Nj(13c, O1H) Coupling Constants of Intramolecularly Hydrogen-Bonded Compounds

nJ(13C,O1H) carbon–hydrogen couplings were measured for a broad series of intramolecularly hydrogen-bonded compounds, some of which display tautomerism. A plot of Jobs(C-3,OH)+Jobs(C-1,OH) vs. δOH showed reasonable correlation both for compounds displaying tautomerism and for those with localized hydrogen bonds. Ketones and aldehydes fall on one line and esters on another line corresponding to a lower sum. The 4Jobs(C-1,OH) coupling depends on orbital overlap between the hydrogen-bonded hydrogen and the carbonyl oxygen. This coupling can therefore also be useful for monitoring twisting of the carbonyl group out of the plane of the hydrogen bond. An interesting finding is that for aromatic compounds 3Jobs(C-2,OH)cis is larger than that for an olefin both having the same 1H OH chemical shift. A plot of Jobs(C-3,OH) vs. Jobs(C-1,OH) is very useful for monitoring tautomerism, and Jobs(C-1′,OH) and Jobs(C-1″,OH) are both well suited for estimating the mole fractions of tautomeric systems. © 1998 John Wiley & Sons, Ltd.