Assessment of theoretical methods for the calculation of methyl cation affinities

The methyl cation affinity (MCA; 298 K) of a variety of neutral and anionic bases has been examined computationally with a wide variety of theoretical methods. These include high-level composite procedures such as W1, G3, G3B3, and G2, conventional ab initio methods such as CCSD(T) and MP2, as well as a selection of density functional theory (DFT) methods. Experimental results for a variety of small model systems are well reproduced with practically all these methods, and the performance of DFT based methods are far superior in comparison to their MP2 analogs for these small models. For larger model, systems including motifs frequently encountered in organocatalysts, the performance deteriorates somewhat for DFT methods, while it improves significantly for MP2, rendering the former methods unreliable for common organic bases. Thus, MP2 calculations performed in combination with basis sets such as 6-31+G(2d, p) or larger, appear to offer a practical and reliable approach to compute MCAs of organic bases.     

Ab initio MD simulations of a prototype of methyl chloride hydrolysis with explicit consideration of three water molecules: a comparison of MD trajectories with the IRC path

Ab initio molecular dynamics simulations at the Hartree-Fock/6-31G level of theory are performed on methyl chloride hydrolysis with explicit consideration of one solute and two solvent water molecules at a temperature of 298 K. The reaction involves the formation of a reactant complex and the energy surface to the transition state is found to be simple. Two types of trajectories toward the product are observed. In the first type, the system reaches an intermediate complex (complex-P1) region after two nearly concerted proton transfers involving the attacking water molecule and the solvent water molecules. These trajectories resemble the intrinsic reaction coordinate trajectory. The thermal motion of the atoms leads the system to another intermediate complex (complex-P2) region. A second type of trajectory is found in which the system reaches the complex-P2 region directly after the proton transfers. In both of these forward trajectories, back proton transfers lead the system to a final complex-F region which resembles protonated methanol. Received: 3 July 1998 / Accepted: 2 September 1998 / Published online: 15 February 1999     

Theoretical evaluation of the substrate-assisted catalysis mechanism for the hydrolysis of phosphate monoester dianions

Quantum chemistry methods coupled with a continuum solvation model have been applied to evaluate the substrate-assisted catalysis (SAC) mechanism recently proposed for the hydrolysis of phosphate monoester dianions. The SAC mechanism, in which a proton from the nucleophile is transferred to a nonbridging phosphoryl oxygen atom of the substrate prior to attack, has been proposed in opposition to the widely accepted mechanism of direct nucleophilic reaction. We have assessed the SAC proposal for the hydrolysis of three representative phosphate monoester dianions (2,4-dinitrophenyl phosphate, phenyl phosphate, and methyl phosphate) by considering the reactivity of the hydroxide ion toward the phosphorus center of the corresponding singly protonated monoesters. The reliability of the calculations was verified by comparing the calculated and the observed values of the activation free energies for the analogous S_N2(P) reactions of F⁻ with the monoanion of the monoester 2,4-dinitrophenyl phosphate and its diester analogue, methyl 2,4-dinitrophenyl phosphate. It was found that the orientation of the phosphate hydrogen atom has important implications with regard to the nature of the transition state. Hard nucleophiles such as OH⁻ and F⁻ can attack the phosphorus atom of a singly protonated phosphate monoester only if the phosphate hydrogen atom is oriented toward the leaving-group oxygen atom. As a result of this proton orientation, the SAC mechanism in solution is characterized by a small Brønsted coefficient value (β_lg=−0.25). This mechanism is unlikely to apply to aryl phosphates, but becomes a likely possibility for alkyl phosphate esters. If oxyanionic nucleophiles of pK_a<11 are involved, as in alkaline phosphatase, then the S_N2(P) reaction may proceed with the phosphate hydrogen atom oriented toward the nucleophile. In this situation, a large negative value of β_lg (−0.95) is predicted for the substrate-assisted catalysis mechanism.