Hydrogen cyanide: theory and experiment

Abstract

The results of large-scale ab initio calculations for hydrogen cyanide are compared with available experimental data. The equilibrium bond lengths of the electronic ground state are obtained with an accuracy of ≈ 0.0005 Å and the wavenumbers of the fundamental vibrational transitions have errors of 2.1 cm −1 ( ν 1), 1.3 cm −1 ( ν 2) and 5.8 cm −1 ( ν 3). An accurate three-dimensional CCSD(T) electric dipole moment is reported and, by combination of experiment and theory, the equilibrium dipole moment is determined as being μ e = − 3.0146(5) D. The calculated transition dipole moments of the ν 3 bands of HCN isotopomers agree with experiment, and the intensity anomaly found experimentally for H 12C 14N and H 12C 15N is reproduced by theory for the first time. Accurate equilibrium geometries are reported for the two lowest electronic states of HCN + (X̃ 2Π and A ̃ 2Σ + ). By combination of experimental and theoretical data, the equilibrium excitation energy of the first excited singlet state (Ã 1 A″) is obtained as T e = 53 266 ± 30 cm −1. The equilibrium geometry of the lowest triplet state (ã 3A′) is r e = 1.100 A ̊ , R e = 1.288 A ̊ and α e = 120.9 °. Its T e value is recommended as being 38 500 ± 500 cm −1.