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Intermolecular and intramolecular effects on the 1 H and 13 C shielding in some gaseous hydrocarbons at various temperatures—theory and discussion
Author(s) -
Raynes William Thomas,
Bennett Brian
Publication year - 1991
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/mrc.1260290916
Subject(s) - chemistry , intermolecular force , intramolecular force , conformational isomerism , electromagnetic shielding , proton , density functional theory , molecule , chemical shift , thermodynamics , computational chemistry , stereochemistry , physics , quantum mechanics , organic chemistry
Values of the first‐order derivatives of proton and 13 C shielding with respect to gas density for several hydrocarbons over the temperature range 180–370 K are fitted to a long‐standing theory. The theory includes consideration of (a) the ‘site factor’ allowing for the position of the resonating nucleus in the molecule and (b) the presence of higher order interactions in the intermolecular potential. The resulting B coefficients are improved as a result of (b) and in the case of 13 C shielding there is almost totally satisfactory agreement. Values of dσ 0 /d T , the temperature dependence of the 1 H and 13 C shielding extrapolated to zero density, are discussed in terms of the displacements of the nuclei from equilibrium geometry. The results suggest that attempts to relate the temperature dependence of chemical shifts of conformationally mobile compounds to the varying populations of a small number of conformers are naive. It is also necessary to take account of contributions from distorted conformers which proliferate increasingly at higher temperatures. The positive values of dσ 0 /d T observed for some carbon nuclei are also believed to be related to torsional motion.