Volume 130, Issue 1, 07 January 2009
Index of content:
- Theoretical Methods and Algorithms
Application of state-specific multireference Møller–Plesset perturbation theory to nonsinglet states130(2009); http://dx.doi.org/10.1063/1.3043364View Description Hide Description
We present molecular applications of a spin free size-extensive state-specific multireference perturbation theory (SS-MRPT), which is valid for model functions of arbitrary spin and generality. In addition to the singlet states, this method is equally capable to handle nonsinglet states. The formulation based on Rayleigh–Schrödinger approach works with a complete active space and treats each of the model space functions democratically. The method is capable of handling varying degrees of quasidegeneracy and of ensuring size consistency as a consequence of size extensivity. In this paper, we illustrate the effectiveness of the Møller–Plesset (MP) partitioning based spin free SS-MRPT [termed as SS-MRPT(MP)] in computations of energetics of the nonsinglet states of several chemically interesting and demanding molecular examples such as LiH, , and . The spectroscopic constants of state of NH and molecular systems and the ground as well as excited states of have been investigated and comparison with experimental and full configuration interaction values (wherever available) has also been provided. We have been able to demonstrate here that the SS-MRPT(MP) method is an intrinsically consistent and promising approach to compute reliable energies of nonsinglet states over different geometries.
Comparison between solutions of the general dynamic equation and the kinetic equation for nucleation and droplet growth130(2009); http://dx.doi.org/10.1063/1.3054634View Description Hide Description
A comparison is made between two models of homogeneous nucleation and droplet growth. The first is a kinetic model yielding the master equations for the concentrations of molecular clusters. Such a model does not make an explicit distinction between nucleation and droplet growth. The second model treats nucleation and growth separately, fully ignoring stochastic effects, and leads to the continuous general dynamic equation (GDE). Problems in applying the GDE model are discussed. A numerical solution of the kinetic equation is compared with an analytic solution of the GDE for two different cases: (1) the onset of nucleation and (2) the nucleation pulse. The kinetic model yields the thickness of the condensation front in size space as a function of supersaturation and dimensionless surface tension. If the GDE is applied properly, solutions of the GDE and the kinetic equation agree, with the exception of very small clusters, near-critical clusters, and the condensation front.
130(2009); http://dx.doi.org/10.1063/1.3054708View Description Hide Description
We demonstrate the prospects of computing two photon absorption cross sections of open-shell systems by applying recently developed spin-restricted time-dependent density functional response theory using the pyrrole radical as an example. The spin multiplicity effects on two photon absorption cross sections of this species are investigated for the doublet, quartet, and sextet states. It is found that irrespective of the exchange-correlation functional employed, the two photon cross sections increase with the increase in spin multiplicity. This result indicates that two photon cross sections of paramagnetic compounds can be controlled by manipulating their spin states and this opens new possibilities for design of hybrid magneto-optical materials.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
130(2009); http://dx.doi.org/10.1063/1.3049782View Description Hide Description
A second example of a barrierless reaction between two closed-shell molecules is reported. The reaction has been investigated with crossed molecular beam experiments and ab initio calculations. Compared with previous results of the reaction [J. Chem. Phys.127, 101101 (2007); J. Chem. Phys.128, 104317 (2008)], a new product channel leading to is observed to be predominant in the title reaction, whereas the anticipated channel is not found. In addition, the product channel, the analog to the channel in the reaction, opens up at collision energies higher than 4.3 kcal/mol. Angular and translational energy distributions of the products are reported and collision energy dependences of the reaction cross section and product branching ratio are shown. The reaction barrier is found to be negligible . Multireference ab initio calculations suggest a reaction mechanism involving a short-lived intermediate which can be formed without activation energy.
130(2009); http://dx.doi.org/10.1063/1.3042269View Description Hide Description
Gas-phase laser excitation spectra of the jet-cooled CCS radical were observed for the electronic transition in a discharge of a gas mixture containing acetylene and in Ar. The observed excitation spectra show complicated rotational structures, consisting of a much larger number of rotational lines than those expected for this size of linear molecule. The excited vibronic levels emit fluorescence, the lifetime of which is much longer than that estimated theoretically, indicating that the upper levels are strongly mixed with many “dark” background levels. Dispersed fluorescence spectra from single vibrational levels of the state show long progressions of the C–S stretching mode, enabling observations of many vibrational levels in the ground electronic state.
130(2009); http://dx.doi.org/10.1063/1.3054185View Description Hide Description
Both cage and noncage structures of ( and 10) clusters are studied using density functional theory. All the cage structures are stable without imaginary vibrational frequency but the global minima are the noncage clusters for most cases. Our results show that oxidation of clusters by at room temperature is exothermic, while oxidation of clusters is endothermic. This is in qualitative agreement with an experimental observation that only clusters are produced in a laser vaporization source under saturated growth conditions. Since clusters have high stability and different structural and bonding properties from those of the bulk , they may serve as good models for predicting or interpreting novel properties of nanomaterials.
130(2009); http://dx.doi.org/10.1063/1.3043365View Description Hide Description
Following our general approach to -doubling specificity in the capture of dipolar molecules by ions [M. Auzinsh et al., J. Chem. Phys.128, 184304 (2008)], we calculate the rate coefficients for the title process in the temperature range . Three regimes considered are as follows: (i) nonadiabatic capture in the regime of high-field Stark effect with respect to the -doubling components, , (ii) adiabatic capture in the regime of intermediate Stark effect, and (iii) adiabatic capture in the limit of very low temperatures in the regime of quadratic Stark effect with respect to the -doubling and hyperfine components. The results predict a high specificity of the capture rates with respect to the -doublet states even under conditions when the collision energy of the partners strongly exceeds the -doubling splitting.
The rotational spectrum of the CCP radical and its isotopologues at microwave, millimeter, and submillimeter wavelengths130(2009); http://dx.doi.org/10.1063/1.3043367View Description Hide Description
The pure rotational spectrum of CCP has been measured at microwave, millimeter, and submillimeter wavelengths , along with its isotopologues (, , and ). The spectra of these species were recorded using a combination of millimeter/submillimeter direct absorption methods and Fourier transform microwave (FTMW) techniques. The phosphorus dicarbides were created in the gas phase from the reaction of red phosphorus and acetylene or methane in argon in an ac discharge for the direct absorption experiments, and using as the phosphorus source in a pulsed dc nozzle discharge for the FTMW measurements. A total of 35 rotational transitions were recorded for the main isotopologue, and between 2 and 8 for the substituted species. Both spin-orbit components were identified for CCP, while only the ladder was observed for , , and . Hyperfine splittings due to phosphorus were observed for each species, as well as carbon-13 hyperfine structure for each of the substituted isotopologues. The data were fitted with a Hund’s case (a) Hamiltonian, and rotational, fine structure, and hyperfine parameters were determined for each species. The bond lengths established for CCP, and , imply that there are double bonds between both the two carbon atoms and the carbon and phosphorus atoms. The hyperfine constants suggest that the unpaired electron in this radical is primarily located on the phosphorus nucleus, but with some electron density also on the terminal carbon atom. There appears to be a minor resonance structure where the unpaired electron is on the nucleus of the end carbon. The multiple double bond structure forces the molecule to be linear, as opposed to other main group dicarbides, such as , which have cyclic geometries.
130(2009); http://dx.doi.org/10.1063/1.3042153View Description Hide Description
The one-center approach for molecular Auger decay is applied to predict the angular distribution of Auger electrons from rotating and fixed-in-space molecules. For that purpose, phase shifts between the Auger decay amplitudes have been incorporated in the atomic model. The approach is applied to the resonant Auger decay of the photoexcitedresonance in carbon monoxide. It is discussed how the symmetry of the final ionic state is related to features in the angular distributions and a parametrization for the molecular frame Auger electronangular distribution is suggested. The angular distribution of Auger electrons after partial orientation of the molecule by the -excitation process is also calculated and compared to available experimental and theoretical data. The results of the one-center approach are at least of the same quality as the available theoretical data even though the latter stem from a much more sophisticated method. As the one-center approximation can be applied with low computational demand even to extended systems, the present approach opens a way to describe the angular distribution of Auger electrons in a wide variety of applications.
130(2009); http://dx.doi.org/10.1063/1.3012353View Description Hide Description
Velocity imaging technique combined with resonance-enhanced multiphoton ionization (REMPI) is used to detect primary photodissociation of propionyl chloride. In one-color experiments at 235 nm, the Cl and fragments are produced rapidly, leading to a fraction of translational energy release of 0.37 and 0.35, anisotropy parameters of 1.1 and 0.8, and quantum yield of 0.67 and 0.33, respectively, when initial excitation of the band is coupled to the repulsive configuration. The resulting propionyl radical with sufficient internal energy may undergo secondary dissociation to yield CO that is characteristic of an isotropic distribution. The REMPI spectra of the CO (0,0) and (1,1) bands are measured, giving rise to a Boltzmann rotational temperature of 1200 and 770 K, respectively, and a Boltzmann vibrational temperature of 2800 K. A minor channel of HCl elimination is not detected, probably because of predissociation in two-photon absorption at 235 nm. In two-color experiments comprising an additional 248 nm photolyzing laser, Cl and are produced with a fraction of translational energy release of 0.43 and 0.40 and anisotropy parameters of 1.0 and 0.6, respectively. The secondary production of CO is not observed although the internal energy partitioned in the propionyl radical is in the proximity of the dissociation barrier. In either experimental scheme, a small component appearing in the center of the Cl and images is proposed to stem from ground statedissociation via internal conversion.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
Femtosecond time-resolved absorption anisotropy spectroscopy on -bianthryl: Detection of partial intramolecular charge transfer in polar and nonpolar solvents130(2009); http://dx.doi.org/10.1063/1.3043368View Description Hide Description
Femtosecond time-resolvedabsorptionanisotropyspectroscopy by multichannel detection has been developed. The charge transfer(CT) character and dynamics of the UV-photoexcited -bianthryl (BA) in heptane, acetonitrile, and ethanol are revealed with this method. The transient absorption spectra are decomposed into two absorption components with different anisotropy values by the absorptionanisotropyspectra. The decomposition results show two absorption bands having different anisotropy values or different directions of the transition dipole moment. One band that has the transition dipole perpendicular to the central C–C bond has almost an identical spectral shape with transient absorption of anthracene in the singlet excited state. This band is assigned to a transition in a locally excited anthracene ring. The other band is broad and structureless. This band is assigned to partial charge transfer (PCT) absorption because its transition dipole moment is parallel to the central C–C bond. Because the PCT band is observed in a nonpolar solvent heptane as well as in polar solvents, the PCT occurs in both nonpolar and polar solvents. The PCT band rises within the instrumental response, indicating that the PCT takes place immediately after the photoexcitation. In acetonitrile, the CT component shows a significant blueshift, indicating the formation of the stabilized CT state from the PCT state. In ethanol, the CT band does not show a spectral shift, suggesting that the stabilization is smaller than in acetonitrile. From these results, a new kinetic model on the intramolecular CT in BA is discussed.
130(2009); http://dx.doi.org/10.1063/1.3049399View Description Hide Description
A hard -well potential is employed to test a recently proposed thermodynamic perturbation theory (TPT) based on a coupling parameter expansion. It is found that the second-order term of the coupling parameter expansion surpasses by far that of a high temperature series expansion under a macroscopic compressibility approximation and several varieties. It is also found that the fifth-order version displays best among all of the numerically accessible versions with dissimilar truncation orders. Particularly, the superiority of the fifth-order TPT from other available liquid state theories is exhibited the most incisively when the temperature of interest obviously falls. We investigate the modification of the phase behavior of the hard -well fluid resulting from a density dependence imposed on the original potential function. It is shown that (1) the density dependence induces polymorphism of fluid phase, particularly liquid-liquidtransition in metastable supercooled region, and (2) along with enhanced decaying of the potential function as a function of bulk density, both the liquid-liquidtransition and vapor-liquid transition tend to be situated at the domain of lower temperature, somewhat similar to a previously disclosed thumb rule that the fluid phase transition tends to metastable with respect to the fluid-solid transition as the range of the attraction part of a density-independence potential is sufficiently short compared to the range of the repulsion part of the same density-independence potential.
130(2009); http://dx.doi.org/10.1063/1.3050294View Description Hide Description
Vibrational optical activity (VOA) is an important property used to determine the absolute configuration of a chiral molecule in condensed phases. In particular, vibrational circular dichroism and Raman optical activity (ROA) are two representative VOA measurement techniques that have been extensively used to study structures and dynamics of biomolecules. Recently, the amide I vibrational circular dichroism of polypeptides was theoretically described by using fragment approximation methods, which are based on the assumption that amide I VOA can be described as a linear combination of those of constituent fragment peptide units. Here, we develop a fragment approximation theory applicable to numerical simulations of Raman and Raman optical activity spectra for the amide I vibrations in polypeptides. For an alanine dipeptide and pentapeptide analogs, we carried out density functional theory calculations of polarizability, magnetic dipole-, and electric quadrupole-ROA tensors. Numerically simulated spectra using the fragment approximation are directly compared to density functional theory results. Furthermore, the simulated ROA spectra of alanine-based right-handed -helix and polyproline II polypeptides are directly compared to the previously reported experimental results. The agreements were found to be excellent, which suggests that the fragment approximation method developed for the numerical simulation of ROA spectrum of polypeptide in solution is valid and useful.
Proton-proton homonuclear dipolar decoupling in solid-state NMR using rotor-synchronized -rotation pulse sequences130(2009); http://dx.doi.org/10.1063/1.3046479View Description Hide Description
We present a theoretical analysis of rotor-synchronized homonuclear dipolar decoupling schemes that cause a -rotation of the spins. These pulse sequences applicable at high spinning rates yield high-resolution protonNMR spectra that are free of artifacts, such as zero lines and image peaks. We show that the scaled isotropic chemical-shift positions of proton lines can be calculated from the zero-order average Hamiltonian and that the scaling factor does not depend on offset. The effects of different adjustable parameters (rf field, spinning rate, pulse shape, offset) on the decoupling performance are analyzed by numerical simulations of proton spectra and by solid-stateNMR experiments on and glycine.
Dynamic polarizability, Cauchy moments, and the optical absorption spectrum of liquid water: A sequential molecular dynamics/quantum mechanical approach130(2009); http://dx.doi.org/10.1063/1.3054184View Description Hide Description
The dynamic polarizability and optical absorption spectrum of liquid water in the 6–15 eV energy range are investigated by a sequential molecular dynamics (MD)/quantum mechanical approach. The MD simulations are based on a polarizable model for liquid water. Calculation of electronic properties relies on time-dependent density functional and equation-of-motion coupled-cluster theories. Results for the dynamic polarizability, Cauchy moments, , , , and dielectric properties of liquid water are reported. The theoretical predictions for the optical absorption spectrum of liquid water are in good agreement with experimental information.
130(2009); http://dx.doi.org/10.1063/1.3046678View Description Hide Description
Raman spectroscopic measurements of simple hydrogen and sII clathrate hydrates have been performed. Both the roton and vibron bands illuminate interesting quantum dynamics of enclathrated molecules. The complex vibron region of the Raman spectrum has been interpreted by observing the change in population of these bands with temperature, measuring the absolute content as a function of pressure, and with isotopic substitution. Quadruple occupancy of the large sII clathratecavity shows the highest vibrational frequency, followed by triple and double occupancies. Singly occupied small cavities display the lowest vibrational frequency. The vibrational frequencies of within all cavity environments are redshifted from the free gas phase value. At 76 K, the progression from ortho- to para- occurs over a relatively slow time period (days). The rotational degeneracy of molecules within the clathratecavities is lifted, observed directly in splitting of the para- roton band. Raman spectra from and hydrates suggest that the occupancy patterns between the two hydrates are analogous, increasing confidence that is a suitable substitute for . The measurements suggest that Raman is an effective and convenient method to determine the relative occupancy of hydrogen molecules in different clathratecavities.
130(2009); http://dx.doi.org/10.1063/1.3050277View Description Hide Description
The vibrational relaxation of embedded in solid Ar has been studied over 4–80 K. The interactionmodel is based on OH undergoing local motions in a cage formed by a face-centered cubic stacking where the first shell atoms surround the guest and connect it to the heat bath through 12 ten-atom chains. The motions confined to the cage are the local translation and libration-rotation of OH and internal vibrations in , their energies being close to or a few times the energies of nearby first shell and chain atoms. The cage dynamics are studied by solving the equations of motion for the interaction between OH and first shell atoms, while energy propagation to the bulk phase through lattice chains is treated in the Langevin dynamics. Calculated energy transfer data are used in semiclassical procedure to obtain rate constants. In the early stage of interaction, OH transfers its energy to libration-rotation intramolecularily and then to the vibrations of the first shell and chain atoms on the time scale of several picoseconds. Libration-to-rotational transitions dispense the vibrational energy in small packages comparable to the lattice frequencies for ready flow. Energy propagation from the chains to the heat bath takes place on a long time scale of 10 ns or longer. Over the solid argon temperature range, the rate constant is on the order of and varies weakly with temperature.
130(2009); http://dx.doi.org/10.1063/1.3054635View Description Hide Description
We use molecular dynamics simulation results on viscous binary Lennard-Jones mixtures to examine the correlation between the potential energy and the virial. In accord with a recent proposal [U. R. Pedersen et al., Phys. Rev. Lett.100, 015701 (2008)], the fluctuations in the two quantities are found to be strongly correlated, exhibiting a proportionality constant, , numerically equal to one-third the slope of an inverse power law approximation to the intermolecular potential function. The correlation is stronger at higher densities, where interatomic separations are in the range where the inverse power law approximation is more accurate. These same liquids conform to thermodynamic scaling of their dynamics, with the scaling exponent equal to . Thus, the properties of strong correlation between energy and pressure and thermodynamic scaling both reflect the ability of an inverse power law representation of the potential to capture interesting features of the dynamics of dense, highly viscous liquids.
Dynamical density functional theory for molecular and colloidal fluids: A microscopic approach to fluid mechanics130(2009); http://dx.doi.org/10.1063/1.3054633View Description Hide Description
In recent years, a number of dynamical density functional theories (DDFTs) have been developed for describing the dynamics of the one-body density of both colloidal and atomic fluids. In the colloidal case, the particles are assumed to have stochastic equations of motion and theories exist for both the case when the particle motion is overdamped and also in the regime where inertial effects are relevant. In this paper, we extend the theory and explore the connections between the microscopic DDFT and the equations of motion from continuum fluid mechanics. In particular, starting from the Kramers equation, which governs the dynamics of the phase space probability distribution function for the system, we show that one may obtain an approximate DDFT that is a generalization of the Euler equation. This DDFT is capable of describing the dynamics of the fluid density profile down to the scale of the individual particles. As with previous DDFTs, the dynamical equations require as input the Helmholtz free energy functional from equilibrium density functional theory(DFT). For an equilibrium system, the theory predicts the same fluid one-body density profile as one would obtain from DFT. Making further approximations, we show that the theory may be used to obtain the mode coupling theory that is widely used for describing the transition from a liquid to a glassy state.
- Surfaces, Interfaces, and Materials
130(2009); http://dx.doi.org/10.1063/1.3046292View Description Hide Description
When an excess charge carrier is added to a one-dimensional (1D) wide-band semiconductor immersed in a polar solvent, the carrier can undergo self-localization into a large-radius adiabatic polaron. We explore the local optical absorption from the ground state of 1D polarons using a simplified theoretical model for small-diameter tubular structures. It is found that about 90% of the absorption strength is contained in the transition to the second lowest-energy localized electronic level formed in the polarization potential well, with the equilibrium transition energy larger than the binding energy of the polaron. Thermal fluctuations, however, can cause a very substantial—an order of magnitude larger than the thermal energy—broadening of the transition. The resulting broad absorption feature may serve as a signature for the optical detection of solvated charge carriers.