneous CO vibrational and H2 rotational transitions are discussed. [ABSTRACT FROM AUTHOR]