Factors that Influence Proton Shifts

The progressive replacement of hydrogen with chlorine on a methane molecule moves the chemical shift to higher frequency (downfield) because of chlorine s ability to remove electron density from the remaining protons bO.23 for CH4,3.05 for CH Cl.5.30 for 

CH2CI2, and 121 for CHCI3. The trend for a series of methyl resonances often can be explained in the same fashion by the inductive or polar effect. The chemical shifts for the series CH3X for which X may be F, HO, H2N, H, Me3Si, and Li, respectively, are 84.26, 3.38, 2.47, 0.23,0.0 (TMS, the standard), and —0.4 (this value is considerably dependent on the solvent the minus sign indicates a lower frequency than TMS), following the electronegativity of the atom attached to CH3. 

Electron density is influenced by resonance (mesomerism), as well as by inductive effects, as seen in unsaturated molecules such as alkenes and aromatics. The donation of electrons through resonance by a methoxy group increases the electron density at the p position of a vinyl ether (3-1) and at the para position of anisole (C6H5OCH3). Thus, the 

Hybridization of the carbon to which a proton is attached also influences electron density. As the proportion of s character increases from sp to sp to sp orbitals, bonding electrons move closer to carbon and away from the protons, which then become deshielded. For this reason, methane and ethane resonate at 8 0.23 and 0.86, respectively, but ethene resonates at 8 5.28. Ethyne (acetylene) is an exception in this regard, as we shall see. Hybridization contributes to shifts in strained molecules, such as cyclobutane (8 1.98) and cubane (8 4.00), for which hybridization is intermediate between sp and sp.  

Although the protons of benzene reside in the deshielded portion of the cone, molecules have been constructed to explore the full range of the effect. The methylene protons of methano[10]annulene (3-3) are constrained to positions above the aromatic IOtt electron 

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Section 3-1 Factors that Influence Proton Shifts 63... 

Because of the proton s chemical importance and its favorable characteristics for NMR detection, the overwhelming bulk of experimental investigation and correlation of chemical shifts has been centered on the proton. However, the situation regarding the determination of proton shielding anisotropies has been unsatisfactory in many respects. Similarly, the field of ab initio theoretical calculations of proton shielding tensors has, until very recently, enjoyed little success. Both these factors are related to the fact that the proton chemical shifts are comparatively small and hence influenced strongly by secondary factors such as neighboring-atom electron distribution and medium effects. 

Apart from structural factors annular tautomerism is strongly influenced by the aggregate state of the compound. Gas-phase and solid state studies of tautomerism have received much attention recently because of the development of methods such as X-ray crystallography, CPMAS NMR, microwave spectroscopy, etc. Low temperature H NMR is the simplest and most straightforward method to determine Kx it only requires that proton transfer be slow enough to observe separate signals for both tautomers, and that the equilibrium is not shifted too much towards one of the tautomers (about 5% of the minor tautomers seems the limit). More detailed information on tautomerism of key azole systems is given below. 

It is clear that there are other factors at work which influence the chemical shifts of different types of protons. 

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