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Various 1,2,5-oxadiazole-1,2,3,4-tetrazole-based high-energy materials were designed and their properties investigated. Their heat of formation, detonation properties, and thermal stability were calculated on the basis of isodesmic reactions, Kamlet-Jacobs equation, and bond dissociation energy, respectively. The results showed that all designed compounds possess high positive heat of formation, and the –N=N–/–N3 substituents were found to be the most effective for improving the heat of formation. The trend of changes in density, detonation pressure, and detonation velocity was almost identical in all compounds, which suggests that density was key to the determination of detonation properties. In view of bond dissociation energy, the –CN/–NH2 substituents improved the thermal stability of the designed compounds, while the other substituents/bridges decreased the stability to some extent. Considering the detonation properties and thermal stability, six compounds (C7, D3, D7, F7, G7, and H7) were selected as potential high-energy materials because they exhibited higher detonation properties and thermal stability than 1,3,5-trinitro-1,3,5-triazinane (RDX). Finally, electronic structures (such as distribution of frontier molecular orbitals and electrostatic potential) of the selected compounds were simulated to obtain detailed information about the electronic structures of the screened compounds.

In this study, density functional theory (DFT) was employed to predict thermochemical and energetic properties for single-walled carbon nanotubes (SWCNTs) decorated with one and more nitroaromatic substituents. Heat of sublimation and crystal density of these molecules were estimated on the basis of molecular surface properties calculated by electrostatic potential (ESP) analysis. Heats of formation in gas and solid phases were calculated using isodesmic approach. These data were used to measure velocity, pressure, and heat of detonation based on valid theoretical equations. Results show that crystal density of these molecules is close to TNT and their heat of formation is much larger. The calculated velocity and pressure of detonation for these molecules are approximately 4–6 km/s and 20–123 kbar, respectively; which are in the range of high explosives. However, their performance is lower than TNT. The detonation performance of these molecules increases as the number of nitroaromatic substituents increases.

The DLPNO-CCSD(T1)/CBS and G4 methods combined with isogyric, isodesmic, and homodesmotic working reactions were applied to calculate gas-phase enthalpies of formation for thirty-three C–H–N–O compounds with well-established experimental values. Unlike the G4 method, the DLPNO calculations, even together with isogyric reactions with small reference species, reproduce the experimental enthalpies of formation within chemical accuracy (≈ 4 kJ/mol). For more than half of the compounds, a better accuracy, within 2 kJ/mol, was achieved. These results make the DLPNO-CCSD(T1)/CBS method a promising tool for accurate prediction of enthalpy of formation of medium-sized organic compounds. Being convinced of its reliability, the DLPNO-CCSD(T1)/CBS method was also used to estimate the enthalpies of formation for some challenging compounds with conflicting values reported in the literature (glycerol, nitromethane, nitrobenzene, 1-aminoadamantane, piperazine, and others).

Following our quest for stable group 14 divalents, novel N-heterocyclic plumbylenes (NHPbs), composed of 2,4,6-cycloheptatriene-2,7-diazaplumbylene (1), benzannulated with one (2), two (3, 4), and three benzene rings (5), are compared and contrasted at the density functional theory level. Results indicate that in going from 1 to 5, the absolute values of singlet-triplet energy gap (ΔEs-t) and band gap (ΔEH-L) increase, while nucleophilicity (N), electrophilicity (ω), and chemical potential (μ) decrease. The most benzannulated structure, ([a,c,e]tribenzo)cyclohepta-2,7-diazaplumbylene (5), turns out as the most stable plumbylene for showing the most negative ΔEs-t of − 194.94 kcal/mol. Furthermore, 5 shows the highest ΔEH-L (− 2.75 eV), with the lowest N (1.31 eV), ω (8.20 eV), μ (− 7.15 eV), and charge of (+ 0.736) on Pb atom. Isodesmic reactions of 1-5 with common transition metal halides, MX2, give forty new metal complexes of 1M-X-5M-X, where M = Pt and Pd, while X = F, Cl, Br, and I. Their complexation energies (ΔECom) indicate that 1-5 are rather stronger ligands with Pt than Pd. Bader’s atoms in molecules (AIM) and NBO analyses show the partial covalent and partial electrostatic nature of Pb-M bonds.

Gas-phase enthalpies of formation of diamantane and some of its derivatives were calculated from isodesmic reactions using adamantane as reference species. The G4 theory was applied to calculate the energies of all molecules involved in the isodesmic reactions. The sublimation enthalpies were predicted by the molecular electrostatic potential (MEP) model. Their values were computed from an equation that describes the relationship between the sublimation enthalpy and computed crystal density, and molecular surface area plus four quantities obtained from the surface electrostatic potential. The coefficients in this equation were determined earlier from least squares fitting to reliable values of sublimation enthalpy of 23 adamantanes. The enthalpy formation of diamantanes in the crystalline phase was estimated on the basis of calculated gas-phase enthalpies of formation and sublimation enthalpies. The difference between experimental and theoretical data and inconsistencies in available experimental data are discussed.

We constructed five novel cyclotetramethylene tetranitramine (HMX) derivatives based on the skeleton of HMX ring. The molecules were fully optimized at the B3LYP/6-311G (d) level. We designed isodesmic reactions to calculate their enthalpies of formation. Then, their heats of detonation, detonation velocities, and detonation pressures were calculated using the Kamlet-Jacobs equations on the theoretical densities and enthalpies of formation. Their oxygen balances were computed to estimate whether the title compounds are oxygen-enriched or oxygen-poor. Finally, their impact sensitivity values were evaluated via a simple equation reported in previous studies. In terms of the quantitative standard as high energy density compounds (HEDCs), the potential candidates of HEDCs were selected.

We first use the second-order Moller–Plesset theory to optimize β-aminoacrolein, β-thioaminoacrolein and a set of their halogenated derivatives, which involve N–H···O and N–H···S hydrogen bonds. Then, four different methods, namely rotation barrier method (RBM), isodesmic reaction method (IRM), geometry corrected method (GCM) and related rotamers method, (RRM) are utilized to estimate the intramolecular hydrogen bond (IMHB) energies. The linear correlations between the estimated IMHB energies and some hydrogen bond strength descriptors such as geometrical, electron density topological and spectroscopic parameters are obtained. According to our data, RRM and RBM have the best linear correlations with all of the hydrogen bond descriptors, while GCM and IRM do not reveal suitable results. It is assumed that the performance or reliability of the estimating methods is determined based on the value of squared regression coefficients (R 2). By adding the results of the previous studies including OH···O, OH···S and NH···O hydrogen bond units to the present obtained data, the methods can be sorted according to their relative reliability as follows: RRM > RBM ≫ GCM > IRM.

Kinetics and thermochemistry of the H-atom abstraction reaction of CH3OCH2CH2Cl with OH radical have been carried out using dual level of methods. Initially, geometry optimization and frequency calculations are performed at M06-2X/6-31+G(d, p) level of theory, and energetic calculations are further refined using CCSD(T)/6-311++G(d, p) level of theory in order to characterized all stationary points on potential energy surface (PES). The result shows that H-atom abstraction from –OCH2 site of CH3OCH2CH2Cl is dominant path. The rate constants are calculated using canonical transition state theory at 298 K, which are found to be in good agreement with the experimental data. We have presented the standard enthalpies of formation for CH3OCH2CH2Cl and the radicals generated during the H-atom abstraction using group-balanced isodesmic reactions scheme. The atmospheric lifetime of title molecule is also calculated.

The gas-phase enthalpies of formation ( $${\Delta_{\text{f}}}H_{ 2 9 8}^\circ$$ ) of guanidine and its 10 amino and nitro derivatives were calculated using the isodesmic reaction method at the Gaussian-4 level of theory. The enthalpies of sublimation ( $${\Delta_{\text{sub}}}H_{ 2 9 8}^\circ$$ ) were estimated in the framework of the Politzer approach that combines the empirical equation for enthalpy of sublimation with the B3LYP/cc-pVTZ calculations of the electronic properties of the molecular surfaces. The enthalpies of sublimation of mono-, di-, and triaminoguanidine were also estimated using experimental data for their salts. On the basis of the calculated $${\Delta_{\text{f}}}H_{ 2 9 8}^\circ$$ (g) and $${\Delta_{\text{sub}}}H_{ 2 9 8}^\circ$$ values, the solid-phase enthalpies of formation were estimated for all guanidine derivatives. A predictive model confirms the available experimental data for guanidine, nitroguanidine, and some of their derivatives. The calculated value of solid-phase enthalpy of formation of high-nitrogen energetic compound 3,6-bis(2-nitroguanidino)-1,2,4,5-tetrazine is also in reasonable agreement with the reported experimental values.

Density functional theory calculations at the B3LYP/6-31G** theoretical level were performed for two series of guanidine-fused bicyclic skeleton derivatives (B-B7 and C-C7). The heats of formation (HOFs) were calculated via isodesmic reaction; the detonation properties were evaluated using the Kamlet–Jacobs equations; the bond dissociation energies were also analyzed to investigate the thermal stability of the cyclic nitramines. The results show that all the derivatives have high positive HOFs; compounds containing –N3 groupshave the highest HOFs, while compounds containing –NF2 groups have the highest density and detonation properties. Taking both of the detonation properties and thermal stabilities into consideration, compounds B1, B2, B5, C3, and C5 can be considered as the potential candidate of high-energy density compounds.

The mass spectral fragmentation of different 1-chloroalkanes (of the 1-chlorohexane-1-chlorooctadecane series) has been investigated, quantifying the relative abundance of the fragment ions. The base peak is dominantly at m/z 91, 93 in each investigated case, although with the increasing chain length, its contribution to the total ion current exhibits some reduction. Among the possible fragmentation products, the five-membered chloronium containing ring is the most stable as measured by an isodesmic reaction, although the six-and seven-membered rings exhibit only slightly reduced stability. The most stable structure of the 1-chlorohexane radical cation has a hydrogen bonded structure with the involvement of chlorine and the HC(δ), pre-forming the five-membered cationic ring. Accordingly, among the reactions leading to alkyl (or H) radical and a chloronium containing ring, this transition structure has the lowest energy, providing explanation for the experimental observations.

The optimized geometries, molecular properties, and stabilities of new noble gas molecules, XXeOH (X = F, Cl, Br, and I), were studied using CCSD, MP2, CAM-B3LYP, and WB97XD methods and large basis sets. All XXeOH molecules showed equilibrium structures with Cs symmetry. The results also showed that some bonds in XXeOH could be presented as a typical ionic bond. An alteration in ion-pair character was observed for IXeOH, showing two OH − and IXe + parts, while in other molecules, they could be presented as XeOH + and X − . Two decomposition routes were proposed for these molecules that showed high exothermic reactions. However, despite their low thermodynamic stabilities, their decomposition rate constants were small and all molecules (except BrXeOH) had high kinetic stabilities, indicating the possibility for identification and characterization of these molecules. However, in addition to the calculation of their vibrational frequencies, NBO atomic charges, and hybridizations, the bonding properties of XXeOH molecules were studied by AIM calculations (to calculate electron densities, bond elipticities, and Laplacian of electron densities) and second-order intramolecular perturbation energies using NBO calculations. Moreover, the ease of formations and relative stabilities of XXeOH molecules were compared using heats of formations, Gibbs free energies of formations and isodesmic reactions. These calculations showed that the stability of XXeOH molecules was decreased from F to I.

The mechanism, kinetics, and thermochemistry of the gas-phase reactions of CF2ClC(O)OCH2CH3,ethyl chlorodifluoroacetate (ECDFA) with the OH radical and Cl atom are investigated. Geometry optimization and frequency calculations have been performed at the MPWB1K/6-31+G(d,p) level of theory and energetic information is refined by using G2(MP2) theory. Transition states are searched on the potential energy surface of reaction channels and each of the transition states is characterized by the presence of only one imaginary frequency. Connections of the transition states between designated local minima are confirmed by intrinsic reaction coordinate calculation. Theoretically calculated rate constants at 298 K using the Canonical Transition State Theory are found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions as working chemical reactions, the standard enthalpies of formation for CF2ClC(O)OCH2CH3, CF2ClC(O)OCH2CH2, and CF3C(O)OCHCH3 are also reported for the first time. The hydrogen abstraction occurs mainly from –CH2 group. The T1 diagnostic calculation suggests that the multi-reference character is not an issue for such systems. The estimated atmospheric life time of ECDFA is expected to be around 24 days.

The energy of intramolecular hydrogen bond (IMHB) is a central subject in chemistry and biochemistry. In contrast with the IMHBs, there is no general accepted procedure to determine the IMHB energy. In the present study, for the first time, we applied all the different adopted models for assessing the energy of IMHB of O–H···O and O–H···S in simple resonance-assisted hydrogen bond systems (the cis enol form of malonaldehyde, thiomalonaldehyde, and a variety of halogenated derivatives), and compared them. The energy of IMHB, by various methods such as related rotamers method (RRM), rotational barrier method (RBM), conformational analysis method (CAM), isodesmic reaction method (IRM), and open–close method (OCM), was estimated. Exploring and comparing the correlations between the IMHB energies with various descriptors of hydrogen bond strength, such as geometrical, topological, molecular orbital, and spectroscopic parameters, were carried out. According to the theoretical results, we found that both RRM and RBM have the best linear correlations with all of the hydrogen bond descriptors (R ≥ 0.90) while the results of other methods (CAM, IRM and OCM) are not suitable (R ≤ 0.80). Surprisingly, we found that the OCM, which has been widely applied in the estimation of the IMHB energy, has the weakest linear dependent with all of the HB descriptors. Consequently, according to the regression coefficients, the order of linearity of these methods is as follows: RRM > RBM > > CAM > IRM > OCM.

The conventional strain energies of 1,2-dihydroazete, 2,3-dihydroazete, 1,2-dihydrophosphete, and 2,3-dihydrophosphete are determined within the isodesmic, homodesmotic, and hyperhomodesmotic models. Optimum equilibrium geometries, harmonic vibrational frequencies, and corresponding electronic energies and zero-point vibrational energies are computed for all pertinent molecular systems using SCF theory, second-order perturbation theory, and density functional theory and employing the correlation consistent basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ. Single-point fourth-order perturbation theory, CCSD, and CCSD(T) calculations employing the cc-pVTZ and the cc-pVQZ basis sets are computed using the MP2/cc-pVTZ and MP2/cc-pVQZ optimized geometries, respectfully, to ascertain the contribution of higher order correlation. Three DFT functionals, B3LYP, wB97XD, and M06-2X, are employed to determine whether they can yield results similar to those obtained at the CCSD(T) level.

The two-dimensional potential energy scan shows that the pseudo-trans conformer of 1,5-dinitrobiuret (DNB) is the most stable form of isolated molecule, while the pseudo-cis conformer is about 7.5 kJ/mol higher in energy. Thus, the structure of gaseous DNB is different from that in crystal state, where the molecules have pseudo-cis conformation. The value of enthalpy of formation of gaseous DNB (−257 ± 5 kJ/mol) is calculated from isodesmic reactions using G4 energies. Combining this value with empirically estimated enthalpy of sublimation, the enthalpy of formation of crystal DNB is predicted to be −415 ± 15 kJ/mol. The bond dissociation enthalpies are calculated for all bonds. The energy of the weakest N–NO2 bonds is equal to 190–200 kJ/mol. Similar calculations were carried out for biuret. The gaseous biuret exists predominantly in the pseudo-trans form. The calculated enthalpy of formation of gaseous biuret agrees well with the experimental one. The correlation of calculated bond energies with corresponding bond distances and electron density is discussed for biuret and DNB.

The heats of formation (HOFs) for a series of furazan-based energetic materials were calculated by density functional theory. The isodesmic reaction method was employed to estimate the HOFs. The result shows that the introductions of azo and azoxy groups can increase the HOF, but the introduction of azo group can increase the more HOF, when compared with azoxy group. The detonation velocities and detonation pressures of the furazan-based energetic materials are further evaluated at B3LYP/6-31G* level. Dioxoazotetrafurazan and azoxytetrafurazan may be regarded as the potential candidates of high-energy density materials because of good detonation performance. In addition, there are good linear correlations between OB and detonation velocities, and OB and detonation pressures. The energy gaps between the HOMO and LUMO of the studied compounds are also investigated. These results provide basic information for the molecular design of novel high-energy density materials.

In this work, the experimental synthesized bipyridines azo-bis(2-pyridine),4,4′-dimethyl-3,3′-dinitro-2,2′-azobipyridine, and N,N′-bis(3-nitro-2-pyridinyl)-methane-diamine and a set of designed bipyridines that have similar frameworks but different linkages and substituents were studied theoretically at the B3LYP/6-31G* level of density functional theory. The gas-phase heats of formation were predicted based on the isodesmic reactions, and the condensed-phase heats of formation and heats of sublimation were estimated in the framework of the Politzer approach. The crystal densities have been computed from molecular packing and results show that incorporation of –N=N–, –N=N(O)–, –CH=N–, and –NH–NH– into bipyridines is more favorable than –CH=CH– and –NH–CH2–NH– for increasing the density. The predicted detonation velocities (D) and detonation pressures (P) indicate that –NH2, –NO2, and –NF2 can enhance the detonation performance, and –NO2 and –NF2 are more favorable. Introducing –N=N–, –N=N(O)–, and –NH–NH– bridge groups into bipyridines is also favorable for improving their detonation performance. The oxidation of pyridine N always but that of –N=N– bridge does not always improve the detonation properties. E4–O, the derivative with –N=N– bridge and two –NF2 substituent groups, has the largest D (9.90 km/s) and P (47.47 GPa). An analysis of the bond dissociation energies shows that all derivatives have good thermal stability.

Density functional theory calculations were performed to find the relationships between the structures and performance of a series of 1,2,4,5-tetrazine-based energetic derivatives. The isodesmic reaction method was employed to estimate the heats of formation (HOFs). The result shows that the azo or azoxy group is one of the most energetic functional groups known and its substitution can drastically increase HOFs of a molecule. The detonation properties were also evaluated by the Kamlet–Jacobs equations based on the theoretical densities and HOFs. Results show that NO2 group is an effective substituent for enhancing the detonation performance. There exist better correlations between OB and detonation velocities and OB and detonation pressures. The energy gaps between the HOMO and LUMO of the studied compounds are also investigated, and from the data we estimated the relative thermal stability ordering of the title compounds.

Based on the full optimized molecular geometric structures at B3LYP/6-31G**, B3LYP/6-31+G**, B3P86/6-31G**, and B3P86/6-31+G** levels, the densities (ρ), detonation velocities (D), and pressures (P) for a series of toluene derivatives, as well as their thermal stabilities, were investigated to look for high energy density compounds (HEDCs). The heats of formation (HOFs) are also calculated via designed isodesmic reactions. The calculations on the bond dissociation energies (BDEs) indicate that the BDEs of the initial scission step are between 48 and 59 kcal/mol, and pentanitrotoluene is the most reactive compound, while 2,4,6-trinitrotoluene is the least reactive compound for toluene derivatives studied. A good linear relationship between BDE/E and impact sensitivity is also obtained. The condensed phase HOFs are calculated for the title compounds. These results would provide basic information for the further studies of HEDCs. The detonation data of pentanitrotoluene show that it meets the requirement for HEDCs.

Gas-phase enthalpies of formation of 122 relatively large organic molecules with up to 15 non-hydrogen atoms have been calculated at the Gaussian-4 (G4) level of theory using the atomization reaction procedure. The calculated values were compared with experimental data published mainly last years. Particular attention has been given to nitro compounds and nitrogen, oxygen, and sulfur containing heterocyclic compounds. The expected accumulation of systematic errors as the molecular size increases was not observed with increasing the number of non-hydrogen atoms from 6 to 15. The largest mean absolute deviation between experimental and G4 enthalpies of formation, 10.7 kJ/mol, was revealed for nitro compounds. All theoretical values for nitro compounds were underestimated by 5–15 kJ/mol. The best agreement with experiment with mean absolute deviation of 4.5 kJ/mol was observed for compounds which types were widely presented in the original test set of G4 method. The mean absolute deviations for nitrogen heterocycles (6.8 kJ/mol) and oxygen and sulfur heterocycles (9.1 kJ/mol) are noticeably larger. Experimental enthalpies of formation of four compounds (N,N-dinitromethanamine, 2,3,5,6-tetrachloronitrobenzene, 2-methyl-2H-tetrazole, and proline) were suggested to be unreliable from comparison with the G4 values calculated from atomization energies and isodesmic reactions.

Quantum chemical calculations employing the MP2/aug-cc-pVDZ model chemistry have been used to investigate the structure of mono- and diprotonated forms of cyclohexyldiamine (CHDA) isomers. The strength of the intramolecular hydrogen bonds and ion–dipole interactions between the amino groups, which play a key role in the conformational behaviour of the monoprotonated forms, is estimated by isodesmic reactions and electrostatic calculations. The energy variations found for the diprotonated forms are explained by the electrostatic repulsions between the amino groups. From the Gibbs energies and enthalpies calculated for the neutral and protonated CHDAs, the gas-phase basicities and proton affinities of the neutral and monoprotonated forms were determined and discussed.

The tetrazole is an important functionality of the most of energetic materials due to 80% nitrogen content, stability, and high enthalpy of formation. The present structure–property relationship study focuses on the optimized geometries of tetrazole derivatives obtained from density functional theory (DFT) calculations at B3LYP/6-31G* levels. The heat of formation (HOF) of tetrazole derivatives have been calculated by designing the appropriate isodesmic reactions. The increase in nitro groups on azole rings shows the remarkable increase in HOF. Density has been predicted by using CVFF force field. Increase in the nitro group increases the density. Detonation properties of the designed compounds were evaluated by using the Kamlet–Jacobs equation based on predicted densities and HOFs. Designed tetrazole derivatives show detonation velocity (D) over 8 km/s and detonation pressure (P) of about 32 GPa. Thermal stability was evaluated via bond dissociation energies (BDE) of the weakest C–NO2 bond at B3LYP/6-31G* level. Charge on the nitro group has been used to assess the sensitivity correlation. Overall, the study implies that designed compounds of this series are found to be stable and expected to be the novel candidates of high energy materials (HEMs).

Based on the full-optimized molecular geometric structures at B3LYP/6-31G* and B3P86/6-31G* levels, the densities (ρ), detonation velocities (D), and pressures (P) for a series of 1,2,3-triazole derivatives, as well as their thermal stabilities, were investigated to look for high energy density compounds (HEDCs). The heats of formation (HOFs) are also calculated via designed isodesmic reactions. The calculations on the bond dissociation energies (BDEs) indicate that the BDEs of the initial scission step are between 53 and 70 kcal/mol, and 4-nitro-1,2,3-triazole is the most reactive compound, while 1-(2′,4′-dinitrophenyl)-5-nitro-1,2,3-triazole is the least reactive compound for 1,2,3-triazole derivatives studied. The condensed phase heats of formation are also calculated for the title compounds. These results would provide basic information for the further studies of HEDCs. The detonation data of 1-(3′,4′-dinitrophenyl)-4-nitro-1,2,3-triazole and 1-(2′,4′-dinitrophenyl)-4-nitro-1,2,3-triazole show that they meet the requirement for HEDCs.

Density functional theory calculations were performed to study the effects of different substituents and nitrogen-containing heterocyclic rings on the heats of formation (HOFs), energetic properties, and thermal stability for a series of furoxan derivatives. The isodesmic reaction method was employed to calculate the HOFs of the derivatives using total energies obtained from electronic structure calculations. The detonation velocities and pressures were evaluated by using the semiempirical Kamlet–Jacobs equations, based on the theoretical densities and HOFs. The bond dissociation energies and bond orders for the weakest bonds were analyzed to investigate the thermal stability of the furoxan derivatives. These results provide basic information for the molecular design of novel high energy density materials.

The structure and energetics of hyaluronan oligomers from di- to decasaccharides have been studied by density functional theory calculations at the B3LYP/6-31G** level. The study covered selected conformers of the sodium salt, anionic disaccharides, and neutral acids in the isolated state and in aqueous solution using the PCM model approach. We investigated the structural changes of the hyaluronan chain when the Na+ ion is removed or replaced by proton. These processes result in some characteristic changes in the glycosidic torsional angles and hydrogen bonding interactions. We evaluated the folding for the hyaluronan chains and obtained values between 2.2 and 3.2, somewhat smaller than reported for the crystals. We found a contraction of the hyaluronan chains upon enlargement in most derivatives attributed partly to the helical character of hyaluronan. The energy consequences of the enlargement have been modeled by isodesmic reactions. The enlargement processes proved to be exothermal and the energies consistent within the gradual enlargement.

We have predicted the optimized geometries, infrared spectrum, and thermodynamic properties for six tetrazole derivates at density functional theory and second-order Moller–Plesset perturbation theory (MP2) level with the 6-31G** basis set. Their heats of formation were calculated accurately using the designed isodesmic and isogyric reactions. The computed total energies, heat of formation, and enthalpy of combustion consistently indicate the stability: 5-(2,4,6-trinitrophenyl)-2H-tetrazole > 5-(2,4,6-trinitrophenyl)-1H-tetrazole > 5-(2,4,6-trinitrophenyl)-5H-tetrazole The similar result for the isomers of 5-(2,4,6-trinitrobenzyl)-tetrazole: 5-(2,4,6-trinitrophenyl)-2H-tetrazole > 5-(2,4,6-trinitrophenyl)-1H-tetrazole > 5-(2,4,6-trinitrophenyl)-5H-tetrazole.

The enthalpies of formation of chlorinated methanes, ethanes, ethylenes, phenols, and benzenes have been calculated at the G3X level of theory using the atomization energy procedure and the method of isodesmic reactions. By comparing the most reliable experimental data on chlorinated hydrocarbons recommended by Manion [Manion JA (2002) J Phys Chem Ref Data 31:123] with the G3X results, the accuracy of theoretical enthalpies of formation is estimated as ranging from ±4 to ±10 kJ/mol. Only for hexachloroethane, the difference between the experimental value and G3X result was outside this range and the experimental enthalpy of formation of hexachloroethane was called into question by theory. The G3X enthalpies of formation of all chlorobenzenes agree well with experimental data which were partly reanalyzed using recent experimental data on enthalpies of sublimation. Based on the G3X results, a set of self-consistent experimental data for chlorobenzenes is recommended. The enthalpies of formation of some polychlorinated dibenzo-p-dioxins were estimated using improved enthalpies of formation for chlorobenzenes. The possible inaccuracy of previously estimated values for polychlorinated aromatic compounds is discussed.

Polyynes and cumulenes from 2-12 atoms have been calculated at the B3LYP/6-311++G** level and their energies compared using an isodesmic reaction. The chain length has been modeled empirically affording an equation that predicts substantial variation for long chains. © 2005 Springer Science+Business Media, Inc.

An ab initio SCF-MO approach has been used to compute the structures, bond orders, and relative and isodesmic reaction energies of two isomeric nitroazatriprismanes and a series of related smaller molecules, with the aim of investigating the conformational preferences and bond strength effects of the NO2 in these systems. In fused or caged systems containing both three- and four-membered rings, the nitro prefers to orient itself perpendicular to the three-membered ring. In contrast to earlier results for highly strained caged hydrocarbons, no significant conformation-dependent bond weakening is found to be associated with the NO2 in single-ring (and fused) systems. The presence of an aza nitrogen adjacent to the site of NO2 substitution in the triprismane framework essentially eliminates the bond weakening observed in the parent nitrated hydrocarbon. Calculated isodesmic reaction energies reinforce our earlier finding that mononitration of secondary and tertiary carbons in strained hydrocarbons and then monoaza analogues is in general slightly stabilizing.

By means of the generic isodesmic reaction C6H5COX + RCH3 → C6H5COCH3 + RX, the phenyl/vinyl thermochemical equivalence, and an experimental determination of the enthalpy of the Curtius rearrangement of acryloyl azide, the resonance energy of acyl azides was ascertained to be ca. 29 kJ mol−1.

Isodesmic reactions involving methyl, phenyl, and vinyl species are shown to be accompanied by small entropy changes. In contrast, the “association≓ reaction of methane with vinyl species has a large entropy change that is quite well estimated by approximate translational entropy differences. The effect of neglecting multiple species on conformational equilibrium returns us to small but still predictable changes.