Gas-phase Enthalpies Of Formation Research Articles

Lattice energy and specific impulse to hydrazoic acid, Pb, Ag and Cu azides

In this work, the gas phase formation enthalpies (kJmol−1) to hydrazoic acid, Pb, Ag and Cu azides were calculated as: HN3 = 310.00; AgN3 = 509.70, Pb(N3)2 = 768.37 and Cu(N3)2 = 855.45. The respective phase transition enthalpies (kJmol−1) are: HN3(l) → HN3(g) = 9.70; AgN3 (s) → AgN3 (g) = 198.70; Pb(N3)2(s) → Pb(N3)2(g) = 284.37 and Cu(N3)2(s) → Cu(N3)2(g) = 268.45. By using and empirical equation, the detonation velocity to cupric azide, Cu(N3)2 (s), was estimated as 7340 ms−1.

Update for Isomerization Stabilization Energies: The Fulvenization Approach.

An alternative approach for calculating aromatic stabilization energies is proposed based on transforming an (anti)aromatic ring into a fulvene isomer. This fulvenization process gives a value of 34.05 kcal·mol-1 for benzene in the singlet state and a value of -17.85 kcal·mol-1 in the triplet state. Additionally, it is possible to use experimental values (as long as they exist) for the calculation as the gas-phase formation enthalpies of benzene and fulvene, whose difference is 33.72 kcal·mol-1. On the other hand, this same approach has been evaluated on several six-membered rings, including those persubstituted, biradicals, azines, and inorganic analogues, giving results in agreement with those reported in the literature using different criteria. Additionally, it is possible to differentiate the aromaticity of the rings in polycyclic aromatic hydrocarbons according to Clar's rules. Assigning the (anti)aromatic character in various nonbenzenoid rings (neutral and charged), except for five- and seven-membered rings, is also possible. The construction of the fulvene isomers in PAHs is set such that nonaromaticity-related effects are not considered. The results show that the fulvenization approach is an effective and efficient approach that can serve as an alternative or complement to existing tools.

Structure-property relationships in alkoxy substituted benzoates from experimental and computational thermochemistry

The energetics of formation and the phase transitions for isomeric methyl and ethyl methoxy‑benzoates were studied. The vapour pressures of methyl 2‑methoxy-benzoate and methyl 4‑methoxy-benzoate were measured by transpiration method. The high-precision combustion calorimetry was used to derive the liquid-phase enthalpy of formation of methyl 2‑methoxy-benzoate. The new results and the data available in the literature were critically evaluated using the structure-property correlations. Reliable correlations were established between the vaporisation enthalpies and the normal boiling temperatures as well as with the Kovats indices. Moreover, the validity of vaporisation enthalpies of alkyl methoxy‑benzoates was proved using structure-property relationships within families of structurally similar molecules. Mutual validation of the experimental and theoretical gas phase enthalpies of formation was performed using high-level quantum-chemical methods. The enthalpic contributions to the enthalpy of formation in the gas phase, resulting from the nearest and non-nearest neighbour interactions of the substituents in the benzene ring, were developed and used to quantify the intra-molecular hydrogen bond strength in the ortho-substituted species.

Enthalpy of Formation of Carbocycles: A Precise Theoretical Determination of Experimentally Imprecise Measurements.

Despite being one of the best-known families of organic compounds, the fact that hydrocarbons exhibit a rich variety of structures owing to branching, cyclization, and the presence of multiple bonds, means that many of their properties are yet to be determined accurately, or even at all. Among cyclic hydrocarbons, those with three-membered rings are particularly interesting because of their strain energy. In this paper, we report accurate calculations of the enthalpy of formation of three-membered carbocycles, whose experimental values have not been obtained by direct measurement of the heat of combustion. For this purpose, we used several accurate composite methods to obtain the gas-phase enthalpies of atomization and derived from them the gas-phase enthalpies of formation, using experimentally determined accurate values for the atoms. Moreover, to minimize the inaccuracy that can possibly arise in this procedure, we also used homodesmotic reactions designed to balance systematic errors in the geometric and electronic structure of some of the species. A careful analysis of the results shows that some of the indirectly derived values reported in the literature are far from the most accurate theoretical outcomes, and we suggest that these new ones should be adopted.

Ab initio calculations on structure and stability of BN/CC isosterism in azulene

Herein, we investigated the thermodynamic stability and opto-electronic properties of a newly BN-doped azulene. The gas-phase formation enthalpies of 11 BN-doped azulene were calculated by the atomization energy method using three computational models (CBS-APNO, CBS-QB3, and G3MP2). The results suggest that AZ-1N9B exhibits the highest stability among the studied isomers. On the other hand, AZ-1B9N and AZ-9B10N display nearly equal stability with relative energies of 19.36 and 19.82 kcal/mol at CBS-QB3, respectively. These two isomers are considered the least stable among the investigated compounds. The frontier molecular orbitals (FMO), ionization energies (IE), and electron affinities (EA) of these isomers were discussed. Additionally, the electronic absorption spectra of the BN-doped azulenes were computed using the TD-B3LYP/6–31 + G(d,p) and TD-CAM-B3LYP level of theories, which using a long-range corrected hybrid functional in acetone. The computational results obtained in this research are align closely with the existing literature, thereby reinforcing the credibility and reliability of our findings.

Thermochemistry, Tautomerism, and Thermal Stability of 5,7-Dinitrobenzotriazoles

Nitro derivatives of benzotriazoles are safe energetic materials with remarkable thermal stability. In the present study, we report on the kinetics and mechanism of thermal decomposition for 5,7-dinitrobenzotriazole (DBT) and 4-amino-5,7-dinitrobenzotriazole (ADBT). The pressure differential scanning calorimetry was employed to study the decomposition kinetics of DBT experimentally because the measurements under atmospheric pressure are disturbed by competing evaporation. The thermolysis of DBT in the melt is described by a kinetic scheme with two global reactions. The first stage is a strong autocatalytic process that includes the first-order reaction (Ea1I = 173.9 ± 0.9 kJ mol-1, log(A1I/s-1) = 12.82 ± 0.09) and the catalytic reaction of the second order with Ea2I = 136.5 ± 0.8 kJ mol-1, log(A2I/s-1) = 11.04 ± 0.07. The experimental study was complemented by predictive quantum chemical calculations (DLPNO-CCSD(T)). The calculations reveal that the 1H tautomer is the most energetically preferable form for both DBT and ADBT. Theory suggests the same decomposition mechanisms for DBT and ADBT, with the most favorable channels being nitro-nitrite isomerization and C-NO2 bond cleavage. The former channel has lower activation barriers (267 and 276 kJ mol-1 for DBT and ADBT, respectively) and dominates at lower temperatures. At the same time, due to the higher preexponential factor, the radical bond cleavage, with reaction enthalpies of 298 and 320 kJ mol-1, dominates in the experimental temperature range for both DBT and ADBT. In line with the theoretical predictions of C-NO2 bond energies, ADBT is more thermally stable than DBT. We also determined a reliable and mutually consistent set of thermochemical values for DBT and ADBT by combining the theoretically calculated (W1-F12 multilevel procedure) gas-phase enthalpies of formation and experimentally measured sublimation enthalpies.

Comprehensive Thermodynamic Study of Alkyl-Cyclohexanes as Liquid Organic Hydrogen Carriers Motifs

Alkyl-cyclohexanes can be considered as suitable model compounds to understand the thermochemistry of aromatic compounds and their hydrogenated counterparts discussed as Liquid Organic Hydrogen Carrier systems. Thermochemical measurements on these hydrogen-rich compounds are thwarted by complications due to the 99.9 % purity limitation and sample size specific to these methods. However, the data on vaporisation and formation enthalpies are necessary to optimize the hydrogenation/dehydrogenation processes. In this work, various empirical and theoretical methods are described to reliably assess the gas phase enthalpies of formation and vaporization enthalpies of alkyl-substituted cyclohexanes. The empirical and quantum-chemical methods have been validated against reliable literature data and provide reasonable estimates with an accuracy comparable to that of the experimental data. The liquid phase enthalpies of formation of differently shaped alkyl-cyclohexanes were derived and used to estimate the energetics of their dehydrogenation reactions. The influence of alkyl substituents on the reaction enthalpy is discussed. The vapour pressures of typical hydrogen-rich compounds at technically relevant temperatures were calculated and compared to vapour pressures of biodiesel fuels measured in this work using the static method.

Enthalpy of formation of 6-phenyl-1,5-diazabicyclo[3.1.0]hexane by combustion calorimetry and theoretical approach for efficient prediction of thermochemistry of diaziridines.

The combustion energy and standard molar enthalpy of formation of crystalline 6-phenyl-1,5-diazabicyclo[3.1.0]hexane (PDABH) were determined using an isoperibolic calorimeter with a static bomb. PDABH is the first diaziridine for which the experimental value of the enthalpy of formation was obtained. This value was validated by the theoretical values of gas phase enthalpy of formation and enthalpy of sublimation. The gas phase enthalpy of formation was calculated using the DLPNO-CCSD(T1)/CBS method in conjunction with isodesmic-type reactions. This method was chosen in comparison to another high quality evaluative method (G4), which has been shown to provide unreliable results for cyclic nitrogen containing compounds. The descriptors of the molecular electrostatic potential (MEP) were used to estimate the enthalpy of sublimation of PDABH. The proposed MEP model is based on experimental enthalpies of sublimation for 75 compounds structurally similar to PDABH. The high-level ab initio calculations of gas phase enthalpies of formation combined with enthalpies of sublimations estimated using descriptors of MEP allow predicting the enthalpies of formation of diaziridines in the solid phase.

Experimental and computational thermochemistry: how strong is the intramolecular hydrogen bond in alkyl 2-hydroxybenzoates (salicylates).

Hydrogen bonding (HB) is a fascinating phenomenon that exhibits unusual properties in organic and biomolecules. The qualitative manifestation of hydrogen bonds is known in numerous chemical processes. However, quantifying HB strength is a challenging task, especially in the case of intra-molecular hydrogen bonds. It is qualitatively well established that the alkyl 2-hydroxybenzoates have strong intra-HB. The thermochemical methods suitable for the determination of intra-HB strength were the focus of this study. The experimental gas phase formation enthalpies for alkyl 2-hydroxybenzoates (including methyl, ethyl, n-propyl and n-butyl) at 298.15 K were derived from a combination of vapour pressure measurements and high-precision combustion calorimetry and validated by the quantum chemical methods G3MP2 and G4. The intra-HB strength in methyl 2-hydroxybenzoate was determined from the evaluated gas-phase enthalpies of formation by comparing the energies of cis- and trans- conformers, by well-balanced reactions, the "para-ortho" method and the "HB and Out" method. All these methods give a common level of intra-molecular hydrogen bond strength of -43 kJ mol-1. The intra-HB strength was found to be independent of the chain length of the alkyl 2-hydroxybenzoates.

Computer-assisted Design of Novel Polyketide Synthase 13 of Mycobacterium tuberculosis Inhibitors Using Molecular Modeling and Virtual Screening

Aims: Polyketide synthase 13 (Pks13) is an essential enzyme in the synthesis of mycolic acids biosynthesis pathway of Mycobacterium tuberculosis (Mtb). Therefore, Pks13 is a promising drug target for tuberculosis treatment. Here we report the in silico design and evaluation of novel Pks13 inhibitors made of benzofuran derivatives with favorable predicted pharmacokinetic profiles.
 Methodology: A 3D model of Pks13-TAMx complexes was prepared for a training set of 18 TAMs with experimentally determined inhibitory potencies (half-maximal inhibitory concentrations ) by using in situ modifications of the crystal structure of the TAM1-Pks13 complex (PDB entry 5V3X). A linear QSAR model was built, correlating computed gas phase enthalpies of formation ( ) of Pks13-TAMx complexes with the in order to find active conformations of the 18 TAMs. Furthermore, taking into account the implicit solvent effect and entropy changes upon ligand binding, a superior QSAR model was brought forth, correlating computed complexation of Gibbs’ free energies . Using the active conformations of the training set TAMs, we built a pharmacophore model (PH4) which was used to virtually screen novel analogs included in a virtual combinatorial library (VCL) of compounds containing benzofuran scaffolds. The PH4 model screened the VCL, which was formerly filtered by Lipinski’s rule-of-five, in order to identify new benzofuran analogs.
 Results: Gas phase QSAR model: , ; superior aqueous phase QSAR model , and PH4 pharmacophore model: , . The PH4-based screening retained 109 new TAM analogues. Finally, the predicted pharmacokinetic profiles of these new analogues were compared to current orally administered antituberculosis drugs, and the former were found to be almost 92 times more active than TAM2 ( .
 Conclusion: This computational approach, which combines molecular mechanics and the Poisson–Boltzmann (PB) implicit solvation theory, the pharmacophore model, the analysis of Pks13-TAMs interaction energies, the in silico screening of VCL compounds, and the inference of ADME properties resulted in a set of new suggested Pks13 inhibitors.

Thermochemistry of per- and polyfluoroalkyl substances.

The determination of gas phase thermochemical properties of per- and polyfluoroalkyl substances (PFAS) is central to understanding the long-range transport behavior of PFAS in the atmosphere. Prior gas-phase studies have reported the properties of perfluorinated sulfonic acid (PFOS) and perfluorinated octanoic acid (PFOA). Here, this study reports the gas phase enthalpies of formation of short- and long-chain PFAS and their precursor molecules determined using density functional theory (DFT) and ab initio approaches. Two density functionals, two ab initio methods and an empirical method were used to compute enthalpies of formation with the total atomization approach and an isogyric reaction. The performance of the computational methods employed in this work were validated against the experimental enthalpies of linear alkanoic acids and perfluoroalkanes. The gas-phase determinations will be useful for future studies of PFAS in the atmosphere, and the methodological choices will be helpful in the study of other PFAS.

A New Enthalpy of Formation Test Set Designed for Organic Fluorine Containing Compounds (Adv. Theory Simul. 8/2022)

A New Enthalpy of Formation Test Set for Finding Organic Fluorine Containing Compounds Density functional theory often performs worse predicting gas phase enthalpy of formation for fluorine compounds than it does for other systems. The numerical bias somewhat lacks a systematic quantification; the evaluation of various commonly used computational levels will facilitate research on fluorine chemistry. In article 2200093, Si, Yu and co-workers propose a new test set for fluorine containing compounds, and quantitatively analyze the differences induced by various computational levels and strategies.

A New Enthalpy of Formation Test Set Designed for Organic Fluorine Containing Compounds

AbstractThe deviations of gas phase enthalpy of formation predicted by density functional theory on the fluorine containing system are larger than those on other systems in most cases. Therefore, an enthalpy of formation test set composed of 41 organic fluorine containing compounds is proposed, and in this work it is called OFCC41 test set. Twenty exchange‐correlation functionals are estimated by CCSD(T)/CBS as benchmark. The first part of the results indicates that CAM‐B3LYP, ωB97X, and M06 are the most suitable functionals to predict enthalpies of formation for fluorine containing compounds. The remainder of this work is primarily concerned with diffuse basis sets and size‐dependent effects. Diffuse basis sets ma‐SVP and ma‐TZVP can generally improve the prediction performance. For very small molecules, using ma‐SVP is sufficient while for larger molecules more completed basis sets are demanded. Size‐dependent effect of prediction accuracy on different molecules is revealed by using a simple strategy, that the different sizes of molecules are distinguished with three intervals and given the corresponding mean absolute errors. This study provides a test set designed for organic fluorinated compounds and contributes to the selection of some functionals suitable for fluorine containing system.

Computational thermochemistry: extension of Benson group additivity approach to organoboron compounds and reliable predictions of their thermochemical properties.

High-level computational data for standard gas phase enthalpies of formation, entropies, and heat capacities are reported for 116 compounds of boron. A comparison of the results with extant experimental and computational benchmark values reveals important trends and clear outliers. Recommendations are made to revise some of the key quantities, such as the enthalpies of formation of orthoboric acid, trimethylthioborate, and triphenylborane, the last of which is found to be considerably in error. The uncertainties associated with the experimental values are found to exceed those of high-level calculations by a clear margin, prompting the redetermination of Benson group additivity contributions for boron-based groups on purely computational grounds. The applicability of the established group contribution values is demonstrated by estimating thermochemical data for large organoboron compounds that cannot be treated with high-level quantum chemical methods and comparing the results with existing experimental and computational values.

Enthalpy of formation of 2-methyltetrahydrofuran: Experimental and computational study

The liquid phase standard molar enthalpy of formation of 2-methyltetrahydrofuran was determined by isoperibolic calorimetry of combustion. The purity of the sample was determined by the g.l.c and the cryoscopic methods. Using available enthalpy of vaporization, the gas phase enthalpy of formation was obtained. This value agrees well with that calculated by DLPNO-CCSD(T1)/CBS method.

Invoking chemical principles to predict the anions of dihydrooxazole family as prospective carbon capture moieties

The present study investigates 17 under-explored isomers of dihydrooxazole family, including oxazoline, oxadiazoline and oxatriazoline and their proposed anions as potential azolate based CO2 capture systems, using quantum chemical calculations. The presence of a π bond and –NH in the five-membered heterocyclic ring renders the isomers of dihydrooxazole family suitable for carbon capture, when deprotonated at the –NH nitrogen in the ring. Relative energies (ΔErel) of the isomers supported by their gas phase enthalpies of formation (ΔfH° (298.15 K)) and natural bond orbital (NBO) analysis, aided in the screening of stable anions suitable for CO2 capture. The deprotonation energies (ΔH298) of the isomers could establish a relationship between stability and basicity of the anions. The CO2-anion interaction with the selected anions was found to be feasible resulting into stable carbamates. The proposed anions could be a new addition to the family of azolate based carbon capture moieties.

Hydrogen storage technologies: Methyl-substituted biphenyls as an auspicious alternative to conventional liquid organic hydrogen carriers (LOHC)

Derivatives of biphenyls, namely mono-, di- and tert- methyl substituted biphenyls, can be considered as promising hydrogen carriers. The absolute vapour pressures of nine methyl-substituted biphenyls were measured using the transpiration method. The vapour pressures available in the literature for methylbiphenyls were collected and analyzed. The standard molar enthalpies of vaporization/sublimation of methyl substituted biphenyls were derived from the temperature dependencies of the vapour pressures. The experimental enthalpies of vaporization were validated using Kovats’s indices. The enthalpies of fusion of four methylbiphenyls were measured by DSC. The standard molar enthalpy of formation of the liquid 3,3′-dimethylbiphenyl was determined using high precision combustion calorimeter. The enthalpies of formation of methylbiphenyls available in the literature were collected and combined with the evaluated enthalpies of vaporization/sublimation to obtain their gas-phase enthalpies of formation. High-level G3MP2 and G4 quantum-chemical methods were used to establish consistency of the experimental and theoretical results. The enthalpies of the reversible hydrogenation/dehydrogenation reactions of the biphenyls were calculated and compared with conventional liquid organic hydrogen carriers.

Rate coefficients for $$\hbox {Ar}^{{\mathbf {+}}}$$ in $$\hbox {Ar/BF}_{{\mathbf {3}}}$$ mixtures

In this paper, we present most probable reactions of $$\hbox {Ar}^{{+}}$$ ion with $$\hbox {Ar/BF}_{{3}}$$ mixtures. Appropriate gas phase enthalpies of formation for the products were used to calculate scattering cross section as a function of kinetic energy. These data are needed for modeling in numerous applications of technologically important $$\hbox {BF}_{{3}}$$ discharges. Results for transport parameters, specially rate coefficients as a function of E/N (E-electric field; N-gas density) were obtained by using the Monte Carlo technique.

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.