Intermolecular and intramolecular effects on the ¹H and ¹³C shielding in some gaseous hydrocarbons at various temperatures—theory and discussion

Abstract

Values of the first-order derivatives of proton and "C shielding with respect to gas density for several hydrocarbons over the temperature range 1 S 3 7 0 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 "C shielding there is almost totally satisfactory agreement. Values of du,/dT, the temperature dependence of the 'H and I3C 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 nesessary to take account of contributions from distorted conformers which proliferate increasingly at higher temperatures. The positive values of duo/dT observed for some carbon nuclei are also believed to be related to torsional motion