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

Abstract

The proton and "C shielding constants in CH, , C,H,, C,H, and some other gaseous hydrocarbons have been studied as functions of density at temperatures in the range 180-370 K. The linear coefficients describing the density dependence of the shielding, after correcting for bulk susceptibility, increase substantially as the temperature is reduced, indicating stronger intermolecular interactions. (Some of the required magnetic susceptibilities were determined in this work by an NMR method). The "C measurements for CH, are close to those of an earlier study; the results for the other gases are new. The linear coefficient is substantially greater for the carbon shielding of the methyl group in propane than for the methylene group at any temperature, but there is virtually no distinction between tbe linear coefficients for the proton shielding in this gas. Values for du,/dT, the temperature dependence of the shielding extrapolated to zero density, are also presented for both proton and "C shielding in the bydrocarboos. They are positive and negative in different instances. It is shown from this and earlier gas-phase studies that standard literature values of the methane "C shielding relative to the ''C shielding in tbe other bydrocarboos are in error.

In Situ NMR Observations of the Photolysis of Cymantrene and Methylcymantrene in Supercritical Fluids: A New Technique Using High-Pressure NMR

“…The bulk susceptibility term, σ b1 , can be calculated according to Smith and Raynes, eq 4. , The term χ m is the molar magnetic susceptibility, experimentally measured to be −19.7 × 10 -6 cm 3 /mol for ethylene and −20.8 × 10 -6 cm 3 /mol for CO 2…”

“…Accordant with previous studies we assume that σ E1 and σ a1 make negligible contributions for the ethylene and CO 2 solvents. , The term σ ex1 is also assumed to be negligible as according to ab initio calculations by Jameson and de Dios this term is not sampled at the intermolecular distances attained in the present work . The bulk susceptibility term, σ b1 , can be calculated according to Smith and Raynes, eq 4. , The term χ m is the molar magnetic susceptibility, experimentally measured to be −19.7 × 10 -6 cm 3 /mol for ethylene and −20.8 × 10 -6 cm 3 /mol for CO 2…”

“…25 The bulk susceptibility term, σ b1 , can be calculated to Smith and Raynes, eq 4. 26,27 The term χ m is the molar magnetic susceptibility, experimentally a Uncertainties are reported as one standard deviation. b Measured in this study, units of ppm cm 3 /mol.…”

“…28 This leads to values of σ b1 ) 82.5 and 87.1 ppm cm 3 /mol for ethylene and CO 2 , respectively. 26 By subtracting these values from σ 1 we arrive at σ loc1 , which in the present case is assumed to be a result of the pairwise dispersion interactions the nucleus is experiencing. The values of σ loc1 in ethylene and in CO 2 are given in the third and sixth columns of Table 1.…”

Density Functional Calculations of Proton Chemical Shifts in Model Peptides

“…Experimental measurements of changes in chemical shifts of nonpolar, magnetically isotropic molecules as a function of density, temperature, and solvent support the existence of a close contact or van der Waals contribution to chemical (de)shielding, that can be modeled with a 1/r 6 dependence. ,− Although this has the same distance dependence as eq 7, two arguments suggest that dispersion effects are not the dominant interaction. First, Hartree−Fock or DFT calculations on rare gas dimers (and other nonpolar interactions) show a deshielding in rough accord with observation, even though dispersion energetics are not correctly modeled at this level of theory.…”