Internal field gradient

Fig. 25. Free-induction decay (FID) after a 90° pulse at t=0 and a series of Carr/Purcell/ Meiboom/Gill (CPMG) spin echoes in linear polyethylene (M, =92,300) at 150 °C. The 180° pulses of the CPMG sequence are indicated by vertical lines. The homogeneity of the external magnetic flux density is demonstrated by an FID of water at 30 °C. The data demonstrate the existence of strong internal field gradients arising from voids [152]...

Terms up to order 1/c are normally sufficient for explaining experimental data. There is one exception, however, namely the interaction of the nuclear quadrupole moment with the electric field gradient, which is of order 1/c. Although nuclei often are modelled as point charges in quantum chemistry, they do in fact have a finite size. The internal structure of the nucleus leads to a quadrupole moment for nuclei with spin larger than 1/2 (the dipole and octopole moments vanish by symmetry). As discussed in section 10.1.1, this leads to an interaction term which is the product of the quadrupole moment with the field gradient (F = VF) created by the electron distribution. 

Pempointner, M., Schwerdtfeger, P. and Hess, B.A. (2000) Accurate electric field gradients for the coinage metal chlorides using the PCNQM method. International Journal of Quantum Chemistry, 76, 371-384. 

Marathe, V.R., Trautwein, A. Calculation of charge density, electric field gradient and internal magnetic field at the nuclear site using molecular orbital cluster theory. In Thosar,... 

Consider a porous medium with magnetic susceptibility difference Ax between the confining solid and the permeating fluid (Figure 3.7.1). Magnetic field gradients will develop in the pore space. The spatial distribution of this internal magnetic... 

Experimentally, an elegant property of the DDIF experiment is its simplicity. As the structural information is encoded by the internal field provided by the static uniform field and the material itself, the pulse sequence requires no applied field gradients and thus suffers from none of their complications, such as eddy currents, mechanical ringing, miscalibrations or non linearity. 

However, the general point is that cell internal structure like that of a cloud is in constant flowing motion controlled by field gradients, chemical or electrical, which we see realised in the transient cytoskeleton. What complex relationship this form has to DNA sequences is difficult to know. It is obvious, as discussed in Section 3.10, that a separation of some of the activities in compartments would be advantageous. 

Because the interactions measured in Mossbauer experiments are products of atomic and nuclear factors, experiments on iodine isotopes have yielded values of the change of nuclear radius between the ground state and the excited state, AR/R, quadrupole moment values Q, and magnetic moment values, fi, as well as electric field gradients and internal magnetic fields. 

All the methods used in this study are response methods. They deserihe the response of an ohservahle sueh as an eleetrie dipole moment /I or quadrupole moment to an external or internal perturhation, e.g., an eleetrie field or field gradient. Response funetions originated in various diseiplines in physies. In statistieal physies, they were used as time-eorrelation functions in the form of Green s functions [44,45]. Linderherg and Ohrn first showed the usefulness of this idea for quantum chemistry [46]. Since then response functions have been derived for many types of electronic wavefunctions. Four of these methods are employed here. 

Maruthe,V.R. Trautwein, A. (1983) Calculation of charge density, electric field gradient and internal magnetic field using molecular orbital cluster theory. In Thosar, B.V. (ed.) Advances in Mossbauer spectroscopy. Elsevier, Amsterdam, 398-449 Matijevic, E. Cimas S. (1987) Formation of uniform colloidal iron(lll) oxides in ethylene... 

To be anisochronous, (1) groups may not be related by symmetry, taking into consideration internal motions which are rapid on the NMR time scale, and (2) there must be sufficient field gradient so that the difference is observable. 

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