Scalar (contact) vs dipolar (pseudocontact) contributions to isotropic shifts.

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Joy Sutton
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1 Scalar (contact) vs dipolar (pseudocontact) contributions to isotropic shifts. Types of paramagnetic species: organic radicals, and complexes of transition metals, lanthanides, and actinides. Simplest case are the lanthanides because the unpaired electrons occupy inner (4f) orbitals that have very little contribution towards covalent bonding. To a good approximation, observed isotropic shifts are predominantly dipolar in nature, for nuclei that are not directly bonded to the metal atom. Contrast with unpaired d-electrons and 5f-electrons (actinides) where both scalar and dipolar contributions may be important. A closer look at the lanthanides... Because spin-orbit coupling is strong, the ground states of lanthanide cations are best described by the value of quantum number J (=L±S depending upon whether the shell is more or less than half full). The sign and magnitude of the expectation value <S z > depends upon whether the orbital contribution is greater or less than the spin contribution.

2 Calculated values of <S z > are shown in the following Figure. Since <S z > is a major contributor to the dipolar shift, also shown is the calculated shift for a nucleus 3 Å away from the metal, with 3cos 2 1 = 1, at 300 K.

3 In order to observe (and hopefully interpret) an isotropic shift, the NMR line should not be broadened significantly. This requires that the electron spin should relax quickly. For NMR purposes we can divide paramagnetic species into 1. Shift Reagents, SR ( rapid electron spin relaxation) 2. Relaxation Reagents, RR ( slower electron spin relaxation) Class 1 complexes have T e -1 's of s -1 they include the lanthanide cations (except Gd 3+ ); transition metal complexes with T ground states, especially low spin d 5 (Fe III, Ru III ), high spin d 6, d 7 (Co II, Fe II ); Ni II. Class 2 complexes (T e -1 's s -1 ) include those with halffilled shells (Mn II, Gd III ) and non-degenerate ground states, VO 2+, Cr III, V II, Cu II ).

4 Shift Reagents (SR) Best examples are chelates of lanthanide cations. The original shift reagents were trischelates of -diketonates such as Eu(dpm) 3 (dpm = [(CH 3 ) 3 CC(O)CHC(O)C(CH 3 ) 3 ] - ) These complexes form Lewis acid-base adducts in a fast exchange process with organic molecules, thereby inducing dipolar shifts in the NMR spectrum of the organic molecule. See example This application of SRs is becoming less important as high-field spectrometers have been developed.

5 Another application of SR has been to probe the structures (conformations) of molecules in solution. This is an enormous challenge, since many factors have to be considered. Assuming that a single acid-base adduct is formed between the SR and the solute molecule, and that the resulting complex has axial symmetry, one can express the ratio of all the observed shifts with respect to the largest observed shift, j, as νi ν j = 2 1 3cos θ 1 3cos i 3 3 r r i j 2 θ j Generally, for complicated molecules, e.g. nucleotides like AMP, several different SRs must be used, together with Relaxation Reagents.

6 A third application of SRs is in in vivo NMR spectroscopy using 23 Na and 31 P. Normal cells maintain an intracellular concentration of Na + of ~10 mm against an extracellular concentration of ~142 mm. Concentration gradient maintained by the action of NaK-ATPase. Abnormal levels of intracellular Na can be determined by Na-NMR using SRs. Here, ionic SRs are employed to prevent them pasing through the cell membrane and to bind Na +. A good example is [Tm(DOTP)] 5- where DOTP is a tetraphosphonate ligand. Infusion of [Tm(DOTP)] 5- into the cell preparation causes the extracellular Na to undergo an isotropic shift, but the intracellular Na is unaffected. The rate of Na transfer in and out of the cell is slow. Thus the Na signal splits into two resonances, Na extra is shifted and Na intra remains in place.

7 It is possible to observe both the P-31 and Na-23 spectra at the same time. This is known as interleaving. Because quadrupolar Na nucleus relaxes much faster than P nucleus, the Na FID can be recorded while the P nucleus is relaxing. Interleaving requires a probe that can be tuned simultaneously to two different frequencies. e.g. on a 300-MHz spectrometer 23 Na at MHz and 31 P at MHz Relaxation reagents have slower electron relaxation times and tend to broaden NMR lines. This effect has an r -6 dependence rather than the r -3 dependence of the dipolar shift. Thus RRs are often used in conjunction with SRs to probe solution structures of molecules. Common examples are Mn 2+ and Gd 3+. NMR imaging depends upon RRs, usually chelates of Gd 3+ that can bind additional water molecules that are in fast exchange with the solvent. The RR is designed to target a specific tissue and thereby enhance the relaxation of water in that tissue.

Nuclear Quadrupole Resonance Spectroscopy. Some examples of nuclear quadrupole moments

Nuclear Quadrupole Resonance Spectroscopy Review nuclear quadrupole moments, Q A negative value for Q denotes a distribution of charge that is "football-shaped", i.e. a sphere elongated at the poles; a