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

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AbstractThe magnitude of the NMR spin‐spin coupling constant, 3J(CH), between a vicinal 13C–1H pair depends, inter alia, on the value of the torsion angle ΦCH(13CCCH) and is influenced by the presence of an electronegative substituent located on the coupling 13C nucleus. The form and magnitude of the effect of the orientation ΨXC 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13CCC) 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 Jcalc. The magnitude of this effect is shown to be strongly dependent on the electronegativity χ of the α‐substituent. Thus, it is shown that Jcalc 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 3J(CH) in cis‐ and trans‐2,2,6,6‐tetradeuterio‐4‐tert‐butylcyclohexanol confirmed some of the theoretical predictions. In the cis compound (ΦCH = 180°, ΨOC = 60°) 3J(C‐1,H‐3eq) is 7.1 Hz, whereas in the trans compound (ΦCH = 180°, ΨOC = 180°) 3J(CH) equals 10.4 Hz, in qualitative agreement with the INDO calculations.