Effective quadrupolar centres

The superscript EQC in the entries of Table 2.7 related to BE indicates that the origin-dependent quantities to which they are associated refer to the so-called effective quadrupolar centre [14], R(EQC co), a particular frequency-dependent vector in the coordinate space defined with respect to a given choice of origin of the coordinates, or . 

After the 2D spectra have been obtained and the axes labelled by a method of choice, the centre of gravity along Fi and F2 can be used to derive the chemical shift and second-order quadrupolar effect, according to Eqs. (57) and (58) or specifically by Eqs. (59) or (60). However, more information can be obtained by summing slices parallel to F2 of specific 2D peaks. These result in quadrupolar MAS lineshapes for each site separately. From the second-order quadrupolar MAS lineshapes, x fd 17 can be unambiguously determined. Lineshape simulations can be performed, for example,... 

In their simulations, L6vesque et used a standard Leimard-Jones interaction potential between hydrogen molecules, and included the effect of quadrupolar interactions by adding a Coulomb interaction term in which each hydrogen molecule is represented by three effective charges q (q = 0.4829e at the position of the protons and q = -e at the centre of mass of the molecule). The adsorbate-adsorbent interaction was modeled with a standard Lennard-Jones potential. In order to partially account for quantum effects at 77 K, a semi-classical approach based on the Feynman-Hibbs effective potential was used ... 

RF pulse and spinning speed effects/ (iv) When the second-order quadrupolar interaction is much smaller than the chemical shift anisotropy, the quadrupolar parameters can be estimated by monitoring the frequency shift of the centre band as a function of the Bo field and the chemical shift anisotropy can be determined by Herzfeld—Berger sideband analysis/ We shall discuss the applicabihty of these methods to Co systems in turn. 

---