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Seven theoretical methods (B3LYP, B3P86, X3LYP, BMK, MP2, CBS-QB3, G3B3) were employed to calculate oxygen–oxygen homolytic bond dissociation enthalpies of 13 peroxides. Comparison of the performances of these methods with the available experimental data showed that ROBMK method is suitable and economical for calculating O–O homolytic bond dissociation enthalpies of peroxides. Then ROBMK method was used to calculate O–O homolytic bond dissociation enthalpies of a series of peroxides. Meanwhile, the α-substituent effect and Hammett relationship were also discussed. [ABSTRACT FROM AUTHOR]

Abstract: The heat of formation of a number of key C1 and C2 chlorofluorocarbons and hydrochlorofluorocarbons have been calculated by G3, G3MP2, G3MP2B3 and G3B3 methods. Based on the results of the atomization approach, it was found that the errors are approximately dependent upon the number of C–F or C–Cl bonds. Moreover, the bond additive correction (BAC) procedure and isodesmic reactions approach improved the accuracy and decreased these system errors significantly. The extended comparison between the BAC procedure and isodesmic reaction approaches had been made; the latter yielded the best results and showed broader applicability. [Copyright &y& Elsevier]

Abstract: Substituent resonance constant, is calculated from substituted benzenes using molecular electrostatic potential (MESP). The reliability of is rigorously verified using isodesmic reactions and found that the energy component of substituent resonance effect is proportional to . Thus the MESP approach enabled the definition to an electrostatic scale of substituent resonance effect. [Copyright &y& Elsevier]

The structural and bonding characteristics of several CO-releasing molecules have been investigated using density functional theories. Based on the nature of the physiological buffer compositions used in experiments, several isodesmic reactions involving chloride (Cl-), hypophosphate (HPO42-), and nitrosyl (NO) ligands are set up to find out whether the ligand-substituted products, which could have direct influence in CO releasing mechanism, are thermodynamically feasible. The isodesmic reactions involving the Cl- and HPO42- ligands are exothermic whereas it is endothermic with the NO ligand. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Quantum chemical calculations are used to compute the heats of formation (HOFs) for 24 nitroester (NE) energetic compounds in which only 5 nitroester energetic compounds have the available experimental heats of formation. The heats of formation of the five compounds are calculated from isodesmic reactions by employing the hybrid density functional theory (DFT) (B3LYP, B3PW91, and B3P86) methods with 6-31G** and 6-311G** basis sets. It is demonstrated that B3PW91/6-31G** method can yield reliable HOFs, which has the mean absolute deviation of 1.1kcal/mol. The HOFs of other 19 nitroester energetic compounds are calculated by using B3PW91/6-31G** method. Through the analysis of the calculated result, it is found that the computed heat of formation decreases when the number of methylene (CH2) group increases for normal chain nitroester compounds. The further study shows that our results about gas-phase heats of formation of nitroester compounds are better than the results of Muthurajan et al. [H. Muthurajan, R. Sivabalan, M.B. Talawar, M. Anniyappan, S. Venugopalan, Prediction of heat of formation and related parameters of high energy materials, J. Hazard. Mater. A133 (2006) 30–45], and Byrd Edward and Rice Betsy [F.C. Byrd Edward, M. Rice Betsy, Improved prediction of heats of formation of energetic materials using quantum mechanical calculations, J. Phys. Chem. A 110 (2006) 1005–1013]. In addition, the condensed phase heat of formation of the nitroester compounds are computed through the same method of Byrd Edward and Rice Betsy. [Copyright &y& Elsevier]

The energetics of ethylenes and 1,3-butadienes may be interrelated by the reaction: RHC=CH2 + H2C=CHR′ → RHC=CH-CH=CHR′ + H2. Shown earlier to be nearly enthalpically thermoneutral for a variety of hydrocarbon cases, we are now interested in the related energetics of halogenated alkenes and alkynes. Using quantum chemical calculations, we have studied this as recast as the isodesmic reactions: 2(H2C=CHX) + H2C=CH-CH=CH2 → p,q-di-X-1,3-butadiene + 2H2C=CH2 2(HC≡CX) + HC≡C-C≡CH → di-X-butadiyne + 2HC≡CH. Here p,q- = 1,3-; 1,4- and 2,3- with X = F, Cl, Br, and I. The halogen and location-dependent deviations from near enthalpic thermoneutrality are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Phosphorus containing functional groups induce an important stabilizing effect on adjacent carbanions. Such an interaction is, however, not observed for the neutral compounds. Using computational methods, we clarify the fundamental properties of these functional groups that lead to these differences. By doing so, a general description of phosphorus containing functional groups of the –PY2 and –POY2 type is obtained. These groups should be seen as highly ionic groups having a σ donor/π acceptor character. [Copyright &y& Elsevier]

Abstract: Heats of formation (HOFs) and strengths of group interactions for 19 polyisocyanoadamantanes were obtained by using the density functional theory (DFT). The adamantane skeleton was chosen for a reference compound in the process of designing isodesmic reactions. It was found that the HOF increases 213–252kJ/mol for each additional number of the isocyano group being added to the adamantane skeleton. The HOFs slightly deviate from the group additivity with respect to the isocyano group. The distance between isocyano groups influences the values of HOFs. The disproportionation energies of neighbor isocyano groups in polyisocyanoadamantanes are in the range of 10.3–15.8kJ/mol, which are also slightly related to the substituent numbers. The average interaction energy between nearest neighbor –NC group in decaisocyanoadamantane is 10.34kJ/mol at the B3LYP/6-311G∗∗ level. The relative stability related to the number of isocyano groups of the title compounds was evaluated based on the calculated HOFs, the energy gaps between the frontier orbitals, the bond orders of the C–NC and C–C bonds, and bond dissociation energies. [Copyright &y& Elsevier]

Abstract: The heats of formation (HOFs) were calculated for a series of neopentyl difluoroamino compounds by using density functional theory (DFT) method with 6-311G∗∗ basis set, as well as semi-empirical methods. In the isodesmic reactions designed for the computation of HOF, the neopentane was chosen as a reference compound. The HOFs increase smoothly as –ONO2 groups being replaced by –NF2 and –NO2. However, HOFs increase dramatically as –ONO2 groups being replaced by –N(NO2)CH3 and –N3 groups. The variations of HOFs are different with different substituents. The semi-empirical MO (MNDO, AM1, and PM3) methods did not produce accurate and reliable results for the HOFs of the title compounds. The relative stability of the title compounds was evaluated based on the calculated HOFs, the energy gaps between the frontier orbitals and the bond order of C–R bond (C–NF2, C–ONO2, C–NO2 and C–N3). The magnitudes of intramolecular group interactions were predicted through the disproportionation energies. Thermal stabilities were evaluated via bond dissociation energies (BDE) at the B3LYP/6-311G∗∗ level. These results provide basic information for the molecular design of novel high energetic density materials. [Copyright &y& Elsevier]

The geometries and energies of substituted sulfenes RCH═SO2 and alkenes RCH═CH2 were calculated at the HF/6-311+G(2d,p) level. An isodesmic reaction was designed to study substituent effects on the stability of sulfenes. The correlation between substituent group electronegativity χBE and the stability of sulfenes was reasonably good. Electropositive and π-acceptor substituents stabilized sulfenes while electronegative substituents destabilized them. [ABSTRACT FROM AUTHOR]

Many different degradation reactions of chlorinated hydrocarbons are possible in natural groundwaters. In order to identify which degradation reactions are important, a large number of possible reaction pathways must be sorted out. Recent advances in ab initio electronic structure methods have the potential to help identify relevant environmental degradation reactions by characterizing the thermodynamic properties of all relevant contaminant species and intermediates for which experimental data are usually not available, as well as provide activation energies for relevant pathways. In this paper, strategies based on ab initio electronic structure methods for estimating thermochemical and kinetic properties of reactions with chlorinated hydrocarbons are presented. Particular emphasis is placed on strategies that are computationally fast and can be used for large organochlorine compounds such as 4,4′-DDT [ABSTRACT FROM AUTHOR]

Both the heats of formation (HOFs) and the group interactions were predicted for a series of adamantane derivatives substituted with difluoroamino groups by using density functional theory (DFT) with 6-31G∗ and 6-311G∗∗ basis sets. The adamantane skeleton was chosen for as a reference compound in the isodesmic reactions designed for calculating the HOFs. The contribution of difluoroamino group to the heat of formation deviates from group additivity. The relationship between HOFs and molecular structures was discussed. It was found that the HOFs decrease more with each difluoroamino group attached to the tertiary carbon than to the secondary carbon of the adamantane skeleton, when the secondary carbon was substituted by one group only. However, the HOFs increase with the second difluoroamino group attached to the secondary carbon. The distance between difluoroamino groups influences the values of HOFs. The group interacting energies of polydifluoroaminoadamantane increase in the range of 8–70 kJ/mol with each difluoroamino group attached. The interaction of neighbor difluoroamino groups discords with the group additivity. The relative stability of the title compounds was assessed based on the calculated HOFs, the energy gaps between the frontier orbitals, and the bond order of C–NF2. The C–NF2 bond is greatly weakened with two NF2 groups being attached to the same secondary carbon. The basis sets influence the HOFs when the substituent number is over 6 or when two substituted groups being attached to the same secondary carbon. [Copyright &y& Elsevier]

Density functional theory (DFT) was employed to evaluate the heats of formation (HOFs) for hexaazaadamantane (HAA) derivatives with &bond;CN, &bond;NC, and &bond;ONO2 groups, respectively. This was done by designing isodesmic reactions at the B3LYP/6-31G* level of theory, where the HAA cage skeletons were kept unbroken to produce more accurate results, and all HOFs for the required reference compounds, NH2CN, NH2NC, NH2ONO2, and (CH2NH)3, were derived from the G3 theory calculation based on the atomization energies. The calculation results show that the HOFs of HAA derivatives are mainly affected by the number and the position of substituent groups, all the obtained HOFs are positive, and the &bond;NC derivatives have the most HOFs among the three types of derivatives with the same number of substituent groups. The detonation velocity (D) and detonation pressure (P) were obtained from the empirical Kamlet–Jacobs equations. All the &bond;NC and &bond;CN derivatives of HAA have lower densities (ρ), heats of explosion (Q), D, and P. However, these properties of &bond;ONO2 derivatives are rather high and vary with the number of &bond;ONO2 groups. Considering the easiness for synthesis and relative stability, 2,4,6,8-hexaazaadamantanenitrate is finally recommended as a potential candidate of a high-energy density compound (HEDC). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]

We have calculated the heats of formation (HOFs) for a series of polyazidocubanes by using the density functional theory (DFT), Hartree-Fock, and MP2 methods with 6-31G* basis set as well as semiempirical methods. The cubane skeleton was chosen for a reference compound, that is, the cubane skeleton was not broken in the process of designing isodesmic reactions. There exists group additivity for the HOF with respect to the azido group. The semiempirical AM1 method also produced reliable results for the HOFs of the title compounds, but the semiempirical MINDO3 did not. The relationship between HOFs and molecular structures was discussed. It was found that the HOF increases 330–360 kJ/mol for each additional number of the azido group being added to the cubane skeleton. The distance between azido groups slightly influences the values of HOFs. The interacting energies of neighbor azido groups in polyazidocubanes are in the range of 2.3∼6.6 kJ/mol, which are so small and less related to the substituent numbers. The average interaction energy between nearest neighbor &bond;N3 groups in the most stable conformer of octaazidocubane is 2.29 kJ/mol at the B3LYP/6-31G* level. The relative stability related to the number of azido groups of the title compounds was assessed based on the calculated HOFs, the energy gaps between the frontier orbitals, and the bond orders of the C&bond;N3 and C&bond;C bonds. The predicted detonation velocity of hepta- and octa-derivatives is over 9 km/s, and the detonation pressure of them is ca. 40 GPa or over. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1263–1269, 2005 [ABSTRACT FROM AUTHOR]

The enthalpies of formation (ΔfH298°) of biphenyl, 2,2′- and 4,4′-dichlorobiphenyls were calculated at the B3LYP density functional theory (DFT) level using isodesmic reactions. The values for biphenyl and 4,4′-dichlorobiphenyl are in good agreement with calorimetric values, whereas, the values for 2,2′-dichlorobiphenyl are noticeably larger than the experimental one. The results of calculations allow to suggest that experimental value of ΔfH298° for 2,2′-isomer is underestimated by 1–2 kcal/mol. [ABSTRACT FROM AUTHOR]

[Display omitted] oth the computation of, and the uncertainties associated, with gas-phase molar formation enthalpies are now quite well established for systems comprised of tens of 'heavy' atoms chosen from the commonest elements. The same cannot be said for derived thermochemical quantities such as entropy, heat capacity and an enthalpy function. Whilst the application of well known statistical thermodynamic relations is mostly understood, the determination of the uncertainty with which such values can be obtained has been little studied — apart, that is, for a general protocol devised by Goldsmith et al. [ J. Phys. Chem. A , 2012 , 116, 9033–9057]. Specific examples from that work are explored here and it is shown that their estimates are overly pessimistic. It is also evident that for some species the calculated thermochemical parameters show very little variation with either the level of theory, or basis set, or treatment of vibrational modes — this renders the inclusion of such species, in databases designed to validate new methods or to be harvested for machine learning purposes, of limited value. [ABSTRACT FROM AUTHOR]

We present density functional theory (DFT) calculations and analysis of some substituted 2,2′, 4,4′, 6,6′- hexanitroazobenzene (HNAB). Geometries of all compounds have been optimized employing by B3LYP method in conjunction with 6-31G** basis set. The heat of formation (Δ f H) is evaluated using isodesmic reaction paths. Density and detonation characteristics including detonation velocity D , detonation pressure P , and detonation energy Q were researched by Kamlet-Jacob equation. We also include study and discuss the relation between sensitivity and molecular/electronic structures. The trigger bonds in the pyrolysis process for HNAB derivatives were calculated and explored. The predicted results indicate that some designed azo bridging (–N N–) compounds outperform traditional energetic materials and may be potential candidates for high-energy materials. [Display omitted] • Substituent effects on the properties related to detonation performance and sensitivity for 2,2′,4,4′,6,6′-hexanitroazobenzene (HNAB) derivatives is proposed. • The sensitivity of HNAB derivatives is explored with molecular/electronic properties. • Several interrelationships of HNAB derivatives is supposed that relate the detonation performance with measured or predicted molecular/electronic properties. Density functional theory (DFT) has been applied to explore the general rules for structure design of azo compounds through energetic properties and stability. Structures of 2,2′, 4,4′, 6,6′- hexanitroazobenzene (HNAB) derivatives were designed and optimized using B3LYP /6-31G(d,p) level, and the heat of formation (Δ f H) were evaluated by constructing isodesmic reactions. Results reveal that the azo bridging (–N N–) is useful for increasing the heat of formation. Kamlet–Jacobs equations were employed to evaluate detonation performance, while the sensitivity was explored by correlating with the bond dissociation energy (BDE), electronic structural parameters (E HOMO , E LUMO , and Δ E LUMO-HOMO), oxygen balance (OB), and charges on nitro group (Q NO2). According to our calculations, some designed HNAB derivatives possess interesting properties, such as good detonation performance and high stability, even beyond RDX and HMX, revealing that the presence of azo bridging (–N N–) backbone together with the introduction of various energetic groups is a promising choice in the design of high energy density materials (HEDMs). [ABSTRACT FROM AUTHOR]

Macromolecular associates, such as membraneless organelles or lipid-protein assemblies, provide a hydrophobic environment, i.e., a liquid protein phase (LP), where folding preferences can be drastically altered. LP as well as the associated phase change from water (W) is an intriguing phenomenon related to numerous biological processes and also possesses potential in nanotechnological applications. However, the energetic effects of a hydrophobic yet water-containing environment on protein folding are poorly understood. Here, we focus on small β-sheets, the key motifs of proteins, undergoing structural changes in liquid–liquid phase separation (LLPS) and also model the mechanism of energy-coupled unfolding, e.g., in proteases, during W → LP transition. Due to the importance of the accurate description for hydrogen bonding patterns, the employed models were studied by using quantum mechanical calculations. The results demonstrate that unfolding is energetically less favored in LP by ~0.3–0.5 kcal·mol−1 per residue in which the difference further increased by the presence of explicit structural water molecules, where the folded state was preferred by ~1.2–2.3 kcal·mol−1 per residue relative to that in W. Energetics at the LP/W interfaces was also addressed by theoretical isodesmic reactions. While the models predict folded state preference in LP, the unfolding from LP to W renders the process highly favorable since the unfolded end state has >1 kcal·mol−1 per residue excess stabilization. [ABSTRACT FROM AUTHOR]

[Display omitted] • Solving the accurate energy barrier calculations for H-abstraction reactions (amines + H) at a low level HF method. • Providing the accurate reaction kinetics for amines combustion modeling. • Evans-Polanyi diagram for each sub-class and the corresponding rate rule are also performed for the 154 reactions. The H-abstraction reaction kinetics of primary, secondary and tertiary amines with H atom are computed using the isodesmic reaction method. The average unsigned deviation of energy barriers is 12.40 kcal mol−1 at the HF/6-31+G(d) level compared to the CCSD(T)/CBS(D-T-Q) method but reduced to 0.75 kcal mol−1 after corrections. The calculated rate constants of all reactions and corresponding rate rules are also presented. This work solves the accurate energy barrier calculations for larger molecules reaction amines + H at a low level HF method and also provides the accurate reaction kinetics for the critical important H-abstraction reactions of amines combustion modeling. [ABSTRACT FROM AUTHOR]

An analysis of the effects induced by F, Cl, and Br-substituents at the α-position of both, the hydroxyl or the amino group for a series of amino-alcohols, HOCH2(CH2)nCH2NH2 (n = 0–5) on the strength and characteristics of their OH···N or NH···O intramolecular hydrogen bonds (IMHBs) was carried out through the use of high-level G4 ab initio calculations. For the parent unsubstituted amino-alcohols, it is found that the strength of the OH···N IMHB goes through a maximum for n = 2, as revealed by the use of appropriate isodesmic reactions, natural bond orbital (NBO) analysis and atoms in molecules (AIM), and non-covalent interaction (NCI) procedures. The corresponding infrared (IR) spectra also reflect the same trends. When the α-position to the hydroxyl group is substituted by halogen atoms, the OH···N IMHB significantly reinforces following the trend H < F < Cl < Br. Conversely, when the substitution takes place at the α-position with respect to the amino group, the result is a weakening of the OH···N IMHB. A totally different scenario is found when the amino-alcohols HOCH2(CH2)nCH2NH2 (n = 0–3) interact with BeF2. Although the presence of the beryllium derivative dramatically increases the strength of the IMHBs, the possibility for the beryllium atom to interact simultaneously with the O and the N atoms of the amino-alcohol leads to the global minimum of the potential energy surface, with the result that the IMHBs are replaced by two beryllium bonds. [ABSTRACT FROM AUTHOR]

Intramolecular interactions are shown to be key for favoring a given structure in systems with a variety of conformers. In ortho-substituted benzene derivatives including a beryllium moiety, beryllium bonds provide very large stabilizations with respect to non-bound conformers and enthalpy differences above one hundred kJ·mol−1 are found in the most favorable cases, especially if the newly formed rings are five or six-membered heterocycles. These values are in general significantly larger than hydrogen bonds in 1,2-dihidroxybenzene. Conformers stabilized by a beryllium bond exhibit the typical features of this non-covalent interaction, such as the presence of a bond critical point according to the topology of the electron density, positive Laplacian values, significant geometrical distortions and strong interaction energies between the donor and acceptor quantified by using the Natural Bond Orbital approach. An isodesmic reaction scheme is used as a tool to measure the strength of the beryllium bond in these systems in terms of isodesmic energies (analogous to binding energies), interaction energies and deformation energies. This approach shows that a huge amount of energy is spent on deforming the donor–acceptor pairs to form the new rings. [ABSTRACT FROM AUTHOR]

The structural, electronic, and energetic characteristics of CHNO and CHOP isomers, and also of several related compounds, were calculated by a nonempirical method. Enthalpies of these isomers, obtained using the method of isodesmic reactions, are given. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have reached at novel tricyclic stannylenes that include singlet (s) and triplet (t) mono-, di-, tri-, tetra-, and pentaaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylenes. Singlet our stanylenes are ground states and more stable than their corresponding triplets. They are compared and contrasted with respect to their geometrical parameters, stability (ΔΕ s-t), heat of hydrogenation (ΔE H), nucleophilicity (N), electrophilicity (ω), and isodesmic reactions at B3LYP/AUG-cc-pVTZ//B3LYP/6-311++G** level of theory. The highest stability belong to the singlet 1,3-diaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene which shows the highest value of ΔE s-t. Boracyclics singlet 1,4 diaza-7-boratricyclo[1.1.1.01,7.07,3.,07,5]hexa-2-stannylene and 1,4,6-triaza-7-boratricyclo[1.1.1.01,7.07,3.,07,5]hexa-2-stannylene with coordinate covalent bond between nitrogen and stannylene center have low ω. We have reached novel borastannylenes with an unprecedented common frame work that can accommodate up to five nitrogen heteroatoms. All species appear as ground state minima on their energy surface, for showing no negative force constant. Singlets 7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylenes (1 s -20 s ) are ground states and more stable than their corresponding triplets (1 t -20 t ). They are compared and contrasted with respect to their geometrical parameters, stability (ΔΕ s-t), heat of hydrogenation (ΔE H), nucleophilicity (N), electrophilicity (ω), and isodesmic reactions at B3LYP/AUG-cc-pVTZ//B3LYP/6–311++G** level of theory. The highest stability belong to the singlet 1,3-diaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene (5) which shows the highest value of ΔE s-t. Boracyclics singlet 1,4,6-triaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene (14 s ) with coordinate covalent bond between nitrogen and stannylene center and 1,4,5,6-teraaza-7-boratricyclo[1.1.1.01,7.07,3.07,5]hexa-2-stannylene (18 s ) with high LP N → LP* Sn interactions have low ω. Also, the highest band gap (ΔΕ HOMO–LUMO) belong to theirs. Our investigation introduces novel stannylene with possible applications in chemistry such as semiconductors, cumulated multi-dentate legends, etc. [ABSTRACT FROM AUTHOR]

• A method is developed to automatically generate transition states. • An Isodesmic reactions correction scheme is designed for accurate and automatic calculation of rate constants. • A procedure is proposed to perform on-the-fly accurate kinetic calculations for reaction class systems. A detailed combustion mechanism of hydrocarbon fuels usually contains a large number of reactions and need to have kinetic data assigned to them, where the rate constants are usually given by approximate estimation leading to inaccuracies. In this work, a method for construction transition state (TS) and an isodesmic reactions correction scheme for accurate and automatic calculation of rate constants have been designed. These methods are tested using the cyclization reactions of QOOH radicals and the concerted elimination reactions of RO 2 radicals. The results show that the automatic TS construction method is effective to construct the initial guesses of the transition states, the isodesmic reactions corrected results are close to the CCSD(T) results and some of the rate constants are significantly different from the rate constants used in the automated mechanism generator, indicating that it is necessary and significant to perform on-the-fly accurate calculation of rate constants in an automated mechanism generator. [ABSTRACT FROM AUTHOR]

Following our quest for novel carbenes, effects of substituting 1 to 5 nitrogen atoms on the stability and reactivity of singlet (s) and triplet (t) forms of 7-boratricyclo[1,1,1,01,7,07,3,07,5]hexa-2-carbylenes (1–20) are compared and contrasted, at B3LYP/aug-cc-pvtz level of theory. All species appear as ground state minima on their energy surface, for showing no negative force constant. Singlets (1s–20s) are ground states and more stable than their corresponding triplets (1t–20t). Reactivity of the species (1s–20s vs. 1t–20t) is discussed in terms of isodesmic reactions, considering nucleophilicity (N), electrophilicity (ω), and heat of hydrogenation. As well as, the addition of nitrogen atoms decreased nucleophilicity (N), while increasing electrophilicity (ω). Despite the enormous steric strain involved in their cubic structures, the most stable scrutinized carbenes appear to be singlet 1,4,5-triaza-7-boratricyclo[1,1,1,01,7,07,3,07,5]hexa-2-carbylene (13) for showing the highest value of ΔEs–t. Such higher stabilization is attributed to a coordinate covalent bond observed between the carbenic center and the boron atom. This study offers new insights into the chemistry of these exotic tricyclic shaped carbenes. [ABSTRACT FROM AUTHOR]

In this work, quantum chemical calculations have been performed on the ionic liquids based on imidazolium, pyridinium, and pyrrolidinium cations with the dicyanamide anion. The dispersion-corrected B3LYP exchange-correlation functional was used to optimize geometries. Enthalpies of formation were calculated at the high-level G3(MP2) theory by employing both the atomization energy and reaction enthalpy approaches. There exists a very good agreement between the predicted enthalpies of formation and available experimental values. For the first time, the enthalpies of formation have been predicted for 21 ion pairs with no available experimental data. The calculated enthalpies of formation are decreased with increasing alkyl chain length. Linear correlations were observed between enthalpy of formation and number of carbon atoms in the alkyl side-chain for all of the studied series of cations. Regardless of the cation type, the contribution of each CH 2 group in the alkyl chain to the enthalpy of formation was also determined. • Enthalpies of formation of dicyanamide based ionic liquids were calculated. • Atomization and isodesmic reactions at the G3(MP2) were used. • Good agreement with experimental data, where available, was achieved. • The calculated enthalpy of formation was found to decrease with increasing alkyl chain length. • A linear correlation was observed between enthalpy of formation and number of carbon atoms in the alkyl side-chain of cations. [ABSTRACT FROM AUTHOR]

• Experimental data for 16 cyclic amines are recommended as a set of reliable benchmarks. • Errors are suggested for 6 species including key compounds (piperazine, s -trimethyltriazine). • Enthalpies of formation and vaporization are firstly estimated for 18 cyclic amines. 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. [ABSTRACT FROM AUTHOR]

The largest unsaturated carbene, cyclonona‐3,5,7‐trienylidene (1H), emerges as a boat‐shaped transition state (TS) for having a negative force constant (FC). Following the quest for stable germylenes (2X, where X = H, F, Cl, Br, and I) with C8H6X4Ge: formula, the density functional theory (DFT) calculations are carried out to assay the substituent effect on their thermodynamic and kinetic possibilities. These singlet and triplet germylenes (2X‐S and 2X‐T, respectively) appear as boat‐shaped minima without imaginary frequencies and the order of their stability estimated by singlet (S)–triplet (T) energy difference (ΔES‐T = ET − ES) is 1H < 2H < 2F< 2Cl < 2I< 2Br. The purpose of the present work is to swiftly switch from T carbene to S germylenes through alteration of the substituents and assess the influence of different halogens on the stability and other properties. The 2Br‐S seems as the most stable species. From kinetic point of view, 2Cl, 2Br, and 2I are more stable than some of synthesized germylenes. The stabilization of the S state is essential to stabilize germylenes. This can be achieved by σ‐electron withdrawing and π‐electron donating substituents, of which stabilizing effect can be further increased in the order of steric effect < positive hyperconjugation < negative hyperconjugation < mesomeric effect. Applying suitable isodesmic reactions, we show the π‐donor/σ‐acceptor halogen substituents stabilize not only the S but also the T states, and these groups stabilize 2X‐S more than 2X‐T. [ABSTRACT FROM AUTHOR]

Five sets of group 14 sextet cyclic unsaturated divalent species including carbenes (1X), silylenes (2X), germylenes (3X), stannylenes (4X), and plumbylenes (5X) are compared and contrasted at B3LYP/AUG‐cc‐pVTZ//B3LYP/6‐311 + G* and MP2/AUG‐cc‐pVTZ//MP2/6‐311 + G* levels of theory, where X = H, F, Cl, and Br. The scrutinized carbenes consisting of cyclonona‐3,5,7‐trienylidene (1H) and its 2,2,9,9‐tetrahalo derivatives (1Cl, 1Br) appear as boat shaped minima except for 1H which turns out as a boat shaped transition state for having a negative force constant. All of carbenes (1H, 1Cl, and 1Br) have triplet ground states except for 1F. In contrast, all of the corresponding 2X‐ 5X metallylenes appear as singlet boat shaped minima on their potential energy surfaces. Stability (estimated by ΔEs‐t, ie, energy gap between singlet [s] and triplet states) appear as a function of size and decreases by going down in group 14 column: 5X > 4X > 3X > 2X > 1X. Exclusive of carbene set, within other four sets stability decreases by going down in group 17 column: Br > Cl > F > H. Hence, singlet 2,2,9,9‐tetrabromocyclonona‐3,5,7‐trienplumbylene, 5Br, appears as the most stable sextet divalent with no tendency for dimerization. Comparison is extended to the structural parameters, band gap, nucleophilicity, electrophilicity, isodesmic reactions, and dimerization. [ABSTRACT FROM AUTHOR]

• Enthalpy of Hydrogenation scheme is used to calculate aromaticity of graphene. • Aromatic stabilization energy (ASE) estimated for naphthalene, polyacenes and graphene. • ASE estimates for graphene range from 2000 to 1 million kcal/mol. • Size estimates for planar graphene range from 22 to 225 nm and agree with experiments. Naphthalene, polyacenes and graphene are considered aromatic. Existing models for polyacenes predict a linearly increasing aromatic stabilization energy (ASE) and give little insights into their high reactivity and decreasing stability. Graphene's aromaticity has been studied only qualitatively suggesting alternate Clar's sextet and two-electrons per ring, and ASE estimates are missing. In this paper, various Enthalpy of Hydrogenation (ΔH hydro) and isodesmic schemes have been proposed and compared for the estimation of naphthalene ASE. Results show that ΔH hydro schemes are simple to design, are equivalent to isodesmic schemes, and unconjugated unsaturated reference systems predict ASE values in agreement with literature reports. Partially aromatic reference systems underestimate ASE. ΔH hydro schemes require calculations for a smaller number of structures, and offer scope for experimental validation, and involve enthalpy differences. Polyacene (X-axis extensions of benzene) ASE estimates (using ΔH hydro scheme) correlate well with experimental instability data and offer new chemical physical insights. ASEs extrapolated from quadratic and logarithmic regression models have been used to estimate the largest polyacene with limiting ASE values. ASE values for Pyrene (Y-axis extension of benzene) and higher analogues (here called pyrene-vertacenes) are estimated using ΔH hydro schemes. Further truncated graphene models and graphene are approximated as combinations of polyacene and pyrene-vertacene units. First ever ASE and molecular sizes (22–255 nM) estimates predict nanometer size ranges for flat graphene in agreement with recent experiments and offer new physical insights. These ASE and size estimates for graphene will prove useful in the design of novel energy (hydrogen) storage, electronic and material science applications. [ABSTRACT FROM AUTHOR]

In this paper, it was found that in comparison with acyclic normal (1R) and abnormal (1′R), one observes cyclic normal (2R) and abnormal (2′R) as well as cyclic‐unsaturated normal (3R) and abnormal (3′R) exhibit a decrease in nucleophilicity (N) and proton affinity (PA) in going from 1R → 2R → 3R, and in going from 1′R → 2′R → 3′R. The singlet‐triplet energy gaps (ΔES‐T) and HOMO‐LUMO energy gaps (ΔEH‐L) increase in going from 1R → 2R → 3R, and in going from 1′R to 2′R. [ABSTRACT FROM AUTHOR]

The enthalpies of formation were calculated for a number of bitetrazole salts using classical quantum chemistry methods and an improved method of isodesmic reactions, and the latter was found most accurate in the assessment of this characteristic. [ABSTRACT FROM AUTHOR]

Detailed theoretical investigation has been performed on the mechanism, kinetics and thermochemistry of the gas phase reactions of CF3CH2OCH3 (HFE-263fb2) with OH radicals using ab-initio and DFT methods. Reaction profiles are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels, respectively. Our calculations reveal that hydrogen abstraction from the CH2 group is thermodynamically and kinetically more facile than that from the CH3 group. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CF3CH2OCH3 and radicals (CF3CHOCH3 and CF3CH2OCH2) are also reported for the first time. The calculated bond dissociation energies for the CH bonds are in good agreement with experimental results. At 298K, the calculated total rate coefficient for CF3CH2OCH3+OH reactions is found to be in good agreement with the experimental results. The atmospheric fate of the alkoxy radicals, CF3CH(O)OCH3 and CF3CH2OCH2O are also investigated for the first time using the same level of theory. Out of three plausible decomposition channels, our results clearly point out that reaction with O2 is not the dominant path leading to the formation of CF3C(O)OCH3 for the decomposition of CF3CH(O)OCH3 radical in the atmosphere. This is in accord with the recent report of Osterstrom et al. [CPL 524 (2012) 32] but found to be in contradiction with experimental finding of Oyaro et al. [JPCA 109 (2005) 337]., (Copyright © 2014 Elsevier Inc. All rights reserved.)