bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational and translational dynamics of flexible macromolecules
Authors: Peters Michael H. , Ying Ruoxian
Published: July 2002

In two previous studies [J. Chem. Phys. 91, 1287 (1989); 95, 1234 (1991)] we had examined the dynamics of coupled, internal translational, and rotational motions in a rigid or pseudorigid N-sphere macromolecular model using the rotational–translational Brownian dynamics algorithm of Dickinson et al. [J. Chem. Soc. Faraday Trans. 2 81, 591 (1985)]. In the present study, those works are generalized to include all possible internal flexible motions in an N-sphere macromolecular model. In general, for the N-sphere system there are 6(N-1) degrees of configurational freedom, although not all modes may be active in any particular application. Using the language of small oscillation theory, the deviations in the generalized coordinates associated with the ‘‘joints’’ connecting the spheres to one another are described in terms of quadratic potentials. From these potentials, the components of the generalized forces (torques and forces) are obtained for subsequent use in the Brownian dynamics algorithm. The degree of flexibility for any particular mode in the N-sphere system can be controlled by a single constant in the associated quadratic potential function. By taking the appropriate limits of these constants, the complete range of flexibility, viz., from ‘‘torque (or force)-free’’ to ‘‘rigid’’ can be approximately realized at any point in the N-sphere system. The model given is, therefore, capable of simulating all types of macromolecular motions. As a specific example, we studied the linear elastic rotator, where all modes (translational and rotational) are constrained except for rotations about the line of center of the spheres. Brownian dynamics results expressed in terms of rotational correlation functions were in good agreement with analytical solutions obtainable for this highly symmetric system. The algorithm given here is believed to be particularly useful in the study of the dynamics of biological macromolecules where ‘‘flexibility’’ is often critical to the functionality of the macromolecule [J. Chem. Phys. 90, 3843 (1989); Macromolecules 16, 421 (1983); 15, 1544 (1982); Chem. Phys. 41, 35 (1979)].
High pressure studies of a hydrogen-transfer photoreaction in a crystalline solid: Acridine/fluorene
Authors: Bromberg Steven E. , Chan I. Y. , Schilke David E. , Stehlik D.
Published: July 2002

We report the effect of pressure on the photoinitiated hydrogen transfer reaction in a mixed crystal of acridine in fluorene. Upon excitation to its triplet state, the aza-atom of acridine abstracts a hydrogen atom from the methylene bridge of fluorene. The reaction rate constant for both the hydro- and deutero- (methylenic) species has been measured as a function of pressure. For both species, the room temperature transfer rate increases exponentially with increasing pressure. Furthermore, the rate of change is the same for the two isotopic species. At 77 K, the H-transfer rate again increases exponentially, but with a smaller slope. In contrast, the D-transfer rate remains too small to affect the precursor triplet lifetime which hardly changes over a pressure range of 36 kbar. The H-transfer rate was also measured as a function of temperature for a few higher pressures. A temperature independent regime is observed at low temperature. This limiting low temperature rate increases with pressure and reflects the tunneling rate from the vibrational ground state. At ambient pressure it was too small to be accessible to measurements. High pressure allows the direct determination of this ground state tunneling rate. The implications of our observations in the tunneling mechanism are discussed.
Novel interfacial phenomena at liquid–liquid interfaces of the three-component surfactant system water+n-tetradecane+C6E2
Authors: Chen L.-J. , Yan W.-J.
Published: July 2002

In this study, we have found, by simply adjusting system temperature, two wetting transitions at different liquid–liquid interfaces in three liquid phases coexisting region of the system H2O+n-tetradecane+diethylene glycol monohexyl ether (C6E2). When the temperature close to the lower critical consolute temperature, an intruding (or wetting) layer of the lower water-rich phase is formed between the upper oil-rich and the middle amphiphile-rich phase, and, with only a small amount of middle phase, the middle phase is formed a lenticular droplet suspending on the interface between the upper and the lower phase. A wetting transition is occurred with increasing temperature and the intruding (or wetting) layer of the lower phase is contracted into several beads suspending on the interface between the upper and the middle phase. These suspending beads of the lower phase vanish as increasing temperature, however, the middle phase still forms a lenticular droplet. With further increasing temperature, before the upper critical consolute temperature is reached, another wetting transition is observed, i.e., the lenticular droplet of middle phase collapses and forms a thin layer separating the upper and the lower phase.
Photoelectron–photofragment ion recoil vector correlations in the dissociative photoionization of PF3 at He i and Ne i ionizing wavelengths
Authors: Powis Ivan
Published: July 2002

A comparative study of the photoionization and dissociation of the A state of PF3+ at two different resonance line ionizing wavelengths (He i and Ne i) is presented. Using a photoelectron–photoion coincidence (PEPICO) technique, energy selected ions are prepared and fragment ion kinetic energy distributions derived. The energetics and dynamics of the ion fragmentation process are discussed and a simple impulsive dissociation model is proposed. An angular correlation between the recoil directions of ejected electrons and fragment ions is identified and is especially pronounced for the He i data. An interpretation in terms of an anisotropic body-frame photoelectron angular distribution is presented, from which the likely presence of a shape resonance in the A state photoionization dynamics around λ=584 Å is inferred. This accords with the observation of resonances in core photoionization studies of PF3.
Dynamics of the oxygen combination reaction on Pt(111) initiated by photodissociation of N2O at 193 nm: O*+O(ad)→O2(g)
Authors: Sawabe Kyoichi , Lee Jihwa , Matsumoto Yoshiyasu
Published: July 2002

The desorption of O2 is observed when a Pt(111) surface with the coadsorbates of oxygen atoms and N2O is irradiated with 193 nm photons. This indicates that an oxygen atom produced by photodissociation of N2O reacts with a chemisorbed oxygen adatom to form an oxygen molecule. The dynamics of the photoinitiated combination reaction of oxygen is studied by time-of-flight spectroscopy.
Preconditioned complex generalized minimal residual algorithm for dense algebraic variational equations in quantum reactive scattering
Authors: Reeves Melissa S. , Chatfield David C. , Truhlar Donald G.
Published: July 2002

Variational basis-set formulations of the quantum mechanical reactive scattering problem lead to large, dense sets of equations. In previous work, we showed that the generalized minimal residual (GMRes) algorithm is sometimes competitive in terms of computer time with direct methods for these dense matrices, even when complex-valued boundary conditions are used, leading to non-Hermitian matrices. This paper presents a preconditioning scheme to accelerate convergence and improve performance. We block the potential energy coupling into a series of distortion blocks, and we employ the outgoing wave variational principle with nonorthogonal basis functions, including both dynamically adapted Green’s functions for the distortion blocks and also square integrable functions. The coefficient matrix of the resulting linear system couples the blocks. We have found that preconditioners formed from diagonal blocks of the coefficient matrix corresponding to the distortion blocks and vibrational blocks are effective at accelerating the iterative method in every test case, by factors of 2.9–20, with an average speedup of a factor of 6.5. The storage requirements and computational efficiency of the new scheme compare favorably to those for preconditioners based on banded matrices of variable bandwidth. The new preconditioners yield converged transition probabilities in less computer time than a direct solver even in cases which do not converge in a reasonable amount of time without preconditioning, and the average speedup compared to the direct solution is a factor of 7.6.
Highly excited vibrational states of acetylene: A variational calculation
Authors: Sibert Edwin L. , Mayrhofer Rudolph C.
Published: July 2002

The vibrational states of a planar model of acetylene are obtained using a discrete variable representation. This representation takes advantage of the dynamic separability of high frequency stretching motions and low frequency bending motions. We obtain bend states up to 8770 cm−1 above the zero point energy converged to within ±2 cm−1.
Flux–flux correlation function study of resonance effects in reactive collision
Authors: Ryaboy Victor , Lefebvre Roland
Published: July 2002

Thermal rate constants for a one-dimensional model of a reactive collision involving a transient resonance are calculated by using autocorrelation functions of the flux operator in a finite basis set representation [Miller, Schwartz, and Tromp (MST), J. Chem. Phys. 79, 4889 (1983)] and performing either integration over time (MST) or Pade extrapolation to zero of an energy parameter [Lefebvre, Ryaboy, and Moiseyev, J. Chem. Phys. 98, 8601 (1993)]. The two procedures prove to be equally successful. We observe that in the time dependent approach, the correlation function of the reactive flux operator shows, as expected, damped oscillations with a period which slightly depends on the temperature. However, these oscillations are decaying on a time scale that is significantly shorter than the resonance lifetime. This finding shows that the flux–flux correlation function approach is applicable to calculations of thermal rate constants for reactions which proceed via formation of intermediate complexes as well as to studies of short time direct reactive processes.
Ion–ion correlations in polyelectrolyte solutions: Hard sphere counterions
Authors: Penfold Robert , Jönsson Bo , Nordholm Sture
Published: July 2002

The intimately related mechanisms of macroion screening and ion–ion correlations in electrolyte solutions are investigated with the aim to explore the simplest theoretical estimations of the latter on the former. The Poisson–Boltzmann theory of macroion screening in electrolyte solutions was developed under two important assumptions. Specifically, the correlated motion of the mobile ions is ignored by identifying the mean field electrostatic potential with the potential of average force and, moreover, only point charges are considered so that finite size effects are not accounted for. Utilizing the generalized van der Waals free energy density functional formulation for inhomogeneous fluids, we obtain simple corrections to the Poisson–Boltzmann result where both these mechanisms are treated in local and nonlocal approximations. By incorporating a somewhat more realistic short range repulsive interaction, the asymptotic instability encountered in the point particle case is avoided and self-consistent density profiles can be computed. Nonlocal approximations are shown to be thermodynamically consistent in terms of the contact theorem for the osmotic pressure, and excellent agreement with Monte Carlo simulation is achieved even at high electrostatic coupling and with large particles.
Pulsed laser evaporation of boron/carbon pellets: Infrared spectra and quantum chemical structures and frequencies for BC2
Authors: Martin Jan M. L. , Taylor Peter R. , Yustein Jason T. , Burkholder Thomas R. , Andrews Lester
Published: July 2002

Pulsed laser evaporation of pellets pressed from boron and graphite powder gave a new 1:4 doublet at 1232.5 and 1194.6 cm−1 in addition to the carbon cluster absorptions reported previously. The 1232.5 cm−1 band dominated boron-10 experiments. The new bands increased as carbon cluster bands decreased with increasing B/C ratio in the pellet and with increasing laser power. Augmented coupled cluster and full-valence complete active space SCF (CASSCF) calculations predict the global minimum BC2 structure to be an asymmetric triangle: however, the vibrationally averaged structure will be an isosceles triangle with a strong symmetric B–C2 stretching frequency near 1200 cm−1. The calculated boron-10/boron-11 frequency ratio (1.0323) is in excellent agreement with the observed ratio (1.0317), and confirms assignment of the 1194.6 cm−1 band to the BC2 ring. Calculations predict linear BCC to be less stable by 6.2±2 kcal/mol and to absorb in the 2000–2050 cm−1 range: the barrier towards rearrangement to the cyclic structure is very low (1.1 kcal/mol). Linear BCC was not detected in these experiments. Computed energetics explain why BC2 is abundant in B/C experiments, but absent in B+C2H2 experiments.
Calculation of dynamic electric dipole polarizability, nuclear electric shieldings, and their Cauchy moments in benzene
Authors: Lazzeretti P. , Malagoli M. , Turci L. , Zanasi R.
Published: July 2002

Theoretical methods based on the random-phase approximation have been applied to evaluate near Hartree–Fock dynamic electric polarizability and shielding tensors of carbon and hydrogen nuclei in the benzene molecule. Cauchy moments of the various properties have been determined. The results obtained in different gauges (dipole length, velocity, and acceleration) are reported.
Deuteron magnetic resonance probe of the D2–He potential energy surface
Authors: Armstrong Robin L. , Bogdan Mircea , Jeffrey Kenneth R. , Bissonnette Carey , McCourt Frederick R. W.
Published: July 2002

Measurements of the nuclear magnetic resonance (NMR) longitudinal relaxation time T1 have been carried out for D2–He gas mixtures at temperatures of 293 and 220 K for two concentrations of D2 in He and the results extrapolated linearly to infinite dilution at each temperature. Full quantum close-coupled scattering calculations have also been carried out for an ab initio D2–He interaction potential. The agreement found between measured and calculated relaxation times is excellent at 293 and good at 220 K. Based on the level of agreement found between theory and experiment, it can be concluded that the molecular hydrogen–helium ab initio potential energy surface employed in the present D2–He calculations as well as in earlier H2–He calculations [J. Chem. Phys. 81, 5275 (1984)] represents well the effects of both the spin–rotation and combined dipolar/quadrupolar intramolecular interactions, which dominate the spin relaxation, respectively, of protons and deuterons in the hydrogen isotopes. At the same time, the present results indicate that the disagreement found earlier [J. Chem. Phys. 92, 5907 (1990)] between calculated and measured deuteron T1 values for HD–He mixtures must arise from the specific nature of the HD–He potential energy surface.
Theory of fluorescence excitation spectra using anharmonic-Coriolis coupling in S1 and internal conversion to S. II. Application to the channel three problem in benzene for the 14112 band
Authors: Helman Adam , Marcus R. A.
Published: July 2002

Rotational lines in the fluorescence excitation spectra of the 14112 band of the first excited singlet state (S1) of benzene are calculated for various J and K. For this purpose, perturbation theory is used to obtain an ‘‘eigenstate’’ in S1. Internal conversion to S0 via Franck–Condon (FC) factors is then calculated. A search procedure is used to obtain the important contributors to this S1 state and to this internal conversion process S1→S0 using the perturbation theory coefficients and the FC factors in the evaluation function. At low J, the calculated lines with K=0 are sharp, other lines being broadened and diminished in intensity. The calculated K=0 lines have a linewidth proportional to J(J+1). For high J, the lines with K=J remain sharp, the other lines being broadened and diminished in intensity. These various results are in general agreement with the experimental findings. The onset of channel three in benzene occurs in the present mechanism via anharmonic-Coriolis coupling in the S1 state plus internal conversion to S0. The calculations suggest that, at low J, parallel Coriolis coupling causes mixing of the in-plane mode-excited ‘‘light state’’ with in-plane modes that are anharmonically coupled to out-of-plane modes.

Dark states with certain excited out-of-plane mode contributions possess large FC factors for the internal conversion to S0. At high J, on the other hand, the in-plane modes are coupled directly to these out-of-plane modes by perpendicular Coriolis coupling. Paths involving two perpendicular Coriolis operators are important at high J in the present calculation—their matrix elements are larger at high J and so they become more competitive relative to purely anharmonic coupling operators. Such two-Coriolis paths at high J are expected to yield multiple excitation in the out-of-plane modes and further enhance the internal conversion. The perpendicular Coriolis coupling is least at J=K and so these lines survive at high J. Two-Coriolis operator paths are calculated to be relatively unimportant at low J. The present calculations, using the same electronic matrix element, account for both the low JK=0 and high JK=J sets of lines being the dominant ones. Aspects regarding further study are discussed.
About origin of Q component on the vibration–rotation band of HF in simple solvents
Authors: Tokhadze K. G. , Mielke Z.
Published: July 2002

The evolution of the fundamental νHF absorption in the spectra of HF/Ar, Xe, CO2, CO mixtures at densities varying from dilute gas to liquid has been studied. The spectral region of the HF librational mode has been simultaneously examined. The results show that molecular complexes play the crucial role in the mechanism of formation of the Q component in the spectra of HX in simple solvents.
Towards the one-particle basis set limit of second-order correlation energies: MP2-R12 calculations on small Ben and Mgn clusters (n=1–4)
Authors: Klopper Wim , Almlöf Jan
Published: July 2002

A principal source of error in electronic structure calculations is the inability of conventional CI (configuration interaction) expansions to describe the electron–electron cusp. This manifests itself in the slow convergence of correlation treatments with finite basis sets which are commonly applied in traditional ab initio quantum chemistry. In this paper we describe results obtained by adding special n-particle functions, which have terms linear in the interelectronic coordinate r12, to the usual trial wave function, which is an expansion in terms of Slater determinants. A vectorized and efficient computer program has been written for putting into practice second-order Mo/ller–Plesset perturbation theory with linear r12 terms (MP2-R12): the sore program. It exploits both direct integral evaluation strategies and techniques that permit the full (also nonabelian) use of molecular point group symmetry. These two ingredients to the program allow for the use of very large Gaussian basis sets in conjunction with the linear r12 terms. As a result we are now able to press into new territories of accuracy. Calculations on Be and Mg clusters illustrate applications of the program. Binding energies are discussed with regard to basis set saturation and with some emphasis on the basis set superposition error (BSSE). The combination of our MP2 basis set limits on one hand with results from CCSD(T) and MRCI calculations with standard basis sets on the other leads to reliable estimates of the binding energies of Be3 (27 kcal/mol), Be4 (88 kcal/mol), Mg3 (8 kcal/mol), and Mg4 (28 kcal/mol). The most extensive MP2-R12 calculations have been performed with very large uncontracted Cartesian Gaussian basis sets. Also, core–core and core–valence correlation effects have been accounted for. In this work we present the results of the first real large-scale calculations employing Hylleraas-type coordinates which have been performed so far on many-electron, polyatomic molecules.
The Jahn–Teller effect in 9-fluorotriptycene
Authors: Furlan Alan , Leutwyler Samuel , Riley Mark J. , Adcock William
Published: July 2002

The vibronic structure in the S1(E)←S0(A1) resonant two-photon ionization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—E⊗e, (A⊕E)⊗e, and (A⊕E)⊗(a2+e). The basic E⊗e interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron-withdrawing effect of the F atom. For the same reason, the exciton splitting between the S1 and the S2 states, as calculated in the (A⊕E)⊗e model, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm−1 are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S0 frequencies, as well as those of triptycene. A 273 cm−1 degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher-energy (200–360 cm−1) part of the R2PI spectrum, and may represent an example of e⊕e multimode coupling within a degenerate electronic state.
Quantum and quasiclassical calculations on the OH+CO→CO2+H reaction
Authors: Clary David C. , Schatz George C.
Published: July 2002

Scattering calculations on the OH+CO→CO2+H reaction are reported using both quantum and quasiclassical methods. The rotating bond approximation is used in the quantum calculations. This method explicitly treats the OH vibration and CO rotation in the reactants and the bending vibration and a local CO stretch in the CO2 product. Analogous quasiclassical trajectory computations are also reported. A potential energy surface obtained as a fit to ab initio data is used. The quantum reaction probabilities are dominated by sharp resonances corresponding to vibrationally excited states of the HOCO complex formed in the reaction. The quantum and quasiclassical lifetimes of these resonances compare quite well with measurements made by Wittig et al. Calculations of differential cross sections, rate coefficients, and CO2 vibrational product distributions are also compared with experimental data. The comparisons of quantum and quasiclassical calculations for models that treat explicitly different numbers of degrees of freedom provide detailed insight into the dynamics of the OH+CO reaction.
Comparison of a Born–Green–Yvon integral equation treatment of a compressible binary polymer blend on a lattice with recent simulations
Authors: Sevian H. M. , Brazhnik P. K. , Lipson J. E. G.
Published: July 2002

Predictions of macroscopic and microscopic thermodynamic properties of a polymer blend using the Born–Green–Yvon (BGY) integral equation treatment of a compressible polymer mixture on a lattice are compared to recent simulations of compressible symmetric binary mixtures on a lattice. The theory is consistent with recent simulation studies which indicate that the critical temperature scales linearly with chain length. In addition, we show that the theory is effective in modeling the qualitative trends in the numbers of different types of pair contacts on the lattice, and also present comparisons of binodal and spinodal curves. These comparisons indicate that quantitative differences between the theory and simulation results are likely associated with the manner in which connectivity is treated in the lattice BGY description. We conclude by discussing avenues whereby connectivity may be incorporated more consistently in the development of the theory.
Surface segregation in a polymer blend. Comparison between Monte Carlo simulation and mean-field theory
Authors: Cifra P. , Bruder F. , Brenn R.
Published: July 2002

Monte Carlo simulation of wetting of a solid wall by a polymer blend in the one phase region, for χ=0, was performed in a compressible system. We varied the wall interaction preference of one of the components and also the blend composition for the fixed interaction with the wall. The results were compared to the prediction of mean-field (MF) theory. We found a good agreement in composition profiles at the wall using the same parameters in the theory and in simulation, without additional fitting. Due to compressibility of our system, interaction of chains with the wall and the coil deformation at the wall small deviations of the total density and thus also of the composition profile from MF theory were found in close vicinity of the wall.
On the dipole moment of PbO
Authors: Dolg Michael , Nicklass Andreas , Stoll Hermann
Published: July 2002

Pseudopotential calculations have been performed for the dipole moment of the PbO molecule, including core-polarization and spin–orbit contributions. Agreement to ∼0.1 a.u. is achieved with accurate relativistic all-electron calculations as well as experimental results, at the self-consistent-field and valence-correlated levels, respectively.
A Gaussian-2 ab initio study of van der Waals dimers R1R2 and their cations R1R+2 (R1, R2=He, Ne, Ar, and Kr)
Authors: Ma Ngai Ling , Li Wai-Kee , Ng C. Y.
Published: July 2002

Predictions for the ionization energies (I.E.s) of van der Waals dimers R1R2 and the bond dissociation energies (D0’s) of the dimer ions R1R2+ (R1, R2=He, Ne, Ar, and Kr) have been calculated using the ab initio Gaussian-2 theoretical procedure. Despite only fair agreements observed between theoretical and experimental geometries, the theoretical I.E.s and D0’s are found to be in excellent agreement with available experimental findings.
The admissible sign of the differential capacity, instabilities, and phase transitions at electrified interfaces
Authors: Partenskii Michael B. , Jordan Peter C.
Published: July 2002

The issue of the allowed sign of the differential capacity C of electrochemical interfaces has a long history dating back to the so-called ‘‘Cooper–Harrison catastrophe.’’ Previously suggested ‘‘electromechanical models’’ are modified to include entropic contributions; the possibility of C<0 for an isolated electric cell is supported by rigorous solution of the model. We also provide new evidence that for an electric cell in contact with a potentiostat (‘‘extended system’’) the overall C of the cell must be positive; the contribution of an individual double layer may still be negative. The previous statistical mechanical derivation of the upper boundary for C−1 is generalized for a quite general model of an electrolyte in contact with hard charged walls. We also discuss the possibility of electric instabilities and phase transitions in an extended system when an isolated prototype possesses a negative capacity branch.
Response to ‘‘Comment on ‘A modified superposition approximation to the three-body distribution function’ ’’ [J. Chem. Phys. 99, 5625 (1993)]
Authors: Hernando J. A.
Published: July 2002

A modified Kirkwood superposition approximation (MKSA) is applied to a MC simulation of hard spheres with η=0.27 and is compared to the KSA. The results are good but not as good as in the LJ case. We conclude that, globally speaking, the MKSA is an improvement with respect to the KSA.
Simulations in planar slits at constant chemical potential
Authors: Svensson Bo , Woodward C. E.
Published: July 2002

In this work we describe Monte Carlo simulations of a fluid confined between planar walls. We derive a free energy difference method (FEDM) in the isotension ensemble. We illustrate the method by varying the separation between the walls at constant chemical potential. It is shown that the FEDM is a viable alternative to grand canonical ensemble (GCE) simulations. Our method appears to be less susceptible to the difficulties that confront the simulation of confined fluids at high density in the GCE.
The adsorption and photochemistry of CD3I on TiO2(110)
Authors: Garrett Simon J. , Holbert Victor P. , Stair Peter C. , Weitz Eric
Published: July 2002

The adsorption and photochemistry of CD3I adsorbed on TiO2(110) at ∼110 K has been studied by means of temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Complex desorption behavior is observed in TPD suggesting the presence of several distinct coverage regimes. At submonolayer coverages there are two coexisting phases: one dominated by adsorbate–adsorbate interaction, the other dominated by adsorbate–substrate interactions. The first completed monolayer corresponds to (3.8±0.3)×1014 molecules  cm−2 and shows only one desorption peak, although this is broad and extends asymmetrically to high temperature indicative of a changing desorption activation energy. With increasing coverage, a discrete, less tightly bound second layer is formed which slowly rearranges to produce three-dimensional clusters of methyl iodide, as indicated by a sharp reduction in the I (3d5/2)/Ti(2p) XPS intensity ratio. There is evidence that this rearrangement does not involve the first monolayer. Irradiation of 1 ML CD3I by 254 or 334 nm photons causes cleavage of the C–I bond and expulsion of I and C containing species into the vacuum. There is retention of ∼20% of the initial I atoms after irradiation at 254 nm. The photodissociation cross section, σ, of adsorbed CD3 I on TiO2(110) at 254 and 334 nm is calculated to be ∼1.1×10−18 cm2 and ∼1.3 ×10−20 cm2, respectively. At 254 nm, the adsorbate and gas phase σ are similar suggesting photodissociation is dominated by adsorbate excitation, but at 334 nm the adsorbate photodissociation cross section is almost an order of magnitude larger than its gas phase counterpart. This suggests that a second photoexcitation mechanism may be contributing to adsorbate photodissociation, possibly involving photogenerated substrate carriers.
Cluster effects in O3/H2O photochemistry: Dynamics of the O+H2O→2OH reaction photoinitiated in the O3⋅H2O dimer
Authors: King David S. , Sauder Deborah G. , Casassa Michael P.
Published: July 2002

The dynamics of the 266 nm photoinitiated reaction of 16O3 and H218O were studied using 16O3⋅H218O van der Waals dimers to orient the initial reagents. In the absence of perturbations, the geometry of the 16O3⋅H218O dimer is such that 266 nm photolysis of cluster-bound ozone initiates glancing O+H2O trajectories, with a 3 Å impact parameter. Laser induced fluorescence probes show that 81±7% (2σ) of the ‘‘new’’-16OH and essentially all of the ‘‘old’’-18OH products were formed with v=0, with a slight preference for the Π(A′) Λ doublets, and average rotational energies of 900±130 and 760±80 cm−1, respectively. Approximately 19% of the ‘‘new’’-16OH products form with v=1 and average rotational energy of 930±210 cm−1. No significant OH scattering anisotropy or other vector correlations were observed. Sub-Doppler resolution experiments showed average kinetic energies for new-16OH(v=0) products about 19% higher than for old-18OH(v=0) products in the same rotational levels; increasing from values of about 500 cm−1 at low rotational levels, to about 1500 cm−1 at the highest rotational levels populated. Similar OH internal and kinetic energies were observed when the clusters were photolyzed at 281.5 nm. These dimer results are very different from those observed for the bimolecular O(1D)+H2O→2OH reaction, photoinitiated in gas phase mixtures of 16O3 and H218O. The gas phase O(1D)+H2O→2 OH reaction produces OH with pronounced recoil anisotropy, these OH products carry far more internal energy than seen in the cluster products, and there is greater disparity between the internal energies of the gas phase 16OH and 18OH products. Evidently, cooperative effects in the cluster environment result in a significant change in reaction path.
Vector properties in photodissociation: Quantum treatment of the correlation between the spatial anisotropy and the angular momentum polarization of the fragments
Authors: Siebbeles Laurens D. A. , Glass-Maujean Michèle , Vasyutinskii Oleg S. , Beswick J. Alberto , Roncero Octavio
Published: July 2002

The dependence of the angular momentum polarization (orientation and alignment) of the fragments on the direction of ejection k, is studied quantum mechanically for molecular photodissociation into two fragments of which one carries an angular momentum j. Explicit expressions in terms of the transition matrix elements for electronic excitation into the final dissociative states are given in the axial-recoil limit and for different photon polarizations. The importance of interference effects due to coherent excitation of dissociative states with different helicity quantum numbers (the projection Ω of j on the recoil direction k) is stressed. It is shown that not only absolute magnitudes but also relative phases of individual transition matrix elements can be determined separately if the spatial anisotropy of the angular momentum polarization is measured.
A path integral study of electronic polarization and nonlinear coupling effects in condensed phase proton transfer reactions
Authors: Lobaugh John , Voth Gregory A.
Published: July 2002

Feynman path integral quantum transition state theory is used to calculate the quantum rate constants for model proton transfer systems in a polar fluid. The effects of intramolecular vibrations on the proton transfer rate, as well as those from solute and solvent electronic polarization, are examined. In the latter study, quantum Drude oscillators are used to model the high frequency electronic polarization of the solvent and a path integral action functional is developed within the context of that model. The results obtained with the corresponding classical Drude model are compared with the correct quantum treatment. From these calculations, it is concluded that proton transfer reactions involving a significant degree of tunneling cannot be quantitatively described without a quantum mechanical polarizable solvent model. Furthermore, the calculations illustrate the intrinsically nonlinear character of the proton tunneling process in polar solvents.
The D+H2(v=1,j)→HD(v′,j′)+H reaction. A detailed quasiclassical trajectory study
Authors: Aoiz F. J. , Buchenau H. K. , Herrero V. J. , Rábanos V. Sáez
Published: July 2002

Thorough quasiclassical trajectory (QCT) calculations have been carried out for the D+H2(v=1,j) exchange reaction. These calculations include integral and differential cross sections, rate constants, reaction probabilities as a function of total energy, opacity functions, and distributions of internal states of the HD product in the range of collision energies from the reaction threshold to 1.5 eV and initial j values from 0 to 12. An overall good agreement with some discrepancies is found between the present QCT results and those from experiments and accurate quantum-mechanical calculations.
Molecular theory of phase equilibria in model associated mixtures. I. Binary mixtures of water and a simple fluid
Authors: Nezbeda I. , Smith W. R. , Kolafa J.
Published: July 2002

The phase equilibrium behavior of a basic statistical mechanical model of a binary mixture of water and a simple fluid are investigated. For water we use a primitive model which incorporates no attractive forces other than hydrogen bonds within a hard-sphere core, and the simple fluid is modeled by a fluid of hard spheres with an attractive mean-field term. The qualitative behavior of the phase equilibria and P−T critical lines of this model are calculated using an accurate theoretical equation of state for primitive water obtained in analytical form using the Wertheim thermodynamic perturbation theory, for a range of values of the mixture’s two underlying parameters, the hard-sphere size ratio and the mean field strength. The model exhibits type IIId and type IV* phase behavior of three qualitatively different types, characterized by the existence of either one or two three-phase lines. The model exhibits a range of phenomena, including tricritical points and double critical end points. The implications of the model results for corresponding real fluid mixtures are also discussed.
Photodissociation of CH2. IV. Predicted resonance Raman emission spectrum of photodissociating 3CH2
Authors: Kroes G. J. , van Hemert M. C.
Published: July 2002

Predicted resonance Raman emission spectra of photodissociating 3CH2 excited in its first absorption band are presented. In the three-dimensional wave packet calculations, we used ab initio potential energy surfaces for both the 3B1 ground state and the first excited 3A1 state, and an ab initio transition dipole moment function. The resonance Raman emission of CH2 is similar to that of Ã state H2O in that it shows a strong progression in the symmetric stretch vibration. It differs from that of Ã state H2O in that it also shows a fairly strong progression in the bending mode, which results from the much larger anisotropy of the excited state potential of CH2 in the exit channel. Because the stretching vibrational wave functions of CH2 are much more normal modelike than those of H2O the ratio of the intensity of emission into the (vs=2, vb=0, va=0 state to the intensity of emission into the (vs=0, vb=0, va=2) state is larger for CH2 by more than an order of magnitude. The measurement of resonance Raman emission spectra of CH2 would yield the energies of the symmetric stretch fundamental and its overtones, which are unknown at present, and present the first empirical evidence concerning the first excited triplet state.
Correlated dipole oscillator sum rules
Authors: Packer Martin J. , Sauer Stephan P. A. , Oddershede Jens
Published: July 2002

The second order polarization propagator approximation (SOPPA) has been applied to the calculation of the dynamic dipole polarizability, α(ω), of the molecules N2, C2H2, CO, and HCN. The dipole oscillator sum rules S(−4) and S(−6), which are the leading contributions to the Cauchy expansion of the polarizability, have also been evaluated at SOPPA level, using a polynomial fit to this expansion. These are the first reported values of the higher sum rules obtained using the SOPPA approach, and we observe excellent agreement with experiment. We also discuss the applicability of scaling or additivity expressions for obtaining the correlated polarizability from static correlated and dynamic uncorrelated quantities.
Segmental dynamics and relaxation of n-octane at solid–liquid interfaces
Authors: Gupta S. , Koopman D. C. , Westermann-Clark G. B. , Bitsanis I. A.
Published: July 2002

In this paper we present our results from a molecular dynamics study of n-octane liquids confined between planar bcc solid surfaces. The systems studied were wide enough to develop a bulklike region throughout the middle portion of the film and two well-separated interfaces. Our work focused on segmental dynamics and relaxation of ‘‘adsorbed’’ octane molecules. In particular, we investigated the role of architectural and dynamical features peculiar to short chain molecules (almost fixed bend angles and restricted torsional rotations) on the dynamics of ‘‘adsorbed’’ chains. We found that the relaxation of octane molecules exhibits the same qualitative trends as those observed in molecular simulations of generic ‘‘bead-spring’’ oligomer films. The most important effect is the dramatic slow down of rotational motions (up to a factor of 1000) for chains adsorbed on strongly physisorbing surfaces (adhesion energy per segment of 1–2 kT). Despite the qualitative similarities with bead-spring chains, the dynamics of realistic short hydrocarbon chains are affected much more strongly by the interfacial environment than their bead-spring counterparts. These stronger effects originate largely from the suppression of torsional angle transitions inside the extremely dense first layer (in cases of strong physisorption). The frequency of torsional transitions was found to be correlated directly with the amount of ‘‘free volume’’ available inside the crowded first layer.
The vibrational second overtones of HF dimer: A quartet
Authors: Chang Huan-C. , Klemperer William
Published: July 2002

We complete the study of the HF stretches (v1 and v2) of (HF)2 at N=v1+v2=3. A previous publication [J. Chem. Phys. 98, 9266 (1993)] reported the observations of the free-HF and hydrogen-bonded-HF stretches at (v1,v2)=(3,0) and (0,3). In this paper, second overtone (ΔN=3←0) spectra of the vibrations mixed between the two HF subunits are presented. Spectroscopic constants of the K subbands and tunneling states (A+ and B+) of the two mixed modes (2,1) and (1,2) are determined from their lifetime-broadened but rotationally resolved manifolds. For the (2,1) mode, we observe only a parallel band, K=0←0, and obtain band origins ν0=11 552.897 cm−1 (A+), 11 552.509 cm−1 (B+), rotational constants B̄=0.220 86 cm−1 (A+), 0.220 94 cm−1 (B+). For the (1,2) mode, a perpendicular band, K=1←0, is observed at ν0=11 536.95 cm−1 (A+), 11 536.93 cm−1 (B+) with B̄=0.222 cm−1 for both A+ and B+ states. The hydrogen interconversion tunneling splittings are determined to be 0.387 and 0.02 cm−1 for the K=0 levels of (2,1) and the K=1 levels of (1,2), respectively, demonstrating a strong dependence on K rotation and the importance of transition-dipole coupling in the tunneling process. Based on our present and previous results, we provide an overview of all the four components of the quartet by comparing five unique characteristics: vibrational symmetry, band origin, relative transition strength, hydrogen interconversion tunneling, and vibrational predissociation. Systematic comparison is also made against ab initio calculations of Jensen, Bunker, Karpfen, Kofranek, and Lischka [J. Chem. Phys. 93, 6266 (1990)]. A brief analysis suggests that the pure overtone modes can be described sufficiently by a local mode picture, whereas the mixed modes have strong normal mode characters. It is also concluded that the ab initio calculations do not reproduce the observations correctly and more adequate representation of the high vibrationally excited states of the HF dimer is required.
The short-time dynamics of solvation
Authors: Stratt Richard M. , Cho Minhaeng
Published: July 2002

At long enough times, the idiosyncratic motions of individual solvent molecules have long since ceased to matter to the process of solvation; the fact that a real solvent is not a featureless continuum just has no bearing on the dynamics. However, at short times, typically times well under a picosecond, the situation is quite different. We show here that at least within the realm of classical mechanics, one can indeed talk about how specific molecular motions contribute to short-time solvation. Precisely how one should think about these motions depends on just how short a time interval one is considering. At the very shortest times, we use the fact that it is possible to express solvation time correlation functions rigorously as power series in time to confirm that the onset of solvation is unequivocally a matter of inertial (free-streaming) motion of individual solvent molecules. We allow for somewhat longer, but still short, time intervals by writing these same correlation functions in terms of the instanteous normal modes of the solvent. The instantaneous-normal-mode expressions allow us to decompose the solvent dynamics into separate, well-defined collective motions, each with its own characteristic abilities to foster solvation. As distinctive as they appear, these two complimentary short-time views are, in fact, equally correct in the inertial regime, a point we establish by proving that two are simply different mathematical representations of the same underlying behavior.
Electron binding energies of linear C3, C5, C7, and C9 clusters
Authors: Ortiz J. V. , Zakrzewski V. G.
Published: July 2002

Electron propagator calculations are performed on the vertical ionization energies and electron affinities of linear C3, C5, C7, and C9 clusters with a variety of correlation approximations and basis sets. Estimates of adiabatic electron affinities are made as well, and are in excellent agreement with experiment. A bound, excited state of C−9 has been found. Correlation effects are large for the electron affinity calculations and for many vertical ionization energies. The Koopmans description of final states is valid for the Feynman–Dyson amplitudes, but for many cationic states there is considerable shakeup character.
A note on the temperature dependence of dielectric strength in supercooled liquids
Authors: Murthy S. S. N.
Published: July 2002

The dielectric strength (Δε=ε0−ε∞) of dipolar liquid is often found to follow a simple relation of the form: Δε=−A+B/T. In the present communication, the temperature dependence of Δε in a large number of van der-Waals liquids has been investigated experimentally throughout the liquid range (from boiling temperature down to the glass transition temperature). Deviation from the above relation has been noticed in the supercooled state, only in the case of liquids which have a significant intramolecular contribution to polarization in true liquid state. The deviation is explained as due to a crossover to a mechanism which by necessity is predominantly, intermolecular in nature in the supercooled state. The present study indicates that this crossover is a result of increase in the liquid viscosity and is not due to any structural change.
Rotational analysis of the SiD2 Ã 1B1–X̃ 1A1 transition observed in a jet
Authors: Fukushima Masaru , Obi Kinichi
Published: July 2002

The SiD2 radical was produced by ArF laser photolysis of C6H5SiD3 in a free-jet expansion, and the laser-induced fluorescence (LIF) excitation spectrum of the Ã 1B1–X̃ 1A1 transition of SiD2 was measured. The LIF excitation spectra of the five vibronic bands, (0,v2′,0)–(0,v2″,0), v2′–v2″= 0–0, 1–0, 2–0, 1–1, and 2–2, were obtained using a narrow-band dye laser with an intracavity étalon, the resolution of which attained to ∼0.03 cm−1. The rotational structures of the vibronic bands were well analyzed by a Hamiltonian including fourth-order terms, and the molecular constants were determined for the vibronic levels, v2=0, 1, and 2, of the Ã 1B1 and X̃ 1A1 states. By comparing the observed rotational line intensities with simulated ones, we found two kinds of intensity anomalies depending on the rotational quantum numbers J and Ka. We conclude that both the anomalies are caused by a predissociation process to the dissociation continuum, Si(3P)+D2, which was proposed in our previous paper [J. Chem Phys. 96, 44 (1992)]. The Ka dependent anomaly was explained by the interaction terms in the Fermi Golden Rule expression for the predissociation process, and the J dependence was interpreted by the final-state density.