Thermodynamics of the Hydroxyl Radical Addition to Isoprene

Abstract

Oxidation of isoprene by the hydroxyl radical leads to tropospheric ozone formation. Consequently, a more complete understanding of this reaction could lead to better models of regional air quality, a better understanding of aerosol formation, and a better understanding of reaction kinetics and dynamics. The most common first step in the oxidation of isoprene is the formation of an adduct, with the hydroxyl radical adding to one of four unsaturated carbon atoms in isoprene. In this paper we discuss how the initial conformation of isoprene, s-trans and s-gauche, influences the pathway to adduct formation. We explore the formation of pre-reactive complexes at low and high temperatures, which are often invoked to explain the negative temperature dependence of this reaction’s kinetics. We show that at higher temperatures the free energy surface indicates that a pre-reactive complex is unlikely, while at low temperatures the complex exists on two reaction pathways. The theoretical results show that at low temperatures all eight pathways possess negative reaction barriers, and reaction energies that range from −36.7 to −23.0 kcal·mol−1. At temperatures in the lower atmosphere, all eight pathways possess positive reaction barriers that range from 3.8 to 6.0 kcal·mol−1, and reaction energies that range from −28.8 to−14.4kcal·mol−1.

A quantum chemical study on the free radical scavenging activity of tyrosol and hydroxytyrosol

“…In all the cases, the deviations from the correct value (hS 2 i = 0.75) were lower than 6 and 0.5% before and after annihilation of the first spin contaminant. It has been established that for differences within 10% error the obtained results can be trusted [85]. Therefore, the spin contamination can be considered negligible for all the radicals species studied in this work and their energy values are reliable.…”

Effects of spin contamination on estimating bond dissociation energies of polycyclic aromatic hydrocarbons

“…The expectation is often used as a measure of the spin contamination of unrestricted open‐shell calculations. Some studies have assumed that if the predicted value deviates by less than 10% from its theoretical value, then the effects of spin contamination are negligible . However, in the present case, assuming a linear scaling for Figure b, a 10% error (i.e., ) would lead to an error in the computed BDE value of about 11 kJ/mol for the UMP2.…”

“…Some studies have assumed that if the predicted value deviates by less than 10% from its theoretical value, then the effects of spin contamination are negligible. [26][27][28] However, in the present case, assuming a linear scaling for Figure 2b, a 10% error (i.e., hS 2 i % 0:825) would lead to an error in the computed BDE value of about 11 kJ/mol for the UMP2. This result suggest that one should be very careful in using spin-unrestricted UMP2 energies values even if hS 2 i is close to the theoretical value.…”

Quantitative infrared absorption cross sections of isoprene for atmospheric measurements

“…While the consensus in the literature supports this conclusion, there has been some dispute regarding the scis conformer (Fig. 2b), its fractional abundance at room temperature, and whether the conformer is planar or gauche (Traetteberg et al, 1984;Bock et al, 1987;Cias et al, 2007;Panchenko and De Mare, 2008;Squillacote and Liang, 2005;Allodi et al, 2008). Compton Squillacote and Liang (2005) using the G3B3 method.…”