Volume 117, Issue 19, 15 November 2002
 ARTICLES

 Theoretical Methods and Algorithms

A new method of molecular dynamic computer simulation at constant temperature and pressure
View Description Hide DescriptionA new method of molecular dynamiccomputer simulation at constant temperature T and pressure P has been proposed. Unlike the earlier techniques of this sort, the new approach does not assume the volume of the crystal to be an independent variable but the one that can be estimated from the coordinates of the atoms in the crystal. The new method was carefully tested on a finite hcp crystal whose LennardJones potential of the twoatom interaction is known. The test proved the efficiency of the new method under different conditions including and

A quantum symmetry preserving semiclassical method
View Description Hide DescriptionSymmetry constraints are built into a semiclassical propagation scheme. It is then applied to treat collisions at 30 K, where quantum selection rules restrict the final rotational states of symmetric molecules to the even manifold. The cross sections for statetostate transitions are calculated for symmetric and nonsymmetric isotopic compositions of All bound and longlived quasibound (trapped behind the centrifugal barrier) states of are considered. This semiclassical method captures symmetry effects and shows satisfactory agreement with the quantum results.

Coupledcluster Raman intensities: Assessment and comparison with multiconfiguration and density functional methods
View Description Hide DescriptionAn extensive study of static and dynamic Raman intensities is presented for normal vibrations of small molecules obtained with different correlated quantum chemical methods: namely, coupledcluster, multiconfiguration selfconsistentfield, and density functional theories. While this is the first systematic study of coupledcluster (CC) Raman intensities considering also the dispersion effect for molecules with more than two atoms, another purpose of this study is the analysis of the accuracy of density functional Raman activities with respect to those from highly correlated ab initio methods in order to evaluate the validity of density functional theory for the calculation of Raman spectra for large molecules. The density functional intensities compare sufficiently well with those from ab initio methods. While the dynamic multiconfigurational intensities always compare well with the experimental values, they are usually smaller than those from density functional and coupledcluster theories. The Raman intensities obtained from staticcoupledcluster calculations are in better agreement with experiment than those from dynamic calculations, which should yield improved results as the dispersion effect is taken into account. Furthermore, Raman intensities obtained from the CC2 model are compared to those from CCSD calculations. It is found that the CC2 Raman activities deviate from the CCSD reference data. Particularly for the coupledcluster Raman intensities the widely used Sadlej basis set leads to results which can be significantly improved on by using larger basis sets.

Itô diffusions on hypersurfaces with application to the SchwarzP surface and nuclear magnetic resonance theory
View Description Hide DescriptionThis work presents a new Brownian dynamics simulation method of translational diffusion on curved surfaces. This new method introduce any implicit defined surface into the stochastic differential equation describing Brownian motion on that surface. The surface curvature will thus enter the force term (A) in the stochastic differential equation describing an Itô process. We apply the method calculating time correlation functions relevant in nuclear magnetic resonance(NMR) relaxation and translational diffusion studies of cubic phases of lyotropic systems. In particularly we study some bicontinuous cubic liquid crystalline phases which can be described as triply periodic minimal surfaces. The curvature dependent spin relaxation of the SchwarzP minimal surface is calculated. A comparison of relaxation is made with the more complex topology of the Neovius surface which is another minimal surface in the same space group, and with parallel displacement of the minimal surface which thus results in a nonminimal surface. The curvature dependent relaxation effects are determined by calculating the translational diffusion modulated timecorrelation function which determine the relaxation rates of a quadrupole nuclei residing in the water–lipid interface. The results demonstrates that spin relaxation data can provide quantitative information about microstructure of biocontinuous cubic phases and that it is sensitive to the topology of the surface and to parallel displacement of the model surface. Consequently, spin relaxation may be used as a complement to xray diffraction in order to discriminate between different microstructures. It is concluded that fast and accurate computer simulations experiments is needed to be able to interpret NMR relaxation experiments on curved surfaces.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Spectroscopy of the lowlying states of the group III–V diatomics, AlP, GaP, InP, and GaAs via anion photodetachment spectroscopy
View Description Hide DescriptionThe lowlying electronic states of AlP, GaP, InP, and GaAs have been probed using anion photoelectron spectroscopy and zero electronic kinetic energy spectroscopy. We observe transitions from the anion and lowlying states to the triplet and states) and singlet and states) manifolds of the neutral species. The spectra of the triplet manifolds are particularly complex, with overlapping spin–orbit and vibrational progressions. Spin–orbit splittings, term energies, and vibrational frequencies are reported and compared to previous electronic structure calculations on the anions and neutrals, as well as to those parameters determined previously for the isovalent homonuclear diatomics and

Photodissociation and ab initio studies of Electronic structure and photoinduced reaction
View Description Hide DescriptionPhotodissociationspectra of cluster ions are examined in the wavelength region of 240–1200 nm. From the comparison with the results of ab initio calculations for the structure and the excitation energies of these clusters, the observed absorption bands are assigned to the transitions derived from the transition of ion. The extensive redshift of the observed spectra is ascribed to the formation of a onecenter ionpair state. In the photolysis of and ions are produced via photoinduced charge transfer and intracluster reaction processes, respectively, in addition to the ion generated by the evaporation of ammonia molecules. For both the intracluster reaction and evaporation are dominant decay processes, while the evaporation is the sole photodissociation channel for larger clusters. The branching fractions of these processes are found to depend strongly on the solvation number n and also on the photolysis wavelength. The energetics and the dynamics of the dissociation processes are discussed in relation to the redox reaction of metal ions.

Symmetry properties of electronically excited states of nitroaromatic compounds
View Description Hide DescriptionWe report polarizationspectroscopy of oriented gas phase nitroaromatic compounds, specifically nitrobenzene and the three isomers of nitrotoluene. Orientation of supersonically cooled molecules was achieved in a uniform electric field. The dependence of the yield of NO on the polarization direction of the excitation laser was used to derive the direction of the transition dipole. The transition to the state centered at 250 nm demonstrated increased yield when the excitation laser was polarized perpendicular to the orientation field. The transition dipole was therefore characterized to be near perpendicular to the permanent dipole. The transition to showed an opposite behavior, indicating that the transition dipole for is primarily parallel to the permanent dipole. Quantitative analysis of the polarization dependence further revealed that in both cases, the transition dipole is not strictly parallel or perpendicular to the permanent dipole, and the tilt is about 20°–30°. This result demonstrates the sophisticated nature of electronically excited states of nitroaromatics. It is therefore not surprising that only two previous theoretical calculations agree with our results, while other calculations and experimental work in condensed phases do not.

Statetostate ArHBr photodissociation quantum dynamics
View Description Hide DescriptionWe report quantum dynamical calculations on the photodissociation process: or Jacobi coordinates associated with the product arrangement have been used as well as a pseudospectral wave packet propagation method. The radial singularity at the origin is either analytically lifted with a discrete Bessel transform approach, or treated within a sine transform strategy. The relative fraction of a bound ArBr product state is high (of the order of 50%) and decreases with energy. Product vibrational populations decrease with vibrational energies. Rotational distributions show oscillatory patterns characteristic of rotational rainbows, whereas angular distributions have maxima at rainbow angles.

Laserinduced dispersed vibration–rotation fluorescence of acetylene: Spectra of ortho and para forms and partial trapping of vibrational energy
View Description Hide DescriptionThe laserinduced dispersed vibration–rotation fluorescence method has been developed further when compared with a previous publication [Saarinen et al., J. Chem. Phys. 110, 1424 (1999)]. More than one order of magnitude better signaltonoise ratio has been achieved in the wavenumber region by taking advantage of directionality of the fluorescence signal. The improvement has been applied to overtone spectroscopy of normal acetylene where for high CH stretching excitations separate spectra of ortho and para forms are obtained containing basically just single CH stretching vibrational quantum transitions from the pumped antisymmetric vibrational and and closelying symmetric vibrational local mode and states. No nuclear spin conversion is observed in these spectra. Two new symmetric vibrational states and (50%)) have been observed and the precision of the spectroscopic parameters of previously published symmetric states has been improved by an order of magnitude. Unexpected fluorescence originating from the antisymmetric CH stretching fundamental state and some associated states of acetylene have also been observed. These spectra are characterized by both ortho and para forms in normal abundance and by unusual intensity patterns due to strong reabsorption of the fluorescence by ground state acetylene molecules in the sample cell. A simple collisional stepdown mechanism is proposed to account for the appearance of the fluorescence band system. The excess vibrational energy in the sample volume is partly trapped in the form of mode energy and it decays from the system by radiation.

Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. III. Comparison with Born–Oppenheimer dynamics
View Description Hide DescriptionIn a recently developed approach to ab initiomolecular dynamics (ADMP), we used an extended Lagrangian to propagate the density matrix in a basis of atom centered Gaussian functions. Results of trajectory calculations obtained by this method are compared with the Born–Oppenheimer approach (BO), in which the density is converged at each step rather than propagated. For NaCl, the vibrational frequency with ADMP is found to be independent of the fictitious electronic mass and to be equal to the BO trajectory result. For the photodissociation of formaldehyde, and the three body dissociation of glyoxal, very good agreement is found between the Born–Oppenheimer trajectories and the extended Lagrangian approach in terms of the rotational and vibrational energy distributions of the products. A 1.2 ps simulation of the dynamics of chloride ion in a cluster of 25 water molecules was used as a third test case. The Fourier transform of the velocity–velocity autocorrelation function showed the expected features in the vibrational spectrum corresponding to strong hydrogen bonding in the cluster. A redshift of approximately 200 cm^{−1} was observed in the hydroxyl stretch due to the presence of the chloride ion. Energy conservation and adiabaticity were maintained very well in all of the test cases.

The Rydberg state of Opticaloptical doubleresonance and Hugginsband ozonephotolysis, resonanceenhanced multiphotonionization studies with a state platform
View Description Hide DescriptionResonanceenhanced multiphotonionization spectra are presented of the transition of the state generated both by photolysis of in the Huggins bands and also by direct excitation of single J levels in an opticaloptical doubleresonance (OODR) experiment. The ozonephotolysisderived spectra reveal a preferential formation of fragments with high J, the rotational distributions exhibiting significant dependence on the photolysis wavelength. Rotational analyses of the OODR and ozonephotolysisderived spectra indicate that the Rydberg state is multiply perturbed by successive vibrational levels of the valence state. The OODR technique allows the first full resolution of the low levels of and the high levels favored by the ozonephotolysis technique are reported here for the first time.

Determination of a flexible (12D) water dimer potential via direct inversion of spectroscopic data
View Description Hide DescriptionWe report the determination of two dimer water potential energy surfaces via direct inversion of spectroscopic data. The first surface,rigid, employs the MCY functional form originally fitted by Clementi and coworkers from ab initio calculations, modified by adjunction of a fifth, uncharged, site to improve the dispersion component. The vibrationrotationtunneling energy levels were computed by means of the pseudospectral split Hamiltonian method that we developed previously. The fitted surface shows considerable improvement as compared to the original one: transitions among the groundstatemanifold are in error by at most and excited state band origins (up to are reproduced to within 0.5 to For the second surface,flexible, we used the same modified MCY functional form, considered now to depend on the 12 internal degrees of freedom, and augmented by the monomerpotential energy terms. The water dimer is described in its full dimensionality by collisiontype coordinates in order to access the whole configuration sampled by this floppy system. Internal motions of the monomers (stretches and bends) are explicitly considered by invoking an adiabatic separation between the slow (intermonomeric) and fast (intramonomeric) modes. This adiabatic formulation allows us to recast the calculations into an equivalent sixdimensional dynamics problem monomers) on an effectivepotential energy surface. The resulting, fitted, fully flexible dimer potential leads to a much better agreement with experiment than does the rigid version, as examplified by the standard deviation on all observed frequencies being reduced by a factor of 3. It is shown that monomer flexibility is essential in order to reproduce the experimental transitions.

Fourier moment analysis of velocitymap ion images
View Description Hide DescriptionAn alternative to inverse Abel transform and forward convolution methods is presented for extracting dynamical information from velocitymap ion images. Unlike most competing methods, that presented here does not require the probed threedimensional distribution to possess cylindrical symmetry. The new method involves analysis of the Fourier moments of images measured in different experimental geometries, and allows speed distributions, angular differential cross sections, and angular momentum alignment and orientation to be determined from raw images of the products of photodissociation and photoninitiated bimolecular reactions. The methodology is developed within the semiclassical framework of Dixon’s bipolar moment formalism [R. N. Dixon, J. Chem. Phys. 85, 1866 (1986)], although it is equally applicable to other common formulations of the product scattering distribution. To allow a comparison of the method with the Abel inversion, which requires that the velocity distribution of the probed product has an axis of cylindrical symmetry, the method is applied to newly acquired experimental images of atomic chlorine produced in the photolysis of NOCl. Extraction of product rotational alignment information is illustrated using newly acquired images of rotationally aligned NO formed by photolysis. Application of the Fourier moment methodology to studies of bimolecular reactions is also demonstrated, using simulated images for the reaction

Theoretical rate constants for the reaction using variational transition state theory on analytical potential energy surfaces
View Description Hide DescriptionVariational transition state theory, within the canonical unified statistical model including multidimensional tunneling corrections, has been applied to derive thermal rate constants in the temperature range 80–2800 K for the title reaction and its deuterated counterpart in the low and highpressure limits. The analytical potential energy surfaces of Bradley and Schatz (BS) and of Yu, Muckerman, and Sears have been employed. In the lowpressure limit, the results are comparable though slightly better for the BS potential surface at the lowest temperatures. In the highpressure limit, the BS potential surface is notably closer to the experimental data, which are only available at temperatures close to and higher than 300 K. The kinetic isotope effect (KIE) at room temperature is better predicted by the BS surface, although both surfaces fail to reproduce the experimental dependence of the KIE on temperature.

Selectivity of the α and β bond fissions for bromoacetyl chloride upon excitation: A combined completeactivespace selfconsistent field and multireference configuration interaction study
View Description Hide DescriptionThe potential energy surfaces for the dissociations into and in the and states have been investigated at the completeactivespace selfconsistent field, density functional theory, and multireference configuration interaction levels with the and ccpVDZ basis sets, which provide some new insights into the mechanism of the photodissociation at 248 nm. It is found that the most probable pathway is the C–Cl α and C–Br β bond fissions, which are a pair of competitive dissociation channels with some preference of the α C–Cl bond cleavage. The C–C α bond fission can take place along the pathway upon photoexcitation at 248 nm, but it is not in competition with the C–Cl α bond cleavage. These results are consistent with the experimental findings. The relative strength of the C–C and C–Cl α bonds is one of the factors that influences the selectivity of the α bond fissions. However, the selectivity is mainly determined by the mechanism of dissociation upon excitation. The preference of the C–Cl α bond fission over the C–Br β bond cleavage was attributed to the nonadiabatic recrossing in previous studies. The present calculations predict that the distance dependence of intramolecular energy relaxation is another important factor that influences the selectivity of the α and β bond cleavages.

Photodissociation spectroscopy of stored ions: Detection, assignment, and closecoupled modeling of nearthreshold Feshbach resonances
View Description Hide DescriptionWe have measured and theoretically analyzed a photodissociationspectrum of the molecular ion in which most observed energy levels lie within the finestructure splitting of the fragment and predissociate, and where the observed irregular line shapes and dipoleforbidden transitions indicate that nonadiabatic interactions lead to multichannel dynamics. The molecules were prepared in low rotational levels of the vibrational ground state by storing a beam at 7.1 MeV in the heavyion storage ring TSR for up to 30 s, which was sufficient for the ions to rovibrationally thermalize to room temperature by spontaneous infrared emission. The internally cold molecules were irradiated with a dye laser at photon energies between 31 600–33 400 and the resulting fragments were counted with a particle detector. The photodissociation cross section displays the numerous Feshbach resonances between the two finestructure states predicted by theory for low rotation. The data are analyzed in two steps. First, from the overall structure of the spectrum, by identifying branches, and by a Le Roy–Bernstein analysis of level spacings we determine the dissociation energy (with respect to the lower finestructure limit) and assign the strongest features to the vibrational levels of the dipoleallowed state. The majority of the 66 observed resonances cannot be assigned in this way. Therefore, in a second step, the complete spectrum is simulated with a closecoupling model, starting from recent ab initio Born–Oppenheimer potentials. For the longrange induction, dispersion and exchange energies, we propose an analytical expression and derive the coefficients. After a systematic variation of just the vibrational defects of the four Born–Oppenheimer potentials involved, the closecoupling model yields a quantitative fit to the measured cross section in all detail, and is used to assign most of the remaining features to the dipoleforbidden state and some to the weakly bound state The model potentials, which reproduce the spectrum and compactly represent the spectroscopic data, should help to predict more accurately scattering in the interstellar medium.

Anharmonic effects on the transition state theory rate constant
View Description Hide DescriptionAccurate rovibrational levels for HCN up to 24 349 cm^{−1} above the bottom of the vibrational well and vibrational levels at the saddle point of the HCN/HNC isomerizationreaction up to 32 809 cm^{−1} above the saddle point have been computed and used to obtain partition functions over the temperature range 200–2500 K. Under the rigidrotor approximation, the rovibrational partition function for HCN is found to be exactly separable into vibrational and rotational contributions to first order. Two approximate approaches, secondorder perturbation theory and simple perturbation theory, the latter of which obviates the need for a direct summation over energy levels, are shown to yield vibrational partition functions for HCN and at the saddle point that agree with the accurate values within 2%. In contrast, the usual harmonic approximation leads to errors of up to 24% in the individual partition functions, resulting in differences of between 3.5% and 6.5% in conventional transition state theoryrate constants calculated with harmonic versus anharmonic vibrations.

Twophoton photodissociation of NO through Rydberg levels in the 265–278 nm region: Spectra and photofragment angular distributions
View Description Hide DescriptionThe spectroscopy and dynamics of the NO photodissociation through Rydberg levels near 74 000 cm^{−1} have been investigated following twophoton excitation. The and levels overlap near 74 070 cm^{−1}. Assignment of the rotational transitions for these levels has been aided by the use of the photoproduct angular distributionsmeasured using product imaging techniques. Product imaging was also used to investigate the and regions assigned by previous investigators. In all cases, the major products were The angular distributions vary strongly with rotational transition and with the assumed intermediate in the twophoton excitation scheme and can, for the most part, be predicted by calculation. They demonstrate that, for the Rydberg levels examined, the major contribution to the twophoton line strength is a Π intermediate, likely the state, with less than a 30% amplitude contribution from either a Σ or Δ intermediate.

Zeroelectronkineticenergy photoelectron spectroscopy of transitionmetal—ether complexes: and
View Description Hide DescriptionThe yttrium complexes with one and two dimethyl ethers and their deuterated derivatives are prepared with laser vaporization molecular beam techniques, identified with photoionizationtimeofflight mass spectrometry, and investigated with pulsedfieldionization zeroelectronkineticenergy (ZEKE) photoelectron spectroscopy and ab initio calculations. Adiabatic ionization potentials and yttriumoxygen stretch and etherbased vibrations are measured for the 1:1 and 1:2 complexes. Fermi interactions are observed from the ZEKE spectra of the 1:1 species. The ground electronic states of the monoligand complexes are determined to be for the neutral and for the singly charged positive ion, both in symmetry, with yttrium binding to oxygen. The coordination of a second ether forms a diligand complex with a linear oxygenyttriumoxygen configuration. This is the first electronicvibrational spectroscopic study of yttriumpolyatomic molecule complexes and weakly bound metal complexes with two or more polyatomic molecules.

Anisole complexes: An experimental and theoretical investigation of the modulation of optimal structures, binding energies, and vibrational spectra in both the ground and first excited states
View Description Hide DescriptionWe present the results obtained from spectroscopic investigations and quantum chemical calculations of the interaction of anisole (methoxybenzene) with small water clusters. The experiments have been carried out using resonant twophoton ionization (R2PI) and IRUV doubleresonance vibrational spectroscopy (IR/R2PI) in the region of the OH stretches. Apart from the vibrational spectra of the water moiety in the clusters, their intermolecular vibrations in the electronically excited state are identified by IR/R2PI hole burning spectroscopy and assigned according to the vibrations calculated for the state and compared with the vibrations calculated for the state. The calculations for the state were carried out at the second order MøllerPlesset level of theory using both the and augccpVDZ basis sets and for the state at the configuration interaction singles (CIS) level with the basis set. In the electronic ground state the interaction of a watermonomer to anisole is mediated through its oxygen atom, and that of a water dimer both through the oxygen atom (σ type of interaction) and the arene ring (π type of interaction). Thus in contrast to the interaction of fluorinated benzenes with water clusters, wherein a conformational transition from an inplane σ to a ontop π bonding emerges starting with a water trimer, this conformational transition appears in case of anisole already with a water dimer. In the excited state of the investigated systems, there is a pronounced weakening of the interaction of the water cluster with the aromatic chromophore, which is also responsible for the blue shift of the electronic transitions. Consequently, the structures of the complexes of anisole with a watermonomer or dimer are very different in both states. The weakening results from a diminished electron density of the oxygen atom and of the π system of anisole in the excited state. The calculated binding energies of the groundstate conformers indicate that these small water clusters are bound more strongly to anisole than to other π systems like benzene, toluene, fluorobenzene, and pdifluorobenzene. The manybody decomposition of the binding energy reveals that a progressive increase in the size of the water clusters results in a weakening of the σ O⋯H interaction and a concurrent strengthening of the πH interaction. The complex containing a cyclic water trimer exhibits in the excited state also a πtype Hbonding interaction, but its stability emerges from a delocalization of the electron density from the water trimer to the anisole oxygen. Excepting the water dimer complexes, there is a good agreement between the calculated OH vibrational frequencies and the experimental IR spectra recorded for the ground state. The surprisingly good agreement of the calculated and the experimentally observed intermolecular modes in the excited state of these complexes provides convincing evidence that the experimental spectra emerge from structures similar to those predicted at the level.