13C-C-H spin-spin coupling constants involving the formyl proton in aldehydes

The two-bond 13C-C-H coupling constants involving the formyl proton of about 40 aldehydes were measured in natural abundance. The sign of the coupling constant was found, by a decoupling technique, to be the same as the directly bonded 13C-II coupling constant. On the basis of molecular orbital theory, the two-bond coupling constants of acetaldehyde, acrolein and propargyl aldehyde were evaluated. From the calculations it was found that the main contribution to the coupling constant comes from the delocalization of the lone-pair electrons of the carbonyl oxygen atom, and the low excitation energy of these electrons. The substituent effects on, and the temperature dependence of, the two-bond coupling constant were qualitatively elucidated along these lines.