No abstract

No abstract

I t is shown that the electric field a t a particular nucleus arising from polar groups in other parts of a molecule can lead to chemical shifts proportional to the first power of the field strength. The change Au in the proton screening constant of an S-I3 bond when it is subjected to an electric field E is approsirnately given by where E , is the colnpollent of E i11 the bond direction. ; 2 field along the S-H boncl drays the electrons in the enriched region bet\vee11 tlie nuclei away from the proton, thereby causlng its resonance to occur a t lower magnetic field strengths, while a field in the H-X direction leads to resonance a t higher fields. The electric field produced by tlie polarization of tleigliboring solvent molecules may also be inlportant ancl lead to a solvent shift related to the dielectric constant of the solvent. The direction of the internal lield, and that due to the solvent, can easily be estimated when the lnolecule is rigid, so that the model can be useful in assigning spectra. I t gives a si~iiple and reasonably accurate account of the obscr\wl spectra of substituted benzenes and of some solvent eltects on the proton resonances of CZS-and trans-1.2-dichloroethene.I11 nuclear inagnetic resonance experiments, nuclei with magnetic moments are effectively used as probes to n-~easure the actual magnetic field, a t the nucleus.[ll a,,,,,where II is the applied magnetic field strength and a is the screening constant. Owing to rapid ~nolecular motion in gases and liquids, the observed screening constant corresponds to an average for all molecular orientations. The molecules which are far removed from the one with the resonant nucleus interact with it as if they formed a continuunl having the bull; properties of the fluid, and they may contribute to u a term proportional to the volume susceptibility; this term depends on the sainple shape, and vanishes for a spherical specimen. The screening constants quoted in this paper have all been "corrected" so as to remove this bullr susceptibility effect. Progress in understanding observed screening constants has lai-gely been based on models in which a is split up into atomic contributions (see Pople, Schneider, and Bernstein (1) and Sailia and Slichter (2)). Thus, where ad and u D are the dia-and para-magnetic shielding of the nucleus whose total screening constant is a ; ad is proportional to the magnetic field produced a t the nucleus by the moment induced through the rotation of the electrons about the nucleus with the Larmor precessional frequency eEI/2?.rzc. The paramagnetic shielding a, arises from the hindrance to the Larmor precession produced by the lack of axial sy~nmetry about the field H ; in a linear molecule, u, is zero when H is along the molecular axis, but in general it is non-zero for an atom in a molecule. U, is the shielding coi~stant proportional to t h e field produced by the magnetic nioments induced in other atoms, and a, is thecoiltributio~l lilfaa7iz~scribt received Se bte~nber I S . 1959. Chemistry, iVatioa7ial Researcl~ C...

Despite the fact that a very wide range of organic radicals have been studied by esr spectroscopy, both in solution and in the solid state, very few have been interpreted for R2CX radicals, where X = Cl, Br, or I. Recently, various R2CC1 radicals have been detected in the liquid state1 and this has enabled others2 to interpret esr spectra for such radicals in the solid state that had previously defied complete interpretation.3 Very recently, a full analysis of the radical FC(CONH2)Cl has been described4 formed in 7-irradiated FCH(CONH2)Cl crystals.5Although H2CBr radicals have been matrix isolated and studied by infrared spectroscopy,6 we know of only one tentative esr identification, in which a multiline esr spectrum for an irradiated single crystal of bromoacetic acid was obtained but was not analyzed.7 The

Contributions to nuclear screening (chemical shifts) arising from molecular interactions with solvent molecules (excluding hydrogen bonding) are discussed in terms of appropriate theoretical models. These include contributions from van der Waals interactions σw, from the magnetic anisotropy of the solvent molecule σa, and from polar effects σE. By a suitable choice of solute-solvent systems it has been possible to demonstrate each of these effects experimentally for proton resonances. For CH4 as a solute, σw was in all cases negative, its magnitude varying with the nature of the solvent and amounting to as much as 0.6 ppm for high molecular weight solvents. In agreement with the theoretical models, σa was found to be positive for disk-shaped solvent molecules and negative for cylindrically symmetrical rod-shaped molecules, its magnitude in extreme cases reaching 0.75 ppm. For CH3CN as a solute, σE was negative and showed the expected dependence on the dielectric constant of the solvent.