Computational Study on the Existence of Organic Peroxy Radical-Water Complexes (RO₂·H₂O)

Abstract

The existence of a series of organic peroxy radical-water complexes [CH3O2.H2O (methyl peroxy); CH3CH2O2.H2O (ethyl peroxy); CH3C(O)O2.H2O (acetyl peroxy); CH3C(O)CH2O2.H2O (acetonyl peroxy); CH2(OH)O2.H2O (hydroxyl methyl peroxy); CH2(OH)CH2O2.H2O (2-hydroxy ethyl peroxy); CH2(F)O2.H2O (fluoro methyl peroxy); CH2(F)CH2O2.H2O (2-fluoro ethyl peroxy)] is evaluated using high level ab initio calculations. A wide range of binding energies is predicted for these complexes, in which the difference in binding energies can be explained by examination of the composition of the R group attached to the peroxy moiety. The general trend in binding energies has been determined to be as follows: fluorine approximately alkyl < carbonyl < alcohol. The weakest bound complex, CH3O2.H2O, is calculated to be bound by 2.3 kcal mol-1, and the strongest, the CH2(OH)O2.H2O complex, is bound by 5.1 kcal mol-1. The binding energy of the peroxy radical-water complexes which contain carbonyl and alcohol groups indicates that these complexes may perturb the kinetics and product branching ratios of reactions involving these complexes.

Perspective: Water cluster mediated atmospheric chemistry

“…4,9,25 Water can form open and closed shell molecular complexes with low binding energies (van der Waals interactions) and more strongly bound complexes (dipolar interactions and hydrogen bonding) with atmospheric species (e.g., O 2 , N 2 , Ar, OH, HO 2 , RO 2 , O 3 , OCS, SO 2 , SO 3 , NO, SH, ClO, NH 3 , HNO 3 , HCl, H 2 SO 4 , organic acids, aldehydes, and ketones, etc.). 19,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] We illustrate results of calculations of the abundance of water complexes with altitude in Figure 1, pointing out that while the temperature minimum at the tropopause enhances the equilibrium constant, it also reduces the partial pressure of water and other monomeric condensable species leading to an overall decrease of the complex abundance with altitude. 4,8,10,12,29 Nevertheless, even low concentrations of weakly bound complexes can have a significant effect on atmospheric chemistry.…”

Computational study of hexanal peroxy radical–water complexes

“…The K eq *[H 2 O] ratio values are indicative of other peroxy-water systems, such as the methyl peroxy, ethyl peroxy, and the acetyl peroxy radicals [10]. One noticeable difference between the 2h3pHEX-H 2 O and 2p3hHEX-H 2 O complexes and those mentioned in Ref.…”

“…The NBO analysis indicates that the peroxy moiety plays no role in the overall stability of the radical-water complexes, which is in stark contrast with interactions observed in HIPs and smaller organic peroxy radical complexes where the peroxy moiety plays a central role complex stabilization. It is of note that smaller peroxy radicals containing an aldehyde group, such as the acetyl and acetonyl peroxy radicals, also indicate a lack of interaction between the carbonyl carbon and the peroxy functionalities [10].…”

“…Recent computational results by Clark et al suggest that many methyland ethyl-sized peroxy radicals exhibit binding energies between À2.5 and À5.1 kcal/mol with water vapor [10]. These strong binding energies are traced to the formation of hydrogen bonds between the radical's peroxy and hydroxy moieties and water, as well as surprisingly strong CHAO interactions between the radical and water that contribute up to 36% of the overall stabilization energy [10].…”

Global analysis of peroxy radicals and peroxy radical-water complexation using the STOCHEM-CRI global chemistry and transport model

“…Although the HO 2 .H 2 O term has been included as an additional term for the calculation of the HO 2 þ HO 2 termination step (Sander et al, 2011), the water influence on the abundances of RO 2 throughout the troposphere has, up to now, been ignored in many atmospheric modelling studies. The energetics and potential impacts of hydroperoxyl radical-water complexes (HO 2 .H 2 O) and organic peroxy radical-water complexes (RO 2 .H 2 O) were presented in previous theoretical chemistry studies (Aloisio and Francisco, 1998;Clark et al, 2008Clark et al, , 2010Archibald et al, 2011) (Aloisio et al, 2000;Kanno et al, 2006). The estimation by Aloisio and Francisco (1998) showed that 29% of all HO 2 radicals present in the atmosphere could participate in water complexation under ambient conditions.…”