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Circular dichroism of nitrate esters Barton, Richard Edgar

Abstract

The circular dichroism of optically active alkyl cyclo-hexyl-, bicyclo [2.2.1] heptyl-, and oxolanyl nitrates was measured. A chirality rule is proposed which correlates the stereochemistry of the nitrate esters with the sign of their 230 nm CD bands: when the planar nitrato chromophore is oriented in right-handed rectangular cartesian coordinates with the alkoxyl oxygen at the origin, the C-O bond on the z axis, and the nitro moiety in the positive yz plane, the sign of the 230 nm circular dichroism band will be positive when the perturbing atoms lie in the positive x-direction. The rule can be effectively applied only when a favoured conformation of the nitrate ester can be determined. Of forty-eight nitrate esters examined, the rule correctly predicted the band sign of twenty-eight; the conformations of eighteen and the band signs of two remain uncertain. No direct contradictions were observed. Menthyl and carvomenthyl nitrates gave "double-humped" bands in the 250-330 nm spectral region and evidence of conformational equilibria. In a series of seven alkylbicyclo-[2.2.1] heptyl nitrates, a positive 230 nm band was observed for compounds containing a 2-endo nitrato group while a negative band was observed for compounds with a 2-exo nitrato group. The influence of temperature and solvent on the molecular ellipticity of the 270 and 230 nm CD bands of nitrate esters was examined. The 270 nm band is the more sensitive to environmental effects, and in some cases its sign may be reversed by changing the solvent or temperature. This is attributed to rotation of the nitrato group about the O-C bond, although non-conformational effects may also be important. Studies on menthyl and fenchyl nitrates showed no appreciable influence of solvent on the energy of the 270 nm electronic transition.

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