High Accuracy ab Initio Calculations of Rotational–Vibrational Levels of the HCN/HNC System

Abstract

Highly accurate ab initio calculations of vibrational and rotational-vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously. An adiabatic Born-Oppenheimer diagonal correction (BODC) correction surface as well as a relativistic correction surface were also calculated. These surfaces were used to compute vibrational and rotational-vibrational energy levels up to 25 000 cm which reproduce the extensive set of experimentally known HCN/HNC levels with a root-mean-square deviation σ = 1.5 cm. We studied the effect of nonadiabatic effects by introducing opportune radial and angular corrections to the nuclear kinetic energy operator. Empirical determination of two nonadiabatic parameters results in observed energies up to 7000 cm for four HCN isotopologues (HCN, DCN, HCN, and HCN) being reproduced with σ = 0.37 cm. The height of the isomerization barrier, the isomerization energy and the dissociation energy were computed using a number of models; our best results are 16 809.4, 5312.8, and 43 729 cm, respectively.

A new spectroscopically-determined potential energy surface and ab initio dipole moment surface for high accuracy HCN intensity calculations

“…Our experience with the water [22,23] and ammonia [24] molecules shows that even a small correction to a high-quality ab initio PES can give us a notable improvement in the resulting energy levels. In this context, we note that the previous empirical PES constructed by Varandas and Rodrigues [25] is actually less accurate than our ab initio one [21].…”

“…For water molecule, we achieved 0.1 cm −1 accuracy ab initio for the rovibrational energies [23]. In the case of HCN/HNC system, calculations with the ab initio PES of I [21] reproduced the extensive set of experimental data below 7000 cm −1 with a root-mean-square deviation σ = 0.37 cm −1 when a semi-empirical non-adiabatic correction was applied and σ = 1.5 cm −1 for levels below 25000 cm −1 using a purely ab initio approach.…”

“…As the ab initio PES for the HCN/HNC system computed by van Mourik et al [15] does not satisfy modern spectroscopic needs, we recently constructed a new one [21]. Despite significantly increasing the accuracy of the predicted rovibrational energy levels, calculations with this PES still do not reach the 0.1 cm −1 level of accuracy which places most computed line positions within their observed line width in practical spectra.…”

“…This approach has been employed successfully for a number of studies [18][19][20]. A reliable ab initio PES is already available for the HCN/HNC system [21] and the present work provides the second PES needed for such calculations.…”

“…Comparison of HCN intensities computed using potential surfaces by van Mourik et al[15], Makhnev et al[21] (I) and using the new fitted PES. The comparison is made for 550 transitions between the states with J ≤ 5.…”