An abinitio study of protonation and alkylation of aminopyridine

STO-3G abinitio calculations show that the first site of protonation in 2-, 3- and 4-aminopyridines is the cyclic nitrogen, whatever the position of the amino group. There is an excellent linear relationship between the calculated proton affinities and the experimental pK_a values (r = 0.995). The protonation site is also confirmed by π→π* transition energy variations as a function of position of the amino-group due to protonation; these effects are connected with the MO structures. Basicity and reactivity in alkylation reveal lone pair localization.