NMR Studies of Asymmetric Ethanic Rotators: 1,2-Disubstituted Propanes

The NMR spectra of several 1,2-disubstituted propanes, particularly 1,2-dichloropropane and 1,2-dibromopropane, are examined in detail in various media. A range of vicinal J couplings is obtained from the computational analyses of solution systems selected to afford an extensive cross section of rotational averaging over the possible conformers. An expression is then given relating the conformational free-energy difference to theoretical couplings of ``frozen'' structures and experimentally evaluated couplings of ``fluid'' structures. This permits the approximate determination of the gauche and trans vicinal couplings for the frozen conformations of a 1,2-disubstituted propane. These are —1.5 cps for the gauche and +15.5 cps for the trans coupling, in very good agreement with the findings of Gutowsky, Belford, and McMahon for other ethanic molecules studied by variable temperature techniques. Ranges of free energies are graphically related to computed couplings obtained from experimental spectra. The stereospecificity of the sign of the long-range couplings in these structures is deduced and predictions are made of the previously unreported signs of the reportedly observed long-range couplings in other related structures. Experimental measurements and predictions of the signs of long-range couplings are made for some 2,2-disubstituted propanes studied to evaluate the role of substitution in determining the magnitudes and signs of the long-range couplings in the 1,2-disubstituted propanes. Geminal J couplings computed from these spectra show novel solution effects. Experimental chemical shift differences are accounted for by considering the neighbor-anisotropy contributions in model structures.