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 l engths 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 ano maly found experimentally for (HCN)-C-12-N-14 and (HCN)-C-12-N-15 is r eproduced by theory for the first time. Accurate equilibrium geometrie s are reported for the two lowest electronic states of HCN+ ((X) over tilde(2) Pi and (A) over tilde(2) Sigma(+)). By combination of experim ental and theoretical data, the equilibrium excitation energy of the f irst 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) o ver tilde(3)A') is r(e) = 1.100 Angstrom, R-e = 1.288 Angstrom and alp ha(e) = 120.9 degrees. Its T-e value is recommended as being 38 500 +/ - 500 cm(-1). (C) 1997 Elsevier Science B.V.