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Carbonyl 17O Chemical Shift in the Proximity of a Methyl Group in Amides: an Experimental and Theoretical Study

✍ Scribed by Rubén H. Contreras; Rodolfo R. Biekofsky; Angel L. Esteban; Ernesto Diez; Jesus San Fabian

Publisher
John Wiley and Sons
Year
1996
Tongue
English
Weight
547 KB
Volume
34
Category
Article
ISSN
0749-1581

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✦ Synopsis

The effect of a cis-N-methyl group on the carbonyl 1 7 0 chemical shift, cis-MSCS, was investigated both from theoretical and experimental points of view in ten amide derivatives. Experimentally, it was observed that the cis-MSCS in N-methylformamide (2) corresponds to a shielding effect of 12.0 ppm with respect to formamide (1). LORG calculations at both the 6-31G" and 6-311G** levels reproduced fairly well this trend, ia. 10.2 and 11.4 ppm, respectively, provided that as the N-methyl group conformation was such that a C-H bond eclipsed the C-N bond (2a). This is the preferential conformation a t the 6-31G*/MP2 level. For other methyl group conformations the LORG calculations did not reproduce that experimental trend. For instance, for an N-methyl C-H bond eclipsing the N-H bond (2b), deshielding cis-MSCSs of 3.7 ppm (6-31G*) and 3.6 ppm (6-311G**) were predicted. Analyses of LORG bond-bond contributions suggested that the interaction that defines 2a as the preferential conformation is an attractive interaction between the in-plane N-methyl C-H bond and the carbonyl oxygen lone pairs. Experimental trends observed for the 1 7 0 chemical shifts measured in the remaining compounds can be rationalized on the same grounds.

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