Electric deflection of clusters of H-bonding molecules. Polarity as a means to study: charge transfer reactions (electron and proton) and hydrophobic effect

E. MARCECA

Waterloo

Seminario; Seminario semanal del Instituto; 2015

Chemistry Department, University Waterloo, Canada.

Effective polarizabilities of Na(sv)n=630 and cresol(sv)n=18 clusters (sv: H2O and NH3) were measured by electric deflection as a function of the particle size. Field-induced shift of the beam intensity profile without the occurrence of broadening revealed that the clusters behave as ?liquid-like? polar objects in the conditions of the experiment (cluster temperatures estimated in the range of 150 K). The average modulus of the cluster dipoles, 0, was determined using the Langevin-Debye theory giving structural and dynamical characteristics of the solvation environment.In Na(sv)n clusters most of the cluster polarity is attributed to the spontaneous promotion of the alkali atom valence electron to a diffuse state stabilized by the cluster solvent field, with the formation of (e, Na+) pairs. Much larger 0 values and a step size dependence are found when the solvent is ammonia, evidencing that while the (e, Na+) structure is rather compact in Na(H2O)n clusters and remains almost unchanged during the solvation process, in Na(NH3)n the unpaired electron abandons the proximity of the Na+ ion and gradually extends and occupies new solvent shells.The polarity of cresol(sv)n clusters evidences incomplete cancelation of individual molecular dipoles. The OH group of cresol takes part of the cluster H-bond network while the aromatic ring and the methyl group conform a hydrophobic moiety immiscible with the polar region. Aqueous p-cresol clusters exhibit larger 0 values than their ammonia homologues suggestingdeformation from their more stable cyclic H-bond ring motifs to transient chain-type structures. However, the polarity is found substantially lower when the solute molecule is o-cresol because in this case water ring motifs are favored by the proximity of the CH3 group (possible due to the mobility hindrance of the solvent molecules). Within the studied size range, the magnitude of cluster dipoles does not reveal the occurrence of proton transfer in the ground state.