Showing papers on "Bond-dissociation energy published in 2009"

Divalent carbon(0) compounds

Abstract: Quantum chemical studies show that there is a class of carbon compounds with the general formular CL 2 where the carbon atom retains its four valence electrons as two lone pairs. The C-L bonds come from L → C donor-acceptor interactions where L is a strong σ-donor. Divalent C(0) compounds (carbones) are conceptually different from divalent C(II) compounds (carbenes) and tetravalent carbon compounds, but the bonding situation in a real molecule may be intermediate between the three archetypes. There are molecules like tetraaminoallenes which may be described in terms of two double bonds (R 2 N) 2 C=C=C(NR 2 ) 2 where the extraordinary donor strength of the dicoordinated carbon atom comes only to the fore through the interactions with protons and Lewis acids. They may be considered as "hidden divalent C(0) compounds". The donor strength of divalent C(0) molecules has been investigated by calculations of the binding energies with protons and with main-group Lewis acids and the bond dissociation energies (BDEs) of transition-metal complexes.

Determination of the ionization and dissociation energies of the hydrogen molecule.

Abstract: The transition wave number from the EF Σ1g+(v=0,N=1) energy level of ortho-H2 to the 54p11(0) Rydberg state below the X+ Σ2g+(v+=0,N+=1) ground state of ortho-H2+ has been measured to be 25 209.997 56±(0.000 22)statistical±(0.000 07)systematic cm−1. Combining this result with previous experimental and theoretical results for other energy level intervals, the ionization and dissociation energies of the hydrogen molecule have been determined to be 124 417.491 13(37) and 36 118.069 62(37) cm−1, respectively, which represents a precision improvement over previous experimental and theoretical results by more than one order of magnitude. The new value of the ionization energy can be regarded as the most precise and accurate experimental result of this quantity, whereas the dissociation energy is a hybrid experimental-theoretical determination.

Accurate ab initio and "hybrid" potential energy surfaces, intramolecular vibrational energies, and classical ir spectrum of the water dimer.

Abstract: We report three modifications to recent ab initio, full-dimensional potential energy surfaces (PESs) for the water dimer [X. Huang et al., J. Chem. Phys. 128, 034312 (2008)]. The first modification is a refit of ab initio electronic energies to produce an accurate dissociation energy De. The second modification adds replacing the water monomer component of the PES with a spectroscopically accurate one and the third modification produces a hybrid potential that goes smoothly in the asymptotic region to the flexible, Thole-type model potential, version 3 dimer potential (denoted TTM3-F) [G. S. Fanourgakis and S. S. Xantheas, J. Chem. Phys. 128, 074506 (2008)]. The rigorous D0 for these PESs, obtained using diffusion Monte Carlo calculations of the dimer zero-point energy, and an accurate zero-point energy of the monomer, range from 12.5 to 13.2 kJ/mol (2.99–3.15 kcal/mol), with the latter being the suggested benchmark value. For TTM3-F D0 equals 16.1 kJ/mol. Vibrational calculations of monomer fundamental e...

Computational Study of Bond Dissociation Enthalpies for Lignin Model Compounds. Substituent Effects in Phenethyl Phenyl Ethers

Abstract: Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH2CH2OPh, PPE) is the simplest example representing the dominant β-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen−carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen−carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon−carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen−carbon bond dissociation enthalpy is substantially lowered by oxygen subs...

A highly reactive p450 model compound I.

Abstract: The detection and kinetic characterization of a cytochrome P450 model compound I, [OFe(IV)-4-TMPyP](+) (1), in aqueous solution shows extraordinary reaction rates for C-H hydroxylations. Stopped-flow spectrophotometric monitoring of the oxidation of Fe(III)-4-TMPyP with mCPBA revealed the intermediate 1, which displays a weak, blue-shifted Soret band at 402 nm and an absorbance at 673 nm, typical of a porphyrin pi-radical cation. This intermediate was subsequently transformed into the well-characterized OFe(IV)-4-TMPyP. Global analysis afforded a second-order rate constant k(1) = (1.59 +/- 0.06) x 10(7) M(-1) s(-1) for the formation of 1 followed by a first-order decay with k(2) = 8.8 +/- 0.1 s(-1). (1)H and (13)C NMR determined 9-xanthydrol to be the major product (approximately 90% yield) of xanthene oxidation by 1. Electrospray ionization mass spectrometry carried out in 47.5% (18)OH(2) indicated 21% (18)O incorporation, consistent with an oxygen-rebound reaction scenario. Xanthene/xanthene-d(2) revealed a modest kinetic isotope effect, k(H)/k(D) = 2.1. Xanthene hydroxylation by 1 occurred with a very large second-order rate constant k(3) = (3.6 +/- 0.3) x 10(6) M(-1) s(-1). Similar reactions of fluorene-4-carboxylic acid and 4-isopropyl- and 4-ethylbenzoic acid also gave high rates for C-H hydroxylation that correlated well with the scissile C-H bond energy, indicating a homolytic hydrogen abstraction transition state. Mapping the observed rate constants for C-H bond cleavage onto the Bronsted-Evans-Polanyi relationship for similar substrates determined the H-OFe(IV)-4-TMPyP bond dissociation energy to be approximately 100 kcal/mol. The high kinetic reactivity observed for 1 is suggested to result from a high porphyrin redox potential and spin-state-crossing phenomena. More generally, subtle charge modulation at the active site may result in high reactivity of a cytochrome P450 compound I.

Metal-phosphine bond strengths of the transition metals: a challenge for DFT.

Abstract: Previous promising tests of the new M06 family of functionals in predicting ruthenium−metal phosphine bond dissociation energies (Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157) have been extended to a series of phosphine complexes of chromium, molybdenum, nickel, and ruthenium for which relevant experimental data are available. In addition to the M06 family of functionals, bond dissociation enthalpies have been calculated using a selection of density functionals and hybrid functionals based on the generalized gradient approximation (GGA), and with or without an empirical term (i.e., DFT-D) accounting for long-range dispersion. For the ruthenium complexes, second-order Moller−Plesset perturbation theory (MP2) has also been applied. Electrostatic and nonelectrostatic solvent effects have been estimated using the polarizable continuum model (PCM), allowing for comparison with experimental data obtained for dissociation reactions in organic solvents. Whereas the GGA and hybrid-GGA functionals grossly...

Divalent silicon(0) compounds.

Abstract: Quantum-chemical calculations of the geometries and electronic structures of a series of dicoordinated silicon compounds SiL(2), in which L is a five-membered cyclic species suggest that the molecules are divalent silicon(0) compounds that possess two L-->Si donor-acceptor bonds and two lone-pair MOs with pi and sigma symmetry at silicon. The classification as a dicoordinate silicon compound with L-->Si<--L donor-acceptor bonds applies not only to molecules in which L is an N-heterocyclic carbene but also when L is a cyclic silylene. The recently synthesized "trisilaallene" (S. Ishida, T. Iwamoto, C. Kabuto, M. Kira, Nature 2003, 421, 725), which has a bending angle of 136.5(o) for the central moiety, and which was written as Si=Si=Si, is probably better considered as a divalent silicon(0) compound. We suggest the name silylones for the latter species in analogy to silylenes which identify divalent Si(II) compounds. This bonding interpretation explains the theoretically predicted large values for the first and second proton affinities and for the large bond dissociation energies for one and two BH(3) ligands. The calculations predict that the first protonation of the divalent silicon(0) compounds takes place at the pi lone-pair orbital, which yields protonated silylones that have a pyramidal arrangement of the ligands at the central tricoordinate silicon atom. Silylones SiL(2) could be interesting ligands for transition-metal compounds. The calculated structures and bonding situation of the analogous carbon compounds are also reported.

Theoretical Bond Dissociation Energies of Halo-Heterocycles: Trends and Relationships to Regioselectivity in Palladium-Catalyzed Cross-Coupling Reactions

Abstract: Selectivity of the palladium-catalyzed cross-coupling reactions of heterocycles bearing multiple identical halogens is mainly determined by the relative ease of oxidative addition. This is related to both the energy to distort the carbon halogen bond to the transition-state geometry (related to the CX bond-dissociation energy) and to the interaction between the heterocycle π* (LUMO) and PdL2 HOMO (J. Am. Chem. Soc. 2007, 129, 12664). The computed bond dissociation energies of a larger series of halo-heterocycles have been explored with B3LYP and higher accuracy G3B3 calculations. Quantitative trends in bond dissociation energies have been identified for five- and six-membered chloro and bromo substituted heterocycles with N, O, and S heteroatoms.

Strong N−H···π Hydrogen Bonding in Amide−Benzene Interactions

Abstract: Among the weak intermolecular interactions found in proteins, the amide N−H···π interaction has been widely observed but remains poorly characterized as an individual interaction. We have investigated the isolated supersonic-jet-cooled dimer of the cis-amide and nucleobase analogue 2-pyridone (2PY) with benzene and benzene-d6. Both MP2 and SCS-MP2 geometry optimizations yield a T-shaped structure with a N−H···π hydrogen bond to the benzene ring and the C═O group above, but far from the C−H bonds of benzene. The CCSD(T) calculated binding energy at the optimum geometry is De = 25.2 kJ/mol (dissociation energy D0 = 21.6 kJ/mol), corresponding to the H-bond strength of the water dimer or of N−H···O hydrogen bonds. The T-shaped geometry is supported by the infrared−ultraviolet depletion spectra of 2PY·benzene: The N−H stretch vibrational frequency is lowered by 56 cm−1, and the C═O stretch vibration is lowered by 10 cm−1, relative to those of bare 2PY, indicating a strong N−H···π interaction and a weak intera...

Thermal decomposition of the benzyl radical to fulvenallene (C7H6) + H.

Abstract: We show that the benzyl radical decomposes to the C7H6 fragment fulvenallene (+H), by first principles/RRKM study. Calculations using G3X heats of formation and B3LYP/6-31G(2df,p) structural and vibrational parameters reveal that the reaction proceeds predominantly via a cyclopentenyl-allene radical intermediate, with an overall activation enthalpy of ca. 85 kcal mol−1. Elementary rate constants are evaluated using Eckart tunneling corrections, with variational transition state theory for barrierless C−H bond dissociation in the cyclopentenyl-allene radical. Apparent rate constants are obtained as a function of temperature and pressure from a time-dependent RRKM study of the multichannel multiwell reaction mechanism. At atmospheric pressure we calculate the decomposition rate constant to be k [s−1] = 5.93 × 1035T−6.099 exp(−49 180/T); this is in good agreement with experiment, supporting the assertion that fulvenallene is the C7H6 product of benzyl decomposition. The benzyl heat of formation is evaluated ...

Binding energies of O2 and CO to small gold, silver, and binary silver-gold cluster anions from temperature dependent reaction kinetics measurements.

Abstract: A detailed analysis of experimentally obtained temperature-dependent gas-phase kinetic data for the oxygen and carbon monoxide adsorption on small anionic gold (Au(n)(-), n = 1-3), silver (Ag(n)(-), n = 1-5), and binary silver-gold (Ag(n)Au(m)(-), n + m = 2, 3) clusters is presented. The Lindemann energy transfer model in conjunction with statistical unimolecular reaction rate theory is employed to determine the bond dissociation energies E(0) of the observed metal cluster complexes with O(2) and CO. The accuracy limits of the obtained binding energies are evaluated by applying different transition-state models. The assumptions involved in the data evaluation procedure as well as possible sources of error are discussed. The thus-derived binding energies of O(2) to pure silver and binary silver-gold cluster anions are generally in excellent agreement with previously reported theoretical values. In marked contrast, the binding energies of O(2) and CO to Au(2)(-) and Au(3)(-) determined via temperature-dependent reaction kinetics are consistently lower than the theoretically predicted values.

Origin of the correlation of the rate constant of substrate hydroxylation by nonheme iron(IV)-oxo complexes with the bond-dissociation energy of the C-H bond of the substrate.

Abstract: A series of hydrogen-abstraction barriers of a nonheme iron(IV)–oxo oxidant mimicking the active species of taurine/α-ketoglutarate dioxygenase (TauD) are rationalized by using a valence-bond curve-crossing diagram (see figure). It is shown that the barriers correlate with the strength of the CH bond. Furthermore, electronic differences explain the differences between nonheme and heme iron(IV)–oxo hydrogen-abstraction barriers. Mononuclear nonheme iron containing systems are versatile and vital oxidants of substrate hydroxylation reactions in many biosystems, whereby the rate constant of hydroxylation correlates with the strength of the CH bond that is broken in the process. The thermodynamic reason behind these correlations, however, has never been established. In this work results of a series of density functional theory calculations of substrate hydroxylation by a mononuclear nonheme iron(IV)–oxo oxidant with a 2 His/1 Asp structural motif analogous to α-ketoglutarate dependent dioxygenases are presented. The calculations show that these oxidants are very efficient and able to hydroxylate strong CH bonds, whereby the hydrogen abstraction barriers correlate linearly with the strength of the CH bond of the substrate that is broken. These trends have been rationalized using a valence bond (VB) curve-crossing diagram, which explains the correlation using electron transfer mechanisms in the hydrogen abstraction processes. We also rationalized the subsequent reaction step for radical rebound and show that the barrier is proportional to the electron affinity of the iron(III)–hydroxo intermediate complex. It is shown that nonheme iron(IV)–hydroxo complexes have a larger electron affinity than heme iron(IV)–hydroxo complexes and therefore also experience larger radical rebound barriers, which may have implications for product distributions and rearrangement reactions. Thus, detailed comparisons between heme and nonheme iron(IV)–oxo oxidants reveal the fundamental differences in monoxygenation capabilities of these important classes of oxidants in biosystems and synthetic analogues for the first time and enable us to make predictions of experimental processes.

Formation Enthalpies and Bond Dissociation Energies of Alkylfurans. The Strongest C—X Bonds Known?

Abstract: Enthalpies of formation, ΔHf(298.15 K), of 2-methyl-, 3-methyl-, 2-ethyl-, 2-vinyl-, 2,3-dimethyl-, 2,4-dimethyl-, and 3,4-dimethylfurans are computed with three compound quantum chemical methods, CBS-QB3, CBS-APNO, and G3, via a number of isodesmic reactions. We show that previously experimentally determined enthalpies of formation of furan itself, 2,5-dimethyl-, 2-tert-butyl-, and 2,5-di-tert-butylfurans are self-consistent but that for 2-vinylfuran is most probably in error. The formation enthalpies of over 20 furyl and furfuryl radicals have also been determined and consequently the bond dissociation energies of a number of C−H, C−CH3, C−F, C−Cl, and C−OH bonds. The ring-carbon−H bonds in alkylfurans are much stronger than previously thought and are among the strongest ever C−H bonds recorded exceeding 500 kJ mol−1. The relative thermodynamic instability of the various furyl radicals means that bonds to methyl, fluorine, and chlorine are also unusually strong. This is as a consequence of the inability...

Hydration energies of zinc(II): threshold collision-induced dissociation experiments and theoretical studies.

Abstract: The first experimentally determined sequential bond dissociation energies of Zn2+(H2O)n complexes, where n = 6−10, are measured using threshold collision-induced dissociation in a guided ion beam t...

Effect of an external electric field on the dissociation energy and the electron density properties: The case of the hydrogen bonded dimer HF...HF.

Abstract: The effect of a homogeneous external electric field parallel to the hydrogen bond in the FH⋯FH dimer has been studied by theoretical methods. The quantum theory of atoms in molecules methodology has been used for analyzing the electron distribution of the dimer, calculated with different hydrogen bond distances and external field magnitudes. It is shown that an electric field in the opposite direction to the dipole moment of the system strengthens the interaction due to a larger mutual polarization between both molecules and increases the covalent character of the hydrogen bond, while an external field in the opposite direction has the inverse effect. The properties of the complex at its equilibrium geometry with applied field have been calculated, showing that dependencies between hydrogen bond distance, dissociation energy, and properties derived from the topological analysis of the electron distribution are analogous to those observed in families of XDH⋯AY complexes. The application of an external field appears as a useful tool for studying the effect of the atomic environment on the hydrogen bond interaction. In the case of FH⋯FH, both the kinetic energy density and the curvature of the electron density along the hydrogen bond at the bond critical point present a surprisingly good linear dependence on the dissociation energy. The interaction energy can be modeled by the sum of two exponential terms that depend on both the hydrogen bond distance and the applied electric field. Moreover, as indicated by the resulting interaction energy observed upon application of different external fields, the equilibrium distance varies linearly with the external field, and the dependence of the dissociation energy on either the hydrogen bond distance or the external electric field is demonstrated to be exponential.

Energies, geometries, and charge distributions of ZN molecules, clusters, and biocenters from coupled cluster, density functional, and neglect of diatomic differential overlap models

Abstract: We present benchmark databases of Zn-ligand bond distances, bond angles, dipole moments, and bond dissociation energies for Zn-containing small molecules and Zn coordination compounds with H, CH3, C2H5, NH3, O, OH, H2O, F, Cl, S, and SCH3 ligands. The test set also includes clusters with Zn-Zn bonds. In addition, we calculated dipole moments and binding energies for Zn centers in coordination environments taken from zinc metalloenzyme X-ray structures, representing both structural and catalytic zinc centers. The benchmark values are based on relativistic-core coupled cluster calculations. These benchmark calculations are used to test the predictions of four density functionals, namely B3LYP and the more recently developed M05-2X, M06, and M06-2X levels of theory, and six semiempirical methods, including neglect of diatomic differential overlap (NDDO) calculations incorporating the new PM3 parameter set for Zn called ZnB, developed by Brothers and co-workers, and the recent PM6 parametrization of Stewart. We found that the best DFT method to reproduce dipole moments and dissociation energies of our Zn compound database is M05-2X, which is consistent with a previous study employing a much smaller and less diverse database and a much larger set of density functionals. Here we show that M05-2X geometries and single-point coupled cluster calculations with M05-2X geometries can also be used as benchmarks for larger compounds, where coupled cluster optimization is impractical, and in particular we use this strategy to extend the geometry, binding energy, and dipole moment databases to additional molecules, and we extend the tests involving crystal-site coordination compounds to two additional proteins. We find that the most predictive NDDO methods for our training set are PM3 and MNDO/d. Notably, we also find large errors in B3LYP for the coordination compounds based on experimental X-ray geometries.

High-Temperature and Nonequilibrium Partition Function and Thermodynamic Data of Diatomic Molecules

Abstract: Accurate molecular partition functions are required to determine both thermodynamic properties for equilibrium and nonequilibrium flow field calculations, and energy level populations for radiative heat transfer in high enthalpy flows, for instance. Thermodynamic functions of diatomic molecules are computed in this study at high temperatures and for nonequilibrium media in the framework of multi-temperature models. Partition functions, average energies, and specific heats of N2, \({\rm N}_2^+\) , NO, O2, CN, C2, CO, and CO+ are calculated up to 50,000 K by direct summation over energy levels using recent and accurate spectroscopic data and dissociation energy values. Estimates are made for the error introduced by neglecting the highest considered electronic states. For nonequilibrium media, the sensitivity of the multi-temperature thermodynamic functions to different schemes for energy partitioning is discussed. In particular, the effects of vibration–rotation coupling are investigated. The tabulated results are available upon request.

Molecules with all triple bonds: OCBBCO, N2BBN2, and [OBBBBO](2-).

Abstract: DFT calculations at the BP86/TZ2P level have been carried out for the compounds OCBBCO, N2BBN2, and [OBBBBO]2−. The calculations predict very short distances and large bond dissociation energies fo...

Dissociation of water during formation of anodic aluminum oxide.

Abstract: According to model computations at the B3LYP/6-311+G** level, an external electric field can facilitate the heterolytic dissociation of properly oriented water molecules significantly. Depending on the models used, the maximum predicted change of the dissociation energy in the field is ca. −3 to −4 kcal nm mol−1 V−1, and decreases with the cosine of the angle between the external field and the breaking OH bond. These microscopic results can be related semiquantitatively to macroscopic observables from mechanistic studies on the pore formation of anodic aluminum oxide, thus lending support to the equifield strength model and field-enhanced water dissociation at the growing oxide surface that has been put forward in these studies.

Decomposition of Methylbenzyl Radicals in the Pyrolysis and Oxidation of Xylenes

Abstract: Alkyl benzyl radicals are important initial products in thermal and combustion reactions of substituted aromatic fuels. The decomposition reactions of the three isomeric methylbenzyl radicals, formed as primary products in xylene combustion, are studied theoretically and are shown to be significantly more complex than previously reported. Thermochemical properties are calculated using the G3X and G3SX model chemistries, with isodesmic and atomization work reactions. G3X atomization calculations reproduce heats of formation for the 14 reference species in the work reactions to a mean unsigned error of 0.23 kcal mol(-1), and maximum error of 0.70 kcal mol(-1), slightly outperforming the G3SX method. For the target molecules the isodesmic and atomization heats of formation agree to within 0.20 kcal mol(-1), on average. We posit that this study approaches the crossover point at which atomization calculations offer improved accuracy over isodesmic ones, for these closed-shell hydrocarbons. Our results suggest that m-xylylene is not the decomposition product of m-methylbenzyl, as was previously reported. Instead, the m-methylbenzyl radical decomposes to p-xylylene (and perhaps some of the less stable o-xylylene) via a ring-contraction/methylene-migration (RCMM) mechanism, with activation energy of around 70 kcal mol(-1). At higher temperatures m-methylbenzyl is predicted to also decompose to 2- and 3-methylfulvenallene + H, with activation energy of around 84 kcal mol(-1). The o-methylbenzyl radical is shown to primarily decompose to o-xylylene + H with bond dissociation energy of 67.3 kcal mol(-1), with fulvenallene + CH3 proposed as a minor product set. Finally, the p-methylbenzyl radical decomposes solely to p-xylylene + H with bond dissociation energy 61.5 kcal mol(-1). Rate expressions are estimated for all reported reactions, based on thermochemical kinetic considerations, with further modeling along with detailed experiments needed to better refine rate constants and branching ratios for methylbenzyl radical decomposition. These calculated reaction mechanisms and rate constants for methylbenzyl radical decomposition are consistent with the experimental data. Our results help explain the ignition behavior of the xylenes, and should lead to improved kinetic models for combustion of these and other alkylated aromatic hydrocarbons.

Heats of Formation of the H1,2OmSn (m, n = 0−3) Molecules from Electronic Structure Calculations

Abstract: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted from high level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set (CBS) limit for the H1,2OmSn (m, n = 0−3) compounds, as well as various radicals involved in different bond breaking processes. To achieve near chemical accuracy (±1.0 kcal/mol), additional corrections were added to the CBS binding energies based on the frozen core CCSD(T) energies including corrections for core−valence, scalar relativistic, and first-order atomic spin−orbit effects. Geometries were optimized up through the CCSD(T)/aV(T+d)Z level. Vibrational zero point energies were computed at the MP2/aV(T+d)Z level. The calculated heats of formation are in excellent agreement with the available experimental data and allow the prediction of adiabatic bond dissociation energies (BDEs) to within ±1.0 kcal/mol. The decomposition mechanisms we...

DFT and ONIOM(DFT:MM) studies on Co-C bond cleavage and hydrogen transfer in B12-dependent methylmalonyl-CoA mutase. Stepwise or concerted mechanism?

Abstract: The considerable protein effect on the homolytic Co-C bond cleavage to form the 5'-deoxyadenosyl (Ado) radical and cob(II)alamin and the subsequent hydrogen transfer from the methylmalonyl-CoA substrate to the Ado radical in the methylmalonyl-CoA mutase (MMCM) have been extensively studied by DFT and ONIOM(DFT/MM) methods. Several quantum models have been used to systematically study the protein effect. The calculations have shown that the Co-C bond dissociation energy is very much reduced in the protein, compared to that in the gas phase. The large protein effect can be decomposed into the cage effect, the effect of coenzyme geometrical distortion, and the protein MM effect. The largest contributor is the MM effect, which mainly consists of the interaction of the QM part of the coenzyme with the MM part of the coenzyme and the surrounding residues. In particular, Glu370 plays an important role in the Co-C bond cleavage process. These effects tremendously enhance the stability of the Co-C bond cleavage state in the protein. The initial Co-C bond cleavage and the subsequent hydrogen transfer were found to occur in a stepwise manner in the protein, although the concerted pathway for the Co-C bond cleavage coupled with the hydrogen transfer is more favored in the gas phase. The assumed concerted transition state in the protein has more deformation of the coenzyme and the substrate and has less interaction with the protein than the stepwise route. Key factors and residues in promoting the enzymatic reaction rate have been discussed in detail.

Dissociation energy of the HOOO radical.

Abstract: The dissociation of the hydrotrioxy (HOOO) radical to OH and O2 has been studied theoretically using coupled-cluster methods. The calculated dissociation energy for the trans-HOOO isomer is 2.5 kcal mol−1 including zero-point corrections. The minimum energy path to dissociation has been explored and an exit barrier has been revealed, which may help to rationalize the apparent disagreement between theory and experiment on the magnitude of the bond energy.

Enthalpy of Formation of the Cyclohexadienyl Radical and the C-H Bond Enthalpy of 1,4-Cyclohexadiene: An Experimental and Computational Re-Evaluation

Abstract: This article discusses the enthalpy of formation of the cyclohexadienyl radical and the C-H bond enthalpy of 1,4-cyclohexadiene

Theoretical investigation on structures, densities, detonation properties, and the pyrolysis mechanism of the derivatives of HNS.

Abstract: The derivatives of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS) are optimized to obtain their molecular geometries and electronic structures at the DFT-B3LYP/6-31G* level. Detonation properties are evaluated using the modified Kamlet−Jacobs equations based on the calculated densities and heats of formation. It is found that there are good linear relationships between the density, detonation velocity, detonation pressure, and number of nitro, amino, and hydroxy groups. The thermal stability and pyrolysis mechanism of the title compounds are investigated by calculating the bond dissociation energies at the unrestricted B3LYP/6-31G* level. For the nitro and amino derivatives of HNS, the C−NO2 bond is a trigger bond during the thermolysis initiation process, while for hydroxy derivatives, it is started from the isomerization reaction of the hydrogen transfer in the O−H bond. According to the quantitative standard of energetics and stability, as high-energy density compounds, 2,2′,3,3′,4,4′,5,6,6′-nonanitrostilbene ...