GEOMETRIES AND STABILITIES OF VARIOUS CONFORMERS OF CYANOMETHYLENE AND OTHER HC2N ISOMERS

Geometrical structures of various conformers (1-10) in HC2N were optimized by the QCISD(T) method with the D95** basis set. Relative stabilities of the singlet and triplet conformers were obtained using single-reference and multi-reference SDCI+Q methods with the general contraction scheme of the [5s3p2d1f(C and N)/3s2p1d(H)] basis set. We agree with the qualitative conclusions of Schaefer and Roos that triplet cyanomethylene is bent. The energy difference between the bent (1) and the linear (2) triplet conformers is 1.3 kcal/mol at the single-reference SDCI+Q level of theory, in good agreement with Schaefer's estimate (0.8 kcal/mol). At the 9-reference SDCI+Q level of theory using the natural orbitals obtained in the single-reference SDCI calculations the energy difference is 2.1 kcal/mol, slightly larger than Schaefer's value. The singlet conformer of a ring structure lies 8.8 kcal/mol above the most stable triplet-bent 1.