C13 Splittings in Proton Magnetic Resonance Spectra. II. Bonding in Substituted Methanes

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Abstract
We have measured indirect spin‐spin couplings between protons and C13 nuclei in a variety of substituted methanes. As found earlier for hydrocarbons, the coupling constants, JC–H, appear to be linearly related both to the percent s character of the carbon atomic orbital participating in the C–H bond and to the bond length. On the basis of the large variations of the JC–H values and of the C–H bond lengths in these compounds we conclude that the state of hybridization of the carbon atom cannot be reliably inferred from the observed, essentially tetrahedral valence angles. This means that the bonds must in many cases be considered as somewhat bent. We investigated the dependence of the C–H bond hybridization in compounds of type CH3X upon the properties of the substituent group. The results can be correlated on the assumption that the effective electronegativity and the size of the atom directly attached to the methyl carbon primarily determine the hybridization.

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