Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study

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The effect of a cis-N-methyl group on the carbonyl 17O 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 w...

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Detalles Bibliográficos
Autores principales: Contreras, Rubén Horacio, Biekofsky, Rodolfo Roberto
Publicado: 1996
Materias:
17O NMR chemical shifts
Ab initio LORG study
Amides
Acceso en línea:https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_07491581_v34_n6_p447_Contreras
http://hdl.handle.net/20.500.12110/paper_07491581_v34_n6_p447_Contreras
Aporte de:
Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA) de Universidad de Buenos Aires
id paper:paper_07491581_v34_n6_p447_Contreras
record_format dspace
spelling paper:paper_07491581_v34_n6_p447_Contreras2023-06-08T15:45:31Z Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study Contreras, Rubén Horacio Biekofsky, Rodolfo Roberto 17O NMR chemical shifts Ab initio LORG study Amides The effect of a cis-N-methyl group on the carbonyl 17O 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 17O chemical shifts measured in the remaining compounds can be rationalized on the same grounds. Fil:Contreras, R.H. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Fil:Biekofsky, R.R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. 1996 https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_07491581_v34_n6_p447_Contreras http://hdl.handle.net/20.500.12110/paper_07491581_v34_n6_p447_Contreras
institution Universidad de Buenos Aires
institution_str I-28
repository_str R-134
collection Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA)
topic 17O NMR chemical shifts
Ab initio LORG study
Amides
spellingShingle 17O NMR chemical shifts
Ab initio LORG study
Amides
Contreras, Rubén Horacio
Biekofsky, Rodolfo Roberto
Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study
topic_facet 17O NMR chemical shifts
Ab initio LORG study
Amides
description The effect of a cis-N-methyl group on the carbonyl 17O 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 17O chemical shifts measured in the remaining compounds can be rationalized on the same grounds.
author Contreras, Rubén Horacio
Biekofsky, Rodolfo Roberto
author_facet Contreras, Rubén Horacio
Biekofsky, Rodolfo Roberto
author_sort Contreras, Rubén Horacio
title Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study
title_short Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study
title_full Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study
title_fullStr Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study
title_full_unstemmed Carbonyl 17O chemical shift in the proximity of a methyl group in amides: An experimental and theoretical study
title_sort carbonyl 17o chemical shift in the proximity of a methyl group in amides: an experimental and theoretical study
publishDate 1996
url https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_07491581_v34_n6_p447_Contreras
http://hdl.handle.net/20.500.12110/paper_07491581_v34_n6_p447_Contreras
work_keys_str_mv AT contrerasrubenhoracio carbonyl17ochemicalshiftintheproximityofamethylgroupinamidesanexperimentalandtheoreticalstudy
AT biekofskyrodolforoberto carbonyl17ochemicalshiftintheproximityofamethylgroupinamidesanexperimentalandtheoreticalstudy
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