Theoretical study of the ground‐state gas‐phase unimolecular decomposition channels of propynoic acid

Abstract

AbstractTheoretical studies have been carried out to elucidate the ground‐state unimolecular decomposition channels for propynoic acid (PA) in the gas phase. In this paper we present details of mechanisms for the decarboxylation and decarbonylation channels of PA. The transformations of PA result in the formation of carbon dioxide and acetylene from the decarboxylation channel while carbon monoxide and hydroxyacetylene, ultimately the ketene, are formed from the decarbonylation pathway. Equilibrium structures and the principal transition states (TSs) for each pathway have been identified and characterized. The Gaussian 98 suite of programs and MOLDEN were used for computation and visualization. All computations of equilibrium and TS structures relevant to the two competing unimolecular decomposition channels (decarboxylation and decarbonylation) were performed with second‐order Møller–Plesset (MP2) and Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP) levels using the 6‐31G(d,p), 6‐311++G (d, p) and Dunning's correlation‐consistent polarized valence basis set (aug‐cc‐pVDZ). The geometries were fully optimized and characterized as minima (0 imaginary frequencies) or first‐order saddle points (1 imaginary frequency) by harmonic vibrational analysis at the MP2/6‐311++G(d,p) and B3LYP/6‐311++G(d,p) levels. All the calculations indicate that the lowest energy decomposition pathway for PA is decarboxylation. Decarboxylation of PA occurs most easily through a three‐step mechanism: conversion of s‐cis‐HCCCO2H to s‐trans‐HCCCO2H, followed by a 1, 3‐hydrogen migration of hydroxyl hydrogen, resulting in a four‐center TS that decays easily to acetylene and carbon dioxide, with an activation barrier of 61.3–79.9 kcal/mol (average 65.67 kcal/mol). Decarbonylation, leading to hydroxyacetylene (and subsequently the ketene) and carbon monoxide, occurs easily via a direct (one‐step) three‐center TS. The direct (one‐step) elimination of carbon monoxide from propynoic acid has a calculated average activation barrier of 67.9–81.7 kcal/mol (average, 74.1 kcal/mol). The decarboxylation pathway is shown to occur with a maximum exothermicity of 28 kcal/mol. On the contrary, the decarbonylation of PA is predicted to occur with an overall endothermicity of ≈29.2 kcal/mol for formation of hydroxyacetylene and carbon monoxide and an average exothermicity of −7.69 kcal/mol for formation of the ketene and carbon monoxide. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004