Spectroscopy paramagnetic term

The expression for the contribution to the spin-orbit induced MCD intensity from perturbation of the ground state is somewhat reminiscent of an expression for the Ag quantity of EPR spectroscopy. The similarity lies in the paramagnetic term, Agp. This term is composed of integrals of a spin-orbit operator over molecular orbitals similar to the expression for the perturbation of the ground state in the presence of spin-orbit coupling (Eqs. 52-56). The paramagnetic contribution to Ag dominates for blue copper proteins and it was suspected that the MCD parameters and Amay have some sort of relationship. It was found that many of the terms that make large contributions to AgP do play a role in the MCD intensity but no simple relationship was found (160). 

How do we know or decide what terms to put in the spin Hamiltonian This is a question of rather far-reaching importance because, since we look at our biomolecular systems through the framework of the spin Hamiltonian, our initial choice very much determines the quality limits of our final results. In other branches of spectroscopy this is sometimes referred to as a sporting activity. We are guided (one would hope) by a fine balance of intellectual inspection, (bio)chemical intuition, and practical considerations. In a more hypochondriacal vein, one could also call this the Achilles heel of the spectroscopy a wrong choice of the model (the spin Hamiltonian) will not lead to an accurate description of nature represented by the paramagnetic biomolecule. 

It was quickly recognized that a nonrelativistic formulation of MCD led to predictions that were qualitatively wrong for some paramagnetic metal halides (18-20). The introduction of spin-orbit coupling created addition contributions to the C term parameters and produced predictions in qualitative agreement with experiment. It was not until some time later that general formulations of the new terms caused by spin-orbit coupling appeared (21-23). The spin-orbit-induced C terms have played an important role in modem applications of MCD spectroscopy as they have proven to be extremely useful in the study of metalloen-zymes (15,24-26). 

The paramagnetic properties of the lanthanides have been exploited in NMR spectroscopy for many years [16,17], The paramagnetic lanthanide induced shift is generally considered as the sum of the contact and pseudocontact terms (Equation 1) [16-18]... 

The experimental methods of Chapter 11, which contain the word "resonance" (e.g., "nuclear magnetic resonance" "electron paramagnetic resonance, etc.), refer to an allowed absorption or emission process (just as in optical spectroscopy), which is measured in a circuit electrically timed to the frequency for the quantum-mechanical transition. Of course, absorption or emission of light by an atom or molecule also occurs only if the light energy matches the energy level difference nevertheless, by tradition the term "resonance" is not used in that case. 

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