Local diamagnetic shielding term

The values of chemical shift given in Figure 26.7 and in Table 26.1 can be easily understood in terms of two factors local diamagnetic shielding and anisotropy. These two factors are discussed in Sections 26.7 and 26.8. 

Here, is a positive term resulting from diamagnetic shielding by electrons localized on atom A, Op is a term (usually negative) which corrects for any deviation from spherical symmetry of the electrons localized on atom A, represents the effect on atom A (shielding or deshielding) of circulation of electrons localized on other atoms, and 0 ° represents the effect on atom A (shield-... 

The local diamagnetic term i.e., shielding restricted only to the electrons of the N atom, and the difference Aafoc with respect to N2 were calculated semiempirically for small N-containing molecules including N2F4 using Pople s model [15] and the atom-dipole model... 

Nakano and Nakatsuji et have calculated dia- and paramagnetic terms for [MnX(CO)s] (X = Hal, H, CN, Me), [Mn(CO)e]+, and [Mn04]". A qualitative agreement is found between the calculated paramagnetic diamagnetic terms are practically con-... 

The atom-plus-ligand local-term approximation has been justified a posteriori for 30) and 57) shielding, since certain expected relationships of chemical shifts, such as additivity of substituent effects, were found to be absent for the observed shifts, but were fulfilled for i.e., after correction for the atom-plus-ligand diamagnetic term. Some examples are given in the following sections. 

The diamagnetic term local electronic circulations around the nucleus induced by the applied field B0. According to the Lenz rule, the resultant intramolecular field opposes B0, so that the nucleus will be shielded. As known from the Lamb formula, eq. 3.3 [9],... 

The proton chemical shifts are dominated, in diamagnetic systems, by the electron population pis of the Is orbital. The magnetic shielding constant for a nucleus is mainly expressed in terms of the so-called local... 

The term (7 i stands for non-local contributions, i.e. for contributions which do not stem from the immediate coordination sphere of vanadium. This term, mainly reflecting the influence of counfer-ions and other matrix effects, is usually negligibly small in NMR. The local term the diamagnetic contribution, which governs variations in proton shielding, also accounts for substantial shielding variations in lithium NMR. In other nuclei, including is dominated by the core electrons, i.e. is a sizable... 

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