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.

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.

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.

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.

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）.

DFT Studies on the Antiradical Potential of Phenolic Compounds

Phenolic compounds are a class of hazardous substances for human.To study the antiradical potential of a series of phenolic compounds from the aspect of position and substituent type,phenol,o⁃dihydroxybenzene(ODB),m⁃dihydroxybenzene(MDB),p⁃dihydroxybenzene(PDB),paranitrophenol(PNP),and o⁃chlorophenol(OCP)were selected as typical targets.In addition,to elucidate the degradation discrepancy of phenolic compounds,quantum chemical calculations(QCCs)were obtained using the B3LYP method along with a 6-311G(d,p)basis set.Calculations indicate that phenol,ODB,MDB,and PDB,with electron⁃donating groups,exhibited high antiradical potential,while PNP and OCP,with electron⁃withdrawing groups,exhibited low antiradical potential.The chemical indices calculations show that para⁃compounds and ortho⁃compounds had high antiradical ability.Moreover,phenol,ODB,MDB,and PDB possessed higher bond dissociation enthalpy(BDE)and lower adiabatic ionization potential(AIP)values compared with those of OCP and PNP.Medium effects,even in vacuo,were also taken into account to reveal the antiradical ability of phenolic compounds.

Theoretical Elucidation on Different Lipid-Oxidation Potentials of Aminoxyl Antioxidants

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

Theoretical Study on the Antioxidant Activity of Curcumin

The computational results for curcumin at the B3LYP/6-31G(d,p) level show that the enol form of curcumin is more stable than the diketo form because of an intramolecular hydrogen bond, which extends the conjugation effect in the enol chain, formed in the enol structure. Cis-diketone form can not be obtained, presumably due to the strong repulsion between the carbonyl dipoles aligned in parallel. According to the phenolic O—H bond dissociation en- thalpy, curcumin in its most stable form can be suggested to be a relatively good antioxidant. In order to avoid overcoming H-bond interaction and to improve the antioxidant activity of curcumin, a catechol moiety was incor- porated into curcumin for designing a novel antioxidant. It is found that the designed molecule is much more effi- cient to scavenge radical than curcumin, comparable to vitamin E. Moreover, the ionization potential of the de- signed molecule is similar to that of curcumin, indicating that the designed molecule can not display the prooxidant effect.