Nuclear Magnetic Shielding in Fluorine and Hydrogen Compounds

Measurements of the nuclear magnetic shielding of F19 and H1 are reported for a number of their respective binary covalent compounds. The applied fields required for the F19 and H1 nuclear magnetic resonances, at fixed frequency, vary in these compounds over a range of 3.98 and 0.122 gauss, respectively, at fields of about 6365 gauss. These ranges are of the same order of magnitude as the theoretical internal diamagnetic corrections for the unbound fluorine and hydrogen atoms. The dependence of nuclear magnetic shielding on the electronic structures of the molecules is discussed qualitatively and some general correlations indicated. One such correlation is that the magnetic shielding of the F19 nucleus decreases with increasing electro-negativity of the atom to which the fluorine is bonded. This and other correlations differ in degree for the proton. The differences may arise from the relatively greater importance of the diamagnetic shielding term for the proton. Specific information regarding molecular and electronic structure is provided in several instances. The tetragonal pyramid structure for IF5 is confirmed and a similar structure demonstrated for BrF5. The appearance of double F19 resonance lines in PF5, IF5, and BrF5 indicates nonequivalent electronic distributions in the structurally distinguishable bonds. The measurements were facilitated by a large permanent magnet, the design and field homogenization of which are described.