[Robin]

Hi there, Ive been doing a series of thionation reactions, converting lactams to thiolactams,  and I've noticed something pretty peculiar. For all of my compounds the thionation significantly de-shields the protons adjacent to it, particularly those directly bonded to the thiolactam at the N position. C NMR of the thiolactams also clearly shows that these carbons are significantly more deshielded than their carbonyl counterparts (typically 170ppm for C=O and almost 200 for C=S).

Does anybody have any ideas as to why this would be? I would have assumed based on electronegativity that Thioketons are less de-shielding than carbonyls?

Thanks for everything!

[phth]

Is the ionization state of the thiolactam going to be the same as a lactam? Sulfur allows additional π δ on top of  π π* bands.  I think that this means that it will exist as the C⁺--S^- tautomer more often creating a higher shifted Cq in the carbon.  This means that the protons at the α position will be resultantly more δ⁺.