Hydrogen cyanide: Theory and experiment

The results of large-scale ab initio calculations for hydro en cyanide are compared with available experimental data. The equilibrium bond lengths of the electronic ground state are obtained with an accuracy of approximate to 0.0005 Angstrom and the wavenumbers of the fundamental vibrational transitions have errors of 2.1 cm(-1) (nu(1)), 1.3 cm(-1) (nu(2)) and 5.8 cm(-1) (nu(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 mu(e) = -3.0146(5) D. The calculated transition dipole moments of the nu(3) bands of HCN isotopomers agree with experiment, and the intensity anomaly found experimentally for (HCN)-C-12-N-14 and (HCN)-C-12-N-15 is reproduced by theory for the first time. Accurate equilibrium geometries are reported for the two lowest electronic states of HCN+ ((X) over tilde(2) Pi and (A) over tilde(2) Sigma(+)). By combination of experimental and theoretical data, the equilibrium excitation energy of the first excited singlet state (A(-1)A'') is obtained as T-e = 53 266 +/- 30 cm(-1). The equilibrium geometry of the lowest tripler state ((a) over tilde(3)A') is r(e) = 1.100 Angstrom, R-e = 1.288 Angstrom and alpha(e) = 120.9 degrees. Its T-e value is recommended as being 38 500 +/- 500 cm(-1). (C) 1997 Elsevier Science B.V.