On the origin of the different activation energies for hydrogen additions at the C and O centres of RCO⁺ ions (R = H, CH₃): A theoretical interpretation

The different activation energies for hydrogen addition at the carbon and oxygen positions of HCO⁺ and CH₃CO⁺ are explained using frontier orbital arguments.The reactions are considered as "nucleophilic" attack of the hydrogen atom. Addition at the carbon centres yields H₂CO⁺· and CH₃CHO⁺·, respectively, and is favoured in comparison to addition all the oxygen (which would result in the formation of RCOH⁺·) because of the larger LUMO coefficient at the carbon centre. This explanation is in agreement with the calculated and experimentally derived activation energies.That the SOMO(H)-LUMG(RCO⁺) interaction is indeed the dominant one is demonstrated by the transformation matrix between the MOs of the educt and transition state.