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

✍ Scribed by Aart A. van Beuzekom; Frank A. A. M. de Leeuw; Cornelis Altona

• Publisher: John Wiley and Sons
• Year: 1990
• Tongue: English
• Weight: 708 KB
• Volume: 28
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260280113

✦ Synopsis

Abstract

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^CCCH) 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^C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 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.