Addition Elimination Channels Research Articles

Theoretical study on the corrosion of thorium nuclear fuel by water: The effect of two-state reaction mechanism

The title reaction was investigated by DFT calculations. This process realizes in two steps. In the first step, singlet and triplet potential energy surfaces cross each other. We found that two-state reaction (TSR) mechanism is involved, enabling singlet product to be favored energetically. Then, ThO continues to react with H2O to form ThO2. Only the activation energy of singlet addition–elimination channel matches well with experimental reports. This work illustrates the occurrence of TSR mechanism in the corrosion of actinide nuclear fuel and further suggests a potential yet crucial impact on related fields. It deserves particular attention.

Kinetics and Mechanism of the Reaction of Fluorine Atoms with Pentafluoropropionic Acid

The kinetics of the reaction between fluorine atoms and pentafluoropropionic acid has been studied experimentally at T = 262-343 K. The overall reaction rate constant decreases with temperature: k1(T) = 6.1 × 10(-13) exp(+1166 K)/T) cm(3) molecule(-1) s(-1). The potential energy surface of the reaction has been studied using quantum chemistry. The results were used in transition state theory calculations of the temperature dependences of the rate constants of the two channels of the reaction. The abstraction channel ultimately producing HF, C2F5, and CO2 is dominant at the experimental temperatures; the addition-elimination channel producing C2F5 and CF(O)OH becomes important above 1000 K.

A computational study on the mechanism and kinetics of the reaction between CH3CH2S and OH

Addition–elimination channels CH3CHS + H2O, CH2CH2 + HSOH, CH3CHSO + H2 and CH3CH2SH + O are the major channels in the reaction between CH3CH2S and OH.

Branching ratio of gas-phase reaction of CH3C(O)CHCl2 + OH: A theoretical dynamic study

A theoretical study on the atmospheric reaction of hydroxyl radical with CH3C(O)CHCl2 is investigated. Geometries and frequencies calculation have been performed at the MP2/cc-pVDZ level of theory for all stationary points, and energy values have been improved by single-point calculations using the BMC-CCSD level. Two different reaction channels have been found, abstraction and addition–elimination channels. The rate constants for this reaction have been calculated within the temperature range of 200–360K at atmospheric pressure by means of canonical variational transition state theory incorporating with the small-curvature tunneling correction. The overall rate constant of the CH3C(O)CHCl2+OH reaction turns out finally to be the sum of the H-abstraction and OH-addition rate constant. It was found that the contributions of the addition–elimination channel to the overall reaction can be ignore, suggesting that in the upper troposphere and lower stratosphere, this channel will be less important. The reaction proceeds through the H abstraction exclusively from chloromethyl group within the whole range of 200–360K, which may be relevant to atmospheric chemistry. Our results show that the variational effect is small and the small-curvature tunneling effect is only important in the lower temperature range. Agreement between the calculated and experimental data available at 298K is good.

Isomeric C5H11Si+ ions from the trimethylsilylation of acetylene: An experimental and theoretical study

The gas phase offers a unique medium to conduct the electrophilic addition reaction of (CH3)3Si+ (trimethylsilylium ion) to acetylene. However, this deceptively simple reaction displays a remarkable dependence on the gas phase pressure, revealing the interplay of competitive pathways. In FT-ICR mass spectrometry at ca 10−8mbar, the nascent (CH3)3Si+–acetylene complex undergoes a rearrangement process yielding the CH2C(CH3)Si(CH3)2+ ion. This structure has been assigned on the basis of the ion-molecule reactivity displayed by the sampled C5H11Si+ adducts, matching the one of the model ion obtained from 2-(trimethylsilyl)propene. Whereas the absolute values of kinetic rate constants could not discriminate between isomeric species, the branching ratios for competitive addition–elimination channels in the reaction with i-C3H7OH and t-C4H9OCH3 were found to be diagnostic of different structures. The pathways leading from the (CH3)3Si+–acetylene complex primarily formed to the candidate C5H11Si+ isomers have been investigated by ab initio quantum chemical calculations at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311G(2d,p) level. The energy profiles show that the path to the CH2C(CH3)Si(CH3)2+ isomer is associated to the lowest activation energy barrier, below the reactants energy level. The energy released in the (CH3)3Si+–acetylene association process, remaining stored in the complex formed at low pressure, thus allows the isomerization to a species holding the positive charge on electropositive silicon. Interestingly, the most stable of the conceivable isomers, (E)-(CH3)CHCHSi(CH3)2+, is not accessed because of an activation energy barrier protruding above the reactants energy level. The combined information of ion-molecule reactivity and ab initio calculations of potential isomers and rearrangement pathways has thus afforded a comprehensive view of the (CH3)3Si+ addition reaction to acetylene under various pressure regimes.

The OH-initiated atmospheric oxidation of divinyl sulfoxide: A theoretical investigation on the reaction mechanism

The potential energy surfaces for the OH+divinyl sulfoxide reaction in the presence of O2/NO are theoretically characterized at the CCSD(T)/6-311+G(d,p)//BH&HLYP/6-311++G(d,p)+ZPE level of theory. Various possible pathways including the direct hydrogen abstraction channels and the addition–elimination channels are considered. The calculations show that the exclusive feasible entrance channel is the formation of adduct CH2(OH)CHS(O)CHCH2 (IM1) in the initial reaction pathways. In the atmosphere, the newly formed adduct IM1 can further react with O2/NO to form the dominant products HCHO+C(O)HS(O)CHCH2 (P9). The calculated results confirm the experimental studies.

Kinetics and mechanism of the reaction of fluorine atoms with trifluoroacetic acid

The kinetics of the reaction between fluorine atoms and trifluoroacetic acid has been studied experimentally at T=258–343K. The overall reaction rate constant decreases with temperature: k1(T)=1.09×10−12 exp((+1034K)/T) cm3molecule−1s−1. The potential energy surface of the reaction has been studied using quantum chemistry. The results were used in transition state theory calculations of the temperature dependences of the rate constants of the two channels of the reaction. The abstraction channel ultimately producing HF is dominant at the experimental temperatures; the addition–elimination channel producing CF3 and CF(O)OH becomes important above 1000K.

Computational study on the reaction of CH3SCH2CH3 with OH radical: mechanism and enthalpy of formation

The reaction mechanism of CH3SCH2CH3 with OH radical is studied at the CCSD(T)/6-311+G(3df,p)//MP2/6-31+G(2d,p) level of theory. Three hydrogen abstraction channels, one substitution process and five addition–elimination channels are identified in the title reaction. The result shows hydrogen abstraction is dominant. Substitution process and addition–elimination reactions may be negligible because of the high barrier heights. Enthalpies of formation [ $$ \Updelta_{f} H_{(298.15{\text{K}})}^{o} $$ ] of the reactants and products are evaluated at the CBS-QB3, G3 and G3MP2 levels of theory, respectively. It is found that the calculated enthalpies of formation by the aforementioned three methods are in consistent with the available experimental data. Rate constants and branching ratios are estimated by means of the conventional transition state theory with the Wigner tunneling correction over the temperature range of 200–900 K. The calculation shows that the formations of P1 (CH2SCH2CH3 + H2O) and P2 (CH3SCHCH3 + H2O) are major products during 200–900 K. The three-parameter expressions for the total rate constant is fitted to be $$ k_{\text{total}} = 1.45 \times 10^{ - 21} T^{3.24} \exp ( - 1384.54/T) $$ cm3 molecule−1 s−1 from 200 to 900 K.

Reaction of Cl with CF 3CH 2OCHO: A mechanistic and kinetic study

The mechanism and kinetics of the reaction CF 3CH 2OCHO + Cl are investigated theoretically by using a dual-level direct dynamics method. The geometries and frequencies are calculated at the B3LYP/6-311G(d,p), B3LYP/6-311+G(d,p), and MP2/6-31G(d,p) levels, and high-level single-point energy calculations are performed at the BMC-CCSD level. It is shown that the reaction proceeds exclusively via two hydrogen abstraction channels, while the Cl addition–elimination channel is unfavorable. Furthermore, the rate constants of the two H-abstraction channels are evaluated by means of canonical variational transition-state theory (CVT) with the small-curvature tunneling (SCT) correction over a temperature range of 200–1000 K. The overall rate constant at room temperature is in good agreement with the experimental value. This study show that the formyl-H-abstraction channel is the major reaction pathway at low temperatures, while as the temperature increases, methylene-H-abstraction channel becomes more important and competes with the former. The three parameter rate-temperature expression is fitted to be k = 0.17 × 10 −20 T 3.13exp(−23.9/ T) cm 3 molecule −1 s −1 (200–1000 K). The present study may assist in further laboratory work.

Kinetics of the Gas-Phase Reaction of OH with Chlorobenzene

The kinetics of the reaction of hydroxyl radicals with chlorobenzene was studied experimentally using a pulsed laser photolysis/pulsed laser induced fluorescence technique over a wide range of temperatures, 298-670 K, and at pressures between 13.33 and 39.92 kPa. The bimolecular rate constants demonstrate different behavior at low and high temperatures. At room temperature, T = 298.8 +/- 1.5 K, the rate constant is equal to (6.02 +/- 0.34) x 10(-13) cm3 molecule(-1) s(-1); at high temperatures (474-670 K), the rate constant values are significantly lower and have a positive temperature dependence that can be described by an Arrhenius expression k1(T) = (1.01 +/- 0.35) x 10(-11) exp[(-2490 +/- 170 K)/T] cm3 molecule(-1) s(-1). This behavior is consistent with the low-temperature reaction being dominated by reversible addition and the high-temperature reaction representing abstraction and addition-elimination channels. The potential energy surface of the reaction was studied using quantum chemical methods, and a transition state theory model was developed for all reaction channels. The temperature dependences of the high-temperature rate constants obtained in calculations using the method of isodesmic reactions for transition states (IRTS) and the CBS-QB3 method are in very good agreement with experiment, with deviations smaller than the estimated experimental uncertainties. The G3//B3LYP-based calculated rate constants are in disagreement with the experimental values. The IRTS-based model was used to provide modified Arrhenius expressions for the temperature dependences of the rate constant for the abstraction and addition-elimination (Cl replacement) channels of the reaction.

Theoretical study and rate constant calculation for the O(3P) + C2H5CN reaction

The complicated microscopic reaction mechanisms of O(3P) with C2H5CN on the ground electronic state energy surface have been investigated at the G3(MP2) level of theory based on the geometric parameters optimized at the B3LYP/6-311 + G(d, p) level. Two kinds of H-abstraction and addition–elimination channels are considered, namely methylene-H abstraction, methyl-H abstraction, C-addition/elimination and N-addition/elimination. The kinetics of the title reaction have been studied using the TST and multichannel RRKM methodologies over a wide temperature range of 200–2000 K. The results show that the methylene-H abstraction process is predominant for the whole reaction. With an increase of temperature, H-abstraction from the methyl position channel should be taken into account. The C-addition/elimination process provides a few contributions to the title reaction compared with two kinds of H-abstraction channels over the whole temperature region and the N-addition/elimination channel can be negligible due to the high entrance barrier and unstable products.

Oxidation pathways in the reaction of diacetylene with OH radicals

We present a portion of the potential energy surface of the reaction of diacetylene with OH radicals, calculated using RQCISD(T) and two basis set extrapolation schemes. Based on this surface, we performed calculations of the rate coefficients using an RRKM/master-equation formalism. After a small (1 kcal/mol) adjustment to the energy barrier of the association reaction, our calculated rate coefficients of the high-pressure limit agree very well with previous direct measurements. However, our calculations at high temperatures are considerably smaller than the values inferred in previous studies. The non-Arrhenius behavior and significant pressure dependence of the rate coefficients above 800 K is due to the competition between stabilization, abstraction and addition–elimination channels. At low temperatures, the reaction proceeds mostly to the addition products, as well as to CO and propargyl. Above 1200 K, direct hydrogen abstraction and production of H atoms become important.

Electronic structure study of the initiation routes of the dimethyl sulfide oxidation by OH

In the present work the potential energy surface (PES) corresponding to the different initiation routes of the oxidation mechanism of DMS by hydroxyl radical in the absence of O(2) has been studied, and connections among the different stationary points have been established. Single-point high level electronic structure calculations at lower level optimized geometries have been shown to be necessary to assure convergence of energy barriers and reaction energies. Our results demonstrate that the oxidation of DMS by OH turns out to be initiated via three channels: a hydrogen abstraction channel that through a saddle point structure finally leads to CH(3)SCH(2) + H(2)O, an addition-elimination channel that firstly leads to an adduct complex (AD) and then via an elimination saddle point structure finally gives CH(3)SOH and CH(3) products, and a third channel that through a concerted pathway leads to CH(3)OH and CH(3)S. The H-abstraction and the addition-elimination channels initiate by a common pathway that goes through the same reactant complex (RC). Our theoretical results agree quite well with the branching ratios experimentally assigned to the formation of the different products. Finally, the calculated equilibrium constants of the formation of the complex AD and the hexadeuterated complex AD from the corresponding reactants, as a function of the temperature, are in good accordance with the experimental values.

Correlation of the rates of solvolysis of phenyl chlorothionoformate and phenyl chlorodithioformate

The specific rates of solvolysis of phenyl chlorothionoformate (PhOCSCl) are remarkably similar to those previously reported for phenyl chlorothioformate (PhSCOCl). When analyzed using the extended Grunwald-Winstein equation over the usual range of solvent types, these solvolyses show essentially identical divisions into the solvents favoring the addition-elimination channel and those favoring the ionization channel. The introduction of one sulfur caused a partial shift away from the addition-elimination pathway, which was dominant over the full range of solvents for phenyl chloroformate (PhOCOCl). Consistent with these results, introduction of the second sulfur within phenyl chlorodithioformate (PhSCSCl) leads to a completion of this shift, such that an extended Grunwald-Winstein treatment of the specific rates of solvolysis now shows the ionization pathway to be dominant over the full range of solvents.Key words: Grunwald-Winstein equation, solvent nucleophilicity, solvolysis, phenyl chlorothionoformate, phenyl chlorodithioformate.

Vibrational energy disposal in reactions of fluorine atoms with hydrides of groups III, IV and V

Abstract The HF infrared chemiluminescence from the reactions of F atoms with B 2 H 6 , CH 4 , CH 3 F, CH 2 F 2 , CH 2 Cl 2 , CH 3 ONO. CH 3 NO 2 , NH 3 (and ND 3 ). PH 3 and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH 4 and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF( v ) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH 4 in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH 4 is well established, the combination of all of these data provides absolute rate constants for HF( v ) formation at 300 K. The ND 3 reaction was studied to obtain information on more vibrational levels in order to better estimate the HF( v = 0) and DF( v = 0) components of the ammonia distributions. With NH 3 and ND 3 there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF( v ) distributions are inverted and f v > = 0.60. Differences between the HF( v ) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl 2 secondary reaction also was observed; the HCl( v ) distribution was v 1 : v 2 : v 3 : v 4 : v 5 - 0.47: 0.23: 0.18: 0.08: 0.04.