Conformational analysis of organic carbonyl compounds. Part 6. Theoretical examination of the conformational properties (ground and transition states) and solvent effects on 2-formyl-furan, -thiophene, and -pyridine

Abstract

ab-initio MO calculations in different AO basis sets and with full geometry optimization were performed on 2-formyl-furan, -thiophene, and -pyridine. The energy content of more stable conformations and transition states for conformer interconversion, located as saddle-points in the potential energy surface, was obtained. The relative conformer stability is correctly predicted in the STO-3G minimal basis set and in the more extended 3-21G and 6-31G (tested only for the furan derivative) basis sets, yet energy differences between conformers close to the experimental ones are obtained only when the extended sets are employed. A minor effect on the energetics of these molecules is caused by geometry optimization. On the molecular geometry the effect of AO-basis set implementation is significant only as regards the exocyclic C–C bond length, which is shorter in the larger set. An analysis of the effects which can determine the relative conformer stability shows that electrostatic interactions, estimated from calculated atomic point charges, are mainly responsible for the difference in behaviour, observed experimentally, of the conformational equilibrium in these compounds. Using the classical solvent effect theory, the contribution of electrostatic and dispersion solute–solvent interactions and cavity formation to the solvation energy of the ground- and transition-states was calculated: the most important contribution comes from electrostatic effects, as can be seen in previous approaches where only this term has been taken into account. The calculated solvation energies are strongly dependent on the dipole moments for ground- and transition-states, but nevertheless the trend of the experimental solvent effects on the conformational equilibrium is correctly predicted from a qualitative point of view, for the three derivatives examined. The effect of solvent polarity on activation energies was also obtained and enables comparison of the behaviour of the compounds examined, even though an experimental comparison and a qualitative agreement could be found only in the case of furan-2-carbaldehyde.