Quantum and quasiclassical dynamics calculations have been performed for the reaction of HBr with CH3. The accurate ab-initio-based potential energy surface function developed earlier for this reaction displays a potential...

The cross sections of the F + C2H6 → HF + C2H5 reaction obtained using quantum dynamics calculations agree with the quasi-classical trajectory results and the full-dimensional quantum rates are in good agreement with the experimental and theoretical results.

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We investigate the bimolecular nucleophilic substitution (S N 2) reaction of F − with CH 3 CH 2 Cl in aqueous solution using combined multilevel quantum mechanism (ML-QM) theories with molecular mechanics (MM). The synchronized, atomic-level structural and charge evolutions are analyzed along the reaction path. The potential mean force along the reaction path in water is calculated at high-accuracy CCSD(T)/aug-cc-pVTZ/MM level of theory with a free energy barrier of 16.8 kcal/mol and a free energy of reaction of −9.7 kcal/mol. The water solvent hinders the reactivity by raising its reaction barrier by 15.1 kcal/ mol, of which 13.6 kcal/mol comes from solvent energy contribution and 1.5 kcal/ mol comes from the polarization effect. This indicates that the water solvent plays an essential role on this reaction in aqueous solution. We also predict the potential mean force profile based on the gas-phase reaction path and the solvation free energies of the stationary points; the comparison between our calculated result at CCSD(T)/ MM level shows an excellent agreement with the predicted one with the free energy barrier at 16.2 kcal/mol and the free energy of reaction at −8.3 kcal/mol.

Enzymes play a fundamental role in many biological processes. We present a theoretical approach to investigate the catalytic power of the haloalkane dehalogenase reaction with 1,2-dichloroethane. By removing the three main active-site residues one by one from haloalkane dehalogenase, we found two reactive descriptors: one descriptor is the distance difference between the breaking bond and the forming bond, and the other is the charge difference between the transition state and the reactant complex. Both descriptors scale linearly with the reactive barriers, with the three-residue case having the smallest barrier and the zero-residue case having the largest. The results demonstrate that, as the number of residues increases, the catalytic power increases. The predicted free energy barriers using the two descriptors of this reaction in water are 23.1 and 24.2 kcal/mol, both larger than the ones with any residues, indicating that the water solvent hinders the reactivity. Both predicted barrier heights agree well with the calculated one at 25.2 kcal/mol using a quantum mechanics and molecular dynamics approach, and also agree well with the experimental result at 26.0 kcal/mol. This study shows that reactive descriptors can also be used to describe and predict the catalytic performance for enzyme catalysis.