Recebido em 12/2/03; aceito em 10/11/03 NMR INVESTIGATION AND THEORETICAL CALCULATIONS OF THE EFFECT OF SOLVENT ON THE CONFORMA-TIONAL ANALYSIS OF 4',7-DI-HYDROXY-8-PRENYLFLAVAN. The NMR conformational study of 4',7-di-hydroxy-8-prenylflavan 1 was carried out in acetone-d6, DMSO-d6 and CDCl 3 which enabled the proposition of three conformations, namely 1a, 1b and 1c, differing in the position of the prenyl group. Geometry optimizations performed using AM1 method showed that 1a (∆H f = -86.2 kcal/mol) is as stable as 1b (∆H f = -85.1 kcal/mol) and 1c (∆H f = -85.4 kcal/mol). When the solvent was included, the calculations showed that the solute-solvent interactions could be explained either in the light of the electronic intermolecular delocalization or the electrostatic character between solute and solvent. Theoretical calculations (HF/6-31G*, DFT/BLYP/6-31G*, and DFT/B3LYP/6-31G*) showed that the combination of these types of interactions present in each solute-solvent system, dependent on the chemical properties of the solvent, lead to different spatial arrangements of the prenyl group, which in turn determined the conformation of 1.