Evaluation of a Combined Quantum Chemical Method Used in Calculating O?H Bond Dissociation Enthalpy

O? H bond dissociation enthalpies (BDE) for a variety of substituted phenols were calculated using a combined quantum chemical method. It is found that the calculated O? H BDE correlated well with the recommended values, except for ortho‐tert‐butyl substituted phenols. For the electron‐donating group substituted phenols the calculated O? H BDE are slightly higher than the recommended values, however, for the electron‐withdrawing group substituted phenols the calculated O? H BDE are slightly lower than the recommended values.     

Accurate Prediction of IrH Bond Dissociation Enthalpies by Density Functional Theory Methods

The iridium hydride complexes have been extensively used in organic reactions, such as oxidation and hydrogenation reactions. In many of these reactions, the dissociation or formation of Ir? H bond plays an important role in determining the overall reaction rates and yields. In the present study, the accuracy of different theoretical methods for prediction of Ir? H bond strengths has been examined on the basis of the previously reported Ir? H BDEs of 17 different complexes. Comparing the performance of different DFT functionals (e.g. B3LYP, TPSS, M06), different basis sets (including the different effective core potentials (ECP) on Ir and I atoms, and the total electron basis sets on the other atoms), and different solvation models (SMD, CPCM, and IEFPCM) in solution phase single point calculations, we found that the gas‐phase calculation with TPSS/(LanL2DZ: 6‐31G(d)) method is relatively more accurate than the other gas‐phase calculation methods, and can well simulate the Ir? H BDEs in low‐polarity solvents (such as chlorobenzene and dichloroethane). Finally, efforts were put in analyzing the structure‐activity relationships between the ligand structure (around Ir center) and the Ir? H BDEs. We wish the present study could benefit future studies on the Ir‐H complexes involved organic reactions.     

First Total Synthesis of (±)‐Latifolin and Its Antioxidant Mechanism

The first total synthesis of (±)‐latifolin has been accomplished in six steps and 47.8% overall yield. To understand the relative importance of phenolic O? H and benzhydryl C? H hydrogen on the antioxidant activity of latifolin, 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) radical scavenging assay and density functional theory (DFT) studies were carried out. On scavenging DPPH radical in ethanol, the activity of latifolin ( 1 ) bearing phenolic hydrogen is remarkably higher than analogue 10 bearing no phenolic hydrogen. Therefore, Phenolic hydrogen is responsible for latifolin's antioxidant activity rather than benzhydryl C? H hydrogen. Furthermore, the 5‐OH BDE is lower than 2′‐OH and 7‐CH BDEs by a DFT calculation, respectively. Based on theoretical results it is definitely concluded that the phenolic 5‐OH plays a major role in the antioxidant activity of latifolin.     

PCM study of the solvent and substituent effects on bond dissociation energies of the O?NO Bond—A DFT study

A PCM continuum model, at the B3LYP, B3P86, and B3PW91 three‐parameter hybrid DFT methods with 6‐311G** basis set, is used to study the bond dissociation energies (BDEs) of benzyl nitrites. Compared the computed results with the experimental values, it is noted that B3PW91 functional is the best method to compute the BDEs of benzyl nitrites. The solvent and substituent effects on the BDEs of the O? NO bond are analyzed, and it is shown that the BDE of the O? NO bond decreases with the increment of the Hammett constants of substituent groups on benzene for benzyl nitrites except C₆H₅CH₂O? NO. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Si-H, P-H 和 S-H 键离解能Alfa取代基效应研究

CBS-Q and G3 methods were used to generate a large number of reliable Si--H, P---H and S--H bond dissociation energies （BDEs） for the first time. It was found that the Si--H BDE displayed dramatically different substituent effects compared with the C--H BDE. On the other hand, the P---H and S--H BDE exhibited patterns of substituent effects similar to those of the N--H and O--H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH3 would strengthen the Si--H bond whereas increasing the positive charge on P and S of XPH2 and XSH would weaken the P---H and S--H bonds. Meanwhile, increasing the positive charge on Si of XSiH2^＋ stabilized the silyl radical whereas increasing the positive charge on P and S in XPH＂ and XS＊ destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals.     

Diffusion Monte Carlo Study of Bond Dissociation Energies for BH2, B(OH)2, BCl2, and BCl

On basis of bond dissociation energies (BDEs) for BH₂, B(OH)₂, BCl₂, and BCl, the diffusion Monte Carlo (DMC) method is applied to explore the BDEs of HB-H, HOB-OH, ClB-Cl, and B-Cl. The effect of the choice of orbitals, as well as the backflow transformation, is studied. The Slater-Jastrow DMC algorithm gives BDEs of 359.1±0.12 kJ/mol for HB?H, 410.5±0.50 kJ/mol for HOB-OH, 357.8±1.46 kJ/mol for ClB-Cl, and 504.5±0.96 kJ/mol for B-Cl using B3PW91 orbitals and similar BDEs when B3LYP orbitals are used. DMC with backflow corrections (BF-DMC) gives a HB-H BDE of 369.9±0.12 kJ/mol which is close to one of the available experimental value (375.8 kJ/mol). In the case of HOB-OH BDE, the BF-DMC calculation is 446.0±1.84 kJ/mol that is closer to the experimental BDE. The BF-DMC BDE for ClB-Cl is 343.2±2.34 kJ/mol and the BF-DMC B-Cl BDE is 523.3±0.33 kJ/mol, which are close to the experimental BDEs, 341.9 and 530.0 kJ/mol, respectively.     

The DFT study on Rh–C bond dissociation enthalpies of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III)alkyl

Rhodium transition-metal-organic cooperative catalysis, which has been intensively studied by many chemists, represents a great success in C–H bond activation because of high efficiencies and selectivities. Typically, in the reaction mechanism of aldehyde and alkene catalyzed by Rh(I) complex and 2-amino-3-picoline, two kinds of metala-cyclic transition-metal complexes of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III) alkyl are generally formed. The two complexes play an important role in the overall reaction, in which the Rh–C bond formations are involved. So it is meaningful to understand the strength of Rh–C bond, which can be measured by the homolytic bond dissociation enthalpies (BDEs). To this end, we first calculated 16 relative Rh–C BDEs of Tp′Rh(CNneopentyl)RH (Tp′?=?hydridotris-(3,5-dimethylpyrazolyl)borate) by 19 density functional theory (DFT) methods. Furthermore, the 5 absolute Rh–C BDEs of Rh transition-metal complexes were also calculated. The results show that the B97D3 is the most accurate method to predict the relative and absolute Rh–C BDEs and the corresponding RMSE values are the smallest of 2.8 and 3.3?kcal/mol respectively. Therefore, the Rh–C BDEs of (iminoacyl)rhodium(III)hydride and (iminoacyl)rhodium(III)alkyl as well as the substituent effects were investigated by using the B97D3 method. The results indicated that the different substituents exhibit different effects on different types of Rh–C BDEs. In addition, the analysis including the natural bond orbital (NBO) as well as the energies of frontier orbitals were performed in order to further understand the essence of the Rh–C BDE change patterns.     

A theoretical investigation on the structures,densities, detonation properties,and pyrolysis mechanism of the nitro derivatives of phenols

The nitro derivatives of phenols 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 density, detonation velocity, detonation pressure, and the number of nitro and hydroxy groups. Thermal stability and pyrolysis mechanism of the title compounds are investigated by calculating the bond dissociation energies (BDEs) at the unrestricted B3LYP/6‐31G* level. The activation energies of H‐transfer reaction is smaller than the BDEs of all bonds and this illustrates that the pyrolysis of the title compounds may be started from breaking O? H bond followed by the isomerization reaction of H transfer. Moreover, the C? NO₂ bond with the smaller bond overlap population and the smaller BDE will also overlap may be before homolysis. According to the quantitative standard of energetics and stability as a high‐energy density compound, pentanitrophenol essentially satisfies this requirement. In addition, we have discussed the effect of the nitro and hydroxy groups on the static electronic structural parameters and the kinetic parameter. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Theoretical Study on the Photodegradation Mechanism of Nona‐BDEs in Methanol

Polybrominated diphenyl ethers (PBDEs) have received special environmental concern due to their potential toxicity to humans and wildlife worldwide, however, it is difficult to reveal their dominant photochemical degradation pathways by experiment. We explored the reaction mechanisms of photochemical degradation–debromination of three nona‐BDEs in methanol using theoretical calculations, in which time‐dependent density functional theory (TDDFT) combined with the polarizable continuum (PCM) model is applied. The selectivity of debromination was studied, and the major octa‐BDE products photochemically debrominated from nona‐BDEs were identified. We find that the debromination reaction results from the electronic transitions from π to σ* orbitals when nona‐BDEs are exposed to UV‐light in the sunlight region, at which point the two low‐lying excited states for each nona‐BDE are πσ*(5Br) and πσ*(4Br), which correlate to the σ* orbitals located on the penta‐Br and tetra‐Br substituted phenyls, respectively. Our calculations indicate that each nona‐BDE may degrade to form three kinds of octa‐BDE products via the πσ*(5Br) state, whereas only one kind of octa‐BDEs can be formed via the πσ*(4Br) state. Our calculations can interpret the recent experiments successfully.     

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。     

S=O homolytic bond dissociation enthalpies in sulfoxides

The S=O bond dissociation enthalpies (BDE) were calculated using high-level ab initio methods including G3, G3B3, CBS-Q, CBS-4M, CCSD(T), and MP2. Based on the comparison of these theoretical values and experimental ones, the performances of a number of density functional theory (DFT) methods were then assessed. It was found that the B3P86 method gave the lowest root of mean square error. We therefore used this method to calculate the S=O BDEs of a number of substituted sulfoxides. The electronic effect of the substituents and the remote substituents effect of aryl-substituted sulfoxides on S=O BDE were investigated. In addition, a molecular orbital analysis of typical molecules was conducted in order to investigate the electronic effect on S=O BDEs. We also predicted several S=O BDE values of heteroaromatic substituted sulfoxides using the B3P86 method.     

Compound‐specific bromine isotope analysis of brominated diphenyl ethers using gas chromatography multiple collector/inductively coupled plasma mass spectrometry

The bromine isotope composition is potentially diagnostic in both degradation monitoring and source apportionment of organobromines in the environment. A method for compound‐specific bromine isotope analysis (δ⁸¹Br) based on gas chromatography multiple collector inductively coupled plasma mass spectrometry (GC/ICPMS) was developed for common brominated diaromatic compounds. Brominated diphenyl ethers (BDEs) in Bromkal 70‐5DE, a technical flame‐retardant mixture containing mainly BDEs #47, #99 and #100, were used as test substances, with standard bracketing for the samples achieved through co‐injected monobromobenzene (MBB) with a known δ⁸¹Br of ?0.39‰ vs. Standard Mean Ocean Bromine (SMOBr). Three different heated transfer lines were constructed and tested to achieve efficient conduction of the BDEs from the gas chromatograph to the ICPMS instrument. The MBB was analyzed with a precision of 0.4‰ (1 s, n = 18). The precision for BDEs was 1.4–1.8‰ (1 s, n = 10–12 depending on the congener). The lower precision for the BDEs than for MBB may reflect the heat required to prevent condensation of the analytes in ICP torch assembly. The use of an internal standard of similar chemical structure to the analytes alleviates this problem, as illustrated by a difference of 0.3 ± 0.7‰ (1 s, n = 6) between the δ⁸¹Br values of co‐injected methoxy BDE‐47 and BDE‐47 extracted from whale blubber. Improvements in precision and accuracy may be achieved by the use of a more efficient heating of the torch assembly in conjunction with a set of internal standards that match the target compounds. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical investigation of the structures and properties of fluoromethyl peroxyl radicals

The equilibrium geometries, harmonic vibrational frequencies, charge distributions, spin density distributions, dipole moments, electron affinities (EAs), and C? O bond dissociation energies (BDEs) of HO, CH₃O, CH₂FO, CHF₂O, and CF₃O peroxyl radicals have been calculated using ab initio molecular orbital theory and density functional theory (DFT) at the B3LYP level. The C? H bond dissociation energies of the parent fluoromethanes have been calculated using the same levels of theory. Both the MP2(full) and B3LYP methods, using the 6‐31G(d,p) basis set, are found to be capable of accurately predicting the geometries of peroxyl radicals. Electron correlation accounts for ～25% of the C? H BDE of fluoromethanes and for ～50% of the C? O BDE of the corresponding peroxyl radicals. The B3LYP/6‐31G(d,p) method is found to be comparable to high ab initio levels in predicting C? O BDEs of studied peroxyl radicals and C? H BDEs of the parent alkanes. The progressive fluorine substitution of hydrogen atoms in methyl peroxyl radicals results in shortening of the C? O bond, lengthening of the O? O bond, an increase (decrease) of the spin density on the terminal (inner) oxygen, a decrease in the dipole moments, and an increase in electron affinities. Both C? O BDEs and EAs of peroxyl radicals (RO) correlate well with Taft σ* substituent constants for the R group in peroxyl radicals. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Kinetic and thermodynamic properties of the aminoxyl (NH2O*) radical

The product of one-electron oxidation of (or H-atom abstraction from) hydroxylamine is the H2NO* radical. H2NO* is a weak acid and deprotonates to form HNO-*; the pKa(H2NO*) value is 12.6+/-0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N2 as the sole nitrogen-containing product. The following rate constants were determined: kr(2H2NO*)=1.4x10(8) M-1 s-1, kr(H2NO*+HNO-*)=2.5x10(9) M-1 s-1, and kr(2HNO-*)=4.5x10(8) M-1 s-1. The HNO-* radical reacts with O2 in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O2-*), with a rate constant of ke(HNO-*+O2-->HNO+O2-*)=2.2x10(8) M-1 s-1. Both O2 and O2-* seem to react with deprotonated hydroxylamine (H2NO-) to set up an autoxidative chain reaction. However, closer analysis indicates that these reactions might not occur directly but are probably mediated by transition-metal ions, even in the presence of chelators, such as ethylenediamine tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA). The following standard aqueous reduction potentials were derived: E degrees (H2NO*,2H+/H3NOH+)=1.25+/-0.01 V; E degrees (H2NO*,H+/H2NOH)=0.90+/-0.01 V; and E degrees (H2NO*/H2NO-)=0.09+/-0.01 V. In addition, we estimate the following: E degrees (H2NOH+*/H2NOH)=1.3+/-0.1 V, E degrees (HNO, H+/H2NO*)=0.52+/-0.05 V, and E degrees (HNO/HNO-*)=-0.22+/-0.05 V. From the data, we also estimate the gaseous O-H and N-H bond dissociation enthalpy (BDE) values in H2NOH, with BDE(H2NO-H)=75-77 kcal/mol and BDE(H-NHOH)=81-82 kcal/mol. These values are in good agreement with quantum chemical computations.     

Thermal activation of methane and ethene by bare MO·+ (M=Ge, Sn, and Pb): a combined theoretical/experimental study

The thermal ion/molecule reactions (IMRs) of the Group 14 metal oxide radical cations MO ^{. +} (M=Ge, Sn, Pb) with methane and ethene were investigated. For the MO ^{. +}/CH₄ couples abstraction of a hydrogen atom to form MOH⁺ and a methyl radical constitutes the sole channel. The nearly barrier‐free process, combined with a large exothermicity as revealed by density functional theory (DFT) calculations, suggests a fast and efficient reaction in agreement with the experiment. For the IMR of MO ^{. +} with ethene, two competitive channels exist: hydrogen‐atom abstraction (HAA) from and oxygen‐atom transfer (OAT) to the organic substrate. The HAA channel, yielding C₂H₃ ^. and MOH⁺ predominates for the GeO ^{. +}/ethene system, while for SnO ^{. +} and PbO ^{. +} the major reaction observed corresponds to the OAT producing M⁺ and C₂H₄O. The DFT‐derived potential‐energy surfaces are consistent with the experimental findings. The behavior of the metal oxide cations towards ethene can be explained in terms of the bond dissociation energies (BDEs) of MO⁺? H and M⁺? O, which define the hydrogen‐atom affinity of MO⁺ and the oxophilicity of M⁺, respectively. Since the differences among the BDEs(MO⁺? H) are rather small and the hydrogen‐atom affinities of the three radical cations MO ^{. +} exceed the BDE(CH₃? H) and BDE(C₂H₃? H), hydrogen‐atom abstraction is possible thermochemically. In contrast, the BDEs(M⁺? O) vary quite substantially; consequently, the OAT channel becomes energetically less favorable for GeO ^{. +} which exhibits the highest oxophilicity among these three group 14 metal ions.     

Computational I(III)—X BDEs for Benziodoxol(on)e‐based Hypervalent Iodine Reagents: Implications for Their Functional Group Transfer Abilities

The first comprehensive I(III)―X (X = F, Br, CN, N₃, CF₃, etc.) bond dissociation energy (BDE) scales for benziodoxol(on)e‐based hypervalent iodine reagents have been developed by virtue of DFT calculations. Excellent correlation is observed between the I(III)―X BDEs and the X―H BDEs, offering a powerful avenue to quickly estimate the group‐transfer ability of a novel benziodoxol(on)e‐based hypervalent reagent.     

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.     

PCM study of the solvent and substituent effects on bond dissociation energies of the C?NO bond

Quantum chemical calculations are used to estimate the equilibrium C? NO bond dissociation energies (BDEs) for eight X? NO molecule (X = CCl₃, C₆F₅, CH₃, CH₃CH₂, iC₃H₇, tC₄H₉, CH₂CHCH₂, and C₆H₅CH₂). These compounds are studied by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86) methods together with 6‐31G** and 6‐311G** basis sets and the complete basis set (CBS‐QB3) method. The obtained results are compared with the available experimental results. It is demonstrated that B3P86/6‐31G** and CBS‐QB3 methods are accurate for computing the reliable BDEs for the X? NO molecule. Considering the inevitably computational cost of CBS‐QB3 method and the reliability of the B3P86 calculations, B3P86 method with 6‐31G** basis set may be more suitable to calculate the BDEs of the C? NO bond. The solvent effects on the BDEs of the C? NO bond are analyzed and it is shown that the C? NO BDEs in a vacuum computed by using B3PW91/6‐311G** method are the closest to the computed values in acetontrile and the average solvent effect is 1.48 kcal/mol. Subsequently, the substituent effects of the BDEs of the C? NO bond are further analyzed and it is found that electron denoting group stabilizes the radical and as a result BDE decreases; whereas electron withdrawing group stabilizes the group state of the molecule and thus increases the BDE from the parent molecule. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009