Volume 110, Issue 16, 22 April 1999
 COMMUNICATIONS


Characterization of the radical by anion photoelectron spectroscopy
View Description Hide DescriptionThe ground and first excited states of the radical are characterized by photoelectron spectroscopy of and at 266 nm. The electron affinity of is 4.226±0.013 eV. Based on the recently determined bond dissociation energy of the ground state is bound by 0.143±0.06 eV. The first excited state of lies 0.27 eV above the ground state. A vibrational progression is seen in the ground state band of the photoelectron spectrum. The addition of an argon atom to reduces the contribution of hot bands to the photoelectron spectrum, facilitating the interpretation of the vibrational structure. Simulations indicate that the ground state is linear with a symmetric stretch frequency of 115±5 cm^{−1} and is likely to be centrosymmetric.
 Top

 THEORETICAL METHODS AND ALGORITHMS


Jastrow corrected timedependent selfconsistent field approximation
View Description Hide DescriptionAn improved approximation to the timedependent Schrödinger equation is developed by correcting the timedependent selfconsistent field ansatz with a Jastrow prefactor defined via a set of variationally determined timedependent parameters and a linearly independent set of prespecified spatial functions. The method is applicable in any number of dimensions, conserves norm and energy, is without parametric singularities, possesses an internal estimate of the accuracy, and has computational costs that scale algebraically with the number of degrees of freedom. The new formalism is applied to a twodimensional double well potential to demonstrate the improved accuracy of the method. An extension of the method to electronically nonadiabatic problems is also presented.

Uniform Jshifting approach for calculating reaction rate constant
View Description Hide DescriptionThis paper presents a new uniform Jshifting approach for accurate calculation of rate constant in quantum dynamics study of chemical reaction. Instead of using a fixed shifting constant B in the standard Jshifting approach, the current method employs a temperaturedependent shifting constant which is obtained through an optimization procedure at a given temperature. By utilizing the calculated reaction probabilities at only a few total angular momentum values of the current approach automatically gives uniformly accurate rate constant across the entire range of temperature. Numerical studies of several benchmark reaction systems, including the H+ +OH and +CN reactions, show explicitly that the uniform Jshifting approach is far superior to the standard Jshifting approach and it provides a robust method for accurate and efficient calculation of reaction rate constant in rigorous quantum dynamics study of chemical reaction.

Nuclear magnetic shieldings in solution: Gauge invariant atomic orbital calculation using the polarizable continuum model
View Description Hide DescriptionWe present the gauge invariant atomic orbital (GIAO) calculation of nuclear magnetic shieldings for solvated molecules described within the polarizable continuum model (PCM). The performance of the PCMGIAO approach is tested in a benchmark calculation of isotropic and shielding constants for and in vacuo and in water, both at the Hartree–Fock and density functional levels of theory. Various aspects of the calculation of solvent effects on these properties, such as the dependence on the basis set, the electron correlation, and the size of the molecular cavity embedding the solute, are taken into account and discussed. An interpretation of the gastosolution shielding variations in terms of a combined action of the solvent reaction field and the shielding polarizabilities is also given.

Diffusionassisted longrange reactions in confined systems: Projection operator approach
View Description Hide DescriptionThe diffusionassisted longrange reversible reaction equation is solved for the pair survival probability using a projection operator method in terms of the diffusion propagator in the absence of reaction. For a localized (delta function) reaction sink, the wellknown analytical solution is immediately reproduced from the operator expression. It is emphasized that the mean reaction time approach, often used to approximate the overall reaction rate, is not adequate for a nonequilibrium initial condition. The general operator solution for a delocalized sink is shown to reduce to a closed matrix form, provided the propagator has a discrete spectrum of eigenmodes. The matrix solution is exact and applies for an arbitrary functional form and strength of the reaction sink. Although matrices of infinite dimensions are involved, they can be truncated at a certain finite dimension to attain any prescribed precision. Convergence of the truncated matrix solution is fast and often only a few of the lowest eigenmodes are sufficient to obtain quantitatively reasonable results. Several longrange reaction models are analyzed in detail revealing the breakdown of the widely used closure approximation obtained as a firstorder Padé approximation of the operator solution.

Gaussian3 theory using density functional geometries and zeropoint energies
View Description Hide DescriptionA variation of Gaussian3 (G3) theory is presented in which the geometries and zeropoint energies are obtained from B3LYP density functional theory instead of geometries from secondorder perturbation theory and zeropoint energies from Hartree–Fock theory This variation, referred to as is assessed on 299 energies (enthalpies of formation,ionization potentials,electron affinities,proton affinities) from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The average absolute deviation from experiment for the 299 energies is 0.99 kcal/mol compared to 1.01 kcal/mol for G3 theory. Generally, the results from the two methods are similar, with some exceptions. theory gives significantly improved results for several cases for which MP2 theory is deficient for optimized geometries, such as CN and However, does poorly for ionization potentials that involve a Jahn–Teller distortion in the cation because of the geometries. The G3(MP2) method is also modified to use geometries and zeropoint energies. This variation, referred to as has an average absolute deviation of 1.25 kcal/mol compared to 1.30 kcal/mol for G3(MP2) theory. Thus, use of density functional geometries and zeropoint energies in G3 and G3(MP2) theories is a useful alternative to MP2 geometries and HF zeropoint energies.

Coherent stabilization of zeroelectronkineticenergy states
View Description Hide DescriptionThe accuracy of zeroelectronkineticenergy (ZEKE) photoelectron spectroscopy rests on the ultralong lifetimes of the highn, highlRydberg states that are responsible for the ZEKE signal. However, a fewphoton process cannot excite electrons directly from the lowlground state to the highl ZEKE manifold. In this paper we show that using the dynamics of Rydberg Stark states in slowly time dependent external fields it is possible to control coherently the angular momentum of Rydberg electrons, and therefore also their lifetime. We derive explicitly two different schemes based on simple, short electric dc pulses, which populate precisely those highl, longlived Rydberg states that are necessary for accurate ZEKE experiments. The highl states that we construct are also Stark eigenstates, therefore a moderate dcexternal field can eventually enforce cylindrical symmetry and lock the ZEKE electrons in the stable, longlived highlmanifold.

Gaussian basis sets for use in correlated molecular calculations. IX. The atoms gallium through krypton
View Description Hide DescriptionValence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions (although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms). After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: ccpVDZ: ccpVTZ: ccpVQZ: ccpV5Z: Augmented sets were obtained by adding diffuse functions to the above sets (one for each angular momentum present in the set), with the exponents of the additional functions optimized in calculations on the atomic anions. Test calculations on the atoms as well as selected molecules with the new basis sets show good convergence to an apparent complete basis set limit.

Electronicstructure calculations by firstprinciples densitybased embedding of explicitly correlated systems
View Description Hide DescriptionA firstprinciples embedding theory that combines the salient features of density functional theory(DFT) and traditional quantum chemical methods is presented. The method involves constructing a DFTbased embedding potential and then using it as a oneelectron operator within a very accurate ab initio calculation. We demonstrate how DFT calculations can be systematically improved via this procedure. The scheme is tested using two closed shell systems, a toy model and the experimentally well characterized CO/Cu(111) system. Our results are in good agreement with near full configuration interaction calculations in the former case and experimental adsorbate binding energies in the latter. This method provides the means to systematically include electron correlation in a local region of a condensed phase.

Density functional calculations of nuclear magnetic shieldings using the zerothorder regular approximation (ZORA) for relativistic effects: ZORA nuclear magnetic resonance
View Description Hide DescriptionWe present a new relativistic formulation for the calculation of nuclear magnetic resonance(NMR) shielding tensors. The formulation makes use of gaugeincluding atomic orbitals and is based on density functional theory. The relativistic effects are included by making use of the zerothorder regular approximation. This formulation has been implemented and the NMR shifts of HgMeCN, HgMeCl, HgMeBr, HgMeI, and have been calculated using both experimental and optimized geometries. For experimental geometries, good qualitative agreement with experiment is obtained. Quantitatively, the calculated results deviate from experiment on average by 163 ppm, which is approximately 3% of the range of NMR. The experimental effects of an electron donating solvent on the mercury shifts have been reproduced with calculations on and In addition, it is shown that the mercuryNMR shieldings are sensitive to geometry with changes for of approximately 50 ppm for each 0.01 Å change in bond length, and 100 ppm for each 10 change in bond angle.
 Top

 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


The state of SO
View Description Hide DescriptionLaser induced fluorescence and 1+1 resonance enhanced multiphoton ionisationspectra of the transition of SO radicals prepared by an electric discharge in a supersonic jet expansion are presented. Rotational constants are given for A state vibrational levels with extending to within 190 cm^{−1} of the A state dissociation limit. The Rydberg–Klein–Rees curve derived from these constants shows significant anharmonicity, even around the equilibrium geometry. In addition, several small local perturbations of the rotational structure are observed. Collision free fluorescence lifetimes are determined for the complete range of vibrational states, and are found to fall smoothly from 29.5 μs for to 6.45 μs for Combining these data with earlier measurements leads to a better determination of the transition dipole moment over the range 1.4–2.0 Å.

Stereodynamics of the vibrational channel
View Description Hide DescriptionThe stateselected differential cross section (DCS) and rotational angular momentumpolarization for the reaction have been measured by utilizing the polarized Dopplerresolved laserinduced fluorescence probing technique. Stereodynamics of the reaction channel forming the newly formed OH in the specific vibrational level is discussed on the basis of the vector properties. A nearly isotropic DCS for the product most probably indicates that the reaction is dominated by an insertion mechanism involving a collisional HOOH complex with a lifetime comparable to its rotational period. The extremely asymmetrical energy partitioning between the two OH fragments, therefore, suggests that the redistribution of the available energy does not occur on a time scale comparable to the rotational period of the complex. Furthermore, it has been found that the product rotational angular momentum vector is predominantly perpendicular to the collision plane spanned by k and (the relative velocity vectors of the reactants and products, respectively) both for the forward and backwardscattered products. It suggests that the initially excited bending motion of the H–O–O moiety in the collisional HOOH complex primarily contributes to the product rotation.

Experimental and theoretical study of line mixing in methane spectra. I. The broadened band at room temperature
View Description Hide DescriptionLinemixing effects have been studied in the band of perturbed by at room temperature. New measurements have been made and a model is proposed which is not, for the first time, strictly empirical. Three different experimental set ups have been used in order to measure absorption in the 2800–3200 cm^{−1}spectral region for total pressures in the 0.25–2 and 25–80 atm ranges. Analysis of the spectra demonstrates the significant influence of line mixing on the shape of the Q branch and of the P and Rmanifolds. A model is proposed which is based on statetostate collisional transfer rates calculated from the intermolecular potentialsurface with a semiclassical approach. The linecoupling relaxation matrix is constructed from these data and two additional parameters which are fitted on measured absorption. Comparisons between measurements and spectra computed accounting for and neglecting line mixing are made. They prove the quality of the approach which satisfactory accounts for the effects of pressure and of rotational quantum numbers on the spectral shape under conditions where modifications introduced by line mixing are important. For high rotational quantum number lines, the main features induced by collisions are predicted but some discrepancies remain; the latter may be due to improper linecoupling elements but there is strong evidence that the use of inaccurate line broadening parameters also contributes to errors in calculated spectra.

Temperature, pressure, and perturber dependencies of linemixing effects in infrared spectra. III. Second order rotational angular momentum relaxation and Coriolis effects in bands
View Description Hide DescriptionThe energy corrected sudden approach is used in order to deduce collisional parameters and to model infrared quantities in bands of and mixtures in the 200–300 K temperature range. Measured linebroadening coefficients and absorption in the Qbranch of the band at moderate pressure are first used for the determination (from a fit) of the time constant associated with the relaxation of the second order traceless tensor of the rotational angular momentum (all other collisional quantities have been determined previously). The results obtained are consistent with previous (calculated) temperature dependent values of the depolarized Rayleigh cross sections. The model is then successfully tested through computations of absorption in the and bands at elevated densities. Analysis of linemixing effects is made, including study of the influence of interbranch transfers and of Coriolis coupling. Differences between the effects of collisions with He and Ar are pointed out and explained.

Ab initio potential energy surfaces for and
View Description Hide DescriptionThe threedimensional ground statepotential energy surfaces for and have been calculated using the single and double excitation coupledcluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)]. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions. Single point calculations for approximate minima have also been performed with several other basis sets including the quadruple zeta basis set (augccpVQZ) with bond functions. For and the CCSD(T) results show that the linear configuration is lower in energy than the Tshaped one. For the CCSD(T) approach predicts the Tshaped configuration to be lower in energy. The linear configuration has been found to be more sensitive than the Tshaped one to the changes of the Cl–Cl bond length with the interaction becoming weaker when the Cl–Cl bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. The interaction in the Tshaped configuration becomes slightly stronger for shorter Cl–Cl bonds. For and the larger zeropoint vibrational energy of the linear configuration is responsible for making the Tshaped configuration the ground vibrational state. Vibrational effects further increase the difference in energy between the ground state Tshaped configuration of and its linear counterpart.

Theoretical study on quantum control of photodissociation and photodesorption dynamics by femtosecond chirped laser pulses
View Description Hide DescriptionWe have theoretically studied the effect of chirping onephoton incident laser pulses on (I) the branching ratio of the HOD molecule in the photochemical reaction and (II) the UV photodesorption dynamics of and from Cu(111). As was predicted in our previous 1D model, wave packet calculations have demonstrated that it is possible, in practice, to control the branching ratio of reaction (I) and to greatly enhance the desorption probability of the photodesorption reaction (II) by negatively chirped laser pulses. It was found that two characteristics of (negatively) chirped laser pulses contribute to this remarkable effect; the mechanism of adiabatic rapid passage for the population transfer between the ground and excited states, and the intrapulse pumpdump process for determining the branching ratio and photodesorption yield.

Structural transition in hot small clusters
View Description Hide DescriptionAt relatively high temperatures (higher than the melting temperature of a liquid), clusters existing in the supersaturated vapor are characterized by an intense internal motion of molecules. The virtual chains model of small “hot” clusters is proposed, which assumes that the number of bonds in small clusters is minimal, and that their structure is chainlike. Interpolation formulas for extensive thermodynamic functions of a cluster containing arbitrary number of atoms are found. Validity of model assumptions are verified by the molecular dynamics simulation for the ensemble with constant temperature and pressure. Simulation results are discussed, among which are the average potential energy of a cluster, the radial distribution function, and topological structure of clusters. Numerical results validate the basic assumption of proposed model.

Benchmark enthalpies of formation and binding energies of protonbound pairs between HCN and HCN, and HF
View Description Hide DescriptionThe relative thermochemicalproperties of cluster ions (solvation enthalpies,entropies, and free energies) can be obtained from experimental techniques such as high pressuremass spectrometry and selectedion flow tube mass spectrometry.Theory can play an important role in these studies by providing both accurate binding energies of the smaller members of the cluster families and insight into the structure and bonding in the cluster ions. This study assesses the performance of a variety of levels of ab initio and density functional theories for predicting the structures and energies of one family of cluster ions, the protonbound dimers between HCN and HCN, and HF. The theoretical procedures were assessed based on their performance relative to highlevel treatments such as QCISD(T) correlation, the basis set, and G2 energy calculations. The results of the assessment indicate that optimized geometries are sufficient for the calculation of binding energies and heats of formation with advanced methods such as G2. Further increases in basis set size and electron correlation improve the geometries of the dimers, but these geometric changes have little impact on the final highlevel energy calculations. The heats of formation and binding energies of the clusters are best described by G2 theory, but modified versions of G2 such as G2(MP2) and G2(MP2,SVP) also provide reliable values. Calculated binding energies of these four protonbound dimers are compared to available experimental values from the literature, and the effect of basis set superposition error is examined

Global ninedimensional potential energy surface for the system. I. Ab initio multiple reference single and double excitation configuration interaction computations
View Description Hide DescriptionThe groundstate adiabatic potential energy surface was computed for 2836 conformations of using a multiple reference single and double excitation configuration interaction program and an extended basis set. The transition state obtained is that of a pentagon with symmetry. We find the energy barrier to lie at 81 kcal/mol within ±1 kcal/mol if a multireference Davidson’s correction is employed, while the at this level was 109 kcal/mol. The calculations presented here clearly show the energy of in symmetry to be 28 kcal/mol below the dissociation energy of and therefore a bimolecular mechanism, in the presence of hydrogen atoms, could indeed be considered for the exchange process:

Global ninedimensional potential energy surface for the system. II. Fit to an analytical expression
View Description Hide DescriptionA global ninedimensional potential energy surface (GPES) for the adiabatic ground state of the system which fits the ab initio data, reported in the previous paper of this series, has been obtained (rootmeansquare error 2.2 kcal/mol). The global fitting procedure is an extension of the corresponding procedure for triatomic and tetraatomic systems including the functional form previously proposed by the authors. The GPES obtained here is totally symmetric with respect to permutations of the hydrogen atoms and satisfies the criteria needed to be used in scattering calculations.
