The proposed research is divided into four parts, each of which involves the application of dynamic NMR spectroscopy to problems in conformational analysis. The projects of the first three parts are expected to provide experimental evidence that """"""""aromaticity"""""""" is important for stabilizing the Z conformations of esters, secondary amides (proteins), and related compounds. Vinyl, ethynyl, phenyl, and cyclopropyl groups, and the acidic protons of thioacids, are expected to be incapable of providing the third pair of electrons to complete the aromatic sextet, and compounds containing these groups will be studied to determine whether the populations of E-isomers are large, as expected. The ability to control the conformation of aromatic amides is known to be useful in the design of drugs, and the proposed research is expected to provide a set of groups which can be used to change the conformation of certain esters, amides, and related compounds, in addition to contributing to an understanding of the conformational preferences of three important classes of biological compounds (amides, esters, and thioesters). In part I, low-temperature carbon and proton NMR will be used to study E-Z isomerism in dilute solutions of N-alkylforamides and in a series of thiolformate esters. Part II describes a study by the same method of conformational equilibration and reversible covalent association in several dithiocarboxylic acids. In part III, the conformational equilibria and barriers of alkyl thionitrites will be studied and compared with those of alkyl nitrites. The barriers for nitrites and thionitrites will provide evidence regarding the relative importance of oxygen and sulfur pi-bonding. Part IV is an NMR study of hindered rotation about the triple bond in alkynes of the type (CH3)2N-C=CCOR, which will provide information about the ability of the triple bond to transmit conjugation between a donor and an acceptor.
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