Theoretical study on second hyperpolarizability for cationic pyridine derivatives

The molecular second hyperpolarizabilities γ of cationic 3- or 4-substituted pyridine derivatives have been investigated. The γ values are calculated by an ab initio molecular orbital (MO) method at correlated levels including CCSD(T). The magnitude of γ for the pyridinium cation is shown to be smaller than that of pyridine. It is found that γ for the pyridinium cation changes dramatically with substitution, whereas γ for neutral pyridine does not show such large substituent effects. This remarkable substituent effect on γ of cationic pyridine derivatives is one of the special features of cationic species.