Abstract
Semi-empirical (PM3, AM1) and ab initio (B3LYP/6-31G(d,p)) calculations of the structure and energies of isolated 3-nitrobenzaldehyde (3-NBA) reveal that the planar trans and cis isomers are in equilibrium, with the trans being more stable by about 2.2 kJ/mol. The experimental dipole moments of 3-NBA in solvents of dielectric permittivity ε ranging from 1.9 to 25 have been determined. The increase in the mean dipole moment with increasing ε can be well explained in terms of the trans⇔ cis equilibrium and dipolar isomer–solvent interaction. The heterogeneous dielectric medium model with two concentric dielectric layers proved to be the best description for this interaction. On the other hand, the intrinsic experimental Gibbs energy for the trans⇔ cis conversion obtained from this model was higher than the quantum-chemical values. It is suggested that the quadrupolar interaction acts oppositely to the dipolar one stabilizing the more quadrupolar trans isomer.
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