Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 1. Electronic Structure Calculations and Thermochemistry

Abstract

Alkyl nitrates (RONO 2 ) are minor products formed in the atmospheric reactions of alkyl peroxy radicals (RO 2 •) with nitric oxide. The major products are alkoxy radicals (RO•) and NO 2 . The alkyl nitrate channel is important in the troposphere because RONO 2 formation results in removal of NO x and trapping of free radicals; both effects reduce the rate of ozone production. We have used electronic structure calculations at the G3 and B3LYP/6-311++G** levels to calculate the geometries, energies, and vibrational frequencies for major stationary points on the potential energy surfaces for R ) H, CH 3 , C 2 H 5 , n-C 3 H 7 , i-C 3 H 7 , and 2-C 5 H 11 . Selected calculations have been made at the G2 and QCISD(T)/cc-pVTZ levels. Reaction energies are found to be rather insensitive to the size of the alkyl group. Corrections to the reaction energies are estimated, and a generic set of reaction energies are suggested. The B3LYP/6-311++G** barriers for the isomerization of ROONO to RONO 2 are found to be much too high to account for observed nitrate formation.

A theoretical examination of the isomerization of BrONO2 to BrOONO

“…The isomerization energy barrier for the HOONO M HONO 2 reaction is reported to be 39.0 kcal mol À1 , which is 14.8 kcal mol À1 higher than that of the equivalent bromine reaction. We can also compare our results to those of Lohr et al [30] who studied isomerization of ROONO to RONO 2 at the B3LYP/6-311+G(d,p) level of theory for several alkyl groups (R = CH 3 , C 2 H 5 , C 3 H 7 , C 5 H 11 ) included in the investigation of the reaction of alkyl peroxy radicals with NO. Transition states that connect cis-perp isomers of ROONO and RONO 2 are reported and are also very similar to bromine TS1 calculated here.…”

“…The B3LYP method has yielded remarkably good accuracy for numerous applications to the transition state geometries and energies [25][26][27], although it tends to underestimate the barrier heights in some cases. Calculations using the B3LYP functional have also been performed on the isomerization of ROONO to RONO 2 , where R is hydrogen [28], chlorine [29] or alkyl groups [30], and thus they provide a common ground for comparison.…”

Thermochemistry for enthalpies and reaction paths of nitrous acid isomers

“…HNO 2 is not considered to be formed from HONO via an isomerization process, because the isomerization barrier of 55.8 kcal mol −1 is higher than the HO NO bond energy and it is too high to occur under atmospheric conditions [19a,23,37,38a]. The high barrier is further supported by the similar barriers for isomerization to organonitrates from the corresponding nitrites [3,31,33] for gas phase, nonhydrated reactions.…”

Nitrogen Oxides: Vehicle Emissions and Atmospheric Chemistry

“…These include quantum chemical calculations [22,26] of the structures and relative energies of the intermediates, as well as master equation [5,16,35,36] and classical trajectory simulations [11,25] of the chemical reactions. The ROONO * !…”