| Summary: | 碩士 === 國立成功大學 === 化學系專班 === 94 === Weinhold and co-workers suggested the X－H bond length in X－H…Y hydrogen bonded complexes is controlled by a balance of two main factors acting in opposite directions. One is n(Y)→σ＊(X－H) hyperconjugative interaction leading to X－H bond lengthening, and the other is rehybridization leading to X－H bond shortening. We concentrate on the C－H bond length of methane substituted by halogens. All computations are performed using Gaussian98 program, and all discussions in this paper are based on B3LYP/6-31＋G＊ calculations.
Without hydrogen bond acceptors, the C－H bond length is mainly affected by s-character of C atom. When hydrogen bond acceptor is present, at larger distance, the C－H bond length shortening is due to the increase in the s-character of the C－H bond, and at shorter distance, the C－H bond length lengthening is due to n(Y)→σ＊(X－H) hyperconjugative interaction. If we change with different hydrogen bond acceptors(Y), the C－H bond length is mainly affected by hyperconjugative interaction. The C－H bond length even has linear relation with E(2). If we use the same hydrogen bond acceptor(Y), the C－H bond length keeps the same in spite of different substitutions on C atom of methane. If we change the central atom, the C－H bond length still has linear relation with E(2), but the slope is different. The larger electronegativity of the central atom is, the smaller the slope is.
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