Substitution patterns coupling constants

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. 

The most recent application of 1,1-ADEQUATE of which the author is aware is the early 2011 report of Schraml et al.69 The isomeric S-(2-pyrrole) cysteine S-oxide (25) and S-(3-pyrrole)cysteine S-oxide (26) both have AMX proton spin systems with comparable coupling constants that do not allow differentiation of the substitution of the pyrrole ring. The 13C resonances of the two molecules are likewise quite similar and are also not amenable to the unequivocal assignment of the substitution pattern. In contrast, the Vcc derived connectivity information from the 1,1-ADEQUATE spectrum provides an unequivocal assignment of the substitution pattern for the isomeric structures. 

Arnold s scale is derived for the action of a single substituent on the benzylic 7c-system. It cannot be used to estimate the influence of several substituents on the system under consideration. In this way it is, therefore, not possible to gain insight into the problem of captodative stabilization of a radical centre. The investigation of the spin-density distribution in benzylic radicals has been extended (Korth et al., 1987) to include multiple substitution patterns. Three types of benzylic radicals were considered a,p-disubsti-tuted a-methylbenzyl radicals [17], a-substituted p-methylbenzyl radicals [18] and a-substituted benzyl radicals [19]. In [17] and [18] the hyperfine coupling constants of the methyl hydrogens were used to determine the spin-density... 

One-bond carbon-carbon spin-spin coupling constants are included in a review by Krivdin and KalabinR". They are analyzed in terms of hybridization, substitution effects, lone pair effects and steric effects as well as respective applications to structural determination. The carbon-carbon spin-spin coupling constants between carbons that are separated by more than one bond were reviewed by Krivdin and DeUa and are discussed in terms of experimental techniques, the effects of hybridization, substituent effects, steric effects and respective additivity patterns. 

The H-NMR spectra of pavines can provide appreciable assistance in structural elucidation. The oxygenation pattern of a pavine may be deduced from a careful examination of the methine (H j) and methylene (H. f) proton absorptions (Ic). In the case of 2,3,8,9 substitution, the abc and def protons furnish two superimposable ABX patterns. A doublet integrating for two protons at the lower field end of the system at approximately 8 4.0 represents the bridgehead protons, Ha and Hj. At 60 MHz, it appears as if these protons are coupled to only one of the neighboring protons (J = 6 Hz) (18,20,25). Furthermore, the geminal hydrogens couple to each other with a coupling constant of 17 Hz (18,20). On... 

These values for the ring coupling constants vary only within these small ranges listed above and provide therefore a trustworthy tool for determining substitution patterns. This is perhaps best seen by consideration of a heterogeneous group of thiophene containing structures (116)—(125). 

Indirect evidence for the validity of this assumption is provided by the structure [rf(Si-Si)=2.229(l) A, 359.9°], and especially the 1/si,Si coupling constant of the disilaoxirane 81 derived from the unsymmetrically substituted disilene 2996. With a Jsi,Si value of 123 Hz, this coupling constant is appreciably larger than those observed for other disilanes with a similar substitution pattern (85 Hz) and approaches the value of 160 Hz for the disilene 29. 

The appearance of benzenoid signals in analyzable patterns, the coupling constants of benzenoid protons, the comparison of spectra with those of analogous compounds, the solvent and concentration dependence of the 7-proton resonance, spin decoupling experiments, and some additional factors which are summarized below, may lead to the proper assignment of the benzenoid signals, and thus to the substitution pattern. 

The calculated hyperfine coupling constants (B3LYP/6-31G //MP2/6-31G ) for the type B transition state and the distorted minimum clearly show that this species must be considered a type A structure. The hyperfine coupling pattern of the lowest-energy minimum [ai = —1.43 mT a = 1.98 mT) shows a trend similar to the experimental splittings of the fran.s-l,2-dimethylcyclopropane radical cation a, 2 = —1.19 mT a = 2.18 mT), whereas the pattern calculated for the transition state (fl2,3 = 0.55 mT) is incompatible with that model (Figure 18). The distorted structure type calculated for the methyl-substituted systems seems to prevail also under other conditions (see below). 

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