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28 November 2009

Volume 131, Issue 20,  partial issue

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Cover image from Helen O. Leung, Mark D. Marshall, and Brent K. Amberger, J. Chem. Phys. 131, 204302 (2009).

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ARTICLES

Theoretical Methods and Algorithms

Obtaining Hartree–Fock and density functional theory doubly excited states with Car–Parrinello density matrix search

Wenkel Liang, Christine M. Isborn, and Xiaosong Li

J. Chem. Phys. 131, 204101 (2009) (6 pages)

Online Publication Date: 23 November 2009

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The calculation of doubly excited states is one of the major problems plaguing the modern day excited state workhorse methodology of linear response time dependent Hartree–Fock (TDHF) and density function theory (TDDFT). We have previously shown that the use of a resonantly tuned field within real-time TDHF and TDDFT is able to simultaneously excite both the alpha and beta electrons to achieve the two-electron excited states of minimal basis H2 and HeH+ [C. M. Isborn and X. Li, J. Chem. Phys. 129, 204107 (2008)]. We now extend this method to many electron systems with the use of our Car–Parrinello density matrix search (CP-DMS) with a first-principles fictitious mass method for wave function optimization [X. Li, C. L. Moss, W. Liang, and Y. Feng, J. Chem. Phys. 130, 234115 (2009)]. Real-time TDHF/TDDFT is used during the application of the laser field perturbation, driving the electron density toward the doubly excited state. The CP-DMS method then converges the density to the nearest stationary state. We present these stationary state doubly excited state energies and properties at the HF and DFT levels for H2, HeH+, lithium hydride, ethylene, and butadiene.
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31.15.xr, 31.15.ee, 34.80.-i, 31.50.Df

Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Fourier transform microwave spectroscopy and molecular structure of the 1,1-difluoroethylene-hydrogen fluoride complex

Helen O. Leung, Mark D. Marshall, Tasha L. Drake, Tadeuz Pudlik, Nazir Savji, and Daniel W. McCune

J. Chem. Phys. 131, 204301 (2009) (8 pages)

Online Publication Date: 23 November 2009

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Fourier transform microwave rotation spectra in the 7–21 GHz region are obtained for the complex formed between 1,1-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted 13C species obtained in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used, in combination with information from the analogous complex, 1,1-difluoroethylene-acetylene, to determine a structure for CH2CF2–HF. This structure is similar to that obtained for vinyl fluoride-HF [G. C. Cole and A. C. Legon, Chem. Phys. Lett. 400, 419 (2004)] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,1-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. A comparison of the structures of 1,1-difluoroethylene complexes with the protic acids HF, HCl, and HCCH demonstrates that the hydrogen bond length increases with decreasing gas-phase acid strength, whereas a comparison of HF complexes with vinyl fluoride, 1,1-difluoroethylene, and 1,1,2-trifluoroethylene indicates that the nucleophilicity of the F atoms decreases with increasing fluorine substitution, but that the secondary interaction length is remarkably similar in all three complexes.
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33.20.Bx, 33.15.Bh, 33.15.Pw, 33.15.Mt, 34.20.Gj, 33.15.Fm, 33.15.Dj

Fourier transform microwave spectroscopy and molecular structure of the trans-1,2-difluoroethylene-hydrogen fluoride complex

Helen O. Leung, Mark D. Marshall, and Brent K. Amberger

J. Chem. Phys. 131, 204302 (2009) (8 pages)

Online Publication Date: 23 November 2009

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Guided by ab initio calculations, Fourier transform microwave rotation spectra in the 6.5–22 GHz region are obtained for the complex formed between trans-1,2-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted 13C species in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used to determine a structure for trans-CHFCHF–HF. This structure is similar to that obtained for 1,1-difluoroethylene-HF [H. O. Leung et al., J. Chem. Phys. 131, 204301 (2009)] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,2-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. Because the two F atoms and the two H atoms in trans-1,2-difluoroethylene form electrostatically equivalent pairs, the structure of the complex with HF provides insight into the contribution of steric effects to the observed geometries of fluoroethylene-protic acid complexes. A comparison of the observed hydrogen bond lengths and deviations from linearity in 1,1-difluoroethylene-HF and trans-1,2-difluoroethylene-HF suggests that the F atoms in trans-1,2-difluoroethylene are more nucleophilic than those in 1,1-difluoroethylene and that the H atoms are similarly more acidic. Ab initio calculations of electrostatic potentials mapped onto total electron density surfaces for these two molecules support these conclusions.
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33.20.Bx, 33.15.Bh, 31.15.A-, 31.15.ae, 31.15.aj, 33.15.Pw

Construction of theoretical hybrid potential energy curves for LiH(X  1Sigma+)

Ian L. Cooper and Alan S. Dickinson

J. Chem. Phys. 131, 204303 (2009) (9 pages)

Online Publication Date: 23 November 2009

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Various all-electron and valence-electron potential energy curves for LiH(X  1Sigma+) are compared and assessed. Hybrid potential energy curves are constructed from all-electron potentials at short range and a valence-electron calculation otherwise. This approach provides for the X state of LiH an overall potential curve, which is ionic at equilibrium, and presents an avoided crossing with the excited A state, leading to neutral dissociation products. The classical turning points predicted by these purely theoretical hybrid potentials are compared with those of the experimentally based inverted-perturbation approach (IPA) potentials for both 7LiH and 7LiD. Predicted vibrational energy-level spacings show reasonable (<~1  cm−1) agreement with the corresponding IPA values. Rotation and vibration-rotation transition energies arising from the most accurate hybrid potential are shown to compare very favorably with recent high-resolution spectroscopic data on 7LiH and 7LiD.
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31.50.Df, 34.50.Ez, 33.20.Sn, 33.20.Tp

H atom formation from benzene and toluene photoexcitation at 248 nm

Tamás Kovács, Mark A. Blitz, Paul W. Seakins, and Michael J. Pilling

J. Chem. Phys. 131, 204304 (2009) (12 pages)

Online Publication Date: 23 November 2009

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The technique of excimer laser excitation/Lyman alpha H atom laser induced fluorescence was used to investigate the formation of H atoms from the 248 nm photoexcitation of benzene and toluene. The H atom signal dependence on laser excitation energy demonstrated that it is produced from two photon photolysis of the aromatics; absorption of the first photon populates the bound 1B2u level followed by absorption from this level to a dissociative level, which produces H atoms, among other potential channels. Analysis of the data yields the second photon absorption cross section to produce H and is equal to 1.0 and 5.2×10−19  cm2 for benzene and toluene, respectively. In addition, the yield of H atoms was observed to be pressure dependent. This is because at sufficiently high pressures the nanosecond lifetime of the 1B2u state can be pressure quenched and hence may compete with the absorption of the second photon. The yields of H atoms were determined as a function of pressure for a range of the laser energies and with various collider gases. The analysis of these data allowed the total absorption cross section for the second photon to be determined and is equal to 2.8 and 1.7×10−17  cm2 for benzene and toluene, respectively. In addition, the rate constants for quenching 1B2u with various gases (He, Ar, N2, and O2) were determined. This large absorption coefficient for the second photon implies that with a pulsed laser source of 248 nm it is difficult to avoid aromatic photodissociation. We highlight a few previous studies that may need to be reevaluated in the light of the results from this study.
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33.80.Gj, 33.80.Rv, 33.50.Dq

Binary rototranslational hyper-Rayleigh spectra of H2–He gas mixture

J.-L. Godet, T. Bancewicz, W. Glaz, G. Maroulis, and A. Haskopoulos

J. Chem. Phys. 131, 204305 (2009) (6 pages)

Online Publication Date: 23 November 2009

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The collision-induced rototranslational hyper-Rayleigh spectra of gaseous H2–He mixture are computed and discussed in the binary regime. As the input data we use our ab initio computed H2–He collision-induced first dipole hyperpolarizability tensor Deltabeta(R). Both the vector and the septor part of the H2–He hyper-Rayleigh spectra are evaluated at room temperature (T=295  K). The spectra are calculated assuming the full quantum computations based on the Schrödinger equation of the relative translational motion in the isotropic H2–He potential as well as using semiclassical methods.
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33.20.Fb, 34.50.Ez, 33.15.Kr

The effect of intracluster photoelectron interactions on the angular distribution in I−·CH3I photodetachment

Matthew Van Duzor, Jie Wei, Foster Mbaiwa, and Richard Mabbs

J. Chem. Phys. 131, 204306 (2009) (8 pages)

Online Publication Date: 23 November 2009

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I− and I−·CH3I velocity mapped photodetachment images are recorded over wavelengths between 270 and 370 nm. Spectral similarities, in conjunction with ab initio calculations show that the cluster anion comprises an atomic iodide anion moiety solvated by a relatively unperturbed CH3I molecule. Between 340–370 nm and at 280 nm, free I− is produced via a process analogous to dissociative electron attachment within the cluster anion. More strikingly, the photoelectron angular distribution for each species at a given electron kinetic energy is very different, despite detachment occurring from the iodide 5p orbital in each case. These observations reveal the effect of interaction of the photoelectron with the neutral cluster residue and are discussed in terms of resonances associated primarily with the CH3I molecule.
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33.80.Eh, 31.15.A-, 33.60.+q, 36.40.Mr, 33.80.Gj

Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Stabilizing a 22 karat nanogolden cage

Q. Wang, Q. Sun, and P. Jena

J. Chem. Phys. 131, 204501 (2009) (5 pages)

Online Publication Date: 23 November 2009

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Since the discovery of C60 fullerene, considerable efforts have been devoted to find other elements with similar hollow cage structures. However, search for hollow metallic cages with a diameter similar to that of C60 fullerene has been elusive. We describe a procedure for the rational design of metallic cages by suitably choosing their size, composition, and charge state. A 22 karat nanogolden cage with a diameter of about 8.5 Å and consisting of 12 Al and 20 Au atoms is found to be metastable, which can be stabilized by embedding a Mn4 cluster. In contrast to bulk Mn, which is antiferromagnetic, and isolated Mn4 cluster, which is ferromagnetic with a giant magnetic moment of 20µB, the Mn4@Al12Au20 endohedral complex exhibits magnetic bistability with 0µB and 14µB configurations being energetically nearly degenerate. These results, based on density functional theory, open the door to design a novel class of endohedral complexes with possible applications.
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61.46.Bc, 75.50.Cc, 75.30.Cr

Surfaces, Interfaces, and Materials

Single-layered chrysotile nanotubes: A quantum mechanical ab initio simulation

Philippe D'Arco, Yves Noel, Raffaella Demichelis, and Roberto Dovesi

J. Chem. Phys. 131, 204701 (2009) (7 pages)

Online Publication Date: 23 November 2009

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Chrysotile single-layered nanotubes, obtained by wrapping the Mg3Si2O5(OH)4 lizardite monolayer along the (n,−n) hexagonal lattice vector, are simulated at the ab initio level by using an all electron 6-31G* basis set and the B3LYP functional for n varying from 14 to 24 (the nanotube radius R referred to the oxygen connecting the Mg and Si layers increases from 20 to 35 Å). Because of the full exploitation of the helical symmetry, recently implemented in the CRYSTAL code, the computational cost for the full self-consistent field (SCF) and gradient calculation increases only by a factor of 2 and 1.2, respectively, when passing from the lizardite monolayer [18 atoms and 236 AOs (atomic orbitals) in the unit cell] to the (24, −24) tube (864 atoms and 11 328 AOs). The total energy of the tubes is always larger than that of the lizardite monolayer; the difference DeltaE decreases very rapidly with n; for the largest tube here considered (n=24) DeltaE is as small as 2.7 kJ/mol per formula unit (f.u.); extrapolating to larger n values, at about R=50  Å, DeltaE becomes smaller than 1 kJ mol f.u. Very large energy gains are observed for small n values during optimization after rolling, mainly due to the rotation of the SiO4 tetrahedra that are in the inner part of the cylinder (“normal rolling”); such a rigid rotation accounts for about 85% of the overall relaxation energy. “Inverse rolling” tubes (SiO4 on the external wall of the tube) are shown to be less stable than the corresponding “normal” tubes.
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61.50.Ah, 61.46.Fg

Polymers and Complex Systems

Atomic motions in poly(vinyl methyl ether): A combined study by quasielastic neutron scattering and molecular dynamics simulations in the light of the mode coupling theory

S. Capponi, A. Arbe, F. Alvarez, J. Colmenero, B. Frick, and J. P. Embs

J. Chem. Phys. 131, 204901 (2009) (12 pages)

Online Publication Date: 23 November 2009

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Quasielastic neutron scattering experiments (time-of-flight, neutron spin echo, and backscattering) on protonated poly(vinyl methyl ether) (PVME) have revealed the hydrogen dynamics above the glass-transition temperature. Fully atomistic molecular dynamics simulations properly validated with the neutron scattering results have allowed further characterization of the atomic motions accessing the correlation functions directly in real space. Deviations from Gaussian behavior are found in the high-momentum transfer range, which are compatible with the predictions of mode coupling theory (MCT). We have applied the MCT phenomenological version to the self-correlation functions of PVME atoms calculated from our simulation data, obtaining consistent results. The unusually large value found for the lambda-exponent parameter is close to that recently reported for polybutadiene and simple polymer models with intramolecular barriers.
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61.41.+e, 61.43.Bn, 82.35.Jk, 64.70.pj

Critical Casimir interaction of ellipsoidal colloids with a planar wall

S. Kondrat, L. Harnau, and S. Dietrich

J. Chem. Phys. 131, 204902 (2009) (10 pages)

Online Publication Date: 23 November 2009

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Based on renormalization group concepts and explicit mean field calculations, we study the universal contribution to the effective force and torque acting on an ellipsoidal colloidal particle, which is dissolved in a critical fluid and is close to a homogeneous planar substrate. At the same closest distance between the substrate and the surface of the particle, the ellipsoidal particle prefers an orientation parallel to the substrate, and the magnitude of the fluctuation-induced force is larger than if the orientation of the particle is perpendicular to the substrate. The sign of the critical torque acting on the ellipsoidal particle depends on the type of boundary conditions for the order parameter at the particle and substrate surfaces and on the pivot with respect to which the particle rotates.
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82.70.Dd, 05.70.Jk

Biological Molecules, Biopolymers, and Biological Systems

Free energies of stable and metastable pores in lipid membranes under tension

Wouter K. den Otter

J. Chem. Phys. 131, 205101 (2009) (9 pages)

Online Publication Date: 23 November 2009

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The free energy profile of pore formation in a lipid membrane, covering the entire range from a density fluctuation in an intact bilayer to a large tension-stabilized pore, has been calculated by molecular dynamics simulations with a coarse-grained lipid model. Several fixed elongations are used to obtain the Helmholtz free energy as a function of pore size for thermodynamically stable, metastable, and unstable pores, and the system-size dependence of these elongations is discussed. A link to the Gibbs free energy at constant tension, commonly known as the Litster model, is established by a Legendre transformation. The change of genus upon pore formation is exploited to estimate the saddle-splay modulus or Gaussian curvature modulus of the membrane leaflets. Details are provided of the simulation approach, which combines the potential of mean constraint force method with a reaction coordinate based on the local lipid density.
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87.16.dm, 87.14.Cc, 87.15.ap

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