Effect of the orientation of an α‐substituent on vicinal 13 C–; 1 H spin–spin coupling constants

The magnitude of the NMR spin‐spin coupling constant, 3 J (CH), between a vicinal 13 C– 1 H pair depends, inter alia , on the value of the torsion angle Φ CH ( 13 CCCH) and is influenced by the presence of an electronegative substituent located on the coupling 13 C nucleus. The form and magnitude of the effect of the orientation Ψ XC of such an α‐substituent were examined. The coupling constant between C‐1 and a hydrogen atom located on C‐3 in a series of α‐substituted propanes were studied by means of the semi‐empirical INDO method. In the calculations both Φ and Ψ(X 13 CCC) were systematically varied in steps of 30°. These calculations reveal that the variation of Ψ at a constant Φ has a pronounced effect on the calculated coupling constant J calc . The magnitude of this effect is shown to be strongly dependent on the electronegativity χ of the α‐substituent. Thus, it is shown that J calc depends on Φ and Ψ, in addition to χ. The resulting set of two‐dimensional Karplus‐type surfaces can be described by an equation that contains only nine adjustable parameters. Measurement of 3 J (CH) in cis ‐ and trans ‐2,2,6,6‐tetradeuterio‐4‐ tert ‐butylcyclohexanol confirmed some of the theoretical predictions. In the cis compound (Φ CH = 180°, Ψ OC = 60°) 3 J (C‐1,H‐3eq) is 7.1 Hz, whereas in the trans compound (Φ CH = 180°, Ψ OC = 180°) 3 J (CH) equals 10.4 Hz, in qualitative agreement with the INDO calculations.