Ab initio global potential, dipole, adiabatic, and relativistic correction surfaces for the HCN–HNC system

Abstract

Ab initio semiglobal potential energy and dipole moment hypersurfaces for the isomerising HCN-HNC system are computed, using a grid of 242 points, principally at the all-electron cc-pCVQZ CCSD͑T͒ level. Several potential energy hypersurfaces ͑PES͒ are presented including one which simultaneously fits 1527 points from earlier ab initio, smaller basis CCSD͑T͒ calculations of Bowman et al. ͓J. Chem. Phys. 99, 308 ͑1993͔͒. The resulting potential is then morphed with 17 aug-cc-pCVQZ CCSD͑T͒ points calculated at HNC geometries to improve the representation of the HNC part of the surface. The PES is further adjusted to coincide with three ab initio points calculated, at the cc-pCV5Z CCSD͑T͒ level, at the critical points of the system. The final PES includes relativistic and adiabatic corrections. Vibrational band origins for HCN and HNC with energy up to 12 400 cm Ϫ1 above the HCN zero-point energy are calculated variationally with the new surfaces. Band transition dipoles for the fundamentals of HCN and HNC, and a few overtone and hot band transitions for HCN have been calculated with the new dipole surface, giving generally very good agreement with experiment. The rotational levels of ground and vibrationally excited states are reproduced to high accuracy.

Complete experimental rovibrational eigenenergies of HCN up to 6880 cm−1 above the ground state

“…This system involves relatively high energies 83 of 5281 cm −1 (the position of the second minimum relative to the first one) and 16 798 cm −1 (barrier height measured relative to the HCN well) for the heights in a double well potential. An interesting question of molecular physics is how the spectroscopic constants change when the two isomer molecules are excited from their ground states in the two potential wells up to isomerization energies.…”

“…Figure 2 shows the vibrational angular momentum dependence of the difference between the experimental and ab initio vibrational eigenenergies E vib = T v,(J =0)meas − T v,(J =0)abinitio for the potential surface calculated by Tennyson and co-workers. [83][84][85] The error plot shows a strong vibrational angular momentum dependence. For high l, the ab initio vibrational eigenenergies agree better with the experimental data than for low l. For the HNC eigenenergies the error plot shows a similar dependence 18 but for HNC the error is smaller for the low l states.…”

Spectroscopic characterization of isomerization transition states

“…Extensive experimental term values for both isomers are available up to 10000 cm −1 above the HCN minimum (26). To continue the analysis up to and beyond the barrier energy, we used spectroscopically assigned ab initio eigenenergies (26)(27)(28). In Ref.…”

“…HCN/HNC Effective Frequency Analysis Two different types of data were used for the HCN/HNC ω eff analysis. The first data set is the product of assigning the eigenenergies (26) generated by a high level variational ab initio calculation (27,28), up to and above the isomerization barrier, to a maximum energy of 18000 cm −1 . The second data set consists of experimental eigenenergies up to 10000 cm −1 from HOTGAME (Hot Gas Molecular Emission) experiments of one of the authors (26,57).…”

A new ab initio ground-state dipole moment surface for the water molecule

“…The most accurate ab initio dipole data available is due to Partridge and Schwenke, 11 who subsequently found it necessary to perform a systematic refit of their data 23 giving a surface, denoted here as SP2000, which gives very good transition intensities over a wide range of frequencies. 24,25 Ab initio studies of the ground-state PES of water, 13,15,26 and indeed of other molecules, [27][28][29][30] have shown the importance of including corrections [31][32][33] to standard ab initio procedures in the form of core-core and core-valence corrections, 11,13 first-order 34 and higher-order relativistic corrections, 35 adjustments due to the limited precision of the Born-Oppenheimer approximation 26,36,37 and even quantum electrodynamic effects. 38 So far none of these corrections have been considered for the DMS of water.…”