Nuclear Spin-Spin Coupling via Nonbonded Interactions. 2. γ-Substituent Effects for Vicinal Coupling Constant Involving 13C

Article abstract of DOI:10.1021/ja00521a002

The well-known γ effect on 13C chemical shielding is shown to have a counterpart in vicinal coupling constants ³J_CX involving a 13C nuclear spin and a trans-oriented nuclear spin X.Experimental and theoretical results for vicinal 13C1-C2-C3-1H and 13C1-C2-C3-13C coupling constants in a number of C3-substituted aliphatic compounds in the trans arrangement and alicyclic compounds show a substantial decrease relative to the parent compound.By means of modified INDO-FTP molecular orbital procedures, which permit investigation of the importance of nonbonded interactions, it is shown that the γ-substituent effects arise in a complex way from the nonbonded interactions associated with groups bonded to the C3 or γ-carbon atom.A major factor is the positive contribution to ³J_CX(180 grad) from the hydrogen atoms on the γ-carbon atom of the parent compound.Methyl groups on C3 make negative contributions; a major factor is the interaction between the hydrogen atoms of the C1 methyl and the hydrogen atoms of the methyl groups on the C3 carbon atom.It is further shown that other substituents also make negative contributions, and that hydrogen atoms on the γ substituents are not essential for the observation of a negative γ effect on vicinal constants.The γ-susbtituent effects on vicinal 13C-X coupling constants appear to be general phenomena of substantial magnitude; their recognition is important for conformational and other coupling-constant studies as they lead to results which appear to be anomalous.