CARBONYL O-17 CHEMICAL-SHIFT IN THE PROXIMITY OF A METHYL-GROUP IN AMIDES - AN EXPERIMENTAL AND THEORETICAL-STUDY

The effect of a cis-N-methyl group on the carbonyl O-17 chemical shift , cis-MSCS, was investigated both from theoretical and experimental po ints 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 a t both the 6-31G and 6-311G** levels reproduced fairly well this tren d, i.e. 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 at the 6-31G/MP2 level. For other methyl group conformations the LORG calculations did not reprodu ce that experimental trend. For instance, for an N-methyl C-H bond ecl ipsing the N-H bond (2b), deshielding cis-MSCSs of 3.7 ppm (6-316) an d 3.6 ppm (6-311G*) were predicted, Analyses of LORG bond-bond contri butions suggested that the interaction that defines 2a as the preferen tial conformation is an attractive interaction between the in-plane N- methyl C-H bond and the carbonyl oxygen lone pairs. Experimental trend s observed for the O-17 chemical shifts measured in the remaining comp ounds can be rationalized on the same grounds.