Trends In Bond Dissociation Energies Research Articles

Vibrational spectroscopy and dissociation dynamics of cyclohexyl hydroperoxide.

Organic hydroperoxides (ROOH) are ubiquitous in the atmospheric oxidation of volatile organic compounds (VOCs) as well as in low-temperature oxidation of hydrocarbon fuels. The present work focuses on a prototypical cyclic hydroperoxide, cyclohexyl hydroperoxide (CHHP). The overtone OH stretch (2νOH) spectrum of jet-cooled CHHP is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of the resulting OH products. A distinctive IR feature is observed at 7012.5 cm-1. Two conformers of CHHP are predicted to have similar stabilities (within 0.2 kcal mol-1) and overtone OH stretch transitions (2νOH), yet are separated by a significant interconversion barrier. The IR power dependence indicates that absorption of three or more IR photons is required for dissociation of CHHP to cyclohexoxy (RO) and OH radical products. Accompanying high-level single- and multi-reference electronic structure calculations quantitatively support the experimental results. Calculations are extended to a range of organic hydroperoxides to examine trends in bond dissociation energies associated with RO + OH formation and compared with prior theoretical results.

Trends in Bond Dissociation Energies for the Homolytic Cleavage of Successive Molecular Bonds

The students in our physical chemistry course pointed out an interesting trend in the bond dissociation energies (BDEs) of successive homolytic cleavages of hydrogens from methane and water. Namely, while there is an increase in BDE from CH4 to CH3, there is a decrease in BDE from H2O to OH. In order to explain this trend, we employed a theoretical approach that could be used in an undergraduate setting. We compare our theoretical results with experimental and highly accurate theoretical values readily available in the literature. Having validated our theoretical approach, we use equilibrium molecular structure as evidence of the change in dominating attraction and repulsion effects of the central atom nucleus and nonbonding electrons and explain the anomaly in BDEs in undergraduate chemistry terms.

Trends in bond dissociation energies of brominated flame retardants from density functional theory

Bond dissociation enthalpies (BDEs) are computed using the Density Functional Theory (DFT) for a selected set of C–Br, C–O, and C–Br bonds susceptible to homolysis in the thermal degradation of four brominated flame retardants (BFRs): decabromo-diphenyl, decabromo-diphenylethane, 1,2-bis(2,4,6-tribromophenoxy)-ethane, and 3,4,5,6-tetrabromo-1,2-diethyl-phtalate. Based on BDEs computed at M06/cc-pVTZ level, choice based on a previous benchmark, we find debromination as the dominant decomposition pathway of brominated diphenyls and brominated phtalates, whereas scission to form brominated phenoxyls and benzyl radicals is preferred in aromatic BFRs containing ether and alkyl bridges, respectively. Trends in computed BDEs are rationalized in terms of (de)localization of spin density in the electronic structure of the resulting radicals.

Trends in Bond Dissociation Energies of Alcohols and Aldehydes Computed with Multireference Averaged Coupled-Pair Functional Theory

As part of our ongoing investigation of the combustion chemistry of oxygenated molecules using multireference correlated wave function methods, we report bond dissociation energies (BDEs) in C1-C4 alcohols (from methanol to the four isomers of butanol) and C1-C4 aldehydes (from methanal to butanal). The BDEs are calculated with a multireference averaged coupled-pair functional-based scheme. We compare these multireference BDEs with those derived from experiment and single-reference methods. Trends in BDEs for the alcohols and aldehydes are rationalized by considering geometry relaxations of dissociated radical fragments, resonance stabilization, and hyperconjugation. Lastly, we discuss the conjectured association between bond strengths and rates of hydrogen abstraction by hydroxyl radicals. In general, abstraction reaction rates are higher at sites where the C-H bond energies are lower (and vice versa). However, comparison with available rate data shows this inverse relationship between bond strengths and abstraction rates does not hold at all temperatures.

Theoretical Approaches To Estimating Homolytic Bond Dissociation Energies of Organocopper and Organosilver Compounds

Although organocopper and organosilver compounds are known to decompose by homolytic pathways among others, surprisingly little is known about their bond dissociation energies (BDEs). In order to address this deficiency, the performance of the DFT functionals BLYP, B3LYP, BP86, TPSSTPSS, BHandHLYP, M06L, M06, M06-2X, B97D, and PBEPBE, along with the double hybrids, mPW2-PLYP, B2-PLYP, and the ab initio methods, MP2 and CCSD(T), have been benchmarked against the thermochemistry for the M-C homolytic BDEs (D(0)) of Cu-CH(3) and Ag-CH(3), derived from guided ion beam experiments and CBS limit calculations (D(0)(Cu-CH(3)) = 223 kJ·mol(-1); D(0)(Ag-CH(3)) = 169 kJ·mol(-1)). Of the tested methods, in terms of chemical accuracy, error margin, and computational expense, M06 and BLYP were found to perform best for homolytic dissociation of methylcopper and methylsilver, compared with the CBS limit gold standard. Thus the M06 functional was used to evaluate the M-C homolytic bond dissociation energies of Cu-R and Ag-R, R = Et, Pr, iPr, tBu, allyl, CH(2)Ph, and Ph. It was found that D(0)(Ag-R) was always lower (~50 kJ·mol(-1)) than that of D(0)(Cu-R). The trends in BDE when changing the R ligand reflected the H-R bond energy trends for the alkyl ligands, while for R = allyl, CH(2)Ph, and Ph, some differences in bond energy trends arose. These trends in homolytic bond dissociation energy help rationalize the previously reported (Rijs, N. J.; O'Hair, R. A. J. Organometallics2010, 29, 2282-2291) fragmentation pathways of the organometallate anions, [CH(3)MR](-).

The effect of orbital type and active space size on valence bond structure weights and bond dissociation energies

Valence bond (VB) theory calculations have been performed on a series of small molecules and the bond dissociation energies (BDEs) and Chirgwin–Coulson structure weights have been obtained. The number of electrons included in the VB active space has been varied to include all valence electrons, only valence electrons of the same symmetry as the target bond, or only the bonding electrons. In addition, VB orbitals were either held strictly localized on a particular fragment (fragment localized orbitals) or allowed to delocalize onto neighbouring fragments (overlap enhanced orbitals). Structure weights and BDEs thus obtained were compared to previous breathing orbital VB (BOVB) and experimental results. Although none of the methods used herein give particularly accurate results due to neglect of dynamic electron correlation, including only bonding electrons in the VB active space with the use of overlap enhanced orbitals sufficiently reproduces trends in BDEs and structure weights with minimal computational effort. It is hoped that the computational ease along with the qualitative accuracy of this method will allow for the chemical insight of VB theory to be applied to larger molecules.

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

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 pi* (LUMO) and PdL(2) 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.

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.

Computational study on the difference between the Co–C bond dissociation energy in methylcobalamin and adenosylcobalamin

The bond dissociation energies of the Co-C bonds in the cobalamin cofactors methylcobalamin and adenosylcobalamin were calculated using the hybrid quantum mechanics/molecular mechanics method IMOMM (integrated molecular orbital and molecular mechanics). Calculations were performed on models of differing complexities as well as on the full systems. We investigated the origin of the different experimental values for the Co-C bond dissociation energies in methylcobalamin and adenosylcobalamin, and have provided an explanation for the difficulties encountered when we attempt to reproduce this difference in quantum chemistry. Additional calculations have been performed using the Miertus-Scrocco-Tomasi method in order to estimate the influence of solvent effects on the homolytic Co-C bond cleavage. Introduction of these solvation effects is shown to be necessary for the correct reproduction of experimental trends in bond dissociation energies in solution, which consequently have no direct correlation with dissociation processes in the enzyme.

Periodic Trends in Bond Dissociation Energies. A Theoretical Study

Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X). For A-X bonds involving electronegative atoms (A = C, N, O, F, P, S, and Cl) the bond energies achieve a maximum around Z(X) = 5. The same behavior is observed when BDEs are plotted against the electronegativity chi(X) of the atom X. Thus, for A-X bonds (A = Li, Be, B, Na, Mg, Al, Si), the BDEs for a fixed A increases, grosso modo, as the electronegativity differences between X and A increase, with some exceptions, which reflect the differences in the relaxation energies of the radicals produced upon the bond cleavage. A similar trend, albeit less pronounced, is found for single A-X bonds, where A = C, N, O, F, P, S, and Cl. However, there is an additional feature embodied in the enhancement of the strength of the A-boron bonds due to the ability of boron to act as a strong electron acceptor. The trends in bond lengths and charge densities at the bond critical points are in line with the aforementioned behavior.

Alkali ion carbonyls: sequential bond energies of Li +(CO) x ( x = 1−3), Na +(CO) x ( x = 1, 2) and K +(CO)

The sequential bond dissociation energies for Li +(CO) x ( x = 1−3), Na +(CO) x ( x = 1,2), and K +(CO) are determined by examining the collision-induced dissociation reactions with argon in a guided ion beam mass spectrometer. Analysis of the kinetic energy dependent cross sections yield values for the (CO) x−1 M +CO bond dissociation energies (BDEs) of 0.57 ± 0.13, 0.37 ± 0.04 and 0.36 ± 0.04 eV for M  Li ( x = 1–3, respectively), 0.33 ± 0.08 and 0.25 ± 0.03 eV for M  Na ( x = 1 and 2, respectively), and 0.19 ± 0.05 eV for K +CO. In addition, the M +Ar BDEs are determined by examining the ligand exchange reaction of M +(CO) with Ar and are found to be 0.34 ± 0.14, 0.16 ± 0.09 and 0.14 ± 0.07 eV for M  Li, Na and K, respectively. The trends in BDEs can be explained readily in terms of electrostatic bonding interactions. A comparison of the alkali metal ion—CO BDEs with those of transition metal ion carbonyls reveals the contributions of s-dσ hybridization and d-π ∗ back-donation in the chemical bonding of the latter systems.

Sequential Bond Dissociation Energies of M+(NH3)x (x = 1−4) for M = Ti−Cu

The sequential bond dissociation energies (BDEs) for M+(NH3)x (x = 1−4) for M = Ti−Cu are determined by examining the collision-induced dissociation reactions with xenon in a guided ion beam mass spectrometer. In the cases of Fe, Co, Ni, and Cu, the BDE for the second ammonia molecule is determined to be greater than the BDE for the first ammonia molecule. For all metal ions but Ti+, the BDEs for the first two ammonia molecules are large in comparison to the BDEs for the third and fourth ammonia molecules. In general, the results of this study for the BDEs of the first and second ammonia molecules agree well with the results of previous experimental and theoretical studies. Previous studies are available only for M+(NH3)3 (M = V, Mn, Ni, and Cu) and M+(NH3)4 (M = V and Cu) complexes, such that this study provides the first determination of all other (NH3)2M+−NH3 and (NH3)3M+−NH3 BDEs. The trends in BDEs are discussed in terms of hybridization, dative interactions, and spin changes and compared to trends for other comprehensively studied ligands, H2O and CO.

Energetics of the Homolytic C−H and C−Cl Bond Cleavages in Polychlorobenzenes: The Role of Electronic and Steric Effects

Geometries of (poly)chlorosubstituted benzenes and phenyl radicals are optimized at the BLYP/6-311G** level of theory. The radicals, which are all planar and of the σ-type, possess geometries that are influenced by both electronic and indirect steric effects. The total energies of the species under study are quantitatively analyzed with simple additive schemes involving the chlorine−chlorine and chlorine−trivalent carbon interactions. Comparisons with the few available experimental data reveal that the computed C−H and C−Cl bond dissociation energies (BDEs) of benzene and its chloro- derivatives are systematically too low by ca. 5 kcal/mol and that the experimental C−Cl BDE of 1,3-dichlorobenzene is most probably in error. The substituents are predicted to facilitate the homolytic C−Cl bond cleavage by up to 6.6 kcal/mol while making the C−H cleavage less favorable by as much as 3.8 kcal/mol. The trends in BDEs are readily accounted for by a superposition of electronic end steric effects. In all cases, the C−Cl bond cleavages are found to require significantly less energy than the C−H ones, implying kinetic control of the aryl radical formation in the course of pyrolysis of (poly)chlorobenzenes.

Solvation of Transition Metal Ions by Water. Sequential Binding Energies of M+(H2O)x (x = 1-4) for M = Ti to Cu Determined by Collision-Induced Dissociation

Thresholds for collision-induced dissociation of M^+(H_2O), (x = 1-4, M = Ti to Cu) with xenon are measured by using guided ion beam mass spectrometry. In all cases, the primary product is endothermic loss of one water molecule. The cross-section thresholds are interpreted to yield 0 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the clusters, and dissociation lifetimes. Overall, the results presented here are consistent with previously reported values for x = 1 and 2 and resolve several discrepancies in these values. Theory is shown to accurately predict the BDEs of x = 1 and 2 as well as the sign of the difference between them. For all ions but Mn^+, the bond dissociation energies (BDEs) of the first and second water molecules are large compared with those of the third and fourth water molecules. Trends in BDEs are discussed in terms of hybridization and spin changes occurring at the transition metal center.

Sequential Bond Energies of Water to Na+ (3s0), Mg+ (3s1), and Al+ (3s2)

Thresholds for collision-induced dissociation of M^+(H_2O), (x = 1-4, M = Na, Mg, and Al) with xenon are measured by using guided ion beam mass spectrometry. In all cases, the primary product is endothermic loss of one water molecule. The cross-section thresholds are interpreted to yield 0 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the clusters, and dissociation lifetimes. Our results are in good agreement with previous experimental results for Na^(H_2O)_x, and in reasonable agreement with theoretical predictions for all systems. Trends in bond dissociation energies with increasing ligation and for the various metals studied are discussed.

Predicted bond energies in peroxides and disulfides by density functional methods

We have performed self-consistent linear combination of Gaussian-type orbitals-density functional calculations for the molecules YY, RY, RYY, RYR′, RYYR, and RYYR′ with Y=O,S and R,R′=H,CH3. The structures were optimized within the local spin density approximation while the Y–Y, Y–C, and Y–H bond dissociation energies (BDE) were calculated with both a local exchange-correlation energy functional and a gradient corrected functional. Comparison of results obtained with the local and gradient corrected exchange-correlation functionals provides more experience on the successes and failures of gradient corrections. Trends in BDEs and the nature of bonding in oxygen and sulfur containing analog molecules are analyzed on the basis of two observations: (1) the O atom is more electronegative than S, C, and H atoms; (2) a S atom can have a valency larger than two and has a greater ability for multiple bonding than oxygen. Finally, comparison with a number of experimental results suggest that the C–S BDE in CH3S, the S–S BDE in CH3SSCH3, and the enthalpy of formation of CH3S should be reexamined.