Signs of spin–spin coupling constants between aldehydic and ring protons in 2,6-dinitrobenzaldehyde. Evidence for a hyperconjugative contribution to JpH,CHO

Abstract

The spin–spin coupling constant between the aldehydic proton and the proton in the para position, JpH,HCO is negative in 2,6-dinitrobenzaldehyde. JpH,CHO is also very likely negative in 2,6-dichlorobenzaldehyde. It is suggested that steric interaction with the ortho substituents forces the aldehyde group out of a coplanar conformation and leads to an interaction of the aldehydic C–H bond with the π system of the ring. Tentative values of θ, a measure of the deviation from coplanarity, are given.