Conformational Analysis Part 26—An Objective Method for Determining Conformer Populations and Coupling Constants in NMR Spectroscopy
✍ Scribed by Raymond J. Abraham; Paul Leonard; Timothy A. D. Smith; W. Anthony Thomas
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 701 KB
- Volume
- 34
- Category
- Article
- ISSN
- 0749-1581
No coin nor oath required. For personal study only.
✦ Synopsis
An objective method of obtaining conformer populations and coupling constants from molecules rapidly interconverting between a number of conformers is given. The method relies on the acquisition of a sufficient data set by varying the solvent and/or temperature until the system is over-determined with more equations than unknowns. No assumptions concerning 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 illustrative examples are given of 1,2-dichloro-l-fluoroethane (1) 1,1,2-trifluoroethane (2) and trans-2-fluoro-3-hydroxytetrahydronaphthalene (3). The analysis of the vicinal H-H and H-F couplings of 1 in seven solvents of different polarities gave the couplings and relative energies of the three interconverting conformers. Extrapolation to the vapour using solvation theory gives AE (g -+ t ) = 0.8 kcal mol-' and AE (g' + t ) = 1.1 kcal mol-' (1 kcal = 4.184 kJ). A similar analysis of the solvation dependence of the vicinal couplings of 2 shows that the vicinal F-F couplings cannot be used to derive the conformer energies. An alternative treatment based on solvation theory gives the vicinal H-H and H-F couplings in the two interconverting conformers. The direct analysis of the solvent dependence of the couplings of 3 in seven solvents gives the conformer couplings in the two equilibrating half-chair conformers and the energy differences AE (ax, ax + eq, eq) which vary from 1.2 kcal mol-' in CCl, solution to 0.21 kcal mol ' in DMSO.