Major Reaction Channel Research Articles

Ab initio direct dynamics studies on the reaction Br + SiH 4

The dynamical properties of the reaction Br + SiH 4 → products have been investigated theoretically. The optimized geometries, frequencies and energies of reactants, transition states and products are calculated at the MP2/6-311G(d,p) and MP2/6-311G(2d,p) levels. The energy profiles are refined by performing a series of single-point energy calculations at the MP4(SDQ)/6-311+G(2df,2p)//MP2/6-311G(d,p) level. The rate constants are calculated by using canonical variational transition state theory (CVT) incorporating with the small-curvature tunneling (SCT) correction in the temperature range 200–3000 K, and the results are in excellent agreement with the available experimental values. H-abstraction reaction channel is the major channel for the title reaction.

Ultrafast Electron Diffraction: Structural Dynamics of the Elimination Reaction of Acetylacetone

Ultrafast electron diffraction is reported for the elimination reaction of acetylacetone. For this multipath reaction, upon 266 nm excitation, it is revealed that the major reaction channel is fragmentation from the β carbon, producing OH and 3-penten-2-on-4-yl radicals, with a time constant of 247 ± 34 ps. We determine the structures involved and the changes in bond lengths. These structure-dynamics correlations in different states provide a mechanistic description of the complex reaction landscape.

Theoretical investigation of reaction mechanism for CH 2(X̃ 3B 1) with NO radical

CH 2 radical is an important species in the process of hydrocarbon burning and NO is the major environmental pollutant, so the study about them has practical significance. In this paper the reaction of CH 2(X̃ 3B 1) with NO radicals has been studied theoretically using ab initio MO method. The structure parameters, vibrational frequencies and energy of each stationary point for all reaction channels are given using the G2MP2 method, and the particular reaction mechanism has been given by the potential energy surface which is drawn according to the relative energy. The reaction takes place as the NO radical attacks breadthwise the center of CH 2(X̃ 3B 1) to form an energy-rich intermediate H 2CNO* (IM1), in turn, the products have been produced by complicated isomerization or decomposition reaction. The heat of reaction for each channel is in agreement with the experiment result. The production of CO+NH 2 and H+HNCO are the major reaction channels according to the potential energy surface, and this result is in accordance with the experiment.

A combined crossed beam and theoretical investigation of O(3P) + C3H3 --> C3H2 + OH.

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with propargyl radicals (C3H3) has first been investigated in a crossed beam configuration. The radical reactants O(3P) and C3H3 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor propargyl bromide, respectively. A new exothermic channel of O(3P) + C3H3 --> C3H2 + OH was identified and the nascent distributions of the product OH in the ground vibrational state (X 2Pi:nu" = 0) showed bimodal rotational excitations composed of the low- and high-N" components without spin-orbit propensities. The averaged ratios of Pi(A')/Pi(A") were determined to be 0.60 +/- 0.28. With the aid of ab initio theory it is predicted that on the lowest doublet potential energy surface, the reaction proceeds via the addition complexes formed through the barrierless addition of O(3P) to C3H3. The common direct abstraction pathway through a collinear geometry does not occur due to the high entrance barrier in our low collision energy regime. In addition, the major reaction channel is calculated to be the formation of propynal (CHCCHO) + H, and the counterpart C3H2 of the probed OH product in the title reaction is cyclopropenylidene (1c-C3H2) after considering the factors of barrier height, reaction enthalpy and structural features of the intermediates formed along the reaction coordinate. On the basis of the statistical prior and rotational surprisal analyses, the ratio of population partitioning for the low- and high-N" is found to be about 1:2, and the reaction is described in terms of two competing addition-complex mechanisms: a major short-lived dynamic complex and a minor long-lived statistical complex. The observed unusual reaction mechanism stands in sharp contrast with the reaction of O(3P) with allyl radical (C3H5), a second significant conjugated hydrocarbon radical, which shows totally dynamic processes [J. Chem. Phys. 117, 2017 (2002)], and should be understood based upon the characteristic electronic structures and reactivity of the intermediates on the potential energy surface.

Ruthenium diimine complexes in unusual charge states formed in collisional electron transfer

Ruthenium diimine complexes, Ru(L) 3 2+ (L=bipy=2,2′-bipyridine or phen=1,10-phenanthroline), were formed in the gas phase by electrospray ionization and accelerated through a 50-kV electrostatic potential. In collisions between Ru(L) 3 2+ and O 2, fragment ions were produced in addition to Ru(L) 3 + and Ru(L) 3 3+ formed in collisional electron transfer. Cross-sections for the electron stripping and electron capture reactions are 10±0.2 Å 2 and 1.7±0.1 Å 2, respectively, for L=bipy and 3.8±0.2 Å 2 and 0.8±0.1 Å 2, respectively, for L=phen. In collisions between Ru(L) 3 2+ and Na electron capture was the major reaction channel and resulted in Ru(L) 3 + as well as Ru(L) 3 − (as a result of multiple-collision conditions). Cross-sections are of the order of 100 Å 2 for the formation of one-electron reduced Ru(L) 3 + ions. The kinetic energy released by the dissociation of Ru(L) 3 2+ and Ru(L) 3 3+ into Ru(L) 2 +/2+ and L + after collisional activation was measured to be 1.5 eV for Ru(bipy) 3 2+, 3.7 eV for Ru(phen) 3 2+, 8 eV for Ru(bipy) 3 3+, and 6 eV for Ru(phen) 3 3+.

Ab initio direct dynamics studies on the reaction of H atom with CH3CH2Cl.

The multiple channel reaction H + CH(3)CH(2)Cl --> products has been studied by the ab initio direct dynamics method. The potential energy surface information is calculated at the MP2/6-311G(d,p) level of theory. The energies along the minimum energy path are further improved by single-point energy calculations at the PMP4(SDTQ)/6-311+G(3df,2p) level of theory. For the reaction, four reaction channels (one chlorine abstraction, one alpha-hydrogen abstraction, and two beta-hydrogen abstractions) have been identified. The rate constants for each reaction channel are calculated by using canonical variational transition state theory incorporating the small-curvature tunneling correction in the temperature range 298-5000 K. The total rate constants, which are calculated from the sum of the individual rate constants, are in good agreement with the experimental data. The calculated temperature dependence of the branching fractions indicates that for the title reaction, H-abstraction reaction is the major reaction channel in the whole temperature range 298-5000 K.

Dual-level direct dynamics studies for the hydrogen abstraction reaction of 1,1-difluoroethane with O(3P)

Abstract We present dual-level direct dynamics calculations for the CH 3 CHF 2 + O( 3 P) hydrogen abstraction reaction in a wide temperature range, based on canonical variational transition-state theory including small curvature tunneling corrections. For this reaction, three distinct transition states, one for α-abstraction and two for β-abstraction, have been located. The potential energy surface information is obtained at the MP2(full)/6-311G(d,p) level of theory, and higher-level single-point calculations for the stationary points are preformed at several levels, namely QCISD(T)/6-311+G(3df,3pd), G2, and G3 using the MP2 geometries, as well as at the G3//MP4SDQ/6-311G(d,p) level. The energy profiles are further refined with the interpolated single-point energies method at the G3//MP2(full)/6-311G(d,p) level. The total rate constants match the experimental data reasonable well in the measured temperature range 1110–1340 K. It is shown that at low temperature α-abstraction may be the major reaction channel, while β-abstraction will have more contribution to the whole reaction rate as the temperature increases.

Ab Initio Direct Dynamics Studies on the Reactions of H Atoms with CCl4 and CHCl3

By means of ab initio direct dynamics method, the reactions H + CCl4 → CCl3 + HCl (1) and H + CHCl3 → products (2) have been investigated theoretically to reveal their dynamical properties. The minimum energy paths (MEPs) of both reactions are calculated at the BH&H-LYP/6-311G(d,p) level, and the energies along the MEPs are further refined at the PMP4/6-311+G(3df,2p) (single-point) level. For reaction 2, both reaction channels, H-abstraction and Cl-abstraction, have been identified. Thus, the rate constants for each reaction channel as well as reaction 1 are calculated by the canonical variational transition state theory incorporating the small-curvature tunneling correction in the temperature range 200−5000 K. For reaction 1, the theoretical rate constants are in good agreement with the experimental values over the measured temperature range. For reaction 2, the total rate constants, which are calculated from the sum of the individual rate constants, are in excellent agreement with the experimental ones, and the temperature dependence of the branching ratios indicate that, at low-temperature range (T < 667 K), H-abstraction is the major reaction channels, whereas Cl-abstraction channels will significantly contribute to the whole reaction rate as the temperature increases.

Mechanistic investigation on the multi-channel reaction of Cl+CH 2CO

All the geometries, vibrational frequencies and energies of different stationary points involved in the multi-channel reaction of the CH 2CO with Cl are calculated by B3LYP/6-311++G** and the results agree with the experimental values. The vibrational mode analysis is used to elucidate the relationships of the reactant, the transition states, intermediate and the products. Through the analysis, the major reaction channel and the minor reaction channel are confirmed. A new study method of analyzing reaction mechanism is presented.

Selectivity in the Chemistry of Oxygen‐Centered Radicals — The Formation of Carbon—Oxygen Bonds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Kinetics and Mechanism of the OH + C6H6 Reaction: A Detailed Analysis with First-Principles Calculations

The capability of various high-level DFT and ab initio MO methods to predict molecular and energetic parameters has been critically tested for the OH radical reaction with benzene. G3 theory proved to be the most accurate in estimating reaction barriers and enthalpies. For the first time, the molecular structure, stabilization energy and nature of the bonding interactions of the [C6H6···OH] prereaction complex have been characterized. Accurate thermochemistry for major reaction channels was established by making combined use of experimental and theoretical parameters. A multistep kinetic model for the OH-addition channel was proposed, and the effects of T, P, and reaction time on the apparent rate constants were evaluated by weak collision master equation/RRKM analysis. Available experimental kinetic data for all relevant reactions are critically analyzed and correlated with modeled effective rate constants.

Direct ab initio dynamics calculations on the rate constants for the hydrogen-abstraction reaction of C 2 H 5 F with O ( 3 P)

The hydrogen-abstraction reaction C2H5F+O → C2H4F+OH has been studied by a dual-level direct dynamics method. For the reaction, three reaction channels, one for α-abstraction and two for β-abstraction, have been identified. The potential-energy surface information is obtained at the MP2(full)/6-311G(d,p) and PMP2(full)/6-311G(3df,3pd) (single-point) levels. By canonical variational transition-state theory, rate constants for each reaction channel are calculated with a small-curvature tunneling correction. The total rate constant is calculated from the sum of the individual rate constants and the temperature dependence of the branching ratios is obtained over a wide range of temperatures from 300 to 5,000 K. The agreement of the rate constants with experiment is good in the experimental temperature range from 1,000 to 1,250 K. The calculated results indicate that at low temperatures α-abstraction is most likely to be the major reaction channel, while β-abstraction channels will significantly contribute to the whole reaction rate as the temperature increases.

Atom-radical reaction dynamics of O(3P)+C3H5→C3H4+OH: Nascent rovibrational state distributions of product OH

The reaction dynamics of ground-state atomic oxygen [O(3P)] with allyl radicals (C3H5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3P) and C3H5 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3P)+C3H5→C3H4+OH was observed and the nascent internal state distributions of the product OH (X 2Π:υ″=0,1) showed substantial bimodal internal excitations of the low- and high-N″ components without Λ-doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3P) with C3H5 is the formation of acrolein (CH2CHCHO)+H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart C3H4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N″-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N″-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process.

Direct evidence for base-mediated decomposition of alkyl hydroperoxides (ROOH) in the gas phase.

The reaction of F(-) with CH(3)OOH has been studied in the gas phase using a tandem flowing afterglow-selected ion flow tube apparatus. The reaction is rapid (k = 1.23 x 10(-9) cm(3) s(-1), 49% efficiency), and formation of HO(-) + CH(2)O + HF is the major reaction channel observed (85%). Isotopic labeling, reactions of F(-) with larger alkyl hydroperoxides, and computational studies demonstrate that the major product ion, HO(-), is formed via a concerted elimination mechanism that appears to be general to all alkyl hydroperoxides possessing an alpha-hydrogen. This mechanism represents a base-mediated decomposition of alkyl hydroperoxides in the gas phase that may have important implications for solution and biochemical reactions. The reverse reaction, CH(3)OO(-) + HF is also efficient (k = 2.43 x 10(-9) cm(3) s(-1)). The major product ensemble HO(-) + CH(2)O + HF (81%) is identical to that of the forward reaction, and represents a novel neutral-catalyzed decomposition of the anion.

Theoretical studies on O( 3P)+CH 2Cl reaction mechanism

The reaction of oxygen atom with chlorinated methyl radical has been studied using density functional theory (DFT) method at 6-311++G ∗∗ level. All the geometries, vibrational frequencies and energies of different stationary points are calculated by B3LYP/6-311++G ∗∗ and the results agree with the experimental values. The vibrational frequencies and modes of the reactant, intermediates, transition states and products have been calculated, the changes of these frequencies and modes analyzed and the changes in the vibrational force constants are assigned. The relationship and the change among them can confirm the mechanism of the reaction and the process of electron transfer. Through the analysis, the major and minor reaction channels are confirmed. A new study method of analyzing reaction mechanism is presented.

Selectivity in the Chemistry of Oxygen-Centered Radicals - The Formation of Carbon-Oxygen Bonds

In recent years, powerful new methods for the generation of alkoxyl radicals under mild and neutral conditions have been developed. This progress has led to a thorough investigation of most O-radical elementary reactions.Today, sufficiently reliable thermodynamic and kinetic data are available either from experimental or from theoretical studies in order to predict alkoxyl radical reactivities and selectivities in synthesis. For instance, alkoxyl radicals readily add to carbon-carbon and carbon-nitrogen double bonds. Due to generally low activation barriers and strongly negative reaction enthalpies, inter- and intramolecular addition reactions proceed under kinetic control and are associated with high rate constants. Nevertheless, intramolecular 5-exo-trig additions, i.e. cyclizations, proceed with an astonishing degree of diastereoselectivity and often provide complementary selectivities if compared to commonly used methods such as the bromine cyclization of alkenols. Therefore, several useful applications of O-radical cyclizations in the synthesis of functionalized tetrahydrofurans have been discovered in the last few years. A second major reaction channel of alkoxyl radicals is associated with the homolysis of a β-C-C bond. This fragmentation proceeds under thermodynamic control and affords a carbonyl compound besides an alkyl radical from the starting alkoxyl radical. Regioselectivities for C-C bond homolysis may be predicted by considering strain release (cyclic carbon framework) and the stability of the newly formed carbon radical (cyclic and open chain carbon skeletons). The third major group of alkoxyl radical-based transformations are connected with homolytic substitutions such as intramolecular 1,5-hydrogen shifts which have been applied with considerable success to remote functionalization reactions. In view of the diversity of alkoxyl radical reactions it is the aim of this review to organize basic principles of this type of chemistry and to present its latest useful application in organic synthesis.

Comparison of the collision-induced dissociation of duplex DNA at different collision regimes: Evidence for a multistep dissociation mechanism

The dissociation mechanism of duplex DNA has been investigated in detail by collision-induced dissociation experiments at different collision regimes. MS/MS experiments were performed either in a quadrupole collision cell (hybrid quadrupole-TOF instrument) or in a quadrupole ion trap with different activation times and energies. In addition to the noncovalent dissociation of the duplex into the single strands, other covalent bond fragmentation channels were observed. Neutral base loss from the duplex is favored by slow activation. In fast activation conditions, however, the major reaction channel is the noncovalent dissociation into single strands, which is highly entropy-favored. Fast activation regimes can favor the entropy-driven noncovalent dissociation, while in slow heating conditions the competition with enthalpy-driven covalent fragmentation can completely hinder the dissociation of the complex. We also evidence that the noncovalent dissociation of DNA duplex is a multistep process involving a progressive unzipping, preferentially at terminal positions. This is proposed to be a general feature for complexes containing a high number of contributing interactions organized at the interface of the ligands. The overall (observed) dissociation kinetics of noncovalent complexes can therefore depend on a complicated mechanism for which a single transition state description of the kinetics is too simplistic.

Reactions of Group V Transition Metal Oxide Cluster Ions with Ethane and Ethylene

Reactions of mass-selected group V transition metal oxide cluster ions (VxOy±, NbxOy±, and TaxOy±) with ethane (C2H6) and ethylene (C2H4) were investigated. The major reaction channels observed during the reactions of C2 hydrocarbons with MxOy+ were association and oxygen transfer. The association channel, MxOyC2Hn+, where n = 4 or 6, was common to most of the group V transition metal oxide cluster cations examined. However, a reaction channel corresponding to the loss of an oxygen atom from the mass-selected metal oxide cluster, producing MxOy-1+, was only observed during the reactions of (V2O5)n+, where n = 1, 2, or 3, with ethane and ethylene and during the reactions of ethylene with Nb2O5+. It is proposed that this reaction pathway is oxygen transfer from the mass-selected metal oxide cluster cation to the neutral hydrocarbon. This oxygen transfer channel is the major pathway observed during the course of reactions of V2O5+, V4O10+, and V6O15+ with ethane and ethylene, but this reaction pathway is minor or nonexistent in the case of reactions of C2H6 and C2H4 with stoichiometrically equivalent niobium and tantalum oxide cluster cations. Additionally, the reactions of MxOy- with C2 hydrocarbons were also examined. In contrast to the cation results, no reaction products were observed in studies of the interaction of group V transition metal oxide cluster anions with ethane and ethylene. The studies reveal that the identity of the metal, charge state, cluster stoichiometry, and geometric structure strongly influence the ability of the metal oxide cluster to transfer an oxygen atom to the neutral C2 hydrocarbon.

Reaction of OH radical with mono‐, di‐, and trichloroacetaldehyde: an ab initio study

AbstractThe hydrogen abstraction reactions between chlorine‐substituted acetaldehydes and OH radicals have been investigated by using ab initio molecular orbital theory. Equilibrium geometries and transition‐state structures have been optimized at the (U)MP2/6‐311G(d,p) level. Activation barriers and heats of reaction for different reaction channels have been estimated from the single‐point calculations at the (U)MP2/6‐311G(2df,2p) level. Three, two, and one hydrogen abstraction channel have been found for the mono‐, di‐, and trichloroacetaldehyde, respectively. At a higher temperature region, hydrogen abstraction from the formyl group is found to be the major reaction channel for all the three chloroacetaldehydes. The effect of halogen substitution on reactivity toward hydrogen abstraction has been discussed. © 2001 John Wiley &amp; Sons, Inc. J Comput Chem 22: 1509–1521, 2001

Ab initio molecular orbital/Rice–Ramsperger–Kassel–Marcus theory study of multichannel rate constants for the unimolecular decomposition of benzene and the H+C6H5 reaction over the ground electronic state

The potential energy surface for the unimolecular decomposition of benzene and H+C6H5 recombination has been studied by the ab initio G2M(cc, MP2) method. The results show that besides direct emission of a hydrogen atom occurring without an exit channel barrier, the benzene molecule can undergo sequential 1,2-hydrogen shifts to o-, m-, and p-C6H6 and then lose a H atom with exit barriers of about 6 kcal/mol. o-C6H6 can eliminate a hydrogen molecule with a barrier of 121.4 kcal/mol relative to benzene. o- and m-C6H6 can also isomerize to acyclic isomers, ac-C6H6, with barriers of 110.7 and 100.6 kcal/mol, respectively, but in order to form m-C6H6 from benzene the system has to overcome a barrier of 108.6 kcal/mol for the 1,2-H migration from o-C6H6 to m-C6H6. The bimolecular H+C6H5 reaction is shown to be more complicated than the unimolecular fragmentation reaction due to the presence of various metathetical processes, such as H-atom disproportionation or addition to different sites of the ring. The addition to the radical site is barrierless, the additions to the o-, m-, and p-positions have entrance barriers of about 6 kcal/mol and the disproportionation channel leading to o-benzyne+H2 has a barrier of 7.6 kcal/mol. The Rice-Ramsperger-Kassel-Marcus and transition-state theory methods were used to compute the total and individual rate constants for various channels of the two title reactions under different temperature/pressure conditions. A fit of the calculated total rates for unimolecular benzene decomposition gives the expression 2.26×1014 exp(−53 300/T)s−1 for T=1000–3000 K and atmospheric pressure. This finding is significantly different from the recommended rate constant, 9.0×1015 exp(−54 060/T) s−1, obtained by kinetic modeling assuming only the H+C6H5 product channel. At T=1000 K, the branching ratios for the formation of H+C6H5 and ac-C6H6 are 29% and 71%, respectively. H+C6H5 becomes the major channel at T⩾1200 K. The total rate for the bimolecular H+C6H5 reaction is predicted to be between 4.5×10−11 and 2.9×10−10cm3 molecule−1 s−1 for the broad range of temperatures (300–3000 K) and pressures (100 Torr–10 atm). The values in the T=1400–1700 K interval, ∼8×10−11 cm3 molecule−1 s−1, are ∼40% lower than the recommended value of 1.3×10−10 cm3 molecule−1 s−1. The recombination reaction leading to direct formation of benzene through H addition to the radical site is more important than H disproportionations at T&amp;lt;2000 K. At higher temperatures the recombination channel leading to o-C6H4+H2 and the hydrogen disproportionation channel become more significant, so o-benzyne+H2 should be the major reaction channel at T&amp;gt;2500 K.