Low Bond Dissociation Energy Research Articles

Structure and reactivity of V(2)O(5): bulk solid, nanosized clusters, species supported on silica and alumina, cluster cations and anions.

Vanadyl bond dissociation energies are calculated by density functional theory (DFT). While the hybrid (B3LYP) functional results are close to the available reference data, gradient corrected functionals (BP86, PBE) yield large errors (about 50 to 100 kJ mol(-1)), but reproduce trends correctly. PBE calculations on a V(20)O(62)H(24) cluster model for the (001) surface of V(2)O(5) crystals virtually reproduce periodic slab calculations. The low bond dissociation energy (formation of oxygen surface defect) of 113 kJ mol(-1)(B3LYP) is due to substantial structure relaxations leading to formation of V-O-V bonds between the V(2)O(5) layers of the crystal. This relaxation cannot occur in polyhedral (V(2)O(5))(n) clusters and also not for V(2)O(5) species supported on silica or alumina (represented by cage-type models) for which bond dissociation energies of 250-300 kJ mol(-1) are calculated. The OV(OCH(3))(3) molecule and its dimer are also considered. Radical cations V(2)O(5)(+) and V(4)O(10)(+) have very low bond dissociation energies (22 and 14 kJ mol(-1), respectively), while the corresponding radical anions have higher dissociation energies (about 330 kJ mol(-1)) than the neutral clusters. The bond dissociation energies of the closed shell V(3)O(7)(+) cation (165 kJ mol(-1)) and the closed shell V(3)O(8)(-) anion (283 kJ mol(-1)) are closest to the values of the neutral clusters. This makes them suitable for gas phase studies which aim at comparisons with V(2)O(5) species on supporting oxides.

Bond Dissociation Energies of the N−H Bond and Rate Constants for the Reaction with Alkyl, Alkoxyl, and Peroxyl Radicals of Phenothiazines and Related Compounds

The results of a detailed thermodynamic and kinetic investigation on the homolytic reactivity of phenothiazine, phenoxazine, and phenoselenazine, of several substituted phenothiazines, and of related tricyclic aromatic amines are reported. All these compounds give, by hydrogen atom abstraction from the N−H group, persistent aminyl radicals. Equilibration of each of these radicals with the parent amine and a reference compound having an easily abstractable hydrogen allowed us to determine, by using EPR spectroscopy, the N−H Bond Dissociation Energies (BDE) of the amines. These are characterized by low BDE values (in some cases lower than the O−H bond strength of α-tocopherol, i.e 78.3 kcal/mol) and therefore are very good hydrogen atom transfer reagents. To check the efficiency of tricyclic amines as antioxidants and as polymerization inhibitors, absolute rate constants were determined for their reaction with alkyl, alkoxyl, and peroxyl radicals by using competitive techniques in the first two cases and by autoxidation studies under controlled conditions in the last one. All amines have been found to be highly reactive toward these radicals which makes them very good autoxidation and polymerization inhibitors.

Theoretical investigation of the weakly bonded donor—acceptor complexes X3B—H2, X3B—C2H4, and X3B—C2H2 (X = H, F, Cl)

Quantum mechanical calculations at the MP2/6-311G(2df,2pd)//MP2/TZ2P level of theory are reported for the compounds X3B—D with X = H, F, Cl, and D = H2, C2H4, and C2H2. The calculated binding energies show the trend BH3 >> BF3 > BCl3 for the Lewis acids and C2H4 > C2H2 > H2 for the Lewis bases. BCl3 and BF3 form weakly bonded van der Waals complexes with C2H4 and C2H2 with similar binding energies De between 4.2 kcal mol-1 for F3B—C2H4 and 3.2 kcal mol-1 for Cl3B—C2H2. The dihydrogen complexes have lower bond dissociation energies of 0.9 kcal mol-1 for Cl3B—H2 and 0.7 kcal mol-1 for Cl3B—H2. The BH3 complexes are significantly more strongly bonded and show the onset of true chemical bonding. The NBO method finds a polarized 2-electron 3-centre bond for the three H3B—D compounds. The most strongly bonded complex is H3B—C2H4 (De = 15.0 kcal mol-1), while H3B—C2H2 (De = 8.5 kcal mol-1) and H3B—H2 (De = 5.7 kcal mol-1) are more weakly bonded species. The contribution of the zero-point vibrational energy shows H3B—H2 to be scarcely bound. The CDA method suggests that H2, C2H4 and C2H2 are stronger donor than acceptor ligands. Calculated vibrational spectra are reported also.

On the toxicity of phenols to fast growing cells. A QSAR model for a radical-based toxicity

The cytotoxicities of a series of simple phenols as well as estrogenic phenols such as octyl and nonyl phenols, Bisphenol A, diethylstilbestrol, estradiol, estriol, equilin and equilenin were studied in a fast growing murine leukemia cell line. The use of calculated homolytic bond dissociation energies (BDE) as the electronic parameter led to the development of a Quantitative Structure–Activity Relationship model with superior results; one which established the importance of relatively low BDE values in enhancing toxicity to rapidly multiplying cells. The correlation equation that emerged is as follows: log 1/C = –0.19BDE + 0.21 log P + 3.11. It suggests that toxicity is closely related to mostly homolytic cleavage of the phenolic O–H bond and overall hydrophobicity of the phenol.

A benzyl–carbon bond dissociation energy of one-third the normal value: a quantitative study of benzyl–carbon bond homolysis in [CoIII(L2–CH2Ph)I], a cobalt-to-carbon benzyl migration product derived from the coenzyme B12model complex [CoIII(L1)(CH2Ph)I

Benzyl–carbon bond thermal homolysis studies using the TEMPO radical-trapping method reveals a low benzyl–carbon bond dissociation energy (BDE) in [CoIII(L2–CH2Ph)I] of 25 ± 3 kcal mol–1, a value one-third the normal BDE of ca. 76 kcal mol–1 and approximately equal to the cobalt–carbon homolytic BDE in the parent complex [CoIII(L1)(CH2Ph)I]; such little-documented low BDEs relate back to Gomberg's classic example of Ph3C–C6H5CPh2 and its ca. 11 kcal mol–1 carbon–(p-phenyl)carbon BDE (1 cal = 4.184 J).

Theoretical study of some BXHn compounds where X stands for B, C, N, O or F

In this work, we investigate theoretically the energetics of some of the simplest boron-boron boron-carbon, boron-nitrogen, boron-oxygen and boron-fluorine compounds. Optimized structures have been carried out at the HF (RHF and UHF) and MP2 6-31G** level . Our attention has been focused on the heats of formation. They have been obtained at the MP4 level of the theory, using basis sets including polarized (d, f) and diffuse (+) functions. Bond dissociation energies have also been obtained. They arc closely related to the electronic structures. Singlet BB, HBBH, BCH and HBC compounds exibit particularly low bond dissociation energies, but their fundamental states arc known to be triplet instead of singlet state.

Chemical Considerations in the Incineration of Chlorinated Methanes I-Methyl Chloride

The high temperature product distributions from reactions of methane and methyl chloride have been measured under pyrolysis, preignition oxidation and flame conditions. For pyrolysis and preignition data, 3% fuel/zero oxygen or stoichiometric oxygen/10% N2/Ar were heated behind reflected shocks to temperatures between 1200-2600 K at a density of approximately 2.5 ± 0.25 × 10−5 mol/cm3. Flame studies were conducted at atmospheric pressure for methane/air and methyl chloride/methane/air mixtures with equivalence ratios of 1.15 and 1.35 and for four methyl chloride/methane ratios ranging from pure methane to pure methyl chloride. Methyl chloride is more easily decomposed than methane in either pyrolysis or preignition oxidation. This is because of the lower bond dissociation energy of methyl chloride (Do (CH3-Cl) = 334 kJ/mol) as compared to methane (Do(CH3-H) = 435 kJ/mol). In the flame environment, the methane and methyl chloride disappear at approximately the same rate. The presence of chlorine decreases the measured ethane concentration and promotes the formation of acetylene which may explain the propensity of soot formation from chlorinated hydrocarbons.

Hepatotoxicity of bromotrichloromethane— bond dissociation energy and lipoperoxidation

Bromotrichloromethane (BrCCl 3) is much more potent than CCl 4 or CHCl 3 as a liver poison. Bond dissociation energies for cleavage of H-CCl 3, Cl-CCl 3, and Br-CCl 3 are in the order H-CCl 3 > Cl-CCl 3 > Br-CCl 3. A low bond dissociation energy implies a greater tendency of the given bond to cleave homolytically. In vitro, BrCCl 3 is 200 times more potent than CCl 4 in promoting peroxidative decomposition of rat liver microsomal lipids. CHCl 3 has virtually no potency as a pro-oxidant in vitro. BrCCl 3 produces in rats about three times the degree of liver microsomal lipid peroxidation than does CCl 4, at equivalent doses. CHCl 3 does not produce lipid peroxidation in vivo. Administration of BrCCl 3 in very low doses (0.025 ml/kg of body weight) produces a dramatic fall in liver microsomal cytochrome P-450. Rats with low levels of cytochrome P-450 due to prior administration of BrCCl 3 are completely resistant to large, and otherwise lethal doses of CCl 4. A lethality study is presented which suggests that BrCCl 3 may have extrahepatic sites of action.

Recoil Tritium Reactions: Variations in Hot Abstraction Yield with C–H Bond Type

The yield of the reaction T* + RH yields HT + R, from various RH molecules was measured under essentially constant conditions of tritium atom spectra. The results showed that variation in relative HT yield is clearly an important factor in the variation of HT/RT ratios, and may possibly be the only important factor. Comparison of HT yields with corresponding dissociation energies of the RH bonds showed a generally correct correlation of higher HT yield with lower bond dissociation energies. Relative yields of hot HT per C --H bond from recoil tritium reactions with cyclopropene, propane, butane, cyclobutene, and other molecules are tabulated.