CH2Cl2 + OH− Reaction in Aqueous Solution: A Combined Quantum Mechanical and Molecular Mechanics Study

Atomic-Level Mechanism, Solvent Effect, and Potential of the Mean Force of the F- + CH3CH2Cl SN2 Reaction in Aqueous Solution

We investigate the bimolecular nucleophilic substitution (S_N2) reaction of F^- with CH₃CH₂Cl 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.

Theoretical investigation of the SN2 mechanism of X- [X = SH, PH2] + CH3Y [Y = F, Cl, Br, I] reactions in water

We investigated the S_N2 Walden-inversion mechanism of X^- (X = SH, PH₂) + CH₃Y (Y = F, Cl, Br, I) reactions in water using multi-level quantum mechanics (ML-QM) and molecular mechanics (MM) methods. The potentials of the mean force were mapped using not only the density functional theory (DFT)/MM method but also a high-level, accurate CCSD(T)/MM method using the aug-cc-pVTZ basis set. In particular, for the PH₂^- + CH₃I reaction, although the backside attack Walden-inversion mechanics were not observed in the gas phase, we found that this mechanism takes place in water. The atomic-level dynamics of the concerted S_N2 mechanism and the stationary points along the reaction paths were characterized. For these reactions in water, their Walden-inversion barriers are higher than their corresponding ones in the gas phase, indicating that the aqueous solution hinders their reactivity. For the reactions with the same nucleophile X^- in water, the reaction barrier heights with different leaving groups are in the order of F > Cl > Br > I. For the same leaving group Y with different nucleophiles SH^- and PH₂^-, the reaction barrier with SH^- is greater than that of PH₂^- due to the former having higher electronegativity than the latter.

Investigation of the CH3Cl + CN(-) reaction in water: Multilevel quantum mechanics/molecular mechanics study

The CH3Cl + CN(-) reaction in water was studied using a multilevel quantum mechanics/molecular mechanics (MM) method with the multilevels, electrostatic potential, density functional theory (DFT) and coupled-cluster single double triple (CCSD(T)), for the solute region. The detailed, back-side attack SN2 reaction mechanism was mapped along the reaction pathway. The potentials of mean force were calculated under both the DFT and CCSD(T) levels for the reaction region. The CCSD(T)/MM level of theory presents a free energy activation barrier height at 20.3 kcal/mol, which agrees very well with the experiment value at 21.6 kcal/mol. The results show that the aqueous solution has a dominant role in shaping the potential of mean force. The solvation effect and the polarization effect together increase the activation barrier height by ∼11.4 kcal/mol: the solvation effect plays a major role by providing about 75% of the contribution, while polarization effect only contributes 25% to the activation barrier height. Our calculated potential of mean force under the CCSD(T)/MM also has a good agreement with the one estimated using data from previous gas-phase studies.

A multilayered-representation, quantum mechanical/molecular mechanics study of the CH3Cl + F− reaction in aqueous solution: the reaction mechanism, solvent effects and potential of mean force

A multi-layered representation, hybrid quantum mechanical and molecular mechanics method study of the CH₃Cl + F⁻ → CH₃F + Cl⁻ reaction in water.

Solvent effects and potential of mean force: a multilayered-representation quantum mechanical/molecular mechanics study of the CH3Br + CN− reaction in aqueous solution

The potential of mean force for the CH₃Br + CN⁻ reaction was obtained at the CCSD(T)/MM level of theory using a multilayered-representation quantum mechanical/molecular mechanics approach, as well as the reactant, transition state and product complexes along the reaction pathway in aqueous solution.

A multilayered-representation quantum mechanical/molecular mechanics study of the S(N)2 reaction of CH3Br + OH(-) in aqueous solution

The bimolecular nucleophilic substitution (S(N)2) reaction of CH(3)Br and OH(-) in aqueous solution was investigated using a multilayered-representation quantum mechanical and molecular mechanics methodology. Reactant complex, transition state, and product complex are identified and characterized in aqueous solution. The potentials of mean force are computed under both the density function theory and coupled-cluster single double (triple) (CCSD(T)) levels of theory for the reaction region. The results show that the aqueous environment has a significant impact on the reaction process. The solvation effect and the polarization effect combined raise the activation barrier height by ~16.2 kcal/mol and the solvation effect is the dominant contribution to the potential of mean force. The CCSD(T)/MM representation presents a free energy activation barrier height of 22.8 kcal/mol and the rate constant at 298 K of 3.7 × 10(-25) cm(3) molecule(-1) s(-1) which agree very well with the experiment values at 23.0 kcal/mol and 2.6 × 10(-25) cm(3) molecule(-1) s(-1), respectively.

Hybrid quantum mechanical and molecular mechanics study of the S(N)2 Reaction of CCl4 + OH- in aqueous solution: the potential of mean force, reaction energetics, and rate constants

The bimolecular nucleophilic substitution reaction of CCl(4) and OH(-) in aqueous solution was investigated on the basis of a combined quantum mechanical and molecular mechanics method. A multilayered representation approach is employed to achieve high accuracy results at the CCSD(T) level of theory. The potential of mean force calculations at the DFT level and CCSD(T) level of theory yield reaction barrier heights of 22.7 and 27.9 kcal/mol, respectively. Both the solvation effects and the solvent-induced polarization effect have significant contributions to the reaction energetics, for example, the solvation effect raises the saddle point by 10.6 kcal/mol. The calculated rate constant coefficient is 8.6 × 10(-28) cm(3) molecule(-1) s(-1) at the standard state condition, which is about 17 orders magnitude smaller than that in the gas phase. Among the four chloromethanes (CH(3)Cl, CH(2)Cl(2), CHCl(3), and CCl(4)), CCl(4) has the lowest free energy activation barrier for the reaction with OH(-) in aqueous solution, confirming the trend that substitution of Cl by H in chloromethanes diminishes the reactivity.