Equilibrium geometry of isocyanomethylene (HCNC) and comparison to the troublesome isomer cyanomethylene (HCCN)

Abstract

Inspired by the recent experimental study of the radical anions HCCN− and HCNC− and by earlier examinations of HCCN, the equilibrium geometry of the HCNC molecule has been investigated using both self-consistent field (SCF) and configuration interaction methods including single and double excitations (CISD). The largest basis set used was a triple-ζ plus double polarization with diffuse functions and higher angular momentum functions appended to each atom [TZ2P(f,d)+diff]. Using this basis, the H–C–N equilibrium angle is predicted to be 128.5° at the CISD level of theory. Additionally, the zero point vibrational energy (ZPVE) corrected energy separation of the bent and linear conformations was predicted to be 10.1 kcal mol−1 at the CISD level of theory with the largest basis set employed. The barrier to linearity is 7.7 kcal mol−1 at the CCSD level of theory and 6.9 kcal mol−1 at the CCSD(T) level of theory, employing the CISD optimized geometries with a basis that was comprised of triple-ζ plus double polarization with higher angular momentum functions appended to each atom [TZ2P(f,d)]. These results were compared to those obtained in previous ab initio investigations of HCCN, which has been dubbed a quasilinear molecule by the most recent experimental investigators. HCNC is predicted to lie 22.2 kcal mol−1 above HCCN at the CISD level of theory, with a the TZ2P(f,d) basis. The differences between the two isomers are discussed and HCNC is predicted to be a definitively bent molecule, rather than quasilinear.