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

• Published in: Chemistry : a European journal Vol. 7; no. 1; pp. 169 - 175
• Main Authors: Pliego, Jr, Josefredo R., Riveros, José M.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag GmbH 05-01-2001; WILEY‐VCH Verlag GmbH
• Subjects: ab initio calculations; gas-phase reactions; hydrolysis
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/1521-3765(20010105)7:1<169::AID-CHEM169>3.0.CO;2-E

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 ion 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 BAC2 and SN2 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.
The archetypal gas‐phase reaction of OH− with HCOOCH3 is calculated to go through a complex that can undergo hydrolysis by either a BAC2 or SN2 mechanism, see for example one intermediate. RRKM calculations reveal that the branching ratio is dependent on the internal energy of the system in excellent agreement with experiment.