Influence of Molecular Parameters on Rate Constants of Thermal Dissociation/Recombination Reactions: The Reaction System CF4 ⇄ CF3 + F.

The possibilities to extract incompletely characterized molecular parameters from experimental thermal rate constants for dissociation and recombination reactions are explored. The reaction system CF4 (+M) ⇄ CF3 + F (+M) is chosen as a representative example. A set of falloff curves is constructed and compared with the available experimental database. Agreement is achieved by minor (unfortunately not separable) adjustments of reaction enthalpy and collisional energy transfer parameters.

The Thermal Dissociation-Recombination Reactions of SiF4, SiF3, and SiF2: A Shock Wave and Theoretical Modeling Study.

Monitoring UV absorption signals of SiF2 and SiF, the thermal dissociation reactions of SiF4 and SiF2 were studied in shock waves. Rationalizing the experimental observations by standard unimolecular rate theory in combination with quantum-chemical calculations of the reaction potentials, rate constants for the thermal dissociation reactions of SiF4, SiF3, and SiF2 and their reverse recombination reactions were determined over broad temperature and pressure ranges. A comparison of fluorosilicon and fluorocarbon chemistry was finally made.

Quantum-Chemical Kinetic Study of the Unimolecular Decomposition Reactions of 1-Bromo-3-Chloropropane.

The pressure and temperature dependence of the thermal decomposition of 1-bromo-3-chloropropane has been theoretically investigated. The reaction takes place majorly through the elimination of HBr. Molecular properties of 1-bromo-3-chloropropane and transition states were derived from MN15/6-311++G(3df,3pd) and G4 quantum-chemical calculations. The resulting rate constants obtained from the unimolecular reaction rate theory for the high- and low-pressure limits of reaction BrCH2CH2CH2Cl → CH2CHCH2Cl + HBr at 400-1000 K were k∞ = 6.1 × 1013 exp(-57.2 kcal mol-1/RT) s-1 and k0 = [BrCH2CH2CH2Cl] 1.45 × 10-1 (T/1000 K)-7.9 exp(-55.9 kcal mol-1/RT) cm3 molecule-1 s-1. A value of -26.3 ± 1.0 kcal mol-1 for the standard enthalpy of formation of 1-bromo-3-chloropropane at 298 K was derived.

Role of the Recombination Channel in the Reaction between the HO and HO2 Radicals.

The kinetics of the gas phase recombination reaction HO + HO2 + He → HOOOH + He has been studied between 200 and 600 K by using the SACM/CT model and the unimolecular rate theory. The molecular properties of HOOOH were derived at the CCSD(T)/aug-cc-pVTZ ab initio level of theory, while relevant potential energy features of the reaction were calculated at the CCSD(T)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVDZ level. The resulting high and low pressure limit rate coefficients are k∞ = 3.55 × 10-12 (T/300)0.20 cm3 molecule-1 s-1 and k0 = [He] 1.55 × 10-31 (T/300)-3.2 cm3 molecule-1 s-1. The rate coefficients calculated over the 6 × 10-4 - 400 bar range are smaller at least in a factor of about 60 than the consensus value determined for the main reaction channel HO + HO2 → H2O + O2, indicating that the recombination pathway is irrelevant.

Thermal Decomposition of 3-Bromopropene. A Theoretical Kinetic Investigation.

A detailed kinetic study of the gas-phase thermal decomposition of 3-bromopropene over wide temperature and pressure ranges was performed. Quantum chemical calculations employing the density functional theory methods B3LYP, BMK, and M06-2X and the CBS-QB3 and G4 ab initio composite models provide the relevant part of the potential energy surfaces and the molecular properties of the species involved in the CH2═CH-CH2Br → CH2═C═CH2 + HBr (1) and CH2═CH-CH2Br → CH2═CH-CH2 + Br (2) reaction channels. Transition-state theory and unimolecular reaction rate theory calculations show that the simple bond fission reaction ( 2 ) is the predominant decomposition channel and that all reported experimental studies are very close to the high-pressure limit of this process. Over the 500-1400 K range a rate constant for the primary dissociation of k2,∞ = 4.8 × 10(14) exp(-55.0 kcal mol(-1)/RT) s(-1) is predicted at the G4 level. The calculated k1,∞ values lie between 50 to 260 times smaller. A value of 10.6 ± 1.5 kcal mol(-1) for the standard enthalpy of formation of 3-bromopropene at 298 K was estimated from G4 thermochemical calculations.

EXAFS and DFT study of the cadmium and lead adsorption on modified silica nanoparticles.

Silica nanoparticles of 7 nm diameter were modified with (3-aminopropyl) triethoxysilane (APTES) and characterized by CP-MAS (13)C and (29)Si NMR, FTIR, zeta potential measurements, and thermogravimetry. The particles were shown to sorb successfully divalent lead and cadmium ions from aqueous solution. Lead complexation with these silica nanoparticles was clearly confirmed by EXAFS (Extended X-ray Absorption Fine Structure) with synchrotron light measurements. Predicted Pb-N and Pb-C distances obtained from quantum-chemical calculations are in very good agreement with the EXAFS determinations. The calculations also support the higher APTES affinity for Pb(2+) compared to Cd(2+).

Quantum chemical and kinetics study of the thermal gas phase decomposition of 2-chloropropene.

A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant.

Hydration of barium monohydroxide in (H2O)(1-3) clusters: theory and experiment.

The ionization energies (IEe's) of small BaOH(H2O)m clusters (m = 1-3), as generated in a laser vaporization-supersonic expansion source have been determined by laser photoionization experiments over the 3.65-4.55 eV energy range. Complementary ab initio studies show that the IEe's are in good agreement with computed adiabatic ionization energies and that BaOH(H2O)m structures with a direct coordination of the Ba atom to water molecules are favored over those that are characterized by H-bonded networks involving H2O molecules and the OH group of BaOH. Additional calculations have been performed on the hydration energies for the most stable isomers of the relevant BaOH(H2O)1-3 clusters. A comparison is made between the closed-shell title system and the results of related theoretical studies on the open-shell alkali monohydroxides, which allows for an interpretation of the opposite trends that are found in the cluster size dependence of the vertical ionization energies for both series of systems, and highlights the role of the BaOH unpaired electron in its ionization process. Altogether, the present evidence suggests for the initial steps of the BaOH hydration process to be dominated by electrostatic and polarization interactions between the Ba(+) and OH(-) ion cores, which become both increasingly solvated upon sequential addition of water molecules.

Triplet state of 4-methoxybenzyl alcohol chemisorbed on silica nanoparticles.

The knowledge of photochemical kinetics in colloidal systems is important in understanding environmental photochemistry on dispersed solid surfaces. As model materials for the chemically sorbed organic compounds present in natural environments, modified silica nanoparticles (NPs) were obtained here by condensation of the silanol groups of fumed silica nanoparticles with 4-methoxybenzyl alcohol. These particles were characterized by different techniques. To evaluate their toxicity, the inhibition of the natural luminescence emission of the marine bacterium Vibrio fischeri in suspensions of the particles was measured. Laser flash-photolysis experiments (λ(exc) = 266 nm) performed with NP suspensions in acetonitrile-aqueous phosphate buffer mixtures showed the formation of the lowest triplet excited state of the chemisorbed organic groups (λ(max) = 390 nm). DFT calculations of the absorption spectrum of this radical support the assignment. From the calculated triplet energy, a thermodynamically favorable energy transfer from these triplet states to oxygen to yield singlet molecular oxygen is predicted. A value of 0.09 was measured for the quantum yield of singlet molecular oxygen generation by air-saturated suspensions of the nanoparticles in the mixture of solvents acetonitrile-aqueous phosphate buffer. The quantum yield of singlet molecular oxygen generation by the free 4-methoxybenzyl alcohol in the same solvent is 0.31.

Photoionization and ab initio study of Ba(H2O)n (n = 1-4) clusters.

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.

On the mechanism of Re(I)-carboxylate bond cleavage by perchloric acid: a kinetic and spectroscopic study.

We have studied the reaction between pz-CO(2)-Re(CO)(3)(bpy) and perchloric acid in acetonitrile by following the UV-vis and IR spectral changes in the reaction mixture. A fast equilibrium was found to be established between solvated protons, pz-CO(2)-Re(CO)(3)(bpy), and the protonated intermediate [pz-C(OH)O-Re(CO)(3)(bpy)](+) which finally yields pz-COOH and Re(CO)(3)(bpy)(CH(3)CN)(+) as reaction products. This intermediate has been characterized by UV-vis and IR spectroscopies and by DFT calculations. The fully optimized DFT/CPCM structures for pz-CO(2)-Re(CO)(3)(bpy) and [pz-C(OH)O-Re(CO)(3)(bpy)](+) were compared with the X-ray structure of pz-CO(2)-Re(CO)(3)(bpy). The structural parameters associated with the carboxyl group in the protonated intermediate are between those of pz-CO(2)-Re(CO)(3)(bpy) and pz-COOH. Multivariate curve resolution methods were employed to obtain the spectrum of the protonated intermediate and the concentration profiles from the full matrix of time-resolved UV-vis spectra. The proposed mechanism was numerically simulated by using Runge-Kutta methods. Model parameters were estimated by nonlinear regression fitting of the concentration profiles, yielding values of log(K) = 4.9 ± 0.3 and k = 0.16 ± 0.03 min(-1) for the formation equilibrium constant and the decay rate constant of the protonated intermediate, respectively.

Theoretical study of the equilibrium structure, vibrational spectrum, and thermochemistry of the peroxynitrate CF2BrCFBrOONO2.

The results of a theoretical study of the molecular structure and conformational mobilities of the peroxynitrate CF(2)BrCFBrOONO(2) and its radical decomposition product CF(2)BrCFBrOO are reported in this paper. The most stable structures were calculated from ab initio G3(MP2)B3 and G4(MP2) methods and from density functional theory at the B3LYP/6-311+G(d) and B3LYP/6-311+G(3df) levels of theory. The equilibrium conformation of CF(2)BrCFBrOONO(2) indicates that the bromine atoms lie in position anti to each other and possess a COON dihedral angle of 114°. A quantum statistical analysis shows that about 40% of the internal rotors can freely rotate at room temperature. Our best values for the standard enthalpies of formation of CF(2)BrCFBrOONO(2) and CF(2)BrCFBrOO at 298 K obtained from isodesmic reactions at the G3(MP2)//B3LYP/6-311+G(3df) level of theory are -144.7 and -127.0 kcal mol(-1). From these values and the enthalpy of formation of the NO(2) radical, a CF(2)BrCFBrOO-NO(2) bond dissociation enthalpy of 26.0 ± 2 kcal mol(-1) was estimated.

Gaussian-3 study of the thermochemistry of the germane and its fluoro chloro derivatives.

The thermochemistry of all fluoro and chloro substituted germane molecules have been theoretically studied. Computed G3//B3LYP standard enthalpies of formation at 298 K obtained from isodesmic reaction schemes were compared with values derived via total atomization energies. Bond dissociation energies and energy barriers for the lowest dissociation pathways were estimated at 0 K. From these data, the most probable dissociation products at 0 K were predicted for the thermal decomposition reactions of the gaseous species. Calculated results are compared with experimental determinations as well as other theoretical data when available. The following isodesmic enthalpies of formation in kcal x mol(-1) were calculated for 10 new germane species simultaneously substituted with fluoro and chloro atoms, for which no previously available computations were found in the literature: GeHFCl2, -125.8; GeCl2F, -104.3; GeHFCl, -67.5; GeF2Cl2, -186.3; GeF3Cl, -242.9; GeH2FCl, -89.7; GeCl3F, -159.6; GeHClF2, -168.0; GeF2Cl, -144.3; GeFCl, -81.1.

Reactions of sulphate radicals with substituted pyridines: a structure-reactivity correlation analysis.

The kinetics of the oxidation of pyridine, 3-chloropyridine, 3-cyanopyridine, 3-methoxypyridine and 3-methylpyridine mediated by SO4 () radicals are studied by flash photolysis of peroxodisulphate, S2O8(2-), at pH 2.5 and 9. The absolute rate constants for the reactions of both, the basic and acid forms of the pyridines, are determined and discussed in terms of the Hammett correlation. The monosubstituted pyridines react about 10 times faster with sulphate radicals than their protonated forms, the pyridine ions. The organic intermediates are identified as the corresponding hydroxypyridine radical adducts and their absorption spectra compared with those estimated employing the time-dependent density functional theory with explicit account for bulk solvent effects. A reaction mechanism which accounts for the observed intermediates and the pyridinols formed as products is proposed.

Falloff curves for the recombination reaction Cl + FC(O)O + M --> FC(O)OCl + M.

The pressure dependence of the recombination reaction Cl + FC(O)O + M --> FC(O)OCl + M has been investigated at 296 K. FC(O)O radicals and Cl atoms were generated by laser flash photodissociation of FC(O)OO(O)CF at 193 nm in mixtures with Cl2 and He or SF6 over the total pressure range 8-645 Torr. The measured FC(O)O radical and F atom yields in the photolysis are 0.33 +/- 0.06 and 0.67 +/- 0.06. The reaction lies in the falloff range approaching the high-pressure limit. The extrapolations toward the limiting low- and high-pressure ranges were carried out using a reduced falloff curves formalism, which includes a recent implementation for the strong-collision broadening factors. The resulting values for the low-pressure rate coefficients are (2.2 +/- 0.4) x 10(-28)[He], (4.9 +/- 0.9) x 10(-28)[SF6], (1.9 +/- 0.3) x 10(-28)[Cl2] and (5.9 +/- 1.1) x 10(-28)[FC(O)OO(O)CF] cm3 molecule(-1) s(-1). The derived high-pressure rate coefficient is (4.4 +/- 0.8) x 10(-11) cm3 molecule(-1) s(-1). For the reaction Cl + FC(O)OCl --> Cl2 + FC(O)O a rate coefficient of (1.6 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) was determined. The high-pressure rate coefficient was theoretically interpreted using SACM/CT calculations on an ab initio electronic potential computed at the G3S level of theory. Standard heat of formation values of -99.9 and -102.5 kcal mol(-1) were computed at the G3//B3LYP/6-311++G(3df,3pd) level of theory for cis-FC(O)OCl and trans-FC(O)OCl, respectively. The computed electronic barrier for the conversion between the trans and cis conformers is 8.9 kcal mol(-1). On the basis of the present results, the above reactions are expected to have a negligible impact on stratospheric ozone levels.