Gas phase acidities of N-substituted amine-boranes

Complexation energies and acidities of 19 primary, secondary and tertiary amine-boranes were investigated using MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) methods. Gas phase acidities for free amines were also calculated. Acidity values for studied complexes range from 327.3 to 349.1 kcal mol(-1) and the most acidic are the ones with direct connection between deprotonation center and a pi-system. Results obtained by both computational methods are in good agreement with each other and with known experimental data. Addition of BH3 increases the acidity of amines by 30 to 50 kcal mol(-1). This enhancement effect was compared to the respective effect witnessed in phosphine-boranes and traced back to changes of charge delocalization on nitrogen. A question about the structural stability of several deprotonated amine-borane anions in the gas phase was also raised.