One-Bond C-C Coupling Constants in Ethers Are Not Primarily Determined by n-sigma~* Delocalization

摘要

One-bond C-H coupling constants (~1J_(CH)) have been studied extensively.A major determinant is the hybridization of the carbon,since only an s orbital has electron density at the nucleus.Another feature is the lower ~1J_(CH) value for an axial C-H bond adjacent to O or N in a six-membered ring,compared to equatorial.This is called the Perlin effect,and it is often used for stereochemical assignment in sugars and related substances.It is invariably attributed to an n-sigma~* (anomeric) interaction,whereby a lone pair is delocalized into the antiperiplanar C-H bond,as suggested in Figure 1.