eV Collision Energy Research Articles

Influence of collision energy and reagent vibrational excitation on the dynamics of the reaction H + LiH

We present a detailed quasiclassical trajectory (QCT) study of the dynamics corresponding to the reaction H + LiH proceeding via depletion and H‐exchange paths on a new potential energy surface of the electronic ground state. The effects of collision energy and reagent initial vibrational excitation on the reaction probability and cross sections are studied over a wide range of collision energies. The QCT‐calculated reaction probability and cross sections are in good agreement with previous time‐dependent wave packet results. More importantly, we found that the vibrational excitation of LiH molecule inhibits the LiH depletion reaction, whereas it promotes the H‐exchange reaction. In addition, the differential cross sections calculated for the depletion reaction at different collision energies and excitation states indicate a strong forward scattering of the product molecule H2. © 2013 Wiley Periodicals, Inc.

Theoretical study of stereodynamics for the D′ + DS(v = 0, j = 0) → D′D + S abstraction reaction

Quasiclassical trajectory (QCT) calculations have been performed for the abstraction reaction, D′ + DS(v = 0, j = 0) → D′D + S on a new LZHH potential energy surface (PES) of the adiabatic 3A″ electronic state [Lü et al. 2012 J. Chem. Phys. 136 094308]. The collision energy effect on the integral cross section and product polarization are studied over a wide collision energy range from 0.1 to 2.0 eV. The cross sections calculated by the QCT procedure are in good accordance with previous quantum wave packet results. The three angular distribution functions, P(θr), P(φr), and P(θr, φr), together with the four commonly used polarization-dependent differential cross sections ((2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21−/dωt)) are obtained to gain insight into the chemical stereodynamics of the title reaction. Influences of the collision energy on the product polarization are exhibited and discussed.

The collision energy effect on the stereodynamics of the Ca + HCl→CaCl + H reaction

The stereodynamics of the reaction of Ca + HCl are calculated at three different collision energies based on the potential energy surface [Verbockhaven G et al. 2005 J. Chem. Phys. 122 204307] using quasi-classical trajectory theory. The polarization-dependent differential cross sections (PDDCSs) (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21−/dωt) and the distributions of P(θr), P(φr), and P(θr,φr) are calculated. The results indicate that the rotational polarization of the CaCl product presents different characteristics for the different collision energies, and the effects of the collision energy on the vector potential, including the alignment, orientation, and PDDCSs, are not obvious.

Guided-Ion-Beam Scattering and Direct Dynamics Trajectory Study on the Reaction of Deprotonated Cysteine with Singlet Molecular Oxygen

We present a study on the gas-phase reaction of deprotonated cysteine with the lowest electronically excited state of molecular oxygen O2[a(1)Δg], including the measurement of the effects of collision energy (E(col)) on reaction cross sections over a center-of-mass E(col) range from 0.1 to 1.0 eV. Deprotonated cysteine was generated using electrospray ionization, and has a carboxylate anionic structure (HSCH2CH(NH2)CO2(-)) in the gas phase. Three product ion channels were observed. The dissociation of HSCH2CH(NH2)CO2(-) to NH2CH2CO2(-) and neutral CH2S has the largest cross section over the entire E(col) range. This product channel is driven by the electronic excitation energy of (1)O2 (the so-called dissociative excitation transfer), and is strongly suppressed by E(col). Two minor channels correspond to the formation of HSCH2C(NH)CO2(-) + H2O2 via abstraction of two hydrogen atoms from HSCH2CH(NH2)CO2(-) by (1)O2, and the formation of OSCH2CH(NH2)CO2(-) radical via elimination of ·OH from an intermediate complex, respectively. Density functional theory calculations were used to locate various complexes, transition states, and products. Quasi-classical direct dynamics trajectory simulations were carried out at E(col) = 0.2 eV using the B3LYP/4-31G(d) level of theory. Trajectory results were used to guide the construction of a reaction coordinate, discriminate between different mechanisms, and provide additional mechanistic insights. Analysis of trajectories highlights the importance of complex mediation at the early stages of all reactions, and suggests a partially concerted mechanism for H2O2 elimination.

Influence of collision energy and reagent vibrational excitation on the stereodynamics of the reaction H + LiH → H2 + Li

Quasi-classical trajectory calculations have been carried out to investigate the stereodynamics of the reaction H+LiH→H2+Li which proceeds on the ground electronic state of LiH2 system, using a recent potential energy surface of Prudente et al. The effects of collision energy and reagent vibrational excitation on the product polarization are studied for the ν=0–3, j=0 states of LiH over a wide collision energy range. It has been found that the collision energy and the vibrational excitation of the reagent remarkably influence the four generalized polarization-dependent differential cross-sections, the distributions of P(θr), P(ϕr), and P(θr,ϕr). The calculated results show that the rotational angular momentum j′ of the product H2 is not only aligned, but also oriented along the direction perpendicular to the scattering plane. The orientation of j′ depends very sensitively on the collision energy and the vibrational excitation of the reagent.

Collision‐induced dissociation of aflatoxins

The aflatoxin mycotoxins are particularly hazardous to health when present in food. Therefore, from an analytical point of view, knowledge of their mass spectrometric properties is essential. The aim of the present study was to describe the collision-induced dissociation behavior of the four most common aflatoxins: B1, B2, G1 and G2. Protonated aflatoxins were produced using atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) combined with high-performance liquid chromatography (HPLC). For the tandem mass spectrometry (MS/MS) experiments nitrogen was used as the collision gas and the collision energies were varied in the range of 9-44 eV (in the laboratory frame). The major APCI-MS/MS fragmentations of protonated aflatoxins occurred at 30 eV collision energy. The main fragmentation channels were found to be the losses of a series of carbon monoxide molecules and loss of a methyl radical, leading to the formation of radical-type product ions. In addition, if the aflatoxin molecule contained an ether- or lactone-oxygen atom linked to a saturated carbon atom, loss of a water molecule was observed from the [M + H](+) ion, especially in the case of aflatoxins G1 and G2. A relatively small modification in the structure of aflatoxins dramatically altered the fragmentation pathways and this was particularly true for aflatoxins B1 and B2. Due to the presence of a C = C double bond connected to the ether group in aflatoxin B1 no elimination of water was observed but, instead, formation of radical-type product ions occurred. Fragmentation of protonated aflatoxin B1 yielded the most abundant radical-type cations.

Oxidation of gas-phase hydrated protonated/deprotonated cysteine: how many water ligands are sufficient to approach solution-phase photooxidation chemistry?

We present a study on the reactions of singlet oxygen O2[a(1)Δg] with hydrated protonated and deprotonated cysteine (Cys) in the gas phase, including measurements of the effects of collision energy (E(col)) and hydration number on reaction cross sections over a center-of-mass E(col) range from 0.05 to 1.0 eV. The aim is to probe how successive addition of water molecules changes the oxidation chemistry of Cys in the gas phase. Hydrated clusters, generated by electrospray ionization, have structures of HSCH2CH(NH3(+))CO2H(H2O)(1,2) and HSCH2CH(NH2)CO2(-)(H2O)(1,2) for protonated and deprotonated forms, respectively. In contrast to (1)O2 reactions with dehydrated protonated/deprotonated Cys of which hydroperoxide products all decomposed, reactions with hydrated protonated/deprotonated Cys yielded stable hydroperoxide products, analogous to photooxidation reaction of Cys in solution. We investigated the number of water ligands necessary to produce a stable hydroperoxide, and found that a single water molecule suffices--that is, to relax nascent, energized hydroperoxide in the hydrated cluster by elimination of water. Hydrated protonated Cys shows higher reaction efficiency than the hydrated deprotonated one, particularly with the addition of the second water ligand. Reactions of hydrated protonated/deprotonated Cys are suppressed by E(col), becoming negligible at E(col) ≥ 0.5 eV. Density functional theory calculations were used to locate reaction coordinates for these systems. Quasi-classical, direct dynamics trajectory simulations were performed for HSCH2CH(NH3(+))CO2H(H2O) + (1)O2 at the B3LYP/4-31G(d) level of theory. Analysis of trajectories highlights the importance of complex mediation in the early stages of the reaction, and illustrates that water can catalyze proton transfer within the hydrated complex.

Collision-induced dissociation mechanisms of [Li(uracil)

The collision-induced dissociation (CID) of the [Li(uracil)](+) complex with Xe is studied by means of quasi-classical trajectory calculations. The potential energy surface is obtained "on the fly" from AM1 semiempirical calculations, supplemented with two-body analytical potentials to model the intermolecular interactions. The simulations show that Li(+) production is the primary channel, in agreement with a previous experimental study [M. T. Rodgers and P. B. Armentrout, J. Am. Chem. Soc., 2000, 122, 8548]. Collision-induced isomerization of [Li(uracil)](+) was found to be very important as well in the 2.5-10 eV collision energy range. Three minor channels are also identified: complex formation between Xe and [Li(uracil)](+), ligand exchange to form LiXe(+), and fragmentations of the uracil ring, which are strongly nonstatistical. Additional quasi-classical trajectory calculations carried out to investigate in more detail the fragmentations of the uracil ring reveal the presence of bifurcations in the potential energy surface, as trajectories starting from a transition state give rise to four different product channels. The integral cross sections for Li(+) production calculated in this work agree well with those obtained in the experiments only for the lowest collision energies, being ∼20 times greater than the experimental values for a collision energy of 10 eV. Finally, the initial translational energy is transferred preferentially to the [Li(uracil)](+) vibrational degrees of freedom, with energy transfer to rotation being modest. The amount of energy transfer to the different degrees of freedom as a function of the collision energy follows quite nicely a model recently proposed by our group.

Characterization of the MgO2+dication in the gas phase: electronic states, spectroscopy and atmospheric implications

Franzreb and Williams at Arizona State University detected recently the MgO(2+) molecular species in the gas phase. Here we report a very detailed theoretical investigation of the low-lying electronic states of this dication including their potentials, spin-orbit, rotational and radial couplings. Our results show that the potential energy curves of the dicationic electronic states have deep potential wells. This confirms that this dication does exist in the gas phase; it is a thermodynamically stable molecule in its ground state, and it has several excited long-lived metastable states. The potential energy curves are used then to predict a set of spectroscopic parameters for the bound states of MgO(2+). We have also incorporated these potentials, rotational and radial couplings in dynamical calculations to derive the cross sections for the charge transfer Mg(2+) + O → Mg(+) + O(+) reaction in the 1-10(3) eV collision energy domain via formation-decomposition of the MgO(2+) dication. Our work shows the role of MgO(2+) in the Earth ionosphere and more generally in atmospheric processes in solar planets, where this reaction efficiently participates in the predominance of Mg(+) cations in these media compared to Mg and Mg(2+).

The effects of collision energy and reagent vibrational excitation on the reactivity of the reaction H + LiH: A quasiclassical trajectory study

Quasi-classical trajectory (QCT) calculations are carried out for the reaction H+LiH (ν=0–3, j=0) using a recent analytical potential energy surface (PES) of Prudente et al. [F.V. Prudente, J.M.C. Marques, A.M. Maniero, Time-dependent wave packet calculation of the LiH+H reactive scattering on a new potential energy surface, Chem. Phys. Lett. 474 (2009) 18–22]. The reaction H+LiH proceeds through the highly exothermic LiH depletion and thermoneutral hydrogen-exchange channels. State-selected and energy-resolved reaction probability and integral reaction cross sections are reported and compared with the recent available literature data. The QCT-calculated probabilities are in good agreement with the previous quantum–mechanical (QM) results, meanwhile, the vibrational excitation of the reagent LiH molecule decreases the reactivity of the LiH depletion channel and increases the reactivity of the hydrogen-exchange channel. Moreover, this decrease and increase can be explained by examining the representative reactive trajectories for the different vibrational excitation of the reagent and the topology of the HHLi PES. Our calculated results indicate that the collision energy and the vibrational excitation of the reagent play an important role on the reactivity of this system.

Product rotational angular momentum polarization in the N + NH (v = 0, j = 0, 3, 6, 9)→N2 + H reaction

Based on the ground state HN2(12A′) potential energy surface (PES), the effects of collision energy and the rotational excitation of the reagent on the product alignment and the orientation for the title reaction are investigated by using quasi-classical trajectory (QCT) method. The distribution of polar angle P(θr) at collision energies 2–40kcal/mol with v=0 and j=0 level has been presented. The three angle distribution functions P(θr), P(ϕr), P(θr,ϕr) and four generalized polarization dependent differential cross sections (PDDCSs) at collision energy of 10kcal/mol with different rotational excitation are plotted. The results show that the product rotational angular momentum j′ is not only aligned, but also oriented along the direction perpendicular to the scattering plane. It has been found that the three angle distributions are sensitive to initially rotational excitation. The HHL mass combination and the property of the PES are used to explain the found results.

Dynamical reaction pathways in Eley-Rideal recombination of nitrogen from W(100)

The scattering of atomic nitrogen over a N-pre-adsorbed W(100) surface is theoretically described in the case of normal incidence off a single adsorbate. Dynamical reaction mechanisms, in particular Eley-Rideal (ER) abstraction, are scrutinized in the 0.1-3.0 eV collision energy range and the influence of temperature on reactivity is considered between 300 and 1500 K. Dynamics simulations suggest that, though non-activated reaction pathways exist, the abstraction process exhibits a significant collision energy threshold (0.5 eV). Such a feature, which has not been reported so far in the literature, is the consequence of a repulsive interaction between the impinging and the pre-adsorbed nitrogens along with a strong attraction towards the tungsten atoms. Above threshold, the cross section for ER reaction is found one order of magnitude lower than the one for hot-atoms formation. The abstraction process involves the collision of the impinging atom with the surface prior to reaction but temperature effects, when modeled via a generalized Langevin oscillator model, do not affect significantly reactivity.

Reactions of Deprotonated Tyrosine and Tryptophan with Electronically Excited Singlet Molecular Oxygen (a1Δg): A Guided-Ion-Beam Scattering, Statistical Modeling, and Trajectory Study

The reactions of deprotonated tyrosine ([Tyr-H](-)) and tryptophan ([Trp-H](-)) with the lowest electronically excited state of molecular oxygen O(2)[a(1)Δ(g)] have been studied in the gas phase, including the measurement of the effects of collision energy (E(col)) on reaction cross sections over a center-of-mass E(col) range from 0.05 to 1.0 eV. [Tyr-H](-) and [Trp-H](-) were generated using electrospray ionization, and both have a pure carboxylate anion structure in the gas phase. Density functional theory calculations and RRKM modeling were used to examine properties of various complexes, transition states, and products that might be important along the reaction coordinate. It was found that deprotonation of Tyr and Trp results in a large effect on their (1)O(2)-mediated oxidation. For [Tyr-H](-), the reaction corresponds to the formation of a hydroperoxide intermediate, followed by intramolecular H transfer and subsequent dissociation to product ion 4-(2-aminovinyl)phenolate, and neutral H(2)O(2) and CO(2). Despite that the reaction is 1.83 eV exothermic, the reaction cross section shows a threshold-like behavior at low E(col) and increases with increasing E(col), suggesting that the reaction bears an activation barrier above the reactants. Quasi-classical, direct dynamics trajectory simulations were carried out for [Tyr-H](-) + (1)O(2) at E(col) = 0.75 eV, using B3LYP/4-31G* level of theory. Trajectories demonstrated the intermediacy of complexes at the early stage of the reaction. A similar product channel was observed in the reaction of [Trp-H](-) with (1)O(2), yielding product ion 3-(2-aminovinyl)indol-1-ide, H(2)O(2) and CO(2). However, the reaction cross section of [Trp-H](-) is strongly suppressed by E(col) and becoming negligible at E(col) > 1.0 eV, indicating that this reaction proceeds without energy barriers above the reactants.

Penning ionization electron spectroscopy of water molecules by metastable neon atoms

The Penning ionization electron spectra in metastable Ne∗(3P0,2) colliding with H2O molecules at 0.040, 0.075, 0.130, 0.200 and 0.300eV collision energies have been measured and compared with the photoelectron spectrum with Ne(I) photons. The results show the formation of the ground X̃(2B1) and the first excited state Ã(2A1) of the H2O+ product ion, with a X̃/Ã branching ratio slightly increasing with the collision energy, and with an average value of 0.29±0.03. The spectra have been analyzed in order to obtain some information and some basic features of the potential energy interaction between the two reacting particles.

Collision energy effect on the H′ + BrH ( ν = 0, j = 0) → H′Br + H reaction: A quasi-classical trajectory study

Theoretical studies on the dynamics of the exchange reaction H′ + BrH ( ν = 0, j = 0) → H′Br + H are performed on potential energy surface (PES) (Kurosaki et al., private communication) for the ground state using the quasi-classical trajectory method. The cross sections, computed at the collision energies ( E c ) of 0.5–2.0 eV, are in good agreement with the earlier quantum wave packet results. The rotational, vibrational, and translational fractions in the total energy and the vibrational distribution for the product molecule are calculated at the same collision-energy range. The results support the repulsive character of the PES. In the considered E c range, it has little chance to occur in an indirect reaction. The alignment and orientation of the product H′Br are investigated in detail with stereodynamics. The results show that E c can effect on both the alignment and the orientation of product.

Effect of Collision Energy on the Reactivity O++T2 → OT++T by the Quasiclassical Trajectory Method

The reactivity O++T2 → OT++T is studied by the quasiclassical trajectory method on the RODRIGO potential energy surface at the collision energies of 1.0, 1.3, 1.6 and 1.9 eV, respectively. The four polarization-dependent generalized differential cross sections (PDDCSs) (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt) and (2π/σ)(dσ21-/dωt) are calculated in the center-of-mass frame. Furthermore, the P(θr) distribution describing the k—j′ correlation, the distribution of dihedral angle P(φr) and the distribution of P(θr, φr), which describes the angular distribution of product rotational vectors in the form of polar plots in θr and φr are also investigated. The results demonstrate that the stereo-dynamical properties of the title reactivity are sensitive to collision energies.

INFLUENCE OF THE COLLISION ENERGY ON STEREODYNAMICS OF THEF+LiH(v = 0, j = 0) →LiF+HREACTION

The stereodynamics of the title reaction based on the ground2A′ potential energy surface (PES) has been investigated using the method of the quasi-classical trajectory (QCT) at different collision energies (23 kcal/mol, 35 kcal/mol and 46 kcal/mol). The vector properties of the angular momentum (described by the distribution of K - J′P(θr), the dihedral angle distribution of K - K′ - J′P(φr) and the angular distribution P(θr, ϕr)) and the four PDDCSs [(2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21-/dωt)] of the product LiF at each collision energy have been presented, respectively. Further, the collision energy effects on the behavior of the product LiF have been discussed and studied.

Dissociative Excitation Energy Transfer in the Reactions of Protonated Cysteine and Tryptophan with Electronically Excited Singlet Molecular Oxygen (a1Δg)

We report a study on the reactions of protonated cysteine (CysH(+)) and tryptophan (TrpH(+)) with the lowest electronically excited state of molecular oxygen (O(2), a(1)Δ(g)), including the measurement of the effects of collision energy (E(col)) on reaction cross sections over the center-of-mass E(col) range of 0.05 to 1.0 eV. Electronic structure calculations were used to examine properties of complexes, transition states and products that might be important along the reaction coordinate. For CysH(+) + (1)O(2), the product channel corresponds to C(α)-C(β) bond rupture of a hydroperoxide intermediate CysOOH(+) accompanied by intramolecular H atom transfer, and subsequent dissociation to H(2)NCHCO(2)H(+), CH(3)SH and ground triplet state O(2). The reaction is driven by the electronic excitation energy of (1)O(2), the so-called dissociative excitation energy transfer. Quasi-classical direct dynamics trajectory simulations were calculated for CysH(+) + (1)O(2) at E(col) = 0.2 and 0.3 eV, using the B3LYP/6-21G method. Most trajectories formed intermediate complexes with significant lifetime, implying the importance of complex formation at the early stage of the reaction. Dissociative excitation energy transfer was also observed in the reaction of TrpH(+) with (1)O(2), leading to dissociation of a TrpOOH(+) intermediate. In contrast to CysOOH(+), TrpOOH(+) dissociates into a glycine molecule and charged indole side chain in addition to ground-state O(2) because this product charge state is energetically favorable. The reactions of CysH(+) + (1)O(2) and TrpH(+) + (1)O(2) present similar E(col) dependence, i.e., strongly suppressed by collision energy and becoming negligible at E(col) > 0.5 eV. This is consistent with a complex-mediated mechanism where a long-lived complex is critical for converting the electronic energy of (1)O(2) to the form of internal energy needed to drive complex dissociation.

Significant nonadiabatic effects in the C + CH reaction dynamics

Rigorous quantum nonadiabatic calculations are carried out on the two coupled electronic states (1(2)A' and 2(2)A') for the C + CH reaction. For all calculations, the initial wave packet was started from the entrance channel of the 1(2)A' state and the initial state of the CH reactant was kept in its ground rovibrational state. Reaction probabilities for total angular momenta J from 0 to 160 are calculated to obtain the integral cross section over an energy range from 0.005 to 0.8 eV collision energy. Significant nonadiabatic effects are found in the reaction dynamics. The branching ratio of the ground state and excited state of C(2) produced is around 0.6, varying slightly with the collision energy. Also, a value of 2.52 × 10(-11) cm(3) molecule(-1) s(-1) for the state selected rate constant k (v = 0, j = 0) at 300 K is obtained, which may be seen as a reference in the future chemical models of interstellar clouds.

Study on the polarization distribution of the O(<sup>1</sup>D)+DBr →OD+Br reaction using quasi-classical trajectory method

Theoretical studies of the stereodynamics for the reation O(1D)+DBr→OD+Br have been carried out using the quasi-classical trajectory method on the ab initio global potential energy surface at 0.22 eV collision energy. The polarization-dependent differential cross-sections (PDDCSs) show the scattering direction of the product. The distribution P ( θr ) of the two vectors correlation between k and j ′ shows that the rotational angular momentum polarization of the product molecule is quite strong. The function P ( or ) which reflects the angle distribution of three vectors correlation k-k′-j′ indicates that j ′ is not only aligned, but also oriented preferentially along the negative direction of the y-axis. There are two obvious distributions at (90°, 90°) and (90°, 270°) about the product rotational angular momentum P ( θr , or ), which indicate that the product molecule is preferentially polarized perpendicular to scattering plane. Comparing O(1D)+DBr reaction with O(1D)+HBr reaction, it can be seen that the degree of polarization distribution of O(1D)+DBr reaction is weaker than that of O(1D)+HBr reaction. That is due to the different quality factor of the two reactions, illuminating that the isotopic effect is relatively evident.