NUCLEAR MAGNETIC RESONANCE 103
The paramagnetic screening constant becomes disproportionately larger
for heavier elements; thus while^1 H, the proton, exhibits screening for its com-
pounds within a range of 20 ppm, thallium (^205 Tl) compound screening con-
stants range over 5500 ppm. Changes in screening of each nucleus do not
increase continuously with atomic number but are periodic, following the
value of〈 1/ r^3 〉 , increasing along each period and then falling markedly at the
beginning of the next. Screening constants change in complex manners depen-
dent upon a number of factors including charge density near the nucleus (^14 N
nucleus is 25 ppm more shielded in NH 3 than in NH 4 +), the infl uence of neigh-
boringπ systems, and oxidation states or coordination number of the nucleus
being observed (^31 P screening increases in the series PCl 3456 <<<PCL+−PCl PCl).
Usually, screening increases for substituted main group elements as the elec-
tronegativity of the substituent increases. The nephelauxetic effect (expansion
of the electron cloud and increasing electron delocalization in legend – metal
bonding) changes the screening effect down the halogen group, thus, while the
difference between AlCl 4 − and AlBr 4 − is 22 ppm, that between AlBr 4 − and AlI 4 −
is 47 ppm.
Anisotropic magnets may be formed in chemical bonds within a molecule
so that nuclei in the vicinity may be screened or descreened. Anisotropic
behavior would be found in the vicinity of a carbonyl bond, for instance. The
benzene ring exhibits ring current anisotropy, leading to large descreening
(downfi eld shifts) of benzene protons. Molecules containing electric dipoles
perturb molecular orbitals and therefore the screening of a nuclei. The closer
the nucleus is to the bond generating the electric fi eld, the more they are
descreened. In 1 - chloropropane the descreening shifts, compared to CH 4 , are
α - CH 2 3.24 ppm, β - CH 2 1.58 ppm, and CH 3 0.83 ppm.
The electron ( s = 1/2) has a very large magnetic moment that affects the
NMR spectrum of any molecule possessing a paramagnetic center. If paramag-
netic transition metal ions are present in the molecule, large effects are
observed. The screening constants cover a much larger range than is normal
for the nucleus under study because the unpaired electrons apparently can
delocalize throughout the molecule and appear at or “ contact ” nuclei. The
large resonance frequency shifts that result are called contact shifts. For proton
spectra these shifts may have magnitudes of several hundred parts per million
(ppm). The behavior has great utility in simplifying complex NMR spectra as
illustrated here for some lanthanide elements. Octahedral complexes of lan-
thanide ions such as europium (Eu), dysprosium (Dy), praseodymium (Pr),
and ytterbium (Yb), complexed with organic ligands to render the metals
soluble in organic solvents normally used for NMR samples, are added to the
system being studied. Because the lanthanides may assume higher coordina-
tion numbers, the so - called shift reagent may react with suitable donor sites
(such as O or N) within the target molecule. The interaction produces a pseu-
docontact shift caused by the anisotropic magnetic moment of the shift reagent
(similar to the neighbor anisotropy effect described above). The shift may not
only move the resonance of protons to different locations in the spectrum but