The stable geometries of pyridine, N-alkylpyridinium cations, and the ion-pairs of N-alkylpyridinium cations with F^－, Cl^－, Br^－, and have been investigated by performing density functional theory calculations at the B3LYP/6-31＋G(d) level. The calculated results indicate that the structure of the pyridine ring in N-alkylpyridinium cations is generally similar to that of the neutral pyridine molecule, i.e. the pyridine ring retains its aromaticity, and the N-alkyl side chain has little effect on the structures of the pyridine ring. It was also found that the anions were inclined to move to the vicinities of the C(5)—H fragment or between C(2)—H and N-methyl above the pyridinium ring in ionic liquids. There are always multiple H-bondings between the anion and cation with partial charge transfer. Furthermore, the longer the N-alkyl side chain is, the weaker the hydrogen bond between the cation and anion will be.

Key words: pyridinium cation, pyridinium, density functional theory, H-bonding