Chemical shift electron density, effect

As is typical of all inductive effects, this type of deshielding decreases rapidly with increasing distance from the electronegative atom. Thus, the methyl group of ethyl chloride resonates at 8 = 1.33 ppm. In general, factors influencing electron density in the proximity of the proton are reflected in the chemical shift. Electron deficiency is associated with deshielding and therefore results in downfield shifts from TMS. 

The HT yield linearly correlated (Tang et al. 1971) with proton nuclear magnetic resonance (NMR) chemical shift, indicating the importance of electronic density effect in the reaction. 

Making allowance for those effects gives a good correlation between the chemical shifts and the it- and/or tr-electron density of the carbon atom bearing the proton (133, 236,237). 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Relativistic quantum mechanics yields the same type of expressions for the isomer shift as the classical approach described earlier. Relativistic effects have to be considered for the calculation of the electron density. The corresponding contributions to i/ (0)p may amount to about 30% for iron, but much more for heavier atoms. In Appendix D, a few examples of correction factors for nonrelativistically calculated charge densities are collected. Even the nonrelativistically calculated p(0) values accurately follow the chemical variations and provide a reliable tool for the prediction of Mossbauer properties [16]. 

The electron density i/ (0)p at the nucleus primarily originates from the ability of s-electrons to penetrate the nucleus. The core-shell Is and 2s electrons make by far the major contributions. Valence orbitals of p-, d-, or/-character, in contrast, have nodes at r = 0 and cannot contribute to iA(0)p except for minor relativistic contributions of p-electrons. Nevertheless, the isomer shift is found to depend on various chemical parameters, of which the oxidation state as given by the number of valence electrons in p-, or d-, or /-orbitals of the Mossbauer atom is most important. In general, the effect is explained by the contraction of inner 5-orbitals due to shielding of the nuclear potential by the electron charge in the valence shell. In addition to this indirect effect, a direct contribution to the isomer shift arises from valence 5-orbitals due to their participation in the formation of molecular orbitals (MOs). It will be shown in Chap. 5 that the latter issue plays a decisive role. In the following section, an overview of experimental observations will be presented. 

This effect allows one to monitor the perturbation of the tt-c lection system by interaction of the electrophilic phosphorus atom with a Lewis base. Following the same rationale, the still larger chemical shifts of neutral 1,3,2-diazaphospholes and 1,3,2-diazaphospholide anions are considered to reflect predominantly a reduction in n-n transition energy due to destabilization of the n(P) orbital with an increasing number of lone-pairs on the NPN-moiety rather than differences in the charge densities or n-electron distribution in the heterocyclic ring [16]. 

The acceptor number, AN, of a solvent is a measure of the power of the solvent to accept a pair of electrons [18], Experimental evaluation of AN involves observing the frequency changes induced by a solvent on the 31P NMR spectrum when triethylphosphine oxide, Et3P=0, is dissolved in the solvent. Donation of an electron pair from the oxygen atom of Et3P=0, as shown in Scheme 1.2, reduces the electron density around the phosphorus, causing a deshielding effect which leads to an increase in chemical shift. Hexane (AN = 0) and SbCls (AN = 100) were used as fixed points to define this scale. 

Further doubt about the validity of the original Grant-Cheney model was expressed by Seidman and Maciel (185), whose INDO calculations of proximity effects in hydrocarbons revealed that there is no simple correlation between carbon chemical shifts and calculated electron-density increases caused by steric C-H bond polarization they report the conformational relation of interacting bonds and groups to be at least equally important, if not more so (185). 

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