Respective contributions of polar vs enthalpy effects in the addition/fragmentation of mercaptobenzoxazole-derived thiyl radicals and analogues to double bonds.

The formation and the reactivity of three selected sulfur-centered radicals formed from mercaptobenzoxazole, mercaptobenzimidazole, and mercaptobenzothiazole toward four double bonds (methyl acrylate, acrylonitrile, vinyl ether, and vinyl acetate) are investigated. The reversibility of the addition/fragmentation reaction in these widely used photoinitiating systems of radical polymerization was studied, for the first time, through the measurement of the corresponding rate constants by time-resolved laser spectroscopy. The combination of these results with quantum mechanical calculations clearly evidences that, contrary to previous studies on other aryl thiyl radicals, the addition rate constants (ka) are governed here by the polar effects associated with the very high electrophilic character of these radicals. However, interestingly, the back-fragmentation reaction (k-a) is mainly influenced by the enthalpy effects as supported by the relationship between the rate constants and the addition reaction enthalpy DeltaHR. The addition and fragmentation rate constants calculated from the transition state theory (TST) are in satisfactory agreement with the experimental ones. Therefore, molecular orbital (MO) calculations offered new opportunities for a better understanding of the sulfur-centered radical reactivity.

A photodissociation reaction: Experimental and computational study of 2-hydroxy-2,2-dimethylacetophenone.

The photophysical parameters controlling the cleavage process of 2-hydroxy-2,2-dimethylacetophenone (HDMA) were investigated in detail. Time-resolved picosecond absorption experiments show that the formation of the triplet state occurs within 20 ps after excitation and decays within hundreds of picoseconds depending on the solvent polarity. Molecular modeling reveals that three stable conformations exist in the ground state, the most stable one exhibiting an intramolecular hydrogen bond that modifies the electronic properties of the molecule. This explains quite well the steady-state absorption properties. The conformation of the most stable triplet state is twisted by 180 degrees with respect to the ground state. Computation of the potential energy surface along the molecular coordinate for the dissociation reaction evidences an electronic state crossing yielding a final sigmasigma* state, in perfect agreement with the state correlation diagram. Optimization of the transition state allows the calculation of the activation energy and the use of the transition-state theory leads to an estimate of 100 ps for the cleavage process in the gas phase. Single-point energy calculations using a solvent model predict an increase of the dissociation rate constant with the increase of the solvent polarity, in good agreement with the value deduced from kinetic measurements.

Illusory inferences: a novel class of erroneous deductions.

The mental model theory postulates that reasoners build models of the situations described in premises, and that these models normally make explicit only what is true. The theory has an unexpected consequence: it predicts the occurrence of inferences that are compelling but invalid. They should arise from reasoners failing to take into account what is false. Three experiments corroborated the systematic occurrence of these illusory inferences, and eliminated a number of alternative explanations for them. Their results illuminate the controversy among various current theories of reasoning.

Pragmatic schemas and the selection task: to reason or not to reason.

Cheng and Holyoak (1985) have proposed that people possess classes of linguistically based schemas that have an internal structure that is determined by pragmatic considerations. They found that when permission schemas ("If you want to do P, then you must do Q") are used in the selection task, the success rate is much superior to what is usually observed. According to Cheng and Holyoak, this is due to the fact that the permission schema is defined by a set of production rules that give the same answers to problems of conditional inference as those of formal logic. In order to test this hypothesis specifically, 160 university students were given one of two tests. The first contained two sets of inferential reasoning tasks, one using a permission schema, the second using a relation of multiple causality. The second test employed the same two conditional relations, but in an appropriate context. The results indicated that subjects did better on the reasoning task with the schema of multiple causality when presented in context, but, as predicted, their performance was much worse on the inferential reasoning task with the permission schema, which generated a higher proportion of logically incorrect responses. These results suggest that contrary to what has been affirmed, permission schemas might not have a logical structure that is equivalent to conditional logic. A second experiment examined selection task performance using the same two relations in context. Performance on the permission schema was superior to that found with the relation of multiple causality. This confirmed previous results indicating that permission schemas do improve selection task performance, but also suggests that this effect is not related to understanding of conditional reasoning.