Hydrogen cyanide loss from [CH 5H 2] + cations: 1,2-elimination versus Beckmann rearrangement

Abstract

The [CH 5N 2] + ion isomers have been reexamined using ab initio molecular orbital calculations. On the basis of MP4SDTQ/6-311++G (2d,2p) electronic energies using MP2/6-31G (d,p) geometries, the standard heats of formation are proposed as follows (Δ H 0 f,298, in kJ mol −1): H 3CNHNH +, 903 ± 12; H 3CNNH + 2 904 ± 12; H 2CNHNH + 2, 860 ± 12; H 2CNNH + 3, 874 ± 12; HNNH + 2, 974 ± 8 (experimental 950); and H 3CNNH, 187 ± 12 (experimental, 188 ± 8). These values lead to the proton affinities of diimides, PA(HNNH) = 764 ± 8 kJ mol −1 and PA(H 3CNNH) = 813 ± 12 kJ mol −1. The potential energy surfaces corresponding to unimolecular rearrangements and fragmentations of these ions have also been explored in detail. The present theoretical analysis confirms our recent proposition that the loss of HCN from metastable hydrazonium cations (H 2CNNH + 3) can better be rationalized in terms of a gas-phase Beckmann rearrangement than a 1,2-cis-elimination.