Notes 2475 sorption of Kr on LiF calculated from the isotherms at -183 and -195°are shown in Figure 2. The AHV value is the heat of vaporization of liquid krypton at its normal boiling point.8 If one takes the initial higher heat as due to some surface heterogeneities and the latter part of that including contributions of lateral interactions, then the minimum in the heat curve probably corresponds to the adsorption of individual molecules on the surface. This confirms the above prediction.
Schneider1 for the neighbor anisotropy effects of the halogens on the C13 chemical shifts of the methyl groups in haloethanes. However, the magnetic anisotropy model cannot predict opposite signs for the contribution to long-range shielding at two points which both lie on the axis of symmetry of the susceptibility tensor. In fact, the trends of opposite sign in the a and ß acetylenic resonances can be considered evidence against á rationalization of these shifts based on a simple model of magnetic anisotropy. Caution has previously been expressed in reference to employing the rather flexible concept of magnetic anisotropy in explanations of C13 chemical shifts,20•21 and Schaefer, Reynolds, and Yone-moto21 have suggested contributions to C13 shifts in aliphatic and aromatic halogen compounds due to intramolecular effects of the van der Waals type. The present results seem to emphasize the need for a critical consideration of the importance of anisotropy effects and the need for alternative explanations. It appears that more work, both experimental and theoretical, may be necessary before a satisfactory explanation for the present data will be possible, or before a completely acceptable discussion of earlier data can be given.22
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