Search

The article is devoted to the description of using of two density functional methods B3LYP/6-31+G(2df, p) and wB97X/Def2TZVPP, the geometric parameters and vibration frequencies of the CONO2 group, as well as the enthalpy of formation and dissociation energy of the O-NO2 bond in the molecules of isomeric butyl nitrates (butyl nitrate, 2-butyl nitrate, isobutyl nitrate, tert-butyl nitrate) are determined. The authors show that the observed changes in the geometric parameters of the CONO2 group in the series of compounds studied are insignificant. According to the B3LYP method, they are 4.1 pm for the C–O bond, 0.7 pm for the O–NO2 bond, and about 1 degree for the ONO angle. Another method used in this work (wB97X) predicts similar values of changes in the series. At the same time, the results of calculation make it possible to reliably distinguish, by the value of the geometric parameters, the nitrate groups attached to the primary secondary and tertiary carbon atoms, respectively. It is noted that the highest values of enthalpies of formation and energies (enthalpies) of dissociation are observed for compounds in which nitrate groups are attached to the primary carbon atom. The correlation dependences in the change in the enthalpies of formation of compounds and radicals, which are formed upon homolytic cleavage of the O-NO2 bond, have been revealed. The tendencies of change in the series convey both methods used in the work in the same way. However, a comparison of the results with the available experimental data on the kinetics of thermal decomposition of the studied aliphatic nitrates shows that the most reliable estimates are the wB97X/Def2TZVPP method. The calculated values obtained on the basis of another B3LYP method used in the work are underestimated, by about 3-4 kcal/mol. It was found that of the stretching vibrations of the CONO2 group, only asymmetric vibrations of N-O are characteristic.

Enthalpies of the C−N bond dissociation of nitrobenzene and twenty-seven mono substituted nitrobenzenes along with their formation enthalpies were calculated using different multilevel (G2, G3, G3B3, CBS-QB3) and density functional theory-based B3LYP and wB97XD techniques. The gas phase enthalpies of formation of mono substituted phenyl radicals were also calculated using these methods. The calculated values of reaction enthalpies were compared with available experimental data. We have concluded based on results of present and recent article, that B3LYP level of theory with 6-31G(d′f,p′) basis sets should proposed as the accurate method for calculating formation enthalpies of mono substituted nitrobenzenes as well as enthalpies of C–NO2 bond scission. Based on satisfactory correlation between experimental and calculated data we proposed revision of experimental activation energy of C–NO2 bond rupture in o-chloronitrobenzene, o- and m-nitrotoluene as wells as p-nitroaniline. Enthalpies of formation of some substituted phenyl radicals were estimated using experimental enthalpies of C–NO2 bond cleavage together with formation enthalpies of mono substituted nitrobenzenes and NO2 moiety.

Enthalpies of formation of twenty seven mono substituted nitrobenzenes were calculated employing various DFT and composite methods using atomization approach and two types of isodesmic reactions. The overall best agreement with experiment was achieved by using Gn multilevel techniques. It was found that the best performance among DFT methods within atomization approach demonstrates long range corrected wB97XD level theory. The average absolute deviation value calculated for B3P86/6-311G(d,p) which have been recently reported as an very accurate method for calculating formation enthalpies of poly and mono substituted nitrobenzenes exceeds 50 kcal/mol. Employing of isodesmic reactions tends to it improve estimations of DFT levels, though agreement between theoretical data obtained using this methodology with experiment depends strongly on the type of reaction and the quality of experimental Δ f H 0 ,298 values available for compounds involved in these bond balanced reactions.

Present theoretical study has been carried out with objective to reveal effect of chlorine and fluorine substituents on C–N bond strength in nitromethane. Gn, W1 and CBS-QB3 composite techniques were employed to compute accurate values of formation enthalpies of halogenated nitromethanes, corresponding radicals and ΔrH0(C–N) values. Calculations on DFT and ab initio levels were performed using B3LYP and QCISD methods. It was found that substitution of hydrogen by fluorine atoms causes only slightly decreasing of ΔrH0(C–N), whereas accumulation of chlorine atoms tends to decrease C–N bond strength significantly. Formation enthalpies of halogenated nitromethanes and corresponding radicals were calculated using both atomization approach and involving isodesmic reactions.

Geometric parameters and formation enthalpy and the enthalpy of the radicals formed during the homolytic breakage of C-NO2 bond in 37 aromatic nitro compounds were calculated using different bases of the hybrid density functional method B3LYP, as well as the composite CBS-QB3 methods. On the basis of thermochemical data, were calculated the C-NO2 bond dissociation energy and the activation energy of the radical gas-phase decomposition. Donor substituents were shown to cause an increase in the C-NO2 bond dissociation energy, while the acceptors decrease it. The values of activation energies of gas-phase decomposition of aromatic nitro compounds calculated basing on the C-NO2 bond dissociation energy are in good agreement with experiment.

In recent paper we reported results concerning the gas phase enthalpies of formation of C1–C4 mononitroalkanes calculated using different multilevel and density functional theory (DFT)-based B3LYP techniques. The enthalpies of the C−N bond dissociation of these nitroalkanes had been also calculated. In present paper calculated activation enthalpies and enthalpies of reaction for HONO elimination from C2–C4 mononitroalkanes are represented. DFT/B3LYP, ab initio QCISD levels of theory together with G3, G3B3 G4 and CBS-QB3 were employed in this study. G4 calculated enthalpies of formation and reaction enthalpies of C–N bond rupture are performed in addition to activation and reaction enthalpies for HONO elimination. Comparison with available experimental activation energies showed that G3 and G3B3 estimations are of higher accuracy. The agreement of the CBS-QB3 calculations with experiment is also satisfactory. G4 calculated data demonstrated the worse agreement and did not produce correct trends for reaction enthalpies of C–N bond rupture.

The gas phase enthalpies of formation of C 1 –C 4 mononitroalkanes were calculated using different multilevel (G1, G2, G3, G3B3, CBS-QB3) and density functional theory (DFT)-based B3LYP techniques. The enthalpies of the C–N bond dissociation of these nitroalkanes were also calculated. The calculated values of the formation and reaction enthalpies were compared with available experimental data. It was found that the G3 and G3B3 procedures gave accurate results for the formation enthalpy of nitroalkanes and radical products. The agreement of the CBS-QB3 calculations with experiment is also satisfactory, whereas less accurate than G3 and G3B3 estimations. The G3 and G3B3 multilevel techniques showed good results for the reaction enthalpies of the C–N bond dissociation.