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Straightforward deprotonation of soluble tetramethylammonium salts with alkyllithium reagents gives lithiomethyl trimethylammonium reagents. Coordination of the Li cation is crucial to the stability of these 'N--C ylides'. These reagents were used to prepare epoxides, aziridines and allylic alcohols. [ABSTRACT FROM AUTHOR]

The density functional theory (DFT) calculation is performed on 1,1,3,5,5-pentanitro-1,5-bis(difluoramino)-3-azapentane substituted. The heat of formation (HOF) is predicted by B3LYP and B3P86 methods with the 6-311G** and 6-311++G** basis sets via isodesmic reactions. With NF and ONO substitution for NO, HOFs clearly decrease. Furthermore, we designed a detonation reaction for each compound and computed Δ H, the heat of explosion ( Q), and Δ G for each reaction. The general trend is that Q increases as ONO and NO groups are replaced by the NF groups. [ABSTRACT FROM AUTHOR]

PhSF51 reacts with NO2+BF4-/TfOH in CH2Cl2 (DCM) at room temperature to give 1-nitro-3-(pentafluorosulfanyl)benzene 2 in near quantitative yield. The dinitro derivative 4 is synthesized from 2 by reaction with NO2+BF4-/TfOH at 70 °C. The p-MeC6H4SF5 is mononitrated at room temperature with NO2+BF4-/DCM and dinitrated with NO2+BF4-/TfOH. Substrate selectivity (...) in competitive nitration for PhSF5/PhCF3 and PhSF5/PhNO2 with NO2+BF4- in DCM at room temperature was determined at 21.3 and ~1 respectively. Relative stability of the corresponding benzenium ions were gauged by DFT from the isodesmic proton transfer reaction SF5-C6H6+ + R-C6H5 → SF5-C6H5 + R-C6H6+ (R = CF3 and NO2). These studies indicate that reactivity of ArSF5 in SEAr is similar to ArNO2. [ABSTRACT FROM AUTHOR]

Photodeoxygenation of dibenzothiophene S-oxide (DBTO) has been suggested as a clean way to generate atomic oxygen in solution. Sulfoxide bond dissociation enthalpies (BDEs) are important to the quantum yield and mechanism of this photodeoxygenation. In this study, BDE of substituted DBTO molecules with various functional groups were determined using M062X/aug-cc-pV(T + d)Z and MP2/aug-cc-pV(T + d)Z levels of theory. The sulfoxide BDE was determined using an isodesmic reaction. The observed effect of substitution was that functional groups with negative σpara values (electron donating) strengthen the sulfoxide bond and that substituents with positive σpara values (electron withdrawing) weaken the sulfoxide bond. The Mulliken charge on sulfur for DBTO and the corresponding dibenzothiophene were identified as a suitable indicator of the predicted S-O BDE. Moreover, steric and hydrogen bonding interactions were found to affect sulfoxide BDE for certain functional groups when located at the 1 or 4 positions of the substituted DBTO. Copyright © 2014 John Wiley & Sons, Ltd. [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]

Different high-level quantum chemical composite methods such as the Gaussian-n and Weizmann-n theories have been used to derive enthalpy of formation values from calculated atomization energies and scheme isodesmic reactions of peroxide pollutants. The resulting values of ΔH°f298K de HOOH, HOOF, HOOCl, HOONO, FOOF, FOOCl, FOONO, ClOOCl y ClOONO son -32,2 ± 0,8; -10,9 ± 1,1; -8,4x10 - 2 ± 1,2; -2,9 ± 0,7; 8,4±1,6; 21,5±1,3; 19,7±1,3; 32,3 ± 1,4 and -31,3 ± 0,8 kcal mol-11, respectively. A comparison with previous experimental and theoretical values are presented highlighting the high chemical accuracy with low computational cost. [ABSTRACT FROM AUTHOR]

Highlights: [•] Old data on DADP and TATP enthalpies of formation have been revised. [•] Combining Gaussian-4 (G4) theory with an isodesmic reaction scheme allowed calculated enthalpies of formation of TATP and DADP. [•] Oxygen bomb calorimetry measurements allowed experimental enthalpies of formation of the peroxides. [•] Both experimental and calculated values show a satisfactory agreement between each other. [•] The newly obtained enthalpies reasonably account for the observed derivative parameters: heats of decomposition, combustion, and explosion. [ABSTRACT FROM AUTHOR]

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]

Accurate gas-phase enthalpies offormation (ΔfH298°) of 20 common α-aminoacids, seven uncommon aminoacids, and three small peptides were calculated by combining G4 theorycalculations with an isodesmic reaction approach. The internal consistencyover a set of ΔfH298°(g) values was achievedby sequential adjustment of their values through the isodesmic reactions.Four amino acids, alanine, β-alanine, sarcosine, and glycine,with reliable internally self-consistent experimental data, were chosenas the key reference compounds. These amino acids together with about100 compounds with reliable experimental data (their accuracy wassupported by G4 calculations) were used to estimate the enthalpiesof formation of remaining amino acids. All of the amino acids withthe previously established enthalpies of formation were later usedas the reference species in the isodesmic reactions for the otheramino acids. A systematic comparison was made of 14 experimentallydetermined enthalpies of formation with the results of calculations.The experimental enthalpies of formation for 10 amino acids were reproducedwith good accuracy, but the experimental and calculated values for4 compounds differed by 11–21 kJ/mol. For these species, thetheoretical ΔfH298°(g) values were suggestedas more reliable than the experimental values. On the basis of theoreticalresults, the recommended values for the gas-phase enthalpies of formationwere also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpiesof sublimation were evaluated for all compounds by taking into accountthe literature data on the solid-phase enthalpies of formation andthe ΔfH298°(g) values recommended in our work.A special attention was paid to the accurate prediction of enthalpiesof formation of amino acids from the atomization reactions. The problemsassociated with conformational flexibility of these compounds andharmonic treatment of low frequency torsional modes were discussed.The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactionsis largely the result of a fortuitous mutual compensation of variouscorrections used in the atomization reaction procedure. [ABSTRACT FROM AUTHOR]

We synthesised and characterised eleven complexes of the biomimetic bis(pyrazolyl)methanes (2-pyridinyl)bis(pyrazolyl)methane [HC(Pz)2(Py)] and (2-quinolinyl)bis(pyrazolyl)methane [HC(Pz)2(Qu)] with the transition metals cobalt, iron, copper and zinc. With the pyridinyl ligand HC(Pz)2(Py), we obtained the bifacial complexes [Co{HC(Pz)2(Py)}2]Cl2 ·2CH3CN ( C1), [Co{HC(Pz)2(Py)}2][CF3SO3]2 ( C2), [Fe{HC(Pz)2(Py)}2]Cl2 ·2CH3OH [ C3], [Cu{HC(Pz)2(Py)}2]Cl2 ·2CH3OH ( C4a) and [Cu{HC(Pz)2(Py)}2][CuCl3H2O] ( C4b) and the monofacial complex [CuCl2{HC(Pz)2(Py)}] ·CH3OH ( C5); with the quinolinyl ligand HC(Pz)2(Qu), we obtained the bifacial complexes [Co{HC(Pz)2(Qu)}2][CoCl4] ·4CH3CN ·0.88CH2Cl2 ( C6) and [Fe{HC(Pz)2(Qu)}2][FeCl4] ·2CH3CN ( C7), the monofacial complexes [FeCl2{HC(Pz)2(Qu)}] ( C8) and [CuBr2{HC(Pz)2(Qu)}] ( C9) and the bifacial zinc complex [Zn{HC(Pz)2(Qu)}2][ZnCl4] ( C10). We explored the donor properties of the free ligands and the ligands in these complexes and found that the delicate donor competition is an intriguing field. By second-order perturbation theory, pyrazolyl units are the stronger donors, although pyridinyl units are more basic. Isodesmic reactions indicate that the pyridinyl donors give a stronger complex stabilisation. This small bias can be disturbed by subtle variations of the coordination geometry, which is important in the bioinorganic context. [ABSTRACT FROM AUTHOR]

Highlights: [•] First detail theoretical study using DFT and ab initio methods. [•] Hydrogen abstraction from the CH2 group is more facile. [•] Thermochemistry including BDE (D 0 298) and Δ f H°298 are also reported. [•] Fate of alkoxy radicals are also reported for the first time. [•] Present study solves the controversy on CH2 versus CH3 group contributions. [ABSTRACT FROM AUTHOR]

The theoretical pKa values of the derivatives of anilinium and pyridinium ions in 7 solvents are presented. For this purpose, the usage of isodesmic reaction scheme using the DFT/B3LYP approach with IEFPCM solvation was evaluated. We have shown that the suitable selection of reference species has the primary influence on the resulting data. For the studied anilinium ion derivatives the nonsubstituted anilinium ion seems to be a satisfactory reference system. The calculated values are in good accordance with the available experimental data with the RMS error of 1.00 and 0.99 pKa units in water and THF, respectively. The highest error in predicted pKa value is less than 2.0 pKa units in all cases. The chemical accuracy of the applied treatment is limited in the case of nitroaniline ions and the maximal therotetical uncertainty for derivatives of the pyridinium ion is within 2.1 pKa units. Our theoretical results enable us to predict the values of pKa for the solvents, where the experimental data are not completely available. Also the influence of the chemical structure on the accuracy of the applied method was discussed. [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]

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]

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

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

Coupled-cluster investigations (CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ) of singlet cyclobutadiene and fifteen-substituted analogs were conducted. A local minimum with a square frame does not exist on their potential surfaces. The well-known rectangular D minimum, the square D transition state, and two additional stationary points were found on cyclobutadiene's potential surface. This included a transition state with a rhombic carbon ring and C symmetry, separating two equivalent puckered C local minima. The predicted barriers were 19.7 and 19.8 kcal/mol at the CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ levels, respectively. The relative strain energies of rectangular D cyclobutadiene and all fifteen-substituted analogs were obtained from isodesmic reactions. Progressive substitution with methyl or BH groups continuously lowers ring strain while increasing substitution with fluorines or trifluoromethyl groups steadily increases ring strain. C(BH) is 16.6 and 13.3 kcal/mol less strained than cyclobutadiene while CF is 17.7 and 21.5 kcal/mol more strained at the levels above. Cyclobutadiene is more strained than both cyclopropene and cyclobutene by 12.2 and 37.0 kcal/mol, respectively. Electron density contours indicate that fluorine substitution raised the electron density especially in the short C=C ring bonds above/below the ring plane (π-electrons) but not in the ring plane (σ-electrons). BH-substitutions lower the ring π-electron density with little effect in the ring plane. Methyl substituents have little effect on electron densities. All rings retain a strong bond alternation tendency (rectangular) whether substituted with electron-donating or -attracting groups. One-bond coupling constants and the percent p-character in ring C-to-C and C-to-substituent bonds are described. [ABSTRACT FROM AUTHOR]

The thermochemical properties of superbase species derived from cyclopropeneimine are estimated by computations on systems of isodesmic reactions. Proton affinities, gas phase basicities, and p Ka values are well represented by computations incorporating G4 and CBS-QB3 schemes augmented for large systems by DFT calculations with functionals M06-2X and ωB97DX in the cc-pVTZ Dunning basis. Our calculations show that relative base strengths in gas are enhanced by alkyl substitution, either by methyl groups or larger species. For acetonitrile solution, alkyl substitution seems to weaken the base. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A number of hexaarylbenzene compounds were studied theoretically, in order to compare energy changes as a result of the toroidal delocalization effect that is characteristic of all these species. The energy was studied taking advantage of locally designed isodesmic reactions. Results indicate that the amount of aromaticity manifested by each substituent is a factor that should be considered when assessing the quantity of energy dissipated from each aromatic center. The influence of different substituents on electronic delocalization is also analyzed, as well as the role played by their frontier molecular orbitals. [ABSTRACT FROM AUTHOR]

Various conventional reactions in polymer chemistry have been translated to the supramolecular domain, yet it has remained challenging to devise living supramolecular polymerization. To achieve this, self-organization occurring far from thermodynamic equilibrium-ubiquitously observed in nature-must take place. Prion infection is one example that can be observed in biological systems. Here, we present an 'artificial infection' process in which porphyrin-based monomers assemble into nanoparticles, and are then converted into nanofibres in the presence of an aliquot of the nanofibre, which acts as a 'pathogen'. We have investigated the assembly phenomenon using isodesmic and cooperative models and found that it occurs through a delicate interplay of these two aggregation pathways. Using this understanding of the mechanism taking place, we have designed a living supramolecular polymerization of the porphyrin-based monomers. Despite the fact that the polymerization is non-covalent, the reaction kinetics are analogous to that of conventional chain growth polymerization, and the supramolecular polymers were synthesized with controlled length and narrow polydispersity. [ABSTRACT FROM AUTHOR]

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

Protein aggregation leading to formation of amyloid fibrils is a symptom of several diseases like Alzheimer’s, type 2 diabetes and so on. Elucidating the poorly understood mechanism of such phenomena entails the difficult task of characterizing the species involved at each of the multiple steps in the aggregation pathway. It was previously shown by us that spontaneous aggregation of hen-eggwhite lysozyme (HEWL) at room temperature in pH 12.2 is a good model to study aggregation. Here in this paper we investigate the growth kinetics, structure, function and dynamics of multiple intermediate species populating the aggregation pathway of HEWL at pH 12.2. The different intermediates were isolated by varying the HEWL monomer concentration in the 300 nM—0.12 mM range. The intermediates were characterized using techniques like steady-state and nanosecond time-resolved fluorescence, atomic force microscopy and dynamic light scattering. Growth kinetics of non-fibrillar HEWL aggregates were fitted to the von Bertalanffy equation to yield a HEWL concentration independent rate constant (k = (6.6±0.6)×10−5 s−1). Our results reveal stepwise changes in size, molecular packing and enzymatic activity among growing HEWL aggregates consistent with an isodesmic aggregation model. Formation of disulphide bonds that crosslink the monomers in the aggregate appear as a unique feature of this aggregation. AFM images of multiple amyloid fibrils emanating radially from amorphous aggregates directly confirmed that on-pathway fibril formation was feasible under isodesmic polymerization. The isolated HEWL aggregates are revealed as polycationic protein nanoparticles that are robust at neutral pH with ability to take up non-polar molecules like ANS. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rate coefficients for H with methyl esters at the methoxy group are calculated. [•] Calculated rate coefficients are validated with available data. [•] Length of alkyl in methyl ester does not affect reactivity of the reaction class. [•] Rate constants are fitted in Chemkin format for combustion kinetic modeling. [ABSTRACT FROM AUTHOR]

The aromatic character of three different flowers of general formula [ n:( p, p)], namely [8:(5,7)], [8:(6,6)] and [8:(5,6)], has been evaluated by means of geometric (HOMA index), energetic (heats of formation) and magnetic criteria (NICS index, exaltation of magnetic susceptibility). Also, the reactivity descriptors within the DFT approach -absolute hardness, electrophilicity, Fukui functions - have been computed. All the different methods used for estimating the aromaticity led to a unitary conclusion. Two fullerene structures, patched by the most stable 8-Sumanene flower, have been designed and evaluated by means of ab initio computations [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Carbocations and carbenes, as electron-deficient species, require electron donation from the remainder of the molecule to the carbon center by whatever means available. Classical interactions include resonance, polar and polarizability effects, but neighboring group participation of several sorts can also serve as stabilizing factors. Simple carbocations with directly attached electron-withdrawing groups (EWGs), that is, EWG-CH2+ ions, comprise one group by which these interactions may be probed. This article provides computational evidence at the MP2/6-311 + G** level for variable but significant stabilizing interactions between the carbocation center and common EWGs, many via bridging (partial or symmetrical), neighboring group participation, homoconjugation or π interactions. Bridging from atoms possessing nonbonding electron pairs is a common motif. Removal of bridging by application of geometric constraints nullifies bridging stabilization, but does not eliminate the possibility of other stabilizing interactions, for example polar, polarizability and π donation from the EWG. The potential for π donation from strong EWGs has not been widely appreciated in the past. A carbocation stabilization enthalpy, CSE+, is defined as the enthalpy of the isodesmic reaction CH3+ + R-H → CH4 + R+. Comparisons are made with singlet carbenes bearing EWGs for which many structural features are similar. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The acid-base dissociation constant (pKa) can be related to the solubility and binding of drugs. However, measuring accurate pKa values is a challenging task. In this study, we have examined the pKa of various organic superbases: naphthalenes, cyclic guanidines, vinamidines, and acyclic guanidines computationally. We have calculated the pKa of such superbases by employing two methods: a conventional thermodynamic cycle and a second method based on an isodesmic reaction. The thermodynamic cycle involves computation of solvation free energy by using gas-phase free energy and the difference in solvation free energies (ΔGsolv) between products and reactants. Calculations performed with the isodesmic reaction approach do not use the free energy of solvation; hence, the accuracy of the approach is less sensitive to solvent molecules and global charges of the calculated species. The root-mean-square errors (RMSE) predict that the pKa of the studied organic superbases are more accurate when calculated with the isodesmic reaction approach. [ABSTRACT FROM AUTHOR]

We have investigated the energetic, structural, and other physical–chemical properties (aromaticity, intrinsic strain, hydrogen bond interaction) of 1,4-anthraquinone ( 1), its better known isomer 9,10-anthraquinone ( 2) and the derivatives 9-hydroxy-1,4-anthraquinone ( 3) and 9-methoxy-1,4-anthraquinone ( 4). In particular, the standard enthalpy of formation in the gas phase at 298.15 K of 1,4-anthraquinone was determined [ $$\Updelta_{\text{f}}^{{}} H_{\text{m}}^{\text{o}} \left( {{\text{g}},{\mathbf{1}}} \right) \, = \, - 4 4. 9 { } \pm { 5}. 7\;{\text{kJ}}\;{\text{mol}}^{ - 1} ]$$ Δ f H m o ( g , 1 ) = − 4 4.9 ± 5.7 kJ mol − 1 ] . Using isodesmic/homodesmotic reaction schemes, we have experimentally estimated: (i) the stabilization energy of 1 (162.2 ± 7.2 kJ mol −1) and 2 (193.2 ± 5.2 kJ mol −1), (ii) strength of intramolecular hydrogen bonding in 3 (HB = 79.8 ± 10.8 kJ mol −1), and (iii) additional strain energy due to peri-oxygen interaction in 4 (−34.2 ± 7.6 kJ mol −1). A computational study of these species, at the B3LYP/6-311++G(3df,2p) level, sheds light on structural, aromatic, intrinsic strain, or hydrogen bond effects and further confirmed the consistency of the experimental results. [ABSTRACT FROM AUTHOR]

A series of water-compatible C3-symmetric tricarboxamides endowed with a large number of EO side chains separated from the amide groups by different paraffinic linkers has been synthesized. The addition of a small amount of water breaks the N–H···OC amide H-bonds but induces the formation of aggregates by a solvophobic effect. The lack of highly directional H-bonding interactions results in an isodesmic supramolecular polymerization with a calculated Gibbs free energy of −31.26 and −36.79 kJ mol–1for 1and 3, respectively. [ABSTRACT FROM AUTHOR]

A comprehensive study on the isomerization, decomposition and ring-closure reaction kinetics, as well as thermodynamic properties of methyl pentanoate radicals were carried out in this work. The M06–2X/cc-pVTZ level of theory was employed to optimize geometries and analyze frequencies for all the stationary points on the potential energy surfaces. The CCSD(T) method with two basis sets, cc-pVDZ and cc-pVTZ, was used to calculate single point energies, which were further extrapolated to the complete basis set (CBS) limit to construct the potential energy surfaces. The energy calculation results indicated that isomerization of δ-R to α-R, decomposition of γ-R to CH 2 C(=O)OCH 3 and propene, and ring-closure reaction of δ-R to 1-methoxycyclopentanoxyl radical are the most energetically favored reactions among the studied isomerization, decomposition and ring-closure reaction channels, respectively. High pressure limit, and temperature and pressure dependent rate coefficients were determined by solving the one-dimensional energy-dependent master equations with Tsinghua University Minnesota Master Equation program (TUMME). The rate coefficients and branching ratios were found to be significantly affected by temperature and pressure. The decomposition reactions yielding small unsaturated esters or small ester radicals dominate at high temperatures, while the isomerization reactions proceeding via five- and six-membered ring saddle points, as well as the ring-closure reactions play more important roles at low temperatures. Significant discrepancies are observed between the theoretically calculated rate coefficients and estimated results. The thermochemical properties of methyl pentanoate radicals were calculated using isodesmic reaction method and atomization method together with the multistructural torsional anharmonicity partition functions. This study provides accurate kinetic and thermodynamic data on methyl pentanoate radicals, which are expected to advance our understanding of combustion chemistry of methyl pentanoate and larger methyl esters. [ABSTRACT FROM AUTHOR]

Recently, an unusual elongation of the C-S bond was observed experimentally for some sulfur-containing heterocycles. Using a superior ab initio (SCS-MP2/cc-pVTZ) level of theory, we showed that the phenomenon can be explained by a contribution of a donor–acceptor adduct of a carbene with an unsaturated ligand. One may achieve further elongation of the C-S bond, eventually turning it to a coordinate one, by increasing the stability of each part of the system as, e.g., in the utmost case of spiro adducts with Arduengo carbenes. The effect of carbene stability was quantified by employing the isodesmic reactions of carbene exchange. [ABSTRACT FROM AUTHOR]

[Display omitted] • Conformers of nine fluorinated alcohols were determined by theoretical calculations. • Standard enthalpies of formation were predicted from isodesmic reactions. • Enthalpy changes of H-abstraction reactions by OH radicals were computed. • H-abstraction from CH 2 group is the channel thermodynamically most favorable. Molecular conformations, harmonic vibrational frequencies and standard enthalpies of formation for CH 3-n F n CF 2 CH 2 OH, CH 3-n F n CH 2 OH and CH 3-n F n OH (n = 1,2,3) partially fluorinated alcohols were computed at different levels of theory using isodesmic and isogiric work reactions. The lack of data and large uncertainties in the reported thermodynamic properties motivated the present study. As average at the G4 and CBSQB3 levels of theory, standard enthalpies of formation at 298 K of −313.9±1.8, −258.2±1.8 and −208.4±1.8 kcal mol−1 were computed for CF 3 CF 2 CH 2 OH, CHF 2 CF 2 CH 2 OH and CH 2 FCF 2 CH 2 OH, respectively; values of −213.0±1.7, −154.5±1.7 and −100.4±1.7 kcal mol−1 were obtained for CF 3 CH 2 OH, CHF 2 CH 2 OH and CH 2 FCH 2 OH; while −216.9±1.2, −159.8±1.2 and −100.9±1.7 kcal mol−1 were derived for CF 3 OH, CHF 2 OH and CH 2 FOH. The corresponding enthalpies of formation presented here for CF 3 CF 2 CH 2 OH and CH 2 FCF 2 CH 2 OH were predicted for the first time. Hydrogen abstraction reactions enthalpies by OH radicals were also estimated and comparisons with previously reported results were performed. [ABSTRACT FROM AUTHOR]

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

The article presents a study which focuses on the description of the hydrogen-bonded supramolecular polymer, The study uses the nuclear magnetic resonance spectroscopy (NMR) spectroscopy for the characterization of the Z isomer of UPy moieties. Results show that the formation of the supramolecular polymer is induced by the isodesmic growth mechanism.

The study of the organization of small π-conjugated molecules is imperative to understanding and controlling its properties for various applications. Coronene bisimides (CBIs) are potential candidates for novel liquid-crystalline materials and active n-type semiconductor molecules in organic electronics. To understand the self-assembly of this seldom-studied chromophore, we have designed two derivatives of CBIs bearing chiral and achiral 3,4,5-trialkoxyphenyl groups at the imide position, named as CBI-GCH and CBI-GACH, respectively. CBI-GCH self-assembles mainly through π-stacking and van der Waals interactions in nonpolar methylcyclohexane to result in long 1D fibrillar stacks. The mechanism of supramolecular polymerization was probed by using chiroptical studies, which showed an isodesmic pathway for CBI-GCH. The thermodynamic parameters that govern the self-assembly are detailed. CBI-GACH also shows similar self-assembly behavior as its chiral counterpart. X-ray diffraction studies of both molecules reveals a 2D hexagonal columnar arrangement. The coassembly of CBI-GCH and CBI-GACH shows chiral amplification (sergeant and soldiers experiment) with saturation at 30-50 % of the chiral derivative, which was further used to study the dynamics of the assembly. Thus, this study presents a rare report of chiral amplification in an isodesmic system. [ABSTRACT FROM AUTHOR]

Accurategas-phase enthalpies of formation (ΔfH298°)of 29 azides are recommended by combining G4 theory calculations withan isodesmic reaction approach. The internal consistency over a setof ΔfH298°values was achieved by sequentialadjustment of their values through the isodesmic reactions. The HN3was chosen as a key reference compound. Of the experimentaldata available for 16 compounds, our predictive values agree wellwith 9 of them, while the deviations from 25 to 55 kJ/mol are observedfor 7 compounds; possible systematic errors in the experimental datafor phenyl azide, 2-azidoethanol, azidocyclopentane, azidocyclohexane,3-azido-3-ethylpentane, 2-azido-2-phenylpropane, and 1-azidoadamantaneare discussed. The recommended enthalpies of formation of organicazides were used as reference values to estimate the enthalpy of formationof four nitrogen-rich carbon nitrides. The calculations do not supportthe high value of the solid-state enthalpy of formation of TAAT (4,4′,6,6′-tetra(azido)azo-1,3,5-triazine);its value is estimated to be 300–400 kJ/mol lower than thatmeasured experimentally. [ABSTRACT FROM AUTHOR]

A Theoretical study on the mechanism of the reactions of CFClC(O)OCH with the OH radical and Cl atom is presented. Geometry optimization and frequency calculations have been performed at the MPWB1K/6-31+G(d,p) level of theory and energetic information is further refined by calculating the energy of the species using G2(MP2) theory. Transition states are searched on the potential energy surface involved during the reaction channels and each of the transition states are characterized by presence of only one imaginary frequency. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation. Theoretically calculated rate constants at 298 K and atmospheric pressure using the canonical transition state theory (CTST) are found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions as working chemical reactions, the standard enthalpies of formation for CFClC(O)OCH CFClC(O)OCH and CFC(O)OCH are also reported for the first time. [ABSTRACT FROM AUTHOR]

As part of a search for high energy density materials (HEDMs), a series of purine derivatives with nitro groups were designed computationally. The relationship between the structures and the performances of these polynitropurines was studied. Density functional theory (DFT) at the B3LYP/6-311G** level was employed to evaluate the heats of formation (HOFs) of the polynitropurines by designing an isodesmic reaction method. Results indicated that the HOFs were influenced by the number and positions of substituent groups. Detonation properties were evaluated using the Kamlet-Jacobs equations, based on the theoretical densities and heats of formation of the polynitropurines. The relative stabilities of the polynitropurines were studied via the pyrolysis mechanism and the UB3LYP/6-311G** method. Homolysis of the ring-NO bond is predicted to be the initial step in the thermal decomposition of these purine derivatives. Considering their detonation properties and relative stabilities, the tetranitropurine (D1) derivatives may be regarded as potential candidates for practical HEDCs. These results may provide useful information for further investigations. [ABSTRACT FROM AUTHOR]

Abstract: Enthalpies of formation of twenty seven mono substituted nitrobenzenes were calculated employing various DFT and composite methods using atomization approach and two types of isodesmic reactions. The overall best agreement with experiment was achieved by using Gn multilevel techniques. It was found that the best performance among DFT methods within atomization approach demonstrates long range corrected wB97XD level theory. The average absolute deviation value calculated for B3P86/6-311G(d,p) which have been recently reported as an very accurate method for calculating formation enthalpies of poly and mono substituted nitrobenzenes exceeds 50kcal/mol. Employing of isodesmic reactions tends to it improve estimations of DFT levels, though agreement between theoretical data obtained using this methodology with experiment depends strongly on the type of reaction and the quality of experimental Δf H 0 ,298 values available for compounds involved in these bond balanced reactions. [Copyright &y& Elsevier]

Abstract: We report a study on the energetics and structural properties of naphthalene-based proton sponges and their corresponding protonated cations. In particular, we have determined the experimental standard enthalpies of formation in the gas phase at T = 298.15K, (g), for the neutral and protonated DMAN [1,8-bis (dimethylamino)-naphthalene], (221.0±7.3) and (729.0±11.1)kJ·mol−1, respectively. A reliable experimental estimation of enthalpy associated with “strain” effect and hydrogen bond intramolecular (included within “enhanced basicity”, EB) contributions to the basicity of DMAN, were deduced from isodesmic reactions, −(29.1±4.6) and (87.1±11.9)kJ·mol−1, respectively. The gas-phase basicities (GB) of naphthalene-based proton sponges are compared with the corresponding aqueous basicities (pK a), covering a range of 149kJ·mol−1 in GB and 11.5 in pK a. Density functional calculations at the M05-2X/6-311++G(d,p) level of theory were used to check the consistency of the experimental results and also to estimate the unavailable GB values of the considered species. [Copyright &y& Elsevier]

Abstract: Isodesmic reactions and the energy of intramolecular interactions in bromoalkoxycyclohexanes (alkoxy=OMe, OEt, O i Pr and O t Bu) were computed to evaluate the effect of bromine and alkoxy groups when bonded together in a cyclohexane ring to give cis and trans-1,2, 1,3 and 1,4 isomers. According to the enthalpy energies obtained from the isodesmic reactions, the bromine atom is preferred to be introduced axially in methoxycyclohexane to give trans-1-bromo-2-methoxycyclohexane; otherwise, equatorial introduction of the bromine atom is favoured. Either direct or indirect intramolecular interactions involving OR and Br, obtained from comparison with the conformational energies of the monosubstituted cyclohexanes, indicate a preference for bromine axially oriented, except for the cis-1,3 isomer, in which the OR and Br groups experience high steric hindrance to each other. The effect of R is generally invariant. [Copyright &y& Elsevier]

Density functional theory calculations at B3LYP/6-31G** and B3P86/6-31G** levels were performed to predict the densities ( ρ), detonation velocities (D), pressures (P) and the thermal stabilities for a series of 1,2,4-triazole derivatives for looking high energy density compounds (HEDCs). The heats of formation (HOFs) are also calculated via designed isodesmic reactions. The calculations on the bond dissociation energies (BDEs) indicate that the position of the subsitutent group has great effect on the BDE and the BDEs of the initial scission step are between 31 and 65 kcal/mol. In addition, the condensed phase heats of formation are also calculated for the title compounds. These results would provide basic information for further studies of HEDCs. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

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

With density function theory BLYP/DNP method, together with homodesmotic reactions and isodesmic reactions, we calculated the resonance energies of some explosives, including eight nitro compounds which contains benzene rings, three nitro compounds which contains azaheterocycles (2,4-dinitroimidazole (2,4-DNI), 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) and 2,4,6-trinitro-1,3,5-triazine) and one nitrogen-rich energetic compound of 3,3'-azobis(6-amino-s-tetrazine) (DAAT). The results indicate that their resonance energies are in relation to their shock sensitivity which measuring their threshold pressures of initiation, that is, the lower the resonance energy is, the higher the shock sensitivity of the explosive behaves. And this measuring method according to resonance energy is based on the global property of the molecule instead of the local one, such as one nitro group in the molecule. It is meaningful to calculate resonance energies of these kind of compounds quickly and accurately because resonance structures exist widely in these organic compounds and resonance energies may play a significant role in determining their shock sensitivity, and it is helpful in the rational design or synthesis of high energy and insensitive materials. [ABSTRACT FROM AUTHOR]

In this work, we define an isodesmicity index, which allows the evaluation of the conservation of the error within a given bond conserving reaction. This index is reaction-specific and, therefore, does not rely on the nature of the reference systems. Several aspects of the index are tested, to verify that this measurement shows the desired behavior. The index is a tool, which should allow the user to establish the 'best' bond conserving process for, for instance, the determination of a heat of formation. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012 [ABSTRACT FROM AUTHOR]

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

Abstract: DFT calculations using MPWB1K method with COSMO continuum solvation model have been carried out to quantify the trans influence of various X ligands (E X) in [PtIICl3X]n− complexes as well as the mutual trans influence of two X and Y ligands (E XY) in [PtIICl2XY]n− complexes. A quantitative structure energy relationship (QSER) is derived for predicting the E XY using E X and E Y and this relationship showed a strong similarity to a QSER derived for predicting E XY of [PdIICl2XY]n− complexes. Quantification of the contributions of E X and E XY to the bond dissociation energy of the ligand X (BDE X) in complexes of the type [MIIX(Y)X′(Y′)] (M = Pd, Pt) is also achieved. The BDE X of any ligand X in these complexes can be predicted using the equations, viz. BDE X(Pd) = 1.196E X − 0.603E XY − 0.118E X’Y’ + 0.442D X + 15.169 for Pd(II) complexes and BDE X(Pt) = 1.420E X − 0.741E XY − 0.125E X’Y’ + 0.498D X + 13.852 for Pt(II) complexes, where D X corresponds to the bond dissociation energy of X in [MIICl3X]n− complexes. These expressions suggest that the mutual trans influence from X and Y is more dominant than the mutual trans influence from X′ and Y′ and both factors contribute significantly to the weakening of M–X bond. We also obtained a strong linear relationship between E X and the electron density ρ(r) at the bond critical point of M–Cl bond trans to the X in [MIICl3X]n− and this allows us to express the BDE X(Pd) and BDE X(Pt) in terms of only the ρ(r) and D X. We have demonstrated that using a database comprising of D X and the ρ(r), the bond dissociation energy of X in complexes of the type [MIIX(Y)X′(Y′)] can be predicted. [Copyright &y& Elsevier]

The mechanism and kinetics of the reaction of CH3OCF2CF2OCH3 with OH radicals have been studied theoretically by a direct density functional theory dynamics method. All possible H-abstraction channels and displacement processes taking place on two different conformers of CH3OCF2CF2OCH3 have been taken into consideration. The potential energy surface information including the optimized geometries and harmonic vibrational frequencies of all the stationary points and barrier heights involved in these channels were obtained at the BB1K/6-31+G(d,p) level of theory. The rate constants were calculated using improved canonical variational transition state theory (ICVT) with the small-curvature tunneling correction (SCT) over the temperature range of 200-2000 K. The overall rate constant for the title reaction, which was obtained by considering the weight factor of each conformer from the Boltzman distribution function, is in reasonable agreement with the available experimental value. Three-term Arrhenius expression is fitted to be kT = 1.56 × 10-20T2.47exp(-124.64/T)cm³ molecule-1 s-1 (200-2000 K). Also, the enthalpies of formation of the reactant CH3OCF2CF2OCH3 and product radicals CH3OCF2CF2OCH2 and CH3OCF2CF2O, which lack experimental or theoretical data, were evaluated via applying isodesmic reactions. [ABSTRACT FROM AUTHOR]

The energies of combustion and fusion of 5-cyano-5 H-dibenzo[ a,d]cycloheptene ( 1) and (5E,11E)-dibenzo[ a,e]cyclooctene-5,11-dicarbonitrile ( 2) were measured by means of microbomb calorimetry and DSC, respectively. The derived enthalpies of formation in solid state are 320 ± 18 for nitrile 1 and 470 ± 31 kJ mol for nitrile 2, respectively. The experimental enthalpies of formation are discussed in relationship with values calculated at the G3(MP2)//B3LYP level of quantum chemical theory, by means of group additivity and isodesmic reactions. The two nitriles are not stabilized by dibenzoannelation. [ABSTRACT FROM AUTHOR]

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