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463. PSEUROT: Computer-Assisted Pseudorotational
Analysis of Five-Membered Rings by Means of Vicinal
Proton Spin-Spin Coupling Constants
by Frank A. A. M. de Leeuw and Cornelis Altona, Department of Chemistry, State University, 2300 RA Leiden, The Netherlands 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 . 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 and for each conformer. The general approach involves corrections for the effects exerted by electronegativity and orientation of substituents on, 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. (in press, 1983). ___________ FORTRAN IV (IBM) Lines of Code: 660 |