Vibrational frequencies of the HCCN molecule. A near degeneracy between bent cyanocarbene and linear allene-related geometries

The geometrical structure and vibrational frequencies of the ground triplet electronic state of HCCN have been examined at a wide range of levels of ab initio electronic structure theory. The potential energy surface of HCC bending is very flat for HCCN owing to a competition between linear allene OHC=C=N and bent carbene HCC=N valence structures. Evidence is presented that both the restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF) methods treat this potential surface in a somewhat uneven manner. When the effects of electron correlation are included, however, RHF- and UHF-based methods converge to a similar set of structural and energetic predictions. The most reliable levels of theory suggest the HCCN is a quasi-linear molecule, with THETA/sub e/(HCC) approx. = 138/sup 0/ and a barrier to linearity of only about 2 kcal/mol.