We present here a cell model for evaluating Gibbs energy barriers corresponding to bimolecular reactions (or processes of larger molecularity) in which a loss of translational degrees of freedom takes place along the reaction coordinate. With this model, we have studied the Walden inversion processes: Xa- + H3CXb --> XaCH3 + Xb- (X = F, Cl, Br, and I). In these processes, our model yields an increase of about 2.3-3.4 kcal/mol in Gibbs energy in solution corresponding to the loss of the translational degrees of freedom when passing from separate reactants to the TS in good agreement with experimental data. The corresponding value in the gas phase is about 6.7-7.1 kcal/mol. When the difference between these two figures is used to correct the results obtained by the standard UAHF implementation of the continuum model, the theoretical results are brought significantly closer to the experimental ones. This seems to indicate that for these reactions the parametrization used does not adequately introduce the increase in Gibbs energy corresponding to the constriction of the translational motion of the species along the reaction coordinate when passing from the gas phase to solution. Therefore, we believe that continuum models could perform much better if we released the parametrization process from the task of taking into account the constriction in translation motion in solution, which could be more adequately evaluated using the cell model proposed here, thus allowing it to focus on better reproducing all the remaining solvation effects.

We present a molecular dynamics study of the interactions between two molecules of naphthalene present at air-water versus air-ice interfaces. In agreement with the inference from our previous experimental work [Kahan, T. F.; Donaldson, D. J. J. Phys. Chem. A 2007, 111, 1277], the results suggest that self-association of the molecules is more likely to take place on the ice surface than on the water surface. A shorter average distance between the two naphthalene molecules, in conjunction with a stronger interaction energy and free energy of association, point to a stronger tendency to self-associate on ice than on water. The distinct behavior at the two interfaces appears be due to more favorable interactions between naphthalene molecules on liquid water surfaces than on ice surfaces.

The present study aims to analyse the effect of motivational variables and previous academic achievement on students’ future choice of chemistry once this subject becomes optional in the educational system. Toward this goal, the translation and adaptation of the Science Motivation Questionnaire II (SMQII) has been undertaken in our investigation to measure students’ motivation towards physics and chemistry. The sample comprised 1060 secondary school Spanish students divided into two groups: 695 students who chose the subject when it became optional for the first time and 365 who decided to leave it. Factor analysis confirmed the original structure of latent variables in our sample, providing validity for this adaptation to a new language and context. A segmentation analysis confirmed that career motivation was the best predictor of students’ retention in physics and chemistry before previous academic achievement and the rest of the motivational variables. Although significant gender differences were found in self-efficacy and self-determination, these seem not to be relevant in students’ choice.

We present the results of computations on the dissociation of HCl and HNO(3) at the air-water interface. Molecular dynamics simulations of the acid molecule and 200 water molecules were propagated for several nanoseconds, and the resulting structures were used as input to QM/MM geometry optimization runs. Approximately 20-30 water molecules were included along with the acid in the QM portion of the calculation, which was carried out at the B3LYP/6-31+G(d) level. Whereas dissociation to ions is always spontaneous in the bulk, we find that acid molecules confined to the water surface dissociate only with the participation of two additional water molecules, forming a "critical cluster" about the solute. Thus acid dissociation may occur in the near-surface zone at a dynamic air-water interface, in agreement with our earlier experimental conclusions [Clifford et al., Phys. Chem. Chem. Phys.. 2007, 9, 1362].