Hydroxyalkoxy Radicals: Importance of Intramolecular Hydrogen Bonding on Chain Branching Reactions in the Combustion and Atmospheric Decomposition of Hydrocarbons

Abstract

During both the atmospheric oxidation and combustion of volatile organic compounds, sequential addition of oxygen can lead to compounds that contain multiple hydrogen-bonding sites. The presence of two or more of these sites on a hydrocarbon introduces the possibility of intramolecular H-bonding, which can have a stabilizing effect on the reactants, products, and transition states of subsequent reactions. The present work compares the absolute energies of two sets of conformations, those that contain intramolecular H-bonds and those that lack intramolecular H-bonds, for each reactant, product, and transition state species in the 1,2 through 1,7 H-migrations and Cα-Cβ, Cα-H, and Cα-OH-bond scission reactions in the n-hydroxyeth-1-oxy through n-hydroxyhex-1-oxy radicals, for n ranging from 1 to 6. The difference in energy between the two conformations represents the balance between the stabilizing effects of H-bonds and the steric cost of bringing the two H-bonding sites together. The effect of intramolecular H-bonding and the OH group is assessed by comparing the net intramolecular H-bond stabilization energies, the reaction enthalpies, and barrier heights of the n-hydroxyalkoxy radical reactions with the corresponding alkoxy radicals values. The results suggest that there is a complex dependence on the location of the two H-bonding groups, the location of the abstraction or bond scission, and the shape of the transition state that dictates the extent to which intramolecular H-bonding effects the relative importance of H-migration and bond scission reactions for each n-hydroxyalkoxy radical. These findings have important implications for future studies on hydrocarbons with multiple H-bonding sites.

High-Pressure Limit Rate Rules for α-H Isomerization of Hydroperoxyalkylperoxy Radicals

“…Previous studies reported that decreases in energy resulting from HB can range from 1.5-4 kcal 28,47 . The effect of this energy stabilization on the kinetics will be discussed in a separate study.…”

Photoelectron spectroscopy of the hydroxymethoxide anion, H2C(OH)O−

“…The consequences of the anion and the vibrationally averaged structure of neutral H 2 C(OH)O • both having C s symmetry leads to vibrational symmetry selection rules in the electronic spectra, resulting in vibrational modes 10 (in plane HCH rock), 7 (out of plane HCH rock), and 2 (HCH asymmetric stretch) not being active in the photoelectron spectrum (see Supplementary Material for mode description). Moreover, the selection rules of the torsional transitions are determined by the parity enforced by the symmetry of the potential, as emphasized by the double-headed arrows shown in Fig.…”

“…[1][2][3][4][5][6][7] Many experimental and theoretical studies have focused on the effects of adding various substituents onto the methoxy radical, the simplest alkoxy radical, looking for trends in tropospheric reaction kinetics, unimolecular processes, and overall reactivity. [1][2][3][4][5]8 The hydroxymethoxy radical (H 2 C(OH)O • ) is an oxygenated alkoxy radical, proposed to be formed from the UV photofragmentation of hydroxymethyl hydroperoxide (H 2 C(OH)OOH), a product from the tropospheric oxidation of alkenes.…”

A fluorescent paramagnetic Mn metal–organic framework based on semi-rigid pyrene tetracarboxylic acid: sensing of solvent polarity and explosive nitroaromatics

“…While fluorescent MOFs are being increasingly explored for applications such as chiral sensing (Wanderley et al, 2012;Hu et al, 2014), biosensing (Wu, Gong et al, 2012;Wu, Wang et al, 2012;Xu et al, 2012;Wei et al, 2013;Hu et al, 2014), sensing of explosives (Lan et al, 2009;Xu et al, 2011;Nagarkar et al, 2013;Pramanik et al, 2013;Gole et al, 2014;Hu et al, 2014;Zhang et al, 2014;Liu et al, 2015;Zhang et al, 2015), detection of volatile organic compounds (Hu et al, 2014;Zhang et al, 2014;Zhan et al, 2014) and ionic species (He et al, 2013;Tang et al, 2013;Hu et al, 2014), luminescent thermometers (D'Vries et al, 2013; Rao et al, 2013; Shustova et al, 2013; Hu et al, 2014) etc., we are excited about the opportunities provided by fluorescent MOFs based on paramagnetic metal ions for the detection of paramagnetic molecular free radicals. The latter are invaluable chemical intermediates that exist in different environments, such as those in combustion (Cramer & Campbell, 1949;Valavanidis et al, 2008;Davis & Francisco, 2014), in the atmosphere (Heard & Pilling, 2003;Crounse et al, 2013) and in biological systems (Valko et al, 2007;Alamed et al, 2009;Hong et al, 2010;Panasenko et al, 2013;Song et al, 2014).…”