Volume 109, Issue 3, 15 July 1998
 COMMUNICATIONS


Low temperature CO oxidation triggered by the gasphase D atom incident on Pt(111) covered with and CO
View Description Hide DescriptionExposing an saturated Pt(111) surface at 85 K to a beam of D atom leads to desorption of and A series of post Dexposure thermal desorption spectra shows that is produced by consecutive Daddition reactions via adsorbed OD intermediate, i.e., ++ When CO is coadsorbed with on Pt(111) at 85 K, the incident D atom also induces prompt desorption of but not CO. We propose that CO is oxidized by the nascent hot and formed in a highly exothermic initiation reaction with an energy release of ∼−4.6 eV before they become accommodated to the surface. Possible mechanisms for desorption are also briefly discussed.
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 THEORETICAL METHODS AND ALGORITHMS


Variational resonance valence bond study on the ground state of using the Heisenberg model
View Description Hide DescriptionA detailed variational resonance valence bond (RVB) study is performed for the ground state of the molecule in the framework of the Heisenberg model. It is shown that the 12 500dimensional Kekulé space can be divided into two subspaces of respective dimensions 5828 and 6672, of which the first one recovers 99.82% of the energy of the full Kekulé space. This 5828dimensional subspace is derived from the main Kekulé function, which is formed from spin pairs on hexagon–hexagon bonds only, by simple rotations of the three spin pairs in disjoint sets of hexagonal rings of in all possible ways. This indicates that the concept of the stability of the aromatic sextet still plays an important role even in this nonalternant system. Further, the inclusion of some longer range RVB functions like Dewartype functions and functions involving Claus structures is investigated, and the effect on the groundstate energy as well as on the nearest neighbor correlation functions is examined.

Rigorous formula for the fast calculation of the electron repulsion integral over the solid harmonic Gaussiantype orbitals
View Description Hide DescriptionA rigorous general formula for calculating the electron repulsion integral (ERI) over the solid harmonic (SH) Gaussiantype orbitals (GTOs) can be derived. A general algorithm can be obtained from this formula named as accompanying coordinate expansion (ACE) b3k3. This algorithm is capable of computing very fast SHERIs, especially for SH contracted GTOs. Numerical assessment is shown for the (LLLL) class of SHERIs It is found that the present ACEb3k3 algorithm is the fastest among all algorithms in the literature in the floatingpointopration (FLOP) count assessment when the degree of contraction is large.

Derivation of the phase factor and geometrical phase for an state degenerate system
View Description Hide DescriptionThis work considers the case of degenerate vibronic coupling of states all at one single point. For this case we first derived the extended singlestate Born–Oppenheimer equation [M. Baer and R. Englman, Chem. Phys. Lett. 265, 105 (1997)] and then showed that such a system, like the twostate system, can be described in terms of a single phase factor and is characterized by a welldefined geometrical phase.

Calculation of DFTGIAO NMR shifts with the inclusion of spinorbit coupling
View Description Hide DescriptionA formulation for the calculation of nuclear magnetic resonance(NMR) shielding tensors, based on density functional theory(DFT), is presented. Scalarrelativistic and spinorbit coupling effects are taken into account, and a Fermicontact term is included in the NMR shielding tensor expression. Gaugeincluding atomic orbitals (GIAO) and a frozencore approximation are used. This formulation has been implemented, and and NMR shifts of hydrogen and methyl halides have been calculated and show good agreement with experiment. NMR shifts of transition metal carbonyls have been calculated and show improved agreement with experiment over previous scalarrelativistic calculations. For the metal carbonyls it is shown explicitly that the combination of spinorbit coupling and magnetic field mixes spin triplet states into the ground state, inducing a spin density that then interacts with the nuclei of the metal carbonyl via the Fermicontact term. Results indicate that the Fermicontact contribution to the NMR of the metal carbonyl ions increases with increasing oxidation state of the ion. It is reasoned that as the oxidation state increases, π back bonding decreases and σ bonding increases, within the metal–carbon bond, thus facilitating a greater transfer of spin density from the metal to the carbon nucleus, and thus increasing the Fermicontact contribution to the NMR shielding of the carbon.
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 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Laser spectroscopy of free pentacene molecules (I): The rotational structure of the vibrationless transition
View Description Hide DescriptionThe rotational structure of the vibrationless transition of pentacene has been investigated using a strongly collimated seeded supersonic argon beam. Because single rotational lines could not be completely resolved, a band contour analysis was performed. The rotational constants of the electronic ground state were found to be under the asymmetric rotor approximation whereas the differences to the first electronic excited state are A new value of the band origin was determined to be and the band type was confirmed to be of type as proposed by symmetry arguments. Good agreement between observed and calculated spectra was obtained assuming planarity in both ground and excited state. From the fit procedure a rotational temperature of about 7 K was deduced. The nuclear statistical weights of the electronic ground state are reported.

The state of revisited: Observation and analysis of high vibrational levels
View Description Hide DescriptionCW optical–optical double resonance has been employed to study the upper portion of the state of The use of fluorescence and ion detection enabled the observation of vibrational levels ranging from to 48 and rotational levels ranging from to 25. For optical–optical double resonance experiments the Franck–Condon factors between these levels and useful state levels (levels that can be reached from the thermally populated levels of the ground state) were exceedingly small. The sensitive ionization detection made possible observation of this upper, anharmonic region of the potential. From the data a Rydberg–Klein–Rees potential curve was constructed which represents over 99% of the potential well depth. A coefficient has been calculated and represents the first experimental determination of a long range coefficient for a doubly excited state in diatomic lithium.

The effect of spinorbit coupling on the magnetic properties of
View Description Hide DescriptionExcited states of singlet and triplet have been calculated using multiconfigurational wave functions. The effects of orbital relaxation are determined by optimizing orbitals for all states separately and comparing to stateaveraged calculations, and are found to be small. Dynamic electron correlation included through secondorder perturbation theory is found to have a considerable effect on excited state relative energies, but not on the ordering of states. Spinorbit coupling effects are introduced by a oneelectron operator which uses an effective nuclear charge to replace twoelectron effects. The resulting splittings of the lowest energy triplet state components are 0.027 and respectively. The former is due to the angular momentum operator which acts along the Ti–Ti axis; the latter is due almost entirely to the angular momentum operator which acts in the direction perpendicular to the plane of the Ti–H–Ti bridge. An overall ferromagnetic effect of on the ground state singletfirst excited triplet energy gap is predicted. Orbital interactions responsible for spinorbit coupling effects are identified.

Anomalous enrichment of ions by laser ablation of graphite in Ar jet
View Description Hide DescriptionThe formation mechanisms of and ions by laser ablation of graphite are investigated using timeofflight (TOF) quadrupolemass spectroscopy. The laser ablation is performed in conjunction with a pulsed Ar expansion to elucidate the thirdbody effect on the formation of the small carbon cluster ions. Drastic enhancement of the ion signal is observed by an increase of the local pressure near the target, indicating that ions are efficiently formed by recombination of carbon atoms and subsequent ionization. The branching ratio of and ions and their mean translational energies are different from those of neutrals. Also, the TOF spectra for ions show multiple peak structures, which implies that different mechanisms are involved in the formation of ions.

Electron impact ionization of the hydroxyl radical
View Description Hide DescriptionWe employed the fastneutralbeam technique in a measurement of absolute partial cross sections for the electronimpact ionization and dissociativeionization of the hydroxyl free radical from threshold to 200 eV. The deuterated OD radical rather than the protonated OH radical was used as a target in our studies in order to allow a better separation of the various product ions in our apparatus. The total (single) OD ionization cross section was found to have a value of slightly less than at 70 eV. The ionization of OD is dominated by the formation of parent ions with a parent ionization cross section of at this energy. A comparison of the experimentally determined total single OD ionization cross section with a calculated OH cross section using a modified additivity rule showed good agreement in terms of the absolute value and the cross section shape (at least above 50 eV). In the course of this work, we also measured the partial ionization cross sections for the molecule and found good agreement between our cross sections and the most recent measurements of Straub et al. [J. Chem. Phys. 108, 109 (1998)] as well as with recent calculations.

All the adiabatic bound states of
View Description Hide DescriptionWe calculated all 2967 even and odd bound states of the adiabatic ground state of using a modification of the potential energy surface of Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. The calculation was performed by harmonic inversion of the Chebyshev correlation functiongenerated by a DVR Hamiltonian in Radau coordinates. The relative error for the computed eigenenergies (measured from the potential minimum), is or better, corresponding to an absolute error of less than about . Near the dissociation threshold the average density of states is about for each symmetry. Statistical analysis of the states shows some interesting structure of the rigidity parameter as a function of energy.

Theory for magnetic linear dichroism of electronic transitions between twofolddegenerate molecular spin levels
View Description Hide DescriptionMagnetic linear dichroism (MLD) spectroscopy is a relatively new technique which previously has been almost exclusively applied to atoms. These investigations have revealed that the study of MLD, in conjunction with electronic absorption and magnetic circular dichroism(MCD)spectroscopies, provides significant additional information concerning the electronic structure of atoms. More recent measurements have indicated that MLD is also observable from transition ions in inorganic compounds and metalloproteins. While the theory for atomic MLD has been worked out in considerable detail during the last two decades, an MLD theory of practical utility for the analysis of the spectra derived from the majority of paramagnetic molecules is not available. In the present contribution, the MLD of an electricdipoleallowed transition between twofolddegenerate molecular spin levels is analyzed, assuming nonsaturating conditions. As for atomic systems, it is found that the MLD of a single molecule is dominated by the term However, this term vanishes in the powder average evaluated for a randomly oriented ensemble of molecules, leading to a drastic reduction of the MLD differential absorption for systems with spin compared to that observed for systems with higher groundstate spin. It is found that MLD and MCDspectroscopies on solution samples have complementary spinstate specific sensitivities which suggest that the two methods can be used to selectively probe the individual metal sites in multicenter metalloprotein assemblies.

Investigations of pure rotational transitions of selfperturbed and perturbed by He. I. Measurement, modeling, and quantum calculations
View Description Hide DescriptionHigh resolution stimulated gain Raman spectroscopy is used to investigate the collisional parameters of pure rotational lines of in pure and mixture. Measurements are performed between 300 and 1000 K in a density regime where the lines are essentially collisionally broadened (typically 10 amagat). For the first time, these highly accurate measurements of the frequencies of pure rotational lines allow one to correct previously measured values that did not take into account the collisional frequency shift. For both collisional systems, the shifting coefficients exhibit a linear behavior with the square root of temperature, similar to the behavior already observed in the branch. The broadening coefficients of the branch increase nonlinearly with temperature contrary to the branch. For the system, both these new data and previously measured data are analyzed using a modeling of the broadening coefficients in terms of elastic and inelastic contributions. These different contributions are analyzed as a function of temperature and of the rotational quantum number Preliminary quantum calculations are used to assess the validity of the model. Further calculations will be presented in paper II.

A study of and using B3LYPDFT and CCSD(T) methods: The structures and electron affinities of small germanium clusters
View Description Hide DescriptionThe structures of the anionic germanium clusters and the corresponding neutral clusters have been investigated using B3LYPDFT and CCSD(T) methods. The basis set is employed for the dimers and trimers, while the smaller basis set is used for clusters with The most stable structures for the germanium cluster anions and are found to be and respectively. In the case of our calculations show that the low lying and states are within 1 kcal/mol of each other and both states are candidates for the ground state of the anion. The adiabatic electron affinities calculated for the clusters with are within 0.1 eV of the corresponding experimental values. Furthermore, the adiabatic excitation energies computed at the CCSD(T) level for the low lying states of and compare quite well with the assignments of the bands observed in the photoelectron spectra of and by Burton, Xu, Arnold, and Neumark [J. Chem. Phys. 104, 2757 (1996)].

Structures and stability of hydrated clusters of hydrogen chloride,
View Description Hide DescriptionThe molecular structures of the hydrated clusters of the HCl molecule, are examined by employing density functional molecular orbital methods. The most stable structures of the clusters are found to be of the proton nontransferred type. In the case of the cluster, the proton nontransferred and proton transferred structures have nearly similar energies. There are several stable isomers for the case and the structures of these isomers are found to be all proton transferred. The relative stabilities of the direct ionpair and the indirect ionpair are discussed in conjunction with their structures. The prediction of the IR spectra of the stable clusters clearly indicate the large redshifts of the H–Cl stretching and hydrogenbonded O–H stretching frequencies.

Absorption and fluorescence of OClO in solid Ne, Ar, and Kr. I. Vibrationally unrelaxed emission
View Description Hide DescriptionDispersed laserinduced fluorescence of the transition of OClO in solid Ne in the spectral range 500–770 nm was recorded when the origin at was excited. Progressions with spacings near 939 and are associated with vibrational modes and of the state. A simultaneous fit of both modes yields and When the line of the system at was excited, vibrationally unrelaxed emission was observed in the spectral region 480–600 nm. Excitation of the line at generated weak vibrationally unrelaxed progressions. The visible absorptionspectrum of OClO in solid Ne in the region 415–488 nm was recorded with a Fouriertransform spectrometer, yielding and for the state. Simultaneous fits considering either only and modes or all three modes yield corresponding spectral parameters. Similar experiments were performed with OClO in solid Ar and Kr. Pronounced increases in in Ar and in Kr) and in Ar and in Kr) and a decrease in and redshifted, respectively) from that in the gas phase indicate substantial perturbation of the state in solid Ar and Kr. An absorption continuum underlying the system is attributed to absorption to the state above the predissociation barrier. The phonon interaction increases and the threshold of the continuum decreases as the matrix host is altered from Ne to Ar to Kr.

Laser photolysis of OClO in solid Ne, Ar, and Kr. II. Site selectivity, mode specificity, and effects of matrix hosts
View Description Hide DescriptionIrradiation of lowlying vibronic levels of of OClO isolated in solid Ne, Ar, or Kr produces ClOO. Destruction of OClO and production of ClOO in various matrix sites were monitored with IR absorption lines near 1100 and respectively. For OClO in solid Ar at 5 K, site selectivity is observed after irradiation at the vibronic line associated with a specific site; decreased intensity of the IR line associated with a specific site of OClO correlates well with increased intensity of the corresponding line of ClOO. Similar behavior is observed for OClO in solid Kr except that formation of ClOO in one site is invariably dominant. The nature of the carriers of two major sites of ClOO (corresponding to 1416.7 and in solid Ar, 1412.1 and in solid Kr) is discussed. Temporal profiles of absorbance and were probed after various periods of irradiation until approximately 20%–60% of OClO was destroyed. Excitation of the vibrational mode of OClO enhances photodissociation of OClO appreciably. Rates of destruction of OClO in solid Ar or Kr are much smaller than that in solid Ne. The apparent conversion factor, is greater for OClO in solid Ne than that in solid Ar or Kr. Effects of matrix host on photolysis are discussed.

The ground state spectroscopic parameters and molecular geometry of
View Description Hide DescriptionThe rotation spectrum of stibine, in the ground vibrational state was recorded in the region between 30 and at an unapodized resolution of about transitions were measured and assigned up to and for both and isotopomers. The data of each molecule were analyzed together with the 1 rotational transitions reported in the literature, a few splitting transitions and “perturbation allowed” transitions measured recently with Fourier transform microwave spectroscopy, and ground state combination differences from the analysis of the high resolution spectra of the stretching and bending fundamental bands. The theoretical models adopted for the analysis included or interaction terms and correspond to two different reductions of the rotational Hamiltonian. From the obtained parameters, the and the equilibrium structure were also determined for this molecule.

Modespecific tunneling splittings in 9hydroxyphenalenone: Comparison of two methods for direct tunneling dynamics
View Description Hide DescriptionA benchmark comparison is presented of two direct dynamics methods for protontunneling, namely variational transitionstate theory with semiclassical tunneling corrections (VTST/ST) and the instanton method. The molecules chosen for the comparison are 9hydroxyphenalenone and  which have 64 vibrational degrees of freedom and show large tunneling splittings for the zeropoint level and several vibrationally excited levels of the electronic ground state. Some of the excitedlevel splittings are larger and some smaller than the zerolevel splitting, illustrating the multidimensional nature of the tunneling.Ab initio structure and force field calculations at the Hartree–Fock/631G** level are carried out for the two stationary points of the tunneling potential, viz. the equilibrium configuration and the transition state. The VTST/ST calculations are based on both the small and the largecurvature approximation; the additional quantumchemical calculations required at intermediate points of the potential are performed at the semiempirical modified neglect of differential overlap (MNDO)/H2 level. The VTST/ST computations use the MORATE 6.5 code developed by Truhlar and coworkers. The instanton dynamics calculations are based on the method we previously developed and applied to tropolone, among others. It uses the transition state rather than the equilibrium configuration as reference structure and approximates the least action analytically. The computations use our “dynamics of instanton tunneling” (DOIT) code. It is found that the largecurvature approximation and the instanton method both reproduce the observed zerolevel splitting of the isotopomer if the calculated barrier is reduced by a factor 0.87. With this adjusted barrier, the instanton method also reproduces the zerolevel and excitedlevel splittings of the isotopomer. However, both the small and the largecurvature approximations severely underestimate all these splittings. These methods, which use relatively inflexible trajectories, do not handle the isotope effect well and also are not developed to the point where they can deal satisfactorily with vibronic level splittings. In addition, there is a striking difference in efficiency between the two methods: the MORATE 6.5 code took 40 h on an R8000 workstation to perform the dynamics calculations, whereas the DOIT code took less than 1 min and produced superior results. The main reason for this superior performance is ascribed to the effective use made of the leastaction principle by the instanton method and to the avoidance of the adiabatic approximation, which is not valid for modes with a frequency equal to or lower than the tunnelingmode frequency.

Dynamics of tautomerism in porphine: An instanton approach
View Description Hide DescriptionAb initio calculations are reported of the rate of tautomerization by doublehydrogen transfer of porphine and three of its isotopomers. Both synchronous (onestep) and asynchronous (twostep) hydrogen tunneling mechanisms are considered. Geometries and force fields are calculated at the stationary points by means of a nonlocal density functional method that yields accurate equilibrium structures and vibrational spectra. Potentialenergysurfaces are constructed in terms of all 73 inplane normalmode coordinates at the transition state, the mode with imaginary frequency being taken as the reaction coordinate. Hydrogen tunneling calculations are performed by means of a simplified instanton method that has proved reliable in calculations on smaller systems. The full multidimensional potential is used, and adiabatic separation of the normal modes from the reaction coordinate is avoided. The coordinates of the transverse modes are coupled linearly to the reaction coordinate and all modes are allowed to mix freely with each other along the reaction path. Direct evaluation of the instanton path is not necessary. To calculate the tunnelingrate constant, it is sufficient to evaluate the onedimensional instanton action along the reaction coordinate and to correct it for coupling with transverse vibrations. This makes the method computationally very efficient compared to other multidimensional approaches. For the synchronous mechanism, the calculations closely follow the previously established procedure, but for the asynchronous mechanism, generalization to an asymmetric barrier is required. This is achieved by dividing the normalmode displacements that determine the couplings into symmetric and antisymmetric components which enhance and suppress the tunneling rate, respectively. The relative energies at the stationary points of the densityfunctional potential are calculated both by density functional theory(DFT) and by the Hartree–Fock method at the DFT geometry. The two methods yield results that are quite different. Comparison with a large set of experimental data comprising four isotopomers and a wide range of temperatures, indicates that neither method yields accurate energies but that some adjustment of the barrier height and the cis–trans energy difference is necessary to obtain satisfactory rate constants for the asynchronous mechanism. The other calculated parameters are used without adjustment. All parameters are combined to construct the potential required for the instanton calculations. A good fit to all available kinetic data is obtained, indicating that the method accounts accurately both for the isotope and the temperature dependence of the rate of tautomerization. It is shown that, in order to achieve this result, it is essential to include all linear couplings, since the balance between symmetric couplings, which enhance the tunneling rate, and antisymmetric couplings, which suppress it, varies between isotopomers. All dynamics calculations are performed with a newly developed code, which is designed to use the output of standard quantumchemical codes and requires only minutes of CPU time on a standard workstation.
