Aromatic systems coupling constants

Single-bond 13C- H coupling constants for six-membered heteroaromatic compounds lie in the approximate range 150-220 Hz, the magnitude varying with substituent electronegativity. Data for 

70MI20100 730MR(5)251 80CCC2766 760MR(8)21 80CCC2766 87PS(29)187 

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The normal pattern of coupling constants for aromatic six-membered rings is found in the heterocyclic aza systems, except that the ortho coupling to a proton a to a heterocyclic nitrogen is reduced from 7-8 Hz to 4.5-6 Hz. The J2.3 of pyrylium salts is still lower... 

The coupling constants of ortho ( Jhh = 7 Hz), meta Jhh =1-5 Hz) and para protons CJhh I Hz) in benzene and naphthalene ring systems are especially useful in structure elucidation (Table 2.5). With naphthalene and other condensed (hetero-) aromatics, a knowledge of zig zag coupling = 0.8 Hz) is helpful in deducing substitution patterns. 

Finally, a brief word about aldehydes. They are included at the end of this group for convenience only and should be spotted easily. Aldehydes bound to aromatic rings give sharp singlets at 10.2-9.9 ppm, whilst in alkyl systems, they give sharp signals at 10.0-9.7 ppm, which couple to adjacent alkyl protons with a relatively small coupling constants (2-4 Hz). 

One of the most commonly studied systems involves the adsorption of polynuclear aromatic compounds on amorphous or certain crystalline silica-alumina catalysts. The aromatic compounds such as anthracene, perylene, and naphthalene are characterized by low ionization potentials, and upon adsorption they form paramagnetic species which are generally attributed to the appropriate cation radical (69, 70). An analysis of the well-resolved spectrum of perylene on silica-alumina shows that the proton hyperfine coupling constants are shifted by about four percent from the corresponding values obtained when the radical cation is prepared in H2SO4 (71). The linewidth and symmetry require that the motion is appreciable and that the correlation times are comparable to those found in solution. 

In aromatic systems, the coupling constant between protons attached to an aromatic ring is characteristic of the relative position of the coupled protons i.e. whether they are ortho, meta or para. 

The H-NMR spectra of all 1.1-carbo or l.l-hetero-X -phosphorins show a doublet between 5 = 7,5 and 8 = 8,5 ppm with/p c-c-H = 30 to 50 Hz which is due to the protons at C-3 and C-5 of the phosphorin ring. The low-field signals usually appear somewhat lower than those of X -phosphorins. The position of these signals suggests the existence of a ring current induced by the aromatic X -phosphorin system. However, the vinyl protons of X -phospha-cyclohexadiene -2,5 or -2,4 derivatives also absorb at relative low fields (p. 135). Much more characteristic are the P—H coupling constants, which are about six times as large in X -phosphorins as in X -phosphorins (/p c c-H = 5—7 Hz). Indeed, they provide an excellent help in the identification of X -phosphorins. 

In general there is a good correlation between bond distances in fused aromatic compounds and bond orders. Another experimental quantity that correlates well with the bond order of a given bond in an aromatic system is the nmr coupling constant for coupling between the hydrogens on the two carbons of the bond.74... 

Let us now look at a more complex spin system, in which we have two different coupling constants to the same proton. In the trisubstituted aromatic 3-amino-4-methoxybenzoic acid shown in Figure 4.18, we have an AMX spin system and this leads to a 1,3- (or meta) coupled doublet for at 6 7.26, a 1,2- (or ortho) coupled doublet for at 5 6.85 and a... 

The extensive delocalization and aromatic character of pyridones, pyrones, etc. are shown by their chemical shift and coupling constant values (Table 8). By contrast, pyrans and thiins show chemical shifts characteristic of alkenic systems (Table 9). For these and for rings containing only a single endocyclic bond (Table 10), H NMR spectroscopy offers a most useful tool for structure determination. 

However, the same sequence of downfield shifts, namely NR < S < O < Se, is observed. Thus, at least the heterocyclic moiety appears to be aromatic. The ratio of the vicinal coupling constants 75 6 / 745 (0.70-0.74) is significantly less than for compounds of the Kekule series (ca. 0.9), much higher than observed for a polyenic TT-system (cyclohexa-1,3-diene displays a value of 0.52), and close to the... 

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