Ab initio molecular orbitals studies and NBO analysis of conformational preference in 2-hydroxypiperidine

The conformational properties of 2-hydroxypiperidine have been investigated by means of ab initio molecular orbital and hybrid-density functional theory methods and then followed by NBO analysis. Four stable conformations have been found, Ee, Ea, Ae and Aa, where the energy difference between the most stable conformation (Ee) and the least stable one (Aa) was only 1.02 kcal mol−1 according to MP2/6-311+G(d,p) and G2 calculations. Origin of the conformational preference was investigated by NBO analysis of donor–acceptor (bond–antibond or lone pairs–antibond) interactions. The results showed that the conformational preference is better interpreted in terms of electrostatic interactions and steric effects, rather than hyperconjugation or anomeric effects.