Homolytic Dissociation Research Articles

Naphthalene hydrogenation over a NiMo/γ-Al 2O 3 catalyst: Experimental study and kinetic modelling

A kinetic study of the liquid phase naphthalene hydrogenation over a commercial and sulphided NiMo/γ-Al 2O 3 was performed using experimental data obtained in a Robinson–Mahoney reactor at 523–583 K and 2.0–4.0 MPa. The effect of H 2S on the hydrogenation rate was investigated varying its partial pressure from 0.09 to 0.89 MPa. Under these conditions, an exponential decrease in the hydrogenation rate was observed as the H 2S partial pressure increased. The naphthalene conversion ranged from 6 to 89% with tetralin as the main hydrogenation product with selectivities of at least 89%. Reaction networks were formulated based on stepwise hydrogenation mechanisms assuming that both hydrogen and hydrogen sulphide could be dissociated either homolytically or heterolytically on both coordinatively unsaturated metal ion sites (CUS) and sulphur anions (SA) of the sulphided catalyst. Langmuir–Hinshelwood rate equations were derived and discriminated through model regression to experimental data. Two models best describe the experimental data according to a statistical analysis and the physical meaning of the parameter estimates. These models correspond to the third hydrogen addition chemisorbed on CUS and supplied from either homolytic or heterolytic hydrogen dissociation, as rate-determining step. The calculated catalyst surface species concentrations indicate that hydrogen and sulphydril groups are the most abundant species under the investigated operating conditions while the hydrocarbons surface concentration is almost negligible.

Uncontrolled pseudoliving free-radical polymerization of methyl methacrylate in the presence of butyl-p-benzoquinones

Polymerization of methyl methacrylate initiated by dicyclohexyl peroxydicarbonate in the presence of tri-n-butylboron and butyl-p-benzoquinone or 2,5-di-tert-butyl-p-benzoquinone occurred with no induction period. The yields and molecular masses of the polymers linearly increased with an increase in the conversion degree, which suggests the free-radical mechanism of “living” chain polymerization. The poly(methyl methacrylate) macrochains of the prepolymers contained sterically hindered aromatic structures with labile C-O bonds. The latter underwent reversible homolytic dissociation to give a growth-inducing radical and sterically hindered aryloxyls. Pseudoliving free-radical polymerization in the presence of the prepolymer (macroinitiator) was studied at 45, 60, and 80 °C.

Photochemical behaviour of triclosan in aqueous solutions: Kinetic and analytical studies

The mechanism of the direct photolysis of the anti-microbial triclosan in aqueous solutions was investigated by using steady state and laser flash photolysis. Quantum yields were determined for the disappearance of triclosan and formation of chloride anions in steady state irradiations in the absence and in the presence of oxygen as well as a function of pH. The photoreactivity was found to be efficient with the anionic form and in the absence of oxygen. Following laser flash photolysis (226 nm), three transients were found (triclosan triplet state, solvated electron and phenoxyl radical). Several primary and secondary stable photoproducts were elucidated by means of LC/MS/MS data. They were found to arise from four main photochemical processes: isomerisation, cyclization (leading to the formation of dioxin derivatives), dimerisation of the phenolic moiety and hydrolysis. The ionic chromatography showed that the loss of chloride anion in triclosan phototransformation represents an important degradation pathway. The formation of oligomeric products was also observed for prolonged irradiation time. A detailed mechanism for the formation of the primary products is proposed and discussed. The very important photocyclization reaction is more likely involving the triplet state pathway and the homolytic dissociation of the ether bridge occurs from the singlet excited state pathway.

Evaluation of fundamental processes in macromolecular structures radiolysis using quantum-chemical methods

The behavior of macromolecular structures in ionizing fields was analyzed by quantum-chemical methods. In this study, the primary radiolytical effect was analyzed using a two-step radiolytical mechanism: (a) ionization of molecule and spatial redistribution of atoms in order to reach a minimum value of energy, characteristic to the new quantum state and (b) neutralization of the molecule by capture of one electron and its rapid dissociation into free radicals. Chemical bonds suspected to break have minimum homolytic dissociation energies.

Homolytic Dissociation of the Vulcanization Accelerator Tetramethylthiuram Disulfide (TMTD) and Structures and Stabilities of the Related Radicals Me2NCSn• (n = 1−4)

The homolytic dissociation of the important vulcanization accelerator tetramethylthiuram disulfide (TMTD) has been studied by ab initio calculations according to the G3X(MP2) and G3X(MP2)-RAD theories. Homolytic cleavage of the SS bond requires a low enthalpy of 150.0 kJ mol-1, whereas 268.0 kJ mol-1 is needed for the dissociation of one of the C-S single bonds. To cleave one of the SS bonds of the corresponding trisulfide (TMTT) requires 191.1 kJ mol-1. Me2NCS2* is a particularly stable sulfur radical as reflected in the low S-H bond dissociation enthalpy of the corresponding acid Me2NC(=S)SH (301.7 kJ mol-1). Me2NCS2* (2B2) is a sigma radical characterized by the unpaired spin density shared equally between the two sulfur atoms and by a 4-center (NCS2) delocalized pi system. The ESR g-tensors of the radicals Me2NCSn* (n = 1-3) have been calculated. Both TMTD and the mentioned radicals form stable chelate complexes with a Li+ cation, which here serves as a model for the zinc ions used in accelerated rubber vulcanization. Although the binding energy of the complex [Li(TMTD)]+ is larger than that of the isomeric species [Li(S2CNMe2)2]+ (12), the dissociation enthalpy of TMTD as a ligand is smaller (125.5 kJ mol-1) than that of free TMTD. In other words, the homolytic dissociation of the SS bonds of TMTD is facilitated by the presence of Li+ ions. The sulfurization of TMTD in the presence of Li+ to give the paramagnetic complex [Li(S3CNMe2)2]+ is strongly exothermic. These results suggest that TMTD reacts with naked zinc ions as well as with the surface atoms of solid zinc oxide particles in an analogous manner producing highly reactive complexes, which probably initiate the crosslinking process during vulcanization reactions of natural or synthetic rubber accelerated by TMTD/ZnO.

A complete active space self-consistent field study of the photochemistry of nitrosamine

Photodissociation mechanisms of nitrosamine (NH2NO) have been studied at the complete active space self-consistent field level of theory in conjunction with atomic-natural-orbital-type basis sets. In addition, the energies of all the critical points and the potential energy curves connecting them have been recomputed with the multiconfigurational second-order perturbation method. Ground state minimum of nitrosamine has a C1 nonplanar structure with the hydrogen atoms of the amino moiety out of the plane defined by the N-N-O bonds. Electronic transitions to the three lowest states are allowed by selection rules: (i) S0-->S3 (7.41 eV) has an oscillator strength of f=0.0006 and it is assigned as an (npO)0-->(piNO*)2 transition, (ii) S0-->S2 (5.86 eV) has an oscillator strength of f=0.14 and it is assigned as an npN-->piNO* transition, and (iii) S0-->S1 (2.98 eV) has an oscillator strength of f=0.002 and it is assigned as an npO-->piNO* transition. It is found that N-N bond cleavage is the most likely process in all the photochemical relevant states, namely, S1 (1 1A"), S2 (2 1A'), and T1 (1 3A"). While S1 and T1 yield exclusively homolytic dissociation: NH2NO-->NH2 (1 2B1)+NO(X 2Pi), on S2 the latter process constitutes the major path, but two additional minor channels are also available: adiabatic homolytic dissociation: NH2NO-->NH2 (1 2A1)+NO(X 2Pi), and adiabatic oxygen extrusion: NH2NO-->NH2N (1 3A1)+O(3P). The excited species NH2 (1 2A1) experiences a subsequent ultrafast decay to the ground state, the final products in all cases the fragments being in their lowest electronic state. We have not found a unimolecular mechanism connecting excited states with the ground state. In addition, homolytic dissociation in the ground state, tautomerizations to NHNOH and NHNHO, and intersystem crossings to T1 are considered. The most favorable process on this state is the isomerization to NHNOH.

Nonradical Mechanisms for the Uncatalyzed Thermal Functionalization of Silicon Surfaces by Alkenes and Alkynes: A Density Functional Study

We propose a new concerted mechanism for the uncatalyzed hydrosilylation of terminal alkenes and alkynes, alternative to the conventional radical-based mechanism. Density functional calculations have been carried out on these and on previously proposed alternative mechanisms for the hydrosilylation of ethylene and acetylene by suitable finite size clusters as models of the thermal functionalization of -SiH3, =SiH2, and [triple bound] SiH groups in flat Si(100) and Si(111) and porous silicon surfaces by alkenes and alkynes. For each step involved in the considered hydrosilylation pathways, we optimized the geometries of reactants and products and located the corresponding transition states. The calculated activation energies for the concerted pathways of ethylene and acetylene are, respectively, 57.6 and 60.9 kcal mol(-1) on -SiH3 and in the ranges 62-63 and 58-61 kcal mol-1 on =SiH2 and 64-66 and 56-61 kcal mol(-1) on SiH. These values are much lower than the activation energies calculated for the corresponding homolytic dissociation of the Si-H bond, which is the preliminary step in the radical path, 85.6, 82-83, and 79-81 kcal mol(-1), respectively, for -SiH3, =SiH2, and [triple bound] SiH groups. Our results thus suggest that the thermal hydrosilylation of alkenes and alkynes on silicon surfaces, for which a radical-based mechanism is currently accepted, may occur through a concerted mechanism.

Density Functional Theory Calculations of the Dissociation of H2 on (100) 2H-MoS2 Surfaces: A Key Step in the Hydroprocessing of Crude Oil

Hydrogen activation on the (100) surface of MoS2 structures was investigated by means of density functional theory calculations. Linear and quadratic synchronous transit methods with a conjugate gradient refinement of the saddle point were used to localize transition states. The calculations include heterolytic and homolytic dissociation of hydrogen; that is, an H2 molecule dissociates on an MoS2 catalyst surface into two hydrogen atoms, which react further with the catalyst surface under formation of either one Mo-H and one S-H (heterolytic) or of two S-H surface groups (homolytic). Our results favor the heterolytic adsorption of hydrogen. Ni- and Co-promoted MoS2 have been considered to investigate the secondary promotional effect on the H2 dissociation. The authors observed a negative secondary promotional influence on the H2 dissociation in the case of Ni-promoted MoS2, whereas Co shows a positive effect.

Computational approaches to the dynamics of ions and electrons in materials under extreme conditions

Recent computational approaches to the atomic scale simulation of materials under extreme conditions are presented, with an emphasis on some tools relevant to the study of both shock and irradiation response, i.e. that are useful to describe chemical reactions or electronic excitations. For reactivity, recently introduced analytic reactive force fields are outlined. An alternative formalism including electronic degrees of freedom through hybrid atomic orbitals and providing a satisfactory description of the charge distribution upon homolytic dissociation processes is put forward. Preliminary results are presented to illustrate this electron distribution model. Although parameterized only against ab initio data calculated on equilibrium molecular structures, the latter yields consistent atomic charges for transition state structures and neutral homolysis products. Finally, an attempt at including the electron dynamics in shock wave simulations is outlined.

Reductive bond cleavage of chloro-substituted 10-methyl- tribenzotriquinacenes: Transition between concerted and stepwise mechanisms

The electrochemical reduction of 1-chloro-, 1,4-dichloro- and 1,4,7-trichloro-10-methyl-tribenzotriquinacenes in ACN and DMF was found to occur by concerted electron transfer and bond breaking process, showing irreversible cyclic voltammetric peaks. However, the transition of the process to the stepwise mechanism upon increasing the driving force by raising the scan rate was established. Both C Cl bonds are reduced successively in the dichloro compound, but only two bonds overall were reduced in the trichloro compound. The reduction mechanism is discussed. The homolytic dissociation energy of C Cl bond in monochloro compound was estimated to be equal to 2.9 eV.

Effects of Ionization, Metal Cationization and Protonation on 2‘-Deoxyguanosine: Changes on Sugar Puckering and Stability of the N-Glycosidic Bond

The influence of oxidation, protonation, and metal cationization with Cu(+) and Cu(2+) on the strength of the N-glycosidic bond in 2'-deoxyguanosine has been studied by means of quantum chemical calculations. In all cases, the N9-C1' bond distance increases (0.03-0.06 A) upon introducing positive charge in the guanine moiety, the observed variations being more important for the dicationic systems. Binding energies show that the effect of X(n)(+) in guanine hinders the homolytic dissociation, whereas it largely favors the heterolytic process. With respect to the deoxyribose ring, it has been found that metal binding, oxidation, and protonation do not significantly change the values of the phase angle of pseudorotation P. However, the glycosyl torsion angle chi varies considerably (from 242.0 degrees to 189.8 degrees) as a consequence of a stabilizing guanine-sugar (H8-O4') interaction due to the increase of acidity of guanine C8-H8 upon cationization.

Hydrogen adsorption on a Mo 27S 54 cluster: A density functional theory study

Density functional theory computations have been carried on the hydrogen adsorption mechanism on a Mo 27S 54 single layer cluster, which has a size (15–20 Å) close to that of real catalysts. For one molecule of H 2 adsorption, the most stable adsorption form ( E ads = −27.2 kcal/mol) is the homolytic dissociation on the S edge with the hydrogen atoms keeping away from the plane consisting of all Mo atoms, followed by the heterolytic dissociation ( E ads = −26.1 kcal/mol) on the intersection of S and Mo edges with the formation of Mo c–H and S c–H bonds. At high coverage with two and three H 2, however, dissociated hydrogen adsorption on the Mo sites are much more favored thermodynamically than on the S sites. Moreover, the corner sites are more favored thermodynamically for hydrogen adsorption and formation of coordinatively unsaturated sites than the edge sites. In addition, the activation energy of H 2 dissociation and hydrogen transfer processes have been computed to be 2.7–19.2 kcal/mol, and these rather low barriers indicate the enhanced mobility of the adsorbed hydrogen on the surface.

Reply to comment on ‘The enthalpy of the O–H bond homolytic dissociation: Basis-set extrapolated density functional theory and coupled cluster calculations’

Based on a recent re-evaluation of the O–H bond dissociation enthalpy (BDE) of phenol by Mulder et al. as 86.7 ± 0.7 kcal/mol, Di Labio and Mulder put under dispute the reliability of basis-set extrapolated coupled cluster calculations with single and double excitations (CCSD) for predicting the gas-phase BDEs of phenol and cathecol. Here, we stress again that our results support the experimental values for phenol recommended by Santos and Simões (88.7 kcal/mol) and by DeTuri and Erwin (90.1 ± 3.1 kcal/mol). We also verified that perturbative inclusion of triple excitations do not change our previous conclusions. We provide further elements that indicate why the recommended value of Mulder et al. for the phenol O–H BDE is not a definitive reference value.

Comment on “The enthalpy of the O–H homolytic dissociation: Basis-set extrapolated density functional theory and coupled cluster calculations” by B.J. Costa Cabral and S. Canuto [Chem. Phys. Lett. 406 (2005) 300–305

Costa Cabral and Canuto [B.J. Costa Cabral, S. Canuto, Chem. Phys. Lett. 406 (2005) 300] have studied the O–H bond dissociation enthalpy in water, hydrogen peroxide, methanol, phenol and catechol using a number of theoretical methods. Their choice of experimental O–H bond dissociation enthalpies for phenol and catechol are not the best available values and led them to several erroneous conclusions about the performance of methodologies they tested. In this work, we present more rigorous experimental O–H bond dissociation enthalpies for phenol and catechol and discuss the implications these data have on the conclusions presented by Costa Cabral and Canuto. We also demonstrate the importance of the inclusion of higher-order excitations in the coupled-cluster treatment of bond dissociation enthalpy of the O–H bond in H 2O 2 and HOO .

Liquid phase oxidation of alkanes using Cu/Co-perchlorophthalocyanine immobilized MCM-41 under mild reaction conditions

Amino-functionalized MCM-41 (NH 2-MCM-41) was used to immobilize Cu/Co-Cl 16Pc complex, i.e. Cu/Co-AM(PS) for liquid phase oxidation of alkanes under mild reaction conditions. Higher rates of reaction and better catalytic activity values were obtained for Cu/Co-AM(PS) as compared to Cu/Co-Cl 16Pc grafted on (i) amino-functionalized SiO 2 [Cu/Co-ASiO 2] and (ii) non-functionalized MCM-41 [Cu/Co-M(I)] catalysts along with neat metal complex under identical conditions. The catalysts were evaluated by comparing two different oxidants: (i) TBHP and (ii) O 2/aldehyde. The rate of conversion and percent selectivity differ for the above two oxidants due to differences in stability of radical species and in their homolytic/heterolytic pathways. The homolytic dissociation of oxygen favors a higher rate of conversion in the case of TBHP, whereas the heterolytic mechanism favors a higher selectivity for cyclohexanone in the case of O 2/aldehyde. The catalysts were characterized by XRD, MAS NMR, N 2-adsorption, microanalysis, UV–vis, FTIR and cyclic voltammetry. The UV–vis spectra reveal a blue shift for the metal phthalocyanine-immobilized samples, indicating unimolecular dispersion of metal complex within the channels of MCM-41. Cyclic voltammetry results suggest some coordinative interaction of the amino group of NH 2-MCM-41 with the metal on grafting with the complex.

Density functional theory calculation for H 2 dissociation on MoS 2 and NiMoS cluster models

Quantum chemical calculations for H 2 dissociation on NiMoS and MoS 2 catalysts were carried out using the ab initio density functional theory and the pseudopotential approaches. The catalysts were modeled by XMo 4S 17H 16, X=Ni, Mo cluster models. Two dissociation route, the heterolytic and the homolytic, were studied on the MoS 2 cluster model. The results show that the homolytic dissociation is favored over the heterolytic route. On the NiMoS cluster model, the homolytic route is less favored thermodynamically than in the MoS 2. The topological analysis of the electron density indicates the presence of S 2 groups on the MoS 2 and NiMoS surfaces, as well as a Mo–Mo bond in the MoS 2 model. No Ni–Mo bond was found in the NiMoS cluster model. The presence of Ni activates the Mo–S bond, produces a polarization at this bond and decreases the chemical hardness of the solid.

Sidewall Functionalization of Single‐Walled Carbon Nanotubes by Organometallic Chromium‐Centered Free Radicals

The interaction of organometallic chromium‐centered free radicals generated by the homolytic dissociation of (pentamethylcyclopentadienyl)chromiumtricarbonyl dimer in toluene with single‐walled carbon nanotubes (SWNT) was investigated using ESR spectroscopy. Low values of g‐factors of the radical species formed from chromium‐centered free radicals and SWNT as well as invariability of disorder mode (D band) intensity in Raman spectra of pristine and functionalized SWNT after this reaction indicated that chromium‐centered free radicals added to the surface of nanotubes through rather oxygen atoms than to sidewall carbon atoms. This is the first chromium‐derivatization of carbon nanotubes.

Exploring excited-state hydrogen atom transfer along an ammonia wire cluster: Competitive reaction paths and vibrational mode selectivity

The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline(NH(3))(3) cluster involves a crossing from the initially excited (1)pipi(*) to a (1)pisigma(*) state. The nonadiabatic coupling between these states induces homolytic dissociation of the O-H bond and H-atom transfer to the closest NH(3) molecule, forming a biradical structure denoted HT1, followed by two more Grotthus-type translocation steps along the ammonia wire. We investigate this reaction at the configuration interaction singles level, using a basis set with diffuse orbitals. Intrinsic reaction coordinate calculations of the enol-->HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire, as well as large O-H...NH(3) hydrogen bond contraction and expansion. The calculations also predict an excited-state proton transfer path involving synchronous proton motions; however, it lies 20-25 kcal/mol above the ESHAT path. Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate: Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these T(n) states branches the reaction back to the enol reactant side, decreasing the ESHAT yield. In fact, a product yield of approximately 40% 7-ketoquinoline.(NH(3))(3) is experimentally observed. The vibrational mode selectivity of the enol-->HT1 reaction step [C. Manca, C. Tanner, S. Coussan, A. Bach, and S. Leutwyler, J. Chem. Phys. 121, 2578 (2004)] is shown to be due to the large sensitivity of the diffuse pisigma(*) state to vibrational displacements along the intermolecular coordinates.

Controlled nitric oxide photo-release from nitro ruthenium complexes: The vasodilator response produced by UV light irradiation

Preliminary pharmacological studies of various nitric oxide (NO) photo-releasing agents are reported based on the flash-photolysis studies of the nitro ruthenium complexes cis-[Ru II(NO 2)L(bpy) 2] + (bpy = 2,2′-bipyridine and L = pyridine, 4-picoline and pyrazine) and [Ru II(NO 2)(bpy)(terpy)] + (terpy = terpyridine) in physiological medium. The net photoreactions under these conditions are two primary photoproducts, in ( I) there is Ru II–NO 2 photoaquation, where the photoproducts are Ru II–H 2O plus NO 2 - and ( II) homolytic dissociation of NO from a coordinated nitrito to derive the Ru II–OH 2 specie and NO. Based on photochemical processes, the nitro ruthenium complexes were incorporated in water in oil (W/O) microemulsion and used in the vasorelaxation induced experiment. Denuded rat aortas were contracted with KCl and nitro ruthenium complexes in microemulsion were added. Perfusion pressures were recorded while arteries were irradiated at 355 nm The time to reach maximum relaxation was longer for [Ru II(NO 2)(bpy)(terpy)] + complex (ca. 50 min, n = 6) than for cis-[Ru(NO 2)L(bpy) 2] + with L = py and 4-pic complex (ca. 28 min, n = 6) and cis-[Ru(NO 2)(bpy) 2 (pz)] 2+ complex (ca. 24 min, n = 5).

The enthalpy of the O–H bond homolytic dissociation: Basis-set extrapolated density functional theory and coupled cluster calculations

The O–H bond homolytic dissociation of water, hydrogen peroxide, methanol, phenol, and cathecol is investigated by density functional theory (DFT) and ab initio coupled cluster calculations. DFT results are based on several recently proposed functionals, including B98, PBE, VSXC, and HCTH. The dependence of DFT results on the basis-set size is discussed using correlation-consistent polarized (cc-pVXZ) basis-sets ( X = 2–5). A scheme proposed by Truhlar is used to extrapolate CCSD energies. Basis-set extrapolated CCSD results for the O–H bond homolytic dissociation enthalpies of phenol and cathecol are in excellent agreement with experimental information.