Hydrogen Bonding Directed Reversal of 13 C NMR Chemical Shielding.

The deshielding or downfield 13 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 13 C NMR of thiocarbonyl carbon in thioamides upon H-bonding encouraged us to explore the physical origin of the reversal of 13 C NMR chemical shielding. Careful NMR analysis shows that sulfur and selenium-centered H-bonds (S/SeCHBs) induce a shielding effect on the 13 C C=S(Se) while changing from amides to thioamides or selenoamides. In addition, natural chemical shielding (NCS) analysis shows that the σ 11 and σ 22 components of the isotropic shielding tensor (σ) have a crucial role in this unusual shielding.