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7 Oct 2006

Volume 125, Issue 13, Articles (13xxxx)

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back to top Gas Phase Collision Processes

Mode- and bond-selective reaction of Cl(math) with CH3D: C–H stretch overtone excitation near 6000 cm−1

Robert J. Holiday, Chan Ho Kwon, Christopher J. Annesley, and F. Fleming Crim

J. Chem. Phys. 125, 133101 (2006); http://dx.doi.org/10.1063/1.2352742 (8 pages) | Cited 28 times

Online Publication Date: 2 October 2006

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Experiments explore the influence of different C–H stretching eigenstates of CH3D on the reaction of CH3D with Cl(math). We prepare the ∣110〉∣0〉(A1,E), ∣200〉∣0〉(E), and ∣100〉∣0〉+ν3+ν5 eigenstates by direct midinfrared absorption near 6000 cm−1. The vibrationally excited molecules react with photolytic Cl atoms, and we monitor the vibrational states of the CH2D or CH3 radical products by 2+1 resonance enhanced multiphoton ionization. Initial excitation of the ∣200〉∣0〉(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to ∣100〉∣0〉+ν3+ν5 excitation, indicating mode-selective chemistry in which the C–H stretch motion couples more effectively to the H-atom abstraction coordinate than bend motion. For two eigenstates that differ only in the symmetry of the vibrational wave function, ∣110〉∣0〉(A1) and ∣110〉∣0〉(E), the ratio of reaction cross sections is 1.00±0.05, showing that there is no difference in enhancement of the H-atom abstraction reaction. Molecules with excited local modes corresponding to one quantum of C–H stretch in each of two distinct oscillators react exclusively to form C–H stretch excited CH2D products. Conversely, eigenstates containing stretch excitation in a single C–H oscillator form predominantly ground vibrational state CH2D products. Analyzing the product state yields for reaction of the ∣110〉∣0〉(A1) state of CH3D yields an enhancement of 20±4 over the thermal reaction. A local mode description of the vibrational motion along with a spectator model for the reactivity accounts for all of the observed dynamics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.40.Bj Oscillations, chaos, and bifurcations
82.80.Gk Analytical methods involving vibrational spectroscopy
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

State-to-state quantum reactive scattering for four-atom chemical reactions: Differential cross section for the H+H2O→H2+OH abstraction reaction

Dong H. Zhang

J. Chem. Phys. 125, 133102 (2006); http://dx.doi.org/10.1063/1.2217439 (4 pages) | Cited 27 times

Online Publication Date: 2 October 2006

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The time-dependent wave packet method was extended to calculate the state-to-state differential cross section for the title four-atom abstraction reaction with H2O in the ground rovibrational state. One spectator OH bond length was fixed in the study, but the remaining five degrees of freedom were treated exactly. It was found that (a) the differential cross section changes from being strongly backward peaked at low collision energy to sideward scattering at E = 1.4 eV, and (b) the rotational state-resolved differential cross section for H2 differs substantially from that for OH.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Hf Product distribution
82.20.Pm Rate constants, reaction cross sections, and activation energies

A crossed molecular beam study on the dynamics of F atom reaction with SiH4

Guanlin Shen, Xueming Yang, Jinian Shu, Chung-Hsin Yang, and Yuan T. Lee

J. Chem. Phys. 125, 133103 (2006); http://dx.doi.org/10.1063/1.2217438 (5 pages) | Cited 2 times

Online Publication Date: 2 October 2006

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In this report, the dynamics of the F+SiH4 reaction has been studied using the universal crossed molecular beam method. Angular resolved time-of-flight spectra have been measured for all reaction products in a single set of experiments. Two different reaction channels have been observed: HF+SiH3 and SiH3F+H. Product angular distributions as well as energy distributions were determined for these two product channels. Experimental results show that the HF product is forward scattered relative to the F atom beam direction, while the SiH3F product is backward scattered relative the F atom beam direction, suggesting that two reaction channels proceed with distinctive reaction dynamics. The relative branching ratios of the two channels have also been estimated.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Dx Analytical methods involving electronic spectroscopy
82.20.Hf Product distribution

Mechanism and control of the F+H2 reaction at low and ultralow collision energies

J. Aldegunde, J. M. Alvariño, M. P. de Miranda, V. Sáez Rábanos, and F. J. Aoiz

J. Chem. Phys. 125, 133104 (2006); http://dx.doi.org/10.1063/1.2212418 (12 pages) | Cited 15 times

Online Publication Date: 2 October 2006

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This article uses theoretical methods to study the dependence on stereodynamical factors of the mechanism and reactivity of the F+H2 reaction at low and ultralow collision energies. The impact of polarization of the H2 reactant on total and state-to-state integral and differential cross sections is analyzed. This leads to detailed pictures of the reaction mechanism in the cold and ultracold regimes, accounting, in particular, for distinctions associated with the various product states and scattering angles. The extent to which selection of reactant polarization allows for external control of the reactivity and reaction mechanism is assessed. This reveals that even the simplest of reactant polarization schemes allows for fine, product state-selective control of differential and (for reactions involving more than a single, zero orbital angular momentum partial wave) integral cross sections.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

Energy transfer between azulene and krypton: Comparison between experiment and computation

V. Bernshtein and I. Oref

J. Chem. Phys. 125, 133105 (2006); http://dx.doi.org/10.1063/1.2207608 (11 pages) | Cited 16 times

Online Publication Date: 2 October 2006

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Trajectory calculations of collisional energy transfer between excited azulene and Kr are reported, and the results are compared with recent crossed molecular beam experiments by Liu et al. [J. Chem. Phys. 123, 131102 (2005); 124, 054302 (2006)] . Average energy transfer quantities are reported and compared with results obtained before for azulene-Ar collisions. A collisional energy transfer probability density function P(E,E′), calculated at identical initial conditions as experiments, shows a peak at the up-collision branch of P(E,E′) at low initial relative translational energy. This peak is absent at higher relative translational energies. There is a supercollision tail at the down-collision side of the probability distribution. Various intermolecular potentials are used and compared. There is broad agreement between experiment and computation, but there are some differences as well.
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34.50.Ez Rotational and vibrational energy transfer
34.20.Gj Intermolecular and atom-molecule potentials and forces
34.10.+x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.)

A crossed-beam study of the F+HD→HF+D reaction: The resonance-mediated channel

Shih-Huang Lee, Feng Dong, and Kopin Liu

J. Chem. Phys. 125, 133106 (2006); http://dx.doi.org/10.1063/1.2217374 (10 pages) | Cited 20 times

Online Publication Date: 2 October 2006

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This is the last report of our extensive studies on the title reaction. Presented here are the state-to-state differential cross section determinations at 11 collision energies, ranging from 1.30 to 4.53 kcal/mol. Together with previously reported results at six lower energies (0.4–1.18 kcal/mol), this perhaps represents one of the most comprehensive set of data from a single investigation for any chemical reaction. The information contents of this set of data are examined in detail, from which the dynamical consequences of reactive resonances are elucidated. Qualitative interpretations of some of the major findings are proposed. Observations that need further theoretical investigations for better physical understanding are pointed out.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Rp State to state energy transfer

Rotationally resolved reactive scattering: Imaging detailed Cl+C2H6 reaction dynamics

Cunshun Huang, Wen Li, and Arthur G. Suits

J. Chem. Phys. 125, 133107 (2006); http://dx.doi.org/10.1063/1.2202827 (9 pages) | Cited 14 times

Online Publication Date: 2 October 2006

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The hydrogen atom abstraction reaction of Cl (math) with ethane has been studied using the crossed molecular beam technique with dc slice imaging at collision energies from 3.2 to 10.4 kcal/mol. The products HCl (v,J) (v = 0, J = 0–5) were state-selectively detected using 2+1 resonance enhanced multiphoton ionization. The images were used to obtain the center-of-mass frame product angular distributions and translational energy release distributions. Two general features were found in all probed HCl quantum states at 6.7 kcal/mol collision energy, and these features have distinct translational energy release and angular distributions, as described for HCl (v = 0, J = 2) in a recent preliminary report [ Li et al., J. Chem. Phys. 124, 011102 (2006) ]. The results for HCl (v = 0,J = 2) at four collision energies were also compared to investigate the energy-dependent dynamics. We discuss the reaction in terms of a variety of models of polyatomic reaction dynamics. The dynamics of this well studied system are more complicated than can be accounted for by a single mechanism, and the results call for further theoretical and experimental investigations.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Hf Product distribution
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment

Nonadiabatic effects in the H+D2 reaction

Rui-Feng Lu, Tian-Shu Chu, Yan Zhang, Ke-Li Han, António J. C. Varandas, and John Z. H. Zhang

J. Chem. Phys. 125, 133108 (2006); http://dx.doi.org/10.1063/1.2202826 (6 pages) | Cited 15 times

Online Publication Date: 2 October 2006

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The state-to-state dynamics of the H+D2 reaction is studied by the reactant-product decoupling method using the double many-body expansion potential energy surface. Two approaches are compared: one uses only the lowest adiabatic sheet while the other employs both coupled diabatic sheets. Rotational distributions for the reaction H+D2 (υ = 0,j = 0)→HD(υ′ = 3,j′)+D are obtained at eight different collision energies between 1.49 and 1.85 eV; no significant difference are found between the two approaches. Initial state-selected total reaction probabilities and integral cross sections are also given for energies ranging from 0.25 up to 2.0 eV with extremely small differences being observed between the two sets of results, thus showing that the nonadiabatic effects in the title reaction are negligible at least for small energies below 2.0 eV.
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82.20.Gk Electronically non-adiabatic reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Bc State selected dynamics and product distribution

Exact quantum calculations of the kinetic isotope effect: Cross sections and rate constants for the F+HD reaction and role of tunneling

Dario De Fazio, Vincenzo Aquilanti, Simonetta Cavalli, Antonio Aguilar, and Josep M. Lucas

J. Chem. Phys. 125, 133109 (2006); http://dx.doi.org/10.1063/1.2221695 (8 pages) | Cited 13 times

Online Publication Date: 3 October 2006

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In this paper we present integral cross sections (in the 5–220 meV collision energy range) and rate constants (in the 100–300 K range of temperature) for the F+HD reaction leading to HF+D and DF+H. The exact quantum reactive scattering calculations were carried out using the hyperquantization algorithm on an improved potential energy surface which incorporates the effects of open shell and fine structure of the fluorine atom in the entrance channel. The results reproduce satisfactorily molecular beam scattering experiments as well as chemical kinetics data for both the HF and DF channels. In particular, the agreement of the rate coefficients and the vibrational branching ratios with experimental measurements is improved with respect to previous studies. At thermal and subthermal energies, the rates are greatly influenced by tunneling through the reaction barrier. Therefore exchange of deuterium is shown to be penalized with respect to exchange of hydrogen, and the isotopic branching exhibits a strong dependence on translational energy. Also, it is found that rotational excitation of the reactant HD molecule enhances the production of HF and decreases the reactivity at the D end, obtaining insight on the reaction stereodynamics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Tr Kinetic isotope effects including muonium
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Hf Product distribution

Inelastic scattering from glyoxal: Collision kinematics rather than the interaction potential dominates rotational channel selection

Samuel M. Clegg and Charles S. Parmenter

J. Chem. Phys. 125, 133110 (2006); http://dx.doi.org/10.1063/1.2336222 (13 pages) | Cited 1 time

Online Publication Date: 3 October 2006

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Relative cross sections have been obtained for the rotationally and rovibrationally inelastic scattering of S1 trans-glyoxal (CHO–CHO) in its zero point level with K′ = 0 from the target gases H2, D2, and He. Emphasis is placed on using crossed molecular beam conditions that provide several choices of collision kinematics (center-of-mass collision energy, relative velocity, center-of-mass collision momentum) for each collision pair. The cross sections define the state-to-state competition among numerous rotational channels involving destination states with ΔK′ ranging from 1 to >15 for collisions with each target gas and under every kinematic condition. They also resolve a similar rotational competition among rovibrational channels where the torsion ν7′ is collisionally excited. The cross section sets also allow the relative overall magnitudes of the two types of scattering to be compared. The primary motivation of these experiments concerns the rotationally inelastic scattering. Earlier studies with rare gases and fixed kinematics demonstrated that the distribution of rotational cross sections is remarkably similar from one collision pair to another. The new data show that the competition among rotational channels actually has a small but distinct dependence on kinematic conditions. Data analysis shows that the dependence is a systematic function of the available collision momentum and entirely unrelated to the identity of the target gases, including the heavier rare gases used in earlier studies. The competition among the rotational energy transfer channels and its kinematic heritage is discussed in the context of a classical hard ellipse model of linear momentum to angular momentum conversion much used with room temperature systems. When adapted to our beam conditions, the resulting account of the rotational scattering is accurate and provides insight into the collisional details.
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34.50.Ez Rotational and vibrational energy transfer

Intermolecular interactions of H2S with rare gases from molecular beam scattering in the glory regime and from ab initio calculations

David Cappelletti, Alessandra F. A. Vilela, Patricia R. P. Barreto, Ricardo Gargano, Fernando Pirani, and Vincenzo Aquilanti

J. Chem. Phys. 125, 133111 (2006); http://dx.doi.org/10.1063/1.2218513 (8 pages) | Cited 17 times

Online Publication Date: 4 October 2006

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Integral cross sections for collisions of rotationally hot H2S molecules with rare gas atoms (Ne, Ar, and Kr) have been measured, in the collision energy range of 10–60 kJ mol−1, using a molecular beam apparatus operating under high resolution both in angle and in velocity. A well resolved glory pattern has been measured which permitted the accurate characterization of the intermolecular potentials both at long range (in the attractive region) and at intermediate distances (in the well region). Considering the conditions used in the experiments, the obtained potentials must be considered very close to the spherical averages of the full intermolecular potential energy surfaces. Extensive ab initio calculations have also been carried out in parallel in order to characterize energy minima in the potential energy surfaces and energy barriers associated to the motion of the rare gas atoms around H2S. An assessment of the relative role of the various interaction components has been also attempted: the combined analysis of experimental and theoretical results suggests that H2S-rare gas aggregates are mainly bound by nearly isotropic noncovalent interactions of the van der Waals type.
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31.15.A- Ab initio calculations
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Mt Rotation, vibration, and vibration-rotation constants

Parity-dependent rotational rainbows in D2–NO and He–NO differential collision cross sections

Arjan Gijsbertsen, Harold Linnartz, and Steven Stolte

J. Chem. Phys. 125, 133112 (2006); http://dx.doi.org/10.1063/1.2234771 (10 pages) | Cited 4 times

Online Publication Date: 4 October 2006

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The (j′,math′,ϵ′) dependent differential collision cross sections of D2 with fully state selected (j = 1/2, Ω = 1/2, ϵ = −1) NO have been determined at a collision energy of about 550 cm−1. The collisionally excited NO molecules are detected by (1+1′) resonance enhanced multiphoton ionization combined using velocity-mapped ion-imaging. The results are compared to He–NO scattering results and tend to be more forward scattered for the same final rotational state. Both for collisions of the atomic He and the molecular D2 with NO, scattering into pairs of rotational states with the same value of n = j′−ϵϵ′/2 yields the same angular dependence of the cross section. This “parity propensity rule” remains present both for spin-orbit conserving and spin-orbit changing transitions. The maxima in the differential cross sections—that reflect rotational rainbows—have been extracted from the D2–NO and the He–NO differential cross sections. These maxima are found to be distinct for odd and even parity pair number n. Rainbow positions of parity changing transitions (n is odd) occur at larger scattering angles than those of parity conserving transitions (n is even). Parity conserving transitions exhibit—from a classical point of view—a larger effective eccentricity of the shell. No rainbow doubling due to collisions onto either the N-end or the O-end was observed. From a classical point of view the presence of a double rainbow is expected. Rotational excitation of the D2 molecules has not been observed.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
34.50.Gb Electronic excitation and ionization of molecules
33.20.Sn Rotational analysis
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Ab initio/Rice-Ramsperger-Kassel-Marcus study of the singlet C4H4 potential energy surface and of the reactions of C2(X math) with C4H4(X math) and C(math) with C3H4 (allene and methylacetylene)

A. M. Mebel, V. V. KisIov, and R. I. Kaiser

J. Chem. Phys. 125, 133113 (2006); http://dx.doi.org/10.1063/1.2227378 (15 pages) | Cited 7 times

Online Publication Date: 4 October 2006

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Ab initio modified Gaussian-2 G2M(RCC,MP2) calculations have been performed for various isomers and transition states on the singlet C4H4 potential energy surface. The computed relative energies and molecular parameters have then been used to calculate energy-dependent rate constants for different isomerization and dissociation processes in the C4H4 system employing Rice-Ramsperger-Kassel-Marcus theory and to predict branching ratios of possible products of the C2(math)+C2H4, C(math)+H2CCCH2, and C(math)+H3CCCH reactions under single-collision conditions. The results show that C2 adds to the double CC bond of ethylene without a barrier to form carbenecyclopropane, which then isomerizes to butatriene by a formal C2 “insertion” into the C–C bond of the C2H4 fragment. Butatriene can rearrange to the other isomers of C4H4, including allenylcarbene, methylenecyclopropene, vinylacetylene, methylpropargylene, cyclobutadiene, tetrahedrane, methylcyclopropenylidene, and bicyclobutene. The major decomposition products of the chemically activated C4H4 molecule formed in the C2(math)+C2H4 reaction are calculated to be acetylene+vinylidene (48.6% at Ecol = 0) and 1-buten-3-yne-2-yl radical [i-C4H3(X math),H2CCCCH∙]+H (41.3%). As the collision energy increases from 0 to 10 kcal/mol, the relative yield of i-C4H3+H grows to 52.6% and that of C2H2+CCH2 decreases to 35.5%. For the C(math)+allene reaction, the most important products are also i-C4H3+H (55.2%) and C2H2+CCH2 (30.1%), but for C(math)+methylacetylene, which accesses a different region of the C4H4 singlet potential energy surface, the calculated product branching ratios differ significantly: 65%–69% for i-C4H3+H, 18%–14% for C2H2+CCH2, and ∼ 8% for diacetylene+H2.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Qt Isomerization and rearrangement
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies

Activation of methane by gold cations: Guided ion beam and theoretical studies

Feng-Xia Li and P. B. Armentrout

J. Chem. Phys. 125, 133114 (2006); http://dx.doi.org/10.1063/1.2220038 (13 pages) | Cited 23 times

Online Publication Date: 4 October 2006

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The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its math (5d10) electronic ground state level but with some math (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (math) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C–H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+–CH2) = 3.70±0.07 and D0(Au+–CH3) = 2.17±0.24. Ab initio calculations at the B3LYP/HW+/6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.
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82.20.Kh Potential energy surfaces for chemical reactions
31.50.Df Potential energy surfaces for excited electronic states
82.20.Pm Rate constants, reaction cross sections, and activation energies
31.15.A- Ab initio calculations
31.15.E- Density-functional theory

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+–rare gas collisions: An experimental and trajectory study

Jianbo Liu, Brady W. Uselman, Jason M. Boyle, and Scott L. Anderson

J. Chem. Phys. 125, 133115 (2006); http://dx.doi.org/10.1063/1.2229207 (15 pages) | Cited 12 times

Online Publication Date: 4 October 2006

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A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by ∼ 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2++Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+–NO2 charge state.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Bc State selected dynamics and product distribution
82.20.Fd Collision theories; trajectory models
82.20.Rp State to state energy transfer
82.20.Kh Potential energy surfaces for chemical reactions

Direct observation and reactions of Cl3 radical

Shinichi Enami, Takashi Yamanaka, Satoshi Hashimoto, Masahiro Kawasaki, Simone Aloisio, and Hiroto Tachikawa

J. Chem. Phys. 125, 133116 (2006); http://dx.doi.org/10.1063/1.2217440 (6 pages) | Cited 1 time

Online Publication Date: 4 October 2006

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The broad absorption of Cl3 radical was observed between 1150 and 1350 nm using cavity ring-down spectroscopy at 213–265 K and 50–200 Torr with He, N2, Ar, or SF6 diluents. The absorption intensity of Cl3 increased at lower temperature and higher pressure. SF6 was the most efficient diluent gas. The temperature dependent equilibrium constants for Cl3 formation from Cl+Cl2 were theoretically calculated at the MP4SDQ/6-311+G(d) level. Observed decay time profiles of Cl3 and the pressure dependence of Cl3 formation are explained by the equilibrium reaction and a decay reaction of Cl+Cl3.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Dx Analytical methods involving electronic spectroscopy
82.60.Hc Chemical equilibria and equilibrium constants
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Hf Product distribution

Reaction dynamics of OH+(math)+C2H2 studied with crossed beams and density functional theory calculations

Li Liu, Courtney Martin, and James M. Farrar

J. Chem. Phys. 125, 133117 (2006); http://dx.doi.org/10.1063/1.2212417 (7 pages) | Cited 1 time

Online Publication Date: 4 October 2006

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The reactions between OH+(math) and C2H2 have been studied using crossed ion and molecular beams and density functional theory calculations. Both charge transfer and proton transfer channels are observed. Products formed by carbon-carbon bond cleavage analogous to those formed in the isoelectronic O(math)+C2H2 reaction, e.g., mathH2+HCO+, are not observed. The center of mass flux distributions of both product ions at three different energies are highly asymmetric, with maxima close to the velocity and direction of the precursor acetylene beam, characteristic of direct reactions. The internal energy distributions of the charge transfer products are independent of collision energy and are peaked at the reaction exothermicity, inconsistent with either the existence of favorable Franck-Condon factors or energy resonance. In proton transfer, almost the entire reaction exothermicity is transformed into product internal excitation, consistent with mixed energy release in which the proton is transferred with both the breaking and forming bonds extended. Most of the incremental translational energy in the two higher-energy experiments appears in product translational energy, providing an example of induced repulsive energy release.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Hf Product distribution
82.20.Db Transition state theory and statistical theories of rate constants

Dynamics of ionization of H2 by Ne*(math) investigated by electron spectroscopy

Joseph H. Noroski and P. E. Siska

J. Chem. Phys. 125, 133118 (2006); http://dx.doi.org/10.1063/1.2206781 (9 pages) | Cited 1 time

Online Publication Date: 5 October 2006

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The Penning ionization reaction Ne*(2p53s math)+H2→[NeH2]++e− has been studied in crossed supersonic molecular beams with electron-energy analysis at four collision energies E = 1.83, 2.50, 3.16, and 3.89 kcal/mol. The electron kinetic-energy spectra, which directly reflect the ionizing transition region, show resolved peaks assignable to v′ = 0–4 of H2+. The vibrational populations deviate systematically from Franck-Condon behavior, suggesting that the discrete-continuum coupling increases with H2 bond stretching. Each peak displays both increasing breadth and increasing blueshift with increasing E, and the blueshift also increases with increasing v′. The first two properties are consistent with a predominantly repulsive excited-state potential-energy surface, while the last is speculated to be a reflection of the rHH dependence of the ionic surface. Quantum scattering calculations based on ab initio potential surfaces for the excited and ionic states in spherical and infinite-order-sudden rigid rotor approximations are in semiquantitative agreement with the measurements. Discrepancies suggest changes in the imaginary, absorptive part of the excited surface, which probably can be best effected by multiproperty fitting calculations.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Ej Quantum theory of reaction cross section

Direct ab initio molecular dynamics study on a microsolvated SN2 reaction of OH−(H2O) with CH3Cl

Hiroto Tachikawa

J. Chem. Phys. 125, 133119 (2006); http://dx.doi.org/10.1063/1.2229208 (10 pages) | Cited 9 times

Online Publication Date: 5 October 2006

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Reaction dynamics for a microsolvated SN2 reaction OH−(H2O)+CH3Cl have been investigated by means of the direct ab initio molecular dynamics method. The relative center-of-mass collision energies were chosen as 10, 15, and 25 kcal/mol. Three reaction channels were found as products. These are (1) a channel leading to complete dissociation (the products are CH3OH+Cl−+H2O: denoted by channel I), (2) a solvation channel (the products are Cl−(H2O)+CH3OH: channel II), and (3) a complex formation channel (the products are CH3OH⋯H2O+Cl−: channel III). The branching ratios for the three channels were drastically changed as a function of center-of-mass collision energy. The ratio of complete dissociation channel (channel I) increased with increasing collision energy, whereas that of channel III decreased. The solvation channel (channel II) was minor at all collision energies. The selectivity of the reaction channels and the mechanism are discussed on the basis of the theoretical results.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Bc State selected dynamics and product distribution
82.20.Wt Computational modeling; simulation
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Quasiclassical trajectory study of the reaction H+CH4(ν3 = 0,1)→CH3+H2 using a new ab initio potential energy surface

Zhen Xie, Joel M. Bowman, and Xiubin Zhang

J. Chem. Phys. 125, 133120 (2006); http://dx.doi.org/10.1063/1.2238871 (8 pages) | Cited 25 times

Online Publication Date: 5 October 2006

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Detailed quasiclassical trajectory calculations of the reaction H+CH4(ν3 = 0,1)→CH3+H2 using a slightly updated version of a recent ab initio-based CH5 potential energy surface [ X. Zhang et al., J. Chem. Phys. 124, 021104 (2006) ] are reported. The reaction cross sections are calculated at initial relative translational energies of 1.52, 1.85, and 2.20 eV in order to make direct comparison with experiment. The relative reaction cross section enhancement ratio due to the excitation of the C–H antisymmetric stretch varies from 2.2 to 3.0 over this energy range, in good agreement with the experimental result of 3.0±1.5 [ J. P. Camden et al., J. Chem. Phys. 123, 134301 (2005) ]. The laboratory-frame speed and center-of-mass angular distributions of CH3 are calculated as are the vibrational and rotational distributions of H2 and CH3. We confirm that this reaction occurs with a combination of stripping and rebound mechanisms by presenting the impact parameter dependence of these distributions and also by direct examination of trajectories.
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82.20.Kh Potential energy surfaces for chemical reactions
82.20.Fd Collision theories; trajectory models
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Crossed molecular beam studies on the reaction dynamics of O(math)+N2O

Yu-Ju Lu, Chi-Wei Liang, and Jim J. Lin

J. Chem. Phys. 125, 133121 (2006); http://dx.doi.org/10.1063/1.2202828 (8 pages) | Cited 1 time

Online Publication Date: 5 October 2006

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The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(math) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The mathO product of the isotope exchange channel, math+mathO→math+mathO, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.20.Tr Kinetic isotope effects including muonium
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
back to top Spectroscopy

The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Sheunn-Jiun Tang, Yung-Ching Chou, Jim Jr-Min Lin, and Yen-Chu Hsu

J. Chem. Phys. 125, 133201 (2006); http://dx.doi.org/10.1063/1.2199827 (15 pages) | Cited 10 times

Online Publication Date: 2 October 2006

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Forty three vibronic levels of C2H2+, math math, with υ4 = 0–6, υ5 = 0–3, and K = 0–4, lying at energies of 0–3520 cm−1 above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+1′ two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the math math, 4ν3 (K = 1–2), 5ν3 (K = 1), ν2+4ν3 (K = 0), and 47 206 cm−1 (K = 1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (υ4 = 0–3, K = 0–2) had been reported previously by Pratt et al. [ J. Chem. Phys. 99, 6233 (1993) ]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with υ4+υ5 ⩽ 2, except for the unobserved upper math component of the 2ν4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm−1. The vibronic coupling parameter ε5 (about whose magnitude there has been controversy) was determined to be −0.02737. For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the υ4+υ5 = 3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with υ4+υ5 ⩽ 5, an rms deviation of 0.34 cm−1 was obtained using 20 parameters. An interesting finding is that Hund’s spin-coupling cases (a) and (b) both occur in the Σu components of the ν4+2ν5 combination level. The ionization potential of C2H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91 953.77±0.09 cm−1 (3σ).
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33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Probing the electronic structure of UO+ with high-resolution photoelectron spectroscopy

Vasiliy Goncharov, Leonid A. Kaledin, and Michael C. Heaven

J. Chem. Phys. 125, 133202 (2006); http://dx.doi.org/10.1063/1.2213262 (8 pages) | Cited 14 times

Online Publication Date: 2 October 2006

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The pulsed field ionization–zero kinetic energy photoelectron technique has been used to observe the low-lying energy levels of UO+. Rotationally resolved spectra were recorded for the ground state and the first nine electronically excited states. Extensive vibrational progressions were characterized. Ω+ assignments were unambiguously determined from the first rotational lines identified in each vibronic band. Term energies, vibrational frequencies, and anharmonicity constants for low-lying energy levels of UO+ are reported. In addition, accurate values for the ionization energies for UO [48643.8(2) cm−1] and U [49957.6(2) cm−1] were determined. The pattern of low-lying electronic states for UO+ indicates that they originate from the U3+(5f3)O2− configuration, where the uranium ion-centered interactions between the 5f electrons are significantly stronger than interactions with the intramolecular electric field. The latter lifts the degeneracy of U3+ ion-core states, but the atomic angular momentum quantum numbers remain reasonably well defined.
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33.60.+q Photoelectron spectra
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

Hui Wu, Wen-kai Zhang, Wei Gan, Zhi-feng Cui, and Hong-fei Wang

J. Chem. Phys. 125, 133203 (2006); http://dx.doi.org/10.1063/1.2352746 (12 pages) | Cited 9 times

Online Publication Date: 3 October 2006

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Knowledge of the ratios between different polarizability βi′j′k′ tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the βi′j′k′ tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.
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68.03.Kn Dynamics (capillary waves)
68.03.Hj Liquid surface structure: measurements and simulations
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
63.50.-x Vibrational states in disordered systems
78.30.-j Infrared and Raman spectra

Gold as hydrogen: Structural and electronic properties and chemical bonding in Si3Au3+/0/− and comparisons to Si3H3+/0/−

Boggavarapu Kiran, Xi Li, Hua-Jin Zhai, and Lai-Sheng Wang

J. Chem. Phys. 125, 133204 (2006); http://dx.doi.org/10.1063/1.2216707 (7 pages) | Cited 19 times

Online Publication Date: 4 October 2006

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A single Au atom has been shown to behave like H in its bonding to Si in several mono- and disilicon gold clusters. In the current work, we investigate the Au/H analogy in trisilicon gold clusters, Si3Au3+/0/−. Photoelectron spectroscopy and density functional calculations are combined to examine the geometric and electronic structure of Si3Au3−. We find that there are three isomers competing for the ground state of Si3Au3− as is the case for Si3H3−. Extensive structural searches show that the potential energy surfaces of the trisilicon gold clusters (Si3Au3−, Si3Au3, and Si3Au3+) are similar to those of the corresponding silicon hydrides. The lowest energy isomers for Si3Au3− and Si3Au3 are structurally similar to a Si3Au four-membered ring serving as a common structural motif. For Si3Au3+, the 2π aromatic cyclotrisilenylium auride ion, analogous to the aromatic cyclotrisilenylium ion (Si3H3+), is the most stable species. Comparison of the structures and chemical bonding between Si3Au3+/0/− and the corresponding silicon hydrides further extends the isolobal analogy between Au and H.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.60.+q Photoelectron spectra
31.15.E- Density-functional theory
31.50.Bc Potential energy surfaces for ground electronic states
33.15.Fm Bond strengths, dissociation energies
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