G2(+)_M study on N -alkylamino cation affinities of neutral main-group element hydrides: Trends across the periodic table

We have made an extensive theoretical exploration of gas-phase N-alkylamino cation affinities (NAAMCA), including amino cation affinities (AMCA) and N-dimethylamino cation affinities (NDMAMCA), of neutral main-group element hydrides of groups 15-17 and periods 2-4 in the periodic table by using the G2(+)_M method. Some similarities and differences are found between NAAMCA and the corresponding alkyl cation affinities (ACA) of H_nX. Our calculations show that the AMCA and NDMAMCA are systematically lower than the corresponding proton affinities (PA) for H_nX. In general, there is no linear correlation between NAAMCA and PA of H_nX. Instead, the correlations exist only within the central elements X in period 2, or periods 3-4, which is significantly different from the reasonable correlations between ACA and PA for all H_nX. NAAMCA (H_nX) are weaker than NAAMCA (H_n-1X^-) by more than 700 kJ/mol and generally stronger than ACA (H_nX), with three exceptions: H₂ONR ₂ ⁺(R = H, Me) and HFNH₂ ⁺. These new findings can be rationalized by the negative hyperconjugation and Pauli repulsion. © 2014 American Chemical Society.