Destabilised carbocations: A comparison of the C2H4NS+ and C2H4NO+ potential energy surfaces

Ab initio molecular orbital calculations at the HF/6-31G(d,p) and MP2(full)/6-311G(d,p) levels are reported for the C2H4NO+ and C2H4NS+ potential energy surfaces. All structures have been subjected to geometry optimisations at both levels of theory and have been characterised by harmonic frequency calculations. On the C2H4NS+ surface the S-bridged thioformamidylmethyl cation, 3a, has the lowest energy while on the C2H4NO+ surface the 'solvated ion' H2C=NH2+... CO, 8b, is lowest. On each surface the formamidylmethyl cation, 3, is in a deep well. On both surfaces there is an ion (structure 7) in which a three-membered ring contains both heteroatoms. On the C2H4NS+ surface 7a is 39.3 kcal mol(-1) above 3 and there is a substantial barrier (25.8 kcal mol(-1)) for rearrangement of 7a to 3a. The oxygen analogue, 7b, is 62.6 kcal mol(-1) above 3b. Dissociation of the alpha-aminothioacetyl cation, 6a, is endothermic (by 29.0 kcal mol(-1)) whereas dissociation of the alpha-aminoacetyl cation is exothermic by 12.1 kcal mol(-1). (C) 1997 Elsevier Science B.V.