Chemical shifts heteroaromatic rings

The N chemical shifts of ring nitrogens in heteroaromatic compounds usually fall in the low field half of the spectrum. For benzo(pyridazines) in DMSO the following shifts, which are typical for azo-type groups, have been observed (values relative to a nitromethane external standard) pyridazine (-1-20), 3-methylpyridazine (N-1, -1-19 N-2, -1-15), phthalazine (-flO), cinnoline (N-1, -f 44 N-2, -1-41), and benzo[c]cinnoline (-1-60). The resonances of the unsymmetrical compounds were assigned on the basis of the two-bond N H coupling of about 10-11 Hz observed for N-1 of... 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Apart from the A-methyl group, three double-bond equivalents and three multiplets remain in the chemical shift range appropriate for electron rich heteroaromatics, Sh = 6.2 to 6.9. A-Methyl-pyrrole is such a compound. Since in the multiplets at Sh = 6.25 and 6.80 the Jhh coupling of 4.0 Hz is appropriate for pyrrole protons in the 3- and 4-positions, the pyrrole ring is deduced to be substituted in the 2-position. 

Chemical shift data for a number of simple pseudoazulenes are listed in Table IV. The signals for the protons of the pseudoazulenic skeleton are in the region of other heteroaromatic compounds. Also the NMR spectrum of 49a shows a peak for the pseudoazulene ring protons at i = 4.26 ppm (<5 = 5.74 ppm)135 and that of70a at r = 3.25 ppm (<5 = 6.75 ppm).175 Many authors argue against substantial aromatic character for these pseudoazulene... 

Table A3.4 Chemical shifts (TMS) of protons attached to aromatic and heteroaromatic rings...

TABLE D.5 Chemical Shifts of Protons on Heteroaromatic Rings... 

The 1,2- and 1,3-dichalcogenolylium ions exhibit H NMR chemical shifts that are consistent with a diamagnetic ring current in these molecules (Section 4.35.3.5.1). These compounds are formally 6tt heteroaromatics and would be expected to display some aromatic character. 

As was true for the hydrogens attached to heteroaromatic rings and polycyclic aromatic hydrocarbons (Section 6.5), the carbons of such rings also appear in the aromatic region. Some examples are given below,1 and these can be used as base values when calculating chemical shifts for substituted derivatives of these compounds ... 

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