Protonation effect on C-N bond length of alkylamines studied by molecular orbital calculations

Molecular orbital calculations were performed for the six saturated alkylamines (CH 3 NH 2 , (CH 3 ) 2 NH, (CH 3 ) 3 N, CH 3 CH 2 NH 2 , (CH 3 ) 2 CHNH 2 , (CH 3 ) 3 CNH 2 ), their protonated cations (CH 3 NH 3+ , (CH 3 ) 2 NH 2+ , (CH 3 ) 3 NH + , CH 3 CH 2 NH 3+ , (CH 3 ) 2 CHNH 3+ , (CH 3 ) 3 CNH 3+ ), and (CH 3 ) 4 N + using the Hartree-Fock, second-order M0ller-Plesset, and density functional theory methods with the 6-311+G(d,p) basis set. Protonation lengthens the C-N bonds of the amines by 0.05 - 0.08 A and shortens the C-C bonds of CH 3 CH 2 NH 2 , (CH 3 ) 2 CHNH 2 , and (CH 3 ) 3 CNH 2 by ca. 0.01 A.