Medium Effects in Nuclear Magnetic Resonance. IV. A Refined Theory for Nonpolar Gases

Abstract

Following the general theory for medium NMR shifts in gases as outlined in Part I of this series, two improvements are proposed. For the calculation of the van der Waals interaction a repulsive term has been added to the customary attractive term. Wherever intermolecular distances are required, a properly averaged distance between the site of the measured proton in the solute molecule and the center of the solvent molecule is used rather than the distance between the centers of the molecules. The implications of these corrections for nonpolar molecules are discussed, and new values for the bond parameters B are proposed.

Medium Effects in NMR. VIII. Temperature and Pressure Dependence of 19F Chemical Shifts in Pure CF4, SiF4, and SF6 Gases and in Gaseous Mixtures

“…Buckingham 4 has given a general equation for the electric field effect and Bothner-By5 has dealt with van der Waals interactions on screening. Raynes, Buckingham, and Bernstein 2 have given a detailed treatment of polar and van der Waals terms using a Stockmayer potential, comprising of a Lennard-Jones (6)(7)(8)(9)(10)(11)(12) central-force potential and the interaction energy of two dipoles, to explain IH resonance shifts quite satisfactorily. The same model was applied by Petrakis and Bernstein 6 • 7 to 19F resonance shifts and was found to be quite satisfactory within the experimental errors with a few exceptions.…”

31P nuclear magnetic resonance chemical shifts of elemental phosphorus in the gas phase

“…The short-range repulsive overlap forces cause an increase in shielding, i.e. 6xa> 0, as this interaction pushes the electrons back towards the nucleus [16]. In contrast, the term 3 2 is considered to be negative as, during the collision of a solute molecule with another molecule, the symmetry of the electron shell is reduced.…”

Intermolecular interactions in nuclear magnetic resonance. X. A site-specific continuum model for the gas-to-liquid shifts of nonpolar solutes. Applications to proton medium shifts and the determination of cavity radii

“…In Table 7, the factor 2p22/3~212 of [9] has been calculated for a nu~nber of strongly polar solvents. It is seen that the calculated aE?…”

“…Therefore, Ssllcll will be equal to S,,,i, of previous site factor calculations (9)) To find the total site factor the result of [6] needs to be integrated.…”

“…(8,9) pretend to correct for the fact that the nuclei nleasured are not normt~lly located at the center of mass of the solute molecule. These nuclei may therefore be expected to experience an average squared electric field differed from that calculated at the center of Illass.…”