Homolytic Bond Dissociation Research Articles

Quantum chemical study on nitroimidazole, polynitroimidazole and their methyl derivatives

The insensitive explosive candidates, nitroimidazoles, polynitroimidazoles and their methyl derivatives, are investigated using density functional theory (DFT). The homolytic bond dissociation energies (BDEs) corresponding to –NO 2 group removal from carbon or nitrogen site on imidazole ring were calculated at B3P86/6-311G** level, and the weakest bond has been determined. Further, a correlation is developed between impact sensitivity h 50 and the ratio (BDE/ E) of the weakest bond BDE to the total energy E, and we extrapolate this relationship to predict the impact sensitivities for compounds where experiments are not available. It is found that most of the title compounds are insensitive towards impact stimuli with their h 50 larger than 60.0 cm. Heats of formation (HOFs) for the 21 title compounds at 298 K in gas are also determined both at B3LYP/6-311G** and B3P86/6-311G** levels using isodesmic work reactions. The calculated BDEs and HOFs consistently indicate that C-nitro-substituted imdazole is more stable than the corresponding N-substituted one, and the introduction of methyl on C increases the stability whereas the methyl attached to N atom decreases the stability.

Generation of Thymine Radicals from 6-Alkoxy-5-bromo-5,6-dihydrothymine Derivatives by Radiolytic Reduction and Photolytic Dehalogenation

The reduction of 6-alkoxy-5-bromo-5,6-dihydrothymine derivatives ( 1a, b) by hydrated electrons (e aq (-)) generated in the radiolysis of deoxygenated aqueous solution was investigated. As the major products, 1-(6'-alkoxy-5',6'-dihydrothymin-5'-yl)thymines ( 6a, b), 5-(hydroxymethyl)uracil ( 8), 6-alkoxy-5,6-dihydrothymines ( 9a, b), and thymine ( 10) were produced in sufficient yields. This product distribution is indicative of the generation of 6-alkoxy-5,6-dihydrothymin-5-yl radicals ( 2a, b) as primary intermediates that undergo elimination of alkoxide ions (RO (-)) into thymine radical cations ( 3) followed by deprotonation at the N1 to form N-centered thymine radicals ( 4). The transient absorption spectra of the 5-yl radicals 2a- c were observed by means of nanosecond laser flash photolysis of 1a, b and 5-bromo-6-ethoxy-5,6-dihydrothymidine ( 1c) in deoxygenated aqueous solution, in which homolytic C5-Br bond dissociation occurred. In contrast to the reaction characteristics in aqueous solutions, the dimeric products were not obtained in acetonitrile, probably because in-cage hydrogen abstraction from the C5 methyl group by bromine atom leads to formation of methide type intermediates 20.

Theoretical study on oxygen–oxygen homolytic bond dissociation enthalpies of peroxides

Seven theoretical methods (B3LYP, B3P86, X3LYP, BMK, MP2, CBS-QB3, G3B3) were employed to calculate oxygen–oxygen homolytic bond dissociation enthalpies of 13 peroxides. Comparison of the performances of these methods with the available experimental data showed that ROBMK method is suitable and economical for calculating O–O homolytic bond dissociation enthalpies of peroxides. Then ROBMK method was used to calculated a series of peroxides, and the α-substituent effect and Hammett relationship were discussed.

Substituent effects on O–H and S–H bond dissociation enthalpies of disubstituted phenols and thiophenols

AbstractThe O–H and S–H homolytic bond dissociation enthalpies of a set of disubstituted phenols and thiophenols (NH2, OH, CH3, Cl, CF3, and NO2) have been computed by a density functional theory procedure with the 6‐311++G(d,p) basis set. A very good agreement between our results and available experimental ones is observed. The effect of substituents on structure, charges and BDEs are investigated and their correlation with Hammett parameters is studied. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

47.2: Distinguished Paper: Structural Identification of Chemical Products and Mechanism of Operational Degradation of OLEDs

AbstractBased on the observed changes in chemical compositions of fluorescent and phosphorescent carbazole‐derived OLEDs during operation, the operational degradation mechanism is proposed. Substantial losses of chemical components, identification of degradation products, and photochemical studies point to the excited state homolytic bond dissociation followed by radical additions as key mechanism steps.

A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET‐LRP of vinyl monomers

AbstractA quantum‐chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron‐withdrawing groups as well as for a nonactivated monomer containing an electron‐donor group. Two examples of sulfonyl and N‐halide initiators are also reported. The homolytic inner‐sphere electron‐transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET‐LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron‐withdrawing groups in SET‐LRP. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1607–1618, 2007

A cost-effective basis-set extrapolation scheme: Application to the energetics of homolytic bond dissociation

A composite procedure based on density functional theory (DFT) geometry optimizations and coupled cluster calculations with single and double excitations and perturbative treatment of triple excitations (CCSD(T)) is proposed for the evaluation of homolytic bond dissociation enthalpies (BDEs). The performance of several functionals for predicting the structure and vibrational frequencies of a selected set of closed- and open-shell species was investigated. By using the correlation consistent cc-pVTZ basis-set, it was found that B3LYP and VSXC geometries are in good agreement with experiment. B3LYP/cc-pVTZ geometries were then selected for CCSD(T) single-point energy calculations. The Hartree-Fock (HF) contribution to the total energy was estimated at the HF/cc-pV6Z level and also by using a (x = D(2),T(3),Q(4),5,6) ad hoc extrapolation. Complete basis-set values for CCSD(T) correlation energies were evaluated through dual (x, x + 1;x = 2) extrapolation schemes relying on calculations with the cc-pVxZ basis-set. The results illustrate the importance of the extrapolation schemes and show that (2, 3) extrapolated BDEs are more accurate than those calculated with the cc-pV6Z basis-set.

Betanin, the main pigment of red beet: Molecular origin of its exceptionally high free radical-scavenging activity

In the present study, the pH-dependent free radical-scavenging activity of betanin in the Trolox equivalent antioxidant capacity (TEAC) assay was determined. It was found that at a pH > 4 betanin is about 1.5–2.0-fold more active than some anthocyanins considered very good free radical scavengers as determined in the TEAC assay. The increase in the TEAC values of betanin with increasing pH is discussed in terms of its calculated phenolic OH homolytic bond dissociation energy (BDE) and ionization potential (IP). The results suggest that the exceptionally high antioxidant activity of betanin is associated with an increasing of its H-donation and electron-donation ability when going from cationic state to mono-, di- and tri-deprotonated states present at basic solutions.

A Novel Exploration of the Hartree–Fock Homolytic Bond Dissociation Problem in the Hydrogen Molecule by Means of Electron Localization Measures

Most introductory quantum chemistry textbooks discuss the well-known problem of the inappropriate description given by the restricted Hartree–Fock (HF) method of the homolytic bond dissociation. This weakness of the restricted HF method is generally addressed by analyzing the difference between the energies at long internuclear distances obtained with the configuration interaction (CI) and HF methods, which requires an exhaustive understanding of the methodology. In this article we provide a new insight into this subject using localization and delocalization indices defined in the framework of the atoms-in-molecules theory to analyze the homolytic bond dissociation in the hydrogen molecule. It is shown that the restricted HF requirement of molecular orbitals to be occupied simultaneously by a couple of electrons with different spin is responsible for localization to hold on the same value while dissociation is happening, thus reflecting the well-known deficiency of the HF method to deal with bond dissociation. On the other hand, when the CI method is used, the localizability of the electrons in the system turns into the intuitive scheme expected for homolytic bond dissociation.

Electronic structures and reactivities of iodinating agents in the gas phase and in solutions: A density functional study

The electronic and spatial structures of a broad spectrum of neutral compounds with X-Hal (X = N, O, Cl; Hal = Cl, Br, I) bonds and their protonated forms and of different electronic states of triiodide cation, I3 +, were determined from density functional B3LYP/6 311G* quantum chemical calculations. The effects of the structure of these compounds on the parameters of electrophilic reactivity were revealed and the thermochemical characteristics of homolytic and heterolytic X-Hal bond dissociation and of iodine transfer in hydroxyl-containing solvents were calculated. Due to low homolytic bond dissociation energies of X-I, the formation of molecular iodine and triiodide cation I3 + becomes thermodynamically favorable and the cation should act as iodinating agent alternative to acylhypoiodites and N-iodoimides. The solvation effects of MeOH and CH2Cl2 on the X-Hal bond homolysis and heterolysis were determined using the PCM model.

Quantum chemical study on 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) and some of its constitutional isomers

Presently, certain isomeric compounds of NTO and their tautomers have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p) and ROB3P86/6-311G(d,p) levels and also ab initio calculations at RHF/6-311G(d,p) level. The optimized geometries, vibrational frequencies, electronic structures and some thermodynamical values for the presently considered NTO isomers have been obtained in their ground states. Also, detonation performances were evaluated by the Kammlet–Jacobs equations, based on the calculated densities and heat of formation values. The homolytic bond dissociation energies (BDEs) (at ROB3P86/6-311G(d,p) level) of N NO 2 and C NO 2 for the molecules were calculated. Moreover, aromatic character of NTO and its isomers and tautomers were investigated by performing NICS calculations using the gauge invariant atomic orbital (GIAO) approach at the B3LYP/6-31G(d,p) and B3LYP/cc-pVDZ levels.

S–H bond dissociation enthalpies: The importance of a complete basis set approach

The S–H homolytic bond dissociation enthalpies (BDEs) for several compounds of reference were estimated by different theoretical methods including CCSD(T), CBS-QB3, and the multi-coefficient extrapolated density functional theory multi-level approaches, MCG3-MPWB and MCG3-TS. Emphasis was placed on the importance of extrapolating theoretical BDEs to complete basis set. A very good agreement between S–H BDEs from CCSD(T) calculations and experiment is observed when a simple dual extrapolation scheme to complete basis set proposed by Truhlar is adopted. For thiophenol, our CCSD(T) estimate for the S–H BDE (347.2 kJ mol −1) supports a recent experimental value obtained from time-resolved photoacoustic calorimetry (349 ± 5 kJ mol −1).

Ab initio and DFT study on 1,4-dinitroglycoluril configurational isomers: cis-DINGU and trans-DINGU

cis-1,4-Dinitroglycoluril (1,4-dinitrotetrahydroimidazo[4,5-d]imidazole-2,5-(1H,3H)-dione, C4H4N6O6, known as DINGU in the literature) is an important explosive regarded as one of the potential ingredients for LOVA (low vulnerability ammunition) applications. However, there is no study concerning trans-DINGU, to the best of our knowledge. Thus, in the present study the structural and electronic properties of the configurational isomers of DINGU (cis- and trans-isomers) have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p), UB3LYP/6-31G(d,p) levels and also ab initio calculations at RHF/6-31G(d,p) and UHF/6-31G(d,p) levels. The optimized geometries, vibrational frequencies, electronic structures, and some thermodynamical values for the presently considered DINGU isomers have been obtained in their ground states. Comparing the calculated energy data (total electronic energy, heat of formation values and homolytic bond dissociation values of N–NO2) and the calculated bond lengths, cis-DINGU was found to be a more stable molecule than the trans-isomer.

Design principles for α-tocopherol analogues

An (RO)B3LYP/LANL2DZdp//B3LYP/LANL2DZ model for the prediction of the homolytic bond dissociation enthalpy (BDE) and adiabatic ionisation potential (IP) of phenolic antioxidants containing heavy chalcogens has been developed. The model has been used to probe the relationship between geometry, chalcogen substitution and activity for a series of alpha-tocopherol analogues of varying ring size. From this, a series of design principles for cyclic antioxidants has emerged, embodied by the compound 4-hydroxy-2,2,3,5,6-pentamethylbenzoselenete (4c). This compound is predicted to have a BDE comparable to alpha-tocopherol, and should act in a dual chain-breaking and hydroperoxide-decomposing manner, by analogy with other selenide antioxidants. The stability of chalcogen-substituted benzoxetes was considered, and the as yet unsynthesised benzotelluretes are predicted to be stable. Finally, an attempt was made to determine antioxidant mechanism by considering calculated BDE and IP data together with experimental rate data.

Conversions of Ruthenium(III) Alkyl Complexes to Ruthenium(II) through Ru−Calkyl Bond Homolysis

Single-electron oxidation of the Ru(II) complexes TpRu(L)(L‘)(R) (L = CO, L‘ = NCMe, and R = CH3 or CH2CH2Ph; L = L‘ = PMe3 and R = CH3) with AgOTf leads to alkyl elimination reactions that produce TpRu(L)(L‘)(OTf) and organic products that likely result from Ru−Calkyl bond homolysis. Density functional calculations on TpRu(CO)(NCMe)(Me) and its Ru(III) cation indicate that the Ru−CH3 homolytic bond dissociation enthalpy is substantially reduced (48.6 to 23.2 kcal/mol) upon oxidation.

Unusual structural and energetic features of homolytic bond dissociation: from tetrakis(disyl)diphosphine to tetrakis(di-tert-butylsilyl)hydrazine

Quantum chemical calculations of the structures and thermodynamics of homolytic dissociation of the central P-P and N-N bonds in tetrakis(disyl)diphosphine and tetrakis(di-tert-butylsilyl)hydrazine have been performed. The theory predicted negative standard enthalpies for homolytic bond dissociation in both cases, -71.0 and -108.4 kJ mol(-1) for the diphosphine and hydrazine, respectively, using the ONIOM (MP2/6-31+G*:B3LYP/3-21G*) level. The dissociation is accompanied by considerable structural changes in the radicals as compared to the corresponding fragments of the parent molecules, resulting in low dissociation enthalpies. The most pronounced changes in both radicals are the relaxation of bond angles in the substituents and a conformational change in the orientation of the substituent groups. In addition, the bis(di-tert-butylsilyl)aminyl radical displays a considerable increase in Si-N-Si angle and shortening of the Si-N bonds upon dissociation. These changes are not associated with any appreciable delocalisation of the lone electron, as the spin density is found from the B3LYP/3-21G* calculations to be largely concentrated on the nitrogen atom. It has been also shown that although the dissociation energies are low for both compounds, the intrinsic energies of the central bonds are still high, 140.6 kJ mol(-1) for the P-P bond in tetrakis(disyl)diphosphine and 490.6 kJ mol(-1) for the N-N bond in tetrakis(di-tert-butylsilyl)hydrazine, using the ONIOM method. The calculations predict that the dissociation of tetrakis(disyl)diphosphine would have negative free energy even without taking relaxation of the fragments into account, while the full potential of releasing about 306 kJ mol(-1) of energy stored in the ligands of tetrakis(di-tert-butylsilyl)hydrazine is only fully realised upon a considerable separation of the fragments.

Homolytic bond dissociation energies of 4‐substituted bicyclo[2.2.2]octanyl compounds: inductive/field effects on the stabilities of organic radicals

AbstractThe inductive/field effects on homolytic bond dissociation energies (BDEs) were studied for the first time using substituted 4‐X‐bicyclo[2.2.2]octane‐Y—Z systems. A variety of very different chemical bonds (i.e. Z—Y: Z = CH2, NH, O, SiH2, PH, S; Y = H, F, Li) were considered, and popular substituents including H, CH3, F, OH, NH2, SH, CN and NO2 were utilized. High‐quality BDE values were obtained for the first time for many bicyclo[2.2.2]octane systems from carefully calibrated G3B3/B3LYP calculations. Significant effects of the substituents at the 4‐position of bicyclooctane were found for the Z—Y BDEs of bicyclooctanyl‐Z—Y systems. Nice Hammett‐type correlations were obtained for these substituent effects using the inductive/field F constants. It was found that the reaction constants (i.e. ρ values) of the Hammett correlations varied dramatically from −1.96 to +23.01 kJ mol−1 for different Z—Y systems: The ρ values for the Z—H BDEs were about ∼1.0–5.0 kJ mol−1; the ρ values for the Z—F BDEs were about −2.0 to −1.0 kJ mol−1; the ρ values for the Z—Li BDEs were ∼13.0–23.0 kJ mol−1. The substituent effects on both the stability of the parent molecules before homolysis and the stability of the radical products after homolysis were demonstrated to be important for the BDEs. It was shown that the inductive/field substituent effects on BDEs could not be explained by the electronegativity or bond polarity theories. Nevertheless, we developed a theoretical model on the basis of the classic electrostatic theories for the inductive/field effects. This model successfully explained the intriguing inductive/field substituent effects on BDEs. Copyright © 2004 John Wiley & Sons, Ltd.

Theoretical Investigations into the Intermediacy of Chlorinated Vinylcobalamins in the Reductive Dehalogenation of Chlorinated Ethylenes

The reductive dehalogenation of perchloroethylene and trichloroethylene by vitamin B(12) produces approximately 95% (Z)-dichloroethylene (DCE) and small amounts of (E)-DCE and 1,1-DCE, which are further reduced to ethylene and ethane. Chloroacetylene and acetylene have been detected as intermediates, but not dichloroacetylene. Organocobalamins (RCbls) have been proposed to be intermediates in this process. Density functional theory based approaches were employed to investigate the properties of chlorinated vinylcobalamins and chlorinated vinyl radicals. They reveal that all vinyl radicals studied have reduction potentials more positive (E degrees >or= -0.49) than that of the Co(II)/Co(I) couple of B(12) (E degrees = -0.61 V), indicating that any (chlorinated) vinyl radicals formed in the reductive dehalogenation process should be reduced to the corresponding anions by cob(I)alamin in competition with their combination with Co(II) to yield the corresponding vinylcobalamins. The computed Co-C homolytic bond dissociation enthalpies (BDEs) of the latter complexes range from 33.4 to 45.8 kcal/mol. The substituent effects on the BDEs are affected by the stabilities of the vinyl radicals as well as steric interactions between (Z)-chloro substituents and the corrin ring. The calculated E degrees values of the cobalamin models were within approximately 200 mV of one another since electron attachment is to a corrin ring pi-orbital, whose energy is relatively unaffected by chloride substitution of the vinyl ligand, and all were >500 mV more negative than that of the Co(II)/Co(I) couple of B(12). Reduction of the base-off forms of vinyl- and chlorovinylcobalamin models also involves the corrin pi* orbital, but reduction of the base-off dichlorovinyl- and trichlorovinylcobalamin models occurs with electron attachment to the sigma(Co)(-)(C*) orbital, yielding calculated E degrees values more positive than that of the calculated Co(II)/Co(I) couple of B(12). Thus, cob(I)alamin is expected to reduce these base-off vinyl-Cbls. Heterolytic cleavage of the Co-C bonds is much more favorable than homolysis (>21 kcal/mol) and is significantly more exergonic when coupled to chloride elimination.

Thermodynamic Studies of HRh(depx)2 and [(H)2Rh(depx)2](CF3SO3): Relationships between Five-Coordinate Monohydrides and Six-Coordinate Dihydrides

Four new rhodium complexes, [Rh(depx)2](CF3SO3), [(H)2Rh(depx)2](CF3SO3), [HRh(depx)2(CH3CN)](CF3SO3)2, and HRh(depx)2 (where depx = α,α‘-bis(diethylphosphino)xylene), have been synthesized and characterized. The pKa values of [(H)2Rh(depx)2]+ (30.6 ± 0.3) and [HRh(depx)2(CH3CN)]2+ (11.5 ± 0.4) in acetonitrile were determined by equilibrium reactions with appropriate bases. The hydride donor ability of [(H)2Rh(depx)2]+ (71.6 ± 1.0 kcal/mol) was determined by the heterolytic cleavage of hydrogen by [HRh(depx)2(CH3CN)]2+ to form [(H)2Rh(depx)2]+ in the presence of thioacetamide or benzamide. These equilibrium measurements, the half-wave potentials for the Rh(+1/0) and Rh(0/−1) couples of [Rh(depx)2]+, and the reversible one-electron oxidation of HRh(depx)2 (all measured at 22 ± 1.5 °C) were used to establish nine heterolytic and homolytic bond-dissociation free energies in acetonitrile. From these relationships, and those established previously for analogous cobalt and platinum complexes, we conclude that t...

Effect of Substituents on the P−H Bond Dissociation Enthalpies of Phenylphosphines and Proton Affinities of Phenylphosphine Anions: A DFT Study

The homolytic bond dissociation enthalpies (BDEs) for the P−H bonds in substituted phenylphosphines (X−C6H4−PH2) have been estimated using the (RO)B3LYP procedure with the 6-311++G(2df,2p) basis set. The BDEs for the P−H bond in PH3, N−H bond in NH3 and X−C6H4−NH2 (X = H, CH3, and NO2), and the As−H bond in AsH3 have also been determined by the same computational procedure and compared with the experimental results to assess the reliability of our method. The proton affinities (PAs) of the arylphosphine anions (X−C6H4−PH-) have also been determined at the same level of theory. The effect of substituents on the BDE(P−H) and the PA values have been analyzed in terms of the substituent effects on the stability of the phenylphosphines, radicals, and anions. Additionally, accurate estimations for the electron affinity values of phenylphosphine radicals have been performed. We also explore the correlations of Hammett's substituent constants with the BDEs of the P−H bonds of phenylphosphines and the PA values of arylphosphine anions.