Volume 114, Issue 12, 22 March 2001
Index of content:
- Theoretical Methods and Algorithms
Electronic excitation and ionization spectra of azabenzenes: Pyridine revisited by the symmetry-adapted cluster configuration interaction method114(2001); http://dx.doi.org/10.1063/1.1351880View Description Hide Description
Electronic excited and ionized states of pyridine were reinvestigated by the symmetry-adapted cluster configuration interaction (SAC-CI) method using an extended basis set and a wide active space. The present SAC-CI results for the singlet and triplet excited states are greatly improved and agree well with the experimental observations, providing a firm assignment of all low-lying and valence excited states observed in the vacuum ultraviolet spectrum and electron energy-lossspectrum. The ionization potentials were reexamined by the SAC-CI general-R (R represents excitation operator) method. The first four ionization potentials are greatly improved compared with our previous results obtained by the SAC-CI single- and double-R (SD-R) method. The present theoreticalionization potentials are in good agreement with the experimental values in high-resolution synchrotron photoelectron spectrum for energy regions up to 25 eV (which contain outer- and inner-valence regions), and give a detailed theoretical assignment for the photoelectron spectra.
Electron propagator method with a multiconfigurational second-order perturbation theory wave function as the initial state in the fermion operator block114(2001); http://dx.doi.org/10.1063/1.1349077View Description Hide Description
We have developed an electron propagator method using a multiconfigurational second-order perturbation theory (CASPT2) wave function as the initial state [electron propagator CASPT2 (EPCASPT2)] in the fermion operator block (block 1). In the other blocks a multiconfigurational self-consistent field wave function is the initial state. We apply our new method to directly determine the low-lying vertical ionization potentials of Be, and We compare our results with the results of the calculations using multiconfigurational spin tensor electron propagator (MCSTEP), full configuration interaction (FCI), and multireference configuration interaction (MRCI) methods with the same geometries and basis sets. The calculations are performed using complete active space (CAS) choices that are usually excellent for MCSTEP ionization potential (IP) calculations and also for CAS choices that are inadequate for MCSTEP IP calculations. We show that EPCASPT2 generally improves MCSTEP IPs compared to ΔFCI when the MCSTEP IPs are in very good to excellent agreement with ΔFCI IPs and that EPCASPT2 can effectively mimic ΔFCI even when the CAS choice for the initial state is inadequate for MCSTEP.
114(2001); http://dx.doi.org/10.1063/1.1342217View Description Hide Description
The statistics of single molecule blinking events often reveal underlying quantum mechanisms. The golden rule rate expression for quantum transitions is shown to be the inverse of the mean waiting time. The distribution function for the waiting time is related to the density of states such that simple power-law distribution functions can be predicted based on the functional form for the density of states. Explicit formulas are derived for waiting time distribution functions in three kinetic processes: Quantum tunneling, intersystem conversion, and nonstationary electron transfer.
The calculation of thermal rate constants for gas phase reactions: A semiclassical flux–flux autocorrelation function (SCFFAF) approach114(2001); http://dx.doi.org/10.1063/1.1344890View Description Hide Description
A semiclassical approach to the calculation of thermal rate constants, based on the flux–flux autocorrelation function method, is presented with its applications. The autocorrelation function is generated along classical trajectories using a classical interpretation of the Boltzmannized flux operator. The activation energies for considered reactions are calculated using the G2/MP2 procedure. The forces are generated using a new parametrization of the PM3 NDDO Hamiltonian optimized for accurate gradients. Thermal rate constants for hydrogen abstraction from ethane and haloethanes by hydroxyl radical serve as a first test of this approach. Calculated results are in good agreement with cumulative rate constants for all systems considered over a range of temperature including room temperature. The approach is able to distinguish between α and β abstraction with a result for fluoroethane at room temperature that is consistent with the available experiment and trends that are in line with those expected.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
Hydrogen bonding and stacking interactions of nucleic acid base pairs: A density-functional-theory based treatment114(2001); http://dx.doi.org/10.1063/1.1329889View Description Hide Description
We extend an approximate density functional theory(DFT) method for the description of long-range dispersive interactions which are normally neglected by construction, irrespective of the correlation function applied. An empirical formula, consisting of an term is introduced, which is appropriately damped for short distances; the corresponding coefficient, which is calculated from experimental atomic polarizabilities, can be consistently added to the total energy expression of the method. We apply this approximate DFT plus dispersion energy method to describe the hydrogen bonding and stacking interactions of nucleic acid base pairs. Comparison to MP2/6-31G*(0.25) results shows that the method is capable of reproducing hydrogen bonding as well as the vertical and twist dependence of the interaction energy very accurately.
114(2001); http://dx.doi.org/10.1063/1.1350918View Description Hide Description
The adatom dynamics in exohedral fullerene complexes of rare-gas atoms are studied with a three degrees of freedom model. The eigenvalue problem of the corresponding quantum Hamiltonian is solved and the electric-dipole spectra for and in the low-temperature range from 5 to 40 K are simulated. The most important spectral features are related to the degree of angular anisotropy in the adatom– interaction. The and complexes present very simple spectra which can be assigned in terms of three-mode oscillators; the corresponding motion takes place in the deep hexagon wells (also in the pentagon wells for of the interaction potential. On the contrary, the complex shows more complicated spectra with important tunneling effects due to the smaller angular anisotropy of the interaction. The onset of almost free internal rotation takes place in this complex at rather low energies, and this gives rise to a low-frequency rotational band in the spectra at temperatures above
114(2001); http://dx.doi.org/10.1063/1.1350936View Description Hide Description
H-atom channels in the photodissociation of jet-cooled ethyl radical via the state are studied near 245 nm by using the high-n Rydberg-atom time-of-flight technique. Bimodal product translational energy release and energy-dependent angular distribution suggest two dissociation pathways. A slow and isotropic channel corresponds to unimolecular dissociation of the radical, presumably after internal conversion. A previously unobserved fast and anisotropic channel is consistent with direct H-atom scission via a nonclassical H-bridged transition state from the state to yield The fast/slow branching ratio is ∼0.2. Site-selective loss of the β hydrogen atom is confirmed by using the partially-deuterated radical.
114(2001); http://dx.doi.org/10.1063/1.1351884View Description Hide Description
The potential-energysurface of the interstellar molecule is explored at the B3LYP/6-311G(d) level of theory. Thirteen isomers including the linear, three-membered ring, four-membered ring, A-like, Y-like, and cage-like structures are located as minima connected by 23 interconversion transition states. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by single-point energy calculations at the MP4SDTQ, CCSD(T), and QCISD(T) levels with the 6-311G(2df) basis set. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d) level, the lowest-lying isomer is a linear structure CCCCN 1 followed by a CCC three-membered ring structure 4 with exocyclic CCN bonding that lies only 2.8 kcal/mol higher. The third and fourth low-lying isomers possess a CCC three-membered ring structure 5 with exocyclic CNC bonding at 21.4 kcal/mol and a linear structure CCCNC 2 at 23.4 kcal/mol, respectively. All the four isomers 1, 2, 4, and 5 and another high-lying isomer 3 with a linear CCNCC structure at 62.5 kcal/mol are shown to have considerable kinetic stability towards isomerization and dissociation. Thus, all the five isomers may be experimentally observable. Possible formation of these five stable isomers in interstellar space is discussed. Finally, our calculations show that the Møller–Plesset methods fail to predict even qualitatively the energetic properties between the four low-lying isomers 1, 2, 4, and 5, in comparison with the QCISD and CCSD results. This paper indicates that may be the first interstellar molecule with stable low-lying cyclic isomers among the radical series to be detectable in near future. The results presented in this paper may provide useful information for future laboratory and interstellar identification of various isomers.
114(2001); http://dx.doi.org/10.1063/1.1331570View Description Hide Description
A recently developed new approach for the determination of the work of critical cluster formation in nucleation is applied here to the description of the kinetics of condensation of gases. This method is a generalization of the classical Gibbs’ approach retaining its advantages and avoiding its shortcomings. For an illustration, the method is developed here for the case of condensation in a one-component van der Waals gas. The surface tension between liquid and gas is described by a modification of Macleods equation. However, any other relationships specifying the state of the system under consideration, which may be considered eventually as more appropriate, can be employed as well. For relatively small supersaturations, the classical Gibbs’ results (employing the capillarity approximation) are retained as a special case. However, similarly to the van der Waals–Cahn and Hilliard and more recent methods of density functional calculations in the determination of the work of critical cluster formation, for initial states, approaching the spinodal curve, the work of critical cluster formation is shown to tend to zero. In the intermediate range of supersaturations, it leads to smaller values of the work of critical cluster formation as predicted by the classical theory. The method can be extended also straightforwardly to the description of condensation in multicomponent gases as well as to bubble formation in liquids.
Nearside–farside analysis of differential cross sections using Jacobi functions of the first and second kinds: Application to rotationally inelastic scattering114(2001); http://dx.doi.org/10.1063/1.1335658View Description Hide Description
We report the first nearside–farside (NF) analysis of angular scattering for an inelastic molecular collision in which the partial wave series for the scattering amplitude is expanded in a basis set of reduced rotation matrix elements where θ is the scattering angle, J is the total angular momentum quantum number, and are initial and final helicity quantum numbers, respectively. The practical implementation of the NF theory is described in detail; it exploits in an essential way the properties of a function that we denote and call a reduced rotation matrix element of the second kind. The caustic structure of and is taken into account via a restricted nearside–farside decomposition of the scattering amplitude. The theory is used to analyze polarization and degeneracy averaged differential cross sections for the collision system, treated as an The analysis always provides a clear physical interpretation of the scattering (except sometimes for for phenomena such as diffraction oscillations and potential rainbows, as well as for more complicated (unnamed) interference effects. We also report results for some approximations to the theory. Mathematical properties of the required for the analysis are derived.
114(2001); http://dx.doi.org/10.1063/1.1349058View Description Hide Description
Coating of magnetic clusters by gold atoms is becoming an experimental technique of increasing interest for passivation and stabilization of these small metal particles. To computationally investigate the effect of goldcoating, we have studied the magnetic clusters and employing an all-electron scalar-relativistic density functional method. We examine two series of octahedral clusters with increasing gold coverage of up to a monolayer: and Structural features, binding energies, and goldadsorption energies are determined and discussed. The different atomic radii of Au and Ni lead to rather short Au–Au and relatively long Ni–Ni distances in these clusters. The Au–Ni contacts are found to be the longest nearest-neighbor distances; a detailed analysis indicates these bonds to be the strongest in these Au-covered Niclusters. The atomization energies change only slightly with increasing Au coverage. Also, the effect of goldadsorption on the magnetic properties of the Ni cores is analyzed. For the series the magnetism decreases with n, while for a maximum cluster magnetization is calculated for incomplete gold coverage. This different behavior of the two cluster series can be traced to differing numbers of unpaired electrons per atom in the pure Niclusters and to an increased magnetic moment due to the adsorption of isolated Au atoms. Both series exhibit a residual magnetism at full monolayer coverage of the Ni cores.
Theoretical study of the water activation by a cobalt cation: Ab initio multireference theory versus density functional theory114(2001); http://dx.doi.org/10.1063/1.1336568View Description Hide Description
The reaction mechanism of with has been studied by the ab initio multireference-based theory (MR–SDCI and MC–QDPT) and the density functional theory (B3LYP and BLYP). In the energetics derived by the MR–SDCI(+Q) plus the B3LYP zero-point vibrational energy, the ion–dipole complex, is initially formed with the binding energy of 38.2 (triplet) and 34.1 (quintet) kcal/mol, which is the most stable complex in the respective potential energy surfaces. Then, activates one O–H bond of leading to the insertion complex, There are three possible dissociation channels from i.e., and The third dissociation is expected to occur through the transition state of a four-centered structure, with the activation barrier of 61.6 (triplet) and 49.2 (quintet) kcal/mol, although this dissociation has not been detected in the experiment. The ground state of is predicted to be and the lowest triplet state is with the energy level of 20.8 kcal/mol above. The B3LYP provides the energetics qualitatively similar to the MR–SDCI(+Q) ones through the reactions, with the maximum deviation of 13 kcal/mol. The calculated results are consistent with experimental observations.
Double-resonant photoionization efficiency spectroscopy: A precise determination of the adiabatic ionization potential of DCO114(2001); http://dx.doi.org/10.1063/1.1349080View Description Hide Description
We report the first high-resolution measurement of the adiabatic ionization potential of DCO and the fundamental bending frequency of Fixing a first-laser frequency on selected ultraviolet transitions to individual rotational levels in the (000) band of the intermediate Rydberg state of DCO, we scan a second visible laser over the range from 20 000 to to record double resonance photoionization efficiency (DR/PIE) spectra. Intermediate resonance with this Rydberg state facilitates transitions to the threshold for producing ground-state cations by bridging the Franck–Condon gap between the bent neutral radical and linear cation. By selecting a single rotational state for ionization, double-resonant excitation eliminates thermal congestion. Spectroscopic features for first-photon resonance are identified by reference to a complete assignment of the band system of DCO. Calibration with HCO, for which the adiabatic ionization threshold is accurately known, establishes an experimental instrument function that accounts for collisional effects on the shape of the photoionization efficiency spectrum near threshold. Analysis of the DR/PIE threshold for DCO yields an adiabatic ionization threshold of By extrapolation of vibrationally autoionizing Rydberg series accessed from the component of the intermediate state, we determine an accurate rotationally state-resolved threshold for producing This energy, together with the threshold determined for the vibrational ground state of the cation provides a first estimate of the bending frequency for as Assignment of the (010) autoionization spectrum further yields a measurement of an energy of for the (2-1) rotational transition in the state of
Symmetry specificity in the unimolecular decay of the complex: Two-mode quantum calculations on a coupled-cluster [CCSD(T)] potential energy surface114(2001); http://dx.doi.org/10.1063/1.1350902View Description Hide Description
The decay of resonance states in the complex-forming nucleophilic substitution reaction is investigated by means of two-dimensional quantum mechanical calculations on a coupled-cluster [CCSD(T)] potential energy surface. The dynamics calculations employ Radau coordinates to describe the two C–Cl stretching degrees of freedom, filter diagonalization, and an absorbing (optical) potential. The resonance widths and the corresponding decay rates vary by several orders-of-magnitude, reflecting the large degree of separability of the intramolecular and the intermolecular mode. The decay is found to be strongly symmetry specific: For energies above the reaction barrier, the smallest rates of the ungerade states are about two orders-of-magnitude smaller than the smallest rates of the gerade states. An explanation is given in terms of an adiabatic model formulated in hyperspherical coordinates. The nonadiabatic coupling elements, which control the energy transfer between the two modes and therefore determine the decay rate, are substantially larger for the gerade states. Ultimately, the differences are caused by the different structures of the gerade and the ungerade wave functions at the barrier.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
The temperature dependent dielectric function of liquid benzene: Interpretation of THz spectroscopy data by molecular dynamics simulation114(2001); http://dx.doi.org/10.1063/1.1350446View Description Hide Description
The dielectric function of liquid benzene at −4 °C, 21 °C, and 65 °C has been calculated from molecular dynamics simulations. The simulated dielectric loss curves reproduce the experimental temperature and density trends. In order to investigate the detailed influence of temperature and density changes as well as the underlying molecular mechanism we have taken advantage of different partitioning and projection schemes for the total dipole moment time correlation function (TCF). The study of the n-body partitioning showed that the temperature dependence of the two- and three-body contributions at can be explained solely by density change arguments. The molecular projection scheme showed that the dielectric loss is governed by out-of-plane libration at all temperatures. In-plane libration was found to contribute significantly only below 2 THz. Below 1 THz, diffusion, manifested as the negative cross correlation between the out-of-plane and the in-plane TCF’s, plays a role very different from that of the directly observed diffusion in dipolar liquids. It has further been established that it is highly problematic to carry out an analysis of the dielectric loss function in terms of the molecular axis rotational TCF’s which is a common procedure for the absorptionspectrum. This problem was, however, solved by employing a molecular projection scheme.
An ab initio study of the lattice distortions induced by nonisovalent and substitutional impurities in crystalline NaCl114(2001); http://dx.doi.org/10.1063/1.1352729View Description Hide Description
A theoretical analysis of the lattice distortions induced by nonisovalent and substitutional impurities in crystalline NaCl, and of the off-center equilibrium position adopted by those impurities in their ground electronic configuration is presented. The calculations are based in the cluster approach, and involve large active clusters embedded in an accurate quantal representation of the crystalline environment. The charge compensation problem is dealt with by considering several allocations of a cationic vacancy in the host lattice. The obtained distortions involve in all cases the concerted movement of a large number of host crystal ions. Those distortions are presented and discussed in terms of simple packing and Madelung considerations.
The medium response to an impulsive redistribution of charge in solid argon: Molecular dynamics simulations and normal mode analysis114(2001); http://dx.doi.org/10.1063/1.1352077View Description Hide Description
Excitation of the Rydberg state of NO leads to an extensive rearrangement of the environment, which we have investigated by classical molecular dynamics simulations and normal modeanalysis, using pair potentials from the literature. We find that the medium response is independent of the details at long range of the excited state NO potential, stressing the fact that it is mainly driven by the short range repulsive forces between the Rydberg electron and the matrix atoms. We establish the inertial character of the first shell response in the initial 100–150 fs after excitation, as the next shells are silent over this time scale. The expansion of the first shell at early times, induces the propagation of a supersonic wave along the (011) axis of the crystal, which define 12 linear chains of atoms with the impurity. The early time response is followed by vibrational coherences with a complex behavior. The normal modesanalysis of the crystal shell by shell shows good agreement with the power spectra of the MD trajectories. It allows us to identify the most significant modes in the medium response. Overall, the dynamics of the system may be regarded as that of a supermolecule, embedded in an Ar lattice and undergoing vibrational energy redistribution.
Ion pair correlations near critical points of ionic fluids: Experimental investigation of the static permittivity114(2001); http://dx.doi.org/10.1063/1.1349093View Description Hide Description
We report on data for the concentration dependence of the static dielectric constant of solutions of tetra-n-butylammonium naphtylsulfonate (TBNAS) in toluene derived from measurements of the frequency-dependent complex permittivity. The system shows an upper consolute point at and a mole fraction of the salt. The measurements were performed along a slightly supercritical isotherm at 338.15 K at mole fractions increases with increasing salt concentration. At the critical point compared with for pure toluene. In the concentration range covered by the experiments, the effective dipole moments calculated from these data are of the order of which is distinctly lower than an estimate for the isolated molecule, The difference is attributed to compensation effects due to preferred antiparallel ion pair orientations, as described by a Kirkwood–Fröhlich-type theory with correlation factors of At the critical point we find Due to these orientational correlations, dipole–dipole interactions between ion pairs may provide a significantly smaller contribution to the free energy than predicted by theory. This may explain the good performance of theories that ignore dipole–dipole interactions between pairs.
- Surfaces, Interfaces, and Materials
114(2001); http://dx.doi.org/10.1063/1.1349895View Description Hide Description
The state-selective dissociative sticking coefficient on Pt(111) surfaces for in the ro-vibrational levels has been measured using thermal energyatomic scattering. Continuous wave laser excitation of a molecular beam of seeded in He with a 1.5 μm color center laser, tunable around 6000 cm−1 and coupled to the beam by means of a resonant build-up cavity, allows pumping of up to 11% of the molecules to the excited ro-vibrational state. The laser/molecular beam combination allows precise control over the translational energy as well as the vibrational state of the methane that impinges on the clean Pt(111) surface. The intensity of the specular reflection of the incident helium beam is used to quantitatively monitor the coverage of chemisorbed methane on the platinum surface as a function of time (exposure). The sticking coefficient of with 5.4 kJ/mol normal translational energy is found to increase from to upon excitation (the overtone of the asymmetric stretch of methane). This represents a ∼30 fold enhancement in reactivity of the methane with the Pt(111) surface upon vibrational excitation. We also measured the changes of obtained by varying the nozzle temperature and methane concentration over a tenfold range of energy. We find that 72 kJ/mol of vibrational energy in the excited is approximately equivalent to at least 30 kJ/mol of normal translational energy. This corresponds to a utilization efficiency of the vibrational energy of greater than 40%. In the only other measurement of this kind published in the literature, [L. B. F. Juurlink, P. R. McCabe, R. R. Smith, C. L. DiCologero, and A. L. Utz, Phys. Rev Lett. 83, 868 (1999)] for the fundamental excitation of of on Ni(100) surfaces, a comparable value for the vibrational energy utilization efficiency was found (59%). Further work is necessary to determine if this result is general and if and how it may change by changing the vibrational mode excited by the laser.
114(2001); http://dx.doi.org/10.1063/1.1350580View Description Hide Description
The adsorption of was used to investigate the porosity/morphology of thin films of amorphous solid water. Molecular beams were used to vapor deposit amorphous solid water films on a Pt(111) crystal at a variety of incident growth angles. The amount of adsorbed by the amorphous solid water depends very sensitively on the growth angle and thermal history of the film. For normal and nearly normal incidence growth, the water films are relatively dense and smooth and adsorb only a small amount of For larger growth angles, the films are porous and adsorb large quantities of with apparent surface areas as high as ∼2700 m2/g. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The observations have important implications for laboratory studies which use vapor deposited amorphous solid water films as analogs for astrophysical icy bodies such as comets.