Rearrangement energies for radicals of azido nitroaromatic compounds

Dissociation energies of C–N₃ bonds have been determined on the basis of data on the enthalpies of formation for a series of azido nitroaromatic compounds and the enthalpies of formation of radicals. Using fundamental relationships of chemical physics, a procedure has been suggested to calculate the energy of rearrangement of molecule fragments into radicals on the basis of special properties of rearrangement energy and the sums of average thermochemical energies for bonds comprising radical fragment in molecule. This calculation procedure provided a possibility to determine the energy of the N₃ moiety transformation into N₃ radical and the rearrangement energies of nitroaromatic radicals.     

Bond energies and the enthalpies of formation of mono- and polyradicals in nitroakanes 3. Nitroalkanes C<Subscript>4</Subscript>–C<Subscript>7</Subscript>

Based on the experimentally determined values and published data, the enthalpies of formation of nitroalkanes C₄–C₇ in the standard state and in the gas phase were recommended. The dissociation energies of bonds in these compounds were determined taking into account the enthalpies of atomization and the energies of nonvalent interactions of nitro groups with one another. The calculated values were compared with the available thermal decomposition kinetic data. The dissociation energies of bonds in C₄–C₇ nitroalkane radicals 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 nitroalkanes C₁–C₇ and their radicals are established.     

The energy of peroxide bonds in peroxy substituted silanes and the enthalpy of formation of siloxy radicals. The influence of the nature of substituents on the energy of peroxide bonds

The energies of peroxide bonds and the enthalpies of formation of siloxy radicals were determined from the enthalpies of formation of peroxy substituted silanes by solving thermochemical equations. The influence of the nature of substituents on the energy of peroxide bonds was analyzed.     

Lattice energetics and thermochemistry of phenyl acridine-9-carboxylates and 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates

The melting points and melting enthalpies of nine phenyl acridine-9-carboxylates—nitro-, methoxy- or halogen-substituted in the phenyl fragment—and their 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonate derivatives were determined by DSC. The volatilisation temperatures and enthalpies of phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice energies and enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonate ions, obtained by the DFT method, and the corresponding enthalpies of sublimation and/or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well with the experimentally obtained enthalpies of sublimation. The crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whilst the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.     

The determination of enthalpies of formation of organic free radicals from bond dissociation energies. 5. Organosulfur radicals

The formation enthalpies (H _f°) of 12 organosulfur radicals (R^·) were determined for the first time from the published values of dissociation energies of R—X bonds.     

Theoretical study of the competition between various mechanisms of gas-phase decomposition in the series of primary N-nitramines

Nonempirical and density-functional methods were used to determine geometric parameters, enthalpies of formation of compounds and radicals, dissociation energies of the N-NO₂ bonds of primary N-nitramines and N,N-dinitramines. The tendencies toward variation of the geometric structure, enthalpies of formation, and dissociation energy in the series of primary N-nitramines were analyzed. Alternative mechanisms of the gas-phase thermal destruction to give experimentally observed reaction products were studied for the example of N-methylnitramine and its homologues.     

Substituent effects on enthalpies of formation of nitrogen heterocycles: 2-substituted benzimidazoles and related compounds

The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-tert-butylbenzimidazole (2tBuBIM) and 2-phenylimidazole (2PhIM) are reported and the results compared with those of benzene derivatives and a series of azoles (imidazoles, pyrazoles, benzimidazoles and indazoles). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311++G(d,p) on B3LYP/6-31G optimized geometries. The comparison of experimental and calculated values of all studied compounds bearing H (unsubstituted), methyl (Me) ethyl (Et), propyl (Pr), isopropyl (iPr), tert-butyl (tBu), benzyl (Bn) and phenyl (Ph) groups show remarkable homogeneity. The remarkable consistency of both the calculated and experimental results allows us to predict with reasonable certainty the missing experimental values. The crystal and molecular structure of the 2-benzylbenzimidazole (2BnBIM) has been determined by X-ray analysis. The observed molecular conformation permits the crystal being built up through N-H...N hydrogen bonds and van der Waals contacts between the molecules. An attempt has been made to relate the crystal structure to the enthalpies of sublimation.     

Energy of the O–NO2 bond dissociation and the mechanism of the gas-phase monomolecular decomposition of aliphatic alcohol nitroesters

The equilibrium geometrical parameters, enthalpies of the formation of compounds and radicals, and the dissociation energies of the O–NO₂ bond for nitroesters of mono- and polyatomic aliphatic alcohols have been determined by the density functional B3LYP method. The basic tendencies in the changes of parameters of the geometrical and electronic structure of molecules, enthalpies of the formation and dissociation energies have been analyzed. Various mechanisms of the initial event of the gas-phase decomposition of nitroesters of mono- and polyatomic aliphatic alcohols have been studied.     

Thermochemistry and crystal lattice energetics of phenyl acridine-9-carboxylates and 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates

The melting enthalpies and melting points of phenyl acridine-9-carboxylate, its eleven alkyl-substituted derivatives in the phenyl fragment and eight 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates derived from them, were determined by DSC. The volatilisation enthalpies and temperatures of twelve phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium and trifluoromethanesulphonate ions, obtained by quantum chemistry methods, and the corresponding enthalpies of sublimation or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well to the experimentally obtained enthalpies of sublimation. Analysis of crystal lattice enthalpy contributions indicates that the crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whereas the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.     

Enthalpies of formation, bond dissociation energies, and molecular structures of the n-aldehydes (acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal) and their radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.     

Molecular structure and mechanisms of unimonomolecular decomposition of primary <Emphasis Type="Italic">N</Emphasis>-nitramines

By means of nonempirical and the density functional methods the geometrical parameters, the enthalpies of formation of the compounds and radicals, and the dissociation energies of the N-NO₂ bond in primary and secondary N-nitramines were evaluated. The tendencies to the variation of spatial arrangement, of the formation enthalpies, and of the dissociation energies in the series of simplest N-nitramines were analyzed. Alternative mechanisms of the initial stage of the gas phase unimolecular decomposition were considered. It is noted that among all the processes of unimolecular decomposition the formation and destruction of aci-form according to the complex multy-stage mechanism was the most energetically favored.     

Relative energy of organic compounds IV. Radicals,carbocations, and carbanions

The structure-dependent energies of organic radicals, cations, and anions are deduced from their calculated relative enthalpies and are compared to the relative enthalpies of their parent compounds. The use of relative enthalpies to express the relative energies of organic radicals, cations, and anions proved to be as fruitful as in the case of their parent organic compounds. The same energy-determining structural factors may have stronger, weaker, or even opposite effects in the radicals, cations, or anions than those in their parent molecules.     

Molecular energetics of alkyl substituted pyridine <Emphasis Type="Italic">N</Emphasis>-oxides

The standard (p° = 0.1 MPa) energies of combustion in oxygen, at T = 298.15 K, for the solid compounds 2-methylpyridine-N-oxide (2-MePyNO), 3-methylpyridine-N-oxide (3-MePyNO) and 3,5-dimethylpyridine-N-oxide (3,5-DMePyNO) were measured by static-bomb calorimetry, from which the respective standard molar enthalpies of formation in the condensed phase were derived. The standard molar enthalpies of sublimation, at the same temperature, were measured by Calvet microcalorimetry. From the standard molar enthalpy of formation in gaseous phase, the molar dissociation enthalpies of the N–O bonds were derived, and compared with values of the dissociation enthalpies of other N–O bonds available for other pyridine-N-oxide derivatives.     

Unusual structural and energetic features of homolytic bond dissociation: from tetrakis(disyl)diphosphine to tetrakis(di-tert-butylsilyl)hydrazine

Quantum chemical calculations of the structures and thermodynamics of homolytic dissociation of the central P-P and N-N bonds in tetrakis(disyl)diphosphine and tetrakis(di-tert-butylsilyl)hydrazine have been performed. The theory predicted negative standard enthalpies for homolytic bond dissociation in both cases, -71.0 and -108.4 kJ mol(-1) for the diphosphine and hydrazine, respectively, using the ONIOM (MP2/6-31+G*:B3LYP/3-21G*) level. The dissociation is accompanied by considerable structural changes in the radicals as compared to the corresponding fragments of the parent molecules, resulting in low dissociation enthalpies. The most pronounced changes in both radicals are the relaxation of bond angles in the substituents and a conformational change in the orientation of the substituent groups. In addition, the bis(di-tert-butylsilyl)aminyl radical displays a considerable increase in Si-N-Si angle and shortening of the Si-N bonds upon dissociation. These changes are not associated with any appreciable delocalisation of the lone electron, as the spin density is found from the B3LYP/3-21G* calculations to be largely concentrated on the nitrogen atom. It has been also shown that although the dissociation energies are low for both compounds, the intrinsic energies of the central bonds are still high, 140.6 kJ mol(-1) for the P-P bond in tetrakis(disyl)diphosphine and 490.6 kJ mol(-1) for the N-N bond in tetrakis(di-tert-butylsilyl)hydrazine, using the ONIOM method. The calculations predict that the dissociation of tetrakis(disyl)diphosphine would have negative free energy even without taking relaxation of the fragments into account, while the full potential of releasing about 306 kJ mol(-1) of energy stored in the ligands of tetrakis(di-tert-butylsilyl)hydrazine is only fully realised upon a considerable separation of the fragments.     

Theoretical study of the formation of naphthalene from the radical/π-bond addition between single-ring aromatic hydrocarbons

The experimental investigations performed in the 1960s on the o-benzyne + benzene reaction as well as the more recent studies on reactions involving π-electrons highlight the importance of π-bonding for different combustion processes related to PAH's and soot formation. In the present investigation radical/π-bond addition reactions between single-ring aromatic compounds have been proposed and computationally investigated as possible pathways for the formation of two-ring fused compounds, such as naphthalene, which serve as precursors to soot formation. The computationally generated optimized structures for the stationary points were obtained with uB3LYP/6-311+G(d,p) calculations, while the energies of the optimized complexes were refined using the uCCSD(T) method and the cc-pVDZ basis set. The computations have addressed the relevance of a number of radical/π-bond addition reactions including the singlet benzene + o-benzyne reaction, which leads to formation of naphthalene and acetylene through fragmentation of the benzobicyclo[2,2,2]octatriene intermediate. For this reaction, the high-pressure limit rate constants for the individual elementary reactions involved in the overall process were evaluated using transition state theory analysis. Other radical/π-bond addition reactions studied were between benzene and triplet o-benzyne, between benzene and phenyl radical, and between phenyl radicals, for all of which potential energy surfaces were produced. On the basis of the results of these reaction studies, it was found necessary to propose and subsequently confirm additional, alternative pathways for the formation of the types of PAH compounds found in combustion systems. The potential energy surface for one reaction in particular, the phenyl + phenyl addition, is shown to contain a low-energy channel leading to formation of naphthalene that is energetically comparable to the other examined conventional pathways leading to formation of biphenyl compounds. This channel is the first evidence of a reaction which involves an aromatic radical adding to the nonradical π-bond site of another aromatic radical which leads directly to a fused ring structure.     

Enthalpies of the formation of aliphatic carbonyl-containing radicals

A collection of data on enthalpies of the formation(Δ_f H ^o) of aliphatic carbonyl-containing radicals is analyzed and expanded. The Δ_f H ^o values for 29 carbonyl-containing radicals are determined for the first time, and are strongly revised for 17 carbonyl-containing radicals using the literature data on the dissociation energies of the bonds in molecules. The data is analyzed on the basis of the structureproperty (enthalpy of formation) relation within the additive-group approach, with the determination and specification of the parameters. It is concluded that the Δ_f H ^o values of carbonyl-containing radicals calculated from the obtained parameters (a total of 96 compounds was considered) agree well with the experimental data.     

Correlations between F NMR chemical shifts and bond dissociation energies, enthalpies of formation and group electronegativities

Correlations between ¹⁹F NMR chemical shifts (Φ) in fluorohalohydrocarbons and enthalpies of formation, bond dissociation energies, group contribution to enthalpies of formation and enthalpies of formation of free radicals are presented. A good correlation between these properties has been found with the square root of Φ. A relationship between fluorine chemical shifts and group electronegativities is also discussed.     

Bond dissociation energies in nitramines

The enthalpies of formation in the standard state and in the gas phase were recommended for a series of secondary nitramines and n-butyldinitramine on the basis of the experimental enthalpies of combustion and vaporization and literature data. An analysis of the main thermochemical values (the enthalpies of formation in the gas phase and the enthalpies of atomization) showed that the energy properties of the nitramine group are independent of the structure of the molecules studied and of the number of functional groups in them. The enthalpies of formation of the alkylnitramine radicals were determined. The values obtained make it possible to calculate the bond dissociation energies in the nitramines and their radicals of different structures.     

A radiation-chemical and quantum-chemical study of ascorbic acid interaction with α-hydroxyethyl radicals

The effect of ascorbic acid and its analogue 5,6-O-isopropylidenyl-2,3-O-dimethylascorbic acid (I), which has been synthesized for the purpose and does not contain mobile hydrogen atoms, on the formation of the products of continuous radiolysis of deaerated ethanol and its aqueous solutions has been studied. The ionization potentials, the molecular orbital energies, the enthalpies of homolytic dissociation of C-H and O-H bonds, and the enthalpies of H atom addition to the C=O group of the test compounds have been calculated by ab initio methods. The array of the experimental and calculated theoretical data suggests that both ascorbic acid in the undissociated form and compound I can oxidize α-hydroxyethyl radicals, whereas the monoanion of ascorbic acid acts as a reducing agent in the reactions with these transient radicals. The reduction of α-hydroxyethyl radicals in aqueous solutions by the ascorbic acid monoanion can follow both the hydrogen transfer and electron transfer mechanisms.     

Thermodynamic properties of 5(nitrophenyl) furan-2-carbaldehyde isomers

Background

The aim of the current work was to determine thermo dynamical properties of 5(2-nitro phenyl)-furan-2-carbaldehyde, 5(3-nitro phenyl)-furan-2-carbaldehyde and 5(4-nitro phenyl)-furan-2-carbaldehyde.

Results

The temperature dependence of saturated vapor pressure of 5(2-nitro phenyl)-furan-2-carbaldehyde, 5(3-nitro phenyl)-furan-2-carbaldehyde and 5(4-nitro phenyl)-furan-2-carbaldehyde was determined by Knudsen’s effusion method. The results are presented by the Clapeyron–Clausius equation in linear form, and via this form, the standard enthalpies, entropies and Gibbs energies of sublimation and evaporation of compounds were calculated at 298.15 K. The standard molar formation enthalpies of compounds in crystalline state at 298.15 K were determined indirectly by the corresponding standard molar combustion enthalpy, obtained using bomb calorimetry combustion.

Conclusions

Determination of the thermodynamic properties for these compounds may contribute to solving practical problems pertaining optimization processes of their synthesis, purification and application and it will also provide a more thorough insight regarding the theoretical knowledge of their nature.

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Graphical abstract: Generalized structural formula of investigated compounds and their formation enthalpy determination scheme in the gaseous state