An ab Initio Study on Reactivity of Fluoroethane with Hydroxyl Radical:  Application of G2 Theory

Abstract

The reaction of fluoroethane with hydroxyl radical has been investigated by ab initio molecular orbital theory, and two possible reaction pathways were examined, R and abstractions. Optimized geometries and harmonic vibrational frequencies have been calculated for reactants, products, and transition states up to the (U)MP2/ 6-31G(d,p) level of theory. Accurate energies of reactants, products, and transition states are obtained using G2 theory. The calculated barrier height and reaction enthalpy of R abstraction are 1.35 and -18.36 kcal mol -1 , respectively. A very good agreement is found with available experimental data. Two rotamers were found as the transition-state structures involved in the abstraction reaction. This reaction has higher barrier heights, 2.66 and 3.73 kcal mol -1 , respectively, and it is less exothermic than R abstraction, with reaction enthalpy of -14.42 kcal mol -1 . One rotamer is stabilized by the delocalization of electrons from a fluorine atom into the O-H bond. The transition-state interactions between reacting units have been analyzed in terms of the natural bond orbital (NBO) method. The main delocalization process for all transition-state structures seems to be between the oxygen nonbonding orbitals and the NBOs of the reactive C(1)-H(3) bond. The results on fluoroethane reactivity with hydroxyl radical have been compared with previous results on chloroethane and ethane. The influence of halogen substitution on the barrier heights and reaction enthalpies has been rationalized in terms of three different effects: the electron density redistribution in the reactive C(1)-H(3) bond, the stabilization of radical product, and the participation of halogen atom in breaking the reactive C(1)-H(3) bond. Finally, the C-H bond dissociation energies of fluoroethane and chloroethane have been calculated at G2, UMP4, and UMP2/6-311+G(2d,p)//UMP2/6-31G(d,p) levels of theory as well as the G2 enthalpies of formation of R-and -fluoroethyl radicals since the experimental values are not available yet.

Modeling lifetime and degradability of organic compounds in air, soil, and water systems (IUPAC Technical Report)

“…The tremendously fast developments in computing technology, hardware, and software have enabled the calculation of the energy profiles of gas-phase reactions with OH radicals. The abstraction of hydrogen atoms by hydroxyl radicals, as well as its addition to the double bonds, have been a subject of theoretical investigations with semiempirical [38][39][40] and accurate ab initio [41][42][43][44][45][46][47][48][49][50][51][52] MO calculations during the last decade. The early efforts have focused on studying structural and energetic properties of reactants, products, and transition-state structures of hydroxyl radical reactions with small hydrocarbons and their halogenated derivatives [41][42][43][48][49][50][51]53].…”

Reactivity of haloethanes with hydroxyl radicals: Effects of basis set and correlation energy on reaction energetics

“…Two pathways R1 and R2 are feasible for reactions of fluoroethane 19,20 and chloroethane 21 with the hydroxyl radical; that is, the abstraction of hydrogen from ␣ and ␤ carbon atoms:…”

“…The geometry of one rotamer resembles, to a large extent, the transitionstate structure of an ␣-abstraction reaction. 19 ᎐ 21 The other rotamer, obtained by the rotation around the reactive C-H bond, is stabilized by a through-space interaction, which lowers the barrier height for the ␤ abstraction by 1.45 and 0.96 kcal mol y1 for fluoroethane 19,20 and chloroethane, 21 respectively. This interaction is due mainly to the delocalization of electrons from the halogen nonbonding orbital into the antibond orbital of the OH radical.…”

“…This interaction is due mainly to the delocalization of electrons from the halogen nonbonding orbital into the antibond orbital of the OH radical. 20,21 w Ž298K. x The reaction barrier heights E and the 0 w Ž298K.…”

Using Variable and Fixed Topological Indices for the Prediction of Reaction Rate Constants of Volatile Unsaturated Hydrocarbons with OH Radicals

“…Recently, several quantitative structure property relationship (QSPR) prediction models were developed to predict reaction rate constants for the reaction of OH radicals with different organic species. These models estimated reaction rate constants based on empirical fragment contribution technique [4][5][6], bond dissociation energy [7][8][9], NMR chemical shift data [10], ionization potentials [11][12][13], molecular orbital calculations [14][15][16][17][18][19][20][21] and various structural descriptors [22][23][24][25][26][27]. A comprehensive overview [28] of these method and their partial evaluations were recently published [29].…”