C70 Fullerene Cage as a Novel Catalyst for Efficient Proton Transfer Reactions between Small Molecules: A Theoretical study

摘要

When acids are supplied with an excess electron (or placed in an Ar or the more polarizable Nsub2/sub matrix) in the presence of species such as NHsub3/sub, the formation of ion-pairs is a likely outcome. Using density functional theory and first-principles calculations, however, we show that, without supplying an external electron or an electric field, or introducing photo-excitation and -ionization, a single molecule of HCl or HBr in the presence of a single molecule of water inside a Csub70/sub fullerene cage is susceptible to cleavage of the σ-bond of the Br?nsted-Lowry acid into Xsup-/sup and Hsup+/sup ions, with concomitant transfer of the proton along the reaction coordinate. This leads to the formation of an Xsup-/sup···sup+/supHOHsub2/sub (X?=?Cl, Br) conjugate acid-base ion-pair, similar to the structure in water of a Zundel ion. This process is unlikely to occur in other fullerene derivatives in the presence of Hsub2/subO without significantly affecting the geometry of the carbon cage, suggesting that the interior of Csub70/sub is an ideal catalytic platform for proton transfer reactions and the design of related novel materials. By contrast, when a single molecule of HF is reacted with a single molecule of Hsub2/subO inside the Csub70/sub cage, partial proton transfers from HF to Hsub2/subO is an immediate consequence, as recently observed experimentally. The geometrical, energetic, electron density, orbital, optoelectronic and vibrational characteristics supporting these observations are presented. In contrast with the views that have been advanced in several recent studies, we show that the encaged species experiences significant non-covalent interaction with the interior of the cage. We also show that the inability of current experiments to detect many infrared active vibrational bands of the endo species in these systems is likely to be a consequence of the substantial electrostatic screening effect of the cage.