Magnetic Circular Dichroism Studies of Bipyridyls

Abstract

Abstract The magnetic circular dichroism (MCD) and UV absorption spectra of 2,2′- and 4,4′-bipyridyl in cyclohexane and sulfuric acid were measured at room temperature. The MCD spectrum of 2,2′-bipyridyl is different from that of 4,4′-bipyridyl, reflecting the difference in molecular symmetry between them. The absorption and MCD spectral profiles of 2,2′-bipyridyl in cyclohexane are different from those of 2,2′-bipyridyl in mineral acids, in this system protonation taking place on the nitrogen atoms to give a mono- or di-cation with low symmetry. On the contrary, the MCD spectra of 4,4′-bipyridyl in cyclohexane and in acidic media at pH 0.98 exhibit a similar spectral profile to each other due to the D2 symmetry being retained in 4,4′-bipyridyl. Theoretical calculations were carried out on the basis of the PPP-SCF-CI method to get a reasonable explanation for the observed MCD spectra. The assignments of the MCD bands of these compounds are given and their dihedral angles are estimated.