Coupling constant aromatic rings

Assignments based on 2-D correlations can often be confirmed by information found in the 1-D spectra. For instance, if two protons neighbor each other as evidenced in the COSY spectrum, their proton resonances should have the same coupling constant. Aromatic ring protons often show an additional hyperfine splitting, which helps to distinguish ring protons from other electron-rich proton resonances. Labile protons found in N-H and... 

Proton chemical shifts and spin coupling constants for ring CH of fully aromatic neutral azoles are recorded in Tables 3-6. Vicinal CH—CH coupling constants are small where they have been measured (in rather few cases) they are found to be 1-2 Hz. 

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. 

First the five protons (integral) of the //NMR spectrum (Sfj = 7.50 - 7.94) in the chemical shift range appropriate for aromatics indicate a monosubstituted benzene ring with typical coupling constants 8.0 Hz for ortho protons, 1.5 Hz for meta protons.). The chemical shift values especially for the protons which are positioned ortho to the substituent Sn = 7.94) reflect a -M effect. Using the CH COLOC plot it can be established from the correlation signal hclS = 66.AI7.94 that it is a benzoyl group A. 

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). 

Multinuclear NMR data have been recorded for molsidomine 12 and its metabolite SIN-1 13, confirming that both are closed ring structures and that the positive charge is accommodated at the N3 position <1996CHE1358>. The c C4-C5 coupling constants for selected 1,2,3-oxadiazoles lie between 69.4 and 89 Hz, but correlation of these values with measures of bond order and aromaticity is difficult due to substituent effects <2000MRC617, 2002JST269>. 

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. 

Similarly in condensed polynuclear aromatic compounds and heterocyclic compounds, the magnitude of the coupling constants between protons in the aromatic rings reflects the relative position of the coupled protons. 

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