Gas-phase Enthalpies Of Formation Research Articles

The structure, thermodynamic instability and energetics of NI3, its specific impulse and a strategy for its stabilization

The present work is dedicated to deepen the causes of the physicochemical instability of NI3. As a support study, a theoretical investigation is performed. The quantum chemical calculations were performed by using SE(PM6), HF/6-31G* and DFT/M06-2X/6-311G** approach. The calculated gas phase formation enthalpy to NH3.NI3 is 91.75 kJmol−1. The sublimation enthalpy for the adduct is calculated as 237.75 kJmol−1. By structure calculations, it is shown that the stabilization of NI3 in the NH3.NI3 adduct is consequence of the I-N-I angle increase. The enthalpy for the reaction NH3.NI3 (s) → NH3(g) + NI3 (s) is calculated as 46 kJmol−1. Pure NH3:NI3 detonated only three minutes after dried at room conditions (humidity = 65%, temperature = 39 ºC). On the other hand, the NH3:NI3 polyvinylchloride resin, polyvinyl acetate resin or polysiloxane resin “entrapped” samples, do not detonate after 30 h. By using the empirical equation Is = 17.562 η + 125.551, where Is is the specific impulse (s) and η is the absolute chemical hardness (eV), the specific impulse for NH3.NI3 can be calculated as 185.52 s (HF/6-31G* data) and 163.75 s (DFT/M06-2X/6-311G** data). To NI3the Is calculated values are 201.68s and 168.75 s, respectively. The decrease in the specific impulse values from NI3 to NI3:NH3 is explained as consequence of the increase in the polarizability of the entire system.

In silico Design of Novel N-hydrosulfonylbenzamides inhibitors of dengue RNA-dependent RNA polymerase showing favorable predicted pharmacokinetic profiles

Background: In recent years, there has been a growing interest in Denv NS5 inhibition, with several reported RdRp inhibitors such as sulfonylbenzamides, non-nucleo-side inhibitors without any 3D-QSAR pharmacophore (PH4) available. In this context, we report here, in silico design and virtual evaluation of novel sulfonylbenzamides Denv RdRp inhibitors with favorable predicted pharmacokinetic profile. Methods: By using in situ modifications of the crystal structure of 5-(5-(3-hydroxyprop-1-yn-1-yl)thiophen-2-yl)-4- methoxy-2-methyl-N-(methylsulfonyl) benzamide (EHB)-RdRp complex (PDB entry 5HMZ), 3D models of RdRp-EHBx complexes were prepared for a training set of 18 EHBs with experimentally determined inhibitory potencies (half-maximal inhibitory concentrations IC50exp). In the search for active conformation of the EHB1-18, linear QSAR model was prepared, which correlated computed gas phase enthalpies of formation ∆∆HMM of RdRp-EHBx complexes with the IC50exp. Further, considering the solvent effect and entropy changes upon ligand binding resulted in a superior QSAR model correlating computed complexation Gibbs free energies (∆∆Gcom). The successive pharmacophore model (PH4) generated from the active conformations of EHBs served as a virtual screening tool of novel analogs included in a virtual combinatorial library (VCL) of compounds with scaffolds restricted to phenyl. The VCL filtered by the Lipinski’s rule-of-five was screened by the PH4 model to identify new EHB analogs. Results: Gas phase QSAR model: -log10(IC50exp) = p IC50exp =-0.1403 x ∆∆HMM _ 7.0879, R2 = 0.73; superior aqueous phase QSAR model: p IC50exp = -0.2036 x ∆∆Gcom + 7.4974, R2 = 0.81 and PH4 pharmacophore model: p IC50exp = 1.0001 x p IC50pre -0.0017, R2 = 0.97. The VCL of more than 30 million EHBs was filtered down to 125,915 analogs Lipinski’s rule. The five-point PH4 screening retained 329 new and potent EHBs with predicted inhibitory potencies p IC50pre up to 30 times lower than that of EHB1 (IC50exp = 23nM). Predicted pharmacokinetic profile of the new analogs showed enhanced cell membrane permeability and high human oral absorption compared to the alone drug to treat dengue virus. Conclusions: Combined use of QSAR models, which considered binding of the EHBs to RdRp, pharmacophore model and ADME properties helped to recognize bound active conformation of the sulfonylbenzamide inhibitors, permitted in silico screening of VCL of compounds sharing sulfonylbenzamide scaffold and identify new analogs with predicted high inhibitory potencies and favorable pharmacokinetic profiles. Keywords: ADME properties prediction, Dengue, 3-(5-ethynylthiophen-2-yl)-N-hydrosulfonylbenzamides, in silico screening, RNA-dependent RNA polymerase.

Nitro-, Cyano-, and Methylfuroxans, and Their Bis-Derivatives: From Green Primary to Melt-Cast Explosives.

In the present work, we studied in detail the thermochemistry, thermal stability, mechanical sensitivity, and detonation performance for 20 nitro-, cyano-, and methyl derivatives of 1,2,5-oxadiazole-2-oxide (furoxan), along with their bis-derivatives. For all species studied, we also determined the reliable values of the gas-phase formation enthalpies using highly accurate multilevel procedures W2-F12 and/or W1-F12 in conjunction with the atomization energy approach and isodesmic reactions with the domain-based local pair natural orbital (DLPNO) modifications of the coupled-cluster techniques. Apart from this, we proposed reliable benchmark values of the formation enthalpies of furoxan and a number of its (azo)bis-derivatives. Additionally, we reported the previously unknown crystal structure of 3-cyano-4-nitrofuroxan. Among the monocyclic compounds, 3-nitro-4-cyclopropyl and dicyano derivatives of furoxan outperformed trinitrotoluene, a benchmark melt-cast explosive, exhibited decent thermal stability (decomposition temperature >200 °C) and insensitivity to mechanical stimuli while having notable volatility and low melting points. In turn, 4,4′-azobis-dicarbamoyl furoxan is proposed as a substitute of pentaerythritol tetranitrate, a benchmark brisant high explosive. Finally, the application prospects of 3,3′-azobis-dinitro furoxan, one of the most powerful energetic materials synthesized up to date, are limited due to the tremendously high mechanical sensitivity of this compound. Overall, the investigated derivatives of furoxan comprise multipurpose green energetic materials, including primary, secondary, melt-cast, low-sensitive explosives, and an energetic liquid.

Multireaction Approach to Quantum Thermochemistry.

Gas-phase enthalpies of formation are frequently obtained from electronic-structure calculations. For a compound containing elements that are not permanent gases, experimental data are needed to connect the ab initio energies with the elements in their standard states. Usually only one chemical reaction, such as atomization, is used to make this connection. Using a single reaction has the supposed advantage of producing a single value, but it remains challenging to estimate the associated uncertainty. Here, we propose using several chemical reactions instead of one. This yields several values for the target enthalpy of formation. The relative reliability of each reaction is estimated and used for weighted averaging of the corresponding enthalpies of formation. The resulting weighted mean is an estimate for the target quantity, and the standard deviation of the weighted mean is a reasonable estimate for the associated standard uncertainty. Two implementations are explored here and applied predictively to dihydrolevoglucosenone (cyrene), which has been proposed as a "green" polar aprotic solvent.

Structural and Energetic Insights on Two Dye Compounds: 1-Acetyl-2-Naphthol and 2-Acetyl-1-Naphthol.

The energy involved in the structural switching of acyl and hydroxyl substituents in the title compounds was evaluated combining experimental and computational studies. Combustion calorimetry and Knudsen effusion techniques were used to determine the enthalpies of formation, in the crystalline state, and of sublimation, respectively. The gas-phase enthalpy of formation of both isomers was derived combining these two experimental data. Concerning the computational study, the G3(MP2)//B3LYP composite method was used to optimize and determine the energy of the isomers in the gaseous state. From a set of hypothetical reactions it has been possible to estimate the gas-phase enthalpy of formation of the title compounds. The good agreement between the experimental and computational gas-phase enthalpies of formation of the 1-acetyl-2-naphthol and 2-acetyl-1-naphthol isomers, provided the confidence for extending the computational study to the 2-acetyl-3-naphthol isomer. The structural rearrangement of the substituents in position 1 and 2 in the naphthalene ring and the energy of the intramolecular hydrogen bond are the factors responsible for the energetic differences exhibited by the isomers. The gas phase tautomeric keto ↔ enol equilibria of the o-acetylnaphthol isomers were analyzed using the Boltzmann’s distribution.

Thermal Stability of Bis-Tetrazole and Bis-Triazole Derivatives with Long Catenated Nitrogen Chains: Quantitative Insights from High-Level Quantum Chemical Calculations.

Azobis tetrazole and triazole derivatives containing long catenated nitrogen atom chains are of great interest as promising green energetic materials. However, these compounds often exhibit poor thermal stability and high impact sensitivity. Kinetics and mechanism of the primary decomposition reactions are directly related to these issues. In the present work, with the aid of highly accurate CCSD(T)-F12 quantum chemical calculations, we obtained reliable bond dissociation energies and activation barriers of thermolysis reactions for a number of N-rich heterocycles. We studied all existing 1,1'-azobistetrazoles containing an N10 chain, their counterparts with the 5,5'-bridging pattern, and the species with hydrazo- and azoxy-bridges, which are often present energetic moieties. The N8-containing azobistriazole was considered as well. For all compounds studied, the radical decomposition channel was found to be kinetically unfavorable. All species decompose via the ring-opening reaction yielding a transient azide (or diazo) intermediate followed by the N2 elimination. In the case of azobistetrazole derivatives, the calculated effective activation barriers of decomposition are ∼26-33 kcal mol-1, which is notably lower than that of tetrazole (∼40 kcal mol-1). This fact agrees well with the low thermal stability and high impact sensitivities of the former species. The activation barriers of the N2 elimination were found to be almost the same for the azobis compounds and the parent tetrazole, and the effective decomposition barrier is determined by the thermodynamics of the tetrazole-azide rearrangement. In comparison with 1,1'-azobistetrazole, the hydrazo-bridged compound is more stable kinetically due to the lack of pi-conjugation in the azide intermediate. In turn, the azoxy-bridged compounds are entirely unstable due to tremendous azide stabilization by the hydrogen bond formation. In general, the 5,5'-bridged species are more thermally stable than their 1,1'-counterparts due to a much higher barrier of the N2 elimination. Apart from this, the highly accurate gas-phase formation enthalpies were calculated at the W1-F12 level of theory for all species studied.

Experimental and computational thermochemical study of dimethoxyacetophenones

The standard (po = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, for 2′,4′-, 2′,5′-, 2′,6′- and 3′,4′-dimethoxyacetophenones, in the condensed phase, were derived from measurements of the standard massic energies of combustion using a static bomb combustion calorimeter. The standard molar enthalpies of sublimation or vaporization, at T = 298.15 K, were determined by high temperature Calvet microcalorimetry. Combining the results measured by these two techniques, the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived.Computational studies for the experimentally studied compounds were also performed, as a complement to the experimental work, being also extended to the 2′,3′- and 3′,5′-dimethoxyacetophenone isomers. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3 level.Group additivity values and corrections for functional group interactions in the benzene ring were derived and used for predicting enthalpies of formation for dimethoxyacetophenones. The applicability of simple group-additivity procedures to tri-substituted benzenes was discussed.

Bond dissociation energies of the diatomic late transition metal sulfides: RuS, OsS, CoS, RhS, IrS, and PtS.

The spectra of RuS, OsS, CoS, RhS, IrS, and PtS have been recorded near their respective bond dissociation energies using resonant two-photon ionization spectroscopy. The spectra display an abrupt drop to baseline when the bond dissociation energy (BDE) is exceeded. It is argued that spin-orbit and nonadiabatic interactions among the myriad of states that result from the ground and low-lying separated atom limits cause the molecules to predissociate rapidly as soon as the ground separated atom limit is exceeded in energy. Thus, the observed sharp predissociation thresholds are assigned as the 0 K BDEs of the molecules. With this assumption, the BDEs are assigned as follows: 4.071(8) eV (RuS), 4.277(3) eV (OsS), 3.467(5) eV (CoS), 3.611(3) eV (RhS), 4.110(3) eV (IrS), and 4.144(8) eV (PtS). Using thermochemical cycles, the gas-phase enthalpies of formation at 0 K, ΔfH0 K°, were calculated to be 531.8(4.3) kJ mol-1 (RuS), 651.2(6.3) kJ mol-1 (OsS), 365.3(2.2) kJ mol-1 (CoS), 481.5(2.1) kJ mol-1 (RhS), 546.7(6.3) kJ mol-1 (IrS), and 438.9(1.5) kJ mol-1 (PtS). The ionization energies of RuS, CoS, and RhS were also calculated using data on the BDEs of the associated cations and were found to be 8.39(10) eV (RuS), 8.40(9) eV (CoS), and 8.46(12) eV (RhS). Combining these data with predissociation measurements of other transition metal sulfide BDEs, the periodic trends in the transition metal sulfide BDEs are discussed and the BDEs of the transition metal sulfides are compared to those of the corresponding selenides. The BDEs of the sulfides are found to be 15.4% greater than those of the corresponding sulfides.

Thermochemistry of phthalic acids: Evaluation of thermochemical data with complementary experimental and computational methods

Available from literature primary experimental results on temperature dependences of vapour pressures of phthalic, isophthalic, and terephthalic acids have been collected and they have been treated uniformly in order to derive sublimation enthalpies at the reference temperature 298.15 K. In order to evaluate the available experimental data, enthalpies of sublimation of isophthalic and terephthalic acid were obtained from the temperature dependence of vapour pressure measured by the transpiration method. Enthalpies of formation of crystalline isophthalic acid and 4-methylphthalic acid were derived by using combustion calorimetry. Gas phase enthalpies of formation of phthalic acids were calculated by the composite quantum-chemical methods G4 and G3MP2. These values were used for evaluation of experimental results and recommendation of reliable benchmark properties for thermochemical calculations. Mutual interactions of carboxylic substituents in the benzene ring were derived and discussed.

Self-Consistent Component Increment Theory for Predicting Enthalpy of Formation.

The gas-phase enthalpy of formation (ΔHf) plays a fundamental role in predicting reaction thermodynamics and constructing kinetic models. With advances in computational power and method development, chemically accurate quantum chemistry methods that can predict ΔHf values for small molecules are available; however, large molecules are still out of reach. Increment theories provide a means of extending the prediction capability of high-level methods by decomposing the molecular ΔHf into the additive contributions from individual atoms, bonds, groups, or components. Here, we introduce a novel component increment theory, topology-automated force-field interaction component increment theory (TCIT), in which all component contributions are derived exclusively from Gaussian-4 (G4) results for algorithmically generated model compounds. In a benchmark evaluation of noncyclic compounds from the Pedley, Naylor, and Kline experimental ΔHf dataset, TCIT exhibits consistently lower signed and absolute errors compared with the conventional Benson group increment theory (BGIT). These results pave the way for future extensions of TCIT to ring-containing, ionic, and radical species for which experimental data scarcity currently limits the application of BGIT.

Cyclic aliphatic amines: A critical analysis of the experimental enthalpies of formation by comparison with theoretical calculations

The accuracy of experimental data on enthalpies of formation and vaporization of cyclic aliphatic amines was assessed by theoretical calculations. The gas-phase enthalpies of formation were calculated using the Gaussian-4 (G4) method combined with isodesmic reactions. The enthalpies of vaporization were estimated by group additivity and molecular electrostatic potential models and then were used to calculate the enthalpies of formation in the liquid state whenever experimental results were unavailable or unreliable. Evidence of experimental errors was derived taking into account the discrepancies between calculated and experimental enthalpies of formation which were significantly larger than expected from the computational method. Special attention was made to the possible inaccuracy in the experimental data for piperazine, which is the key compound in the thermochemistry of nitrogen heterocycles.

Accurate Thermochemistry of Novel Energetic Fused Tricyclic 1,2,3,4-Tetrazine Nitro Derivatives from Local Coupled Cluster Methods.

Highly accurate theoretical values of formation enthalpies and bond energies are crucial for reliable predictions of performance and detonation-related phenomena of energetic materials (EM). However, high-level ab initio calculations even for medium-sized important EMs still remain a demanding challenge. In the present work, we studied in detail the gas-phase thermochemistry of novel high-energy polynitro derivatives of 5/6/5 structural frameworks comprised of fused 1,2,3,4,-tetrazine and two 1,2,4-triazole or pyrazole rings. To this end, we proposed and benchmarked a "bottom-up" approach. First, highly accurate multilevel procedures W2-F12 and/or W1-F12 in conjunction with the atomization energy approach were utilized for smaller species. In turn, for medium-sized species (up to 24 non-H atoms), these values were complemented with the enthalpies of isodesmic reactions calculated using the recently proposed domain-based local pair natural orbital (DLPNO) modifications of coupled cluster techniques. The benchmarks on a number of atomization energies and enthalpies of isodesmic reactions reveal that the DLPNO-CCSD(T)/aVQZ approach does not deteriorate the quality of the W1-F12 and W2-F12 procedures and exhibits overall accuracy close to "chemical" (∼1 kcal mol-1). We obtained a set of accurate and mutually consistent gas-phase formation enthalpies for 12 energetic heterocyclic species. Among them, the gas-phase formation enthalpy of 1,2,9,10-tetranitrodipyrazolo[1,5-d:5',1'-f][1,2,3,4]tetrazine, a novel promising EM, turned out to be ΔfHgas0 = 214.5 kcal mol-1, which is ∼12 kcal mol-1 higher than the best theoretical estimates available in the literature. The formation enthalpy of another novel EM, a fused tricyclic 1,2,3,4-tetrazine with two nitro-1,2,4-triazole moieties, was predicted to be ΔfHgas0 = 213.5 kcal mol-1, which is also ∼4 kcal mol-1 higher than the reported value. Apart from this, we considered the thermodynamics of radical reactions (viz., C-NO2 bond scission) and the thermochemistry of the corresponding radicals. The difference between DLPNO-CCSD(T)/aVQZ and CCSD(T)-F12/VTZ-F12 benchmark values did not exceed 1 kcal mol-1. In a more general sense, the use of DLPNO-CCSD(T) in conjunction with the bottom-up approach is promising for quantitative thermochemical calculations of energetic materials composed of species up to several dozens of CHNO atoms.

Structure-Based Design and in Silico Screening of Virtual Combinatorial Library of Benzamides Inhibiting 2-trans Enoyl-Acyl Carrier Protein Reductase of Mycobacterium tuberculosis with Favorable Predicted Pharmacokinetic Profiles.

Background: During the previous decade a new class of benzamide-based inhibitors of 2-trans enoyl-acyl carrier protein reductase (InhA) of Mycobacterium tuberculosis (Mt) with unusual binding mode have emerged. Here we report in silico design and evaluation of novel benzamide InhA-Mt inhibitors with favorable predicted pharmacokinetic profiles. Methods: By using in situ modifications of the crystal structure of N-benzyl-4-((heteroaryl)methyl) benzamide (BHMB)-InhA complex (PDB entry 4QXM), 3D models of InhA-BHMBx complexes were prepared for a training set of 19 BHMBs with experimentally determined inhibitory potencies (half-maximal inhibitory concentrations IC50exp). In the search for active conformation of the BHMB1-19, linear QSAR model was prepared, which correlated computed gas phase enthalpies of formation (∆∆HMM) of InhA-BHMBx complexes with the IC50exp. Further, taking into account the solvent effect and entropy changes upon ligand, binding resulted in a superior QSAR model correlating computed complexation Gibbs free energies (∆∆Gcom). The successive pharmacophore model (PH4) generated from the active conformations of BHMBs served as a virtual screening tool of novel analogs included in a virtual combinatorial library (VCL) of compounds containing benzamide scaffolds. The VCL filtered by Lipinski’s rule-of-five was screened by the PH4 model to identify new BHMB analogs. Results: Gas phase QSAR model: −log10(IC50exp) = pIC50exp = −0.2465 × ∆∆HMM + 7.95503, R2 = 0.94; superior aqueous phase QSAR model: pIC50exp = −0.2370 × ∆∆Gcom + 7.8783, R2 = 0.97 and PH4 pharmacophore model: = 1.0013 × p − 0.0085, R2 = 0.95. The VCL of more than 114 thousand BHMBs was filtered down to 73,565 analogs Lipinski’s rule. The five-point PH4 screening retained 90 new and potent BHMBs with predicted inhibitory potencies IC50pre up to 65 times lower than that of BHMB1 (IC50exp = 20 nM). Predicted pharmacokinetic profile of the new analogs showed enhanced cell membrane permeability and high human oral absorption compared to current anti-tuberculotics. Conclusions: Combined use of QSAR models that considered binding of the BHMBs to InhA, pharmacophore model, and ADME properties helped to recognize bound active conformation of the benzamide inhibitors, permitted in silico screening of VCL of compounds sharing benzamide scaffold and identification of new analogs with predicted high inhibitory potencies and favorable pharmacokinetic profiles.

Electron-induced ionization of undeuterated and deuterated benzoic acid isopropyl esters and nicotinic acid isopropyl esters: Some implications for the mechanism of the McLafferty rearrangement.

Electron ionization mass spectra, ionization, and appearance energies and bond energies (as dissociation energies) are reported for benzoic acid-1-methyl-ethyl ester (BAIPE), benzoic acid-1-deutero-1-methyl-ethyl ester (BAIPED1), benzoic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (BAIPED6) as well as nicotinic acid-1-methyl-ethyl ester (NAIPE), nicotinic acid-1-deutero-1-methyl-ethyl ester (NAIPED1), and nicotinic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (NAIPED6). Ionization energies of 9.39 eV for BAIPE, 9.40 eV for BAIPED1, 9.26 eV for BAIPED6 as well as 9.70 eV for NAIPE, 9.79 eV for NAIPED1, and 9.65 eV for NAIPED6 were determined. A gas-phase formation enthalpy of = (-4.10 ± 0.1) eV for BAIPE is calculated as well as = (-3.35 ± 0.1) eV for NAIPE. Molecular ions show two main fragmentation pathways. The first is a classical McLafferty rearrangement, characterized by the transfer of one γ-hydrogen atom from the isopropyl ester chain leading to the ions of the corresponding acid and neutral propene. The second is the double hydrogen transfer from the ester chain leading to the formation of the protonated acid and a C3H5√ allyl radical. For BAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of ≥98%, whereas the C-1-hydrogen is transferred with a probability of ≤2%. For NAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of 90%. Experimental proton affinities of PA = (8.75 ± 0.2) eV for benzoic acid and PA = (8.43 ± 0.2) eV for nicotinic acid are derived. For the protonation of the carbonyl group, B3LYP DFT calculations yielded PA = 8.66 eV for benzoic acid and PA = 8.41 eV for nicotinic acid. The overall fragmentation mechanism is explained with the initial formation of a 1,5-distonic ion by transfer of the first hydrogen. For the transfer of the second hydrogen, an intermediate ion/neutral complex is formulated.

Accurate Prediction of Bond Dissociation Energies and Barrier Heights for High-Energy Caged Nitro and Nitroamino Compounds Using a Coupled Cluster Theory.

Highly accurate theoretical values of bond energies and activation barriers of primary decomposition reactions are crucial for reliable predictions of thermal decomposition and detonation-related phenomena of energetic materials (EM). However, due to the prohibitive computational cost, high-level ab initio calculations had been impractical for a large number of important EMs, including, e.g., hexanitrohexaazaisowurtzitane (CL-20). In the present work, we obtained accurate bond dissociation energies and the activation barriers for primary decomposition reactions for a series of novel promising caged polynitroamino and polynitro EMs, viz., CL-20, TEX, octanitrocubane (ONC), and hexanitro derivatives of adamantane, using the recently proposed domain-localized pair natural orbitals (DLPNO) modifications of coupled cluster techniques. DLPNO-CCSD(T) allows for routine quadruple-ζ basis set quality coupled cluster calculations for the species comprised of ∼30 non-H atoms. The benchmarks on a number of simpler congeners of CL-20 and ONC revealed that the DLPNO approach does not deteriorate the quality of the quadruple-ζ coupled cluster procedure. With the aid of this technique, the full set of gas-phase primary decomposition reactions for all 9 conformers of CL-20 was considered. For all species studied, C-NO2 or N-NO2 radical decomposition channels dominate over molecular counterparts. The best theoretical results reported in the literature so far, viz., density functional theory energies of nitro group radical elimination in CL-20 and ONC, underestimate the value by ∼10 kcal mol-1. We also present reliable and accurate gas-phase formation enthalpies for CL-20, ONC, and related species. In a more general sense, these results offer a new level of predictive computational kinetics for polynitro-caged energetic materials.

Computational thermochemistry of oxatriquinane and its analogues

The standard gas phase enthalpies of formation, electron affinities, first ionization energies and partial atomic charges for oxatriquinane and some of its isoelectronic tricyclic analogues have been calculated using high-level, ab initio methods: G4, OVGF and CCSD. G4 method has been used to obtain thermochemical data, while OVGF and CCSD methods were used to calculate vertical ionization energies and electron affinities. We suggest that several unknown oxatriquinane analogues are synthetically accessible and of comparable thermodynamic stability to those synthesized recently and have predicted some of their properties.

Enthalpies of formation of diamantanes in the gas and crystalline phase: comparison of theory and experiment

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.

Theoretical Calculation of the Thermodynamic Properties of 20 Amino Acid Ionic Liquids.

The thermodynamic properties of gas-phase amino acid ionic liquids (AAILs) containing 20 amino acids ([AA]-) and 1-ethyl-3-methylimidazolium ([Emim]+) are studied using a combination of the ab initio method, molecular dynamics simulations, Born-Haber (BH) cycle analysis, and isodesmic reactions. The M06-2X/TZVP method is used to explore the structure and dissociation enthalpies of [Emim][AA] by considering dispersion interaction, and the MP2/Aug-cc-pVTZ method is used to correct these enthalpies. The vaporization enthalpies of all 20 AAILs are calculated by molecular dynamics simulations, and the gas-phase formation enthalpies (Δf H) of the 20 [AA]- anions and [Emim]+ cation are calculated by the density functional theory/M06-2X method and isodesmic reaction approaches. To obtain the Δf H of the AAILs, interconnections in the corresponding BH cycles are evaluated. A systematic study of the 20 [Emim][AA] ion pairs provides some initial factors contributing to the thermodynamic properties of AAILs: including length of the alkyl chain, interatomic electronic effects, steric repulsion from the cyclic group, and H-bonds formed by functional groups. Generally speaking, the results of this work provide insights into the structure-property relationships of not only ILs but also any ionic or molecular substance.

Thermochemical and structural studies of gallic and ellagic acids

We report a study on the energetics and structural properties of gallic (1) and ellagic (2) acids. The experimental values of standard enthalpy of formation in solid state at 298.15 K, ΔfHm0 (cd) of 1 as (−985.0 ± 2.9 kJ·mol−1) and 2 as (−1377.9 ± 4.7 kJ·mol−1) have been determined. The vapour pressure of 1 have been measure by Knudsen effusion methodology and the derived enthalpy of sublimation, ΔcdgHm0, was combined with the ΔfHm0 (cd) in order to derive its gas-phase enthalpy of formation, ΔfHm0(1,g) = −835.7 ± 4.0 kJ·mol−1. Quantum chemical calculations, at DFT (M05-2X) and composite ab initio Gn (n = 3, 4) levels of theory, provided the consistency of the experimental results and a plausible estimation of ΔfHm0 (g) of 2 as (−1128.6 ± 6.4 kJ·mol−1), which was deduced from the isodesmic-reactions methodology.

Theoretical study of bromine halocarbons: Accurate enthalpies of formation

The gas phase enthalpies of formation of sixty-five C1 and C2 bromine compounds have been calculated using G3, G4, the correlation consistent Composite Approach (ccCA) and CCSD(T)/CBS. Several compounds investigated have importance in atmospheric chemistry due to their global warming potentials. Compounds investigated include chlorine and fluorine containing bromine compounds, and bromine hydrocarbons. Computational methods have been compared to experimental and theoretical values when available. All methods investigated calculate enthalpies of formation that are in agreement with available ATcT, each with a greater than 0.999 R2 value and mean absolute deviations (MADs) of 1.2 kcal/mol, 0.6 kcal/mol, 0.7 kcal/mol, and 0.6 kcal/mol for G3, G4, ccCA, and CCSD(T), respectively. The importance of molecular spin-orbit corrections is noted. The molecular spin-orbit correction for tetrabromomethane increased the enthalpy of formation by 2.7 kcal/mol to an enthalpy of formation of 27.5 kcal/mol when using CCSD(T)/CBS.