Lowest Doublet Potential Energy Surface Research Articles

Calculation of the rate constants for hydrogen abstraction reactions by Hydroperoxyl radical from Methanol, and the investigation of stability of CH3OH.HO2 complex

Master equation (ME) with Lennard-Jones potential utilized to simulate the collision between CH3OH and HO2 radical in the presence of bath gas. The reaction mechanism explored through the lowest doublet potential energy surface at CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level of theory that is barrierless and forms shallow energized intermediate at the entrance channel. The investigation of the fractional population showed that lifetime of CH3OH.HO2 intermediate (INT*) is fairly short due to its shallow depth, and at the low temperature, most reaction takes place by re-dissociation back to reactants and also when the pressure is high enough, the INT* is thermalized and comes into equilibrium with the bath gas, so that equilibrium constant is well-defined that is in accordance with Christiansen’s results. The rate constant of products agrees with reported experimental values in literature. ME predicts the formation of CH2OH + H2O2 and CH3O + H2O2 as the major products; these results agree with previous studies.

A theoretical study on the kinetics of multichannel Multiwell reaction of H2S(1A1) with HO2(2A′′)

The mechanism and kinetics of the reaction of hydrogen sulphide (H2S(1A1)) with hydroperoxyl radical (HO2(2A″)) on the lowest doublet potential energy surface have been theoretically studied. The potential energy surface for possible pathways has been investigated by employing Complete Basis Set (CBS), DFT, and CCSD(T) methods. Three possible pathways are suggested for the title reaction. The most probable entrance channel consists of formation of a hydrogen-bonded pre-reaction complex (vdw1) and two energised intermediates. Multichannel RRKM-Steady State Approximation and CVT calculations have been carried out to compute the rate constants over a broad range of temperature from 200 K to 3000 K to cover the atmospheric and combustion conditions and pressure from 0.1 to 2000 Torr. No sign of pressure dependence was observed for the title reaction over the stated range of pressure. We have shown that the major products of the title reaction are H2O2 and SH while at higher temperatures, formation of the other products such as H2O, HOS, HSOH and OH are feasible, too. Our calculated overall rate constant is in agreement with the reported experimental data in the literature.

Theoretical Investigation of the Radical–Radical Reaction of O(3P) + C2H3 and Comparison with Gas-Phase Crossed-Beam Experiments

Herein, we present an ab initio study of the prototypal radical-radical reactions of ground-state atomic oxygen [O((3)P)] with the vinyl (C2H3) radical using density functional theory and a complete basis set model. Two distinctive pathways on the lowest doublet potential energy surfaces (PESs) were predicted to be in competition: addition and abstraction. The barrierless addition of O((3)P) to the hydrocarbon radicals leads to energy-rich intermediate formation followed by subsequent isomerization and decomposition to yield various products: CH2CO (ketene) + H, CO + CH3, C2HOH (acetylenol) + H, (3,1)CCHOH + H, H2O + C2H, (3,1)CH2 + HCO, H2CO (formaldehyde) + CH, C2H2 (acetylene) + OH, and (3,1)CCH2 + OH. The competing but minor H-atom abstraction mechanisms produce C2H2 + OH and (1,3)CCH2 + OH. The optimized structures of the reactants, products, intermediates, and transition states and the reaction mechanisms were obtained on the lowest doublet PESs. The major pathway was predicted to be the formation of CH2CO + H through the low-barrier, single-step cleavages of the addition intermediates. The Levine-Bernstein prior method, statistical surprisal approach, and microcanonical Rice-Ramsperger-Kassel-Marcus theory were applied to deduce the energy distributions of H atoms and OH products and quantitative rate constants. On the basis of the statistical theory and the population analysis, the predicted energy distributions were compared to the kinetic energy release of H and the preferential population of the Π(A') component of OH products reported in recent gas-phase crossed-beam investigations (Park, M. J.; Jang, S. C.; Choi, J. H. J. Chem. Phys. 2012, 137, 204311), and their kinetic and dynamic characteristics were discussed.

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas-Phase Radical-Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas-phase radical-radical dynamics of the reaction of ground-state atomic oxygen [O((3)P), from the photodissociation of NO(2)] with secondary isopropyl radicals [(CH(3))(2)CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O((3)P)+(CH(3))(2)CH→C(3)H(6) (propene)+OH, is examined by high-resolution laser-induced fluorescence spectroscopy in crossed-beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X(2)Π) products with low- and high-N'' components in a ratio of 1.25:1. No significant spin-orbit or Λ-doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS-QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential-energy surface: direct abstraction, giving the dominant low-N'' components, and formation of short-lived addition complexes that result in hot rotational distributions, giving the high-N'' components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast-flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O((3)P) with primary and tertiary hydrocarbon radicals allows molecular-level discussion of the reactivity and mechanism of the title reaction.

H2NSi radical: structures, isomerization pathways and electronic states characterization

Using state-of-the-art theoretical methods, the stable isomers of H2NSi which are relevant for astrophysics, astrochemistry and ammonia silicon surface chemistry, were investigated. These computations are performed using configuration interaction ab initio methods and the aug-cc-pVQZ and cc-pVQZ basis sets. Calculations confirm the existence of three stable isomers: H2NSi, trans-HSiNH and H2SiN. The intramolecular isomerization and the H-abstraction reaction pathways on the lowest doublet potential energy surfaces are given. Insight into the pattern of the lowest doublet and quartet electronic states of these molecular species are also presented.

Interaction of Ga3 cluster with molecular hydrogen: combined DFT and CCSD(T) theoretical study

We have explored the lowest doublet and quartet potential energy surfaces (PES) for the reaction of gallium trimer with H2. This reaction was studied experimentally by Margrave and co-workers in a noble gas matrix. The detailed reaction paths ending up with the low-energy Ga3H2 hydride isomers have been predicted based on the high level ab initio coupled-cluster calculations (CCSD(T)) with large basis set. We have found that the reaction occuring on the lowest doublet PES is described by the activation barrier for H2 cleavage of about 15 kcal/mol, consistent with experiment. In the most stable Ga3H2 hydride structure, whose formation is exothermic by 15 kcal/mol, both H atoms assume three-fold bridged positions. The diterminal planar structure of Ga3H2, proposed experimentally from the observed IR spectra, is found to be only 1 kcal/mol less stable than the dibridged form.

Theoretical investigations of the cyanogen anion

Large ab initio calculations are performed on neutral cyanogen (C 2N 2) and its negatively charged ion ( C 2 N 2 - ). Four stable isomers are found on the lowest doublet potential energy surface of the anion: trans NCCN − X ∼ 2B u, trans CNCN − X ∼ 2A′, trans CNNC − X ∼ 2B u and trans NNCC − X ∼ 2A′. A set of spectroscopic data is derived for them at the RCCSD(T)/aug cc-pVTZ level of theory. The vertical and the adiabatic electron affinities are also deduced. High level ab initio calculations show that no metastable electronically excited states may exist either in the doublet or in the quartet states manifolds of C 2 N 2 - species.

Production of HNC from the CH(X 2Π) + NH(X 3Σ −) reaction: Direct dynamics study

The production mechanism and dynamics of the HNC molecule from the radical–radical CH + NH reaction on the lowest doublet potential energy surface has been theoretically studied using electronic structure calculations. We have applied a direct dynamics simulation technique at the B3LYP/6-311++G(d,p) level of theory. The accuracy of the B3LYP-level potential energy surface was systematically confirmed from the comparison to more accurate MRCI-level and CCSD(T)-level calculations. More than 250 classical trajectories were integrated and we have found that the CH + NH reaction somewhat favors the HNC + H production channel over the HCN + H production despite that the energy level of HCN + H is lower than that of HNC + H. Detailed reaction mechanisms will be discussed on the basis of the trajectory results.

A Theoretical Investigation of the Gas‐Phase Oxidation Reaction of the Saturated tert‐Butyl Radical

The radical-radical reaction mechanisms and dynamics of ground-state atomic oxygen [O(3P)] with the saturated tert-butyl radical (t-C4H9) are investigated using the density functional method and the complete basis set model. Two distinctive reaction pathways are predicted to be in competition: addition and abstraction. The barrierless addition of O(3P) to t-C4H9 leads to the formation of an energy-rich intermediate (OC4H9) on the lowest doublet potential energy surface, which undergoes subsequent direct elimination or isomerization-elimination leading to various products: C3H6O + CH3, iso-C4H8O + H, C3H7O + CH2, and iso-C4H8 + OH. The respective microscopic reaction processes examined with the aid of statistical calculations, predict that the major addition pathway is the formation of acetone (C3H6O) + CH3 through a low-barrier, single-step cleavage. For the direct, barrierless H-atom abstraction mechanism producing iso-C4H8 (isobutene) + OH, which was recently reported in gas-phase crossed-beam investigations, the reaction is described in terms of both an abstraction process (major) and a short-lived addition dynamic complex (minor).

Crossed-beam radical-radical reaction dynamics of O(P3)+C3H3→H(S2)+C3H2O

The radical-radical oxidation reaction, O(3P)+C3H3 (propargyl)-->H(2S)+C3H2O (propynal), was investigated using vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed-beam configuration, together with ab initio and statistical calculations. The barrierless addition of O(3P) to C3H3 is calculated to form energy-rich addition complexes on the lowest doublet potential energy surface, which subsequently undergo direct decomposition steps leading to the major reaction products, H+C3H(2)O (propynal). According to the nascent H-atom Doppler-profile analysis, the average translational energy of the products and the fraction of the average transitional energy to the total available energy were determined to be 5.09+/-0.36 kcal/mol and 0.077, respectively. On the basis of a comparison with statistical prior calculations, the reaction mechanism and the significant internal excitation of the polyatomic propynal product can be rationalized in terms of the formation of highly activated, short-lived addition-complex intermediates and the adiabaticity of the excess available energy along the reaction coordinate.

A study of the radical-radical reaction dynamics of O(P3)+t-C4H9→OH+iso-C4H8

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with t-butyl radicals (t-C4H9) in the gas phase were investigated using high-resolution laser spectroscopy in a crossed-beam configuration, together with ab initio theoretical calculations. The radical reactants, O(3P) and t-C4H9, were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of the precursor, azo-t-butane, respectively. A new exothermic channel, O(3P)+t-C4H9 --> OH+iso-C4H8, was identified and the nascent rovibrational distributions of the OH (X 2Pi: upsilon" = 0,1,2) products were examined. The population analyses for the two spin-orbit states of F1(2Pi32) and F2(2Pi12) showed that the upsilon" = 0 level is described by a bimodal feature composed of low- and high-N" rotational components, whereas the upsilon" = 1 and 2 levels exhibit unimodal distributions. No noticeable spin-orbit or Lambda-doublet propensities were observed in any vibrational state. The partitioning ratio of the vibrational populations (Pupsilon") with respect to the low-N" components of the upsilon" = 0 level was estimated to be P0:P1:P2 = 1:1.17+/-0.24:1.40+/-0.11, indicating that the nascent internal distributions are highly excited. On the basis of the comparison of the experimental results with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major, direct abstraction process leading to the inversion of the vibrational populations, and the minor, short-lived addition-complex process responsible for the hot rotational distributions. After considering the reaction exothermicity, the barrier height, and the number of intermediates along the addition reaction pathways on the lowest doublet potential energy surface, the formation of CH3COCH3(acetone)+CH3 was predicted to be dominant in the addition mechanism.

Reduced-dimensionality and direct trajectory calculations for the C( 3PJ) + NH 2( 2B 1) reaction

Nonadiabatic transitions through spin–orbit interaction for the C( 3 P J ) + H 2( 2B 1) reaction were investigated by ab initio electronic structure calculations and quantum reactive scattering calculations. It has been found that the reactivity for the J = 0 and J = 1 states is quite large. Ab initio direct trajectory calculations on the lowest doublet potential energy surface have also been carried out in order to understand the HNC production mechanism. We have found that HNC is mostly produced via direct mechanism, in which the H elimination occurs directly from the CNH 2 intermediate, initially formed by the addition of C to NH 2.

High-level ab initio studies of unimolecular dissociation of the ground-state N3 radical

A comprehensive study of the unimolecular dissociation of the N(3) radical on the ground doublet and excited quartet potential energy surfaces has been carried out with multireference single and double excitation configuration interaction and second-order multireference perturbation methods. Two forms of the N(3) radical have been located in the linear and cyclic region of the lowest doublet potential energy surface with an isomerization barrier of 62.2 kcal/mol above the linear N(3). Three equivalent C(2v) minima of cyclic N(3) are connected by low barrier, meaning the molecule is free to undergo pseudorotation. The cyclic N(3) is metastable with respect to ground state products, N((4)S)+N(2), and dissociation must occur via intersystem crossing to a quartet potential energy surface. Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic N(3) minima. This strongly suggests that cyclic N(3) possesses a long collision-free lifetime even if formed with substantial internal excitation.

Exploring the dynamics of hydrogen atom release from the radical-radical reaction of O(3P) with C3H5.

The gas-phase radical-radical reaction dynamics of O(3P) + C3H5 --> H(2S) + C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet-laser induced fluorescence spectroscopy in the Lyman-alpha region centered at 121.6 nm. With the aid of the CBS-QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H + C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H + C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition.

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.

Radical–radical reaction dynamics: The OH formation in the reaction of O(3P) with propargyl radical, C3H3

The radical–radical reaction dynamics of ground-state atomic oxygen [O(3P)] with propargyl radicals (C3H3) has first been investigated by applying laser induced fluorescence spectroscopy in a crossed beam configuration, together with ab initio calculations. A new exothermic channel of O(3P)+C3H3→C3H2+OH was identified and the nascent distributions of OH reaction product in the ground vibrational state (X 2Π:υ″=0) showed substantial rotational excitations with an unusual bimodal feature composed of the low- and high-N″ components. No spin–orbit propensities were observed, whereas the averaged ratios of Π(A′)/Π(A″) were determined to be 0.60±0.28. With the aid of the CBS-QB3 level of ab initio theory it is predicted that on the lowest doublet potential energy surface the reaction proceeds through the addition complexes formed through the barrierless addition of O(3P) to C3H3, and that the counterpart of C3H2 of the probed OH product is cyclopropenylidene (1c-C3H2). On the basis of the comparison with statistical prior and rotational surprisal analyses, the ratio of population partitioning for the low- and high-N″ regimes is estimated to be about 1:2 and the reaction is described in terms of two competing addition-complex mechanisms: a major short-lived dynamic complex in the high-N″ regime and a minor long-lived statistical complex in the low-N″ regime.

A theoretical study of the reaction of O(3P) with an allyl radical C3H5

Ab initio calculations of the reaction of ground-state atomic oxygen [O(3P)] with an allyl radical (C3H5) have been carried out using the density functional method and the complete basis set model. On the calculated lowest doublet potential energy surface, the barrierless association of O(3P) to C3H5 forms three energy-rich addition intermediates, which are predicted to undergo subsequent isomerization and decomposition steps leading to various products: C3H4O+H, CH2O+C2H3, C2H4+CHO, C2H2O+CH3, C2H5+CO, C3H4+OH, and C2H4O+CH. The respective reaction mechanisms through the three addition intermediates are presented, and it has been found that the barrier height, reaction enthalpy, and the number of intermediates involved along the reaction coordinate are of extreme importance in understanding such reactive scattering processes. With the aid of Rice–Ramsperger–Kassel–Marcus calculations, the major reaction pathway is predicted to be the formation of acrolein (C3H4O)+H, which is consistent with the previous gas-phase bulk kinetic experiment performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. For the minor C3H4+OH channel, which has been newly found in the recent crossed beam investigations, a second barrierless, direct H-atom abstraction from the central carbon of C3H5 is calculated to compete with the addition process due to the little C–H bond dissociation energy and the formation of a stable allene product. The dynamic and kinetic characteristics of the reaction mechanism are discussed on the basis of the comparison of prior statistical calculations to the nascent internal distributions of the observed OH product.

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.

Structure and charge transfer dynamics of the (Ar–N2)+ molecular cluster

In this paper we have investigated the interaction potential and the charge transfer processes at low collision energies in the (Ar–N2)+ system. The angular dependence of the lowest doublet potential energy surfaces (PES), correlating with Ar+(2Pj)–N2 and Ar–N2+(2Σ,2Π), has been given in terms of spherical harmonics, while the dependence on the intermolecular distance has been represented by proper radial coefficients. Such coefficients, which account for van der Waals, induction, charge transfer, and electrostatic contributions, have been predicted by empirical correlation formulas. The PES so obtained have been employed to calculate cross sections for the charge transfer process Ar++N2→Ar+N2+ at low collision energy (E⩽2 eV). A good agreement between calculated and experimental cross sections is obtained by assuming that the duration of the nonadiabatic transition has to match the time required for the molecular rearrangement into the final vibrational state. As a consequence the efficient formation of product ions into specific vibrational states is limited to well defined ranges of impact parameters. This treatment leads to a unified description of the major experimental findings.

Potential Energy Surfaces for F−H2 and Cl−H2: Long-Range Interactions and Nonadiabatic Couplings

The intermediate and long-range behavior of the three lowest doublet potential energy surfaces for the F(2Pj)-H2 and Cl(2Pj)-H2 systems has been studied, using a harmonic expansion of the potential, where the dependence on the relative orientation of the half-filled orbital of the open-shell atom and the molecular axis has been given in terms of bipolar spherical harmonics, whereas the coefficients modulate the effect of the variation of the intermolecular distance. The contribution of van der Waals, electrostatic, and charge-transfer interactions to the strength and the intermolecular distance dependence of each radial term are derived from previous molecular beam scattering experiments and from correlation formulas. The latter provide the link of these quantities to basic properties of the interacting partners. Besides describing elastic and inelastic channels, these surfaces also provide accurate information on the entrance channel for reactions.