Circular Dichroism of Nitrate Esters: A Planar Symmetry Rule for the Nitrato Chromophore

Abstract

Conformational analysis of nitrate esters based on X-ray and spectroscopic data showed the nitrato group to be coplanar with the α carbon atom and, in the preferred rotamer, the nitro moiety to be antiperiplanar to the β carbon atom. Three optically active absorption bands not apparent in the isotropic electronic spectra were resolved in the circular dichroism spectra. Band I (270 nm) reflected nitrato group libration changing sign and magnitude with changes in temperature and solvent polarity. Differences in rotatory strength of band III (210 nm) for cis- and trans-dinitrates indicated transition moment coupling. A planar symmetry rule for chromophores in the Cs point group correlated the molecular geometry and sign of band II for 42 nitrate esters: when the lowest-energy rotamer of the nitrate ester is viewed down the O—C bond with the nitro group uppermost rotatory contributions to band II (230 nm) are positive for perturbing atoms to the right and negative for those to the left of the nitrato symmetry plane.