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

Abstract

Based on the experimental results and the published data, the enthalpies of formation of ethane and propane nitro derivatives were obtained for both the standard state and gas phase. The bond dissociation energies of the ethane and propane nitro derivatives were calculated using the enthalpies of atomization and the energies of nonvalent interactions of nitro groups. The calculated values were compared with the kinetic data on thermal decomposition. The bond dissociation energies in radicals of the ethane and propane nitro derivatives were also calculated using the enthalpies of atomization and the energies of nonvalent interactions of nitro groups. Regularities of changes in the bond dissociation energies of the ethane and propane nitro derivatives and their radicals were established.Key words: thermochemical properties, bond dissociation energy, nonvalent interactions, enthalpy of radical formation, enthalpy of atomization, energy of substitution, ethane nitro derivatives, propane nitro derivatives.Nitroalkyl radicals are intermediates of many reac tions of synthesis and decomposition of high energy com pounds. 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 equationwhere 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 ).

Use of G4 Theory for the Assessment of Inaccuracies in Experimental Enthalpies of Formation of Aliphatic Nitro Compounds and Nitramines

“…The calculated Δ f H 298 o (g) values for 19−22, 26, 28, and 29 are in internal consistency with the experimental data for mononitroalkanes (1−8), dinitroalkanes (10−18), and tetranitroalkanes (23)(24)(25)27), and thus the theoretical gas-phase enthalpies of formation are recommended for 19 −22, 26, 28, and 29 in Table 2 rather than the experimental data. On the basis of these values and the experimental condensed phase enthalpies of formation, the new values of enthalpy of vaporization or sublimation are recommended in Table 2 Among the seven cyclic and polycyclic nitro compounds (30)(31)(32)(33)(34)(35)(36), the largest difference of 47 kJ·mol −1 is revealed between the experimental and calculated Δ f H 298 o (g) values of 1,1-dinitrocyclopropane (32). Evidence of experimental error for 32 was derived from the results of isodesmic reaction calculations (see Table S2 of Supporting Information).…”

“… a Values calculated from isodesmic reactions are given. b Calculated in this work using the relationships between condensed and gas phase enthalpies of formation: Δ f H 298 o (l) + Δ vap H 298 o = Δ f H 298 o (g) or Δ f H 298 o (cr) + Δ sub H 298 o = Δ f H 298 o (g). c Reference . d Reference . e Estimated on the basis of experimental Δ vap H 298 o values for nitromethane and trinitromethane. f (323 to 353) K. g Estimation. h Reference . i Reference . j G4­(MP2) energies were used in the isodesmic reaction calculations. k Reference . l The value of Δ f H 298 o (cr) = 83.7 kJ·mol –1 is given by the authors, however, a different value, 108.0 kJ·mol –1 , is obtained from the enthalpy of combustion given in ref . m Estimated from empirical equation. n Reference . o Reference . p Reference . q Reference . r Reference . s The enthalpy of sublimation at 373.7 K (ref ) was recalculated to 298.15 K using the C p ,298 o (cr) = 218.0 J·K –1 ·mol –1 (ref ) and C p ,298 o (g) = 171.7 J·K –1 ·mol –1 calculated in this work. t Interpolated from the enthalpies of formation of methyldinitramine ( 50 ) and n -butyldinitramine ( 53 ). u Reference . v Obtained using MP2/cc-pVTZ calculations and the estimated value of Δ sub H 298 o ; this value leads to reasonable agreement with the experimental enthalpy of combustion determined in ref . w Assumed to be equal to the enthalpy of sublimation of biuret. x Estimated by comparison with related compounds, see ref . …”

Assessment of Gaussian-4 theory for the computation of enthalpies of formation of large organic molecules

“…The D f H 298 values were calculated from isodesmic reactions for some other compounds (3,10,14,17) with considerable deviations from experiment. However, for these compounds (Table 5), the results from isodesmic reactions are very close to the experimental values.…”

Effect of molecular structure of nitroalkanes on the C-NO2 bond strength and activation energy of the gas-phase radical decomposition according to the quantum-chemical simulation

“…(2); fairly accurate estimates of the formation enthalpies of mononitroalkanes have been obtained [4][5][6][7]. Accuracy of the estimates of formation enthalpies of the hydrocarbon radicals required for calculation of D(C-N) has been much worse: the two poorly correlated sets of values have been reported in the literature [8][9][10], the difference between the values often exceeding 12-15 kJ/mol.…”