Theoretical density functional and ab initio computational study of different types of hydrogen-bonding

Computational calculations have been carried out at the B3-LYP/6-31 + + G** and RHF/6-31 + + G** levels of theory to determine the equilibrium structures of complexes formed between the tautomers 2-hydroxypyridine and 2- oxopyridine with HCl. Different types of hydrogen-bonding have been found: two closed complexes of type III (proton transfer), each containing two hydrogen bonds i.e. N-H⁺···Cl^-···H-O and C=O-H⁺···Cl^-···H-N, two open complexes of intermediate type I → II: N···H···Cl and C=O···H-Cl, and one open complex of type I:H-O···H-Cl. A comparison of the experimental and calculated IR frequencies demonstrates that the most accurate spectral parameters are predicted by the B3-LYP/6-31 + + G** method, especially when this method is used with a set of different scaling factors.