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.     

基于平均影响值的反向传播神经网络方法用于提高密度泛函理论计算Y—NO键均裂能精度

将基于平均影响值(Mean impact value,MIV)的反向传播神经网络(Back propagation neural netowrk,BPNN)(MIV-BPNN)方法用于提高密度泛函理论(Density functional theory,DFT)计算Y—NO(Y=N,S,O及C)键均裂能的精度.利用量子化学计算和MIV-BPNN联合方法计算92个含Y—NO键的有机分子体系的均裂能.结果表明,相对于单一的密度泛函理论B3LYP/6-31G(d)方法,利用全参数下的BPNN方法计算92个有机分子均裂能的均方根误差从22.25 kJ/mol减少到1.84 kJ/mol,而MIV-BPNN方法使均方根误差减少到1.36 kJ/mol,可见B3LYP/6-31G(d)和MIV-BPNN联合方法可以提高均裂能的量子化学计算精度,并可预测实验上无法获取的均裂能值.     

X-H (X = C, N, O, Si, P, S) 键离解能计算的密度泛函方法研究

使用了不同密度泛函方法计算X-H (X = C, N, O, Si, P, S) 键离解能，并分析不同密度泛函方法的计算精度。研究发现大多数密度泛函方法包括B3LYP, B3P86, B3PW91, G96LYP, PBE1PBE，和BH&HLYP都明显低估键离解能13-25 kJ/mol。该现象与是否使用无限基组无关，因为即使使用无限基组键离解能仍然被低估。因此密度泛函方法不适合用于键离解能的估算。其中B3P86方法的偏差最小。进一步分析表明，使用限制性开壳层计算并无任何优势，在大多数情况下非限制性开壳层计算实际上比限制性开壳层计算要好。最后，我们发现了密度泛函方法对键离解能的低估是系统的，因此建议利用校准后的UDFT/6-311++G(d, p)方法计算化学键离解能。     

DFT study of Co-C bond cleavage in the neutral and one-electron-reduced alkyl-cobalt(III) phthalocyanines

Density functional theory (DFT) has been applied to the analysis of the structural and electronic properties of the alkyl-cobalt(III) phthalocyanine complexes, [CoIIIPc]-R (Pc = phthalocyanine, R = Me or Et), and their pyridine adducts. The BP86/6-31G(d) level of theory shows good reliability for the optimized axial bond lengths and bond dissociation energies (BDEs). The mechanism of the reductive cleavage was probed for the [CoIIIPc]-Me complex which is known as a highly effective methyl group donor. In the present analysis, which follows a recent study on the reductive Co-C bond cleavage in methylcobalamin (J. Phys. Chem. B 2007, 111, 7638-7645), it is demonstrated that addition of an electron and formation of the pi-anion radical [CoIII(Pc*)]-Me- significantly lowers the energetic barrier required for homolytic Co-C bond dissociation. Such BDE lowering in [CoIII(Pc*)]-Me- arises from the involvement of two electronic states: upon electron addition, a quasi-degenerate pi*Pc state is initially formed, but when the cobalt-carbon bond is stretched, the unpaired electron moves to a sigma*Co-C state and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. As in corrin complexes, the pi*Pc-sigma*Co-C states crossing does not take place at the equilibrium geometry of [CoIII(Pc*)]-Me- but only when the Co-C bond is stretched to approximately 2.3 A. The DFT computed Co-C BDE of 23.3 kcal/mol in the one-electron-reduced phthalocyanine species, [CoIII(Pc*)]-Me-, is lowered by approximately 37% compared to the neutral Py-[CoIIIPc]-Me complex where BDE = 36.8 kcal/mol. A similar comparison for the corrin-containing complexes shows that a DFT computed BDE of 20.4 kcal/mol for [CoIII(corrin*)]-Me leads to approximately 45% bond strength reduction, in comparison to 37.0 kcal/mol for Im-[CoIII(corrin)]-Me+. These results suggest some preference by the alkylcorrinoids for the reductive cleavage mechanism.     

Calculation of bond dissociation energies for oxygen containing molecules by ab initio and density functional theory methods

Two ab initio (ROHF and MP2), one local (SVWN), four hybrid (BHandH, BHandHLYP, Becke3LYP, and Becke3P86), and two nonlocal (BLYP and BP86) density functional theory (DFT) methods are used for calculating the dissociation energies of molecules that contain H(SINGLE BOND)O, O(SINGLE BOND)O and O(SINGLE BOND)C bonds. The sensitivity to the basis set of the prediction of bond dissociation energies with DFT methods was tested with Becke3LYP on the H(SINGLE BOND)O dissociation energy of water. The 6–31 + G(d) methods are chosen as the smallest basis set which produces reasonable results. The calculated values for all other ab initio and DFT methods were performed with these basis sets and then compared with the experimental data. The suitability of DFT methods for computing reliable bond dissociation energies of oxygen containing molecules is discussed. © 1996 John Wiley & Sons, Inc.     

First-Principle Calculations of the Adsorption, Dissociation and Diffusion of Hydrogen on the Mg(0001) Surface

First-principle pseudopotential plane wave calculations and the Nudged Elastic Band method based on density functional theory (DFT) have been used in this article to study the dissociation of molecular hydrogen on a Mg(0001) surface and the subsequent diffusion of atomic hydrogen into the magnesium substrate. First, the dissociation pathway of H2 and the relative barrier were investigated. It was shown that physical adsorption rather than chemisorption of molecular hydrogen was observed in the calculation of the dissociation process of molecular hydrogen. Also, the diffusion process of atomic hydrogen on Mg(0001) was presented. The surface effect, which affected the diffusion of hydrogen obviously, was observed. Finally, comparing the values of the activation energies for the steps of dissociation, diffusion, and desorption, our calculation further showed that the dissociation of H2 and the desorption of hydride were the rate-limiting steps.     

Theoretical Studies on the Reaction Mechanism of 1-Chloroethane with Hydroxyl Radical

The reaction mechanism of 1-chloroethane with hydroxyl radical has been inves- tigated by using density functional theory (DFT) B3LYP/6-31G (d, p) method. All bond dissociation enthalpies were computed at the same theoretical level. It was found that hydrogen abstraction pathway is the most favorable. There are two hydrogen abstraction pathways with activation barriers of 0.630 and 4.988 kJ/mol, respectively, while chlorine abstraction pathway was not found. It was observed that activation energies have a more reasonable correlation with the reaction enthalpy changes (△Hr) than with bond dissociation enthalpies (BDE).     

Density functional theory study of the d(10) series (H3P)3M(eta 1-SO2) and (MenPh(3-n)P)3M(eta 1-SO2) (M = Ni, Pd, Pt; n = 0-3): SO2 pyramidality and M-S bond dissociation energies

Quantum mechanical density functional theory (DFT) and coupled DFT/molecular mechanics (QMMM) studies of the compounds (H(3)P)(3)M(eta(1)-SO(2)) and (Me(n)Ph(3-n)P)(3)M(eta(1)-SO(2)) (M = Ni, Pd, Pt; n = 0-3) model the experimental data well, particularly the planar/pyramidal geometries at sulfur. Bond dissociation energy (BDE) calculations confirm that Pd/Pt systems with pyramidal SO(2) ligands exhibit M-S BDEs smaller by 30-50% than Ni systems with planar SO(2). However, scans of the potential energy surfaces show that flexing the planar/pyramidal torsion angle within ranges of 20-30 degrees requires little energy. Bond energy decomposition calculations indicate that the electrostatic Delta E(elstat) term determines the BDE for Pd/Pt molecules where the sulfur is pyramidal, whereas all three terms matter when the sulfur is planar, as for Ni compounds. However, this accounts only for a fraction of the BDE differences; orbital energy matching accounts for the balance.     

Theoretical Elucidation on Different Lipid-Oxidation Potentials of Aminoxyl Antioxidants

To elucidate the different lipid-oxidation potentials of aminoxyl antioxidants, a kind of combined density functional theory (DFT) method was employed to calculate C-H bond dissociation enthalpies (BDEs) of a model linoleic acid (LH) and O-H BDEs of hydrogenated aminoxyls. The higher the O-H BDE is, the more potent the amlnoxyl to abstract the H-atom from LH and the stronger the LH-oxidation potential. Accordingly,the prooxidant activity differences of amlnoxyls were elucidated by the different O--H BDEs of hydrogenated amlnoxyls, which were further clarified in terms of distinct electronic effects of the substituents.     

羰基镍簇Ni(CO)n(n=1-4)的结构和Ni-CO键解离性质的密度泛函理论研究

在密度泛函理论框架下, 应用不同泛函计算了配合物Ni(CO)n(n=1~4)的平衡几何构型和振动频率. 考察了泛函和基组重叠误差对预测Ni—CO键解离能的影响. 计算结果表明, 用杂化泛函能得到与实验一致的优化几何构型和较合理的振动频率. 对Ni(CO)n(n=2~4)体系, 用“纯”泛函, 如BP86和BPW91, 可得到与CCSD(T)更符合、 并与实验值接近的解离能. 当解离产物出现单个金属原子或离子(如金属羰基配合物的完全解离)时, BSSE校正项的计算中应保持金属部分的电子结构一致. 只有考虑配体基组和不考虑配体基组两种情况下金属的电子构型与配合物中金属的构型一致时, 才能得到合理的BSSE校正, 从而预测合理的解离能.     

BAC-MP4 predictions of thermochemistry for gas-phase antimony compounds in the Sb-H-C-O-Cl system

Calibrated by both experimental data and high-level coupled-cluster calculations, the BAC-MP4 methodology was applied to 51 SbL(n) (L = H, CH(3), C(2)H(5), Cl, and OH, n = 1-5) molecules, providing calculated heats of formation and associated thermodynamic parameters. These data identify a linear variation in heats of formation with ligand substitution, trends in bond dissociation energies (BDEs) with ligand identity [BDE(Sb-C(2)H(5)) < BDE(Sb-CH(3)) < BDE(Sb-H) < BDE(Sb-Cl) < BDE(Sb-OH)], and a monotonic decrease in BDE upon successive ligand elimination. The linear variation in BDE is consistent with the behavior of other group V elements, in contrast to the characteristic high-low-high trend of adjacent group III (In) and group IV (Sn) elements. Additionally, these data complement those of previous studies of metal-organic species and provide a foundation of thermochemical data that can aid in the selection of CVD precursors and deposition conditions for the growth of antimony-containing materials.     

Electronic effects on O-H proton dissociation energies of phenolic cation radicals: a DFT study

The electronic effects on O-H proton dissociation energies (PDEs) of para- and meta-substituted phenolic cation radicals have been investigated by density functional theory (DFT) using B3LYP function on a 6-31G(d, p) basis set. The calculation results indicate that electron-donating groups raise the O-H PDE and electron-withdrawing groups reduce the parameter, which are opposite to the electronic effects on O-H bond dissociation energies (BDEs). In addition, the electronic effects on O-H PDE are much stronger than those on O-H BDE. The differences result from the distinct electronic effects on stabilities of phenolic cation radicals and parent phenols. The finding also implies the proton-transfer process is unlikely a rate-controlling step for phenolic antioxidants to scavenge free radicals. Moreover, like O-H BDE, O-H PDE correlate better with the resonance parameter R+ than with field/inductive parameter F. Therefore, O-H PDEs of para-substituted phenolic cation radicals are mainly governed by the resonance effect.     

The C-H and alpha(C-X) bond dissociation enthalpies of toluene, C6H5-CH2X (X = F, Cl), and their substituted derivatives: a DFT study

The homolytic C-H bond dissociation enthalpies (BDEs) of toluene and its para- and meta-substituted derivatives have been estimated by using the (RO)B3LYP/6-311++G(2df,2p)//(U)B3LYP/6-311G(d,p) procedure. The performance of two other hybrid functionals of DFT, namely, B3PWP91 and O3LYP, has also been evaluated using the same basis sets and molecules. Our computed results are compared with the available experimental values and are found to be in good agreement. The (RO)B3LYP and (RO)O3LYP procedures are found to produce reliable BDEs for the C-H bonds in toluene and the C-X (X = F, Cl) bond in alpha-substituted toluene (C6H5-CH2X) and their substituted derivatives. The substituent effect on the BDE values has been analyzed in terms of the ground-state effect and the radical effect. The effect of polarization of the C-H bond on the substituent effect is also analyzed. The BDE(C-H) and BDE(C-X) values for alpha-substituted (X = F and Cl) toluenes with a set of para substituents are presented for the first time.     

Density Functional Calculations of C-NO2 Bond Dissociation Energies for Nitroalkanes Molecules

Bond dissociation energies for the removal of nitrogen dioxide group in some nitroalkane energetic materials have been calculated by using the three hybrid density functional theory （DFT） methods B3LYP, B3PW91 and B3P86 with 6-31g^＊＊ and 6-311g^＊＊ basis sets. The computed BDEs have been compared with the available experimental results. It is found that the B3P86 method with 6-31g^＊＊ and 6-311g^＊＊ basis sets can obtain satisfactory bond dissociation energies （BDEs）, which are in extraordinary agreement with the experimental data. Considering the smaller mean absolute deviation and maximum difference, the reliable B3P86/6-311g^＊＊ method was recommended to compute the BDEs for the removal of nitrogen dioxide group in the nitroalkane energetic materials. Using the method, the BDEs of 8 other nitroalkane energetic materials have been calculated and the maximum difference from experimental value is 1.76 kcal.mo1^-1 （for the BDE of tC4Hg-NOz）, which further proves the reliability of B3P86/6-311g^＊＊ method. In addition, it is noted that the BDEs of C-NO2 bond change slightly for main chain nitroalkane compounds with the maximum difference of only 3.43 kcal mo1^-1.     

Density functional and multiconfigurational ab initio study of the ground and excited states of Os2

Multiconfigurational ab initio methods predict that the ⁵Π_u state as the ground state instead of the ⁷Δ_u state. Although multiconfigurational perturbation theory correctly predicts the ground state, they overestimate the bond dissociation energy (BDE). Only multireference configuration interaction method can reasonably calculate the BDE. The spin‐orbit effect on the spectroscopic constants is not significant. The results calculated by density functional theory (DFT) vary significantly depending on the selection of a DFT functional. No DFT functional gives the same energy ordering as calculated by the second‐order multiconfigurational perturbation theory (CASPT2). The old generalized gradient approximations functionals are well suited for predicting the ground state and calculating the bond length and the vibrational frequency of Os₂. According to the CASPT2 calculation, the ground state of Os₂ has a quadruple bond. © 2014 Wiley Periodicals, Inc.     

Comparisons of multilayer H2O adsorption onto the (110) surfaces of alpha-TiO2 and SnO2 as calculated with density functional theory

Mono- and bilayer adsorption of H2O molecules on TiO2 and SnO 2 (110) surfaces has been investigated using static planewave density functional theory (PW DFT) simulations. Potential energies and structures were calculated for the associative, mixed, and dissociative adsorption states. The DOS of the bare and hydrated surfaces has been used for the analysis of the difference between the H2O interaction with TiO2 and SnO 2 surfaces. The important role of the bridging oxygen in the H2O dissociation process is discussed. The influence of the second layer of H2O molecules on relaxation of the surface atoms was estimated.     

Theoretical Study of Remote Substituent Effects on X-H （X=CH2, NH,O） Bond Dissociation Energies of Azulene

In the study, the X-H （X=CH2, NH, O） bond dissociation energies （BDE） of para-substituted azulene （Y-C10H8X-H） were predicted theoretically for the first time using Density Functronal Theory （DFT） methods at UB3LYP/6-311 ＋ ＋g（2df,2p）//UB3LYP/6-31 ＋g（d） level. It was found that the substituents exerted similar effects on the X-H BDE of azulene as those on benzene, except for 6-substituted 2-methylazulene. Owing to the substituent-dipole interaction, the reaction constants （ρ^＋） of 2- and 6-Y-CIoHsX-H （X=NH and O only） varied violently. The origin of the substituent effects on the X-H BDE of azulene was found, by both GE/RE and SIE theory, to be directly associated with variation of the radical effects, although the ground effects also played a modest role in determining the net. substituent effects.     

C?H Bond dissociation of acetylene: Local density functional calculations

The C? H bond dissociation energy of acetylene was computed by both ab initio approaches and density functional theory in a local density approximation (DFT–LDA ). Structures and energies for acetylene and its dissociation products (the ethynyl and hydrogen radicals) are presented and compared. Using directly computed HCCH and HCC· energies and the exact H· value, the DFT–LDA calculations are found to yield C? H dissociation energies ranging from 129 to 131 kcal/mol, in good agreement with recent experimental and the highest level theoretical results. The DFT–LDA results show little dependence upon the computational procedure used to obtain geometries.     

Bond Dissociation Energies and Electronic Structures in a Series of Peroxy Radicals: A Theoretical Study

Quantum chemical calculations are performed to estimate the bond dissociation energies (BDEs) for 18 peroxy radicals. Since DFT methods are researched to have low basis sets sensitivity, these radicals are studied by utilizing the hybrid density functional theory (DFT) (B3LYP, B3P86, B3PW91 and PBE1PBE) in conjunction with the 6‐311G** basis set and the complete basis set (CBS‐Q) method. On the basis of comparisons of the computational results and the experimental values, we evaluate the effectiveness of above methods. It is demonstrated that CBS‐Q method is the best method for computing the reliable BDEs of C—OO bond, with the average absolute errors of 2.1 kcal/mol. So CBS‐Q method is suitable to predict accurate BDEs of C‐OO bond for peroxy compounds. The computational energy gaps between the HOMO and LUMO of studied compounds are almost identical from the point of view of stability and substantial HOMO‐LUMO gaps for all molecules suggest their electronic stability. In addition, substituent effect on the C—OO BDE of peroxy radicals is analyzed. It is noted that the effects of substitution on the C—OO BDE of peroxy radicals are significant. Our results will shed lights on future theoretical and experimental work.     

Bond dissociation energies in second-row compounds

Heats of formation at 0 and 298 K are predicted for PF3, PF5, PF3O, SF2, SF4, SF6, SF2O, SF2O2, and SF4O as well as a number of radicals derived from these stable compounds on the basis of coupled cluster theory [CCSD(T)] calculations extrapolated to the complete basis set limit. In order to achieve near chemical accuracy (+/-1 kcal/mol), additional corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, a correction for first-order atomic spin-orbit effects, and vibrational zero-point energies. The calculated values substantially reduce the error limits for these species. A detailed comparison of adiabatic and diabatic bond dissociation energies (BDEs) is made and used to explain trends in the BDEs. Because the adiabatic BDEs of polyatomic molecules represent not only the energy required for breaking a specific bond but also contain any reorganization energies of the bonds in the resulting products, these BDEs can be quite different for each step in the stepwise loss of ligands in binary compounds. For example, the adiabatic BDE for the removal of one fluorine ligand from the very stable closed-shell SF6 molecule to give the unstable SF5 radical is 2.8 times the BDE needed for the removal of one fluorine ligand from the unstable SF5 radical to give the stable closed-shell SF4 molecule. Similarly, the BDE for the removal of one fluorine ligand from the stable closed-shell PF3O molecule to give the unstable PF2O radical is higher than the BDE needed to remove the oxygen atom to give the stable closed-shell PF3 molecule. The same principles govern the BDEs of the phosphorus fluorides and the sulfur oxofluorides. In polyatomic molecules, care must be exercised not to equate BDEs with the bond strengths of given bonds. The measurement of the bond strength or stiffness of a given bond represented by its force constant involves only a small displacement of the atoms near equilibrium and, therefore, does not involve any reorganization energies, i.e., it may be more appropriate to correlate with the diabatic product states.