Linear relationship between activation energies and reaction energies for coverage-dependent dissociation reactions on rhodium surfaces

Evidence of a relationship between activation energies and enthalpy changes of various dissociation reactions on transition metals has been reported recently. A reconsideration of density functional theory results for dissociation energies of oxygen and NO on different rhodium surfaces (low-index and stepped) and their dependencies on oxygen precoverage reveal that also here a linear Br?nsted-Evans-Polanyi (BEP) relationship exists. The establishment of such a general concept would be of tremendous importance for the development of detailed, elementary-step reaction mechanisms, because the activation energies of reaction steps as well as their coverage dependencies could be estimated based on the adsorption energies calculated by means of DFT.     

Identification of general linear relationships between activation energies and enthalpy changes for dissociation reactions at surfaces

The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Br?nsted-Evans-Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.     

Cycloalkane and cycloalkene C-H bond dissociation energies

Both C-H bond dissociation energies for cyclobutene were measured in the gas phase (BDE = 91.2 +/- 2.3 (allyl) and 112.5 +/- 2.5 (vinyl) kcal mol-1) via a thermodynamic cycle by carrying out proton affinity and electron-binding energy measurements on 1- and 3-cyclobutenyl anions. The results were compared to those for an acyclic model compound, cis-2-butene, and provide the needed information to experimentally establish the heat of formation of cyclobutadiene. Chemically accurate G3 and W1 calculations also were carried out on cycloalkanes, cycloalkenes, and selected reference compounds. It appears that commonly cited bond energies for cyclopropane, cyclobutane, and cyclohexane are 3 to 4 kcal mol-1 too small and their pi bond strengths, as given by BDE1 - BDE2, are in error by up to 8 kcal mol-1.     

Thermochemistry and bond dissociation energies of ketones

Ketones are a major class of organic chemicals and solvents, which contribute to hydrocarbon sources in the atmosphere, and are important intermediates in the oxidation and combustion of hydrocarbons and biofuels. Their stability, thermochemical properties, and chemical kinetics are important to understanding their reaction paths and their role as intermediates in combustion processes and in atmospheric chemistry. In this study, enthalpies (ΔH°(f 298)), entropies (S°(T)), heat capacities (C(p)°(T)), and internal rotor potentials are reported for 2-butanone, 3-pentanone, 2-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone, and their radicals corresponding to loss of hydrogen atoms. A detailed evaluation of the carbon-hydrogen bond dissociation energies (C-H BDEs) is also performed for the parent ketones for the first time. Standard enthalpies of formation and bond energies are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, and G3MP2B3 levels of theory using isodesmic reactions to minimize calculation errors. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropies and heat capacities. The recommended ideal gas-phase ΔH°(f 298), from the average of the CBS-QB3 and G3MP2B3 levels of theory, as well as the calculated values for entropy and heat capacity are shown to compare well with the available experimental data for the parent ketones. Bond energies for primary, secondary, and tertiary radicals are determined; here, we find the C-H BDEs on carbons in the α position to the ketone group decrease significantly with increasing substitution on these α carbons. Group additivity and hydrogen-bond increment values for these ketone radicals are also determined.     

Bond dissociation energies and bond orders for diatomic alkali halides

The bond dissociation energies for Alkali halides have been estimated based on the derived relations: $$\begin{gathered} D_{AB} = \bar D_{AB} + 31.973{\text{ e}}^{0.363\Delta x} {\text{ and}} \hfill \\ D_{AB} = \bar D_{AB} (1 - 0.2075\Delta xr_e ) + 52.29\Delta x, \hfill \\ \end{gathered} $$ where \(\bar D_{AB} = (D_{AA} \cdot D_{BB} )^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}} \) , Δx represents Pauling electronegativity differences x(_A ?x_B) and r _e is the internuclear distance. A simplified formula relating bond orders, q, to spectroscopic constants is suggested. The formula has the form q = 1.5783 × 10^?3 (ω _e ² r_e/ B_e)^1/2. The ambiguity arising from the Parr and Borkman relation is discussed. The present study supports the view of Politzer that q/(0.5r _e)² is the correct definition of bond order. The estimated bond energies and bond orders are in reasonably good agreement with the literature values. The bond energies estimated with the relations we suggested, for alkali halides give an error of 4.5% and 5.3%, respectively. The corresponding error associated with Pauling's equation is 40.2%.     

The effect of bond functions on dissociation energies

The procedure employing bond functions recently suggested by Wright and Buenker has been applied to the N₂ X ¹Σ_g⁺ potential curve within the CAS SCF+MRSD Ci treatment of electron correlation. The basis set used herein is identical to that employed by these authors in their SCF+CI calculations. The D_e and the shape of the resulting potential curve, as judged by the computed vibrational levels, is not so accurate as would be expected from the results reported by Wright and Buenker. Our results indicate that using the CI superposition errors associated with bond functions to cancel basis set incompleteness depends on the treatment of the electron correlation.     

木质素模化物键离解能的理论研究

采用密度泛函理论B3P86方法,在6-31G(d,p)基组水平上,对木质素结构中的6种连接方式(β-O-4、α-O-4、4-O-5、β-1、α-1、5-5)的63个木质素模化物的醚键(C-O)和C-C键的键离解能E_B进行了理论计算研究。分析了不同取代基对键离解能的影响以及键长与键离解能的相关性。计算结果表明,C-O键的键离解能通常比C-C键的小,在各种醚键中C_α-O键的平均键离解能最小,为182.7 kJ/mol;其次是β-O-4连接中的C_β-O键,苯环和烷烃基上的取代基对醚键的键离解能有较强的弱化作用,C-O键的键长和键离解能的相关性较差。与C-O键相比,C-C键的键离解能受苯环上取代基的影响很小,而烷烃基上的取代基对C-C键的键离解能有较大的影响,C-C键的键离解能和键长之间存在较强的线性关系,C-C键的键长越长,其键离解能越小。     

Determination of bond dissociation energies using mass spectrometry

Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR/MS) offers the opportunity for gas phase cluster formation reactions at very low pressures and at temperatures that are different from room temperature. Reactions take place with single positive‐charge metal ions that are normally +2, +3, +4, etc., charged in solution. The ions formed are detected by measuring the current induced by their cyclotron rotation, but they cannot be physically separated and collected. Collision‐induced dissociation (CID) is widely used for ion‐structure determination via the fragmentation of the excited ions. CID study aims to determine the relationship between the V_pp [peak‐to‐peak voltage of the radiofrequency (rf) pulse] and the mass‐to‐charge (m/z) ratio, which will be used for the calculation of the center‐of‐mass translational kinetic energy (Ek_cm) of the excited ion under investigation. CID studies are restricted to stable ions with relatively high abundance. Nevertheless, with the evolution of computational chemistry, such problems can be overcome whereby CID calculations will be used to provide the substantial parameters for computer software, such as the Gaussian 03 program, for the structure determination of the less stable Ni_xS anions. The latter constitutes the core for our current research. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Remote substituent effects on homolytic bond dissociation energies

In the study we tried to answer two questions. First, does X-Z homolytic bond dissociation energy (BDE) of Y-C6H4-X-Z obey the Hammett relationship? Second, if it does what factors determine the magnitude and sign of the slope (rho+) of Hammett regression against substituent sigma(p)(+) constants? We collected a large number of X-Z BDEs for over one-thousand Y-C6H4-X-Z systems using the RMP2/6-311++G**//UB3LYP/6-31G* method. We found that remote substituent effects on X-Z BDEs are determined by both the ground effect (i.e. stabilization/destabilization of X-Z by the substituents) and the radical effect (i.e. stabilization/destabilization of X. by the substituents). The ground or radical effect is determined by the electron demand of X-Z or X. in the same way as the deprotonation enthalpy of HOOC-C6H4-X-Z or HOOC-C6H4-X. is affected by X-Z or X. . As a result, rho+ (BDE) for X-Z bond homolysis can be quantitatively predicted by using the change in deprotonation enthalpy from HOOC-C6H4-X-Z to HOOC-C6H4-X. .     

Investigation of correlation between impact sensitivities and bond dissociation energies in benzenoid nitro compounds

The geometries of ten benzenoid energetic materials are fully optimized by employing B3LYP and B3P86 methods with the 6–31G** basis set. Bond dissociation energies (BDEs) for the removal of the NO₂ group in benzenoid molecules are calculated at the same level. The calculation results show that the insertion of an electron withdrawing group increases the stability of the molecules, while the insertion of an electron donating group reduces the stability of the molecules. In addition, the relationship between the impact sensitivities and the weakest BDE values is examined. There exists a good linear correlation between the impact sensitivity and the ratio of the BDE value to the molecular total energy.     

Atomic contributions to bond dissociation energies in aliphatic hydrocarbons

This paper explores the atomic contributions to the electronic vibrationless bond dissociation enthalpy (BDE) at 0 K of the central C-C bond in straight-chain alkanes (C(n)H(2n+2)) and trans-alkenes (C(n)H(2n)) with an even number of carbon atoms, where n=2, 4, 6, 8. This is achieved using the partitioning of the total molecular energy according to the quantum theory of atoms in molecules by comparing the atomic energies in the intact molecule and its dissociation products. The study is conducted at the MP2(full)6-311++G(d,p) level of theory. It is found that the bulk of the electronic energy necessary to sever a single C-C bond is not supplied by these two carbon atoms (the alpha-carbons) but instead by the atoms directly bonded to them. Thus, the burden of the electronic part of the BDE is primarily carried by the two hydrogens attached to each of the alpha-carbons and by the beta-carbons. The effect drops off rapidly with distance along the hydrocarbon chain. The situation is more complex in the case of the double bond in alkenes, since here the burden is shared between the alpha-carbons as well as the atoms directly bonded to them, namely, again the alpha-hydrogens and the beta-carbons. These observations may lead to a better understanding of the bond dissociation process and should be taken into account when locally dense basis sets are introduced to improve the accuracy of BDE calculations.     

The effect of bond functions on molecular dissociation energies

Multi-reference Cl calculations are reported for the ground states of HCl and N₂ at their equilibrium distances, and for their separated atoms. Basis sets of double-zeta and double-zeta plus polarization quality are systematically augmented by additional sets of functions located at the bond centers. It is shown that use of bond functions can lead to either an underestimate or an overestimate of the the bond energy. Optimum basis sets for each molecule were obtained, giving D_e values of 4.59 eV for HCl (expt. 4.62 eV) and 9.96 eV for N₂ (expt. 9.905 eV) at the estimated full Cl level. The quality of the potential curves obtained with these basis sets is discussed.     

Investigation of correlation between impact sensitivities and bond dissociation energies in some triazole energetic compounds

In this article, density functional theory has been utilized to study on the correlation between impact sensitivities h _50% and the bond dissociation energies (BDEs) of nine triazole energetic explosives. By employing B3LYP and B3P86 method with the 6-311G** basis set, all the molecules have been fully optimized. The BDEs for removal of the NO₂ group in these compounds have also been calculated at the same level. Computed results show that BDEs calculated by B3LYP method are all less than those by B3P86 method. The relationship between the impact sensitivities and the weakest C–NO₂ bond dissociation energy (BDE) values have been investigated. The results indicate a good linear correlation between the impact sensitivity h _50% and the ratio (BDE/E) of the weakest BDE to the total energy E.     

Effects of geminal disubstitution on C-H and N-H bond dissociation energies

Composite ab initio methods including G3, CBS-Q, and G3B3 were used to calculate the C-H and N-H bond dissociation energies (BDEs) of a variety of disubstituted methane and ammonia molecules. The calculated BDEs were in excellent agreement with the available experimental data. Using these reliable BDEs we studied the effects of geminal disubstitution on C-H and N-H BDEs. It was found that the effects of the two substituents were not additive. Detailed separation of the substituent effects on BDEs to those associated with the parent molecules and those associated with the radicals was then performed using appropriate isodesmic reactions. It was found the geminal substitution effects on the stabilities of methanes, methyl radicals, amines, and amine radicals were all governed by five basic types of energetic effects, namely, hyperconjugation effect (stabilizing), electrostatic attraction (stabilizing) or repulsion (destabilizing), conjugation saturation effect (destabilizing), captodative effect (stabilizing), and steric effect (destabilizing). The conformations of the species played an essential role in determining whether a particular energetic effect could take place. Because the carbon-centered and nitrogen-centered species often had quite different conformational preferences, the geminal substitution effects on these two classes of species were quite dissimilar to each other.     

AM1 calculations of O-H bond dissociation energies in polyoxides

Energies of homolytic cleavage of O-H bonds in 33 compounds of the general formula Ro _n H (n = 2, 3, and 4) were calculated by the AMI method. For hydrotrioxides and hydrotetroxides, the bond dissociation energies are virtually independent of the nature of the substituent R:D(RO _n -H) = 92.3±0.8 kcal mol^–1 (n = 3 and 4).     

Homolytic C-H and N-H bond dissociation energies of strained organic compounds

High-level computations at G3, CBS-Q, and G3B3 levels were conducted, and good-quality C-H and N-H bond dissociation energies (BDEs) were obtained for a variety of saturated and unsaturated strained hydrocarbons and amines for the first time. From detailed NBO analyses, we found that the C-H BDEs of hydrocarbons are determined mainly by the hybridization of the parent compound, the hybridization of the radical, and the extent of spin delocalization of the radical. The ring strain has a significant effect on the C-H BDE because it forces the parent compound and radical to adopt certain undesirable hybridization. A structure-activity relationship equation (i.e., BDE (C-H) = 61.1-227.8 (p(parent)% - 0.75)(2) + 152.9 (p(radical)% - 1.00)(2) + 40.4 spin) was established, and it can predict the C-H BDEs of a variety of saturated and unsaturated strained hydrocarbons fairly well. For the C-H BDEs associated with the bridgehead carbons of the highly rigid strained compounds, we found that the strength of the C-H bond can also be predicted from the H-C-C bond angles of the bridgehead carbon. Finally, we found that N-H BDEs show less dependence on the ring strain than C-H BDEs.     

On the advantages of hydrocarbon radical stabilization energies based on R-H bond dissociation energies

Hydrocarbon radical stabilization energy (RSE) estimates based on the differences in R-H vs CH(3)-H bond dissociation energies have inherent advantages over RSEs based on R-CH(3) vs CH(3)-CH(3), as well as R-R vs CH(3)-CH(3) comparisons, since the R-CH(3) and R-R reference systems are prone to unbalanced contaminating intramolecular interactions involving the R groups. When the effects of steric crowding, branching, protobranching, conjugation, and hyperconjugation are taken into account, R-CH(3) and R-R based RSE values are nearly identical to R-H RSEs. Corrections for electronegativity differences between H and R are not needed to achieve agreement.     

Kinetics of the C-C bond beta scission reactions in alkyl radicals

High pressure limits of thermal rate constants of four C-C bond beta scission reactions of propyl, 1-butyl, 2-butyl and isobutyl radicals were calculated using the canonical variational transition state theory (CVT) with a multi-dimensional small-curvature tunneling (SCT) correction over the temperature range of 300-3000 K. The CCSD(T)/cc-pVDZ//BH&HLYP/cc-pVDZ method was used to provide necessary potential energy surface information. Rate constants for these reactions were used to extrapolate rate constants for reactions in larger alkyls where experimental data are available using the Reaction Class Transition State Theory (RC-TST). Excellent agreement with experimental data confirms the validity of the RC-TST methodology and the accuracy of the calculated kinetic data in this study.