bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonadiabatic transition state theory and multiple potential energy surface molecular dynamics of infrequent events
Authors: Hammes-Schiffer Sharon , Tully John C.
Published: July 2002

Classical transition state theory (TST) provides the rigorous basis for the application of molecular dynamics (MD) to infrequent events, i.e., reactions that are slow due to a high energy barrier. The TST rate is simply the equilibrium flux through a surface that divides reactants from products. In order to apply MD to infrequent events, corrections to the TST rate that account for recrossings of the dividing surface are computed by starting trajectories at the dividing surface and integrating them backward and forward in time. Both classical TST and conventional MD invoke the adiabatic approximation, i.e., the assumption that nuclear motion evolves on a single potential energy surface. Many chemical rate processes involve multiple potential energy surfaces, however, and a number of ‘‘surface-hopping’’ MD methods have been developed in order to incorporate nonadiabatic transitions among the potential energy surfaces. In this paper we generalize TST to processes involving multiple potential energy surfaces. This provides the framework for a new method for MD simulation of infrequent events for reactions that evolve on multiple potential energy surfaces. We show how this method can be applied rigorously even in conjunction with phase-coherent surface-hopping methods, where the probability of switching potential energy surfaces depends on the history of the trajectory, so integrating trajectories backward to calculate the recrossing correction is problematic. We illustrate this new method by applying it in conjunction with the ‘‘molecular dynamics with quantum transitions’’ (MDQT) surface-hopping method to a one-dimensional two-state barrier crossing problem.
A density-matrix divide-and-conquer approach for electronic structure calculations of large molecules
Authors: Yang Weitao , Lee Tai-Sung
Published: July 2002

A density matrix divide-and-conquer method is proposed for electronic structure calculation of large molecules. It is based on partition of density matrix and thus applicable to both density-functional and Hartree–Fock method. Compared to the original formulation with electron density, the present method is more efficient and as accurate.
Classical/quantal method for multistate dynamics: A computational study
Authors: Martinez Todd J. , Ben-Nun M. , Ashkenazi Guy
Published: July 2002

We discuss a classically-motivated method for modeling ultrashort laser pulse optical excitation. The very same method can be used to treat the breakdown of the Born–Oppenheimer approximation. The results are compared to numerically-exact quantum mechanics for a model problem representing excitation from the X (ground) state to the B (excited) state of molecular iodine. Expectation values and final B state populations are predicted quantitatively. The method provides a new way to simulate pump–probe experiments in particular and multistate dynamics in general. The method appears extendible to multidimensional problems. We argue that the increase of effort with dimensionality will be similar to that encountered in classical mechanical simulations as opposed to the exponential scaling of numerically-exact quantum mechanical propagation techniques.
A refined H3 potential energy surface
Authors: Boothroyd Arnold I. , Keogh William J. , Martin Peter G. , Peterson Michael R.
Published: July 2002

In evaluating some low temperature (T<1000 K) thermal rate coefficients for inelastic rotational excitation of H2 by H atoms, Sun and Dalgarno have found a marked sensitivity to the potential energy surface adopted for the calculations. We have investigated the origin of the discrepancies between previous H3 potential energy surfaces and have developed a refined surface which addresses these concerns. New quasiclassical trajectory calculations of cross sections for low energy rotational excitation are reported. The refined surface is based on 8701 ab initio energies, most newly computed for this purpose. It has the same functional form as our earlier (BKMP) surface, but since the fit of the parameters is more fully constrained than for any previous surface it is a more accurate representation. The refined surface matches the ab initio energies with an overall rms error of 0.27 mEh (i.e., 0.17 kcal/mol) and a maximum absolute deviation of 6.2 mEh (for a very compact high energy equilateral triangle conformation). For ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.18 mEh and the maximum absolute deviation is 1.7 mEh. The refined surface is compared critically to four previous surfaces, including the DMBE surface of Varandas et al., in several respects: Legendre expansion coefficients; the interaction region for low energy rotational excitation; near the collinear saddle point; near conical intersections of the ground and first excited state surfaces; the van der Waals well; and compact geometries. We have also compared new first excited state ab initio energies for 1809 conformations with corresponding predictions from the DMBE surface.
Threshold photoionization spectra of benzyl radical: Cation vibrational states and ab initio calculations
Authors: Eiden Gregory C. , Lu Kueih-Tzu , Badenhoop Jay , Weinhold Frank , Weisshaar James C.
Published: July 2002

We have measured threshold photoionization spectra of benzyl+-h7, benzyl+-αd2, and benzyl+-d7 in the ground electronic state (X̃+ 1A1) using resonant two-photon excitation and detection of electrons by pulsed field ionization. The adiabatic ionization potentials of benzyl-h7, benzyl-αd2, and benzyl-d7 are 58 468±5 cm−1, 58 418±5 cm−1, and 58 386±5 cm−1. Excitation through a variety of vibronically mixed Ã 2A2–B̃ 2B2 neutral excited states allows observation of cation vibrations of both a1 and b1 symmetries. We directly measure in-plane fundamentals and infer the frequencies of certain out-of-plane fundamentals from their involvement in combinations or overtones. By comparison with harmonic frequencies from ab initio calculations, we assign 35 of 48 observed levels in the -h7 isotopomer, 15 of 22 levels in -αd2, and 25 of 30 levels in -d7. Ab initio calculations permit a detailed comparison of the geometry, chemical bonding, and vibrational frequencies in the benzyl anion, neutral, and cation. The anion and cation, both closed-shell species, have remarkably similar geometries with relatively short exocyclic CC bond (1.371 Å and 1.372 Å, respectively) and with the aromatic ring compressed along the C2 symmetry axis. The neutral free radical has a longer exocyclic CC bond (1.413 Å) and a more nearly sixfold symmetric ring. The natural resonance theory provides bond orders and resonance-structure weights in all three species. While no single resonance structure dominates in any of the three species, the structure with an exocyclic CC double bond is significantly more important in the anion and cation than in the neutral.
Analytical energy gradient for the reference interaction site model multiconfigurational self-consistent-field method: Application to 1,2-difluoroethylene in aqueous solution
Authors: Sato Hirofumi , Hirata Fumio , Kato Shigeki
Published: July 2002

We have reformulated the reference interaction site model self-consistent-field (RISM-SCF) method to calculate the molecular properties of solute in solutions. The key feature is to introduce the Helmholtz free energy given as a sum of the solute electronic energy estimated by ab initio molecular orbital (MO) methods and the hypernetted-chain (HNC) excess chemical potential of solvation for the extended reference interaction site model (XRISM) method. The variational conditions for the multiconfigurational self-consistent-field (MCSCF) wave functions were derived. It was also shown that, in the case of the Hartree–Fock (HF) method, the present expression gives the same Fock matrix defined previously. Moreover the expression for first derivatives of the free energy with respect to the solute nuclear coordinates were obtained. Sample calculations for the chemical equilibrium of cis- and trans-isomers of 1,2-difluoroethylene are presented. With the use of analytical energy gradients for the MCSCF and HF methods, the solute molecular geometries were determined in an aqueous solution.
Vibronic analysis of fluorescence spectrum of NO2 D̃ 2B2(0,0,0) in the region of 250–550 nm
Authors: Aoki Kaoru , Hoshina Ken-nosuke , Shibuya Kazuhiko
Published: July 2002

The dispersed fluorescence spectrum of NO2 D̃ 2B2(0,0,0) was measured and analyzed in the spectral range of 250–550 nm. The strong fluorescence bands in 250–350 nm correspond to D̃ 2B2(0,0,0)→X̃ 2A1(n1=0–9, n2=0–5, n3=0) with a Franck–Condon maximum at n1=4 and n2=0. The weak and broad bands in 350–410 nm are built on a progression of bending frequency, 710 cm−1. The lower state responsible for this fluorescence was interpreted as admixture evB2 levels generated by a vibronic coupling between a1-vibrational levels on Ã 2B2 and highly excited b2 levels on X̃ 2A1. The medium-intensity bands in 410–550 nm were assigned to D̃ 2B2(0,0,0)→C̃ 2A2(n1=0–2, n2=0–5, n3=0–2) with a Franck–Condon maximum at n1=0, n2=2, and n3=0. The vibrational frequencies of C̃ 2A2 are 1010 cm−1 for symmetric stretch (ω1), 740 cm−1 for bending (ω2), and 250 cm−1 for antisymmetric stretch (ω3). The simple Franck–Condon calculation for D̃ 2B2(0,0,0)→C̃ 2A2(n1,n2,n3) gives the approximate geometry of the C̃ 2A2 state as r(N–O)∼134 pm and θ∼108°. The partial rotational structure of C̃ 2A2(0,0,0) was analyzed using an optical–optical double resonance measurement, which confirms the A2 vibronic symmetry. The origin of NO2 C̃ 2A2 (T0) was determined to be around 16 234 cm−1.
Double screening in polyelectrolyte solutions: Limiting laws and crossover formulas
Authors: Muthukumar M.
Published: July 2002

We have derived general expressions for the Helmholtz free energy of a polyelectrolyte solution and the average size of a labeled chain as functions of polyelectrolyte concentration, salt concentration, and the strengths of excluded volume and electrostatic interaction all varying from very low to very high. We have shown that the bare excluded volume interaction and the electrostatically screened Coulomb interaction between any two segments are both screened by the presence of polyelectrolyte chains at nonzero concentrations. The screening length associated with this excluded volume screening is a function of Debye length and the entropy of the connected polymer and consequently depends on the polyelectrolyte concentration differently at different salt concentrations. By treating the correlation of the monomer density fluctuations, we have derived integral equations coupling the effective interaction between any two segments and the effective step length of a labeled chain. In the limit of the excluded volume screening length and the effective step length being independent of wave vectors, they are related by algebraic equations. These equations reduce to scaling laws with numerical prefactors for semidilute and concentrated polyelectrolyte solutions. Our development provides limiting laws and analytical interpolation formulas for the excluded volume screening length, effective step length, free energy of the polyelectrolyte solution, and the average size of a chain for various polyelectrolyte concentrations in an isotropic solution and for arbitrary strengths of excluded volume and electrostatic interactions. The excluded volume screening leads to an attractive component in the effective potential interaction at intermediate distances between two segmental charges of the same sign. The strength and range of the attractive potential are determined by the concentrations of the polyelectrolyte and the various ions.
Intermolecular potential effects in trajectory calculations of collisions between large highly excited molecules and noble gases
Authors: Lenzer Thomas , Luther Klaus
Published: July 2002

Quasiclassical trajectory calculations have been applied to study intermolecular potential effects in the collisional deactivation of highly vibrationally excited aromatic molecules by noble gases. For benzene+helium and benzene+argon several potential functions of Lennard-Jones 12-6 and EXP-6 type were used. At low well depths ε (for benzene+helium) energy transfer is dominated by the exact shape of the intermolecular potential, especially of the repulsive part, whereas the dependence on ε itself is less important. At higher well depths (for benzene+argon) the energy transfer parameters 〈ΔE〉 and 〈ΔE2〉 scale with ε and are much less sensitive to details of the interaction. New potential parameters based on experimental scattering data for the benzene+helium system are presented, which provide very good agreement with experimental energy transfer parameters for the deactivation of benzene and azulene by helium.
On the nonperturbative theory of pure dephasing in condensed phases at low temperatures
Authors: Reichman David , Silbey Robert J. , Suárez Alberto
Published: July 2002

The nonperturbative treatment of the pure dephasing problem studied by Osad’ko and Skinner and Hsu is reexamined. It is found that these treatments are inadequate for very low temperatures in the case of Ohmic friction. New nonperturbative methods are used to expose interesting pure dephasing behavior at very low temperatures in this case. The methods are shown to coincide with the previous theories at higher temperatures. The experimental detection of such phenomena is discussed.
Electronic absorption spectra of size-selected hydrated electron clusters: (H2O)n−, n=6–50
Authors: Ayotte Patrick , Johnson Mark A.
Published: July 2002

We report photodestruction spectra for the (H2O)n− clusters (n=6, 11, 15, 20, 25, 30, 35, 40, and 50) in the range 0.7–1.6 eV. The spectra are observed to strongly redshift and narrow with decreasing cluster size, with a concomitant increase in peak intensity. The maxima occur well below the vertical detachment energies for n>20 and almost exclusively result from excitation of a bound–bound transition.
High n Rydberg spectroscopy of benzene: Dynamics, ionization energy and rotational constants of the cation
Authors: Neuhauser Robert G. , Siglow Klaus , Neusser Hans Jürgen
Published: July 2002

In a high resolution uv-uv double resonance experiment with two Fourier-transform limited nanosecond laser pulses high Rydberg states up to n>100 were selectively excited and detected by ionization in a pulsed electric field. We were able to identify 64 Rydberg series in benzene C6D6 and 20 series in benzene C6H6 converging to different rotational levels of the ionic molecular core. Their assignment by application of a crosscorrelation analysis yields accurate rotational constants and a precise value for the lowest rotationless ionization energy of 74556.57(5) cm−1 (C6H6) and 74583.51(5) cm−1 (C6H6). All observed series show surprisingly low quantum defects below 0.01 and no strong local perturbations at their crossings points. This points to a weak intramolecular coupling of the series in the absence of an external electric field. In a second series of experiments the influence of external fields on the intensity, the position and dynamics of single n Rydberg peaks is investigated. It is shown that the peak shape and peak position of a single Rydberg peak depends strongly on the applied electric field. Within the envelope of one Rydberg peak different decay times of the pulsed field ionization signal can be found.
Spectral hole burning study of electron–phonon coupling in polymers
Authors: Renge Indrek
Published: July 2002

Persistent hole burning in the S1←S0 transition of tetra-tert-butyl-tetraazaporphine was used to investigate the electron–phonon coupling in a broad range of polymeric solids between 5 and 30–50 K. The maxima of pseudophonon sidebands (νph) are displaced by 5–20 cm−1 from the 0–0 hole. The relationships between the νph values and the velocities of transversal and longitudinal ultrasound waves as well as the Young’s modulus of polymers were established. At the same time νph compare well to inelastic neutron scattering maxima, the first boson peaks in the Raman scattering spectra, and the characteristic modes responsible for extra specific heat and heat conductivity plateau. Mutual correlations of the molecular structure, nanoscopic, and bulk properties in glassy and partially crystalline polymers are pointed out. The quasihomogeneous hole width (Γqh) at fixed temperature (T) increases when νph becomes smaller and the polarity of the host increases. Hole widths measured at 15 and 25 K also display a common linear relationship with total heat content (J/cm3) of the matrix in less polar hosts. Irreversible broadening of holes as a function of excursion temperature was investigated by means of T cycling. The contribution of slow irreversible broadening processes (spectral diffusion) to Γqh does not exceed 20%. The shift of holes burned at 4–8 K upon the rise of T was measured. The pure thermal hole shift was calculated by taking into account the solvent shift contribution due to the density change of the matrix. This pure phonon-induced shift is always bathochromic with increasing T. The T dependence of both the hole width and shift can be equally well fitted with the power law and a coth function. In most systems both the width and shift obey the power law with similar T coefficients of 2.8±0.5 and 2.4±0.5, respectively. A consistent description of the T dependence of the Debye–Waller factor, the hole shift, as well as the width in terms of an anharmonic single-mode model can be achieved for most of the polymers with the same characteristic energy (entering the coth function) which is approximately by a factor of 4 larger than νph. The influence of crystallinity, tacticity, molecular weight, polarity, and chemical structure of the macromolecular host on the strength of electron–phonon coupling is analyzed in detail. The relative importance of the Stark effect and intermolecular dispersive forces in the dynamic modulation of electronic energy levels causing the optical dephasing is discussed.
Extended ab initio studies of the vinylidene–acetylene rearrangement
Authors: Chang Nai-yuan , Shen Min-yi , Yu Chin-hui
Published: July 2002

The ground state vinylidene–acetylene isomerization was investigated by ab initio molecular electronic structure theory. The coupled-cluster method with single, double, and noniterative inclusion of triple excitations [CCSD(T)]; with single, double, and noniterative inclusion of triple and quadruple excitations [CCSD(TQ)]; and with full single, double, and triple excitations (CCSDT) were used to treat the effect of electron correlation. Several correlation-consistent polarized valence basis sets, cc-pVXZ, were employed. Theoretical limiting values of the energetics of the reaction were then deduced from the series of computations. With zero-point energy correction, the energy of reaction is −42.95 kcal/mol and the reaction barrier is 1.5 kcal/mol. Both agree excellently with experimental values.
Temperature programmed desorption: A statistical rate theory approach
Authors: Elliott J. A. W. , Ward C. A.
Published: July 2002

The equation traditionally used to interpret temperature programmed desorption (TPD) spectra, the Polanyi–Wigner equation, does not contain explicitly the coverage and temperature dependence necessary to predict TPD spectra in several important systems including CO–Ni(111). Herein, the statistical rate theory (SRT) approach is used to formulate equations for temperature programmed desorption which are then used to examine TPD spectra reported in the literature for CO–Ni(111). The molecular and material properties for the CO–Ni(111) system have been previously established. One experimental spectrum has been chosen to determine the apparatus constants. The material properties and the apparatus constants are then used in the SRT equations to predict the eight additional TPD spectra for different initial coverages. A critical comparison can then be made between the theory and these eight experimental spectra, since no fitting constants were used in these eight cases. The results show that there is clearly qualitative agreement. The SRT equations are then used along with the heat of adsorption to derive an equation for the pre-exponential factor appearing in the Polanyi–Wigner equation. A prediction is made for the pre-exponential factor that is in agreement with that found empirically. The agreement found between the SRT predictions and the measured spectra indicates that all of the coverage and temperature dependence necessary to predict TPD spectra is given explicitly by the SRT approach. Hence, the experimental support for the SRT approach is enhanced. The SRT equations are then used to predict CO–Ni(111) spectra that would occur if the heating rate were varied.
Reverse micelle to sponge phase transition
Authors: Texter John , Antalek Brian , Williams Antony J.
Published: July 2002

A pseudoternary system exhibiting a single-phase domain extending from water-in-decane to sponge phase to decane-in-water microemulsions as temperature and decane weight fraction (α; relative to brine+decane) are varied has been studied by Chen, Chang, and Strey [J. Chem. Phys. 93, 1907 (1990)]. Conductivity and self-diffusion measurements at 45 °C show that the reverse micelle to sponge phase transition proceeds in a sequence of two continuous transitions defined quantitatively by an order parameter. This is the first study to quantify the onset of sponge phase formation within a single-phase domain, as a function of composition, and to quantify the amount of sponge phase.
The electrostatic persistence length calculated from Monte Carlo, variational and perturbation methods
Authors: Ullner Magnus , Jönsson Bo , Peterson Carsten , Sommelius Ola , Söderberg Bo
Published: July 2002

Monte Carlo simulations and variational calculations using a Gaussian ansatz are applied to a model consisting of a flexible linear polyelectrolyte chain as well as to an intrinsically stiff chain with up to 1000 charged monomers. Addition of salt is treated implicitly through a screened Coulomb potential for the electrostatic interactions. For the flexible model the electrostatic persistence length shows roughly three regimes in its dependence on the Debye-Hückel screening length, κ−1. As long as the salt content is low and κ−1 is longer than the end-to-end distance, the electrostatic persistence length varies only slowly with κ−1. Decreasing the screening length, a controversial region is entered. We find that the electrostatic persistence length scales as ξp/κ, in agreement with experiment on flexible polyelectrolytes, where ξp is a strength parameter measuring the electrostatic interactions within the polyelectrolyte. For screening lengths much shorter than the bond length, the κ−1 dependence becomes quadratic in the variational calculation. The simulations suffer from numerical problems in this regime, but seem to give a relationship half-way between linear and quadratic. A low temperature expansion only reproduces the first regime and a high temperature expansion, which treats the electrostatic interactions as a perturbation to a Gaussian chain, gives a quadratic dependence on the Debye length. For a sufficiently stiff chain, the persistence length varies quadratically with κ−1 in agreement with earlier theories.
Monte Carlo simulation of the dynamics and configurational dependent properties of polymethylene chains
Authors: Linxi Zhang , Agen Xia , Jianmin Xu
Published: July 2002

The dynamics and configurational-dependent properties of polymethylene (PM) chains are studied using the modified bond-fluctuation model and the rotational-isomeric state model. In this article, the tetrahedral lattice model is adopted because it gives a reasonable approximation to the carbon–carbon backbones of PM chain. In our bond-fluctuation model, a Kuhnian bond includes four carbon–carbon bonds. Our statistical mechanics properties of chains are in good agreement with the Flory theory. The relaxation times τ rise with the chain length N by a power law of the form τ (N)−N2 in the absence of excluded volume and τ (N)−N2.17 in the presence of excluded volume, and the diffusion coefficients D behave as D−1/N both in the absence and presence of excluded volume. Our modified bond-fluctuation model can also be used to investigate the glass transition of polymer chains.
Near-infrared band of the nitrate radical NO3 observed by diode laser spectroscopy
Authors: Hirota Eizi , Ishiwata Takashi , Kawaguchi Kentarou , Fujitake Masaharu , Ohashi Nobukimi , Tanaka Ikuzo
Published: July 2002

We have analyzed the near-infrared band of NO3 observed at 7602 cm−1 by using diode laser spectroscopy. Most of the spectral lines were recorded using source-frequency modulation. Zeeman modulation was found useful in selectively detecting some Q branch lines, which provided us with a clue to the assignment of the observed spectra. The band satisfied selection rules for a parallel band and was thus ascribed to a 2A1″–2A2′ vibronic component associated with the 2E′′–X̃ 2A2′ electronic transition, namely, to a transition from the ground vibronic state to the A1″ vibronic state resulting from excitation of the degenerate in-plane bending mode in the 2E′′ electronically excited state manifold. The band was almost free of perturbations, except for some K=6 lines. The least-squares analysis of 581 assigned lines led to molecular parameters of the upper state, where ground-state parameters were fixed to those obtained from the infrared study previously reported [K. Kawaguchi, E. Hirota, T. Ishiwata, and I. Tanaka, J. Chem. Phys. 93, 951 (1990)]. The upper-state B rotational constant gave the effective N–O distance of 1.271 Å, which is to be compared with 1.240 Å in the ground vibronic state. The εbb spin–rotation interaction constant of the upper state was close in magnitude to that in the ground vibronic state, but of opposite sign. This observation indicates that the spin–rotation interaction is primarily caused by that between the 2E′′ excited and the ground electronic states.
Intramolecular excitations in the H2O⋅⋅CO complex studied by diffusion Monte Carlo and ab initio calculations
Authors: Sandler P. , Sadlej J. , Feldmann T. , Buch V.
Published: July 2002

Intramolecular fundamentals of CO⋅⋅H2O are studied using a combination of diffusion Monte Carlo (DMC), and an adiabatic approximation based on frequency separation between inter- and intramolecular vibrations. Variational treatment in a Morse basis is employed for the stretch excitations. DMC is used to calculate the distribution of intermolecular configurations, and to evaluate the (substantial) influence of intermolecular zero-point motion on intramolecular frequency shifts. The dependence of the potential on intermolecular coordinates was derived with the help of ab initio Moller Plesset perturbation theory calculations.
Ion sizes and finite-size corrections for ionic-solvation free energies
Authors: Hummer Gerhard , Pratt Lawrence R. , García Angel E.
Published: July 2002

Free energies of ionic solvation calculated from computer simulations exhibit a strong system size dependence. We perform a finite-size analysis based on a dielectric-continuum model with periodic boundary conditions. That analysis results in an estimate of the Born ion size. Remarkably, the finite-size correction applies to systems with only eight water molecules hydrating a sodium ion and results in an estimate of the Born radius of sodium that agrees with the experimental value.
Phase behavior of semiflexible tangent hard sphere chains
Authors: Fynewever Herb , Yethiraj Arun
Published: July 2002

The isotropic–nematic phase transition in semiflexible hard chain fluids is investigated via an Onsager type density functional theory. The angle-dependent excluded volume of two chains required in the theory is obtained via Monte Carlo simulations. The theory predicts an isotropic to nematic phase transition at lower densities than those predicted by previous theories. These results compare favorably with available simulation data.
Pressure-induced dynamics in solid n-alkanes as probed by optical spectroscopy
Authors: Friebel J. , Friedrich J. , Suisalu A. , Kikas J. , Kuznetsov An. , Laisaar A. , Leiger K.
Published: July 2002

The dependence of frequency, width, and area of spectral holes on pressure were measured at 1.6 K in the pressure range up to 2.5 MPa for dimethyl-s-tetrazine (DMST) doped n-hexane (Shpol’skii system), and as reference systems, for DMST-doped durene (“hard” molecular crystal) and ethanol:methanol glass. For the Shpol’skii system, in addition the inhomogenous fluorescence spectra were measured for normal and high (200 MPa) pressures. The main observations were the following: (i) spectral holes in the Shpol’skii system exhibit very large pressure-induced broadening (up to 65 GHz/MPa) depending essentially on the prehistory (freezing pressure) and exceeding the corresponding values for durene (by far) and glass; (ii) spectral holes in the Shpol’skii system exhibit strong, and to a large extent, reversible, area reduction with applied pressure; and (iii) the inhomogeneous fluorescence lines show quite a moderate (as compared to holes) pressure broadening of about several GHz/MPa. The results for the Shpol’skii system are shown to be inconsistent with existing theories. They are qualitatively explained by pressure-induced dynamics of vacancy defects in the frozen n-alkanes.
Momentum distributions and molecular property information for trans 1,3 butadiene: An electron momentum spectroscopy and density functional theory investigation
Authors: Brunger M. J. , Winkler D. A. , Michalewicz M. T. , Weigold E.
Published: July 2002

The technique of electron momentum spectroscopy (EMS) has been used to measure orbital momentum distributions (MDs) for the complete valence electronic structure of trans 1,3 butadiene. The corresponding theoretical MDs were calculated using a plane wave impulse approximation (PWIA) model for the reaction mechanism and density functional theory (DFT) for the wave function. Seven basis sets, at the local density approximation (LDA) level and, additionally, incorporating nonlocal correlation functional corrections, were studied. The sensitivity of the level of agreement between the experimental and theoretical MDs to the nonlocal corrections is considered. A critical comparison between the experimental and theoretical MDs allows us to determine the “optimum” wave function from our basis sets. This wave function is then used to derive butadiene’s chemically interesting molecular properties, which are subsequently compared to the results of other workers. The sensitivity of the derived molecular property information to the nonlocal correlation functional corrections is also examined.
Molecular bending mode frequencies of the surface and interior of crystalline ice
Authors: Hernandez Justin , Uras Nevin , Devlin J. Paul
Published: July 2002

The Fourier transform infrared (FT-IR) spectra of the bending modes of the three categories of molecules at the surface of ice nanocrystals have been determined for both H2O and D2O samples and the response of the bending mode of the dangling hydrogen surface molecules to association with adsorbates of varying acceptor strengths has been examined. From these combined data for water molecules in a wide range of environments at the ice surface, a clear picture of the dependence of the bending mode frequency on the extent of H-bonding is revealed for ice-related conditions. These frequencies have been examined in terms of the linear equation developed by Falk that relates the water decoupled bend mode frequency to H-bond strength through the average of the symmetric and asymmetric stretch mode frequencies. The published value of 1735 cm−1, for the decoupled bend mode frequency of bulk H2O ice, is consistent with this presentation of the new data, but a new value, higher by at least 40 cm−1 than the accepted value of ∼1220 cm−1, is indicated for D2O ice. A value of 1265 cm−1 is offered as the best estimate of the bend mode frequency of D2O ice.
Non-Fickian diffusion in colloidal glasses
Authors: Hagen M. H. J. , Frenkel D. , Lowe C. P.
Published: July 2002

We have studied numerically the decay of the self-dynamic structure factor (SDSF) for a small particle diffusing in a colloidal glass. We show that, in line with theoretical predictions, the super-Burnett coefficient (characterizing the deviation of the fourth moment of the single particle distribution from its Gaussian value) is finite. However, our results also show that large scale deviations from Fick’s law of diffusion should still be easy to detect experimentally. These deviations take the form of the SDSF decaying algebraically rather than exponentially as Fick’s law would predict.
Disorder-induced exciton scattering in the light-harvesting systems of purple bacteria: Influence on the anisotropy of emission and band→band transitions
Authors: Kumble Ranjit , Hochstrasser Robin M.
Published: July 2002

A time-domain analysis of the effects of pigment inhomogeneity upon the dynamics of optical excitations within bacterial light-harvesting complexes is presented. We focus upon examining the manifestation of disorder scattering in polarized femtosecond spectroscopy and the degree to which exciton delocalization is revealed in emission and transient absorption anisotropy measurements. The time evolution of states prepared by impulsive excitation of a statically disordered circular aggregate model for LH2 antenna complexes have been calculated exactly for varying degrees of pigment inhomogeneity. For a Gaussian distribution of site energies, the dynamics of coherence-loss (scattering) is explored as a function of the ratio of the standard deviation (σ) of the distribution to the intersite interaction energy (β). It is found that modest degrees of disorder (σ/β∼0.4) are sufficient to cause scattering on a sub-100 fs time scale. Results from model calculations of the pump–probe anisotropy strongly suggest that the initial ultrafast emission depolarization component reported for LH1 and LH2 antenna complexes by several groups represents the decay of an initially delocalized exciton, prepared by coherent excitation of eigenstates that become partially localized due to inhomogeneity. A novel approach to studying exciton coherence is proposed based on measurement of the anisotropy of band→band transient absorption transitions, which exhibit a pronounced sensitivity to exciton delocalization.
Methods for geometry optimization of large molecules. I. An O(N2) algorithm for solving systems of linear equations for the transformation of coordinates and forces
Authors: Farkas Ödön , Schlegel H. Bernhard
Published: July 2002

The most recent methods in quantum chemical geometry optimization use the computed energy and its first derivatives with an approximate second derivative matrix. The performance of the optimization process depends highly on the choice of the coordinate system. In most cases the optimization is carried out in a complete internal coordinate system using the derivatives computed with respect to Cartesian coordinates. The computational bottlenecks for this process are the transformation of the derivatives into the internal coordinate system, the transformation of the resulting step back to Cartesian coordinates, and the evaluation of the Newton–Raphson or rational function optimization (RFO) step. The corresponding systems of linear equations occur as sequences of the form yi=Mixi, where Mi can be regarded as a perturbation of the previous symmetric matrix Mi−1. They are normally solved via diagonalization of symmetric real matrices requiring O(N3) operations. The current study is focused on a special approach to solving these sequential systems of linear equations using a method based on the update of the inverse of the symmetric matrix Mi. For convergence, this algorithm requires a number of O(N2) operations with an O(N3) factor for only the first calculation. The method is generalized to include redundant (singular) systems. The application of the algorithm to coordinate transformations in large molecular geometry optimization is discussed.
Determination of bond dissociation energies by threshold ion-pair production spectroscopy: An improved D(HCl)
Authors: Martin J. D. D. , Hepburn J. W.
Published: July 2002

A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (±0.8 cm−1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric fields involved, and scanning the exciting photon energy, it was possible to determine the field-free energetic threshold for H35Cl+hν→H++35Cl−. Using this energy, together with the known values of the ionization potential of H and electron affinity of Cl, a new estimate for the dissociation energy of HCl was obtained: D0(H35Cl)=35 748.2±0.8 cm−1.
Spectroscopic evidence for the coexistence of tetragonal and trigonal minima within the exited state adiabatic potential energy surfaces of hexachlorotellurate and -selenate complexes
Authors: Cremers C. , Degen J.
Published: July 2002

Coexistence of Jahn–Teller minima resulting from the coupling to different accepting modes within the adiabatic potential energy surface (APES) is not possible within the framework of linear vibronic coupling theory. For the lowest exited triplet state T1u3 of inorganic complexes with s2 electronic ground-state configuration, such a coexistence, due to quadratic coupling effects, is discussed. As a direct experimental evidence two vibronic progressions with different accepting modes in the emission spectra resulting from a single electronic state are observed in the emission spectra of the title compounds. The observation of vibronic finestructure in the emission spectra of [TeCl6]2− is reported for the first time.
Monte Carlo simulations of electronic excitation transfer in polymer composites and comparison to theory
Authors: Hussey D. M. , Matzinger S. , Fayer M. D.
Published: July 2002

Monte Carlo (MC) simulations of electronic excitation transfer (EET) among a small number of chromophores covalently incorporated into copolymer molecules are presented and used to test the results of previously developed analytical EET theories that are useful for the study of polymer chain structure [K. A. Peterson and M. D. Fayer, J. Chem. Phys. 85, 4702 (1986)] and phase separation in polymer blends [A. H. Marcus and M. D. Fayer, J. Chem. Phys. 94, 5622 (1991)]. The simulations and theory account for EET among chromophores bound to a single chain and among chromophores attached to different chains. The calculated quantity, 〈Gs(t)〉, which is the probability that an initially excited chromophore is still excited at time t, is related to time-resolved fluorescence depolarization experiments. The theories, particularly the treatment of interchain EET, depend on a series of approximations whose efficacy has not been determined. Close agreement between the MC simulations and the analytical theory are found for a variety of situations, including those that mimic real polymer systems. The limits beyond which agreement is weakened provide specific guidelines for the design of polymer structure and phase-separation experiments.
The origin of the magnetic-field-dependent quadrupolar splitting
Authors: Salsbury Freddie R. , Harris Robert A.
Published: July 2002

We confirm that the recently discovered field-dependent nuclear quadrupolar splitting of Xe131 [T. Meersmann and M. Haake, Phys. Rev. Lett. 81, 1211 (1998)] arises from a distortion of the electron density due to the applied magnetic field. This distortion depends both linearly and quadratically on the applied field. The existence of the former is due to the coupling to the field and the nuclear spin. The latter is a manifestation of the quadratic Zeeman effect. In addition to confirming the order of magnitude for the observed effect, we show that there should be an asymmetry introduced in the spectra due to the linear coupling with the nuclear spin. This effect has not been seen experimentally.
Ab initio study on the reaction mechanism of ozone with the chlorine atom
Authors: Hwang Der-Yan , Mebel Alexander M.
Published: July 2002

Ab initio calculations of the potential energy surface for the Cl+O3 reaction have been performed using the MP2, QCISD(T), CCSD(T), G2, G2M, CASPT2, and MRCI methods with various basis sets. The results show that the reaction pathway can be divided in two parts. The reaction starts on the nonplanar pathway when the Cl atom attacks a terminal oxygen of ozone via TS1, producing a virtual intermediate, a nonplanar chlorine trioxide B. B isomerizes to another virtual intermediate, planar C, which immediately dissociates to ClO+O2 in the coplanar manner. The ClOOO intermediates B and C disappear at the QCISD level of theory. The calculations confirm the direct reaction mechanism for Cl+O3 but the existence of a very flat plateau on the potential energy surface in the region of B, TS2, C, and TS3 can have some effect on the reaction dynamics. TS1 is the critical transition state determining the rate of the Cl+O3 reaction. High level calculations, such as QCISD(T), CCSD(T), MRCI, and CASPT2 with the basis sets from moderate to very large, at the QCISD and CASSCF optimized geometry of TS1, consistently predict the barrier to be about 4–5 kcal/mol, much higher than the experimental value (below 1 kcal/mol). New experimental measurements as well as even higher level theoretical calculations are encouraged in order to resolve this discrepancy.
High pressure studies of the Kramers turnover behavior for the excited-state isomerization of 2-alkenylanthracene in alkane
Authors: Hara Kimihiko , Ito Naoki , Kajimoto Okitsugu
Published: July 2002

The isomerization rate of 2-(2-propenyl)anthracene (22PA) in the lowest excited singlet (S1) state was investigated in supercritical fluids (SCF); ethane and CO2, as well as in a series of liquid n-alkanes at high pressures. Combining the present results with our preceding ones which had been examined in compressed liquid n-alkane solvents [J. Phys. Chem. A 101, 2240 (1997)] we completed the curve of forward isomerization rate constant (kf) against solvent viscosity (η) over the entire friction range. We also discuss the dependence of kf on the inverse of solvent self-diffusion coefficient (D−1), which is used as a measure of solvent collisional frequency. The Kramers turnover behavior of 22PA thus observed was compared with our previous result of 2-vinylanthracene [J. Chem. Phys. 103, 5548 (1995)]. The kf values of both isomerization reactions around the turnover viscosity region never attains those which are predicted by transition state theory.
Exploiting temporal nonlocality to remove scaling bottlenecks in nonadiabatic quantum dynamics
Authors: Ben-Nun M. , Martı́nez Todd J.
Published: July 2002

An extension of the full multiple spawning (FMS) method for quantum non-adiabatic dynamics that capitalizes on the global nature of quantum mechanics and on the deterministic nature of the FMS method is discussed. The FMS method uses a classically motivated time-dependent basis set for the wave function and here we demonstrate that the choice of a temporally nonlocal basis set can reduce the scaling of the dominant effort in ab initio multiple spawning from O(N2) to O(N), where N is the number of basis functions describing the nuclear degrees of freedom. The procedure is applied to a two-dimensional two electronic state model problem and we show that the temporally nonlocal basis set provides accurate expectation values and branching ratios over a broad range of energies.
Femtosecond transient-grating techniques: Population and coherence dynamics involving ground and excited states
Authors: Brown Emily J. , Zhang Qingguo , Dantus Marcos
Published: July 2002

Time-resolved transient grating techniques (TG) arising from four-wave mixing (FWM) processes are explored for the study of molecular dynamics in gas-phase systems ranging from single atoms to large polyatomic molecules. For atomic species such as Ar and Xe, each TG signal shows only a peak at zero time delay when all three incident pulses are overlapped temporally. For diatomic O2 and N2 and linear triatomic CS2 molecules, the TG signals exhibit ground state rotational wave packet recurrences that can be analyzed to obtain accurate rotational constants for these molecules. With heavier systems such as HgI2, ground state vibrational and rotational wave packet dynamics are observed. Resonant excitation allows us to select between measurements that monitor wave packet dynamics, i.e., populations in the ground or excited states or coherences between the two electronic states. To illustrate these two cases we chose the X→B transition in I2. TG measurements yield dynamic information characteristic of vibrational and rotational wave packets from the ground and excited states. Reverse transient grating (RTG) experiments monitor the time evolution of an electronic coherence between the ground and excited states which includes vibrational and rotational information as well. Early time TG signal for the polyatomic samples CH2Cl2, CH2Br2, benzene, and toluene exhibit a coherence coupling feature at time zero followed by rotational dephasing. Differences in the amplitude of these two components are related to the contributions from the isotropic and anisotropic components of the molecular polarizability. A theoretical formalism is developed and used successfully to interpret and simulate the experimental transients. The measurements in this study provide gas-phase rotational and vibrational dephasing information that is contrasted, in the case of CS2, with liquid-phase measurements. This comparison provides a time scale for intramolecular dynamics, intermolecular collisions, and solvation dynamics.
Density functional calculations of nuclear magnetic shieldings using the zeroth-order regular approximation (ZORA) for relativistic effects: ZORA nuclear magnetic resonance
Authors: Wolff S. K. , Ziegler T. , van Lenthe E. , Baerends E. J.
Published: July 2002

We present a new relativistic formulation for the calculation of nuclear magnetic resonance (NMR) shielding tensors. The formulation makes use of gauge-including atomic orbitals and is based on density functional theory. The relativistic effects are included by making use of the zeroth-order regular approximation. This formulation has been implemented and the Hg199 NMR shifts of HgMe2, HgMeCN, Hg(CN)2, HgMeCl, HgMeBr, HgMeI, HgCl2, HgBr2, and HgI2 have been calculated using both experimental and optimized geometries. For experimental geometries, good qualitative agreement with experiment is obtained. Quantitatively, the calculated results deviate from experiment on average by 163 ppm, which is approximately 3% of the range of Hg199 NMR. The experimental effects of an electron donating solvent on the mercury shifts have been reproduced with calculations on HgCl2(NH3)2, HgBr2(NH3)2, and HgI2(NH3)2. In addition, it is shown that the mercury NMR shieldings are sensitive to geometry with changes for HgCl2 of approximately 50 ppm for each 0.01 Å  change in bond length, and 100 ppm for each 10° change in bond angle.