Rj. Abraham et al., CONFORMATIONAL-ANALYSIS .26. AN OBJECTIVE METHOD FOR DETERMINING CONFORMER POPULATIONS AND COUPLING-CONSTANTS IN NMR-SPECTROSCOPY, Magnetic resonance in chemistry, 34(1), 1996, pp. 71-77
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.