A theoretical and experimental study of the conformational behavior of methyl 3-nitrobenzoate in vacuum, in various solvents, and in solid state

The conformational behavior of methyl 3-nitrobenzoate (I) was preliminarily explored using semi-empirical methods. Among eight rotational isomers, It and Ic, resulting from rotation around the C(ar)-C(carbonyl) bond, were the most stable and most populated in vacuum, with proportions of 66% and 34%, respectively. Their molecular structures, barrier to rotation, total energies, and Gibbs energies as well as their dipolar and vibrational properties were then calculated employing DFT theory with the 6-31+G(d,p) basis sets. The subtle differences in the molecular geometries of It and Ic were discussed in terms of attractive interactions between the oxygen atoms of the ester group and the hydrogen atoms of the C(ar)-H bonds. Finally, infrared spectra of I in solvents of different polarity were measured and used to estimate the Gibbs energy of It ⇌ Ic conformational equilibrium. The solvent effect on this equilibrium was quantitatively discussed on the basis of reaction field theory adapted to the structural dielectric medium.