Carbon-proton coupling constants solvent effects

Intermolecular solvent-solute interactions influence the charge distribution on a carbohydrate molecule. Subtle electronic changes that occur as a result of these interactions are responsible for the solvent dependence of carbon -proton coupling constants. The general aspects of solvent effects on NMR parameters have been reviewed,78-79 and consequently, only a very brief outline of the theoretical model within FPT INDO SCF MO formalism is considered here. 

One-bond coupling constants JCH may suffer from slight solvent effects. Table 3.4 shows this behavior for chloroform, whose carbon-proton coupling increases with the polarity of the medium when measured in different solvents, being 208 Hz in cyclohexane and 215 Hz in pyridine [92]. This is attributed to association between chloroform and solvents susceptible to hydrogen bonding. 

The ortho proton-proton coupling constants in benzofuroxan show the expected alternation in magnitude, following the bond orders between the carbon atoms.90,98,99 Substituent effects on coupling constants have been correlated in a series ofbenzo-fused heterocycles, including benzofuroxan.109 The proton resonance spectrum of benzofuroxan in a nematic solvent has been published (somewhat obscurely).110... 

The proton and carbon chemical shifts of twenty-one and twenty different anthocyanins are presented in Table FI. 4.4 and Table FI. 4.5, respectively. These anthocyanins are chosen to illustrate the chemical shifts of the majority of anthocyanin building blocks reported. The linkage positions of the various anthocyanin building blocks may be conspicuous through shift comparison. However, be aware of shift effects caused by variation in solvent, pigment concentration and temperature. Table FI.4.6 contains typical H- H coupling constants of the most common anthocyanidins. 

Scheme 3.18 provides some pertinent proton and carbon chemical shift and coupling constant data for trihalomethanes, including what data are available for fluorodihalomethanes. There are potentially significant solvent effects on proton chemical shifts of all trihalomethanes,... 

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