Proton Chemical Shifts for the Alkyl Derivatives

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Abstract
The chemical shifts of a series of methyl, ethyl, propyl, and isopropyl derivatives have been studied in an effort to determine the distance and angular dependence of any contribution to the chemical shift arising from magnetically anisotropic substituent groups. It was found that electron withdrawal effects play a dominant role in determining the chemical shifts for methyl derivatives. The nearly exactly linear relation of the methyl shifts to electronegativity for the methyl halides seems to rule out any large influence due to magnetic anisotropy. The ethyl shifts were found to be determined by electron withdrawal effects plus a factor which acted equally at the α and β positions. It was found that this factor arises from the carbon‐carbon bond. By assigning a chemical shift due to the presence of the C–C bond, which is dependent in size on the substituent attached to the α carbon, the apparently anomalous frequencies observed for the alkyl derivatives can be accounted for. A possible explanation for the C–C bond shift may be regular changes in the paramagnetic term in the Ramsey equation due to changes in the excitation energy denominator when a C–C bond replaces a C–H bond.