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14 Dec 2008

Volume 129, Issue 22, Articles (22xxxx)

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back to top Theoretical Methods and Algorithms

Energy derivatives in quantum Monte Carlo involving the zero-variance property

A. Badinski, J. R. Trail, and R. J. Needs

J. Chem. Phys. 129, 224101 (2008); http://dx.doi.org/10.1063/1.3013817 (7 pages) | Cited 5 times

Online Publication Date: 8 December 2008

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We give an exact expression for the nth derivative of the expectation value of the energy that satisfies the zero-variance (ZV) principle when the wave function and its first n derivatives are exact. The ZV principle was previously applied to the first energy derivative (“force”) within the variational Monte Carlo and mixed-estimator diffusion Monte Carlo methods. We present a new expression for the force in pure-estimator diffusion Monte Carlo that satisfies the ZV principle and can be evaluated much more efficiently than previous expressions while maintaining comparable accuracy. This expression is the sum of a pure expectation value and a variational expectation value, which separately satisfy the ZV principle. The bias in this force estimator is second order in the deviation of the trial wave function from the diffusion Monte Carlo wave function. Results for small molecules demonstrate the accuracy of the method and its statistical efficiency.
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31.15.xt Variational techniques
33.15.Dj Interatomic distances and angles
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry

Relativistic E×T Jahn–Teller effect in tetrahedral systems

Leonid V. Poluyanov and Wolfgang Domcke

J. Chem. Phys. 129, 224102 (2008); http://dx.doi.org/10.1063/1.3035189 (7 pages) | Cited 4 times

Online Publication Date: 8 December 2008

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It is shown that math states in tetrahedral systems exhibit a linear E×T Jahn–Teller effect which is of purely relativistic origin (that is, it arises from the spin-orbit-coupling operator). The electrostatic interactions give rise to a Jahn–Teller effect which is quadratic in the T displacements. The 4×4 Hamiltonian matrix in a diabatic spin-electron basis is derived by an expansion of the electrostatic electronic Hamiltonian and the Breit–Pauli spin-orbit operator in powers of the Jahn–Teller active normal mode and taking account of symmetry selection rules for the matrix elements. The adiabatic potential-energy functions of the math×T system are doubly degenerate (Kramers degeneracy). For small displacements from the tetrahedral reference geometry, the adiabatic potential-energy surfaces represent a double cone in four-dimensional space, which is a novel topography of Jahn–Teller potential-energy surfaces. The topological phases of the adiabatic electronic wave functions are discussed.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.50.-x Potential energy surfaces
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Relativistic calculation of nuclear magnetic shielding tensor using the regular approximation to the normalized elimination of the small component. III. Introduction of gauge-including atomic orbitals and a finite-size nuclear model

S. Hamaya, H. Maeda, M. Funaki, and H. Fukui

J. Chem. Phys. 129, 224103 (2008); http://dx.doi.org/10.1063/1.3028047 (10 pages) | Cited 5 times

Online Publication Date: 9 December 2008

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The relativistic calculation of nuclear magnetic shielding tensors in hydrogen halides is performed using the second-order regular approximation to the normalized elimination of the small component (SORA-NESC) method with the inclusion of the perturbation terms from the metric operator. This computational scheme is denoted as SORA-Met. The SORA-Met calculation yields anisotropies, Δσ = σ∥−σ⊥, for the halogen nuclei in hydrogen halides that are too small. In the NESC theory, the small component of the spinor is combined to the large component via the operator math⋅mathU/2c, in which math = math+math, U is a nonunitary transformation operator, and c ≅ 137.036 a.u. is the velocity of light. The operator U depends on the vector potential math (i.e., the magnetic perturbations in the system) with the leading order c−2 and the magnetic perturbation terms of U contribute to the Hamiltonian and metric operators of the system in the leading order c−4. It is shown that the small Δσ for halogen nuclei found in our previous studies is related to the neglect of the U(0,1) perturbation operator of U, which is independent of the external magnetic field and of the first order with respect to the nuclear magnetic dipole moment. Introduction of gauge-including atomic orbitals and a finite-size nuclear model is also discussed.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
31.15.xp Perturbation theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Using neural networks, optimized coordinates, and high-dimensional model representations to obtain a vinyl bromide potential surface

Sergei Manzhos and Tucker Carrington, Jr.

J. Chem. Phys. 129, 224104 (2008); http://dx.doi.org/10.1063/1.3021471 (8 pages) | Cited 17 times

Online Publication Date: 9 December 2008

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We demonstrate that it is possible to obtain good potentials using high-dimensional model representations (HDMRs) fitted with neural networks (NNs) from data in 12 dimensions and 15 dimensions. The HDMR represents the potential as a sum of lower-dimensional functions and our NN-based approach makes it possible to obtain all of these functions from one set of fitting points. To reduce the number of terms in the HDMR, we use optimized redundant coordinates. By using exponential neurons, one obtains a potential in sum-of-products form, which greatly facilitates quantum dynamics calculations. A 12-dimensional (reference) potential surface for vinyl bromide is first refitted to show that it can be represented as a sum of two-dimensional functions. To fit 3d functions of the original coordinates, to improve the potential, a huge amount of data would be required. Redundant coordinates avoid this problem. They enable us to bypass the combinatorial explosion of the number of terms which plagues all HDMR and multimode-type methods. We also fit to a set of ∼ 70 000 ab initio points for vinyl bromide in 15 dimensions [ M. Malshe et al., J. Chem. Phys. 127, 134105 (2007) ] and show that it is possible to obtain a surface in sum-of-products form of quality similar to the quality of the full-dimensional fit. Although we obtain a full-dimensional surface, we limit the cost of the fitting by building it from fits of six-dimensional functions, each of which requires only a small NN.
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31.50.-x Potential energy surfaces
31.15.A- Ab initio calculations

Efficient evaluation of analytic Fukui functions

Roberto Flores-Moreno, Junia Melin, J. V. Ortiz, and Gabriel Merino

J. Chem. Phys. 129, 224105 (2008); http://dx.doi.org/10.1063/1.3036926 (6 pages) | Cited 5 times

Online Publication Date: 10 December 2008

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An efficient method for the analytic evaluation of Fukui functions is proposed. Working equations are derived and numerical results are used to validate the method on medium size set of molecules. In addition to the obvious advantages of analytic differentiation, the proposed method is efficient enough to be considered a practical alternative to the finite difference formulation used routinely. The reliability of the approximations used here is demonstrated and discussed. Problems found in other methods for prediction of electrophilic centers are corrected automatically when using the new method.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
02.30.-f Function theory, analysis

Non-Markovian suppression of charge qubit decoherence in the quantum point contact measurement

Ming-Tsung Lee and Wei-Min Zhang

J. Chem. Phys. 129, 224106 (2008); http://dx.doi.org/10.1063/1.3036114 (10 pages) | Cited 5 times

Online Publication Date: 10 December 2008

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A nonequilibrium theory describing the charge qubit dynamics measured by a quantum point contact is developed based on Schwinger–Keldysh’s approach. Using the real-time diagrammatic technique, we derive the master equation to all orders in perturbation expansions. The non-Markovian processes in the qubit dynamics is naturally taken into account. The qubit decoherence, in particular, the influence of the tunneling-electron fluctuation in the quantum point contact with a longer correlation time comparing to the time scale of the qubit dynamics, is studied in the framework. We consider the Lorentzian-type spectral density to characterize the channel mixture of the electron-tunneling processes induced by the measurement, and determine the correlation time scale of the tunneling-electron fluctuation. The result shows that as the quantum point contact is casted with a narrower profile of the spectral density, tunneling electrons propagate in a longer correlation time scale and lead to the non-Markovian processes of the qubit dynamics. The qubit electron in the charge qubit can be driven coherently. The quantum point contact measurement with the minimum deviation of the electron-tunneling processes prevents the qubit state from the decoherence.
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05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
03.67.Lx Quantum computation architectures and implementations
02.50.Ey Stochastic processes

Multiple state transition path sampling

Jutta Rogal and Peter G. Bolhuis

J. Chem. Phys. 129, 224107 (2008); http://dx.doi.org/10.1063/1.3029696 (9 pages) | Cited 13 times

Online Publication Date: 10 December 2008

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We developed a multiple state transition path sampling (TPS) approach in which it is possible to simultaneously sample pathways connecting a number of different stable states. Based on the original formulation of the TPS we have extended the path ensemble to include trajectories connecting not only two distinct stable states but any two states defined within a system. The multiple state TPS approach is useful in complex systems exhibiting a number of intermediate stable states that are interconnected in phase space. Combining this approach with transition interface sampling we can also directly obtain an expression for the rate constants of all possible transitions within the system.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Uv Stochastic theories of rate constants
87.15.hp Conformational changes
87.15.A- Theory, modeling, and computer simulation
87.14.E- Proteins
87.15.R- Reactions and kinetics

Efficient solution of Poisson’s equation using discrete variable representation basis sets for Car–Parrinello ab initio molecular dynamics simulations with cluster boundary conditions

Hee-Seung Lee and Mark E. Tuckerman

J. Chem. Phys. 129, 224108 (2008); http://dx.doi.org/10.1063/1.3036423 (11 pages) | Cited 2 times

Online Publication Date: 11 December 2008

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An efficient computational approach to perform Car–Parrinello ab initio molecular dynamics (CPAIMD) simulations under cluster (free) boundary conditions is presented. The general approach builds upon a recent real-space CPAIMD formalism using discrete variable representation (DVR) basis sets [ Y. Liu et al., Phys. Rev. B 12, 125110 (2003); H.-S. Lee and M. E. Tuckerman, J. Phys. Chem. A 110, 5549 (2006) ]. In order to satisfy cluster boundary conditions, a DVR based on sinc functions is utilized to expand the Kohn–Sham orbitals and electron density. Poisson’s equation is solved in order to calculate the Hartree potential via an integral representation of the 1/r singularity. Excellent convergence properties are achieved with respect to the number of grid points (or DVR functions) and the size of the simulation cell. A straightforward implementation of the present approach leads to near linear scaling [O(N4/3)] of the computational cost with respect to the system size (N) for the solution of Poisson’s equation. The accuracy and stability of CPAIMD simulations based on sinc DVR are tested for a model problem as well as for N2 and a water dimer.
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31.15.ae Electronic structure and bonding characteristics
31.15.xv Molecular dynamics and other numerical methods
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Computation of vibrational energy levels and eigenstates of fluoroform using the multiconfiguration time-dependent Hartree method

Loïc Joubert Doriol, Fabien Gatti, Christophe Iung, and Hans-Dieter Meyer

J. Chem. Phys. 129, 224109 (2008); http://dx.doi.org/10.1063/1.3020716 (9 pages) | Cited 13 times

Online Publication Date: 11 December 2008

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A theoretical study of the vibrational spectrum of the CHF3 molecule is carried out with the aid of the multiconfiguration time-dependent Hartree (MCTDH) algorithm. In order to obtain the eigenvalues and the eigenstates, recent developments in the MCTDH improved relaxation method in a block form are exploited. Around 80 eigenvalues are reported, which are converged with a very high accuracy. The results obtained with our study are compared with those of a previous work using the wave operator sorting algorithm approach. The present investigation exemplifies the robustness and the accuracy of the improved relaxation method.
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33.20.Tp Vibrational analysis
31.15.xr Self-consistent-field methods
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Internal energy of HCl upon photolysis of 2-chloropropene at 193 nm investigated with time-resolved Fourier-transform spectroscopy and quasiclassical trajectories

Chih-Min Chang, Yu-Hsuan Huang, Suet-Yi Liu, Yuan-Pern Lee, Marta Pombar-Pérez, Emilio Martínez-Núñez, and Saulo A. Vázquez

J. Chem. Phys. 129, 224301 (2008); http://dx.doi.org/10.1063/1.3023149 (11 pages) | Cited 1 time

Online Publication Date: 8 December 2008

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Following photodissociation of 2-chloropropene (H2CCClCH3) at 193 nm, vibration-rotationally resolved emission spectra of HCl (υ ≤ 6) in the spectral region of 1900–2900 cm−1 were recorded with a step-scan time-resolved Fourier-transform spectrometer. All vibrational levels show a small low-J component corresponding to ∼ 400 K and a major high-J component corresponding to 7100–18 700 K with average rotational energy of 39±311 kJ mol−1. The vibrational population of HCl is inverted at υ = 2, and the average vibrational energy is 86±5 kJ mol−1. Two possible channels of molecular elimination producing HCl+propyne or HCl+allene cannot be distinguished positively based on the observed internal energy distribution of HCl. The observed rotational distributions fit qualitatively with the distributions of both channels obtained with quasiclassical trajectories (QCTs), but the QCT calculations predict negligible populations for states at small J. The observed vibrational distribution agrees satisfactorily with the total QCT distribution obtained as a weighted sum of contributions from both four-center elimination channels. Internal energy distributions of HCl from 2-chloropropene and vinyl chloride are compared.
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82.50.Hp Processes caused by visible and UV light
82.80.Gk Analytical methods involving vibrational spectroscopy
82.80.Ha Analytical methods involving rotational spectroscopy
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Vibrational signatures of hydrogen bonding in the protonated ammonia clusters NH4+(NH3)1−4

Y. Yang, O. Kühn, G. Santambrogio, D. J. Goebbert, and K. R. Asmis

J. Chem. Phys. 129, 224302 (2008); http://dx.doi.org/10.1063/1.3028211 (8 pages) | Cited 10 times

Online Publication Date: 8 December 2008

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The gas phase vibrational spectroscopy of the protonated ammonia dimer N2H7+, a prototypical system for strong hydrogen bonding, is studied in the spectral region from 330 to 1650 cm−1 by combining infrared multiple photon dissociation and multidimensional quantum mechanical simulations. The fundamental transition of the antisymmetric proton stretching vibration is observed at 374 cm−1 and assigned on the basis of a six-dimensional model Hamiltonian, which predicts this transition at 471 cm−1. Photodissociation spectra of the larger protonated ammonia clusters NH4+(NH3)n with n = 2–4 are also reported for the range from 1050 to 1575 cm−1. The main absorption features can be assigned within the harmonic approximation, supporting earlier evidence that hydrogen bonding in these clusters is considerably weaker than for n = 1.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Ea Infrared spectra
31.15.bw Coupled-cluster theory

The remarkable influence of an “insignificant” quantity: How recoil orbital angular momentum determines product j distributions and (v;j) correlation in H+LH reactions

Anthony J. McCaffery

J. Chem. Phys. 129, 224303 (2008); http://dx.doi.org/10.1063/1.3029665 (10 pages) | Cited 1 time

Online Publication Date: 9 December 2008

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Reactions for which the reactant (r)-to-product (p) mass ratio (μr/μp) is high, the well-known H+LH→HH+L processes, convert most of available energy to product rotation, while that disposed as recoil is often regarded as negligible. In angular momentum (AM) terms, however, this recoil orbital AM (lp) is shown to be a critical component of the overall AM balance. For products of light μp, the maximum value of lp is energy limited and as a result the formation of products in low rotational (jp) states is severely restricted. Here energy constraints on recoil orbital AM and the consequent restrictions on jp-state populations are quantified using novel diagrammatic methods that illustrate how constraints on lp determine the jp states that are allowed or forbidden by the need to conserve energy and AM for each state-to-state transition. The method accurately predicts jBaIj (v = 0,1,2) peaks from crossed-beam Ba+HI experiments, providing a quantitative and physically transparent rationale for the observed BaI rotational distributions. Extension to a wider range of reactions having μr/μp>1 shows that at least some jp are formally forbidden for each given reactant relative velocity or, more accurately, lr. The fraction of inaccessible product states for a given initial velocity rises rapidly with μr/μp (>96% in Ba+HI). The method is also used to demonstrate that recoil orbital AM will be strongly aligned parallel to product rotational AM for high μr/μp, although this correlation is generally lost in the low jp region as the parallel vector requirement is relaxed.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Hf Product distribution
82.60.Cx Enthalpies of combustion, reaction, and formation

A combined experimental/theoretical investigation of the near-infrared photodissociation of IBr−(CO2)n

Matthew A. Thompson, Joshua P. Martin, Joshua P. Darr, W. Carl Lineberger, and Robert Parson

J. Chem. Phys. 129, 224304 (2008); http://dx.doi.org/10.1063/1.3033746 (12 pages) | Cited 4 times

Online Publication Date: 9 December 2008

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We report the collaborative experimental and theoretical study of the time-resolved recombination dynamics of photodissociated IBr−(CO2)n clusters. Excitation of the bare anionic chromophore to the dissociative A′ 2Π1/2 state yields only I− and Br products. Interestingly, however, the addition of a few solvent molecules promotes recombination of the dissociating chromophore on the X 2Σ1/2+ ground state, which correlates asymptotically with Br− and I products. This process is studied experimentally using time-resolved, pump-probe techniques and theoretically via nonadiabatic molecular dynamics simulations. In sharp contrast to previous I2− studies where more kinetic energy was released to the photofragments, the observed recombination times increase from picoseconds to nanoseconds with increasing cluster size up to n = 10. The recombination times then drop dramatically back to picoseconds for cluster sizes n = 11–14. This trend, seen both in experiment and theory, is explained by the presence of a solvent-induced well on the A′ state, the depth of which directly corresponds to the asymmetry of the solvation about the chromophore. The results seen for both the branching ratios and recombination times from experiment and theory show good qualitative agreement.
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82.50.Bc Processes caused by infrared radiation
82.80.Dx Analytical methods involving electronic spectroscopy
82.20.Kh Potential energy surfaces for chemical reactions
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Wt Computational modeling; simulation

Rotationally resolved studies of S0 and the exciton coupled S1/S2 origin regions of diphenylmethane and the d12 isotopologue

Jaime A. Stearns, Nathan R. Pillsbury, Kevin O. Douglass, Christian W. Müller, Timothy S. Zwier, and David F. Plusquellic

J. Chem. Phys. 129, 224305 (2008); http://dx.doi.org/10.1063/1.3028543 (13 pages) | Cited 7 times

Online Publication Date: 9 December 2008

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Rotationally resolved microwave and ultraviolet spectra of jet-cooled diphenylmethane (DPM) and DPM-d12 have been obtained in S0, S1, and S2 electronic states using Fourier-transform microwave and UV laser/molecular beam spectrometers. The S0 and S1 states of both isotopologues have been well fit to asymmetric rotor Hamiltonians that include only Watson distortion parameters. The transition dipole moment (TDM) orientations of DPM and DPM-d12 are perpendicular to the C2 symmetry axes with 66(2)%:34(2)% a:c hybrid-type character, establishing the lower exciton S1 origin as a completely delocalized, antisymmetric combination of the zero-order locally excited states of the toluene-like chromophores. In contrast, the rotational structures of the S2 origin bands at S1+123 cm−1 and S1+116 cm−1, respectively, display b-type Q-branch transitions and lack the central a-type Q-branch features that characterize the S1 origins, indicating TDM orientations parallel to the C2(b) symmetry axes as anticipated for the upper exciton levels. However, rotational fits were not possible in line with expectations from previous work [ N. R. Pillsbury, J. A. Stearns, C. W. Müller, T. S. Zwier, and D. F. Plusquellic, J. Chem. Phys. 129, 114301 (2008) ] where the S2 origins were found to be largely perturbed through vibronic interactions with the S1 symmetric, antisymmetric torsional, and butterfly levels in close proximity. Predictions from a dipole-dipole coupling model and ab initio theories are shown to be in fair agreement with the observed TDM orientations and exciton splitting. The need to include out-of-ring-plane dipole coupling terms indicates that in-plane models are not sufficient to fully account for the excitonic interactions in this bichromophore.
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33.20.Bx Radio-frequency and microwave spectra
33.20.Lg Ultraviolet spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.A- Ab initio calculations

Formation of even-numbered hydrogen cluster cations in ultracold helium droplets

S. Jaksch, A. Mauracher, A. Bacher, S. Denifl, F. Ferreira da Silva, H. Schöbel, O. Echt, T. D. Märk, M. Probst, D. K. Bohme, and P. Scheier

J. Chem. Phys. 129, 224306 (2008); http://dx.doi.org/10.1063/1.3035833 (8 pages) | Cited 5 times

Online Publication Date: 10 December 2008

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Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered Hn+. The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered Hn+. The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered Hn+ up to n ≅ 120 by their mass excess that distinguishes them from Hen+, mixed HemHn+, and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n = 6, pronounced drops in the abundance of even-numbered cluster ions are seen at n = 30 and 114, which suggest icosahedral shell closures at H6+(H2)12 and H6+(H2)54. Possible isomers of H6+ are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory
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36.40.Qv Stability and fragmentation of clusters
36.40.Wa Charged clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
34.80.Gs Molecular excitation and ionization
31.15.V- Electron correlation calculations for atoms, ions and molecules

Ab initio characterization of the conical intersections involved in the photochemistry of phenol

Olivier P. J. Vieuxmaire, Zhenggang Lan, Andrzej L. Sobolewski, and Wolfgang Domcke

J. Chem. Phys. 129, 224307 (2008); http://dx.doi.org/10.1063/1.3028049 (10 pages) | Cited 9 times

Online Publication Date: 10 December 2008

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The nature of the vibronic interactions between the 1ππ∗ (A′), the 1πσ∗ (A″), and the S0 (A′) states at the CIππ∗/πσ∗ and CIπσ∗/ππ conical intersections has been investigated by accurate ab initio calculations. Potential energy surfaces have been constructed at the complete-active-space self-consistent-field and multireference configuration-interaction (MRCI) levels of theory along each of the ten normal coordinates of A″ symmetry that potentially can be coupling modes at these conical intersections. The OH torsion was found to be by far the strongest coupling mode in each case. As for benzene, a “channel three” radiationless decay mechanism associated with a prefulvenic conical intersection, CIpref, was found to exist in phenol. The reaction path connecting the prefulvenic form of phenol with the minimum-energy structure of the S1 state was computed at different levels of theory. The barrier to be overcome for the opening of the prefulvenic decay channel is estimated as 6370 cm−1 at the MRCI level, that is, about 2300 cm−1 above the energy of CIππ∗/πσ∗. With sufficient excess energy in the S1 state, the prefulvenic decay mechanism can be in competition with the hydrogen detachment process.
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82.50.-m Photochemistry
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.xr Self-consistent-field methods
31.50.-x Potential energy surfaces
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Vinylphosphine-borane: Synthesis, gas phase infrared spectroscopy, and quantum chemical vibrational calculations

Brahim Khater, Jean-Claude Guillemin, Abdessamad Benidar, Didier Bégué, and Claude Pouchan

J. Chem. Phys. 129, 224308 (2008); http://dx.doi.org/10.1063/1.3035902 (11 pages) | Cited 4 times

Online Publication Date: 10 December 2008

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Both experimental and theoretical investigations are reported on the infrared spectrum of vinylphosphine-borane (CH2 = CHPH2⋅BH3), a donor-acceptor complex. The gas phase infrared spectra (3500–600 cm−1) have been recorded at 0.5 cm−1 resolution. This first primary α,β-unsaturated phosphine-borane synthesized up to now is kinetically very unstable in the gas phase and decomposes rapidly into two fragments: the free vinylphosphine CH2 = CHPH2 and the monoborane BH3 which dimerizes to form the more stable diborane B2H6. Spectra of free CH2 = CHPH2 and B2H6 compounds were also recorded to assign some vibration modes of the complex in very dense spectral regions. The analysis was completed by carrying out quantum mechanical calculations by density functional theory method at the B3LYP/6-31+G∗∗ level. Anharmonic frequencies and infrared intensities of the two predicted gauche and syn conformers of the vinylphosphine-borane complex were calculated in the 3500–100 cm−1 region with the use of a variational approach, implemented in the P_ANHAR_V1.2 code. Because of the relatively weak interaction between the vinylphosphine and the monoborane, the vibrations of the complex can easily be subdivided into modes localized in the CH2 = CHPH2 and BH3 moieties and into “intermolecular” modes. Localized modes are unambiguously correlated with the modes of the isolated monomers. Therefore, they are described in terms of the monomer vibrations, and the complexation shifts are defined as Δν = νcomplex–νmonomer to make the effect of the complexation precise on each localized mode. In this objective, anharmonic frequencies and infrared intensities of the BH3 monomer and the stable gauche and syn conformers of the free vinylphosphine were obtained at the same level of theory. In the gas phase, only the syn form of the complex was observed and assigned. All theoretically predicted frequencies and complexation shifts in magnitude and direction are in good agreement with experiment. By infrared spectroscopy assisted by quantum chemical calculations, the consequences of the complexation of an α,β-unsaturated phosphine by borane on the physicochemical properties of the formed 12-atom complex have been efficiently evaluated.
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78.30.Jw Organic compounds, polymers
71.20.Rv Polymers and organic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
63.20.Ry Anharmonic lattice modes
63.20.Pw Localized modes

Quantum dynamics of the O+OH→H+O2 reaction at low temperatures

Goulven Quéméner, Naduvalath Balakrishnan, and Brian K. Kendrick

J. Chem. Phys. 129, 224309 (2008); http://dx.doi.org/10.1063/1.3035904 (7 pages) | Cited 9 times

Online Publication Date: 11 December 2008

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We report quantum dynamics calculations of the O+OH→H+O2 reaction on two different representations of the electronic ground state potential energy surface (PES) using a time-independent quantum formalism based on hyperspherical coordinates. Calculations show that several excited vibrational levels of the product O2 molecule are populated in the reaction. Rate coefficients evaluated using both PESs were found to be very sensitive to the energy resolution of the reaction probability, especially at temperatures lower than 100 K. It is found that the rate coefficient remains largely constant in the temperature range of 10–39 K, in agreement with the conclusions of a recent experimental study [ Carty et al., J. Phys. Chem. A 110, 3101 (2006) ]. This is in contrast with the time-independent quantum calculations of Xu et al. [J. Chem. Phys. 127, 024304 (2007) ] which, using the same PES, predicted nearly two orders of magnitude drop in the rate coefficient value from 39 to 10 K. Implications of our findings to oxygen chemistry in the interstellar medium are discussed.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Measurement of the electron attachment rates for SF6 and C7F14 at Te = 0.2 eV in a magnetized Q machine plasma

Robert L. Merlino and Su-Hyun Kim

J. Chem. Phys. 129, 224310 (2008); http://dx.doi.org/10.1063/1.3039078 (5 pages) | Cited 2 times

Online Publication Date: 12 December 2008

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Electron attachment rates for SF6 and C7F14 were measured in a magnetized Q machine plasma at an electron temperature of 0.2±0.02 eV and with neutral gas pressures of P ∼ 10−4 Torr. The rate constants for attachment to SF6 and C7F14 were (7.6±2.0)×10−8 and (2.2±0.9)×10−7 cm3 s−1, respectively.
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34.80.Lx Recombination, attachment, and positronium formation
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Theoretical study of unimolecular decomposition of allene cations

A. M. Mebel and A. D. Bandrauk

J. Chem. Phys. 129, 224311 (2008); http://dx.doi.org/10.1063/1.3037204 (12 pages) | Cited 7 times

Online Publication Date: 12 December 2008

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Ab initio coupled clusters and multireference perturbation theory calculations with geometry optimization at the density functional or complete active space self-consistent-field levels have been carried out to compute ionization energies and to unravel the dissociation mechanism of allene and propyne cations, C3H4n+ (n = 1–3). The results indicate that the dominant decomposition channel of the monocation is c-C3H3++H, endothermic by 37.9 kcal/mol and occurring via a barrier of 43.1 kcal/mol, with possible minor contributions from H2CCCH++H and HCCCH++H2. For the dication, the competing reaction channels are predicted to be c-C3H3++H+, H2CCCH++H+, and CCCH++H3+, with dissociation energies of −20.5, 8.5, and 3.0 kcal/mol, respectively. The calculations reveal a H2-roaming mechanism for the H3+ loss, where a neutral H2 fragment is formed first, then roams around and abstracts a proton from the remaining molecular fragment before leaving the dication. According to Rice–Ramsperger–Kassel–Marcus calculations of energy-dependent rate constants for individual reaction steps, relative product yields vary with the available internal energy, with c-C3H3++H+ being the major product just above the dissociation threshold of 69.6 kcal/mol, in the energy range of 70–75 kcal/mol, and CCCH++H3+ taking over at higher energies. The C3H43+ trication is found to be not very stable, with dissociation thresholds of 18.5 and 3.7 kcal/mol for allene and propyne, respectively. Various products of Coulomb explosion of C3H43+, H2CCCH2++H+, CHCHCH2++H+, C2H22++CH2+, and CCH22++CH2+ are highly exothermic (by 98–185 kcal/mol). The tetracation of C3H4 is concluded to be unstable and therefore no more than three electrons can be removed from this molecule before it falls apart. The theoretical results are compared to experimental observations of Coulomb explosions of allene and propyne.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies
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)

The rotational spectrum of H32SOH and H34SOH above 1 THz

Oliver Baum, Monika Koerber, Oliver Ricken, Gisbert Winnewisser, Sergei N. Yurchenko, Stephan Schlemmer, Koichi M. T. Yamada, and Thomas F. Giesen

J. Chem. Phys. 129, 224312 (2008); http://dx.doi.org/10.1063/1.3034741 (6 pages) | Cited 3 times

Online Publication Date: 12 December 2008

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Accurate spectral data of H32SOH and H34SOH at 1.3 THz were recorded using a synthesizer based multiplier spectrometer. The spectra were analyzed together with data from an earlier study which contain measurements at 1.9 THz. The combination of both data sets allows to determine experimentally the tunneling splitting of energy levels with Ka = 4 and 5 for the first time. The obtained results are essential to test a novel model on torsional tunneling splitting in HSOH. Transitions with Ka = 1←0, Ka = 2←1, and Ka = 3←2 all exhibit strong c-type and somewhat weaker b-type transitions. In contrary, transitions with Ka = 4←3 display only c-type but no b-type transitions. The absence of b-type transitions is completely unexpected and yet not well understood. For the H34SOH isotopolog the data set has been substantially extended by the new measurements of rQ3-branch transitions at 1.3 THz. Based on the new data the accuracy of the H34SOH molecular parameters has been significantly improved.
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33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

A comparative study of two classical force fields on statics and dynamics of [EMIM][BF4] investigated via molecular dynamics simulations

Florian Dommert, Jochen Schmidt, Baofu Qiao, Yuanyuan Zhao, Christian Krekeler, Luigi Delle Site, Robert Berger, and Christian Holm

J. Chem. Phys. 129, 224501 (2008); http://dx.doi.org/10.1063/1.3030948 (10 pages) | Cited 20 times

Online Publication Date: 8 December 2008

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The influences of two different commonly employed force fields on statical and dynamical properties of ionic liquids are investigated for [EMIM][BF4]. The force fields compared in this work are the one of Canongia Lopes and Padua [J. Phys. Chem. B 110, 19586 (2006) ] and that of Liu et al. [J. Phys. Chem. B 108, 12978 (2004) ]. Differences in the strengths of hydrogen bonds are found, which are also reflected in the static ion distributions around the cation. Moreover, due to the stronger hydrogen bonding in the force field of Liu et al., the diffusive motions of cations and anions and the rotational behavior of the cations are slower compared with those obtained with the force field of Canongia Lopes and Padua. Both force fields underestimate the zero-field electrical conductivity, while the experimental dielectric constant can be reproduced within the expected statistical error boundaries.
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61.25.Em Molecular liquids
61.20.Ja Computer simulation of liquid structure
66.10.Ed Ionic conduction
66.10.cd Thermal diffusion and diffusive energy transport

Assessing the dephasing dynamics of water from linear field-resolved pulse propagation experiments and simulations in highly absorbing solutions

Julie A. Gruetzmacher, Rene A. Nome, Andrew M. Moran, and Norbert F. Scherer

J. Chem. Phys. 129, 224502 (2008); http://dx.doi.org/10.1063/1.2990654 (10 pages) | Cited 1 time

Online Publication Date: 9 December 2008

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We measure and simulate electric field distortions resulting from propagation of mid-infrared pulses that are resonant with the OH stretch vibration through optically dense HDO:D2O. These distortions are characterized experimentally by full-field-resolved time- and frequency-domain measurements, specifically cross-correlation frequency-resolved optical gating and spectral interferometry, establishing amplitude and phase of the signal fields. Correlation-function finite-difference time-domain (CF-FDTD) simulations using response functions for the OH-stretching vibration, obtained from nonlinear spectroscopic studies reported by others, show that details of the line shape functions are manifested in the measured (linear-response) spectrograms. The degree of homogeneous or inhomogeneous broadening present in the various model correlation functions is readily apparent in the measured and simulated signals. Surprisingly, the published correlation functions are shown to range from modest inhomogeneous to homogeneous line broadening. The present experimental and simulation approach is very useful for establishing the correct form of energy gap correlation functions and dephasing dynamics of IR and optical transitions. In the case of HDO:D2O, correlation functions with modest inhomogeneous broadening better reflect our measured responses.
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63.50.-x Vibrational states in disordered systems
78.30.C- Liquids
61.20.Gy Theory and models of liquid structure

Structural changes and preferential cage occupancy of ethane hydrate and methane-ethane mixed gas hydrate under very high pressure

Hisako Hirai, Naoya Takahara, Taro Kawamura, Yoshitaka Yamamoto, and Takehiko Yagi

J. Chem. Phys. 129, 224503 (2008); http://dx.doi.org/10.1063/1.3036006 (7 pages)

Online Publication Date: 9 December 2008

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High-pressure experiments of ethane hydrate and methane-ethane mixed hydrates with five compositions were performed using a diamond anvil cell in a pressure range of 0.1–2.8 GPa at room temperature. X-ray diffractometry and Raman spectroscopy showed structural changes as follows. The initial structure, structure I (sI), of ethane hydrate was retained up to 2.1 GPa without any structural change. For the mixed hydrates, sI was widely distributed throughout the region examined except for the methane-rich and lower pressure regions. For the ethane-rich and intermediate composition regions (73 mol % ethane sample and 53% sample), sI was maintained up to 2.1 GPa. With increasing methane component (34% and 30% samples), sI existed at pressures from 0.1 to about 1.0 GPa. Hexagonal structure (sH) appeared in addition to sI at 1.3 GPa for the 34% sample and at 1.1 GPa for the 30% sample. By further increasing the methane component (22% sample), structure II (sII) existed solely up to 0.3 GPa. From 0.3 to 0.6 GPa, sII and sI coexisted, and from 0.6 to 1.0 GPa only sI existed. At 1.2 GPa sH appeared, and sH and sI coexisted up to 2.1 GPa. Above 2.1 GPa, ethane hydrate and all of the mixed hydrates decomposed into ice VI and ethane fluid or methane-ethane fluid, respectively. The Raman study revealed that occupation of the small cages by ethane molecules occurred above 0.1 GPa in ethane hydrate and continued up to decomposition at 2.1 GPa, although it is thought that ethane molecules are contained only in the large cage.
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61.66.Hq Organic compounds
64.75.-g Phase equilibria
78.30.Jw Organic compounds, polymers
62.50.-p High-pressure effects in solids and liquids
64.70.D- Solid-liquid transitions

Statistics and kinetics of single-molecule electron transfer dynamics in complex environments: A simulation model study

Luciana C. Paula, Jin Wang, and Vitor B. P. Leite

J. Chem. Phys. 129, 224504 (2008); http://dx.doi.org/10.1063/1.3036421 (9 pages) | Cited 1 time

Online Publication Date: 9 December 2008

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Dynamics of the environments of complex systems such as biomolecules, polar solvents, and glass plays an important role in controlling electron transfer reactions. The kinetics is determined by the nature of a complex multidimensional landscape. By quantifying the mean and high-order statistics of the first-passage time and the associated ratios, the dynamics in electron transfer reactions controlled by the environments can be revealed. We consider real experimental conditions with finite observation time windows. At high temperatures, exponential kinetics is observed and there are multiple kinetic paths leading to the product state. At and below an intermediate temperature, nonexponential kinetics starts to appear, revealing the nature of the distribution of local traps on the landscape. Discrete kinetic paths emerge. At very low temperatures, nonexponential kinetics continues to be observed. We point out that the size of the observational time window is crucial in revealing the intrinsic nature of the real kinetics. The mean first-passage time is defined as a characteristic time. Only when the observational time window is significantly larger than this characteristic time does one have the opportunity to collect enough statistics to capture rare statistical fluctuations and characterize the kinetics accurately.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.37.Np Single molecule reaction kinetics, dissociation, etc.
82.20.Bc State selected dynamics and product distribution
82.20.Wt Computational modeling; simulation
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