1 9F NMR chemical shifts due to intermolecular interactions in F2C=CFX. A quantitative measure of the nuclear site effect

Abstract

In CF2=CFX (X=H, Cl, Br, I) there are three probe nuclei for studying the magnitudes of intermolecular effects on nuclear shielding. The values of σ1=limρ→0 (∂σ/∂ρ)T, the change in the nuclear shielding due to interactions between pairs of molecules, have been obtained from the resonance frequencies in medium to low density gas samples. The nuclear site effect gives the most exposed one of the three probe nuclei the largest magnitude of σ1, i.e., the F trans to X=Cl, Br, I or the F cis to X=H. The gas-to-liquid shifts show exactly the same ordering as the σ1 in the dilute gas. A model is described which relates a calculated site factor to the observed σ1 values. With this model, the observed density dependence of the NMR isotope shift in D2/HD is calculated. This model provides a mechanism for nonspecific solvent effects on the NMR isotope shift.

Gas-phase NMR spectroscopy

“…The linear correlation was observed in each of three molecules (X = Cl, Br, and I). 15 The observed temperature dependence of <r1 values has generally been in the direction of decreasing magnitude of <rx with increasing temperature, except in the two cases of 129Xe in xenon interacting with CO and N2 molecules. 22 Systems with large magnitudes of <rx tend to have a large change of ox with temperature.2…”

“…(ID where C is the spin-rotation constant, v is the mean relative velocity, d;(1) = U(j + l)]1/2O'0' + 1)>"1/2 (12) and the diagonal matrix P gives the populations of the initial states normalized such that d(1)Pd(1> = 1 (13) (2) The quadrupolar relaxation rate (for low densities only) is given by 3 (…”

Molecular dynamics averaging of Xe chemical shifts in liquids

“…For example, extensive studies of gas to liquid NMR shifts over a wide range of temperatures in simple molecules have established that the 19 F chemical shifts varied with temperature, uniformly decreasing as the density of the liquid decreases with increasing temperature. [52][53][54][55][56][57] The temperature dependence of Xe chemical shifts has been measured in some liquids. 8,16 The MD simulations in the present work provide the theoretical paradigm for the understanding of all such data.…”

Comparative NMR Properties of H2and HD in Toluene-d8and in H2/HD@C60

“…Although the proton T 1 of pure HD (or HD as an impurity in H 2 or D 2 ) gas, , liquid, , and solid ,, has been reported, we are not aware of a previous determination of T 1 for HD in organic media. The detection of both H 2 and HD peaks in the same sample and the resolution of the HD coupling constant, J HD , also makes it possible to measure J HD and the 1 H chemical shift difference between the two species in the two media over a range of temperatures as a probe of the changes of these parameters compared to experimental and theoretical determinations for H 2 and HD in the gas phase.…”