Near ultraviolet optical activity of chiral pyridine derivatives

Abstract

The near ultraviolet optical activity of dissymmetric pyridyl compounds is examined on a theoretical model in which both electronic and vibronic rotatory are considered. Specifically investigated are 2-, 3-, 4- and 2,3-substituted pyridyl derivatives in which the substituent groups include an asymmetric carbon atom. The electronic rotatory strengths of these compounds are calculated using an independent-systems model in which 2-, 3- and 4-methylpyridine and 2,3-dimethylpyridine are treated as chromophoric units and the extrachromophoric substituent atoms (or groups) are treated as perturber centres which alter the spectroscopic properties of the chromophore. The rotatory strengths of the three lowest energy singlet-singlet transitions of the pyridyl chromophore are calculated as functions of several conformational variables associated with the substituent groups. The three lowest singlet states of the pyridyl chromophore are calculated to be relatively close in energy (all three within a range of ∼0–8000 cm^–1) and consequently subject to pseudo Jahn-Teller (PJT) type interactions. The possible influence of PJT interactions on the near ultraviolet chiroptical spectra of dissymmetric pyridyl systems is investigated on a model in which the three lowest singlet electronic states are assumed to be coupled through two vibrational modes. This three state-two mode vibronic model is used in calculating the energies and rotatory strengths of the vibronic levels associated with the PJT distorted systems. Spectra-structure relationships based on previously reported experimental data and on the results calculated in this study are presented and discussed.