Single-crystal study of 13C chemical shielding tensors in dimethyl maleic anhydride and acrylamide

Abstract

Carbon chemical shielding tensors of dimethyl maleic anhydride (DMMA) and acrylamide were determined by single-crystal studies using proton-enhanced NMR. The double-bond carbon tensors in DMMA were found to have their most shielded component approximately perpendicular to the sp 2 plane and the component of intermediate shielding approximately in the bond direction. The most shielded and the intermediate shielding components of the anhydride carbon in DMMA have close values, the intermediate shielding component being approximately perpendicular to the sp 2 plane. For the acrylamide there are two possible assignments for each shielding tensor. One of the two possible assignments of each of the double-bond carbon tensors has the principal directions oriented similarly to those in the DMMA case. One of the two possible assignments of the amide carbon tensor has the orientation of the principal directions quite similar to those encountered in the studies of other tensors of carbons double bonded to oxygen (allowing for deviations caused by molecular deformation). A new kind of rf pulse power amplifier possessing good stability was used in the present work for H decoupling and is described.