Aromatic compounds proton chemical shifts

The two exchangeable Primary Amide protons resonate at low field as either one or two very broad bands. The table of chemical shifts provided below indicates that the aliphatic Primary Amides resonate at slightly higher field than the aromatic compounds. The chemical shift(s) of these protons vary over a relatively wide range of values due to their sensitivity to the concentration of the sample solution, the solvent employed and the temperature at which the solution was examined, in addition to any possible hydrogen bonding effects and other structural considerations. 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds ... 

Proton nuclear magnetic resonance (NMR) chemical shifts of 1,2,3-thiadiazoles give another indication of the aromatic character of these compounds. Compiled in Table 4 are a number of examples of proton chemical shifts for ring-substituted 1,2,3-thiadiazoles. 

The general absorption pattern of quaternary pavines strongly resembles that of the tertiary analogs with the exception of the expected downfield shifts for each of the protons. In particular, the bridgehead protons will move downfield by about 1-1.5 ppm (32,35). N,N-Dimethyl protons will be observed as a singlet between 8 3.3 and 3.7 (29,32,35). The set of empirical rules deduced for aromatic proton chemical shifts in a tertiary system has been shown to apply also to the quaternary system (29). A listing of aromatic proton chemical shifts of some quaternary pavine bases has been presented as a reference for future studies on similar compounds (29). 

A ring current model has been shown to correlate proton and boron-11 chemical shifts in a number of pyramidal boron compounds including pentaborane(9) and its derivatives 180>. This model is analogous to the model used to explain proton chemical shifts in benzene and similar aromatic compounds. 

When investigating an unknown compound, the chemical shift (5, ppm) and pattern of signals, especially those isolated from other in the ID H-NMR spectra (ethylenic and/or aromatic protons, for instance), are very useful for preliminary structural information. Among them, peaks corresponding to the anomeric protons (i.e., those appearing in the region 4.5 to 5.5 ppm) are particularly relevant, because they provide... 

Correlations anticipated in various homonuclear ( H- H) and heteronuclear ( C- H) 2D NMR experiments are conceptualized in Eigure 5.1. A hypothetical model compound (the chemical shifts are not accurate and are for illustrative purposes only) with three aromatic protons and four side-chain protons on its three side-chain carbons is used to illustrate the information available from each experiment. A set of five experiments, in addition to the standard ID proton and carbon spectra, are useful for characterizing any model compound or lignin. The correlation spectroscopy (COSY) experiment correlates directly coupled protons (Figure 5.1a). 

The two following sections develop the theory of molecular diamagnetism and proton chemical shifts in aromatic systems thus setting the stage for the detailed review of the n.m.r. spectra of the currently known compounds within our purview. Particular emphasis will be placed on the qualitative conclusions (which may be drawn from the n. m. r. parameters) regarding aromaticity and anti-aromaticity. 

The relationship between ring proton chemical shifts and 7r-electron densities of aromatic molecules and ions is important. S.C.F. molecular orbital calculations have been used to evaluate ring currents and associated chemical shifts for pyridine and other heterocycles 276a. When corrections are made for polar solvent effects and ring current effects, a roughly linear relationship between chemical shift and electron density is obtained for pyridine and other heterocycles 276 With 4-substituted pyridines, a simple additivity relationship of substituent contribution to the proton shieldings in these compounds can be applied, as for substituted benzenesi277 Correlations of proton chemical shifts and 7r-electron densities have been made for a... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

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