Reaction dynamics simulations of the identity SN2 reaction H2O + HOOH2+→ H2OOH++ H2O. Requirements for reaction and competition with proton transfer

Abstract

Despite the fact that the transition structure of the gas phase S(N)2 reaction H(2)O + HOOH(2)(+)--> HOOH(2)(+)+ H(2)O is well below the reactants in potential energy, the reaction has not yet been observed by experiment. Variational transition state RRKM theory reveals a strong preference for the competing proton transfer reaction H(2)O + HOOH(2)(+)--> H(3)O(+)+ HOOH due to entropy factors. Born-Oppenheimer reaction dynamics simulations confirm these results. However, by increasing the collision energy to around 7.5 eV the probability for nucleophilic substitution increases relative to proton transfer. These observations are explained by the presence of the key common intermediate HOO(H)[dot dot dot]H-OH(2)(+) which leads to effective proton transfer, but can be avoided with increasing collision energy. However, the S(N)2 probability remains below 0.2 since successful passage through the TS requires optimum initial orientation of the reactants, excitation of the relative translational motion and good phase correlation between the O-O vibration and the motion of the incoming water.

Assessment of density-functionals for describing the X−+ CH3ONO2gas-phase reactions with X = F, OH, CH2CN

“…The k(E i , J ) was computed with the Beyer-Swinehart algorithm 60 to determine the number of states implemented in the SuperRRKM program. [62][63][64][65][66] The calculated values of E* as well as the geometries, energies and vibrational frequencies are available in the ESI. These thermal contributions were calculated at 298 K and rotational effects were not considered ( J = 0).…”

“…This procedure to compute E i has been successfully applied to estimate the product distributions of several gas-phase reactions. [62][63][64][65][66] The calculated values of E* as well as the geometries, energies and vibrational frequencies are available in the ESI. †…”

Gas-Phase Nucleophilic and Elimination Reactions in Simple Alkyl Nitrates

“…The rate constants for the two elimination channels can then be estimated by RRKM theory if statistical behavior is assumed for these reactions. This type of approach has been used in the past to determine relative efficiencies of gas-phase S N 2 reactions, to describe the outcome of ionic reactions that proceed through a double minimum well surface energy or neutral processes through long-lived intermediates, and to estimate the branching ratios of gas-phase ionic and neutral reactions …”

Understanding E2 versus SN2 Competition under Acidic and Basic Conditions

“…tential energy surfaces (PES) and the structure of species and transition states (TS). [19][20][21][22][23][24][25][26][27] Activation-strain analyses [28] of the anionic substitution reactions between halides and halomethanes [29] have shown that the S N 2 barrier decreases as the highest-occupied molecular orbital (HOMO) of the nucleophile becomes a better electron donor, that is, as the basicity of the nucleophile becomes stronger, because of a more stabilizing interaction between nucleophile and substrate. Likewise, a weaker bond between carbon and the leaving group (CÀLG) also leads to a lower S N 2 barrier because the energetic strain associated with such a weaker bond is less destabilizing.…”