The Gas-Phase Reaction Between Hydroxide Ion and Methyl Formate: A Theoretical Analysis of the Energy Surface and Product Distribution

Abstract

The potential energy surface for the prototype solvent-free ester hydrolysis reaction: OH- +HCOOCH3 --> products has been characterized by high level ab initio calculations of MP4/6-311 + G(2df,2p)//MP2/6-31 + G(d) quality. These calculations reveal that the approach of an OH- ion leads to the formation of two distinct ion-molecule complexes: 1) the MS1 species with the hydroxide ion hydrogen bonded to the methyl group of the ester, and 2) the MS4 moiety resulting from proton abstraction of the formyl hydrogen by the hydroxide ion and formation of a three-body complex of water, methoxide ion and carbon monoxide. The first complex reacts to generate formate anion and methanol products through the well known B(AC)2 and S(N)2 mechanisms. RRKM calculations predict that these pathways will occur with a relative contribution of 85% and 15% at 298.15 K, in excellent agreement with experimentally measured values of 87% and 13%, respectively. The second complex reacts by loss of carbon monoxide to yield the water-methoxide complex through a single minimum potential surface and is the preferred pathway in the gas-phase. This water-methoxide adduct can further dissociate if the reactants have excess energy. These results provide clear evidence that the preferred pathways for ester hydrolysis in solution are dictated by solvation of the hydroxide ion.

A Theoretical Analysis of the Free-Energy Profile of the Different Pathways in the Alkaline Hydrolysis of Methyl Formate In Aqueous Solution

“…Unlike the analogous gas-phase reaction of OH À with HCOOCH 3 previously characterized [11,12] solvation has two major effects: a) it stabilizes the hydroxide ion dramatically compared with the tetrahedral intermediate; this leads to a sizable activation free energy for the reaction; b) whereas the hydrolysis reaction in the gas phase is initiated by formation of an ion-dipole S N 2 type complex, [48] [HO À ¥¥¥ H 3 COOCH], which either rearranges to the tetrahedral intermediate or proceeds to products by an S N 2 mechanism, [12] the transition state for this mechanism is poorly solvated when compared with the B AC 2 pathway and solvation is responsible for making the S N 2 reaction highly unfavorable.…”

“…[9±12] Furthermore, these energy surfaces are particularly appropriate for analyzing gas-phase reactions within the realms of microcanonical transition-state theory, and application of simple statistical rate theories in the case of methyl formate has been shown to yield excellent agreement with the observed product distribution. [12,13] Over the years, many important and elucidative experimental studies have contributed to our understanding of the mechanistic features of ester hydrolysis in solution. [14±20] Some of the classical mechanistic studies with labeled 18 O reagents established that the hydroxide ion reacts preferentially with esters of carboxylic acids by initial attack at the carbonyl center leading to the formation of a tetrahedral intermediate.…”

“…As a follow up to our investigation of the gas-phase reaction between the hydroxide ion and methyl formate, [12] the present report describes a full analysis of the free-energy profile of the possible pathways for the reaction in aqueous solution outlined in Scheme 1. Our analysis is based on a clustercontinuum method to calculate the free energy of solvation of ions, a hybrid approach recently proposed by us and tested for several organic ions.…”

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).…”

Effect of higher order solvation and temperature on SN2 and E2 reactivity

“…The fundamental differences between gas-phase and solutionphase reactions raise interesting questions about the transition between the gas phase and solution [1]. These differences include extreme variation in both reaction rate constants and mechanisms.…”