INDO theoretical studies of 4-aminopyridine and protonated 4-aminopyridine

Abstract

Geometry optimized INDO calculations have been performed on 4-aminopyridine (2) and protonated 4-aminopyridine (1) in order to compare their optimized geometries, π-bond orders, C(4)–NH₂ rotational barriers, charge densities, and π-electron distributions. It had previously been shown that 4-aminopyridine falls far off a linear correlation of ΔG° and ΔS° for the protonation equilibria of a series of 4-substituted pyridines. The calculations support the concept that a very high rotational barrier in (1) could cause this deviation. The contribution of a quinonoid resonance hybrid (1b) to (1) was greater than (2b) to (2). The π-cloud in (1) is polarized toward the ring nitrogen, relative to (2), and the C(4)–NH₂ rotational barrier in (1)(27.3 kcal mol^–1) is larger than in (2)(14.7 kcal mol^–1).