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21 Jan 2012

Volume 136, Issue 3, Articles (03xxxx)

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J. Chem. Phys. 136, 035101 (2012); http://dx.doi.org/10.1063/1.3671986 (13 pages)

L. Dupuis and Normand Mousseau
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back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Reaction dynamics of Mo + O2 → MoO + O studied by a crossed-beam velocity map imaging technique

Kenji Honma and Yoshiteru Matsumoto

J. Chem. Phys. 136, 034301 (2012); http://dx.doi.org/10.1063/1.3676724 (7 pages)

Online Publication Date: 17 January 2012

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The oxidation reaction dynamics of gas-phase molybdenum atoms by oxygen molecules was studied under a crossed-beam condition. The product MoO was detected by a time-of-flight mass spectrometer combined with laser multi-photon ionization. An acceleration lens system designed for the ion-velocity mapping condition, a two-dimensional (2D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products at three collision energies: 10.0, 17.8, and 50.0 kJ/mol. The angular distributions showed forward and backward peaks, whose relative intensities changed by the collision energy. While two peaks had similar intensities at low collision energies, the forward peak became dominant at the highest collision energy, 50 kJ/mol. The product kinetic energy distributions showed a good correlation with the initial collision energies, i.e., almost the same energy as the collision energy appeared as the product kinetic energy. These results suggested that the reaction proceeds via an intermediate complex, and the lifetime of the complex becomes shorter than its rotational period at high collision energy.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution

State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

Bin Jiang, Daiqian Xie, and Hua Guo

J. Chem. Phys. 136, 034302 (2012); http://dx.doi.org/10.1063/1.3676725 (13 pages) | Cited 2 times

Online Publication Date: 17 January 2012

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State-to-state photodissociation dynamics of H2O in its B band has been investigated quantum mechanically on a new set of non-adiabatically coupled potential energy surfaces for the lowest two 1A′ states of H2O, which are developed at the internally contracted multi-reference configuration interaction level with the aug-cc-pVQZ basis set. Quantum dynamical calculations carried out using the Chebyshev propagator yield absorption spectra, product state distributions, branching ratios, and differential cross sections, which are in reasonably good agreement with the latest experimental results. Particular focus is placed here on the dependence of various dynamical observables on the photon energy. Detailed analyses of the dynamics have assigned the diffuse structure in absorption spectrum to short-time recurring dynamics near the HOH conical intersection. The non-adiabatic dissociation to the ground state OH product via the HOH conical intersection is facile, direct, fast, and produces rotationally hot OH(math) products. On the other hand, the adiabatic channel on the excited state leading to the OH(math) product is dominated by long-lived resonances, which depend sensitively on the potential energy surfaces.
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82.50.-m Photochemistry
82.20.Fd Collision theories; trajectory models
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

On the molecular structure of HOOO

Michael C. McCarthy, Valerio Lattanzi, Damian Kokkin, Oscar Martinez, Jr., and John F. Stanton

J. Chem. Phys. 136, 034303 (2012); http://dx.doi.org/10.1063/1.3673875 (10 pages) | Cited 4 times

Online Publication Date: 17 January 2012

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The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H2O and O2 heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H2 and O2 as precursor gases. Using equal mixtures of normal and 18O2, it has been possible to detect three new isotopic species, H18OOO, HO18O18O, and H18O18O18O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O2 → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r0) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O–O bond (1.684 Å), in agreement with earlier work, a markedly shorter O–H bond distance (0.913 Å), and a more acute ∠HOO angle (92.4°) when compared to equilibrium (re) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (reemp) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen–oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines.
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31.15.bw Coupled-cluster theory
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Bx Radio-frequency and microwave spectra

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Xiaofeng Tang, Xiaoguo Zhou, Manman Wu, Shilin Liu, Fuyi Liu, Xiaobin Shan, and Liusi Sheng

J. Chem. Phys. 136, 034304 (2012); http://dx.doi.org/10.1063/1.3676411 (8 pages) | Cited 2 times

Online Publication Date: 17 January 2012

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Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH3Cl+ ions was investigated in the excitation energy range of 11.0–18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH3+ dissociated from CH3Cl+(A2A1 and B2E) ions were recorded. CH3+ was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH2Cl+ fragment was very low. For dissociation of CH3Cl+(A2A1) ions, a series of homocentric rings was clearly observed in the CH3+ image, which was assigned as the excitation of umbrella vibration of CH3+ ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH3+(11A′) provided a direct experimental evidence of a shallow potential well along the C–Cl bond rupture. For CH3Cl+(B2E) ions, total kinetic energy released distribution for CH3+ fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B2E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH3Cl+, CH3+ formation from CH3Cl+(A2A1) ions was a rapid direct fragmentation, while CH3Cl+(B2E) ions statistically dissociated to CH3+ + Cl via internal conversion to the high vibrational states of X2E.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Ta Mass spectra
33.50.Hv Radiationless transitions, quenching

Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism

Amit R. Sharma, Joel M. Bowman, and David J. Nesbitt

J. Chem. Phys. 136, 034305 (2012); http://dx.doi.org/10.1063/1.3666987 (8 pages) | Cited 2 times

Online Publication Date: 18 January 2012

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We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated “reaction paths.” The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm−1. Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Qt Isomerization and rearrangement
31.15.ae Electronic structure and bonding characteristics
82.20.Kh Potential energy surfaces for chemical reactions
31.15.bw Coupled-cluster theory
33.15.Fm Bond strengths, dissociation energies

Statistical thermodynamics of 1-butanol, 2-methyl-1-propanol, and butanal

Prasenjit Seal, Ewa Papajak, Tao Yu, and Donald G. Truhlar

J. Chem. Phys. 136, 034306 (2012); http://dx.doi.org/10.1063/1.3674995 (10 pages) | Cited 5 times

Online Publication Date: 18 January 2012

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The purpose of the present investigation is to calculate partition functions and thermodynamic quantities, viz., entropy, enthalpy, heat capacity, and Gibbs free energies, for 1-butanol, 2-methyl-1-propanol, and butanal in the vapor phase. We employed the multi-structural (MS) anharmonicity method and electronic structure calculations including both explicitly correlated coupled cluster theory and density functional theory. The calculations are performed using all structures for each molecule and employing both the local harmonic approximation (MS-LH) and the inclusion of torsional anharmonicity (MS-T). The results obtained from the MS-T calculations are in excellent agreement with experimental data taken from the Thermodynamics Research Center data series and the CRC Handbook of Chemistry and Physics, where available. They are also compared with Benson's empirical group additivity values, where available; in most cases, the present results are more accurate than the group additivity values. In other cases, where experimental data (but not group additivity values) are available, we also obtain good agreement with experiment. This validates the accuracy of the electronic structure calculations when combined with the MS-T method for estimating the thermodynamic properties of systems with multiple torsions, and it increases our confidence in the predictions made with this method for molecules and temperatures where experimental or empirical data are not available.
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31.15.bw Coupled-cluster theory
02.70.Rr General statistical methods

Variation of radiative lifetimes of NH2(Ã2A1) with rotational levels in the (0, 8, 0) and (0, 9, 0) vibration bands

Marc N’Doumi and Joshua B. Halpern

J. Chem. Phys. 136, 034307 (2012); http://dx.doi.org/10.1063/1.3676782 (7 pages)

Online Publication Date: 19 January 2012

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Radiative lifetimes from the first electronically excited state of the amidogen free radical, NH2(Ã2A1), are reported for rotational states in selected vibrational levels ν2′ using laser-induced fluorescence. Thermal collision of argon, Ar*(3P0, 3P2) metastable atoms in a microwave discharge-flow system with ammonia (NH3) molecules produced ground state NH2(math2B1). The radiative lifetimes for the deactivation of NH2(Ã2A1) were determined by measuring the decay profiles of NH2(Ã2A1 → math2B1). In addition to the Fermi resonances with the ground state that lengthen the radiative lifetimes, a systematic increase in the radiative lifetimes with rotational quantum number was observed. Furthermore, the average radiative lifetimes of the (0, 9, 0) Γ, τ1 = 18.65 ± 0.47 μs and (0, 8, 0) Φ, τ2 = 23.72 ± 0.65 μs levels were much longer than those of the (0, 9, 0) Σ, τ3 = 10.62 ± 0.47 μs, and (0, 8, 0) Π, τ4 = 13.55 ± 0.55 μs states suggesting increased mixing of the first electronic excited and the ground states.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.-b Photon interactions with molecules
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.50.Dq Fluorescence and phosphorescence spectra

Theoretical predictions of properties and gas-phase chromatography behaviour of bromides of group-5 elements Nb, Ta, and element 105, Db

V. Pershina and J. Anton

J. Chem. Phys. 136, 034308 (2012); http://dx.doi.org/10.1063/1.3676176 (7 pages) | Cited 3 times

Online Publication Date: 19 January 2012

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See Also: Erratum

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Fully relativistic, four-component density functional theory electronic structure calculations were performed for MBr5, MOBr3, MBr6−, KMBr6, and MBr5Cl− of group-5 elements Nb, Ta, and element 105, Db, with the aim to predict adsorption behaviour of the bromides in gas-phase chromatography experiments. It was shown that in the atmosphere of HBr/BBr3, the pentabromides are rather stable, and their stability should increase in the row Nb < Db < Ta. Several mechanisms of adsorption were considered. In the case of adsorption by van der Waals forces, the sequence in volatility of the pentabromides should be Nb < Ta < Db, being in agreement with the sublimation enthalpies of the Nb and Ta pentabromides. In the case of adsorption by chemical forces (on a quartz surface modified with KBr/KCl), formation of the MBr5L− (L = Cl, Br) complex should occur, so that the volatility should change in an opposite way, i.e., Nb > Ta > Db. This sequence is in agreement with the one observed in the “one-atom-at-a-time” chromatography experiments. Some other scenarios, such as surface oxide formation were also considered but found to be irrelevant.
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31.15.es Applications of density-functional theory (e.g., to electronic structure and stability; defect formation; dielectric properties, susceptibilities; viscoelastic coefficients; Rydberg transition frequencies)
34.20.Cf Interatomic potentials and forces
82.80.Bg Chromatography
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