Bond energies and formation enthalpies of mono- and polyradicals in nitroalkanes 1. Nitromethanes

Abstract

The enthalpies of formation of nitromethane derivatives were obtained on the basis of experimental and literature data. The procedure for the calculation of the bond dissociation energies in nitromethanes from the atomization enthalpies and energies of nonvalent interac tions of nitro groups was proposed. The calculated values were compared with the data on the thermal decomposition kinetics. The atomization enthalpies and energies of nonvalent interac tions of nitro groups were also used for the calculation of the bond dissociation energies in radicals.The bond dissociation energies and formation enthal pies of radicals are most important energy characteristics of molecules and intermediate reaction products (inter mediates). These values determine stability of compounds and affect the mechanism and kinetics of conversion of substances, including the intensity of chain processes. 1 The bond dissociation energies and enthalpies of forma tion of radicals and substances in the gas phase are related by the equationwhere D(R 1 -R 2 ) is the dissociation energy of compound R 1 R 2 to radicals R 1 and R 2 ; ΔH°f(R 1 ), ΔH°f(R 2 ), and ΔH°f(R 1 R 2 ) g are the formation enthalpies of radicals R 1 , R 2 , and compound R 1 R 2 in the gas phase under standard conditions. It follows from Eq. (1) that the dissociation energy of bond is equal to the energy that should be tran ferred to the molecule (radical) for its dissociation to rad icals R 1 and R 2 . The dissociation energies of bonds are usually determined by using the kinetic data on the activa tion energies (Е а ) of monomolecular radical reactions of thermal decomposition. It is assumed 1 that the enthalpy of backward process (formation of a molecule from radi cals) is zero. For such compounds as polynitroalkanes, nitrates, and secondary nitroamines, thermal decomposi tion proceeds as the monomolecular radical reaction with the dissociation of the C-NO 2 , O-NO 2 , or N-NO 2 radicals. 2,3 If the enthalpy of the backward reaction of radical association is considered equal to zero, then the dissociation energy of the R-NO 2 bond is equal to the activation energy of thermal decompositionIt was concluded 4 on the basis of the theory of absolute reaction rates that the difference between D and E a in cludes RT and depends on the differences between the heat capacities of the products and reactants in the disso ciation reaction and on the differences of the heat capaci ties of the transition state and initial radicals in recombi nation. The integrals taken of the heat capacity differenc es have different signs, and various molecules at certain temperatures exactly obey Eq. (2).The differences between D and E a can be estimated from the statistical mechanics equations if the parameters of geometric structures, inter nal rotation barriers, and vibration frequencies of poly atomic particles are known. These calculations are rather difficult for the classes of compounds studied in this work and, hence, we will use Eq. (2).Equations (1) and (2) are used for the determination of...

Bond energies and the enthalpies of formation of mono- and polyradicals in nitroalkanes 2. Nitro derivatives of ethane and propane

“…Knowledge of the enthalpies of radical formation is necessary for calculations of enthalpies of chemical reactions and for prediction of the most probable reaction mechanisms. The bond dissociation energies and the en thalpies of formation of radicals and molecules are related by the following equation D(R 1 -R 2 ) = ΔH f°( R 1 ) + ΔH f°( R 2 ) -ΔH f°( R 1 R 2 ), (1) where D(R 1 -R 2 ) is the dissociation energy of the com pound R 1 R 2 to the radicals R 1 and R 2 ; ΔH f°( R 1 ), ΔH f°( R 2 ), and ΔH f°( R 1 R 2 ) are the enthalpies of formation of the radicals R 1 and R 2 , and compound R 1 R 2 in the gas phase under standard conditions, respectively. In the present work, it is assumed that the dissociation energy of the R-NO 2 bond is equal to the activation energy of mono molecular thermal decomposition of nitrogen containing compounds proceeding by a radical mechanism (E a ).…”

“…In the present work, it is assumed that the dissociation energy of the R-NO 2 bond is equal to the activation energy of mono molecular thermal decomposition of nitrogen containing compounds proceeding by a radical mechanism (E a ). 1 Equation (1) is used for determining unknown enthalpies of formation of radicals and bond dissociation energies. The enthalpies of formation of radicals are usually deter mined with the largest error (≥2 kcal mol -1 ).…”

“…1. using Eq. (1) give reliable values of ΔH f° and D if the cor responding kinetic and thermodynamic data are available. Previously, 1 it was proposed to calculate changes in R-NO 2 bond dissociation energies in polyfunctional compounds using the changes in the contributions of nitro groups to the enthalpy of atomization relative to the corresponding contributions in mononitroalkane, in particular, nitromethane.…”

“…The results of similar calculations for radicals also agree with the enthalpies of formation of methane polyradicals. 1 In the present work, the procedure used for D(R-NO 2 ) calculations for molecules and radicals of the nitro de rivatives of ethane and propane is similar to that described earlier. 1 The dissociation energies of R-NO 2 bonds in mononitroalkanes are determined from Eq.…”

“…1 In the present work, the procedure used for D(R-NO 2 ) calculations for molecules and radicals of the nitro de rivatives of ethane and propane is similar to that described earlier. 1 The dissociation energies of R-NO 2 bonds in mononitroalkanes are determined from Eq. (1) using the experimental values of the enthalpies of formation of these substances in the gas phase and the reference data on the enthalpies of formation of the • NO 2 and alkyl radicals.…”