Volume 102, Issue 14, 08 April 1995
Index of content:

An electron spin echo envelope modulation study of the primary acceptor quinone in Zn‐substituted plant photosystem II
View Description Hide DescriptionA Fe^{2+} ion on the acceptor side of plant photosystem II has been substituted by Zn^{2+} and an anion radical of the primary acceptor quinone, Q ^{−} _{ A }, has been studied by electron spin echo method. The electron spin echo modulation shows the interaction of the unpaired electron of Q ^{−} _{ A } with nitrogen nuclei of the histidine and, probably, alanine residues situated nearby. The comparison of the modulation spectra of Q ^{−} _{ A } with those of the anion radical of plastoquinone‐9 stabilized in protonated and deuterated isopropanol matrices allows one to distinguish between the spectrum lines due to the quinone protons and due to the protons of other molecules that form hydrogen bonds with the oxygen atoms of the quinones.

Near‐edge core photoabsorption in polyenes
View Description Hide DescriptionWe present calculations on near‐edge x‐ray absorption fine structures(NEXAFS) in polyenes based on a newly derived direct, atomic orbital, static exchange method. The trends of different NEXAFS features with respect to the site of ionization and with respect to the number of ethylene subunits in the oligomer chain are studied. The trends for energies and intensities are found to be regular and alternant. A substantial reduction of the π to σ intensity ratios with the number of ethylene subunits is predicted, indicating delocalization of the screening π electrons. In contrast to NEXAFS spectra of a heterogeneous group attached to hydrocarbon chains, the NEXAFS spectra of oligomer sequences, here the polyenes, converge slowly. The building block principle is less appropriate than the final state rule for rationalizing the present findings.

Calculation of the photodetachment spectrum of OHCl^{−} using complex L ^{2} functions
View Description Hide DescriptionThe photodetachment spectrum of OHCl^{−} is calculated using a complex L ^{2} approach that was described and tested previously [R. Mayrhofer and J. M. Bowman, J. Chem. Phys. 100, 7229 (1994)]. The calculations are done using the ab initio potential of Koizumi et al. for the neutral system, and the results are compared with the experimental spectrum of the Neumark group, and a previous calculated spectrum, which also used the ab initio potential [M. J. Davis et al., J. Chem. Phys. 101, 4708 (1994)]. The present spectrum consists of two bands of closely spaced lines. These bands are assigned to OHCl quasibound states with zero and 1 quantum of OH stretching vibration. Within each band is a series of lines of varying width, but generally of the order of 10 meV, which are assigned to hindered‐rotor states of the complex. These assignments confirm the ones made by Davis et al., who used a novel hierarchical analysis of the stick spectrum to make spectral assignments. The present spectrum is convoluted with a Gaussian function of width 10 meV to simulate the resolution of the experiment. This artificially broadened spectrum reproduces many of the features of the experimental spectrum, but is substantially narrower than the experimental one. Spectra for the excited Cl^{−}–OH stretch, and also for small changes in the OHCl^{−} equilibrium position are also presented.

Reactive scattering using the multiconfiguration time‐dependent Hartree approximation: General aspects and application to the collinear H+H_{2}→H_{2}+H reaction
View Description Hide DescriptionThe recently developed multiconfiguration time‐dependent Hartree approach (MCTDH) is for the first time applied to quantum reactive scattering. State‐resolved reaction probabilities for the collinear H+H_{2}(ν=0,1)→H_{2}(ν=0,1)+H exchange‐reaction are calculated and are found to be in excellent agreement with previous results obtained by time‐independent methods. To compute the reaction probabilities the initial wavepacket is propagated forward and the final wave packet backwards in time. The Fourier transform at energy E of the time‐dependent overlap of both wave packets is then proportional to the S‐matrix element. Complex absorbing potentials are shown to be easily implemented in the MCTDH scheme. Fixed single‐particle functions are introduced which result in a decrease in computational effort. The MCTDH algorithm requires the potential energy surface to be represented in a particular form. A new scheme for representing potential energy surfaces in a MCTDH adapted form is derived. This scheme can also be used for fitting multidimensional surfaces to given data points.

Time‐dependent discrete variable representations for quantum wave packet propagation
View Description Hide DescriptionWe present an efficient method for exact wave function propagation with several degrees of freedom based on time‐dependent discrete variable representations (TD‐DVR) of the evolution operator. The key idea is to use basis sets that evolve in time according to appropriate reference Hamiltonians to construct TD‐DVR grids. The initial finite basis representation is chosen to include the initial wavefunction and thus the evolution under the bare zeroth order Hamiltonian is described at each time by a single DVR point. For this reason TD‐DVR grids offer optimal representations in time‐dependent calculations, allowing significant reduction of grid size and large time steps while requiring numerical effort that (for systems with several degrees of freedom) scales almost linearly with the total grid size. The method is readily applicable to systems described by time‐dependent Hamiltonians. TD‐DVR grids based on the time‐dependent self‐consistent field approximation are shown to be very useful in the study of intramolecular or collision dynamics.

Unimolecular dynamics of Cl^{−}...CH_{3}Cl intermolecular complexes formed by Cl^{−}+CH_{3}Cl association
View Description Hide DescriptionA previous trajectory study of the dissociation of Cl^{−}...CH_{3}Cl complexes formed by Cl^{−}+CH_{3}Cl association is further analyzed to determine (1) the relationship between classical and quantum Rice–Ramsperger–Kassel–Marcus (RRKM) rate constants for Cl^{−}...CH_{3}Cl→Cl^{−}+CH_{3}Cl dissociation; (2) the importance of anharmonicity in calculating the RRKM dissociation rate constant; (3) the role of angular momentum in interpreting the trajectory distribution N(t)/N(0) of Cl^{−}...CH_{3}Cl complexes versus time; and (4) the pressure‐dependent collision‐averaged rate constant k(ω,E) for Cl^{−}...CH_{3}Cl dissociation. It is found that only the low‐frequency intermolecular modes of Cl^{−}...CH_{3}Cl are initially excited by Cl^{−}+CH_{3}Cl association. Classical and quantum RRKM rate constants for dissociation of this intermolecular complex are in excellent agreement. Anharmonicity lowers the rate constant by a factor of 4–8 from its harmonic value. The dissociation rate for the long‐time tail of the trajectory N(t)/N(0) distribution is much smaller than predicted by a RRKM model, which accurately treats angular momentum. It is suggested that the long‐lived trajectories may arise from motion on vague tori. The trajectory collision‐averaged rate constant k(ω,E) is in agreement with an experimental study at 300 K.

On the dynamics of high Rydberg states of large molecules
View Description Hide DescriptionIn this paper we explore the level structure, the optical excitation modes and the dynamics of a mixed Stark manifold of very high Rydberg states (with principal quantum numbers n=80–250) of large molecules, e.g., 1,4 diaza bicyclo [2,2,2] octane (DABCO) and bis (benzene) chromium (BBC) [U. Even, R. D. Levine, and R. Bersohn, J. Phys. Chem. 98, 3472 (1994)] and of autoionizing Rydbergs of atoms [F. Merkt, J. Chem. Phys. 100, 2623 (1994)], interrogated by time‐resolved zero‐electron kinetic energy (ZEKE) spectroscopy. We pursue the formal analogy between the level structure, accessibility and decay of very high Rydbergs in an external weak (F≂0.1–1 V cm^{−1}) electric field and intramolecular (interstate and intrastate) relaxation in a bound molecular level structure. The onset n=n _{ M } of the strong mixing (in an external fieldF and in the field exerted by static ions) of a doorway state, which is characterized by a low azimuthal quantum number l, a finite quantum defect δ, and a total nonradiative width Γ_{ s }≂Γ_{0}/n ^{3}, with the inactive high lmanifold is specified by n _{ M }≂80.6δ^{1/5}(F/V cm^{−1})^{−1/5}. At n≥n _{ M } the level structure and dynamics are characterized by the product γρ, where ρ is the density of states and γ=Γ_{ sD }(n) is the average decay width of the eigenstates, with the dilution factor D(n)≊n ^{−2} for (lm _{ l }) mixing and D(n)≂n ^{−1} for (l) mixing, whereupon γρ=(Γ_{0}/4δR)(n _{ M }/n)^{5}, being independent of D(n).
The sparse level structure is realized for γρ≪1, while the dense level structure prevails for γρ≳1, resulting in two limiting situations; (a) a dense limit for n≥n _{ M } and a sparse limit for n≫n _{ M }, and (b) a sparse limit for all n≥n _{ M }. The experimental information currently available on the decay dynamics of molecular (DABCO and BBC) and atomic (Ar) Rydbergs for n≥n _{ M } corresponds to case (b). The time‐resolved dynamics was characterized in terms of the excited state total population probability P(t) and the population probability I(t) of the doorway state. P(t), which is interrogated by time‐resolved ZEKE spectroscopy, will exhibit for both the sparse and dense level structures and for all excitation conditions a superposition of exponential temporal decay terms with an average lifetime of ∼ℏ/γ. I(t) can be used to interrogate coherence effects, which in case (b) are manifested in quantum beats, while case (a) corresponds to a giant resonance with a molecular time characterized by the reciprocal energetic spread of the Stark manifold. The experimental data for the onset of strong mixing and for the diluted lifetimes [ℏ/Γ_{ sD }(n) with D(n)∼n ^{−1}] of the high Rydbergs (n∼100–200) of BBC and of DABCO are in accord with the predictions of the theory for the limit of strong (l) mixing. While strong mixing is realized for F̄=Fn ^{5}/3.4×10^{9}δ≳1, we expect that for the weak mixing regime (F̄<1) the dynamics of ultrahigh Rydbergs will be characterized by two distinct (∼ns and ∼μs) time scales. Finally, we emphasize the universality of the model, which provides a unified description of the level structure and dynamics of high Rydbergs of molecules and of autoionizing atoms.

Convergence of molecular potential energy surfaces by interpolation: Application to the OH+H_{2}→H_{2}O+H reaction
View Description Hide DescriptionA recently proposed scheme for interpolating and iteratively improving molecular potential energy surfaces [Ischtwan and Collins, J. Chem. Phys. 100, 8080 (1994)] is evaluated by comparison with an analytic surface for the OH+H_{2}→H_{2}O+H reaction. An improvement in the procedure for constructing the potential surface is suggested and implemented. The most efficient means of converging the surface is determined. It is found that the probability of reaction, for example, may be accurately calculated using of the order of 200–400 data points to define the potential energy surface.

Incorporating backflow into a relaxation theory treatment of the dynamics of nonequilibrium nonadiabatic transition processes
View Description Hide DescriptionAn approximate method for computing the leakage of population from an initial (‘‘donor’’) electronic state, prepared in a nonequilibrium nuclear coordinate distribution, onto a second, nonadiabatically coupled (‘‘acceptor’’), electronic state is presented. This proposed solution, which utilizes a set of coupled integrodifferential equations (commonly referred to as Generalized Master Equations) is an extension of a nonequilibrium golden rule formula derived previously [R. D. Coalson, D. G. Evans, and A. Nitzan, J. Chem. Phys. 101, 486 (1994)]. The Generalized Master Equation approach is able to describe situations where the donor and acceptor potential energy surfaces have similar energy origins, and hence irreversible flow of population from the donor state to the acceptor state is not expected. The accuracy of the method is demonstrated for an exactly solvable spin–boson model of inner sphere electron transfer. In the regime of small nonadiabatic coupling, agreement of the proposed method and path integral calculations is nearly quantitative for symmetric electron transfer processes and systems with weak bias between the energy origins of the donor and acceptor electronic states. Unlike the nonequilibrium golden rule formula, appropriately constructed Generalized Master Equations are capable of capturing the backflow of electronic population from the acceptor to the donor surface and relaxation to Boltzmann equilibrium at long times.

Classical trajectory studies of the reaction CH_{4}+H→CH_{3}+H_{2}
View Description Hide DescriptionTrajectory data are reported for the reaction CH_{4}+H→CH_{3}+H_{2}, designed to provide information that can be used to test approximate quantitative theories for the dynamics of abstraction reactions. A potential function was devised which properly reflects the nuclear permutation symmetry of the process. Microscopic reaction rate coefficients were obtained as functions of fixed rotational and vibrational energy, and of the angular momentum. The data indicated significant uncoupling between the various modes although, at a minimum, the symmetric stretch is directly coupled to the reaction coordinate at the transition state. The data were used to test the assumption that the total angular momentum,J, may be approximated by the orbital angular momentum,L. L is approximately conserved from the reactant to the saddle point configuration in reactive and nonreactive collisions and may be well approximated by J. The angular momentum about the long axis of the reacting system (equivalent to the K quantum number) is not conserved in either reactive or nonreactive trajectories.

Interference control without laser coherence: Molecular photodissociation
View Description Hide DescriptionControl over channel‐specific line shapes and branching ratios in photodissociation is shown to be achievable by irradiating a molecule with two intense cw lasers whose relative phase need not be well defined. Control results from quantum interference between nonlinear pathways induced by the intense fields, within which the relative laser phase cancels. The interference, and hence the product yields, can be manipulated by changing the relative frequencies and intensities of the two lasers. In this paper this theory of high field control is developed, and computations on the photodissociation of Na_{2} are presented. Control over product yields is shown to be extensive, even with inclusion of rotational states. For example, the branching ratio between the Na(3s)+Na(3p) and Na(3s)+Na(4s) products can change by as much as a factor of 10 as the frequencies are tuned.

Sampling of semiclassically quantized polyatomic molecule vibrations by an adiabatic switching method: Application to quasiclassical trajectory calculations
View Description Hide DescriptionWe apply the adiabatic switching (AS) method to determine the polyatomic classical motions that correspond to selected vibrational quantum states on multidimensional, anharmonic potential energy surfaces, and use these semiclassically quantized motions as initial conditions for quasiclassical trajectory(QCT) calculations of state‐to‐state reactiondynamics. Specifically, we calculate the classical motion corresponding to the quantum mechanical zero‐point vibration of deuterated methane, CD_{4}, and run QCT calculations on the H+CD_{4}→HD(v’,j’)+CD_{3}reaction. The distribution of CD_{4} vibrational zero‐point energy (ZPE) associated with the AS‐sampled motions is compared with that from normal‐mode‐sampled motions. The spread of total zero‐point energy in the AS calculations is much narrower than with normal‐mode sampling, and the ZPE’s are appropriately shifted to lower energy due to anharmonic effects. Reverse adiabatic switching is used as an indirect check of the quantum numbers of the adiabatically sampled motion, but numerical limitations made this test inconclusive. The AS method thus appears to be superior to normal‐mode sampling, but this superiority cannot be demonstrated conclusively for the fully anharmonic CD_{4} potential. However, the AS method is shown to perform very well for transformation from one CD_{4} harmonic potential to another and for transformation from an harmonic to an anharmonic, but decoupled potential in which CD_{4} is described by Morse oscillators. Evidence is presented that suggests the AS calculations are limited by numerical inaccuracies or intrinsic features of the potential energy surface, both of which are unavoidable. H+CD_{4}→HD(v’,j’)+CD_{3}QCT calculations of state‐to‐state dynamics using CD_{4} with no ZPE, the ZPE from AS sampling, and the ZPE from normal‐mode sampling are reported and compared.

Theoretical investigations of proton transfer reactions in a hydrogen bonded complex of cytosine with water
View Description Hide DescriptionThe potential energy functions of the electronic ground as well as the lowest ππ* and nπ* excited singlet states of the most stable hydrogen bonded complex of cytosine with water was theoretically investigated along the proton transfer (PT) reaction coordinate. The full geometry optimization was performed along the PT reaction path. In the geometry optimization, the Hartree–Fock method and the configuration interaction scheme with single excitations was used. The energy calculations at the optimized geometries were performed with the complete‐active‐space self‐consistent‐field (CASSCF) method and with second‐order perturbation theory, employing the CASSCF wave function as the reference state (CASPT2), as well as with the Moller–Plesset second‐order theory (MP2) for the ground state. It was found that the cytosine:water complex is stable in the amino‐oxo form against the PT reaction which leads to the imino‐oxo tautomeric form on the ground as well as on the excited PE surfaces. An efficient nonradiative decay channel of electronically excited cytosine resulting from nonadiabatic interactions between the ground and excited singlet states along the reaction coordinate leading to the prefulvenic form was identified. It is argued that both the above mentioned effects under the normal conditions (neutral aqueous solution, room temperature) are responsible for remarkable stability of cytosine in its ‘‘native’’ amino‐oxo form against any damage that may result from exposure to UV radiation.

A density functional theory study of the alkali metal atom–carbon monoxide interactions: Singularity of the Li atom
View Description Hide DescriptionThe LiCO molecule has been previously characterized experimentally in rare gas matrices by a large red shift of the CO stretching mode and a Li–C vibration around 600 cm^{−1} indicating a relatively strong Li–C bonding. Up to now, post Hartree–Fock ab initio methods had described this complex as unstable in its electronic ground state and predicted a linear metastable structure owing to strong induction forces between this complex and the surrounding polarizable rare gas atoms. Electronic and vibrational calculations at the harmonic approximation on this molecule using approximate density functional theory give results which are better in line with the experimental results. Two distinct bounded structures are found to be stable in the electronic ground state for the LiCO complex. The credibility of these calculations is also assessed in two ways; the first one is the comparison between experimental and calculated vibrational analysis on the ν_{CO} modes of the Li(CO)_{ n } (n=2,3) complexes. The second one is the study of the eventual complexation of Na and K atoms with the CO molecule and the comparison with experimental available data.

The vibrational structure of H^{+} _{4} and D^{+} _{4}
View Description Hide DescriptionA potential energy surface (PES) for the H^{+} _{4} system which is a fit to local, high quality ab initio multiple reference single and double excitations configuration interaction (CI) calculations (142 data points) is reported. The potential energy surface obtained here has been calculated by maintaining H^{+} _{3} as a core in its equilibrium geometry and moving the remaining hydrogen atom around it (three‐dimensional potential energy surface). The new surface supposes an improvement on the preceding potential energy surfaces considering both the basis set size and the ab initio method used here. The energy minimum of the potential presented here has been found to be about 2.5 kcal/mol lower than those obtained in previous studies, indicating that H^{+} _{4} and D^{+} _{4} are more stable ions than previously believed. A three‐dimensional vibrational kinetic energy operator in internal coordinates without singularities has been derived. Energy and wave functions of the vibrational levels of the dissociating hydrogen in H^{+} _{4} and D^{+} _{4} systems have been calculated by using the derived potential and kinetic energy operators and integrating the vibrational Hamiltonian with the normal coordinates finite elements method. The vibrational states with energies below the new dissociation limit are reported and characterized, giving a more complete description of the vibrational structure. The number of bound vibrational levels obtained here is 7 for H^{+} _{4} and 24 for D^{+} _{4}.

Excited‐state reaction pathways for s‐cis buta‐1,3‐diene
View Description Hide DescriptionThe topology and energetics of the potential energy surfaces associated with the 2A _{1} and 1B _{2} valence excited states of s‐cis butadiene have been investigated via ab initio quantum chemical computations at a level of theory which includes dynamic correlation effects and extended basis sets. The results support a photochemical ring‐closure mechanism involving 1B _{2} and 2A _{1} reaction/relaxation pathways that are disrotatory. The reaction path on the 2A _{1} surface begins at a 1B _{2}/2A _{1} conical intersection and the ground state photoproducts are produced via radiationless decay at a second 2A _{1}/1A _{1} conical intersection which has been documented in a previous publication. A local C _{ s } equilibrium structure on 1B _{2}potential energy surface has been optimized using the complete active space‐self‐consistent field and configuration interaction singles methods. The 1B _{2}/2A _{1} conical intersection is located near this C _{ s } equilibrium structure and offers a rationalization of the experimentally observed femtosecond lifetime of this state. The observed preferential disrotatory stereochemistry appears to be simply determined by a difference in the energy barriers located along the 2A _{1} disrotatory and conrotatory paths. This finding is in contrast with the generally accepted notion that the stereochemistry is determined by a different rate of internal conversion at a ‘‘disrotatory’’ and ‘‘conrotatory’’ avoided crossing minima. Indeed, no avoided crossing can be located along the 2A _{1} paths.

On the scattering of low‐energy electrons by sulphur hexafluoride
View Description Hide DescriptionThe collision between a beam of electrons and gaseous molecules of sulphur hexafluoride (SF_{6}) are studied from a theoretical and computational point of view, by solving the multichannel scattering problem within the close‐coupling (CC) approach that uses a symmetry‐adapted, single‐center‐expanded (SCE) description of the continuum functions. The static and exchange interactions are treated exactly within the basis set expansion and correlation forces are introduced in local form using a density functional model (FEG) without empirical parameters. Elastic observables (rotationally summed) are obtained over a broad range of energies, both for the integral and differential cross sections, and compared with the existing experiments. The symmetry and nature of the observed resonances are analyzed and discussed. One finds, in general, very satisfactory accord between the present calculations and the measured cross sections.

N_{2}–Kr interaction: A multiproperty analysis
View Description Hide DescriptionAn earlier anisotropic N_{2}–Kr potential energy surface [Chem. Phys. Lett. 88, 197 (1982)] of Lennard‐Jones form, obtained by inversion of molecular beam differential scattering data, has been found to provide calculated transport–relaxation phenomena in good agreement with experiment, but leads to calculated second interaction virial coefficients which are significantly too low, and does not provide adequate agreement with recent microwave van der Waals spectral data. A modification of this surface to include a recent ab initio determination of the C _{6} dispersion coefficient, and to bring in the virial and microwave data, gives a new potential surface that is in good agreement with all available experimental data.

Optimized atomic Lennard‐Jones 6–12 parameters for simulating pVT properties of a realistic polymethylene melt
View Description Hide DescriptionFrom the comparison of experimental low‐pressure pVT data for a short alkane with the results of Monte Carlo simulations in the NpT ensemble of an atomistically detailed model of polymethylene (PM) with explicit hydrogens, we have obtained Lennard‐Jones parameters that allow accurate prediction of pVT behavior for liquid long‐chain alkanes at high pressure. The parameters were obtained from the Slater–Kirkwood formula and fitted to the experimental density of n‐pentane at 0.1 MPa; they faithfully reproduce experimental data for chains up to C_{23}H_{48} (n‐tricosane) and pressures up to 100 MPa over a wide temperature interval.

Multidensity integral equation theory for highly asymmetric electrolyte solutions
View Description Hide DescriptionIntegral equation theory based on a recently developed multidensity formalism [Mol. Phys. 78, 1247 (1993)] is proposed to study highly asymmetric electrolyte(polyelectrolyte)solutions. The system studied consists of large and highly charged polyions and small counterions having one or two elementary charges. The potential energy of interaction between counterions and polyions is separated into two parts, a strongly attractive part responsible for the association and a nonassociative part. Due to the strong asymmetry in size we can treat each counterion as bondable to a limited number of polyions n, while each polyion can bond arbitrary number of counterions. In our cluster expansion appropriate to the problem the diagrams appearing in the activity expansion of the one‐point counterion density are classified in terms of the number of associating bonds incident upon the labeled white counterion circle. The corresponding diagrams for the one‐point polyion density are classified in the usual way. A generalized version of the Ornstein–Zernike equation, which involves n+1 counterion densities and one polyion density, together with hypernetted‐chain‐like (HNC) closure conditions are derived. The simplest two‐density version of the theory yields very good agreement with new and existing computer simulations for both thermodynamical and structural properties of these systems. This good agreement extends into the region of parameter space where the ordinary HNC approximation does not have a convergent solution.