Volume 114, Issue 10, 08 March 2001
 ARTICLES

 Theoretical Methods and Algorithms

Electron localization function studies of the nature of binding in neutral raregas containing hydrides: HKrCN, HKrNC, HXeCN, HXeNC, HXeOH, and HXeSH
View Description Hide DescriptionNeutral raregas containing molecules HRgCN (Rg=Kr, Xe), HXeSH and HXeOH are investigated by means of topological analysis of the electron localization function (ELF). This analysis explains the type of bonding and delocalization of electron density in chemical systems based on the indirect probability of finding two electrons with the opposite spins. The calculations reveals that all studied species are chargetransfer systems with the approximate formulas: and The isomerization process from HRgCN to HRgNC increases the charge separation to 0.72e for Kr and 0.74e for Xe containing molecules. It is shown that the Rg–C, Rg–N, Xe–S, and Xe–O bonds belong to the unshared electron type and are mainly of the electrostatic origin. The minimum of ELF in the Kr–C and Kr–N linkage, studied at the computational level, yields relatively high values of about 0.4 and 0.3, respectively. There is a correlation between larger stability of HRgCN isomers and an increased exchange of the electron density between the lone electron pair of carbon and the nonbonding electron density of xenon: as compared to smaller delocalization in less stable HRgNC isomers. The analysis of the CN group reveals the covalent character of the carbon–nitrogen bond, which is confirmed on the basis of presence of the disynaptic valence attractor positioned near to nitrogen. The HRgCN→HRgNC isomerization results in depletion of the carbon–nitrogen bond and a large saturation of the valence nitrogen basin

On the validity of the extended Koopmans’ theorem
View Description Hide DescriptionA theorem that provides a sufficient condition for the validity of the extended Koopmans’ theorem (EKT) is formulated and proven. Although its verification may be difficult in practice, this condition involves only quantities pertaining to the Nparticle system. A less restrictive necessary condition remains unknown.

Uniform semiclassical IVR treatment of the
View Description Hide DescriptionA new, uniform, semiclassical, initial value representation expression is obtained for the in the case of collinear collisions. The derivation is based on an asymptotic analysis (for large interfragment distances) of a uniform semiclassical integral expression for the time independent scattering wave function. Although this derivation specifically treats the case of the collision of an atom with a harmonic diatom, the final expression is generalized to arbitrary collinear collisions. The various properties of the expression and its relation to existing semiclassical methods are discussed. Numerical tests are performed for the wellknown Secrest–Johnson system. Among other important advantages, the present treatment is a welldefined, uniform, semiclassical approximation that is capable of good accuracy and high computational efficiency, requiring a relatively small number of classical trajectories to obtain converged elements for a given energy and initial state.

A dielectric continuum molecular dynamics method
View Description Hide DescriptionWe introduce a novel method to simulate hydrated macromolecules with a dielectric continuum representation of the surrounding solvent. In our approach, the interaction between the solvent and the molecular degrees of freedom is described by means of a polarization density free energy functional which is minimum at electrostatic equilibrium. After a pseudospectral expansion of the polarization and a discretization of the functional, we construct the equations of motion for the system based on a Car–Parrinello technique. In the limit of the adiabatic evolution of the polarization field variables, our method provides the solution of the dielectric continuum problem “on the fly,” while the molecular coordinates are propagated. In this first study, we show how our dielectric continuum molecular dynamics method can be successfully applied to hydrated biomolecules, with low cost compared to free energy simulations with explicit solvent. To our knowledge, this is the first time that stable and conservative molecular dynamic simulations of solutes can be performed for a dielectric continuum model of the solvent.

Local and nonlocal approximation for a simple quantum system
View Description Hide DescriptionWe apply two commonly used approximation schemes to the problem of a symmetric dimer with dynamic diagonal disorder. Since the exact analytical solution for this simple dissipative quantum system is known, we are in position to test the range of validity and the quality of the approximations and to derive guidelines for the application of the approximations to more complex systems.

Short range interaction potentials between anions in crystals
View Description Hide DescriptionThe ab initio computation of uncorrelated short range twobody anion–anion potentials can yield two apparent anomalies. First, despite the common understanding that the repulsion between two closed shell species arises from the overlap of their wave functions, compression of the anion electron densities sometimes increases even though the overlap is reduced. Second, attractive are occasionally predicted at large ionic separations These apparent anomalies arise because is the sum of a permutation term arising from interionic electron exchange plus a penetration term independent of such exchange, equal to the nonpoint Coulombic electrostatic interaction. This is attractive at realistic and reduced in magnitude by ionic compression. is always repulsive and is decreased by ionic compression except occasionally at large involving an attractive The latter increases are explained by analyzing into two further terms: one involving Uniform electron gasdensity functional predictions of are oversensitive to the ion density, thereby missing compressioninduced enhancements of Ab initio predictions of and are presented both for “optimal” computed using anion wave functions optimal for each crystal geometry and for “frozen” where the entire potential is computed using the anion wave function optimal for a geometry very close to that of the crystal at equilibrium. This data plus the total “frozen” consisting of plus an approximate electron correlation contribution were required to parametrize both previous compressible ion model studies and the refinements presented in the next paper.

A physically transparent and transferable compressible ion model for oxides
View Description Hide DescriptionA new compressible ion model for describing the energetic components of the cohesive energy of a fully ionic crystal is developed and tested using previous ab initio results for three cubic phases of MgO. This model is physically highly transparent and improves on previous compressible ion models in two ways. First, the shortrange cation–anion interaction and the rearrangement energy needed to convert a free ion plus a free electron into an ion having a form optimal for its incrystal environment are decomposed into the major contributions originating from the six outermost anion electrons plus smaller terms generated by the two electrons. This model transfers to the B2 and B3 phases of MgO after parametrization on the ab initio data for the B1 phase even more accurately than previous compressible ion models. Second, the separate modeling of the repulsive (permutation) and attractive (penetration) components of the shortrange anion–anion interactions enables the new model to describe their subtle dependencies on the incrystal anion environment that lie beyond the scope of previous models. The new model is also used to illuminate the behavior of two fluorite oxides.

Toward quantum information processing by nuclear magnetic resonance: Pseudopure states and logical operations using selective pulses on an oriented spin 3/2 nucleus
View Description Hide DescriptionNuclear magnetic resonance spectroscopy has demonstrated significant experimental progress toward the development of quantum computations. The developments so far have taken place mainly through the use of spin nuclei. In this paper we describe the use of a spin nucleus, oriented in a liquid crystal matrix for the creation of pseudopure states and the implementation of a complete set of twoqubit reversible logic gates using singlequantum transitionselective pulses, extending the range of practice of NMR toward quantum computation.

An evaluation of the density functional approach in the zero order regular approximation for relativistic effects: Magnetic interactions in small metal compounds
View Description Hide DescriptionThe performance of the density functional approach in the relativistic zero order regular approximation for the evaluation of electron spin resonance(ESR) parameters in small metal compounds has been evaluated critically by comparison with experimental data and available theoretical results for 22 linear molecules, characterized by a electronic ground state. For most of the molecules studied the calculated magnetic parameters are in good tensors) or reasonable tensors) agreement with experiment. Effects of spinorbit coupling and spin polarization on the calculated hyperfine interaction are investigated. These two effects can only be evaluated separately, since the present method does not allow us to take spinpolarization effects into account in spinorbit coupled density functional calculations. However, while spinpolarization effects are important for all the molecules investigated, spinorbit effects are nonnegligible only for the molecules containing heavier metal atoms. The ESR parameters, evaluated using different “standard” exchangecorrelation potentials, have only shown little dependence on the specific functional. Direct relativistic contributions to the hyperfine parameters are often large, especially for the heavier metals, but also “secondary” contributions to the ligand hyperfine parameters can be large if the ligand is bound to a heavy element.

Excited state potential energy surfaces from the inversion of absorption spectra: Removal of a global singularity
View Description Hide DescriptionThis paper develops inversion algorithms to extract an excited electronic statepotential energy surface based on absorption spectral data from assumed known lower reference electronic states. A global singularity problem is identified which may also play an important role in other inversion algorithms. The means to remove or circumvent the singularity are discussed. Simulations of the proposed inversion algorithms are tested in a onedimensional model system.

Nuclear Fukui function and Berlin’s binding function: Prediction of the Jahn–Teller distortion
View Description Hide DescriptionThe properties of the derivative of the total binding function (the virial of the forces) with respect to the number of electrons and its decomposition at local and atomic level have been analyzed. At local level the binding function is expressed by the Berlin function and the electronic Fukui function The atomic analog is expressed in terms of the nuclear Fukui function (FF) and the nuclear position vectors. A relationship between the local maps of the nuclear FF vectors, and the Jahn–Teller distortion direction is discussed. It is predicted that upon ionization the symmetry of the nearest local stationary point for is for molecules and and for For the benzene anion a symmetry is predicted.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

The photodissociation of and at 220–231 nm investigated by velocity map ion imaging
View Description Hide DescriptionThe photodissociation of the two isotopomers of dimethyl sulfide, and through the first electronic absorption band at wavelengths 220–231 nm has been studied employing velocity map ion imaging to detect the methyl products. Translational energy and recoil angle distributions have been determined for the and fragments either in the vibrational ground state or without product state selection. The measurements indicate that the S–C bondphotolysis yields strongly anisotropic product scattering distributions characterized by β parameters within the range (−0.4, −1.0). The β values closest to the limiting value of −1.0 (corresponding to a dipole perpendicular transition) are observed for the methyl fragments produced in the vibrational ground state. The product recoil energy distributions are centered at ≈1.5–1.7 eV (i.e., 65%–75% of the available energy) and are quite narrow (full width at half maximum, which indicates that methyl and methylthio fragments are born internally cold but with substantial translational excitation. The recoil energy distribution is practically independent of the photolysis wavelength within the interval studied, indicating that changes in the photonenergy correlate primarily with the rovibrational excitation of the (or fragment whose internal dynamics appear to be more active in the dissociation process than that of the (or partner.

Fourier transform infrared emission spectroscopy of VCl
View Description Hide DescriptionThe high resolution spectrum of VCl has been observed in emission in the 3000–9400 cm^{−1} region using a Fourier transformspectrometer. The bands were excited in a high temperature carbon tube furnace from the reaction of vanadium metal vapor and a trace of and the spectra were recorded at a resolution of 0.05 cm^{−1}. The new bands observed in the 6000–8000 cm^{−1} interval have been attributed to VCl. The bands having R heads near 6176, 6589, 7004, 7358, and 7710 cm^{−1} have been assigned as the 0–2, 0–1, 0–0, 1–0, and 2–0 bands, respectively, of the electronic transition. A rotational analysis of the and subbands of the 0–1 and 0–0 vibrational bands has been obtained and molecular constants have been extracted. The remaining two of the five subbands could not be analyzed because of severe overlapping from neighboring subbands. The principal molecular constants for the state obtained from the present analysis are: and Å. Our work represents the first observation of this near infrared electronic transition of VCl.

Structures and spectra of iodide–water clusters An ab initio study
View Description Hide DescriptionTo investigate the structures of extensive ab initio calculations have been carried out. Owing to very flexible potential surfaces of the system (in particular for and 6), the lowest energy structures are characterized from various possible lowlying energy conformers. In contrast to some previously reported structures, we find a new lowest energy structure (followed by a few lowlying energy conformers) for and four nearly isoenergetic conformers for These conformers have surface and nearsurface structures with the coordination number of 4. The present results provide the information of possible structures in recent profuse experiments of infrared spectra of and charge transfer from the excited iodide ion to water molecules. Our predicted ionization potentials and OH stretching frequencies are in good agreement with the experimental data available, while only the cases of the OH frequencies for and the ionization potential for need consideration of conformational change by the temperature effect.

The and electronic states of the HCSi radical: Characterization of the Renner–Teller effect in the ground state
View Description Hide DescriptionThe electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the selfconsistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD including a perturbative expansion for connected triples CCSD(T) levels of theory with a wide range of basis sets. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and Renner–Teller splitting are reported. At our highest level of theory [CCSD(T)/ccpVQZ], the ground electronic state has a linear geometry with and This is in good agreement with the experimental values of and respectively. In the state, HCSi is also found to have a linear geometry with and at the [CCSD(T)/ccpVQZ] level, confirming experimental values of and the observation of C–Si triple bond character. With the same method, the and state C–H stretching vibrational frequencies are predicted to be 3271 and 3319 cm^{−1}, respectively, for which experimental values are not available. The classical splitting value) was determined to be 32.6 kcal/mol (1.41 eV, 11 730 cm^{−1}) and quantum mechanical splitting value) to be 33.5 kcal/mol (1.45 eV, 11726 cm^{−1}), which are in excellent agreement with the experimental value of 11 766.721 cm^{−1} (33.64 kcal/mol, 1.459 eV). The linear excited state of the molecule has a real degenerate bending vibrational frequency, whereas the ground state is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies. The Renner parameter (ε) and average harmonic bending frequency of the state are predicted to be and at the CCSD(T)/ccpVQZ level of theory. The electronic structure analysis of the ground state showed that the HCSi radical is an Atype Renner–Teller molecule.

Finestructure state resolved rotationally inelastic collisions of with Ar: A combined experimental and theoretical study
View Description Hide DescriptionA collaborative experimental and theoretical study of rotationally inelastic collisions of resolved rotational/finestructure levels with argon is presented. Experimental statetostate rate constants were extracted from CH fluorescence spectra upon laser excitation to individual levels in the state in the presence of Ar. Fluorescence detection of the collisioninduced population permits resolution of the finestructure levels at low N, but no Λdoublet discrimination. For the lowest value of N the dominant process is the finestructurechanging transition, and the efficiency of this transition decreases markedly with increasing N. There is an increasing preference for conservation of the finestructure label in transitions as N increases. These rate constants have been compared to and interpreted with theoretical rate constants computed through quantum coupledstates calculations of cross sections based on ab initio potential energy surfaces determined by Kaledin and Heaven (to be published). The tendency to conserve the finestructure label is attributed to the Hund’s case (b) nature of the state, for which the electron spin is a spectator in the collision.

Stark field induced perturbations in the vibrational overtone band of acetylene
View Description Hide DescriptionThe vibrational overtone of acetylene was recorded using an optothermal molecular beamspectrometer. The sample excitation region was surrounded by a buildup cavity and by electrodes which allowed the observation of Stark splittings of the spectral transitions. An analysis of the J dependence of the splittings determined that the is mixed by the Stark field with the IR (infrared) dark state and that the energy of the transition is 11 611.585±0.018 cm^{−1}. The spectra also showed transitions to highly excited bending levels: These transitions were observable because they borrowed intensity from the more intense stretching overtone as they were tuned into coincidence. The bending states were found to have extremely high polarizabilities which may account for previously reported collision dynamics.

Excitation of the symmetry forbidden bending mode in molecular photoionization
View Description Hide DescriptionWe present results on the energy dependence of the vibrational branching ratio for the bending mode in photoionization. Specifically, we determine the intensity ratio by detecting dispersed fluorescence from the electronically excited photoions. The results exhibit large deviations over a very wide energy range, Production of the level of the ion from the ground state is forbidden by symmetry, and while observations of such features are well established in photoelectron spectroscopy, their appearance is normally ascribed to vibronic coupling in the ionic hole state. In this case, we find that such explanations fail to account for the energy dependence of the branching ratio. These deviations indicate that the continuum photoelectron participates in transferring oscillator strength to the nominally forbidden vibrational transition. A theoretical framework is developed for interpreting the experimental data, and Schwinger variational calculations are performed. These calculations demonstrate that the continuum electron is responsible for the observation of the excited bending mode as well as its energy dependence. This is an intrachannel effect that is best described as photoelectroninduced vibronic symmetry breaking. This appears to be a general phenomenon, and it may be useful in illuminating connections between bond angle and photoionization spectroscopies. The magnitude of these deviations display the utility of vibrationally resolved studies, and the extent over which these changes occur underscores the necessity of broad range studies to elucidate slowly varying characteristics in photoionization continua.

Competing isomeric product channels in the 193 nm photodissociation of 2chloropropene and in the unimolecular dissociation of the 2propenyl radical
View Description Hide DescriptionThis paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2chloropropene excited at 193 nm and of the unimolecular dissociation of the 2propenyl radical. Tunable vacuum ultraviolet (VUV)photoionization of the products allows us to distinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2chloropropene: An excitedstate C–Cl fission channel producing fast Cl atoms, a C–Cl fission channel producing slow Cl atoms, and HCl elimination. A minor fission channel contributes as well. The measured branching of the major primary product channels is: The experiments also allow us to resolve selectively the product branching between the unimolecular dissociation channels of the 2propenyl radical, a high energy isomer; we measure how the branching ratio between the two competing C–H fission channels changes as a function of the radical’s internal energy. The data resolve the competition between the unimolecular and product channels from the radical with internal energies from 0 to 18 kcal/mol above the barrier. We find that the barrier to formation from this highenergy radical is higher than the barrier to formation, in agreement with recent theoretical calculations but in sharp contrast to that predicted for the most stable isomer, the allyl radical. The experiments demonstrate a general technique for selectively forming a particular isomer dispersed by internal energy due to the primary photolysis, thus allowing us to determine the branching between unimolecular dissociation channels as a function of the selected radical isomer’s internal energy.

A multifacet mechanism for the reaction: An ab initio molecular orbital/statistical theory study
View Description Hide DescriptionThe mechanism for the reaction has been studied by ab initio molecular orbital calculations at the G2M(cc3) level of theory. Four complexes and four transition states have been found and confirmed by intrinsic reaction coordinate analyses. The commonly assumed sixmembered ring complex formed by hydrogen bonding of the OH radical with –ON(O)OH…OH–, was found to be stable by 8.1 kcal/mol; its decomposition producing was predicted to have a barrier of 11.6 kcal/mol. A fivemembered ring complex, –ON(O)OH…O(H)–, with the H atom of the OH radical placed out of the ring plane, was found to have a stability of 5.3 kcal/mol; it fragments to form with a barrier of 6.6 kcal/mol. Two additional complexes, which are the mirror image of each other with a 7.4 kcal/mol binding energy, were found to be related to the OH exchange reaction with a 13.3 kcal/mol barrier above the complexes. The direct abstraction process producing and was predicted to have a large barrier of 24.4 kcal/mol, insignificant to atmospheric chemistry. The rate constant has been calculated at 200–1500 K and 0–760 Torr. The results show that the reaction has strong pressure and tunneling effects below room temperature. In addition, the rate constants for the decay of OH and OD (in evaluated by kinetic modeling compare reasonably well with experimental data below room temperature. The unusually pronounced kinetic isotope effect observed experimentally, could be accounted for by the combination of the greater tunneling rate in the H system and the large redissociation rate of stabilized complexes in the D system. The rate constant predicted for the production of and in the temperature range 750–1500 K can be effectively represented by the expression