Theoretical Study of the C-CI Bond Dissociation Enthalpy and Electronic Structure of Substituted Chlorobenzene Compounds

Quantum chemical calculations were used to estimate the bond dissociation energies （BDEs） for 13 substituted chlorobenzene compounds. These compounds were studied by the hybrid density functional theory （B3LYP, B3PW91, B3P86） methods together with 6-31G^＊＊ and 6-311G^＊＊ basis sets. The results show that B3P86/6-311G^＊＊ method is the best method to compute the reliable BDEs for substituted chlorobenzene compounds which contain the C-C1 bond. It is found that the C-C1 BDE depends strongly on the computational method and the basis sets used. Substituent effect on the C-C1 BDE of substituted chlorobenzene compounds is further discussed. It is noted that the effects of substitution on the C-C1 BDE of substituted chlorobenzene compounds are very insignificant. The energy gaps between the HOMO and LUMO of studied compounds estimate the relative thermal stability ordering are also investigated and from this data we of substituted chlorobenzene compounds.

Assessment of Contemporary Theoretical Methods for Bond Dissociation Enthalpies

The density functional theory （DFT） is the most popular method for evaluating bond dis- sociation enthalpies （BDEs） of most molecules. Thus, we are committed to looking for alternative methods that can balance the computational cost and higher precision to the best for large systems. The performance of DFT, double-hybrid DFT, and high-level com- posite methods are examined. The tested sets contain monocyclic and polycyclic aromatic molecules, branched hydrocarbons, small inorganic molecules, etc. The results show that the mPW2PLYP and G4MP2 methods achieve reasonable agreement with the benchmark val- ues for most tested molecules, and the mean absolute deviations are 2.43 and 1.96 kcal/mol after excluding the BDEs of branched hydrocarbons. We recommend the G4MP2 is the most appropriate method for small systems （atoms number≤20）; the double-hybrid DFT methods are advised for large aromatic molecules in medium size （20≤atoms number≤50）, and the double-hybrid DFT methods with empirical dispersion correction are recommended for long-chain and branched hydrocarbons in the same size scope; the DFT methods are ad- vised to apply for large systems （atoms number〉50）, and the M06-2X and B3P86 methods are also favorable. Moreover, the differences of optimized geometry of different methods are discussed and the effects of basis sets for various methods are investigated.

Density Functional Theory Calculations on Ni-Ligand Bond Dissociation Enthalpies

The formation and breaking of Ni-L （L=N-heterocyclic carbene, tertiary phosphine etc.） bond is involved in many Ni-catalyzed/mediated reactions. The accurate prediction of Ni-L bond dissociation enthalpies （BDEs） is potentially important to understand these Ni-complex involving reactions. We assess the accuracy of diffierent DFT functionals （such as B3LYP, M06, MPWB1K, etc.） and diffierent basis sets, including both effective core potentials for Ni and the all electron basis sets for all other atoms in predicting the Ni-L BDE values reported recently by Nolan et al. [J. Am. Chem. Soc. 125, 10490 （2003） and Organometallics 27, 3181 （2008）]. It is found that the MPWB1K/LanL2DZ：6-31＋G（d,p）//MPWB1K/LanL2DZ：6-31G（d） method gives the best correlations with the experimental results. Meanwhile, the solvent effect calculations （with CPCM, PCM, and SMD models） indicate that both CPCM and PCM perform well.

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

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 446.04-1.84 k J/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 evaluation of bond dissociation energies for NO2 scission in nitro compounds using density functional and complete basis set methods

By using the density functional theory （B3LYP） and four highly accurate complete basis set （CBS-Q, CBS-QB3, CBS-Lq, and CBS-4M）ab initio methods, the X（C, N, O）-NO2 bond dissociation energies （BDEs） for CH3NO2, C2H3NO2, C2H5NO2, HONO2, CH3ONO2, C2H5ONO2, NH2NO2 （CH3）2NNO2 are computed. By comparing the computed BDEs and experimental results, it is found that the B3LYP method is unable to predict satisfactorily the results of bond dissociation energy （BDE）; however, all four CBS models are generally able to give reliable predication of the X（C, N, O）-NO2 BDEs for these nitro compounds. Moreover, the CBS-4M calculation is the least computationally demanding among the four CBS methods considered, Therefore, we recommend CBS-4M method as a reliable method of computing the BDEs for this nitro compound system.

Quantum chemical calculations of bond dissociation energies for COOH scission and electronic structure in some acids

Quantum chemical calculations are performed to investigate the equilibrium C-COOH bond distances and the bond dissociation energies（BDEs） for 15 acids.These compounds are studied by utilizing the hybrid density functional theory（DFT）（B3LYP,B3PW91,B3P86,PBE1PBE） and the complete basis set（CBS-Q） method in conjunction with the 6311G^＊＊ basis as DFT methods have been found to have low basis sets sensitivity for small and medium molecules in our previous work.Comparisons between the computational results and the experimental values reveal that CBS-Q method,which can produce reasonable BDEs for some systems in our previous work,seems unable to predict accurate BDEs here.However,the B3P86 calculated results accord very well with the experimental values,within an average absolute error of 2.3 kcal/mol.Thus,B3P86 method is suitable for computing the reliable BDEs of C-COOH bond for carboxylic acid compounds.In addition,the energy gaps between the highest occupied molecular orbital（HOMO） and the lowest unoccupied molecular orbital（LUMO） of studied compounds are estimated,based on which the relative thermal stabilities of the studied acids are also discussed.

Density Functional Calculations of C–NO_2 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.

Theoretical Study of the N-NO_2 Bond Dissociation Energies for Energetic Materials with Density Functional Theory

The N-NO2 bond dissociation energies （BDEs） for 7 energetic materials were computed by means of accurate density functional theory （B3LYP, B3PW91 and B3P86） with 6-31G＊＊ and 6-311G＊＊ basis sets. By comparing the computed energies and experimental results, we find that the B3P86/6-311G＊＊ method can give good results of BDE, which has the mean absolute deviation of 1.30kcal/mol. In addition, substituent effects were also taken into account. It is noted that the Hammett constants of substituent groups are related to the BDEs of the N-NO2 bond and the bond dissociation energies of the energetic materials studied decrease when increasing the number of NO2 group.

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.

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 hydro-genation 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 meth-ods 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 re-lationships 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.

DFT Study on Homolytic Dissociation Enthalpies of C-I Bonds

The C-I bond dissociation enthalpies （BDE） of various organic iodides were calculated using high-level theoretical methods including MP2 and CCSD（T） with extrapolated basis set as well as a number of density functional theory methods. After systematic evaluation of the theoretical results against available experimental C-I BDEs, it was found that the MPW LYPIM method gave the lowest root mean square error. We, therefore, used this method to examine the substituent effects on different categories of C（sp3）-I and C（sp2）-I bonds. Fur thermore, the remote substituent effects on the C-I BDEs of substituted iodobenzenes and substituted （iodomethyl）benzenes were also investigated at the same level. The C-I BDEs of typical heteroaromatic iodides including five-membered and six-membered heterocyclic iodides were also examined.

Heterolytic and homolytic C-D bond dissociation energies of NADH models in acetonitrile and primary isotopic effects on hydride versus hydrogen atom transfer reactions

Heterolytic and homolytic C D bond dissociation energies of three NADH models: BNAH-4,4-d 2 , HEH-4,4-d 2 and AcrD 2 in acetonitrile were first estimated by using an efficient method. The results showed that the heterolytic C D bond dissociation energies are 65.2, 70.2, and 81.9 kcal/mol and the homolytic C D bond dissociation energies are 72.66, 70.69, and 74.95 kcal/mol for BNAH-4,4-d 2 , HEH-4,4-d 2 , and AcrD 2 , respectively. According to the bond dissociation energy differences of isotope isomers, an interesting conclusion can be made that the primary kinetic isotope effects are dependent not only on the zero-point energy difference of the isotope isomers, but also on the types of C D bond dissociations, and the C D bond homolytic dissociations should have much larger primary kinetic isotope effects (26.9 28.8) than the corresponding C D bond heterolytic dissociations (3.9-5.4).

Research on C—C Bond Length Distribution in Hydrocarbon Molecules

The C--C bond dissociation energy （BDE） is a very important data in research of hydrocarbon cracking reactions, because it reflects the difficulty level of chemical reactions. But it is very difficult to obtain the C--C bond dissociation energy （BDE） by experiments, so using quantum chemistry calculation such as density functional theory （DFT） to study the C--C bond dissociation energy is a very useful means. The impact of acceptor substituents and donor substituents on the C--C bond length distribution was studied.

Why Static O-H Bond Parameters Cannot Characterize the Free Radical Scavenging Activity of Phenolic Antioxidants: ab initio Study

The static O-H bond parameters including O-H bond length, O-H charge difference, O-H Mulliken population and O-H bond stretching force constant (k) for 17 phenols were calculated by ab initio method HF/6-31G**. In combination with the O-H bond dissociation enthalpies (BDE) of the phenols determined by experiment, it was found that there were poor correlationships between the static O-H bond parameters and O-H BDE. Considering the good correlationship bt tween O-H BDE and logarithm of free radical scavenging rate constant for phenolic antioxidant, it is reasonable to believe that the ineffectiveness of static O-H bond parameters in characterizing antioxidant activity arises from the fact that they cannot measure the O-H BDE.

Density Functional Theory Study of Marine Polybrominated Diphenyl Ethers in Anaerobic Degradation

Polybrominated diphenyl ethers(PBDEs)are a kind of serious pollutants in the ocean.Biodegradation is considered as an economical and safe way for PBDEs removal and reductive debromination dominates the initial pathway of anaerobic degradation.On the basis of experimental study,Octa-BDE 197,Hepta-BDE 183,Hexa-BDE 153,Penta-BDE 99 and Tetra-BDE 47 were selected as the initial degradation objects,and their debromination degradation were studied using density functional theory.The structures were optimized by Gaussian 09 program.Furthermore,the molecular orbitals and charge distribution were analyzed.All C-Br bond dissociation energies at different positions including ortho,meta and para bromine atoms were calculated and the sequence of debromination was obtained.There is a close relationship between molecular structure,charge,molecular orbital and C-Br bond.All PBDEs exhibited similar debromination pathways with preferential removal of meta and para bromines.

Is HOMO Energy Level a Good Parameter to Characterize Antioxidant Activity

Semiempirical quantum chemical method AM1 was employed to calculate the highest occupied molecular orbital (HOMO) energy levels (E-HOMO) for various types of antioxidants. It was verified that the correlation between logarithm of free radical scavenging rate constants (1gks) and E-HOMO substantially arises from the correlation between E-HOMO and O-H bond dissociation energies (BDE) of antioxidants. Furthermore, E-HOMO were poorly correlated with the logarithm of relative free radical scavenging rate constants (1gk(3)/k(1)) for various types of antioxidants that possess complex structures (r = 0.5602). So in a broad sense, E-HOMO was not an appropriate parameter to characterize the free radical scavenging activity of antioxidants.

Relativistic density functional investigation of UX_6(X=F,Cl,Br and I)

The molecular structures and the vibrational frequencies of uranium hexahalides UX6 （X=F, Cl, Br and I） molecules are investigated by using local density approximation （LDA） and generalised gradient approximation （GGA） functions （BP, BLYP and RPBE） in combination with two different relativistic methods （scalar and scalar＋spin-orbit relativistic effects）. The calculated results show that the differences are trivial between scalar and scalar＋spin-orbit relativistic methods. The vibrational frequencies are also compared with existing experimental values, and overall, the RPBE approach gives the smallest error. The bond dissociation energies （BDEs） of UX6 are computed by using the RPBE function, thereby obtaining exact vibrational frequencies. In addition, the calculated magnitudes of the spin orbit effect on the BDE of UX6 （X=F, Cl, Br, and I） are found to be approximately -0.3198, 0.3218, -0.3609 and -0.4415 eV, respectively.

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

The reaction mechanism of 1-chloroethane with hydroxyl radical has been investigated 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 （ΔHr） than with bond dissociation enthalpies （BDE）.