EFFECT OF METHYL SUBSTITUTION ON THE INTRAMOLECULAR EXCHANGE OF HYDROGEN-BONDED PROTONS

Of the two possible monomethyl naphthazarins only the 2-methyl derivat ive has been encountered. Efforts to obtain the 7-methyl derivative al ways lead to the 2-methyl tautomer. Previous studies suggested a near symmetric double minimum potential for the intramolecular proton excha nge with a high barrier and 0.9 kcal mol(-1) difference between the 2- and 7-tautomers. If there were fast exchange, the two tautomers would have been detected. Now, with a much larger and better basis set and better algorithms for finding stationary points, a new study finds a b arrier of 29 kcal mol(-1). Two more (intermediate) minima were found, corresponding to structures in which the protons are on different ring s. However, the energy difference between the absolute minima and the intermediates is so large that the latter do not contribute to the exc hange process. In monomethylnaphthazarin, the difference between the t wo lowest minima was found to be 1.09 to 1.79 kcal mol(-1). The rotati on of the methyl group changes the energy difference between the minim a, but the minimum of the 2-methyl isomer is always below that of the 7-methyl isomer, thus maintaining the asymmetry of the double minimum potential, without accidental degeneracy, in contrast to dimethylnapht hazarins and 6-methyl-9-hydroxyphenalen-1-one, as already reported. Th e exchange process is described by two quantities: the fraction of the proton emerging into the other well (2.7 x 10(-11)) and the frequency with which it leaks (6 x 10(12)). This will make the 7-methyl derivat ive undetectable by NMR. The product of the two factors suggests that the equivalent of a full proton is exchanging every six milliseconds. (C) 1997 Elsevier Science B.V.