Coupling constants aromatics

The NMR spectra of thiophene, and their 2-methyl and 2,5-dimethyl derivatives, have been obtained and completely assigned (Table 38). The initial assignment of chemical shift values for the 2-methyl series was based on additive substituent relationships and C-H coupling constants. Aromatic character in these compounds is inferred from the similarity of the corresponding chemical shift values with those of the ordinary benzenoid aromatics. Failure of the chemical shift data to correlate in every detail with estimates of the Jt-electron charge densities argues for the importance of cr-bond effects <1965JA5333>. 

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... 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

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... 

Table 6 One-bond Coupling Constants (Hz) in the Simple Monocyclic Aromatic Azines (cf. 159 Hz for...

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. 

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

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. 

Table 2.5. Typical HH coupling constants (Hz) of aromatic and heteroaromatic compounds...

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. 

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

The spectra were recorded at 250 MHz in CDCI3, using tetramethylsilane as internal standard (TMS = 0). The multiplicities have been added by the reviewer and are based on the coupling constants indicated and examination of the visually reproduced spectra. The C-6 and C-7 protons and the aromatic protons resonating between 2.4 and 1.8 ppm, and 7.9 and 7.2 ppm, respectively, were not differentiated. 

A third useful example is provided by aromatic residues, where the coupling constant between ortho protons is large (xx-xx Hz), while that between meta protons is much smaller (1-3 Hz). The coupling between para protons is often of the same magnitude as the Hnewidth. 

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). 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

The limits of the original BIRD-FIMBC sequence and the excellent /ci 1 filter quality of the double tuned G-BIRD element is exemplified in the spectra obtained with the molecule 1,3-butadiynyl (ferf-butyl) diphenyl-silane (Figure 6). This molecule provides a stringent test, since it contains all together aliphatic, aromatic and acetylenic protons, and the corresponding ch coupling constant values vary from 125 to 260 Flz. 

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