Solvent effect on the conformational preference of some 1,3-disubstituted acyclic compounds by 1H NMR

The term conformational analysis refers to two aspects of studies, determination of the molecular geometry and energy of the confomers, followed by determination of which steric and electronic interactions are responsible for the stabilization of each conformer.1 This work had as objective to investigate how the solvent and the intramolecular hydrogen bonding (IHB) affect the conformational preference of some 1,3- disubstituted acyclic compounds (Figure 1),2 and what is the effect of the different alkyl substituents on these compounds by 1H NMR. 1H NMR spectra were obtained on a Bruker Avance III spectrometer, operating at 600.17 at concentrations of 0.05 mol L-1 using TMS as reference. The effect of the solvent on the conformational equilibrium of the compounds 1-4 were analyzed using the values of vicinal coupling constants (3JHH) obtained in different solvents along with the theoretical 3JHH values obtained for the stable conformers of each compound using the BHandH/EPR-III. Experimental 3JH1H2 results increase with increasing polarity of the solvent for all compounds. In CCl4 the values are close to 5.60 Hz, whereas in DMSO-d6 and Pyridine-d5, the values are higher, around 6.40 Hz. These results indicate that the conformational equilibria of 1,3-dissubstituted acyclic compounds change to hydrogen-bonding conformations in non-polar solvents and for non-hydrogen-bonding conformations in polar solvents.