CONFORMATIONAL-ANALYSIS .26. AN OBJECTIVE METHOD FOR DETERMINING CONFORMER POPULATIONS AND COUPLING-CONSTANTS IN NMR-SPECTROSCOPY

An objective method of obtaining conformer populations and coupling co nstants from molecules rapidly interconverting between a number of con formers is given, The method relies on the acquisition of a sufficient data set by varying the solvent and/or temperature until the system i s over-determined with more equations than unknowns. No assumptions co ncerning either the conformer populations or couplings are necessary, The general conditions for an over-determined data set are derived for systems interconverting between two and three conformers, and illustr ative examples are given of 1,2-dichloro-1-fluoroethane (1) 1,1,2-trif luoroethane (2) and trans-2-fluoro-3-hydroxy-tetrahydronaphthalene (3) . The analysis of the vicinal H-H and H-F couplings of 1 in seven solv ents of different polarities gave the couplings and relative energies of the three interconverting conformers, Extrapolation to the vapour u sing solvation theory gives Delta E (g'-->t) = 0.8 kcal mol(-1) and De lta E (g'-->t) = 1.1 kcal mol(-1) (1 kcal = 4.184 kJ). A similar analy sis of the solvation dependence of the vicinal couplings of 2 shows th at the vicinal F-F couplings cannot be used to derive the conformer en ergies. An alternative treatment based on solvation theory gives the v icinal H-H and H-F couplings in the two interconverting conformers, Th e direct analysis of the solvent dependence of the couplings of 3 in s even solvents gives the conformer couplings in the two equilibrating h alf-chair conformers and the energy differences Delta E (ax, ax --> eq , eq) which vary from 1.2 kcal mol(-1) in CCl4, solution to 0.21 kcal mol(-1) in DMSO.