Typical Coupling Constants

Some carbon-carbon couplings measured for 13C-7 labeled monosubstituted benzenes [133] and collected in Table 4.59 summarize the trends outlined in Section 3.2.4.2 One-bond carbon-carbon couplings increase with the s character of the hybrid orbitals originating from, and with the electronegativity of the substituent attached to C-l iJcc values follow a similiar pattern. 

Carbon-fluorine coupling constants of fluorobenzene and selected substituted derivatives are collected in Table 4.60 [402], Benzenoid JCF couplings are about 245+15 Hz. They depend on both type and position of the substituents Electron withdrawing groups increase while electron releasing ones decrease one-bond carbon-fluorine coupling in fluorobenzene, particularly when they are ortho and para to fluorine. These observations can be explained by cannonical resonance formulae which take ( + )- and (-)-M effects into account. The data of fluoroanilines (( + )-M) and fluorobenzaldehydes (( — )-M) provide typical examples (Table 4.60). 

13C NMR measurements of 6-dimethylamino-6-methylfulvene, synthesized from 13C-7 labeled phenyldiazomethane via fulvenallene and ethynylcyclopentadiene, supported the proposed mechanism of the ring contraction of phenylcarbene to fulvenallene. The uniform distribution of 13C in the product fulvenallene may be explained in terms of a preequilibrium Before ring contraction, phenylcarbene, a bicyclic intermediate and cycloheptatrienylidene interconvert rapidly [406]  

Instead of 13C labeling, 12C-enriched precursors with a 13C abundance of less than 1.1 % can be used for mechanistic studies. In this case, the spectra are more easily analyzed as no homonuclear carbon-carbon coupling occurs. An illustrative example is the base-catalyzed dehydrohalogenation of 7,7-dichloronorcarene to benzocyclopropene [407]. For this reaction, two pathways A and B have been proposed, mechanism B involving a skeletal rearrangement  

In order to distinguish between the two alternatives, [7-12C]-7,7-dichloronorcarene was prepared by reaction of 1,4-cyclohexadiene with [12C]-dichlorocarbene originating from [12C]-deuteriochloroform  

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Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3,4 and V3.5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The V values correlate quite well with rr-bond orders calculated by MO methods (74CJC833). 

The //NMR spectrum contains five signals with integral levels in the ratios 1 1 1 1 3 four lie in the shift range appropriate for aromaties or heteroaromaties and the fifth is evidently a methyl group. The large shift values (up to Sh = 9.18, aromaties) and typical coupling constants (8 and 5 Hz) indicate a pyridine ring, which accounts for four out of the total five double-bond equivalents. 

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. 

TABLE 5.3 Typical Coupling Constants Not Involving Hydrogen... 

Figure 22 illustrates how relatively simple global quality factors can be used as filters in the search for optimum solutions in the parameter space that defines multiple-pulse sequences. Suppose for typical coupling constants = 10 Hz a multiple-pulse sequence with a constant rf amplitude = 10 kHz is desired that effects efficient Hartmann-Hahn transfer in the offset range of +4 kHz. Here, the simple two-dimensional parameter... 

In theory, valuable information about the Pb(II) coordination environment can be determined from the Pb X coupling constant (J). Typically, coupling constants provide a measure of bond strength and sterics however, insufficient reports have appeared in the literature to date to observe trends for Pb(ll)... 

Coupling is a through-bond phenomenon, as we know from the couplings in cis and trans alkenes, where trans alkenes have much larger coupling constants as their orbitals are perfectly parallel. Another case of perfectly parallel orbitals occurs with trans-diaxial protons in cyclohexanes. Typical coupling constants are 10-12 Hz for trans-diaxial protons, but much smaller (2-5 Hz) for axial/equatorial and equatorial/equatorial protons. 

Figure 14.36 Typical coupling constants for simple proton systems.

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