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Skeletal editing represents an attractive strategy for adding complexity to a given molecular scaffold in chemical synthesis. Isodesmic reactions provide a complementary skeletal editing approach for the redistribution of chemical bonds in chemical synthesis. However, catalytic enantioselective isodesmic reaction is extremely scarce and enantioselective isodesmic reaction to synthesize atropisomeric compounds is unknown. Herein, we report a facile method to synthesize axially chiral aminoaryl quinoxalines through Cu(I)‐catalyzed dearomatization and sequential chiral phosphoric acid (CPA) catalyzed enantioselective isodesmic C−N bond formation and cleavage from indoles and 1,2‐diaminoarenes under mild reaction conditions. In this process, the five‐membered ring of the indole scaffold was broken and a novel quinoxaline skeleton was constructed. This method allows the practical and atom‐economical synthesis of valuable axially chiral aminoaryl quinoxalines in high yields (up to 95 %) and generally excellent enantioselectivities (up to 99 % ee). Notably, this novel type of quinoxaline atropisomers has promising applications in developing axially chiral ligand in asymmetric catalysis. This strategy represents the first example of CPA‐catalyzed enantioselective isodesmic reaction to form axially chiral compounds. [ABSTRACT FROM AUTHOR]

Isodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional‐group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five‐membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology. [ABSTRACT FROM AUTHOR]

In computational thermochemistry, "isodesmic‐type" reactions play a significant role for obtaining accurate thermochemical quantities using low‐cost methods that can be applied to large systems. This review touches on some of the examples. For instance, a series of relative bond dissociation energies (BDEs) have been devised to calculate absolute BDEs with near‐chemical‐accuracy (~5 kJ mol−1) using density functional theory (DFT) methods. To facilitate the applicability of isodesmic‐type reactions, the connectivity‐based hierarchy (CBH) has been developed to automate the systematic generation of isodesmic‐type reaction schemes, and applied to large organic and biomolecular systems. The related netCBH scheme yields accurate reaction energies in complex organic reactions, achieving coupled‐cluster quality results at DFT cost. Isodesmic‐type reactions have been used to obtain heats of formation for medium‐sized fullerenes, with uncertainties of ~20 kJ mol−1 up to C180. In comparison, the literature C60 heat of formation has an uncertainty of 100 kJ mol−1. Importantly, it fills the gap in which heats of formation for those larger fullerenes are not available. These studies showcase how isodesmic‐type reactions propel the accuracy of quantum chemistry computations to a level that rivals or even betters modern experimental determinations, particularly for systems that are difficult to study experimentally. This article is categorized under:Structure and Mechanism > Reaction Mechanisms and CatalysisElectronic Structure Theory > Combined QM/MM MethodsTheoretical and Physical Chemistry > Thermochemistry [ABSTRACT FROM AUTHOR]

We present a systematic assessment of the density functional tight binding (DFTB) method for calculating heats of formation of fullerenes with isodesmic‐type reaction schemes. We show that DFTB3‐D/3ob can accurately predict ΔfH values of the 1812 structural isomers of C60, reproduce subtle trends in ΔfH values for 24 isolated pentagon rule (IPR) isomers of C84, and predict ΔfH values of giant fullerenes that are in effectively exact agreement with benchmark DSD‐PBEP86/def2‐QZVPP calculations. For fullerenes up to C320, DFTB ΔfH values are within 1.0 kJ mol−1 of DSD‐PBEP86/def2‐QZVPP values per carbon atom, and on a per carbon atom basis DFTB3‐D/3ob yields exactly the same numerical trend of (ΔfH [per carbon] = 722n−0.72 + 5.2 kJ mol−1). DFTB3‐D/3ob is therefore an accurate replacement for high‐level DHDFT and composite thermochemical methods in predicting of thermochemical stabilities of giant fullerenes and analogous nanocarbon architectures. [ABSTRACT FROM AUTHOR]

Cyclonona-3,5,7-trienylidene appears as boat-shaped transition state for having a negative force constant, while its singlet state exhibits less stability than the corresponding triplet state. Succeeding the quest for the largest unsaturated stable carbene-like species, theoretical investigations coupled with suitable isodesmic reactions are used to examine the effects of α,αʹ-tetrahalo groups on the thermodynamic along with kinetic viabilities of nine-membered cyclic silylenes. All the singlet and triplet silylenes appear as boat-shaped minima for having positive force constants on their potential energy surfaces and singlet states emerge as ground state, exhibiting more stability than their corresponding triplet states. The order of stability estimated by singlet (S)–triplet (T) energy separation (ΔES–T = ET − ES) emerges as α,αʹ-tetrahydrocarbene < α,αʹ-tetrahydrosilylene < α,αʹ-tetrafluorosilylene < α,αʹ-tetraiodosilylene < α,αʹ-tetrachlorosilylene < α,αʹ-tetrabromosilylene. This research specifies band gap (ΔEHOMO–LUMO) of scrutinized silylenes with this order. Hence, singlet 2,2,9,9-tetrabromosilacyclonona-3,5,7-trienylidene exists as the most stable species. From both thermodynamic and kinetic points of view, this species is more stable than synthesized silylene by Kira. It shows the highest heat of dehydrogenation through isodesmic reaction. The NBO analysis provides significant evidences for the stability of it through positive hyperconjugation, negative hyperconjugation, as well as mesomeric effects. [ABSTRACT FROM AUTHOR]

Highlights • There were pure samples dicyclopropyldinitromethane and tricyclopropylmethane. • Energy of combustion of these compounds was determined by calorimetry of combustion. • The enthalpies of vaporization were estimated using electrostatic potential model. • The formation enthalpies of liquid and gas of these compounds were calculated. • Obtained experimental and calculation values of formation enthalpy were compared. Abstract The energies of combustion of liquid dicyclopropyldinitromethane and liquid tricyclopropylmethane were determined by calorimetry of combustion at T = 298.15 K. The enthalpies of vaporization were estimated on the basis of electrostatic potential model. The formation enthalpies of the compounds in liquid and gas states were obtained from these data. The gas enthalpies of formation were also calculated by methods of the group additivity and isodesmic reactions. The obtained experimental and calculation values were compared. [ABSTRACT FROM AUTHOR]

The simplest ideas of antiaromaticity refer to regular monocyclic systems and the eigenfunctions of the Hückel Hamiltonian for 4n electrons in such systems. The antiaromaticity is expressed in the energy penalty for such idealized systems relative to the Hückel energy for 2n noninteracting π pairs. Observed systems seldom achieve the regular planar geometry assumed in this picture, owing to their ability to ease the antiaromaticity penalty by departures from the regular geometry and also by export of the 4n π electrons' charge to substituents. In this report we estimate numerical values for the stabilization derived from such departures from the structure and the charge distribution of the idealized antiaromatic cyclopropenyl anion for a specific case, 3-dehydro-3-methyl carboxylate cyclopropenyl anion 1(-) using the thermochemical scheme CBSQB3 supplemented by CCSD(T) calculations. According to the isodesmic reaction, the anion 1(-) is destabilized by about 10-15 kcal/mol relative to the saturated 3-dehydro-3-methylcarboxylate cyclopropyl anion 2(-). We propose that the anion relieves a portion of the antiaromatic destabilization by (a) pyramidalization of one carbon of the ring, and (b) export of negative charge into the ester substituent. Both of these responses are expressed in the equilibrium structure of the anion. In the course of the study we estimate the acidity of several related anions and the enthalpy of formation of their neutral conjugate acids, and describe the interconversion of 1 to the dehydrotriafulvalene anion 3(-) by reaction with CO2. [ABSTRACT FROM AUTHOR]

Abstract The enthalpies of formation are a critical fundamental parameter for the combustion modelling. The enthalpies of formation of the species of the high temperature combustion mechanism of the lignocellulosic biofuel, ethyl levulinate are determined by a number of methodologies. Chemical group additivity, atomization and isodesmic worked reactions are employed with total reaction energies computed from theory with the CBS-QB3, CBS-APNO, and G4MP2 methodologies. These calculations reveal that significant differences as high as 6.2 kcal/mol can be found between each method for the large oxygenated hydrocarbons that were tested. It is found that there is a lack of standard reference enthalpies of formation for liquid-fuel size oxygenated hydrocarbons for both molecules and radicals. In their absence, a benchmarking of the computational methodologies employed in this study to values available in the Active Thermochemical Tables for smaller oxygenated species is performed. The results show that the atomization methods performed at each separate theoretical method show mean absolute deviations against the benchmarking data set of between 0.4–0.7 kcal/mol for radicals, and 0.6–1.9 kcal/mol for molecules. By combining the data obtained with each individual method by unweighted averaging, mean absolute deviations of 0.3 and 0.5 kcal/mol for radicals and molecules can respectively result. However, the computed enthalpies of formation via the atomization procedure of larger species, of the ethyl levulinate system show variations of between 4.3–6.2 kcal/mol for molecules and 0.9–3.4 kcal/mol for radicals. As the values generated by the isodesmic reaction procedure show lower variations of 0.5–0.8 kcal/mol for radicals and 2.3–3.8 kcal/mol for molecules. Consequently, in the absence of more appropriate reference data to benchmark computational methodologies, this study recommends the use of isodesmic reactions executed at multiple theoretical methods for the determination of enthalpies of formation. [ABSTRACT FROM AUTHOR]

Structure of six-membered cyclic aminoiminocarbenes (pyrimidine ylidenes) and their group 13–15 homologues was studied by quantum chemical calculations. Isodesmic and dimerization reaction energies, NICS values, frontier molecular orbitals, as well as NBO, Bader’s “atoms in molecules” and Laplacian bond order (LBO) analyses were employed for estimating relative stability of the studied carbenes (carbene homologues). They show a low degree of aromatic delocalization and considerable variation of electronic structure depending on the nature of divalent element. The predicted stability for the series of interest is lower than that for the corresponding five-membered Arduengo carbenic structures, but comparable with Alder’s bis(dimethylamino)carbene. [ABSTRACT FROM AUTHOR]

Molecular structures of 1-hetero-2,5-cyclohexadiene-1-oxide, XO, are studied and compared with their corresponding deoxygenated compound, 1-hetero-2,5-cyclohexadiene, X, using DFT at B3LYP/6-311++G** level. The most stable boat conformers are found for OO, SO, SeO, PO, AsO, S, Se, N, P, As, and the most stable planar conformers are also found for NO, CO, SiO, GeO, O, C, Si, Ge. Isodesmic reactions to determine the stabilities of XO and X are considered. Nuclear independent chemical shifts, NICS, are calculated for the investigation of the homo-aromatic character of XO and X. The optimised geometries show the bonding in the molecules is explicable in terms of basic chemical concepts. The atomic charges calculated are also reasonable based on the concepts of electronegativity and conjugation.

In this article the detonation performance and stability of the benzene derivatives were studied theoretically to facilitate further developments of energetic materials. The gas-phase heats of formation were calculated based on the isodesmic reaction. The solid-phase heats of formation and heats of sublimation were estimated in the framework of the Politzer approach. Molecular stability was calculated by bond dissociation energies and characteristic height. Detonation velocity and pressure were obtained according to Kamlet-Jacobs equations. The results show that all compounds have positive heats of formation, and compounds D1, D2, D3, E, and F have high detonation performance. In addition, the breaking of N‒N bond may be initial step in initiation process. Furthermore, theoretical result suggestions that the most of derivatives have good thermal stability. These calculations could provide basic information that may prove useful for the molecular design of novel high-energy density materials. [ABSTRACT FROM AUTHOR]

Theoretical prediction of four stepwise protonation constants of 1,4,7,10-tetraazadecane (2,2,2-tet) in correct order and with the smallest (largest) deviation of about 0.1 (−0.8) log unit from experimental values was achieved by an explicit application of a competition reaction (CRn) methodology in discrete-continuum solvation model involving four explicit water molecules. This methodology performs best when (1) tested (L(1)) and reference (L(2)) molecules are structurally similar, (2) lowest energy conformers (LECs, selected from all possible tautomers) are used, and (3) a CRn, which assures a balanced charge distribution between reactants and products, Hn−1L(1) + HnL(2) = HnL(1) + Hn−1L(2), is implemented. A 5-step EEBGB protocol was developed to effectively and in shortest time possibly select LECs (E, B and G stands for electronic-energy-, Boltzmann-distribution- and Gibbs-free-energy-based stepwise selection of conformers). The EEBGB protocol (1) reduced (by 94 %) the number of conformers subjected to the frequency calculations (to obtain G values) from 420 MM-selected to 25 used to compute four protonation constants and (2) is of general purpose as it is applicable to any flexible and polycharged molecules. Moreover, in search for LECs, a rapid pre-screening protocol was developed and tested; it was found efficient for the purpose of this study. Additional research protocols, aimed at even better prediction of protonation constants, are also suggested. [ABSTRACT FROM AUTHOR]

The pyrolysis of γ-valerolactone, a cyclic ester readily obtainable from cellulosic biomass and which has potential as a fuel, was investigated from 873 to 1113 K at 0.17 MPa in a tubular continuous flow reactor. Unimolecular decomposition of γ-valerolactone forms 4-pentenoic acid and its selectivity increases upon addition of toluene, a radical scavenger. A kinetic model, consisting of 520 species and 3589 reactions, was developed. Kinetic and thermodynamic data relevant for γ-valerolactone consumption, including hydrogen abstraction, hydrogen addition and fuel radical decomposition, were calculated at the CBS-QB3 level of theory. Experimental and calculated mole fraction profiles are in good agreement. Reaction path analysis shows that γ-valerolactone is mainly consumed by isomerization to 4-pentenoic acid and hydrogen abstraction from the carbon atom α to the carbonyl group and the carbon atom α to the ring oxygen. C C scission of 4-pentenoic acid forming allyl and carboxy methyl is an important source of radicals. [ABSTRACT FROM AUTHOR]

To look for superior and safe high-energy density materials (HEDMs), tris(triazolo)benzene and its R = ‒CN, ‒NHNO2, ‒N(NO2)2, ‒NH2, ‒N(NH2NO2), and ‒N(NO2ONO2) derivatives were studied at the B3LYP/6-311G(d, p) level of density functional theory (DFT). The energy content of the molecules was evaluated by calculating standard heats of formation, using isodesmic reaction. The results showed that all tris(triazolo)benzene derivations had high and positive heat of formation. The bond dissociation energies and bond orders for the weakest bonds were analyzed to investigate the thermal stability of the tris(triazolo)benzene derivatives. Most tris(triazolo)benzene derivatives for Ring-R had large bond dissociation energies (BDEs) which were over 140kJ · mol− 1except R = ‒N(NO2ONO2), which showed that the tris(triazolo)benzene derivatives had good thermodynamic stability. Impact sensitivity was evaluated using frontier orbital energies and characteristic heights (H50). These results indicated that incorporation with ‒NH2is an effective means to decrease molecular sensitivity. Detonation velocities and pressures were estimated using modified Kamlet-Jacobs equations, based on the theoretical densities and heats of formation. These derivatives possess excellent detonation properties, for C, E, and, the detonation velocity are 9.38, and 9.98 km.s−1, and the detonation pressure are 43.68, and 48.41Gpa, respectively, the detonation performances are better than cyclotetramethylenetetranitramine (HMX). The detonation character of A, B, D, G, and F also are very close to that of HMX. Taking detonation performance and stability into consideration, the most of tris(triazolo)benzene derivatives may be good candidates of high energies materials. [ABSTRACT FROM AUTHOR]

A theoretical study on the mechanism and kinetics of the gas phase reactions of CF 3 CHFCF 2 OCH 2 CF 3 (HFE-449mec-f) with the OH radicals and Cl atom have been performed using meta-hybrid modern density functional M06-2X using 6-31+G(d,p) basis set. Two conformers have been identified for CF 3 CHFCF 2 OCH 2 CF 3 and the most stable one is considered for detailed study. Reaction profiles for OH-initiated hydrogen abstraction are modeled including the formation of pre-reactive and post-reactive complexes at entrance and exit channels. Our calculations reveal that hydrogen abstraction from the CH 2 group is thermodynamically and kinetically more facile than that from the CHF group. Using group-balanced isodesmic reactions, the standard enthalpies of formation for HFE-449mecf and radicals generated by hydrogen abstraction, are also reported. The calculated bond dissociation energies for C H bonds are in good agreement with experimental results. The rate constants of the two reactions are determined for the first time in a wide temperature range of 250–450 K. The calculated rate constant values are found to be 9.10 × 10 −15 and 4.77 × 10 −17 cm 3 molecule −1 s −1 for reactions with OH radicals and Cl atom, respectively. At 298 K, the total calculated rate coefficient for reactions with OH radical is in good agreement with the experimental results. The atmospheric life time of HFE-449mec-f is estimated to be 0.287 years. [ABSTRACT FROM AUTHOR]

An azo bridge (-N = N-) can not only desensitize explosives but also dramatically increase their heats of formation and explosive properties. Amino and nitro are two important high energy density functional groups. Here, we present calculations on 1-nitro-1-triazene (NH-N = N-NO). Thermal stability and detonation parameters were predicted theoretically at CCSD(T)/6-311G* level, based on the geometries optimized at MP2/6-311G* level. It was found that the p → π conjugation interaction and the intramolecular hydrogen bonding that exist in the system together increase the thermal stability of the molecule. Moreover, the detonation parameters were evaluated to be better than those of the famous HMX and RDX. Finally, the compound was demonstrated to be a high energy density material. [ABSTRACT FROM AUTHOR]

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

The kinetics of hydrogen atom abstraction reactions of methyl difluoroacetate (CF2HCOOCH3) by OH radical has been studied by quantum mechanical method. The geometry optimisation and frequency calculation of the titled compound was performed with density functional theory using hybrid meta density functional MPWB1K with 6-31+G(d,p) basis set. Transition states have been determined and intrinsic reaction coordinate (IRC) calculation has been performed to ascertain that the transition from reactants to products was smooth through the corresponding transition state. Energy values are refined by making single point energy calculation at G3B3 level of theory and an energy level diagram is constructed. The standard enthalpies of formation of reactants and other species formed during the reaction were calculated using isodesmic method. The rate constants are calculated using canonical transition state theory and the overall rate constant is determined to be 1.35×10−13cm3molecule−1s−1at 298 K and 1 atmospheric pressure. Tunnelling has been taken into account in the determination of the rate constant because it plays a critical role at low temperature especially when transfer of hydrogen takes place. The calculated value is found to be in good agreement with the experimentally determined value of 1.48×10−13cm3molecule−1s−1. [ABSTRACT FROM PUBLISHER]

Abstract: Bending at the valence angle N–Cα–C′ (τ) is a known control feature for attenuating the stability of the rare intramolecular i→i hydrogen bonded pseudo five-membered ring C5 structures, the so called 2.05 helices, at Aib. The competitive 310-helical structures still predominate over the C5 structures at Aib for most values of τ. However at Aib∗, a mimic of Aib where the carbonyl O of Aib is replaced with an imidate N (in 5,6-dihydro-4H-1,3-oxazine=Oxa), in the peptidomimic Piv-Pro-Aib∗-Oxa (1), the C5i structure is persistent in both crystals and in solution. Here we show that the i→i hydrogen bond energy is a more determinant control for the relative stability of the C5 structure and estimate its value to be 18.5±0.7kJ/mol at Aib∗ in 1, through the computational isodesmic reaction approach, using two independent sets of theoretical isodesmic reactions. [Copyright &y& Elsevier]

The density functional theory (DFT) calculations at the M06-2X/6-31++G(d,p) level have been performed to explore the molecular structure, electronic structure, C-H bond dissociation enthalpy, and reaction enthalpies for five isodesmic reactions of a high energy-density endothermic hydrocarbon fuel JP-10. On the basis of the calculations, it is found that the carbonium ion C-6 isomer formed from the catalytic cracking at the C site of JP-10 has the lowest energy, and the R-5 radical generated from the thermal cracking at the C site of JP-10 is the most stable isomer. Furthermore, a series of hypothetical and isodesmic work reactions containing similar bond environments are used to calculate the reaction enthalpies for target compounds. For the same isodesmic reaction, the reaction enthalpy of each carbon site radical has also been calculated. The present work is of fundamental significance and strategic importance to provide some valuable insights into the component design and energy utilization of advanced endothermic fuels. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Density function theory calculations were performed to find comprehensive relationships between the structures and properties of a series of highly energetic polynitroxanthines. The isodesmic reaction method was employed to estimate the gas-phase heat of formation. The solid-state heats of formation for the designed compounds were calculated by the Politzer approach using heats of sublimation. All of the designed compounds were found to possess solid-state heats of formation of >100 kJ mol. Detonation performances were evaluated by the Kamlet-Jacobs equations, based on the predicted densities and solid-state heats of formation. The results indicate that all of the compounds have excellent detonation velocities and pressures. The stabilities of the derivatives were calculated by evaluating their energy gaps, bond dissociation energies, and characteristic heights. The results indicate that all of the compounds have large bond dissociation energies and energy gaps. The characteristic height values of the compounds are more than or close to those of HMX and RDX. Thus, the polynitroxanthine derivatives show good thermodynamic and dynamic stability. Further, the present study may provide useful information on the structure-property relationships of these compounds, and for the development of novel high-energy materials. [ABSTRACT FROM AUTHOR]

Density functional theory studies on cyclic nitramines were performed at B3LYP/6-311G(d,p) level. The crystal structures were obtained by molecular packing calculations. Heats of formation (HOFs) were predicted through designed isodesmic reactions. Results indicate that the value of HOF relates to the number of =N-NO2 group and aza nitrogen atom and increases with the augment of the number of =N-NO2 group and aza nitrogen atom for cyclic nitramines. Detonation performance was evaluated by using the Kamlet-Jacobs equations based on the calculated densities and HOFs. All the cyclic nitramines exhibit better detonation performance than 1,3,5-trinitro-1,3,5-triazacyclohexane and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane. The stability of cyclic nitramines was investigated by the bond dissociation energies. The result shows that the increase of =N−NO2 group or aza nitrogen atom reduces the stability of the title compounds. These results provide basic information for molecular design of novel high energetic density materials. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A series of purine derivatives with nitramine groups are calculated by using density functional theory (DFT). The molecular theory density, heats of formation, bond dissociation energies and detonation performance are investigated at DFT-B3LYP/6-311G** level. The isodesmic reaction method is employed to calculate the HOFs of the energies obtained from electronic structure calculations. Results show that the position of nitramine groups can influence the values of HOFs. The bond dissociation energies and the impact sensitivity are analyzed to investigate the thermal stability of the purine derivatives. The calculated bond dissociation energies of ring-NHNO and NH-NO bond show that the NH-NO bond should be the trigger bond in pyrolysis processes. The H of most compounds are larger than that of CL-20 and RDX. [ABSTRACT FROM AUTHOR]

1-Methyl-2,4,5-trinitro imidazole (MTNI) is a well-known melt cast explosive possessing good thermal stability and impact insensitivity. MTNI has been synthesized from multi-step nitration followed by methylation of imidazole exhibiting low yield. It is desirable to screen the process thermodynamically for evaluating feasibility of the process. In the present investigations, B3LYP method in combination with 3-21G** basis set has been chosen to evaluate the enthalpy of formation for reaction species by designing reasonable isodesmic reactions. Thermodynamic feasibility of the processes has been worked out assuming free energies of reactions as derived from standard enthalpy and entropy of the reaction species. All possible synthesis routes for the target molecule, MTNI has been conceptualized from different precursors/intermediates viz. imidazole, 2-nitroimidazole, 4-nitroimidazole, 1-methyl imidazole and 2,4,5-triiodoimidazole. Various nitrating agents have been employed and their effect studied for deciding the feasibility of the reaction. It has been found that reaction entropy and enthalpy are favorable on usage of NOBF as nitrating agent. The charge on nitro group (−Q) has been used for better understanding of the reactivity of substrates/intermediates. Overall, the study helped in screening several possible routes for MTNI synthesis and identified the thermodynamically feasible process by using NOBF as nitrating agent. [ABSTRACT FROM AUTHOR]

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

Abstract: DFT computations coupled with appropriate isodesmic reactions are employed to investigate the effects of mono-heteroatom substitution, cyclization and unsaturation on the stability, multiplicity and reactivity of amino-, oxy-, silyl-, phosphino-, and thioalkylgermylenes. The results are based on the references to the parent symmetrical acyclic, saturated/unsaturated cyclic dialkylgermylenes. The calculated singlet–triplet energy gaps (ΔE S–T) demonstrate the following trend: (amino≈oxy)>thio>alkyl>phosphino>silyl. Cyclization increases the ΔE S–T of the all germylenes, but unsaturation considerably decreases it. Isodesmic reactions show that π-donor/σ-acceptor amino, oxy and thio stabilize both singlet and triplet germylenes but with a more considerable effect on the former while phosphino acts reversely. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity and proton affinity issues showing silyl and phosphinoalkylgermylenes more nucleophilic and more basic than amino and oxyalkylgermylenes. [Copyright &y& Elsevier]

Density function theory has been employed to study pyridine derivatives at the B3LYP/6-31 G(d,p) and B3P86/6-31 G(d,p) levels. The crystal structures were obtained by molecular mechanics methods. The heats of formation (HOFs) were predicted based on the isodesmic reactions. Detonation performance was evaluated by using the Kamlet-Jacobs equations based on the calculated densities and HOFs. The thermal stability of the title compounds was investigated by the bond dissociation energies and the energy gaps (Δ ELUMO−HOMO) predicted. It is found that there are good linear relationships between detonation velocity, detonation pressure, and the number of nitro group. The simulation results reveal that molecule G performs similar to the famous explosive HMX and molecule D outperforms HMX. According to the quantitative standard of energetics and stability as high energy density materials, molecule D essentially satisfies this requirement. These results provide basic information for molecular design of novel high energetic density materials. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Based on DFT-B3LYP/6-311G** method, the molecular geometric structures of polynitramineprismanes are fully optimized. The detonation performances, energy gaps, strain energies, as well as their stability were investigated to look for high energy density compounds (HEDCs). Our results show that all polynitramineprismanes have high and positive heat of formation. To construct the relationship between stabilities and structures, energy gaps and bond dissociation energies are calculated, and these results show that the energy gaps of prismane derivatives are much higher than that of TATB (0.1630). In addition, the C-C bonds on cage are confirmed as trigger bond in explosive reaction. All polynitramineprismanes have large strain energies, and the strain energies of all compounds are slightly smaller than prismane, which indicated that the strain energies were somewhat released compared to prismane. Considering the quantitative criteria of HEDCs, hexanitramineprismane is a good candidate of high energy compounds. [ABSTRACT FROM AUTHOR]

Three quantum chemistry methods (B3LYP, M05-2X and CBS-4B3*) have been used, in combination with SMD and CPCM continuum solvent models, to calculate the aqueous p K values of common organic compounds (aliphatic alcohols, carboxylic acids, amines, phenols, benzoic acids and pyridines) by using an isodesmic reaction. Good precision is found for all the studied functional groups, resulting mean absolute deviations of 0.5-1 p K units (equivalent to the best results obtained with thermodynamic cycles). It is worthy to note that no explicit water molecules were needed with the isodesmic reaction. In addition, the quality of the results is not strongly dependent on the combination of quantum chemistry method, solvent model and reference species. Therefore, the isodesmic reaction could be successfully used when dealing with gas-phase unstable species, with species that undergo large conformational changes between gas-phase and solution-phase or other difficult cases for the thermodynamic cycles. [ABSTRACT FROM AUTHOR]

Abstract: Following the quest for stable silylenes, we have found high thermodynamic and kinetic viabilities for a series of novel cyclic alkylaminosilylenes [CAASs: including 1N –4N )] over their corresponding cyclic alkylphosphinosilylenes (CAPSs: 1P –4P , respectively), at ab initio and DFT levels, coupled with appropriate isodesmic reactions. Among silylenes scrutinized, 4N immerged the most promising for its higher singlet–triplet energy difference (ΔE S–T), wider band gap (ΔE HOMO–LUMO), higher heat of hydrogenation (ΔE H), significant resistance to dimerization and resistance to rearrangement to its full valence octet isomer, etc. Unsaturation destabilized 2N , 3N , and 3P through cross conjugation. [Copyright &y& Elsevier]

The derivatives of purine are designed through substituting the hydrogen atoms on it for nitro and amino functional groups. Geometries and frequency are analyzed at the B3LYP/6-31 G** level of density functional theory (DFT). Heats of formation (HOF), bond dissociation energy (BDE) and detonation parameters (detonation velocity and detonation pressure) are obtained in detail at the same level. It is found that the BDE values of all derivatives are over 120KJ·mol, and have high positive heats of formation. These derivatives possess excellent detonation properties, for B1, B2, and C, the detonation velocity are 9.58, 9.57, and 9.90 km·s, and the detonation pressure are 43.40, 46.05, and 46.37 Gpa, respectively, the detonation performances are better than cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX). Hence, the derivations of purine may be promising well-behaved high energy density materials. [ABSTRACT FROM AUTHOR]

α-Cyclopropyl stability impacts on singlet and triplet heterocyclic carbenes with acyclic, cyclic, and cyclic-unsaturated structures are compared and contrasted to di- t-butyl as well as t-butylcyclopropylcarbenes through appropriate isodesmic reactions at B3LYP/AUG-cc-pVTZ level. Substitution of one of the t-butyl groups of di- t-butylcarbene with a cyclopropyl alters the ground state multiplicity from triplet to singlet with a singlet-triplet energy separation (Δ Es-t) of 7.2 kcal/mol. Additional heteroatom substitution increases Δ Es-t values for the resulting α-heteroatom cyclopropylcarbenes in the following order: amino > oxy > thio > phophino. α-Cyclopropyl group stabilizes singlet states of all our carbenes two to three times more than their corresponding triplet states. The Δ Es-t values of all the carbenes are increased through cyclization, while the introduction of unsaturation in the rings causes small and rather random changes. To probe the kinetic stability of the species, we calculated the transition states for the opening of cyclopropyl through 1,2- C shift. Interestingly, the 4.1 kcal/mol energy barrier in cyclopropylcarbene is significantly increased in the presence of heteroatoms to 31.2 kcal/mol for aminocyclopropylcarbene. The reactivity of the species is discussed in terms of nucleophilicity and electrophilicity issues showing our carbenes, especially acyclic ones, more nucleophilic than the common N-heterocyclic carbenes. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This study aimed to design novel nitrogen-rich heptazine derivatives as high energy density materials (HEDM) by exploiting systematic structure-property relationships. Molecular structures with diverse energetic substituents at varying positions in the basic heptazine ring were designed. Density functional techniques were used for prediction of gas phase heat of formation by employing an isodesmic approach, while crystal density was assessed by packing calculations. The results reveal that nitro derivatives of heptazine possess a high heat of formation and further enhancement was achieved by the substitution of nitro heterocycles. The crystal packing density of the designed compounds varied from 1.8 to 2 g cm, and hence, of all the designed molecules, nitro derivatives of heptazine exhibit better energetic performance characteristics in terms of detonation velocity and pressure. The calculated band gap of the designed molecules was analyzed to establish sensitivity correlations, and the results reveal that, in general, amino derivatives possess better insensitivity characteristics. The overall performance of the designed compounds was moderate, and such compounds may find potential applications in gas generators and smoke-free pyrotechnic fuels as they are rich in nitrogen content. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

CBS-QB3, two simplified and less computationally demanding versions of CBS-QB3, DFT-B3LYP, and HF quantum chemistry methods have been used in conjunction with the CPCM continuum solvent model to calculate the free energies of proton exchange reactions in water solution following an isodesmic reaction approach. According to our results, the precision of the predicted p K values when compared to experiment is equivalent to that of the thermodynamic cycles that combine gas-phase and solution-phase calculations. However, in the aqueous isodesmic reaction schema, the accuracy of the results is less sensitive to the presence of explicit water molecules and to the global charges of the involved species since the free energies of solvation are not required. In addition, this procedure makes easier the prediction of p K values for molecules that undergo large conformational changes in solvation process and makes possible the p K prediction of unstable species in gas-phase such as some zwitterionic tautomers. The successive p K values of few amino acids corresponding to the ionization of the α-carboxylic acid and α-amine groups, which is one of the problematic cases for thermodynamic cycles, were successfully calculated by employing the aqueous isodesmic reaction yielding mean absolute deviations of 0.22 and 0.19 p K units for the first and second ionization processes, respectively. [ABSTRACT FROM AUTHOR]

The tetrazole is an important functionality of the most of energetic materials due to 80% nitrogen content, stability, and high enthalpy of formation. The present structure-property relationship study focuses on the optimized geometries of tetrazole derivatives obtained from density functional theory (DFT) calculations at B3LYP/6-31G* levels. The heat of formation (HOF) of tetrazole derivatives have been calculated by designing the appropriate isodesmic reactions. The increase in nitro groups on azole rings shows the remarkable increase in HOF. Density has been predicted by using CVFF force field. Increase in the nitro group increases the density. Detonation properties of the designed compounds were evaluated by using the Kamlet-Jacobs equation based on predicted densities and HOFs. Designed tetrazole derivatives show detonation velocity ( D) over 8 km/s and detonation pressure ( P) of about 32 GPa. Thermal stability was evaluated via bond dissociation energies (BDE) of the weakest C-NO bond at B3LYP/6-31G* level. Charge on the nitro group has been used to assess the sensitivity correlation. Overall, the study implies that designed compounds of this series are found to be stable and expected to be the novel candidates of high energy materials (HEMs). [ABSTRACT FROM AUTHOR]

Abstract: Strain has been defined as a source of errors in the determination of heats of formations using bond separation reactions. In this work, we define Ring Conserving Isodesmic Reactions to conserve the strain. First a set of reference systems is defined to reproduce the different cyclic patterns. Those references are then used on bicyclohexadienes to evaluate the effect on the resulting heats of formation. In a last section, the successive heats of hydrogenation of the different systems are analyzed. [ABSTRACT FROM AUTHOR]

In regard to the worldwide interest in synthesis and application of stable carbenes, ab initio and DFT calculations (CCSD(T)/6-311G*//QCISD/6-31G* and B3LYP/AUG-cc-pVTZ//B3LYP/6-31 + G* levels) are employed to reach at a series of symmetrically substituted diheteroatom carbenes. In this way the multiplicity, stability, and reactivity of diethylcarbene and its saturated and unsaturated cyclic congeners are compared and contrasted to the corresponding disilyl-, diamino-, diphosphino-, dioxy-, and dithiocarbenes. The investigations reveal the enlargement of the singlet-triplet energy gaps (Δ E) in the order of (amino ≈ oxy) > thio > phosphino > alkyl > silyl. The observed trend is thoroughly analyzed applying appropriate isodesmic reactions which differentiate the substituent effects on each of the singlet and triplet states of all the carbenes. In contrast to previous reports, it is found that π-donor/σ-acceptor amino substituents stabilize not only the singlet but also the triplet states. The effects of cyclization and unsaturation are also probed in each series of the symmetrically substituted carbenes. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity. Copyright © 2010 John Wiley & Sons, Ltd. High levels of ab initio and DFT calculations on the group 14-16 substituted carbenes reveal the enlargement of the singlet-triplet energy gap (Δ E) in order of amino≈oxy>thio>phosphino>alkyl>silyl. Cyclization has an increasing effect on the Δ E values of the carbenes with the exception of disilyl- and diphosphinocarbenes. The Δ E values of group 15 and 16 cyclic carbenes further increase under the effect of unsaturation. [ABSTRACT FROM AUTHOR]