THIS INFORMATION IS OBSOLETE AND IS PROVIDED ONLY FOR ITS HISTORICAL VALUE

Vicinal proton spin-spin coupling constants play an important role in conformational analysis. With the help of Karplus-like equations, the vicinal proton spin-spin coupling constants can be translated into proton-proton torsion angles. In the case of five- membered rings, the proton-proton torsion angles are related to the endocyclic torsion angles which, in turn, are correlated via the laws of pseudorotation. Consequently, the vicinal couplings can be expressed as a function of the pseudorotation parameters: a phase angle of pseudorotation P and a puckering amplitude Fm. These parameters define all five endocyclic torsion angles.1 When a two-state equilibrium in solution is present, the coupling constants represent time-averaged values of the couplings of the individual conformers. In this case, the coupling constants can be expressed as a function of the 5 conformation parameters involved: the mole fraction of one conformer and two pseudorotation parameters P andFm for each conformer. The general approach involves corrections for effects exerted by electronegativity and orientation of substituents on 3JHH, as well as corrections for deviations from 120° projection symmetry.2

The program PSEUROT calculates the best fit of the conformational parameters involved to the experimental vicinal couplings. Various applications of the program are discussed in Ref. 3.

References:

[1] C. Altona and M. Sundaralingam, J. Am. Chem. Soc., 94, 8205 (1972).

[2] F.A.A.M. de Leeuw and C. Altona, J. Chem. Soc. Perkin II, 375 (1982).

[3] F.A.A.M. de Leeuw and C. Altona, J. Comp. Chem. (1983).

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