Ab initio study of reactions of hydroxyl radicals with chloro‐ and fluoro‐substituted methanes

Ab initio calculations at the unrestricted Hartree–Fock (UHF) level have been performed to investigate the hydrogen abstraction reactions of ^? OH radicals with methane and nine halogen‐substituted methanes (F, Cl). Geometry optimization and vibrational frequency calculations have been performed on all reactants, adducts, products, and transition states at the UHF/6‐31G* level. Single‐point energy calculations at the MP2/6‐31++G* level using the UHF/6‐31G* optimized geometries have also been carried out on all species. Pre‐ and postreaction adducts have been detected on the UHF/6‐31G* potential energy surfaces of the studied reactions. Energy barriers, ΔE^?, reaction energies, ΔE_r, reaction enthalpies, ΔH_r, and activation energies, E_a, have been determined for all reactions and corrected for zero‐point energy effects. Both E_a and ΔH_r come into reasonable agreement with the experiment when correlation energy is taken into account and when more polarized and diffuse basis sets are used. The E_a values, estimated at the PMP2/6‐31++G* level, are found to be in good agreement with the experimental ones and correctly reproduce the experimentally observed trends in fluorine and chlorine substitution effects. A linear correlation between E_a and ΔH_r is obtained, suggesting the presence of an Evans–Polanyi type of relationship. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 426–440, 2001     

Thermodynamic and kinetic characteristics of intermediates of electrode reactions. Comparative laser photoemission study of the kinetics of electron transfer for certain alkylaryl and alkylhalide radicals

Using the laser photoemission technique, a comparative study of thermodynamic and energy characteristics of electron transfer (ET) is carried out for a number of alkylaryl intermediates (IM) (benzyl, benzhydryl radicals) and alkyl halides (chloromethyl and dichloromethyl radicals). It is found that the standard free energies of activation of radicals under study are 0.34–0.38 eV and the preexpoential factors are between 10⁹ and 10¹⁰s^?1 and weakly depend on the solution nature. The reorganization energies of the medium for these IM are estimated in terms of the classical Marcus theory. The results are compared with the literature data on the dissociative ET in monohalogenomethanes CH₃Hal/CH₃Hal^?·, polychloromethanes CH_nHal_{4 ? n} /CH_nHal _4?n ^?· , and some other systems. Possible reasons for the different probabilities of observing reversible ET for IM under study are discussed.     

Direct kinetic study of the reaction of OH radicals with methyl-ethyl-ketone

The low-pressure discharge flow technique with resonance fluorescence monitoring of OH has been applied to study the kinetics of the overall reaction:

Theoretical study on the gas‐phase reactivity of halogenated alkylperoxyl radicals toward alkenes

The electrophilic additions of hydroperoxyl (HO ${}_{\text{2}}^{\text{⋅}}$), alkylperoxyl (RO ${}_{\text{2}}^{\text{⋅}}$), and halogenated alkylperoxyl radicals to ethylene were studied using the AM1 and PM3 semiempirical MO methods at the SCF/UHF level. Reactantlike transition states were predicted for the title additions. The AM1 activation enthalpies (ΔH ${}_{\text{f}}^{\text{*}}$) were found to be increased in the order HO ${}_{\text{2}}^{\text{⋅}}$<CH₃O ${}_{\text{2}}^{\text{⋅}}$<C₂H₅O ${}_{\text{2}}^{\text{⋅}}$<i‐C₃H₇O ${}_{\text{2}}^{\text{⋅}}$. The reactivity of an alkylperoxyl radical toward ethylene was found to be increased as the degree of halogen substitution on the alkyl group increased. A good correlation was established between ΔH ${}_{\text{f}}^{\text{*}}$ and the Taft polar substituent constants, σ*. The Evans–Polanyi correlation between ΔH ${}_{\text{f}}^{\text{*}}$ and ΔH ${}_{\text{r}}^{\text{°}}$ was justified and the validity of the Hammond postulate was indicated. The calculated results were compared with the available experimental findings. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 273–283, 1999     

The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds

A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R¹COR²→>C=O+R¹R²C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH+R¹CH=CH₂→>C=O+R¹C·HCH₃; >R¹CH=CH₂+R²C·HCH₂R³→R²C·HCH₃+R²CH=CHR³. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (adelhydes, ketones, and quinones) from the C−H and O−H bonds were compared. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2178–2184, November, 1998.     

Gas phase rate coefficients and activation energies for the reaction of butanal and 2‐methyl‐propanal with nitrate radicals

Rate coefficients have been determined for the reaction of butanal and 2‐methyl‐propanal with NO₃ using relative and absolute methods. The relative measurements were accomplished by using a static reactor with long‐path FTIR spectroscopy as the analytical tool. The absolute measurements were made using fast‐flow–discharge technique with detection of NO₃ by optical absorption. The resulting average coefficients from the relative rate experiments were k = (1.0 ± 0.1) × 10⁻¹⁴ and k = (1.2 ± 0.2) × 10⁻¹⁴ (cm³ molecule⁻¹ s⁻¹) for butanal and 2‐methyl‐propanal, respectively. The results from the absolute measurements indicated secondary reactions involving NO₃ radicals and the primary formed acyl radicals. The prospect of secondary reactions was investigated by means of mathematical modeling. Calculations indicated that the unwanted NO₃ radical reactions could be suppressed by introducing molecular oxygen into the flow tube. The rate coefficients from the absolute rate experiments with oxygen added were and k = (1.2 ± 0.1) × 10⁻¹⁴ and = (0.9 ± 0.1) × 10⁻¹⁴ (cm³ molecule⁻¹ s⁻¹) for butanal and 2‐methyl‐propanal. The temperature dependence of the reactions was studied in the range between 263 and 364 K. Activation energies for the reactions were determined to 12 ± 2 kJ mole⁻¹ and 14 ± 1 kJ mole⁻¹ for butanal and 2‐methyl‐propanal, respectively. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 294–303, 2000     

Polar and solvation effects in reactions of oxygen atoms and hydroxyl and alkoxyl radicals with oxygen-containing compounds

Experimental data on the activation energies of reactions of H-abstraction from oxygencontaining compounds by oxygen atoms and hydroxyl and alkoxyl radicals in the gas and liquid phases have been analyzed by means of the parabolic model of the transition state. The contribution of polar interaction to the activation energies of the reactions has been calculated. The contribution of solvation to the activation energy has been calculated by comparison of the reaction parameters of the respective reaction in the liquid and gas phases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 38–42, January, 1994.     

Influence of the size of the formed cycle on the reactivity of free radicals in cyclization reactions

Numerous experimental data for the cyclization of free radicals C·H₂(CH₂)_nCH=CH₂ cyclo-[(CH₂)_n+1CH(C·H₂)], and C·H₂(CH₂)_nCH=CHR cyclo-[(CH₂)_n+1C·HCHR] were analyzed in the framework of the parabolic model. The activation energy of thermoneutral (H _e = 0) cyclization E _e0 decreases linearly with an increase in the energy of cycle strain E _rsc: E _e0(n) (kJ mol–1) = 85.5 – 0.44E _rsc(n) (n is the number of atoms in the cycle). The activation entropy of cyclization S ^# also depends on the cycle size: the larger the cycle, the lower S ^#. A linear dependence of S ^# on the difference between the entropies of formation S° of cyclic hydrocarbon and the corresponding paraffin was found: S ^# = 1.00[S°(cycle) – S°(C_nH_2n+2)]. The E _e0 values coincide for cyclization reactions with the formation of the six-membered cycle and the bimolecular addition of alkyl radicals to olefins.     

Theoretical study of mechanism and kinetics for the addition of hydroxyl radical to phenol

The reaction mechanism and kinetics for the addition of hydroxyl radical(OH) to phenol have been investigated using the hybrid density functional(B3LYP) method with the 6-311++G(2dp,2df) basis set and the complete basis set(CBS) method using APNO basis sets,respectively.The equilibrium geometries,energies,and thermodynamics properties of all the stationary points along the addition reaction pathway are calculated.The rate constants and the branching ratios of each channel are evaluated using classical transition state theory(TST) in the temperature range of 210 to 360 K,to simulate temperatures in all parts of the troposphere.The ortho addition pathway is dominant and accounts for 99.8% 96.7% of the overall adduct products from 210 to 360 K.The calculated rate constants are in good agreement with existing experimental values.The addition reaction is irreversible.     

Competition between mono-and bimolecular reactions of artemisinin alkoxyl radicals

The competition between intramolecular and bimolecular reactions of alkoxyl radicals formed from artemisinin was theoretically analyzed. The enthalpies of these reactions were calculated. The activation energies and rate constants of reactions of intramolecular hydrogen atom transfer, decyclization, and decomposition of alkoxyl radicals of artemisinin and several its derivatives, as well as the activation energies and rate constants of reactions of these radicals with the C-H, S-H, and O-H bonds in biological substrates and their analogs were calculated by the intersecting parabolas method The fastest reactions of artemisinin alkoxyl radicals were identified. The full kinetic scheme of transformation of these radicals was proposed. Artemisinin radicals with the free valence on the carbon atom are predominantly formed due to the transformation of the artemisininoxyl radicals. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1502–1510, September, 2006.     

A theoretical study of the mechanism and kinetics of F+N3 reactions.

Both the singlet(1A') and triplet(3A') potential energy surfaces (PESs) of F+N(3) reactions are investigated using the complete-active-space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) methods with a proper active space. The minimum energy crossing point (MECP) at the intersection seam between the 1A' and 3A' PESs is located and used to clarify the reaction mechanisms. Two triplet transition states are found, with one in the cis form and the other one in the trans form. Further kinetic calculations are performed with the canonical unified statistical (CUS) theory on the singlet PES and the improved canonical variational transition-state (ICVT) method on the triplet PES. The rate constants are also reported. At 298 K, the calculated rate constant is in reasonably good agreement with experimental values, and spin-orbit coupling effects lower it by 28 %. The spectroscopic constants derived from the fitted potential-energy curves for the singlet and triplet states of NF are in very good agreement with experimental values. Our calculations indicate that the adiabatic reaction on the singlet PES leading to NF(a(1)Delta)+N(2) is the major channel, whereas the nonadiabatic reaction through the MECP, which leads to NF(X(3)Sigma(-))+N(2), is a minor channel.     

Direct measurements of HOx radicals in the marine boundary layer: testing the current tropospheric chemistry mechanism

OH and HO(2) radicals, atmospheric detergents, and the reservoir thereof, play central roles in tropospheric chemistry. In spite of their importance, we had no choice but to trust their concentrations predicted by modeling studies based on known chemical processes. However, recent direct measurements of these radicals have enabled us to test and revise our knowledge of the processes by comparing the predicted and observed values of the radical concentrations. We developed a laser-induced fluorescence (LIF) instrument and successfully observed OH and HO(2) at three remote islands of Japan (Oki Island, Okinawa Island, and Rishiri Island). At Okinawa Island, the observed daytime level of HO(2) agreed closely with the model estimates, suggesting that the photochemistry at Okinawa is well described by the current chemistry mechanism. At Rishiri Island, in contrast, the observed daytime level of HO(2) was consistently much lower than the calculated values. We proposed that iodine chemistry, usually not incorporated into the mechanism, is at least partly responsible for the discrepancy in the results. At night, HO(2) was detected at levels greater than 1 pptv at all three islands, suggesting the presence of processes in the dark that produce radicals. We showed that ozone reactions with unsaturated hydrocarbons, including monoterpenes, could significantly contribute to radical production.     

interaction of nitro compounds with radicals: a quantum chemical study

Photoreduction of nitro compounds is accompanied by formation of various radical products that can react with the starting nitro compound, thus causing deviation of the decomposition kinetics from the first-order kinetics with respect to the nitro compound. The results of quantum chemical modeling of the reactions of nitro compounds with radicals and the pathways of further transformations of radical adducts formed in the reactions are presented. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 202–206, February, 2006.     

Atmospheric chemistry of CCl2FCH2CF3 (HCFC-234fb): Kinetics and mechanism of reactions with Cl atoms and OH radicals

The atmospheric chemistry of CCl₂FCH₂CF₃ (HFCF-234fb) was examined using FT-IR/relative-rate methods. Hydroxyl radical and chlorine atom rate coefficients of k(CCl₂FCH₂CF₃+OH)= (2.9 ± 0.8) × 10⁻¹⁵ cm³ molecule^–1 s^–1 and k(CCl₂FCH₂CF₃+Cl)= (2.3 ± 0.6) × 10⁻¹⁷ cm³ molecule^–1 s^–1 were determined at 297 ± 2 K. The OH rate coefficient determined here is two times higher than the previous literature value. The atmospheric lifetime for CCl₂FCH₂CF₃ with respect to reaction with OH radicals is approximately 21 years using the OH rate coefficient determined in this work, estimated Arrhenius parameters and scaling it to the atmospheric lifetime of CH₃CCl₃. The chlorine atom initiated oxidation of CCl₂FCH₂CF₃ gives C(O)F₂ and C(O)ClF as stable secondary products. The halogenated carbon balance is close to 80% in our system. The integrated IR absorption cross-section for CCl₂FCH₂CF₃ is 1.87 × 10⁻¹⁶ cm molecule⁻¹ (600–1600 cm⁻¹) and the radiative efficiency was calculated to 0.26 W m⁻² ppb¹. A 100-year Global Warming Potential (GWP) of 1460 was determined, accounting for an estimated stratospheric lifetime of 58 years and using a lifetime-corrected radiative efficiency estimation.     

中温镍水蒸汽转化催化剂积碳动力学与机理研究

研究了833-948K和常压下, C7H16在镍催化剂上进行水蒸汽转化过程中的积碳反应动力学, 获得了相应的动力学方程。并探讨了相应的反应机理, 提出在镍表面同时进行的水蒸汽转化和积碳反应是两个相对独立的反应。指出在较高温度下, 水蒸汽会从简单非解离吸附转变为解离吸附, 这一变化不仅改变了反应级数, 更重要的是它提高了水蒸汽参与表面反应的能力, 特别是加强了氢对积碳反应的作用; 同时庚烷的吸附热也随之产生相当大变化。分析了出现在本温度区内积碳反应负表观活化能的来源。     

The mechanism of the Baeyer-Villiger rearrangement: quantum chemistry and TST study supported by experimental kinetic data

The mechanism of the Baeyer-Villiger rearrangement is modelled for the reaction of propanone with trifluoroperacetic acid, catalyzed by trifluoroacetic acid in dichloromethane, using three DFT methods (B3LYP, BH&HLYP and MPWB1K) and MP2. These results are refined and used to calculate the overall reaction rate coefficient using conventional Transition State Theory. The excellent agreement between the calculated (1.00 x 10(-3) L mol(-1) s(-1)) and the experimental (1.8 x 10(-3) L mol(-1) s(-1)) rate coefficients at the MPWB1K level strongly supports the mechanism recently proposed by our group. This DFT method is then used to study the mechanism of a larger system: cyclohexanone + trifluoroperacetic acid, for which a very good agreement between the calculated and the experimental rate coefficients is also found (1.37 and 0.32 L mol(-1) s(-1), respectively). The modelled mechanism is not ionic but neutral, and consists of two concerted steps. The first one is strongly catalyzed while the second one, the migration step, seems not to be catalyzed for the systems under study. The results of this work could be of interest for understanding other reactions in non-polar solvents for which ionic mechanisms have been assumed.     

Mechanism and kinetics of the reaction of OH radicals with glyoxal and methylglyoxal: a quantum chemistry + CVT/SCT approach.

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from glyoxal and methylglyoxal is presented. Optimum geometries, frequencies, and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all the stationary points, as well as for 12 additional points along the minimum energy path (MEP). Energies were obtained by single-point calculations at the above geometries using CCSD(T)/ 6-311++G(d,p) to produce the potential energy surface. The rate coefficients were calculated for the temperature range 200-500 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism, which involves the formation of a reactant complex. The overall agreement between the calculated and experimental kinetic data is very good. This agreement supports the reliability of the Arrhenius parameters of the glyoxal + OH reaction that are proposed in this work for the first time. The Arrhenius expressions that best describe the studied reactions are k1 = (9.63 +/- 0.23) x l0(-13)exp[(517 +/- 7)/T] and k2 = (3.93 +/- 0.11) x 10(-13)exp[(1060 +/- 8)/T]cm3 molecule(-1)s(-1) for glyoxal and methylglyoxal, respectively.     

UV curing kinetics and mechanism of a highly branched polycarbosilane

The UV curing process in both air and nitrogen atmosphere for the highly branched polycarbosilane system was investigated by differential scanning photo calorimeter. The UV cured products were characterized by Fourier‐transform infrared spectrometry (FTIR). By comparison with the FTIR results of the uncured liquid mixture and the cured samples, the possible cross‐linking reactions were determined. The kinetics of the curing systems was studied. The rate constant k was calculated based on the experimental results. The activation energies in different curing conditions were obtained. According to these results, it was learned that the mechanism for the UV curing in nitrogen was controlled by the photolysis of photoinitiator. Comparably, the UV curing process in air was complicated. It was affected by not only the photolysis of photoinitiator, but also oxygen and tripropane glycol diacrylate. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical study on reaction mechanism of the vinyl radical with nitrogen atom

The complex triplet potential energy surface of the C₂H₃N system is investigated at the UB3LYP and CCSD(T) (single-point) levels in order to explore the possible reaction mechanism of C₂H₃ radical with N(⁴S). Eleven minimum isomers and 18 transition states are located. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are obtained. Starting from the energy-rich reactant C₂H₃+N(⁴S), the first step is the attack of the N atom on the C atom having one H atom attached in C₂H₃ radical and form the intermediate C₂H₃N(1). The associated intermediate 1 can lead to product P₁ CH₂CN+H and P₂ ³CH₂+³HCN by the cleavage of C–H bond and C–C bond, respectively. The most favorable pathway for the C₂H₃+N(⁴S) reaction is the channel leading to P₁, which is preferred to that of P₂ due to the comparative lower energy barrier. The formation of P₃ ³C₂H₂+³NH through hydrogen-abstraction mechanism is also feasible, especially at high temperature. The other pathways are less competitive comparatively.