Gas‐phase acylium ion transfer reactions mediated by a proton shuttle mechanism

Abstract

ABSTRACT:The mechanism and the energy profile of the gas-phase reaction that mimics esterification under acidic conditions have been investigated at different levels of theory. These reactions are known to proceed with rate constants close to the collision limit in the gas-phase and questions have been raised as to whether the typical addition-elimination mechanism via a tetrahedral intermediate can explain the ease of these processes. Because these reactions are common to many organic and biochemical processes it is important to understand the intrinsic reactivity of these systems. Our calculations at different levels of theory reveal that a stepwise mechanism via a tetrahedral species is characterized by energy barriers that are inconsistent with the experimental results. For the thermoneutral exchange between protonated acetic acid and water and the exothermic reaction of protonated acetic acid and methanol our calculations show that these reactions proceed initially by a proton shuttle between the carbonyl oxygen and the hydroxy oxygen of acetic acid mediated by water, or methanol, followed by displacement at the acylium ion center. These findings suggest

Spatial Distributions and Interstellar Reaction Processes

“…It does have a stable entrance channel complex, however, and the vibrational mode with an imaginary frequency at the transition state is essentially a single hydrogen atom motion (reduced mass 1.09 u), meaning that if the complex were collisionally or radiatively stabilized, − protonated methyl formate could be formed via tunneling. , (2) This reaction also requires a large abundance of protonated formic acid, but H 3 + , which plays a central role in interstellar ion–molecule chemistry, − cannot protonate formic acid directly, as this reaction instead results in molecular dissociation. , Instead, protonation of formic acid must occur by another ion, such as HCO + or H 3 O + , either of which has been shown in the laboratory to efficiently form this ion. − (3) Proton transfer between the reactants is exothermic and may be more facile; two studies found that this proton transfer pathway is the dominant reaction channel. , (4) It has also been suggested that the gas-phase esterification reaction does not proceed through the traditional Fischer mechanism, with a tetrahedral intermediate. A recent study suggests that a closely related reaction, that of protonated acetic acid and methanol, may instead proceed through a proton-shuttling mechanism and might not have a reaction barrier. Due to the complexity of this pathway, the authors were unable to determine the complete reaction path; additional calculations are needed to determine whether acid-catalyzed Fischer esterification is a viable alternative mechanism to produce interstellar methyl formate.…”

“…42,43 (4) It has also been suggested that the gasphase esterification reaction does not proceed through the traditional Fischer mechanism, with a tetrahedral intermediate. A recent study 44 suggests that a closely related reaction, that of protonated acetic acid and methanol, may instead proceed through a proton-shuttling mechanism and might not have a reaction barrier. Due to the complexity of this pathway, the authors were unable to determine the complete reaction path; additional calculations are needed to determine whether acidcatalyzed Fischer esterification is a viable alternative mechanism to produce interstellar methyl formate.…”

Probing Elusive Cations: Infrared Spectroscopy of Protonated Acetic Acid

“…The protonated monomer of AA has previously been studied using ab initio calculations and mass spectrometry. − However, despite it being a rather fundamental organic cation, no optical spectra have been reported for it or for the monomer of other simple protonated carboxylic acids, (RCOOH)­H + . Inokuchi and Nishi have measured infrared (IR) photodissociation spectra in the OH stretching region for protonated formic acid cluster ions, (HCOOH) n H + , and their AA analogues, for n ≥ 2 .…”