Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the ÖX̃ emission spectrum

Abstract

We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled-cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero-point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH-overtone states up to 23 063 cm−1. We also report a simulation of the ÖX̃ stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even-bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited à state. The potential for the à state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the à state. In addition, the Ã-state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis-switching arguments of a number of previously unassigned states in the SEP spectrum.

Global Analytical Potential Energy Surfaces for High‐resolution Molecular Spectroscopy and Reaction Dynamics

“…In a more recent work, the theoretical results using the same potential energy surface have been improved (Poulin et al 1996). In a previous paper (Bentley et al 1992), a DVR method was used on a potential energy surface for the HCN/HNC system of similar quality as that developed by Bowman et al (1993), and deviations from experimental vibrational term values were as large as 150 cm −1 . With improved computer power and the development of refined algorithms to solve equation (1), the numerical precision of calculations of vibrational eigenvalues given a PES has increased, in the last years, to better than 1 cm −1 for to up to penta-atomic molecules, typically (Wang and Carrington 2003a,b, Luckhaus 2003, Shirin et al 2003, Richter et al 2007, Iung and Pasin 2007).…”

Hydrogen Isocyanide, HNC: A Key Species in the Chemistry of Titan's Ionosphere?

“…The thresholds for these processes are, respectively, 141 and 465 kJ mol −1 (8510 and 2570Å) (Bowman et al 1993, Hansel et al 1998, Chase 1998). In the model presented here, we shall neglect any inclusion of photoprocessing of HNC, because it is very probable that neither reaction (10a) nor (10b) is a significant loss process for a species likely to be consumed reasonably rapidly by chemical processing as explored in the subsequent sections.…”

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

“…The isomerization reaction HCN HNC is one of the simplest models of a chemical reaction and one of the prototype model systems used for the study of unimolecular reactions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] This system is important because there is an overlap between the two basic scientific "tools"that we can use to gain a fundamental understanding on the isomerization process on a full quantum mechanical basis: It is possible to do high-level ab initio theoretical calculations (only 17 particles), and high resolution spectroscopic data can be obtained for highly excited vibrational states. Regarding the isomerization reaction one of the key questions is how the isomerization manifests itself in the vibration-rotation eigenenergy spectrum of the HCN and HNC molecules.…”