Van Vleck term

The first term is seen to be positive, corresponding to a diamagnetic contribution to the susceptibility, y,, called the Langevin term. The second is seen to be negative, corresponding to a paramagnetic contribution Xp, called the Van Vleck term. We may obtain these terms directly by multiplying Eq. (5-23) by the electron... 

The final result for the Van Vleck term in the susceptibility obtained by Chadi et al. (1975) may be written... 

Normally the TIP contribution (the van Vleck term) is only a minor correction to the underlying diamagnetism (the Langevin term). In complex systems like metalloproteins the diamagnetic contribution may also dominate the temperature-dependent paramagnetism. 

It is assumed that the measured magnetic susceptibility has already been corrected for the underlying diamagnetism (usually using the set of Pascal constants) and also for the temperature-independent paramagnetic (van Vleck) term (its mononuclear estimate being multiplied by the number of magnetic centres). 

Shaltiel et al. (1964a) noticed that Sm produces a positive g-shift for the Gd resonance in Pdo.96Gdo.02Smo.02> opposite to what is expected from a simple model. They pointed to the fact that the temperature-independent van-Vleck part of the susceptibility should produce an additional positive shift. Malik and Vijayaraghavan (1975) confirmed these results and provided more detailed information. They found that the distance between the H5/2 ground state and the H7/2 first excited state of Sm " " is so small that the van-Vleck terms contribute appreciably and as a consequence the z-component of the lanthanide spin changes sign. 

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