Geometry proton transfer

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

Both these methods require equilibrium constants for the microscopic rate determining step, and a detailed mechanism for the reaction. The approaches can be illustrated by base and acid-catalyzed carbonyl hydration. For the base-catalyzed process, the most general mechanism is written as general base catalysis by hydroxide in the case of a relatively unreactive carbonyl compound, the proton transfer is probably complete at the transition state so that the reaction is in effect a simple addition of hydroxide. By MMT this is treated as a two-dimensional reaction proton transfer and C-0 bond formation, and requires two intrinsic barriers, for proton transfer and for C-0 bond formation. By NBT this is a three-dimensional reaction proton transfer, C-0 bond formation, and geometry change at carbon, and all three are taken as having no barrier. 

Another elegant example of the thermal generation and subsequent intramolecular cycloaddition of an o-QM can be found in Snider s biomimetic synthesis of the tetracyclic core of bisabosquals.2 Treatment of the starting material with acid causes the MOM ethers to cleave from the phenol core (Fig. 4.3). Under thermal conditions, a proton transfer ensues from one of the phenols to its neighboring benzylic alcohol residue. Upon expulsion of water, an o-QM forms. The E or Z geometry of the o-QM intermediate and its propensity toward interception by formaldehyde, water, or itself, again prove inconsequential as the outcome is decided by the relative thermodynamic stabilities among accessible products. 

Curioni et al.148 studied the protonation of 1,3-dioxane and 1,3,5-trioxane by means of CP molecular dynamics similations. The dynamics of both molecules was continued for few ps following protonation. The simulation provided a detailed picture the evolution of both the geometry and the electronic structure, which helped to rationalize some experimental observations. CP molecular dynamics simulations were applied by Tuckerman et al.149,150 to study the dynamics of hydronium (H30+) and hydroxyl (OH-) ions in liquid water. These ions are involved in charge transfer processes in liquid water H20 H+. .. OH2 - H20. .. H+-OH2, and HOH. . . OH- -> HO-. . . HOH. For the solvatetd H30+ ion, a picture consistent with experiment emerged from the simulation. The simulation showed that the HsO+ ion forms a complex with water molecules, the structure of which oscillates between the ones of H502 and I L/ij clusters as a result of frequent proton transfers. During a consid-... 

The proton transfer processes described above induce interesting effects on the geometry of these metal complexes upon protonation (see also Section II). If it is assumed that the equatorial cyano ligands form a reference plane and are stationary for any of these distorted octahedral cyano oxo complexes, the protonation/deprotonation process as illustrated in Scheme 3 is responsible for the oxygen exchange at the oxo sites. This process effectively induces a dynamic oscillation of the metal center along the O-M-O axis at a rate defined by kmv, illustrated in Fig. 15. This rate of inversion is determined by the rate at which the proton is transferred via the bulk water from the one... 

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