Hydrogen Bonding Directed Reversal of C NMR Chemical Shielding.

The deshielding or downfield C NMR chemical shift of amide carbonyl carbon upon H-bonding is a widely observed phenomenon. This downfield shift is commonly used as a spectroscopic ruler for H-bonding. However, the very first observation of an upfield C NMR of thiocarbonyl carbon in thioamides upon H-bonding encouraged us to explore the physical origin of the reversal of C NMR chemical shielding. Careful NMR analysis shows that sulfur and selenium-centered H-bonds (S/SeCHBs) induce a shielding effect on the C while changing from amides to thioamides or selenoamides. In addition, natural chemical shielding (NCS) analysis shows that the σ and σ components of the isotropic shielding tensor (σ) have a crucial role in this unusual shielding.