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Hydrogen Bonding Directed Reversal of 13 C NMR Chemical Shielding
Subhrakant Jena1,2, Chinmay Routray1,2, Juhi Dutta1,2
1School of Chemical Sciences, National Institute of Science Education and Research (NISER), PO-Bhimpur-Padanpur Via-Jatni, District-Khurda, PIN - 752050, Bhubaneswar, India.
Abstract:
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 CC=S(Se) while changing from amides to thioamides or selenoamides. In addition, natural chemical shielding (NCS) analysis shows that the σ11 and σ22 components of the isotropic shielding tensor (σ) have a crucial role in this unusual shielding.
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