Volume 129, Issue 8, 28 August 2008
Index of content:
- Theoretical Methods and Algorithms
Symmetry-adapted perturbation theory utilizing density functional description of monomers for high-spin open-shell complexes129(2008); http://dx.doi.org/10.1063/1.2968556View Description Hide Description
We present an implementation of symmetry-adapted perturbation theory (SAPT) to interactions of high-spin open-shell monomers forming high-spin dimers. The monomer spin-orbitals used in the expressions for the electrostatic and exchange contributions to the interaction energy are obtained from density functional theory using a spin-restricted formulation of the open-shell Kohn–Sham (ROKS) method. The dispersion and induction energies are expressed through the density-density response functions predicted by the time-dependent ROKS theory. The method was applied to several systems: NH⋯He, CN⋯Ne, , and NH⋯NH. It provides accuracy comparable to that of the best previously available methods such as the open-shell coupled-cluster method with single, double, and noniterative triple excitations, RCCSD(T), with a significantly reduced computational cost.
129(2008); http://dx.doi.org/10.1063/1.2971041View Description Hide Description
The quantum-classical Liouville equation describes the dynamics of a quantum subsystem coupled to a classical environment. It has been simulated using various methods, notably, surface-hopping schemes. A representation of this equation in the mapping Hamiltonian basis for the quantum subsystem is derived. The resulting equation of motion, in conjunction with expressions for quantum expectation values in the mapping basis, provides another route to the computation of the nonadiabaticdynamics of observables that does not involve surface-hopping dynamics. The quantum-classical Liouville equation is exact for the spin-boson system. This well-known model is simulated using an approximation to the evolution equation in the mapping basis, and close agreement with exact quantum results is found.
129(2008); http://dx.doi.org/10.1063/1.2969102View Description Hide Description
In previous articles [J. Chem. Phys.121, 4501 (2004); J. Chem. Phys.124, 034115 (2006); J. Chem. Phys.124, 034116 (2006); J. Phys. Chem. A111, 10400 (2007); J. Chem. Phys.128, 164115 (2008)] an exact quantum, bipolar wave decomposition, , was presented for one-dimensional stationary state and time-dependent wavepacket dynamics calculations, such that the components approach their semiclassical WKB analogs in the large action limit. The corresponding bipolar quantum trajectories are classical-like and well behaved, even when has many nodes or is wildly oscillatory. In this paper, both the stationary state and wavepacket dynamicstheories are generalized for multidimensional systems and applied to several benchmark problems, including collinear .
Gaussian-based techniques for quantum propagation from the time-dependent variational principle: Formulation in terms of trajectories of coupled classical and quantum variables129(2008); http://dx.doi.org/10.1063/1.2969101View Description Hide Description
In this article, two coherent-state based methods of quantum propagation, namely, coupled coherent states (CCS) and Gaussian-based multiconfiguration time-dependent Hartree (G-MCTDH), are put on the same formal footing, using a derivation from a variational principle in Lagrangian form. By this approach, oscillations of the classical-like Gaussian parameters and oscillations of the quantum amplitudes are formally treated in an identical fashion. We also suggest a new approach denoted here as coupled coherent states trajectories (CCST), which completes the family of Gaussian-based methods. Using the same formalism for all related techniques allows their systematization and a straightforward comparison of their mathematical structure and cost.
Second-order, two-electron Dyson propagator theory: Comparisons for vertical double ionization potentials129(2008); http://dx.doi.org/10.1063/1.2973533View Description Hide Description
The second-order, two-electron Dyson propagator is derived using superoperator theory with a spin-adapted formulation. To include certain ladder diagrams to all orders, the shifted-denominator (SD2) approximation is made. Formal and computational comparisons with other approximations illustrate the advantages of the SD2 procedure. Vertical double ionization potentials (DIPs) for a set of closed-shell molecules are evaluated with the second-order propagator and the SD2 method. The results of the SD2 approximation are in good agreement with experiment. To systematically examine the quality of the results, we compared SD2 and equation-of-motion, coupled-cluster predictions. The average absolute discrepancy is for 36 doubly ionized states.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
129(2008); http://dx.doi.org/10.1063/1.2966004View Description Hide Description
These experiments study the preparation of and product channels resulting from , a key radical intermediate in the bimolecular reaction. The data include velocity map imaging and molecular beam scattering results to probe the photolytic generation of the radical intermediate and the subsequent pathways by which the radicals access the energetically allowed product channels of the bimolecular reaction. The photodissociation of epichlorohydrin at produces chlorine atoms and radicals; these undergo a facile ring opening to the radical intermediate. State-selective resonance-enhanced multiphoton ionization (REMPI) detection resolves the velocity distributions of ground and spin-orbit excited state chlorine independently, allowing for a more accurate determination of the internal energy distribution of the nascent radicals. We obtain good agreement detecting the velocity distributions of the Cl atoms with REMPI, vacuum ultraviolet (VUV) photoionization at , and electron bombardment ionization; all show a bimodal distribution of recoil kinetic energies. The dominant high recoil kinetic energy feature peaks near . To elucidate the product channels resulting from the radical intermediate, the crossed laser-molecular beam experiment uses VUV photoionization and detects the velocity distribution of the possible products. The data identify the three dominant product channels as , (formyl radical), and . A small signal from (ketene) product is also detected. The measuredvelocity distributions and relative signal intensities at , 28, and 29 at two photoionization energies show that the most exothermic product channel, , does not contribute significantly to the product branching. The higher internal energy onset of the product channel is consistent with the relative barriers en route to each of these product channels calculated at the CCSD(T)/aug-cc-pVQZ level of theory, although a clean determination of the barrier energy to is precluded by the substantial partitioning into rotational energy during the photolytic production of the nascent radicals. We compare the measured branching fraction to the product channel with a statistical prediction based on the calculated transition states.
Time-dependent configuration-interaction calculations of laser-driven dynamics in presence of dissipation129(2008); http://dx.doi.org/10.1063/1.2972126View Description Hide Description
Correlated, multielectron dynamics of “open” electronic systems within the fixed-nuclei approximation are treated here within explicitly time-dependent configuration-interaction schemes. Specifically, we present simulations of laser-pulse driven excitations of selected electronic states of LiCN in the presence of energy and phase relaxation. The evolution of the system is studied using open-system density matrix theory, which embeds naturally in the time-dependent configuration-interaction singles (doubles) formalism. Different models for dissipation based on the Lindblad semigroup formalism are presented. These models give rise to lifetimes for energy relaxation ranging from a few hundreds of femtoseconds to several nanoseconds. Pure dephasing is treated using a Kossakowski-like Gaussian model, proceeding on similar time scales. The pulse lengths employed range from very short (tens of femtoseconds) to very long (several nanoseconds). To make long-time propagations tractable, the quasiresonant approximation is used. The results show that despite the loss of efficiency, selective dipole switching can still be achieved in the presence of dissipation when using appropriately designed laser pulses.
129(2008); http://dx.doi.org/10.1063/1.2971184View Description Hide Description
An eight-dimensional time-dependent quantum dynamics calculation is reported to study the isotopic reaction,, on a new modified potential energy surface. Initial-state-selected reaction probability, integral cross section, and rate constants are presented in this isotopic reaction study. Initial-state-selected reaction probability is obtained by summing over all the possible product’s arrangements in this isotopic reaction study. This study shows that vibrational excitations of HD enhance the reactivity, whereas stretching excitations of only have a small effect on the reactivity. Furthermore, the bending excitations of , compared to the ground-statereaction probability, hinder the reactivity. The present results are consistent with those obtained for the reaction. The comparison of these two reactions also shows the isotopic effect in the initial-state-selected reaction probability, integral cross section, and rate constants. The rate constant comparison shows that the reaction has a smaller reactivity than the reaction.
Infrared photodissociation spectroscopy and density-functional calculations of protonated methanol cluster ions: Solvation structures of an excess proton129(2008); http://dx.doi.org/10.1063/1.2963499View Description Hide Description
Solvation structures of an excess proton in protonated methanol cluster ions, , were investigated by photodissociationspectroscopy in the middle infrared region and by using density-functional theory. This work indicates that the excess proton is delocalized between two methanol molecules. Spectral features observed in the range are attributed to vibrational modes involving collective motion of the shared proton and the two ligand molecules. At , broad spectral features in the region above suggest coexistence of isomers in which the excess proton and a methanol molecule are tightly bound to form an ion core, .
Characteristics of the interaction of azulene with water and hydrogen sulfide: A computational study129(2008); http://dx.doi.org/10.1063/1.2973632View Description Hide Description
A computational study was carried out for studying the characteristics of the interaction between azulene and water or hydrogen sulfide. In azulene water complex the water molecule is located with both hydrogen atoms pointing toward the aromatic cloud but displaced to the five-membered ring. Hydrogen sulfide adopts a similar arrangement but located roughly over the central C–C bond of azulene. Calculations show that hydrogen sulfide interacts with azulene more strongly than water, although this is only revealed at the highest levels of calculation. The nature of the interaction is electrostatic and dispersive in the same percentage for watercluster, whereas for hydrogen sulfide dispersion is the dominant contribution. Clusters containing two water molecules are controlled by the possibility of establishing an hydrogen bond. As a consequence, the most stable structure corresponds to the interaction between a water dimer and azulene, with an interactionenergy amounting to . Hydrogen sulfide interaction is stronger with azulene than with itself, so structures with contact and others, where only interacts with azulene, present similar interactionenergies ( for the most stable one).
Toward spectroscopic accuracy for organic free radicals: Molecular structure, vibrational spectrum, and magnetic properties of129(2008); http://dx.doi.org/10.1063/1.2969820View Description Hide Description
The structure, harmonic frequencies, and hyperfine couplings of have been computed by the coupled cluster ansatz using a hierarchical series of basis sets and, in some cases, extrapolation procedures to reach the complete basis set limit. Methods rooted into the density functional theory have been used to estimate anharmonic and environmental effects. The remarkable agreement with experimental hyperfine coupling constants and most of vibrational frequencies confirms the reliability of the computational approach and suggests that one of the observed frequencies probably refers to a different species.
129(2008); http://dx.doi.org/10.1063/1.2973629View Description Hide Description
In this paper, we have computed the rovibrational spectrum of the molecule using a new global potential energy surface, invariant under all permutations of the nuclei, that includes the long range electrostatic interactions analytically. The energy levels are obtained by a variational calculation using hyperspherical coordinates. From the comparison with available experimental results for low lying levels, we conclude that our accuracy is of the order of for states localized in the vicinity of equilateral triangular configurations of the nuclei, and changes to the order of when the system is distorted away from equilateral configurations. Full rovibrational spectra up to the dissociation energy limit have been computed. The statistical properties of this spectrum (nearest neighbor distribution and spectral rigidity) show the quantum signature of classical chaos and are consistent with random matrix theory. On the other hand, the correlation function, even when convoluted with a smoothing function, exhibits oscillations which are not described by random matrix theory. We discuss a possible similarity between these oscillations and the ones observed experimentally.
129(2008); http://dx.doi.org/10.1063/1.2967182View Description Hide Description
A benchmark theoretical determination of the electron affinities of benzene and linear oligoacenes ranging from naphthalene to hexacene is presented, using the principles of a focal point analysis. These energy differences have been obtained from a series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, and MP4SDQ) levels and from coupled cluster calculations including single and double excitations (CCSD) as well as perturbative estimates of connected triple excitations [CCSD(T)], using basis sets of improving quality, containing up to 1386, 1350, 1824, 1992, 1630, and 1910 basis functions in the computations, respectively. Studies of the convergence properties of these energy differences as a function of the size of the basis set and order attained in electronic correlation enable a determination of the vertical electron affinities of the four larger terms of the oligoacene series within chemical accuracy . According to our best estimates, these amount to , , , and when , 4, 5, and 6. Adiabatic electron affinities have been further calculated by incorporating corrections for zero-point vibrational energies and for geometrical relaxations. The same procedure was applied to determine the vertical electron affinities of benzene and naphthalene, which are found to be markedly negative ( and , respectively). Highly quantitative insights into experiments employing electron transmission spectroscopy on these compounds were also amenable from such an approach, provided diffuse atomic functions are deliberately removed from the basis set, in order to enforce confinement in the molecular region and enable a determination of pseudoadiabatic electron affinities (with respect to the timescale of nuclear motions). Comparison was made with calculations employing density functional theory and especially designed models that exploit the integer discontinuity in the potential or incorporate a potential wall in the unrestricted Kohn–Sham orbital equation for the anion.
129(2008); http://dx.doi.org/10.1063/1.2973626View Description Hide Description
The reaction has been investigated by the quasiclassical trajectory(QCT) method on a recent global ab initio potential energy surface [M. Wang et al., J. Chem. Phys.124, 234311 (2006)]. The integral cross section as a function of collision energy and thermal rate coefficient for the temperature range of have been obtained. At the collision energy of , product energy distributions and rovibrational populations are explored in detail, and rotational state distributions show a clear evidence of two reaction mechanisms. One is the conventional rebound mechanism and the other is the stripping mechanism similar to what has recently been found in the reaction of [J. P. Camden et al., J. Am. Chem. Soc.127, 11898 (2005)]. The computed rate coefficients with the zero-point energy correction are in good agreement with the available experimental data.
129(2008); http://dx.doi.org/10.1063/1.2973631View Description Hide Description
The 351.1 nm photoelectron spectrum of the cyclopentadienide ion has been measured, which reveals the vibronic structure of the state of the cyclopentadienyl radical. Equation-of-motion ionization potential coupled-cluster (EOMIP-CCSD) calculations have been performed to construct a diabatic model potential of the state, which takes into account linear Jahn–Teller effects along the normal coordinates as well as bilinear Jahn–Teller effects along the and ring-breathing coordinates. A simulation based on this ab initio model potential reproduces the spectrum very well, identifying the vibronic levels with linear Jahn–Teller angular momentum quantum numbers of . The angular distributions of the photoelectrons for these vibronic levels are highly anisotropic with the photon energies used in the measurements. A few additional weak photoelectron peaks are observed when photoelectrons ejected parallel to the laser polarization are examined. These peaks correspond to the vibronic levels for out-of-plane modes in the ground state, which arise due to several pseudo-Jahn–Teller interactions with excited states of the radical and quadratic Jahn–Teller interaction in the state. A variant of the first derivative of the energy for the EOMIP-CCSD method has been utilized to evaluate the strength of these nonadiabatic couplings, which have subsequently been employed to construct the model potential of the state with respect to the out-of-plane normal coordinates. Simulations based on the model potential successfully reproduce the weak features that become conspicuous in the spectrum. The present study of the photoelectron spectrum complements a previous dispersed fluorescencespectroscopic study Miller and co-workers [J. Chem. Phys.114, 4855 (2001); 4869 (2001)Miller and co-workers [J. Chem. Phys.114, 4869 (2001)] to provide a detailed account of the vibronic structure of cyclopentadienyl. The electron affinity of the cyclopentadienyl radical is determined to be . This electron affinity and the gas-phase acidity of cyclopentadiene have been combined in a negative ion thermochemical cycle to determine the C–H bond dissociation energy of cyclopentadiene; . The standard enthalpy of formation of the cyclopentadienyl radical has been determined to be .
129(2008); http://dx.doi.org/10.1063/1.2973627View Description Hide Description
We investigate the accuracy of first-principles many-body theories at the nanoscale by comparing the low-energy excitations of the carbonfullerenes, , , , , and with experiment. Properties are calculated via the GW–Bethe–Salpeter equation and diffusion quantum Monte Carlo methods. We critically compare these theories and assess their accuracy against available photoabsorption and photoelectron spectroscopy data. The first ionization potentials are consistently well reproduced and are similar for all the fullerenes and methods studied. The electron affinities and first triplet excitation energies show substantial method and geometry dependence. These results establish the validity of many-body theories as viable alternative to density-functional theory in describing electronic properties of confined carbonnanostructures. We find a correlation between energy gap and stability of fullerenes. We also find that the electron affinity of fullerenes is very high and size independent, which explains their tendency to form compounds with electron-donor cations.
129(2008); http://dx.doi.org/10.1063/1.2973628View Description Hide Description
Stimulated Raman pumping has been used to prepare oriented and aligned samples of and under collision-free conditions using the (1,0) , , , , and lines. The -sublevel anisotropies were interrogated by polarized resonance-enhanced multiphoton ionization via the (0,1) , , and lines of the system. The optical excitation schemes employed in this study generate highly oriented and aligned molecular ensembles. We show that the and samples retain their initial polarization for greater than and are therefore suitable candidates for targets or projectiles in future scatteringexperiments.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
129(2008); http://dx.doi.org/10.1063/1.2969816View Description Hide Description
Broadband dielectricmeasurements on leucrose were performed under ambient and high pressure. We showed that in this disaccharide, there are two secondary relaxation modes, a slower one sensitive to pressure and a faster one that is not. This finding clearly indicates that the faster secondary relaxation originates from the intramolecular motion. This conclusion contradicted previous interpretations of this mode observed for trehalose and maltitol, systems very closely related to leucrose. In addition, pressure sensitivity of the slower relaxation confirms our recent interpretation about the character of this process. Furthermore, we discovered that unlike the faster relaxation, the slower secondary relaxation is sensitive to the thermodynamic history of measurements. Finally, monitoring the changes in maximum loss of the slower secondary relaxationmeasured at the same pressure and temperature conditions for glasses obtained via different thermodynamic routes enabled us to draw a conclusion about the density of the formed glasses. Our observations may be helpful in establishing a new method of suppressing crystallization of amorphous drugs.
129(2008); http://dx.doi.org/10.1063/1.2968550View Description Hide Description
We use the reverse Monte Carlo modeling technique to fit two extreme structure models for water to available x-ray and neutron diffraction data in space as well as to the electric field distribution as a representation of the OH stretch Raman spectrum of dilue HOD in ; the internal geometries were fitted to a quantum distribution. Forcing the fit to maximize the number of hydrogen (H) bonds results in a tetrahedral model with 74% double H-bond donors (DD) and 21% single donors (SD). Maximizing instead the number of SD species gives 81% SD and 18% DD, while still reproducing the experimental data and losing only 0.7–1.8 kJ/mole interaction energy. By decomposing the simulated Raman spectrum we can relate the models to the observed ultrafast frequency shifts in recent pump-probe measurements. Within the tetrahedral DD structure model the assumed connection between spectrum position and H-bonding indicates ultrafast dynamics in terms of breaking and reforming H bonds while in the strongly distorted model the observed frequency shifts do not necessarily imply H-bond changes. Both pictures are equally valid based on present diffraction and vibrational experimental data. There is thus no strict proof of tetrahedral water based on these data. We also note that the tetrahedral structure model must, to fit diffraction data, be less structured than most models obtained from molecular dynamics simulations.
Multilevel vibrational coherence transfer and wavepacket dynamics probed with multidimensional IR spectroscopy129(2008); http://dx.doi.org/10.1063/1.2969900View Description Hide Description
Multidimensional infrared (MDIR) spectroscopy of a strongly coupled multilevel vibrational system (dimanganese decacarbonyl) in cyclohexane solution reveals fully resolved excited vibrational state coherences that exhibit slow decay constants. Detailed analysis of the waiting-time dependence of certain cross-peak amplitudes shows modulation at multiple frequencies, providing a direct signature of excited vibrational coherences resulting from coherence transfer. A new signature of coherence transfer is observed as temporally modulated cross-peak amplitudes with more than one modulation frequency. The relative importance of different coherence transfer paths is considered in the context of the orientational response of a system which includes two vibrational modes with parallel dipole moments. Since MDIR spectroscopy enables spectral isolation of individual excited vibrational coherences (i.e., coherences between fundamental excitations), these experiments report directly on the frequency-frequency correlation functions of the excited states relative to each other as well as relative to the ground state. These results highlight the rich information contained in fully exploring three-dimensional third-order spectroscopy, particularly regarding chemically relevant slower dynamics and the importance of intramolecular interactions leading to dephasing by optically dark or low-frequency modes of the molecule.