correction is only partially cancelled by the higher order corrections determined in the coupled cluster procedure. In spite of such a cancellation, the cumulative electron correlation contribution is large and amounts to almost 40% of the tautomer relative stability. This seems to suggest that one cannot obtain a correct relative energy of tautomers of 2( 1 H)-pyridinethione at the lower orders of the many-body perturbation theory because of cancellations of lower order terms with higher order terms. We conclude that the large contribution to the relative stability of the tautomers arising from the low-order (e.g., from second order) perturbation theory may be considerably reduced by higher order terms. As a consequence, the relative stability estimated with the S C F + scaled ZPE + MBPT(2) + higher order coorelation methods would fall closer to the experimentally derived values than the MBPT(2) result, as is probably true for diazinethiones. However, the inclusion of the higher order correlation terms does not seem to be enough to compensate for the systematic discrepancy between the theoretical and the experimental estimates of the relative stability of the tautomers. ConclusionsThe gas-phase and matrix isolation experiments showed that 4(3H)-pyrimidinethione exists, under conditions where intermolecular interactions are minimized, as a mixture of thiol and thione tautomers. The relative concentration of tautomers, in the gas phase ([thiol]/[thione] -3 at 483 K) and in matrix experiments ([thiol]/[thione] -5 at 350 K), depends on the temperature. In both cases, the experimentally derived value of the free energy difference was similar (AF N 4.5 kJ/mol).

The effect of the iodine interphase transport originated from physical processes, such as pressure decrease, vigorous stirring, and/or an inert gas bubbling, on the dynamics of the oscillating Bray-Liebhafsky reaction is studied. Some new experiments were made at 60 °C that confirmed the possibility of suppressing oscillations by means of the iodine vapor removal from the reaction solution. It has been proved that the iodine vaporization is governed by the first-order kinetics with respect to the concentration of aqueous iodine, and a critical rate constant for iodine escape causing inhibition of the oscillatory mode was determined. The experiments were confronted with the models proposed by Kolar-Anic ´and Schmitz with co-workers. Both the models studied reflect the observed phenomenon. A simulation based on the latest model shows very good semiquantitative agreement with experiment, needing only a minor modification of the rate constants suggested by Kolar-Anic ´et al.

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