Question

can you explain spectral (nmr) and electronic differences (properties) between thiocarbonyl and carbonyl compounds? references would be good.

Answer 1

There are significant and interesting differences between compounds with carbonyl and thiocarbonyl groups. The C=S bond length (~1.6 Å)1 is considerably longer than C=O (~1.25 Å). As a result, one would expect the bond strengths to differ significantly.

Another interesting difference is found in the position of the n-π* transition that is shifted into the visible spectrum by the sulfur.3 As a result the thiocarbonyl derivatives have interesting colors with phenyl thiocarbonyl chloride being red (λmax = 530 nm). The transitions are shifted by about 2eV (~50 kcal/mol) to the red as compared to the corresponding carbonyl compounds

Isotropic deshielding of the carbon nucleus in the C=S group relative to C=O is primarily attributable to the decreased energy associated with the σ ↔ π* excitation within the thiocarbonyl fragment. This result is in contrast with the conventional interpretation that the deshielding originates from a red shift in the C=S HOMO–UMO n → π* transition.