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Carbonyl 17 O Chemical Shift in the Proximity of a Methyl Group in Amides: an Experimental and Theoretical Study
Author(s) -
Contreras Rubén H.,
Biekofsky Rodolfo R.,
Esteban Angel L.,
Diez Ernesto,
Fabian Jesus San
Publication year - 1996
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199606)34:6<447::aid-omr903>3.0.co;2-w
Subject(s) - chemistry , methyl group , amide , group (periodic table) , chemical shift , formamide , bond length , crystallography , computational chemistry , stereochemistry , organic chemistry , crystal structure
The effect of a cis ‐ N ‐methyl group on the carbonyl 17 O 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, 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 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 17 O chemical shifts measured in the remaining compounds can be rationalized on the same grounds.