Volume 117, Issue 15, 15 October 2002
 COMMUNICATIONS


Nuclear magnetic resonance molecular photography
View Description Hide DescriptionA procedure is described for storing a twodimensional (2D) pattern consisting of in a spin state of a molecular system and then retrieving the stored information as a stack of nuclear magnetic resonancespectra. The system used is a nematic liquid crystal, the protons of which act as spin clusters with strong intramolecular interactions. The technique used is a programmable multifrequency irradiation with low amplitude. When it is applied to the liquid crystal, a large number of coherent longlived response signals can be excited, resulting in a spectrum showing many sharp peaks with controllable frequencies and amplitudes. The spectral resolution is enhanced by using a second weak pulse with a 90° phase shift, so that the 1024 bits of information can be retrieved as a set of wellresolved pseudo2D spectra reproducing the input pattern.

Accidental conical intersections of three states of the same symmetry. I. Location and relevance
View Description Hide DescriptionAn efficient algorithm for locating conical intersections of three states of the same symmetry is presented. The algorithm, which derives its efficiency from the use of analytic gradient techniques, is used to locate a three state intersection for the excited Rydberg states of the ethyl radical. The existence of a seam of three state conical intersections in a Rydbergmanifold is expected to be a general occurrence.
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 ARTICLES

 Theoretical Methods and Algorithms

On the acceptance probability of replicaexchange Monte Carlo trials
View Description Hide DescriptionAn analysis is presented of the average probability of accepting an exchange trial in the paralleltempering Monte Carlo molecular simulation method. Arguments are given that this quantity should be related to the entropy difference between the phases, and results from simulations of a simple LennardJones system are presented to support this argument qualitatively. Another analysis based on the energy distributions of a replica pair is presented, and an exact expression for the trialmove acceptance probability in terms of the overlap of these distributions is derived. A more detailed expression is presented using an approximation of constant heat capacity, and an asymptotic form for this result, good for large system sizes, is reported. The detailed analyses are in quantitative agreement with the simulation data. It is further shown that treatment of the energy distributions as Gaussians is an inappropriate way to analyze the acceptance probability.

Local correlation mechanisms in ionic compounds: Comparison with xray scattering experiments
View Description Hide DescriptionElectron correlationeffects are often invoked as possible causes of differences between experimental and Hartree–Fock Compton profiles. The shape as well as the magnitude of these differences can be very different, depending on materials. In order to illustrate this, we performed post Hartree–Fock calculations on small LiH and MgO ionic clusters. The subsequent correlation corrections significantly improve Compton profiles and structure factors versus their experimentally determined counterparts. The opposite trends observed at small momenta for experimental deviations to Hartree–Fock Compton profiles of LiH and MgO are then qualitatively explained through an empirical model for a confined twoelectron anion. This model further suggests that the confinement of the embedded anion favors the angular correlation mechanism.

New ideas for using contracted basis functions with a Lanczos eigensolver for computing vibrational spectra of molecules with four or more atoms
View Description Hide DescriptionWe propose new methods for using contracted basis functions in conjunction with the Lanczos algorithm to calculate vibrational (or rovibrational) spectra. As basis functions we use products of eigenfunctions of reduceddimension Hamiltonians obtained by freezing coordinates at equilibrium. The basis functions represent the desired wave functions well, yet are simple enough that matrixvector products may be evaluated efficiently. The methods we suggest obviate the need to transform from the contracted to an original product basis each time a matrixvector product is evaluated. For HOOH the most efficient of the methods we present is about an order of magnitude faster than a product basis Lanczos calculation.

Current density in exchangecorrelation functionals: Application to atomic states
View Description Hide DescriptionAn old and yet unsolved problem in densityfunctional theory is the strong dependence of degenerate openshell atomic energies on the occupancy of the atomic orbitals. This arises from the fact that degenerate atomic orbitals of different do not have equivalent densities. Approximate density functionals therefore give energies depending strongly on which orbitals are occupied. This problem is solved in the present work by incorporating current density into the calculations using a currentdensity dependent functional previously published by the author.

Transition moments and excitedstate firstorder properties in the coupledcluster model CC2 using the resolutionoftheidentity approximation
View Description Hide DescriptionAn implementation of transition moments and excitedstate firstorder properties is reported for the approximate coupledcluster singlesanddoubles model (CC2) using the resolution of the identity (RI) approximation. In parallel to the previously reported code for the ground and excitedstate amplitude equations, we utilize a partitioned form of the CC2 equations and thus eliminate the need to store any intermediates. This opens the perspective for applications on molecules with 30 and more atoms. The accuracy of the RI approximation is tested for a set of 29 molecules for the basis sets in connection with the recently optimized auxiliary basis sets. These auxiliary basis sets are found to be sufficient even for the description of diffuse states. The RI error is compared to the usual basis set error and is demonstrated to be insignificant.

Optimal methods for calculation of the amount of intermolecular electron transfer
View Description Hide DescriptionIn order to determine the optimal methodology for evaluation of the magnitude of intermolecular charge transfer, several methods have been examined: Mulliken population, natural population analysis, atoms in molecules (AIM) as well as charges from electrostatic potentials using a gridbased method (Chelp and Chelpg) procedures using a series of correlation consistent ccpVXZ (X=D, T, Q) basis sets within LCAO MO SCF, MP2, DFT, and coupled cluster theory levels. In contrast to previous nonconclusive comparative studies, the present calculations reveal close matching of the recently available experimental data for six Lewis acidbase adducts with theoretical values derived from the Chelpg approach, whereas for the remaining methods relative errors are almost doubled. On the other hand, AIM and Chelpg results display the best linear correlation coefficients with the experimental data. Since reasonably accurate Chelpg results could be already obtained with SCF or DFT B3LYP methods using ccpVDZ, such an approach opens the way to study intermolecular charge transfer in larger molecular systems. Preliminary results obtained within ccpVDZ basis set and B3LYP functional for pyridine complex do not exceed relative error limits observed for other smaller complexes. Analysis of corresponding interaction energy components calculated consistently in the dimer basis set indicates significant role of electrostatic, exchange and delocalization contributions, with rather negligible correlation term. In contrast to previous findings, the experimentally observed amount of transferred charge seems not to correlate with any interaction energy term.

Approximate simulation of coupled fast and slow reactions for stochastic chemical kinetics
View Description Hide DescriptionExact methods are available for the simulation of isothermal, wellmixed stochastic chemical kinetics. As increasingly complex physical systems are modeled, however, these methods become difficult to solve because the computational burden scales with the number of reaction events. This paper addresses one aspect of this problem: the case in which reacting species fluctuate by different orders of magnitude. By partitioning the system into subsets of “fast” and “slow” reactions, it is possible to bound the computational load by approximating “fast” reactions either deterministically or as Langevin equations. This paper provides a theoretical background for such approximations and outlines strategies for computing these approximations. Two motivating examples drawn from the fields of particle technology and biotechnology illustrate the accuracy and computational efficiency of these approximations.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Photodissociation spectroscopy and dynamics of formaldehyde
View Description Hide DescriptionWe have carried out photodissociationspectroscopy studies of the bimolecular complex in the visible and nearuv regions. The work is supported by electronic structure calculations of the ground and lowlying excited states of the complex. formaldehyde is bound in a geometry with a theoretical bond energy of The complex shows absorption bands that correlate with based and formaldehydebased radiative transitions. The lowestenergy band is assigned as to an excited state of mixed and orbital character. The band exhibits complex vibrational structure with considerable excitation of the outofplane wag and C=O stretch modes; the vibrational frequencies are shifted dramatically from their values in the ground state, showing the effect of a significant weakening of the C=O bond and outofplane distortion of the complex. Excitation in the based band shows predominantly lowfrequency vibrational motions assigned to the intermolecular inplane wag and MgO stretch modes. Birge–Sponer analysis gives the Mg–O bond energy in the ground state as Partially resolved rotational substructure clearly demonstrates a change in geometry from a linear or near linear ground state to a bent excited state [θ(MgOC)=139°±3°]. Spectroscopicrotational constants are in very good agreement with ab initio predictions for this band. The based band also exhibits pronounced vibrational structure including strong Mg–O and C=O stretch signals, consistent with formation of a partial Mg–O σ bond in this state. is the major dissociation product through the uv–visible region. However, in the and absorption bands, we also observe a substantial branching to the reactive dissociation product The reactive branching ratio increases with photon energy through the absorption bands, reaching a reactive quantum yield of ∼ in the band. Our results suggest that there is no significant activation barrier to reaction above the endothermicity.

The structures of small gold cluster anions as determined by a combination of ion mobility measurements and density functional calculations
View Description Hide DescriptionA combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobilitymeasurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initiomolecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies (VDEs) from the literature, we assign structures for the small ions and locate the transition from planar to threedimensional structures. While a unique assignment based on the observed VDEs alone is generally not possible, the collision cross sections provide a direct and rather sensitive measure of the cluster structure. In contrast to what was expected from other metal clusters and previous theoretical studies, the structural transition occurs at an unusually large cluster size of twelve atoms.

Simulation of pump–probe spectroscopy of a highlycharged diatomic molecule: Role of intermediate charged states and electronic and vibrational excitation in the multiple ionization of and strongfield spectroscopy of
View Description Hide DescriptionThis paper is a theoretical study of the effect of a pulsed ir laser on a neutral molecule, leading to the creation of highlycharged molecular ions. We also develop a new quantummechanical model for the ionization of diatomic molecules in the tunneling regime. We consider the effects of the pump pulse duration and also the wavelength of the probe laser on the trication which is metastable, and consider how an experiment could be performed which would lead to the observation of its vibrational spectrum. The treatment considers nuclear wave packet dynamics which begin with vertical ionization from neutral to includes dynamics arising from the intermediate charged state and electronic excitation and dissociation from the trication The dynamical simulations of a pump–probe experiment show modulated signals which can be Fouriertransformed to yield vibrational spectra. The quality of the modulated signal changes dramatically at an intermediate (relative to the vibrational period) pump pulse duration (at ca. 50 fs). Analysis of this effect shows how to maximize the probability of observing a simple vibrational spectrum for a highly charged diatomic created in a laser field.

Discovery of a new class of stable gasphase dianions: Mixed oxygen–carbon cluster
View Description Hide DescriptionSmall doubly charged negative cluster ions were studied both by experimental and by theoretical means. In the experiments these dianions (with were produced by sputtering of a graphite specimen with a 14.5 keV ion beam at an elevated oxygen partial pressure in the vicinity of the sample’s surface. The dianions and the corresponding singly charged ions as well as homonuclear carbon dianions were detected in a doublefocusing mass spectrometer. The yields of the doubly and singly charged mixed oxygencarbon ions increase with the ratio of the arrival rate to the flux density. The abundance distribution of exhibits distinct even–odd alternations with the number of C atoms in the molecule. The flight time through the mass spectrometer of establishes a lower limit with respect to the intrinsic lifetimes of the doubly charged ions. The theoretical studies investigated dianions with the geometries of these species were optimized and the electronic stability was examined by the calculation of the electron detachment energies. In agreement with the low yield observed experimentally, is found to be weakly stable, whereas and are electronically stable gasphase ions. In particular the latter is characterized by a large number of stable isomers.

Electronic and geometric structures of and Initial growth mechanisms of late and early transitionmetal carbide clusters
View Description Hide DescriptionPhotoelectron spectra of and were measured in the energy range below 3 eV. Analyses of these spectra by the densityfunctional theory deduced their electronic states and geometricstructures. The growth mechanisms of the transitionmetalcarbide clusters were discussed on the basis of the structural models obtained. The geometricstructures of exhibit a tendency that carbon atoms aggregate to form a substructure. In contrast, consists of building blocks, which prelude the formation of a vanadiumcarbide network. These features illustrate the differences in the carbideformation processes of the late and the early transition metals, that is, only the latter forms large metalcarbide networks such as metallocarbohedrens and metalcarbidecompounds.

The van der Waals potential energy surfaces and structures of He–ICl and Ne–ICl clusters
View Description Hide DescriptionThe potential energy surfaces of the ground electronic state of rare gas interhalogen van der Waals molecules, Rg–ICl (Rg=He, Ne), are calculated at CCSD(T) (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) level of theory. Calculations have been performed with specific augmented correlation consistent basis sets for the noble atom (Rg), supplemented with an additional set of bond functions. For iodine atom a correlation consistent triple zeta valence basis set in conjunction with largecore Stuttgart–Dresden–Bonn relativistic pseudopotential has been employed. The CCSD(T) results predict the existence of three minima on the Rg–ICl potential energy surfaces at collinear (Rg–ICl), antilinear (Rg–ClI), and near Tshaped configurations, with the collinear structure to be the lowest one. Bound states calculated from the intermolecular potential surfaces show that zeroorder vibrational corrections do not alter the stability of the three structures. Equilibrium intermolecular distances, binding energies, and isomerization barriers are evaluated using the CCSD(T) potentials and compared with previous theoretical and/or experimental results.

Rotary resonance recoupling for halfinteger quadrupolar nuclei in solidstate nuclear magnetic resonance spectroscopy
View Description Hide DescriptionInvestigations were made of rotary resonance recouplings of chemical shiftanisotropy (CSA), heteronuclear dipolar (HTD), and homonuclear dipolar (HMD) couplings involving halfinteger quadrupolar nuclei under magicangle sample spinning condition. Under rotary resonance conditions provided by a low amplitude rf field and a high spinning speed, the spectrum of the central transition coherence of halfinteger quadrupolar nuclei shows recouplings of CSA, HTD, and HMD interactions that depend on the ratio of the rf field to the spinning speed. These new properties can be used to extract electronic and structural information about the sample that are otherwise difficult to extract in the presence of a dominant quadrupolar interaction. An average Hamiltonian theory is used to explain the recoupling properties of various interactions. Experimental implementations of the are demonstrated on model compounds with spin3/2 systems.

Photodissociation of ethylbenzene and npropylbenzene in a molecular beam
View Description Hide DescriptionThe photodissociation of jetcooled ethylbenzene and npropylbenzene at both 193 and 248 nm was studied using vacuum ultraviolet photoionization/multimass ion imaging techniques. The photofragment translational energy distributions from both the molecules obtained at 193 nm show that the probability of portioning energy to product translational energy decreases monotonically with increasing translational energy. They indicate that the dissociation occurs from the ground electronic state. However, the photofragment translational energy distributions from both molecules obtained at 248 nm contain a fast and a slow component. 75% of ethylbenzene and 80% of npropylbenzene following the 248 nm photoexcitation dissociate from electronic excited state, resulting in the fast component. The remaining 25% of ethylbenzene and 20% of npropylbenzene dissociate through the ground electronic state, giving rise to the slow component. A comparison with an ab initio calculation suggests that the dissociation from the first triplet state corresponds to the fast component in translational energy distribution.

Millimeterwave spectroscopy of the internalrotation band of the He–HCN complex and the intermolecular potential energy surface
View Description Hide DescriptionMillimeterwave absorption spectroscopy combined with a pulsedjet expansion technique was applied to measure the internalrotation band of He–HCN in the frequency region of 95–125 GHz. In total 13 rovibrational lines, split into nitrogen nuclear hyperfine structure, were observed for the fundamental internalrotation band, The observed transition frequencies were analyzed including their hyperfine splitting to yield an intermolecular potential energy surface, as improved from the one given by a coupledcluster single double (triple) ab initio calculation. The surface obtained has a global minimum in the linear configuration with a well depth of 30.2 and a saddle point located in the antilinear configuration which is higher by 8.91 in energy than the global minimum. The distance between the He atom and the center of mass of HCN along the minimum energy path shows a strong angular dependence; is 4.169 and 4.040 Å in the linear and antilinear forms, respectively, while it is 3.528 Å in a Tshaped configuration. In the first excited internalrotation state levels with l less than 4 are bound but not for the one with l = 5, according to the energy level diagram calculated from the present potential energy surface, where l denotes the quantum number for the endoverend rotation of the complex. The energy level diagram is consistent with the millimeterwave observation, in which the transitions with were observed but not for those with l equal to or greater than 5. The band origin of the internalrotation band, 98.70 GHz, as defined to be the same as the frequency of the transition, is larger by 11% than the rotational transition frequency of the free HCN molecule.

Catalytic oxidation of hydrogen on free platinum clusters
View Description Hide DescriptionThe adsorption of hydrogen (deuterium) and oxygen on neutral platinum clusters has been investigated in a cluster beam experiment. The beam passes through two lowpressure reaction cells and the clusters, with and without adsorbed molecules, are detected by laser ionization and mass spectrometry. Both and adsorb efficiently on the platinum clusters with only moderate variations with cluster size in the investigated range, i.e., between 7 and 30 atoms. The coadsorption of and results in the formation and desorption of detected as a decreasing number of adsorbed oxygen atoms with an increasing number of collisions with molecules. The waterformation reaction proceeds efficiently on all investigated clusters with more than seven atoms.

Prediction of absolute rate coefficients and product branching ratios for the reaction system
View Description Hide DescriptionComplex chemical reactions in the gas phase can be decomposed into a network of elementary (e.g., unimolecular and bimolecular) steps which may involve multiple reactant channels, multiple intermediates, and multiple products. The modeling of such reactions involves describing the molecular species and their transformation by reaction at a detailed level. Here we focus on a detailed modeling of the reaction, for which molecular beamexperiments and theoretical calculations have previously been performed. In our previous calculations, product branching ratios for a nonrotating isomerizing unimolecular system were predicted. We extend the previous calculations to predict absolute unimolecular rate coefficients and branching ratios using microcanonical variational transition state theory (μVTST) with full energy and angular momentum resolution. Our calculation of the initial capture rate is facilitated by systematic ab initiopotential energy surface calculations that describe the interaction potential between carbon and allene as a function of the angle of attack. Furthermore, the chemical kinetic scheme is enhanced to explicitly treat the entrance channels in terms of a predicted overall input flux and also to allow for the possibility of redissociation via the entrance channels. Thus, the computation of total bimolecular reaction rates and partial capture rates is now possible.