Equilibrium Structure and HCC Bending Potential Energy Function of 3A‘‘ HCCN

Abstract

The molecular parameters of the ground triplet state of cyanocarbene, 3A‘‘ HCCN, have been determined in large-scale ab initio calculations using the coupled-cluster method, RCCSD(T), and basis sets of double-through-quintuple-ζ quality. The equilibrium structure of the molecule was found to be planar and bent, with the trans conformation of the HCCN chain and the parameters re(HC) = 1.069 A, re(CC) = 1.328 A, re(CN) = 1.186 A, ∠e(HCC) = 144.9°, and ∠e(CCN) = 175.4°. The potential energy function for the HCC bending motion (v5 mode) was determined to be strongly anharmonic, with the barrier to linearity of 286 cm-1. Influence of various electron-correlation effects on the shape of the HCC bending potential energy function is discussed. The rotation−bending energy levels of the HCCN and DCCN molecules were then calculated using a semirigid-bender Hamiltonian. For both molecules, the predicted patterns of rotational transitions in the excited ν5 states agree favorably with the experimental data.