Conformational equilibria in 1-, 3-, and 4-methyl-1, 2, 3, 4-tetrahydroisoquinolines (THIQs) and the diastereomeric pairs of their 1, 3-and 1, 4-dimethyl homologs have been determined by measurement of H3/H4 (trans) coupling constants and have been confirmed by molecular mechanics [MMP2 (85)] calculations. The experimental-Δ G° values (a→ e) for the monomethyl compounds (computed values in parentheses) in kcal mol-1 are Me-1, 0.56 ( ...