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Extended Huckel theory molecular orbital calculations of the vicinal proton-proton coupling constants in 1.1and 1.2-difluoroethanes are presented and compared with experimental results. Equations, describing the angular dependence of vicinal couplings in substituted CH-CH fragments, are formulated on the basis of the MO calculations. Empirical parameters for these equations are derived from a least-squares treatment of a large number of experimental coupling constants. Africa THE Karplus equation,l which relates the vicinal coupling have been assumed throughout and standard bond constants to the dihedral angle between the coupling lengths have been used (C-C = 1.54, C-H = 1-09, protons, is a relation used very frequently in proton C-F = 1.36 A). Full details of the calculations are in magnetic resonance studies of the structure and con-ref. 5b. formation of organic compounds. More recent in-The calculated vicinal proton-proton coupling convestigations have indicated that this equation, originally stants for difluoroethanes are in Table 1. Their angular derived for the unperturbed ethane molecule, is inadequate for substituted CH-CH fragments. It fails, for instance, to account for any of the following observations: (1) the dependence of vicinal couplings on the stituent; (2) the increase of vicinal couplings with increasing electronegativity of the substituent in certain TABLE 1 The calculated vicinal Proton-proton coupling constants in 1,1-and 1,Qdifluoroethane as function of the dihedral H-H angle relative positions of the coupling protons and a sub-CH,-CHF, CH,F-CH,F J l H z J l H z J'W 0 4.26 5.01 4.56