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NMR Spectroscopy Investigation of the Cooperative Nature of the Internal Rotational Motions in Acetophenone
Author(s) -
De Luca Giuseppina,
Longeri Marcello,
Pileio Giuseppe,
Lantto Perttu
Publication year - 2005
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500134
Subject(s) - acetophenone , spectroscopy , nuclear magnetic resonance spectroscopy , rotational spectroscopy , chemistry , internal rotation , computational chemistry , rotational dynamics , nuclear magnetic resonance , materials science , chemical physics , physics , stereochemistry , molecule , organic chemistry , quantum mechanics , catalysis , engineering , mechanical engineering
The proton NMR spectrum of the doubly enriched acetophenone‐ carbonyl,methyl ‐ 13 C 2 isotopomer dissolved in a liquid‐crystalline solvent (LXNMR) was analyzed to yield a data set of 19 dipolar couplings. The presence of so many couplings, and in particular the dependence of some of them on the acetyl carbons enabled the investigation of the structure of the acetyl moiety and of possible cooperative motions about the aryl–carbonyl and carbonyl–methyl bonds. Methodological aspects, and approximations relating to the application of the vibrational correction procedure in the presence of large‐amplitude torsional motions, are discussed. Results show that it is possible to discriminate between a continuous and a discrete conformer distribution about the angle ϕ 1 but not among a few proposed continuous shapes of U iso ({ ϕ }). In this study, the use of dipolar couplings with a non‐negligible contribution from the indirect spin–spin coupling tensor J , ( ${D_{{\rm{C8C9}}} }$ in our case), for structural determination is extended from rigid to flexible molecules. The ${{1 \over 2}J_{{\rm{C8C9}}}^{{\rm{aniso}}} }$ contribution was derived theoretically using the density functional theory linear response (DFT‐LR) first‐principles calculation of the J C8C9 spin–spin coupling tensor.