Volume 126, Issue 3, 21 January 2007
Index of content:
- Theoretical Methods and Algorithms
126(2007); http://dx.doi.org/10.1063/1.2428292View Description Hide Description
A hole created in a system, for instance by ionization, can migrate through the system solely driven by many-electron effects. The implementation of the theory of charge migration and the numerical algorithms used are described in detail. A description of the ab initio calculation of charge migration in realistic systems is presented for several examples and the underlying mechanisms of charge migration are identified and interpreted using theoretical models. In all cases studied the migration is found to be ultrafast.
On the quantum theory of electron transfer: Effect of potential surfaces of the reactants and productsa)126(2007); http://dx.doi.org/10.1063/1.2424702View Description Hide Description
In this study the electron transfer theory for the high frequency intramolecular mode and low frequency medium mode has been rederived using different methods. For a single-mode case when the reactant surface is not in a thermal equilibrium but in a thermocoherent state the authors have derived the rate and discussed about the quantum features of the rate. In the limit of very low and very high temperatures the expressions are derived analytically and compared with the case of thermal distribution. Secondly, a Franck-Condon factor for a multimode displaced, distorted, and Duschinsky rotated adiabatic potential surfaces has been derived to obtain the rate. Lastly, a systematic investigation is made to show the temperature dependence on the rate due to displacement, distortion, and Duschinsky rotation of the harmonic potential energy surfaces of the reactant and product manifold.
126(2007); http://dx.doi.org/10.1063/1.2428290View Description Hide Description
Grid-based energy density analysis (grid-EDA) that decomposes the total energy into atomic energies by a space-partitioning function is proposed. The kinetic energy, nuclear attraction, and exchange-correlation functional are evaluated on grid points and are split into atomic contributions. To reduce numerical errors in the conventional scheme of numerical integration, the electronic Coulomb and HF exchange interactions are evaluated by the pseudospectral method, which was first applied to an ab initio method by Friesner [Chem. Phys. Lett.116, 39 (1985)], and are decomposed into atomic contributions. Grid-EDA using the pseudospectral method succeeds in ensuring less than error in total energies for small molecules and providing reliable atomic energy contributions for the problematic lithium cluster, which exhibits a strong basis-set dependence for Mulliken-type EDA. Also, site-dependent atomization energies are estimated by grid-EDA for cluster models such as , , and . Grid-EDA reveals that these models imitate crystal environments reasonably because atomization energies estimated from the inner atoms of the models are close to the experimental cohesive energies.
Using an -dependent Gaussian width in calculations of the globally uniform semiclassical wave function126(2007); http://dx.doi.org/10.1063/1.2424938View Description Hide Description
The globally uniform semiclassical wave function expresses the solution to the time independent Schrödinger equation in terms of fixed width Gaussian wave packets traveling along a set of trajectories. There is a globally uniform wave function (GUWF) for each value of the Gaussian width parameter . Numerical data show that a small Gaussian width is needed in some regions to obtain accurate results, while a broad Gaussian width provides better results in other regions. Since there is a semiclassically valid GUWF for every positive value of , it is reasonable to employ the GUWF corresponding to a Gaussian width that provides good results at each value of . A criterion for the dependent choice of is proposed and tested on one and two dimensional model problems. The results show that the use of an dependent in the GUWF results in improved accuracy for the model problems considered.
126(2007); http://dx.doi.org/10.1063/1.2428294View Description Hide Description
The transition metal (TM) oxygen bond appears very prominently throughout chemistry and solid-state physics. Many materials, from biomolecules to ferroelectrics to the components of supernova remnants, contain this bond in some form. Many of these materials’ properties depend strongly on fine details of the TM–O bond, which makes accurate calculations of their properties very challenging. Here the authors report on highly accurate first principles calculations of the properties of TM monoxide molecules within fixed-node diffusion Monte Carlo and reptation Monte Carlo.
126(2007); http://dx.doi.org/10.1063/1.2426342View Description Hide Description
An extension of the effective-mode theory for the short-time dynamics through conical intersections in macrosystems [L. S. Cederbaum et al., Phys. Rev. Lett.94, 113003 (2005)] is proposed. The macrosystem, containing a vast number of nuclear degrees of freedom (modes), is decomposed into a system part and an environment part. Only three effective modes are needed—together with the system’s modes—to accurately calculate low resolution spectra and the short-time dynamics of the entire macrosystem. Here, the authors propose an iterative scheme to construct a hierarchy of additional triplets of effective modes. This naturally extends the effective-mode formulation. By taking into account more and more triplets, the dynamics are accurately predicted for longer and longer times, and more resolved spectra can be calculated. Numerical examples are presented, computed using various numbers of additional effective modes.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
126(2007); http://dx.doi.org/10.1063/1.2423024View Description Hide Description
The authors report theoretical results on structure, bonding, energy, and infrared spectra of iodine dimer radical anion hydrated clusters, , based on a systematic study following density functional theory. Several initial guess structures are considered for each size cluster to locate minimum energy conformers with a Gaussian split valence basis function (triple split valence 6-311 basis set is applied for iodine). It is observed that three different types of hydrogen bonded structures, namely, symmetrical double hydrogen bonding, single hydrogen bonding, and interwater hydrogen bondingstructures, are possible in these hydrated clusters. But conformers having interwater hydrogen bonding arrangements are more stable compared to those of double or single hydrogen bonded structures. It is also noticed that up to four solvent units can reside around the solute in interwater hydrogen bondingnetwork. At the maximum six units are independently linked to the dimer anion having four double hydrogen bonding and two single hydrogen bonding, suggesting the hydration number of to be 6. However, conformers having units independently linked to the iodine dimer anion are not the most stable structures. In all these hydrated clusters, the odd electron is found to be localized over two I atoms and the two atoms are bound by a three-electron hemibond. The solvation, interaction, and vertical detachment energies are calculated for all clusters. Energy of interaction and vertical detachment energy profiles show stepwise saturation, indicating geometrical shell closing in the hydrated clusters, but solvation energy profile fails to show such behavior. A linear correlation is observed between the calculated energy of interaction and vertical detachment energy. It is observed that formation of -water cluster induces significant shifts from the normal O–H stretching modes of isolated . However, bending mode of remains insensitive to the successive addition of solvent units. Weighted average energy profiles and IR spectra are reported for all the hydrated clusters based on the statistical population of individual conformers at room temperature.
126(2007); http://dx.doi.org/10.1063/1.2424461View Description Hide Description
This paper is aimed at understanding what happens to the propensity functions (rates) of bimolecular chemical reactions when the volume occupied by the reactant molecules is not negligible compared to the containing volume of the system. For simplicity our analysis focuses on a one-dimensional gas of hard-rod molecules, each of length . Assuming these molecules are distributed randomly and uniformly inside the real interval in a nonoverlapping way, and that they have Maxwellian distributed velocities, the authors derive an expression for the probability that two rods will collide in the next infinitesimal time . This probability controls the rate of any chemical reaction whose occurrence is initiated by such a collision. The result turns out to be a simple generalization of the well-known result for the point molecule case : the system volume in the formula for the propensity function in the point molecule case gets replaced by the “free volume” . They confirm the result in a series of one-dimensional molecular dynamics simulations. Some possible wider implications of this result are discussed.
126(2007); http://dx.doi.org/10.1063/1.2428289View Description Hide Description
Stationary points on the ground and first two excited statepotential energy surfaces of 2-naphthol and the hydrogen bonded complex have been located using an approximate method (time dependent density functional based tight-binding) based on time dependent density functional theory (TDDFT). For the obtained extrema, single point calculations at the full TDDFT level were performed for validation purposes. The energetical ordering of the cis and trans forms of 2-naphthol in the different electronic states is found to be in good agreement with available experimental data. Interestingly, also the optical spectrum is correctly predicted by TDDFT in contrast to calculations on unsubstituted acenes in the literature. Besides the stationary points, also the minimum energy paths connecting them are calculated using an adaptive nudged elastic band approach in order to estimate isomerization barriers. While the increase of barrier height going from the ground to the first excited state is already documented, the authors find as a new result a significant barrier reduction upon hydrogen bond formation in the . The observed effect can contribute to the explanation of recent experiments on infrared laser induced cis-transisomerization in 2-naphthol.
Exploring Renner-Teller induced quenching in the reaction : A combined experimental and theoretical study126(2007); http://dx.doi.org/10.1063/1.2409926View Description Hide Description
Experimental rate coefficients for the removal of and in collisions with H and D atoms are presented; all four isotope combinations are considered: , , , and . The experiments were performed in a quasistatic laser-flash photolysis/laser-induced fluorescence system at low pressures. and were generated by photolysis of and , respectively. The total removal rate coefficients at room temperature are in the range of . For two isotope combinations, and , quenching rate coefficients for the production of or were also determined; they are in the range of . The quenching rate coefficients directly reflect the strength of the Renner-Teller coupling between the and electronic states near linearity and so can be used to test theoretical models for describing this nonadiabatic process. The title reaction was modeled with a simple surface-hopping approach including a single parameter, which was adjusted to reproduce the quenching rate for ; the same parameter value was used for all isotope combinations. The agreement with the measured total removal rate is good for all but one isotope combination. However, the quenching rates for the combination are only in fair (factor of 2) agreement with the corresponding measured data.
126(2007); http://dx.doi.org/10.1063/1.2423021View Description Hide Description
The authors show that polar molecules can be adiabatically aligned and oriented by laser pulses more efficiently when the laser frequencies are vibrationally resonant. The aligned molecules are found in a superposition of vibrational pendular states, each associated with the alignment of the rotor in one vibrational state. The authors construct the dressed potential associated with this mechanism. Values of detunings and field amplitudes are given to optimize the degree of alignment and orientation for the CO molecule.
Experimental and theoretical study of the photodissociation reaction of thiophenol at : Intramolecular orbital alignment of the phenylthiyl radical126(2007); http://dx.doi.org/10.1063/1.2424939View Description Hide Description
The photoinduced hydrogen (or deuterium) detachment reaction of thiophenol or thiophenol- pumped at has been investigated using the H (D) ion velocity map imaging technique. Photodissociation products, corresponding to the two distinct and anisotropic rings observed in the H (or D) ion images, are identified as the two lowest electronic states of phenylthiyl radical . Ab initio calculations show that the singly occupied molecular orbital of the phenylthiyl radical is localized on the sulfur atom and it is oriented either perpendicular or parallel to the molecular plane for the ground and the first excited state species, respectively. The experimental energy separation between these two states is in excellent agreement with the authors’ theoretical prediction of at the CASPT2 level. The experimental anisotropy parameter of at the large translational energy of D from the dissociation indicates that the transition dipole moment associated with this optical transition at is perpendicular to the dissociating S–D bond, which in turn suggests an ultrafast dissociation channel on a repulsive potential energy surface. The reduced anisotropy parameter of observed at the smaller translational energy of D suggests that the channel may proceed on adiabatic reaction paths resulting from the coupling of the initially excited state to other low-lying electronic states encountered along the reaction coordinate. Detailed high level ab initio calculations adopting multireference wave functions reveal that the channel may be directly accessed via a photoexcitation at while the key feature of the photodissociation dynamics of the channel is the involvement of the profile as well as the spin-orbit induced avoided crossing between the ground and the state. The S–D bonddissociation energy of thiophenol- is accurately estimated to be . The S–H bonddissociation energy is also estimated to give , which is smaller than previously reported ones by at least . The C–H bond of the benzene moiety is found to give rise to the H fragment. Ring opening reactions induced by the transitions followed by internal conversion may be responsible for the isotropic broad translational energy distribution of fragments.
Comprehensive theoretical studies on the CF3H dissociation mechanism and the reactions of CF3H with OH and H free radicals126(2007); http://dx.doi.org/10.1063/1.2426336View Description Hide Description
The potential energy surfaces for the CF3H unimolecular dissociationreaction and reactions of CF3H with free radical OH and H were investigated at the and levels and by the G3B3 theory. All the possible stationary and first-order saddle points along the reaction paths were verified by the vibrational analysis. The calculations account for all the product channels. The reactionenthalpies obtained at the G3B3 level are in good agreement with the available experiments. Canonical transition-state theory with Wigner tunneling correction was used to predict the rate constants for the temperature range of without any artificial adjustment, and tshe computed rate constants for elementary channels can be accurately fitted with three-parameter Arrhenius expressions. The theoreticalrate constants of the reaction agree with the available experimental data very well. The theoretical and experimental rate constants for the reaction are in reasonable agreement. The H abstraction of CF3H by OH is found to be the main reaction channel for the CF3H fire extinguishing reactions while the CF3H unimolecular dissociationreaction plays a negligible role.
126(2007); http://dx.doi.org/10.1063/1.2423022View Description Hide Description
The ionization behavior of the high Rydberg states of bis(-benzene)chromium in the presence of ac and/or dc fields has been explored. The application of an ac scrambling field at the time of laser excitation lengthens the lifetime of the Rydberg state by almost two orders of magnitude. The lifetime enhancement by the scrambling field is much more effective for than it is for Rydberg states. The pulsed-field ionization of Rydberg states of shows the typical diabatic ionization behavior for low . The two distinct ionization behaviors observed for the relatively low and high Rydberg states suggest that the former originate from the optically accessed Rydberg series, whereas the latter are due to Rydberg series. Based on the understanding of the ionization behavior of bis(-benzene)chromium, the accurate ionization potential is deduced to give . Optimization of the various electric field conditions greatly enhances the spectral sensitivity of the mass-analyzed threshold ionization (MATI) spectroscopy. The high-resolution MATI spectrum of the title molecule obtained here provides precise cationic vibrational frequencies for many skeletal and benzene ring modes. A number of vibrational modes are newly identified, and the ambiguity regarding to some mode assignments is now clearly resolved through the Frank-Condon analysis based on ab initio calculations.
Accurate modeling of sequential decay in clusters over long time scales: Insights from phase space theory126(2007); http://dx.doi.org/10.1063/1.2426341View Description Hide Description
A general theoretical framework for describing the thermally induced sequential decay in atomic clusters is presented. The scheme relies on a full treatment of individual dissociation steps based on phase space theory (PST), built into a kinetic Monte Carlo (kMC) procedure. This combined PST/kMC approach allows one to follow the evolution of several statistical properties such as the size, the angular momentum, or the temperature of the cluster over arbitrarily long time scales. Quantitative accuracy is achieved by incorporating anharmonicities of the vibrational densities of states, the rigorous conservation of angular momentum via the effective dissociation potential, and a proper calibration of the rate constants. The approach is tested and validated on selected Lennard-Jones clusters in various situations. Several approximations, including a mean-field rate equation treatment, are critically discussed; possible extensions are presented.
126(2007); http://dx.doi.org/10.1063/1.2430523View Description Hide Description
The authors investigate the use of absorbing potentials and discrete variable representation grid methods in multichannel time-independent scattering calculations. An exactly solvable, coupled-two-channel problem involving square-well potentials is used to assess the quality of numerical results. Special emphasis is given to the description of scattering resonances and near-threshold regions. Numerical treatment of close vicinities of thresholds requires the introduction of nonequidistant grids through a mapping procedure of the scattering coordinate.
126(2007); http://dx.doi.org/10.1063/1.2426334View Description Hide Description
Elimination of molecular bromine is probed in the transition following photodissociation of at by using cavity ring-down absorption spectroscopy. The quantum yield for the elimination reaction is determined to be . The nascent vibrational population ratio of is obtained to be . A supersonic beam of is similarly photofragmented and the resulting Br atoms are monitored with a velocity map ion-imaging detection, yielding spatial anisotropy parameters of 1.5 and 1.1 with photolyzing wavelengths of 234 and , respectively. The results justify that the excited state promoted by should have an symmetry. Nevertheless, when is prepared in a supersonic molecular beam under a cold temperature, photofragmentation gives no detectable in a time-of-flight mass spectrometer. A plausible pathway via internal conversion is proposed with the aid of ab initio potential energy calculations. Temperature dependence measurements lend support to the proposed pathway. The production rates of between and are also compared to examine the chlorine-substituted effect.
Role of spatial distortions on the quadratic nonlinear optical properties of octupolar organic and metallo-organic molecules126(2007); http://dx.doi.org/10.1063/1.2428308View Description Hide Description
Following on the recent experimental demonstration of a discrepancy between the nonlinear optical (NLO) behavior of several -conjugated chromophores and their assumed octupolar symmetry, the authors investigate how geometrical distortions influence the NLO response of multipolar push-pull molecules. Their analytical model is set on a basis of valence-bond and charge-transfer states to estimate the hyperpolarizability of organic and metallo-organic chromophores using the lowest possible number of variables. Since symmetry breakdown changes the definition of the molecular Cartesian framework, tensorial spherical coordinates are implemented. The evolution of the nonlinear molecular anisotropy with possible rotational deviations is then evaluated for two recently studied chromophores. Zero-frequency calculations show that, outside optical resonance, weak geometrical distortions lead to strong anisotropy variations in agreement with experimental data. Their goal is to underscore which molecular engineering strategies should be applied when designing a photoisomerizable nonlinear octupole.
Density functional theory and multireference configuration interaction studies on low-lying excited states of126(2007); http://dx.doi.org/10.1063/1.2429062View Description Hide Description
Using density functional theory at the level, optimized geometries and energies of the lowest singlet, triplet, and quintet , , , states of the molecule were obtained. has a ground state in symmetry. Adiabatic excitation energies of the low-lying singlet and triplet states range from . The states optimize at bond angles of about 140°, lying only below linear , whereas and , with bond angles of 120° and 96°, respectively, lie below the respective or states. Minima with short O–O distances of , at energies of 4.2 and , were found for and . The minima of the lowest and states are saddle points, suggesting lower-energy structures in symmetry. The quintet states start at energies of . Multireference configuration interaction (MRCI) methods, employing a polarized valence triple-zeta basis set, lead to similar geometries and energies. MRCI vertical excitation energies up to and oscillator strengths are given. The calculated excitation energy of for agrees well with from a fluorescencespectrum. The vertical electron detachment energy of is , in good agreement with from anion photoelectron spectroscopy. An observed second photoelectron band corresponds to and/or , but the assignment of a third band could not be verified. Vibrational frequencies, ionization energies,electron affinities, and dissociation energies are given.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
126(2007); http://dx.doi.org/10.1063/1.2424710View Description Hide Description
Structural and dynamical properties of the hydration of , , and in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that is certainly categorized as a structure maker (breaker), whereas acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of molecules in the first solvation shell of and decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion.