Three-electron binding between carbonyl-like compounds and ammonia radical cation.: Comparison with the hydrogen bonded complex

High-level ab initio calculations were performed on a two-center three-electron (2c-3e) bond between formaldehyde and the radical cation of ammonia. The stability of the resulting complex is found to be 21.8 kcal/mol, only 2.2 kcal/mol higher than the hydrogen bonded complex of the same species. Upon replacing the formaldehyde by thioformaldehyde (H2C=S), the 2c-3e complex is found to be 20 kcal/mol more stable than the corresponding hydrogen bonded complex. The imine group is also studied via the H2C=NH system. This compound exhibits a larger 2c-3e bond energy, 32.0 kcal/mol, but this is still 8.8 kcal/mol higher in energy than the hydrogen bonded complex. This is the first theoretical evidence of a possible 2c-3e complex involving organic unsaturated systems.