Volume 116, Issue 18, 08 May 2002
 COMMUNICATIONS


Photodissociation of ethylbenzene at 248 nm
View Description Hide DescriptionPhotodissociation of jetcooled ethylbenzene at 248 nm was studied using VUV photoionization/multimass ion imaging techniques. The photofragment translational energy distribution obtained at 248 nm showed that after the excitation 75% of the ethylbenzene molecules dissociate from electronic excited state, and the rest 25% of the molecules dissociate through a hot molecule mechanism. This is the first experimental evidence which proves that the dissociation of alkylsubstituted benzenes can occur not only from hot molecule mechanism in this UV region.

Ordering stripe structures of nanoscale rods in diblock copolymer scaffolds
View Description Hide DescriptionWe report a simulation on the formation of ordered stripe structures of nanoscale rods driven by symmetric diblock copolymer melts. Due to the preferential adsorption of one species of the diblock onto the mobile rods, the phase ordering process will couple with the movement of rods. We find that the selfassembly of rods on the copolymer scaffold produces the highly ordered nanowires of rods, and copolymer blends in turn form the welloriented lamellar structure. This is due to the interplay among the microphase separating dynamics in the diblock copolymer, the wetting interaction between rods and diblock copolymer, and the nematic ordering dynamics of rods. We examine the influence of the domain size, the wetting strength, and the rod number density on the formation of such a nanoscale structure. Additionally, we indicate that the orientation of the pattern can be well controlled by external fields acting on the rods. The results suggest that our model system may provide a novel and simple way to control and design the ordering nanowirestructure.
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 ARTICLES

 Theoretical Methods and Algorithms

Passivescalar diffusion in a fluid membrane
View Description Hide DescriptionA lipidbilayer membrane in the fluid phase when agitated by ambient fluids not only undergoes a shape fluctuation but also a twodimensional disordered flow. Thus, besides moving individually, lipid molecules in a twodimensional fluid element can move collectively in the lateral direction. This collective motion, which has been often overlooked in studying diffusion in a fluid membrane, can shift a lipid molecule as well as its Brownian motion. Calculating the selfdiffusion constant of a passive scalar—a particle convected neither affecting the flow nor making the Brownian motion—in a fluid membrane fluctuating around a plane in aqueous environments, we show that the collective motion cannot be neglected in explaining observed values of the selfdiffusion constant of a lipid molecule.

Accurate molecular integrals and energies using combined plane wave and Gaussian basis sets in molecular electronic structure theory
View Description Hide DescriptionThis paper introduces two developments for the application of plane wave basis sets for accurate molecular calculations. (1) An analytical formula is introduced for the momentum space representation of a Coulomb operator truncated to a finite range. Using this operator, interactions between the molecule and its periodic replicas can be exactly eliminated. Examples demonstrating the accuracy of our scheme are given. Calculations using a goodquality plane wave basis yield variational total SCF energies which are lower than those obtained with the ccpvQZ basis for simple twoelectron systems. (2) A new mixedbasis augmented plane wave allelectron method, the plane wave core Gaussian method has been developed which expands the valence part of the molecular orbitals in plane waves, and the corelike part in nonoverlapping compact Gaussians. Analytic equations have been derived for the necessary mixed Gaussian/plane wave electron repulsion integrals. Using such augmented basis set, we were able to reproduce the Gaussianbasis Hartree energies of small molecules to within a few

Improving “difficult” reaction barriers with selfinteraction corrected density functional theory
View Description Hide DescriptionWe examined 11 difficult reactions with selfinteraction corrected density (SIC) functionaltheory. The data set includes dissociation of radicals into symmetric fragments radical hydrogen abstraction proton transfer halogen exchange and closedshell unimolecular dissociation of tetrasine Calculated selfinteraction energies cancel, almost identically, for the reaction energies so that SIC functionals do not lead to a systematic improvement in Selfinteraction correction increases for reaction transition structures, leading to higher calculated activation barriers The average absolute deviation in from ab initio and experimental barriers, is reduced from 14 kcal/mol for Vosko–Wilk–Nusair (VWN) or 12 kcal/mol for revised Perdew–Burke–Ernzerhof (revPBE) functionals to 5.4 (SICVWN) or 3.4 (SICrevPBE) kcal/mol. Reorganization of the electron density, due to removal of selfinteraction, appears to be important. When SIC is included as a perturbation, using selfconsistent densities of the parent functional, the average absolute deviations for the barriers increase to 7.5 or 5.3 kcal/mol. Gradientcorrected functionals (revPBE, BP86) reduce the magnitude of the total selfinteraction correction, by improving the description of the core orbitals. For the valence orbitals, both the magnitudes of the selfinteraction corrections, and their change between reagents and transition structures, are similar for VWN local density approximation, and generalized gradient approximationfunctionals. Reducing the magnitude of the selfinteraction energy for valence electrons thus appears to be a promising direction for the development of chemically accurate exchangecorrelation functionals.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Modeling of nonlinear vibrational relaxation of large molecules in shock waves with a nonlinear, temperaturevarying master equation
View Description Hide DescriptionWe model recent experiments on the vibrational relaxation of oxirane in a shock tube. A master equation is developed which includes selfcollisions of the oxirane, leading to a nonlinear master equation. This master equation is also applied to a more limited study of vibrational relaxation for cyclopropane in a shock tube. The time variation of the temperature dependence of the bath is also included in the calculations. Good agreement between the modeling and experiments are obtained through a fit to the energy transfer parameters. These fits demonstrate that selfcollisions are dominant in promoting the relaxation even for mixtures of Kr and oxirane where the oxirane is 2% and 4% dilute. This dominance comes from two sources: (1) much larger energy transfer per collision for oxirane–oxirane collisions and (2) resonant energy transfereffects. For cyclopropane, some of the good fits show smaller energy transfer characteristics for selfcollisions than buffer gas collisions. Even in these cases selfcollisions are an important part of the energy transfer process through resonant energy transfereffects.

Dynamics of a nonlinear master equation: Lowdimensional manifolds and the nature of vibrational relaxation
View Description Hide DescriptionThe dynamics of nonlinear master equations describing vibrational relaxation in shockheated molecules are studied. The nonlinearity results principally from inclusion of selfcollisions. The master equations were derived in a previous paper by fitting experimental data and besides being nonlinear they vary according to changes in the bath temperature. It is demonstrated that, except for brief transients, the dynamics lie on onedimensional, nonlinear manifolds, including the full time of experimental observation. The onedimensional nature of the dynamics allows for an in depth study of vibrational relaxation. It is shown that vibrational distributions cannot be characterized accurately by a vibrational temperature until they are close to equilibrium and that onedimensional rate laws accurately describe the dynamics on the onedimensional manifold. The latter characteristic is important, because it allows results generated from master equations which include selfcollisions to be easily incorporated into kinetic modeling.

Reaction dynamics of at low energies: Resonant tunneling mechanism
View Description Hide DescriptionThe complete stateresolved differential cross section investigated in a crossedbeam scattering study, is presented for the title reaction at six initial collision energies which are below or near the barrier energy. At low energies, all reactive flux is gated through a trapped resonance state via a tunneling process. Hence, it serves as a benchmark system for better understanding the reactive resonance phenomenon. In addition to highlighting various resonance fingerprints of experimental observable, the concept of resonant tunnelingreaction mechanism is elucidated. Particular emphasis is placed on its distinction from the more conventional transitionstate reaction mechanism.

Phosphorous trapped within buckminsterfullerene
View Description Hide DescriptionUnder normal circumstances, when covalent molecules form, electrons are exchanged between atoms to form bonds. However, experiment and theoretical computations reveal exactly the opposite effect for the formation of group V elements nitrogen and phosphorous encapsulated within a buckminsterfullerene molecule. The carbon cage remains intact upon encapsulation of the atom, whereas the electronic charge cloud of the N or P atom contracts. We have studied the chemical, spin, and thermodynamic properties of endohedral phosphorous and have compared our results with earlier findings for From a combined experimental and theoretical vantage, we are able to elucidate a model for the interaction between the trapped group V atom and the fullerene cage. A picture emerges for the electronic structure of these complexes, whereby an atom is trapped within a fullerene, and interacts weakly with the molecular orbitals of the cage.

An experimental and ab initio investigation of the lowfrequency vibrations of coumaran
View Description Hide DescriptionCoumaran (2,3dihydrobenzofuran) has been studied using a combination of resonantly enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) studies, supported by ab initio molecular orbital calculations, in order to characterize the low wave number vibrational structure of the neutral excited and ionic ground states. These studies focus primarily on the modifying effects of electronic excitation and ionization on the balance of forces driving the and equilibrium structures toward or away from planarity. The results suggest that coumaran retains a puckered structure in the state, having a barrier significantly smaller than that in the electronic ground state, but is apparently pseudoplanar or weakly puckered in the cation ground state. In each state the drive towards or away from planarity results from a competition between decreasing bond order in the aromatic system which increases torsional interactions thereby favoring a higher barrier and an increase in bond order in the furan ring which has the opposite effect. The lack of symmetry in coumaran lifts any restrictions on which outofplane modes can couple, resulting in a rich combination band structure in REMPI and ZEKE spectra, principally involving the ring twisting (44) and the ring pucker (45) vibrational modes. The butterfly mode (43) on the other hand shows surprisingly little activity.

The effect of weak interactions on the ring puckering potential in the coumaran–argon van der Waals complex: Experimental and ab initio analysis of the intermolecular and lowfrequency intramolecular vibrations
View Description Hide DescriptionThe coumaran–argon van der Waals (vdW) complex has been investigated using a combination of REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. Coumaran (2,3dihydrobenzofuran) has a puckered, nonplanar equilibrium structure in the electronic ground state which allows for the formation of two energetically nonequivalent πbound geometrical conformations. The experimental observation of bands attributable to two isomers in the REMPI spectrum is consistent with a significant barrier to planarity existing in both and states. The two isomers are related through the ringpuckering motion but the interaction of the argon atom with the monomer results in an induced asymmetry in the potential which transforms the ringpuckering tunnelling motion in the isolated monomer to a localized vibration near nonequivalent local minima. Both REMPI and ZEKE spectra show rich vibrational structure, characteristic of excitation of the long axis van der Waals bending mode and the stretching mode, as well as combination bands involving the vdW modes with lowfrequency out of plane intramolecular vibrations. The vibrational structure is consistent with a shift in the position of the argon atom along the long axis upon excitation and subsequent ionization, as well as a modest reduction in the van der Waals bond length.

Molecular dynamics simulations of water clusters with ions at atmospheric conditions
View Description Hide DescriptionClustering of water molecules on charged particles has been studied using the method of molecular dynamics simulations. A selected set of model metal and halogen ions, carrying both positive and negative charges, is chosen as nucleation centers for water molecules. The influence of the ion charge, its size, and shortrange interactions on the local structure, and kinetic characteristics are investigated for the ioncentered clusters of 20 and 30 water molecules at 200 and 300 K, respectively. It is shown, based on radial densities, energy, polarization profiles, and orientational distribution functions, that the local water structure in the clusters becomes perturbed to a larger degree around negative ions compared to ions carrying a corresponding positive charge. The electric field of an anion is more effectively screened by the first hydration shell, resulting in a weaker dependence of the relaxation processes on the ion field in the second hydration shell. The dependence of the work of cluster formation on the ion radius is more pronounced in the case of negative ions. The dependence of the properties on the cluster size are investigated. It was found that for the water–alkali ion system potentials used, the dependence of the work of cluster formation on the number of water molecules has a minimum at about The obtained work of cluster formation for the anions was found to be consistently less than that for the cations. Unfortunately, this work of formation does not alone provide an answer to the still unsolved problem of sign preference connected to water condensation on charged particles in atmospheric conditions.

Highly excited states of gerade symmetry in molecular nitrogen
View Description Hide DescriptionHighly excited states of gerade symmetry in molecular nitrogen have been investigated in a resonanceenhanced XUV+Vis (extreme ultraviolet+visible) transition scheme. Nineteen bands have been observed, of which only four involve known states and albeit in new systems. Three of the newly observed states have been assigned as and Level energies are determined with an accuracy of

Ab initio studies on the van der Waals complexes of polycyclic aromatic hydrocarbons. I. Benzene–naphthalene complex
View Description Hide DescriptionThe stable geometries and binding energies of the benzene–naphthalene complex were studied by the pointbypoint method using ab initio calculations at the MP2/631G^{*}(0.25) and MP2/631+G^{*} levels. Mediumsize basis sets were employed not only to save computational time but also to compensate for the tendency of the MP2 method to overestimate the electron correlation energy of aromatic clusters. The use of the 631G^{*}(0.25) and 631+G^{*}basis sets in the test calculation for the benzene dimer yielded results very similar to those from the CCSD(T) calculation. As for the benzene–naphthalene complex, four stable geometries were found: one paralleldisplaced type and three Tshaped ones, with each type similar to the case of the benzene dimer. The global minimum was found to be the paralleldisplaced structure whose energy was −4.88 kcal/mol at the MP2/631G^{*}(0.25) level and −3.94 kcal/mol at the MP2/631+G^{*} level. These values are ∼1.9 times that of the benzene dimer on the same level of calculation. The energy of the most stable Tshaped structure was −4.17 and −3.47 kcal/mol at the MP2/631G^{*}(0.25) and MP2/631+G^{*} levels, respectively, which are ∼1.8 times that of the corresponding Tshaped structure of the benzene dimer.

Ab initio studies on the van der Waals complexes of polycyclic aromatic hydrocarbons. II. Naphthalene dimer and naphthalene–anthracene complex
View Description Hide DescriptionAb initio calculations were carried out for the naphthalene dimer and naphthalene–anthracene complex to determine their stable geometries and binding energies. Two mediumsize basis sets of and were employed at the MP2 level. Five local minima were found for the naphthalene dimer, three of which were paralleldisplaced type and the other two Tshaped type. The global minimum geometry was a paralleldisplaced structure of a twolayer graphitic type point group), not the crossed form Its energy calculated by the and basis sets was −7.62 and −6.36 kcal/mol, respectively. The naphthalene–anthracene complex showed four local minima, two of which were paralleldisplaced type and the other two Tshaped type. The global minimum was a twisted paralleldisplaced form in which the centers of both molecules lie on the same zaxis with their two long axes skewed at an angle of ≈40°. Its energy was −11.30 and −9.52 kcal/mol with the and basis sets, respectively. From these results a set of general rules for the stable geometry of the polycyclic aromatic hydrocarbon clusters were derived, which turned out to be the same as those previously deduced from other systems less directly relevant to polycyclic aromatic hydrocarbons: (1) a facetoface configuration is unstable, (2) the Tshaped structure is stable, (3) the paralleldisplaced structure is also stable. We also found some additional rules: (4) the energies of the Tshaped and paralleldisplaced structures are quite comparable when the molecules are small, but (5) the paralleldisplaced structure becomes more stable than the Tshaped one as the molecules become larger due to the nature of the π–π interaction. The interplanar distance of stable paralleldisplaced structures was about 3.3–3.4 Å, while the planetocenter distances of Tshaped structures was about 5.0–5.1 Å. We also discovered what we would call the integer rule for the binding energy of the polycyclic aromatic hydrocarbon clusters in that the binding energy varied linearly as the number of overlapping hexagons in the paralleldisplaced structures. The ratio of binding energies for the benzene dimer, benzene–naphthalene complex, naphthalene dimer, and naphthalene–anthracene complex were nearly 1:2:3:4.

The singlet–triplet spectroscopy of 1,3butadiene using cavity ringdown spectroscopy
View Description Hide DescriptionThe absorptionspectrum of gasphase 1,3butadiene has been investigated over the region from 20 500 to 23 000 cm^{−1} using cavity ringdown spectroscopy. Resolved vibrational structure and partially resolved rotational structure have been observed for the first time in the gas phase. The origin transition is located at 20 777 cm^{−1}, with a peak absorption cross section of Vibronic bands appear 249, 491, 1166, and 1617 cm^{−1} above the origin. This structure is observed on top of a rising background whose absolute magnitude and wavelength dependence is quantitatively accounted for as Rayleigh scattering. Using the recent calculations of Brink et al. [J. Phys. Chem. A 102, 6513 (1998)] as a guide, the bands 491, 1166, and 1617 cm^{−1} above the origin can be assigned as totally symmetric fundamentals, while the band 249 cm^{−1} above the origin is the first overtone of the symmetry torsion (calculated at 129.6 cm^{−1}) of a planar excited state. The rotational band contour of the origin transition shows several sharp bandheads that appear in doublets with a splitting of 2 cm^{−1}. Only part of this structure can be accounted for as a single vibronic band. The possible explanations for the remaining band heads and the observed Franck–Condon intensities are discussed in terms of the shape of the potential energy surface.

Anion solvation at the microscopic level: Photoelectron spectroscopy of the solvated anion clusters, where Kr, Xe, and
View Description Hide DescriptionThe negative ion photoelectron spectra of the gasphase, ionneutral complexes; and are reported herein, building on our previous photoelectron studies of and Anion solvation energetic and structural implications are explored as a function of cluster size in several of these and as a result of varying the nature of the solvent in others. Analysis of these spectra yields adiabatic electron affinities, total stabilization (solvation) energies, and stepwise stabilization (solvation) energies for each of the species studied. An examination of energetics as a function of cluster size reveals that its first solvation shell closes at with an icosahedral structure there strongly implied. This result is analogous to that previously found in our study of Inspection of stepwise stabilization energy size dependencies, however, suggests drastically different structures for and the former being “Y” shaped, and the latter being linear. While stepwise stabilization energies usually provide good estimates of ion–single solvent dissociation energies, in the cases of and it is possible to determine more precise values. A plot of these anion–solvent dissociation energies shows them to vary linearly with rare gas atom polarizability, confirming the dominance of an ioninduced dipole interaction in these complexes. Extrapolation of this trend permits the estimation of (rare gas atom) interaction energies for helium, neon, and radon, as well. The relative strengths of the molecular solvents, and EG are reflected in their stepwise stabilization energies and in the degree of broadening observed in their photoelectron spectra.

Unexpected simplicity in the dispersed fluorescence spectra of
View Description Hide DescriptionWe have recorded dispersed fluorescence (DF) spectra (18 cm^{−1} resolution) from the rotational level of six vibrational levels in the state of Improvements in our methods of recording and calibrating DF spectra, that have enhanced the quality of our data sets, are briefly discussed. More than 50 fractionated bright state patterns associated with approximately conserved polyad quantum numbers have been extracted from our DF data sets using a spectroscopic pattern recognition technique, extended cross correlation. These polyads extend to internal energies as high as 20 000 cm^{−1} above the zeropoint vibrational level. The polyad fractionation patterns observed at high energy are surprisingly simple relative to the corresponding patterns of Comparison between the DF spectra of and reveals slower intramolecular vibration redistribution in particularly with nonzero quanta of CC stretch excitation. More than 15 patterns were extracted above the energy at which isomerization is predicted to be energetically feasible (∼15 200 cm^{−1}) and the dynamical information encoded in these patterns is addressed. In particular, we have analyzed a subset of the polyads, the pure bending polyads, those with zero quanta of excitation in each of the stretch modes, The observed pure bending levels are reproduced to a rootmeansquare error of <1.5 cm^{−1} by two different effective Hamiltonian models: an 11 parameter normalmode and a 13 parameter localmode model.

Quantum theory of vector correlations in vibrationally mediated photodissociation
View Description Hide DescriptionThe quantum treatment of the photodissociation of aligned, vibrationally excited molecules prepared by polarized laser excitation is presented. A formal expression for the angledependent multipole moments of a photofragment is derived. As a specific example, the projection of the laboratory angular distribution (zeroorder moment) along a probe direction is considered, and Dopplershiftdependent fluxes for various arrangements of the polarization directions of the vibrational excitation and photolysis lasers are computed. The profiles depend upon the photodissociationdynamics only through the conventional recoil anisotropy parameter β, as in onephoton dissociation. More generally, the dynamical information obtainable from measurement of vector correlations in VMP is the same as that which can be extracted in a onephoton dissociation experiment. Nevertheless, measurement of vector correlations for vibrationally mediated photodissociation can provide new information on the dissociationdynamics since different regions of the excited potential energy surface(s) are accessed from those accessed in onephoton dissociation.
 Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

On the application of virtual Gibbs ensembles to the direct simulation of fluid–fluid and solid–fluid phase coexistence
View Description Hide DescriptionVirtual Gibbs ensembles (VGEs), a class of pseudoensemble Monte Carlo methods, are specialized in this work to simulate phase equilibrium for systems for which conventional direct methods (such as twobox Gibbs ensembles and onebox interfacial ensembles) are inadequate. It is shown that by removing the mass/volume balance constraints of conventional Gibbs ensembles, the resulting VGEs can be used to effectively simulate systems wherein (a) the number of molecules or the composition of one of the phases is to be kept constant and (b) the initial choices for the volume and number of molecules would preclude direct methods to convergence to a stable twophase state. Applications of VGEs are presented for the simulation of vapor–liquid, vapor–solid, and liquid–solid equilibrium in single component systems and multicomponent sorption equilibria of gases in polymer melts. VGE simulations of solid–fluid coexistence entail the combination of elements of both interfacial ensembles and Gibbs ensembles.