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We presented a detailed quantum dynamics study for dissociative adsorption of H2 at different sites of Cu(111) surface to investigate the effect of surface corrugation and site specificity. The theoretical study employed a four-dimensional (4-D) ‘‘fixed-site’’ model, in which the lateral coordinates (X,Y) of the center of mass of the diatom are fixed at the impact site, but the remaining four degrees of freedom are explicitly treated in quantum calculations. The inclusion of the azimuthal angle [lowercase_phi_synonym] in the present 4-D model is a significant step forward in theoretical studies beyond the 3-D ‘‘flat surface’’ model. This 4-D ‘‘fixed-site’’ model allows us to investigate explicitly the local corrugation effect that was not possible using the 3-D flat-surface model. We incorporated the latest ab initio data of Hammer et al. in constructing the LEPS potential energy surface, which gives the lowest dissociation barrier over the bridge site. 4-D dynamics calculations are performed in the present study to mimic a normal incidence of H2 at three symmetric sites on Cu(111): bridge, atop, and center sites with the corresponding rotation symmetries. Our results show that a hydrogen impact at a high symmetry site (six-fold atop site) shows little corrugation effect while impact at low symmetry site (two-fold bridge site) shows a large corrugation effect. In particular, our calculation shows that the inclusion of surface corrugation preserves the strong rotational orientation effect observed in flat-surface model calculations. The effect of homonuclear symmetry persists at high symmetry atop site, and to a lesser degree at a low symmetry bridge site. The contour plot of the wavefunction in the current 4-D model shows explicitly that hydrogen atoms following the dissociation of H2 over the bridge site do not settle at the neighboring center site, but migrate to the... [ABSTRACT FROM AUTHOR]

Dynamical resonances in collinear He+H2+ → HeH++H reaction on the McLaughlin–Thompson–Joseph–Sathyamurthy potential energy surface are identified in terms of the transition state spectrum I(E), computed by Fourier transforming the autocorrelation function C(t) for the system. Eigenenergies of quasibound states corresponding to the resonances, starting from the nonreactive regime up to the dissociation threshold are reported. The nature of some of the resonance states is investigated by calculating the eigenfunctions for the corresponding eigenenergies and also their lifetimes. The relation to the corresponding resonant periodic orbits in classical mechanics at those energies is also established. In addition we predict several resonances at higher energies not reported in earlier calculations. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio predictions of the molecular geometry, harmonic vibrational frequencies, and dissociation energies are reported for the germanium hydride cluster ion GeH+7. Seven stationary points were located on the potential energy surface (PES) of GeH+7 using the self-consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included triple excitations [CCSD(T)] methods in conjunction with a double-ζ plus polarization (DZP) and a triple-ζ plus polarization TZ(3d1f,1p) quality basis set. The most stable structure has a C2 symmetry with the two H2 subunits rotating freely about the symmetry axis of the GeH+3 fragment. Our best estimate of the dissociation energy for GeH+7, taking into account the zero point vibrational energy (ZPVE) is 3.10 kcal/mol compared to 4.6 and 1.2 kcal/mol obtained, respectively, for the SiH+7 and CH+7 cluster ions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of a direct reaction between a gas phase H atom and an adsorbed H atom, often referred to as an Eley–Rideal mechanism, is explored using a fully three-dimensional flat surface model for Cu(111). The model is based on a flat-surface approximation for a single electronically adiabatic potential energy surface (PES). This reduces the inherently six-dimensional reactive scattering problem (for a rigid surface) to a three-dimensional problem by introducing three constants of motion. The resulting scattering problem is treated quantum mechanically by solving the time-dependent Schrödinger equation, and also by quasiclassical trajectory calculations. We have considered four different model PESs which are all more or less compatible with available knowledge about the interactions between hydrogen atoms and molecules and Cu(111). We have studied the dependence of the reactive cross section and product translational energy and rovibrational state distributions on the kinetic energy and angle of the incident H atom, and the vibrational state of the adsorbed H atom. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have analyzed the quantum dynamics of the collinear D+H2 reaction in the region of the transition state on the DMBE potential energy surface. Using the spectral quantization method, the dynamical features of the transition state are mapped out through a sequence of hypothetical Franck–Condon spectra. These spectra are generated by time-dependent wave packet dynamics. A comprehensive analysis of the spectra has revealed three distinct classes of peaks associated with (1) conventional reactive resonances, (2) threshold anomalies, and (3) barrier resonances. Individual peaks in the spectra are assigned through the use of time-independent wave functions which are obtained by Fourier transformation of the wave packet at the peak energies. The positions, lifetimes, and wave functions are extracted for all 23 conventional and barrier resonances. A new analytic line shape formula is developed to fit the spectral peaks of the barrier resonances. The exact quantum analysis is also supplemented with a semiclassical treatment based on periodic orbits. Most of the resonance states are found to be associated with single quantized periodic orbits in the transition state region. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed comparison of quasiclassical trajectory (QCT) and quantum mechanical (QM) reaction probabilities and differential cross sections for the H+D2→HD+D reaction at the collision energies of 0.54 and 1.29 eV has been carried out using the same potential energy surface. The theoretical simulation of the recently published experimental results is also reported. The comparisons made here demonstrate the level of agreement between QCT and QM approaches, as well as between theory and experiment for this reaction. [ABSTRACT FROM AUTHOR]

Two approaches, the outgoing wave variational principle (OWVP) and R-matrix propagation (RMProp), are presented for quantum dynamics calculations of inelastic scattering in systems involving two coupled potential energy surfaces (PES). The two formalisms are applied to Na(3p 2P) collisions with H2 at a total energy of 2.426 eV with zero and unit total angular momentum. This provides a challenging test case because the accessible region of the excited-state potential energy surface intersects the ground-state surface conically and involves H–H internuclear distances that are far larger than their equilibrium values in the ground state. We present the details of the formalisms for treating coupled surfaces, and we present converged results for the quenching probability and final vibrational–rotational quantum state distributions of the quenching agent. Convergence of the transition probabilities is established by convergence checks within each formalism, by obtaining the same results with laboratory-frame and body-frame basis functions in the OWVP formalism, and by obtaining the same results with the OWVP as with RMProp. [ABSTRACT FROM AUTHOR]

We present a procedure for partitioning the Hartree–Fock self-consistent-field (SCF) interaction energy into electrostatic, charge transfer, and deformation components. The natural bond orbital (NBO) approach of Weinhold and co-workers is employed to construct intermediate supermolecule and fragment wave functions that satisfy the Pauli exclusion principle, thereby avoiding the principal deficiency of the popular Kitaura–Morokuma energy decomposition scheme. The function counterpoise method of Boys and Bernardi enters the procedure naturally, providing an estimate of basis set superposition error (BSSE). We find that the energy components exhibit little basis set dependence when BSSE is small. Applications are presented for several representative molecular and ion complexes: the weak hydrogen bond of the water dimer, the strong ionic interaction of the alkali metal hydrides, and the moderate donor–acceptor interactions of BH3NH3 and BH3CO. Electrostatic interaction dominates the long-range region of the potential energy surface and charge transfer is strongly stabilizing for fragments within van der Waals contact. The repulsive interaction in the short range region of the potential arises from deformation as the fragment wave functions distort to avoid significant interpenetration. [ABSTRACT FROM AUTHOR]

The two Σ and four Π states of the weakly bound complex Ar–NH3 correlating to j=2, k=±1 ammonia have been observed by tunable far infrared difference frequency-microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH3 about its C3 axis in the complex is estimated to be 25.606(24) cm-1 near the minimum energy (T-shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm-1, respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Σ states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j=1, k=±1 Ar–NH3. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the 2-naphthol·H2O/D2O system was performed. Two different rotamers of 2-naphthol (2-hydroxynaphthalene, 2HN) exist with the O–H bond in cis- and trans-position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6-31G(d,p) basis set, fully energy-minimized geometries were computed for both cis- and trans-2HN·H2O of (a) the equilibrium structures with trans-linear H-bond arrangement and Cs symmetry and (b) the lowest-energy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol·H2O geometries. The H-bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol·H2O at the same level of theory. Extension of the aromatic π-system therefore hardly affects the H-bonding conditions. The barrier height to internal rotation around the H-bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6-31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M-matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a’ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β2 wag mode shows the largest anharmonic contributions. For the torsional mode τ (H2O H-atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant-two-photon ionization and dispersed fluorescence emission spectra of 2HN·H2O and d-2HN·D2O were measured. A detailed assignment of the bands in... [ABSTRACT FROM AUTHOR]

Accurate quantum mechanical dynamics calculations are reported for the reaction probabilities of O(3P)+H2→OH+H with zero total angular momentum on a single potential energy surface. The results show that the reactive flux is gated by quantized transition states up to the highest energy studied, which corresponds to a total energy of 1.90 eV. The quantized transition states are assigned and compared to vibrationally adiabatic barrier maxima; their widths and transmission coefficients are determined; and they are classified as variational, supernumerary of the first kind, and supernumerary of the second kind. Their effects on state-selected and state-to-state reactivity are discussed in detail. [ABSTRACT FROM AUTHOR]

Variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations of the energy and angular momentum dependence of the rate constant for the dissociation of C6H6+ into C6H5+ and an H atom are reported. In these variational calculations both the definition of the reaction coordinate and its value are independently optimized. A model potential-energy surface which interpolates between a Morse potential at short range and an ion-induced dipole potential at long range is employed in these variational calculations. The fully optimized variational results indicate that the transition state for this dissociation occurs at separation distances of about 3–4 Å and that the available phase space in the transition state is typically a factor of 5 lower than that predicted by phase space theory. Experimental measurements were made of the time-resolved product ion intensity resulting from the laser-induced dissociation of a thermal (≊375 K) distribution of benzene cations. An ion cyclotron resonance trap was used over a range of photolysis wavelengths from 266 to 285 nm. The observed time dependences in the product ion signals are a result of both dissociative and radiative relaxation processes with a deconvolution procedure yielding estimated dissociation rate constants. Satisfactory agreement between the theoretical and experimental results, including the previous experimental results of Neusser and co-workers [J. Phys. Chem. 93, 3897 (1989), and references cited therein] is obtained for an assumed dissociation energy of 3.88 eV to the lowest triplet state of C6H5+. [ABSTRACT FROM AUTHOR]

The lowest singlet potential energy surfaces of OBeH2 and its isomers are calculated, using geometry optimization techniques at the Fermi–Sea-multiconfiguration-self-consistent-field level. The reaction paths going from one isomer to the others are mapped and the appropriate transition states are located and verified. It is shown that, although the transition from BeO+H2 to OBeH2 is virtually barrierless, it can be achieved by an approach of molecular hydrogen to BeO from the Be side at right angles only. It is also shown that, should OBeH2 be formed, the transition to other, energetically lower points on the potential energy surface, involves twisting of the H2 moiety, thus making subsequent formation of linear H–O–Be–H the only option. The energy release upon this isomerization is 71 kcal/mol, while the activation barrier is 10 kcal/mol. [ABSTRACT FROM AUTHOR]

A new approach to calculating quantum functional sensitivity maps of transition probabilities is described in this paper. This approach is based on the log-derivative version of the Kohn variational principle and is applied here to the collinear H+H2 hydrogen exchange reaction. The sensitivity maps provide detailed quantitative information about how variations in the potential energy surface affect the state-to-state transition probabilities. The key issues investigated are (i) the evolution of sensitivity structure in the 0.30–1.50 eV range of total energy; (ii) the comparison of sensitivity structure on the Porter–Karplus, the Liu–Siegbahn–Truhlar–Horowitz, and the double-many-body-expansion potential energy surfaces; and (iii) the range of linearity for first order sensitivity predictions. [ABSTRACT FROM AUTHOR]

The reaction rates of the muonium (Mu) atom with HBr and HI in ∼1 atm N2 moderator have been measured over the temperature range 160–490 K using the μSR technique. While both abstraction and exchange reactions are possible, only the abstraction reaction should be observable, being moderately exothermic. Comparisons with the corresponding H(D) reactions reveal small kinetic isotope effects in both reactions, which do not vary strongly with temperature (kMu/kH≊3.5 near 300 K), consistent with the (classical) ratio of mean velocities. Surprisingly, quantum tunneling, normally facile for similarly exothermic reactions of the ultralight Mu atom (mMu/mH≊1/9), appears to be of little importance here. This despite the fact that the (temperature-independent) experimental activation energies are much less than the expected vibrationally adiabatic barrier heights (estimated to be ≊1.5 kcal mol-1) and, particularly in the case of Mu+HI, much less than the corresponding H-atom activation energy: 0.13±0.03 vs 0.70±0.3 kcal mol-1. In the case of reactions with HBr, the experimental Mu- and H-atom activation energies are much more similar: 0.51±0.03 and 0.74±0.12 kcal mol-1, respectively, over comparable temperature ranges. These data pose a conundrum in which several compensating effects related to the much lighter Mu-atom mass seem to be involved. Theoretical calculations are urgently required.In our view the topography of the potential-energy surface(s) for H2X is poorly known, particularly in the region of the barrier. It may be that the abstraction barriers for both Mu+HI and Mu+HBr are considerably later and even smaller than current calculations indicate, resulting in a cancellation of the effects of zero-point-energy shifts and quantum tunneling at the transition state. Differences in skewing angles between Mu and H+HX could favor a shorter tunneling path for the H-atom reaction, possibly compensating for its... [ABSTRACT FROM AUTHOR]

We present the results of quasiclassical trajectory (QCT) and quantum centrifugal sudden hyperspherical (CSH) scattering calculations for the Cl+HCl→ClH+Cl reaction using a semiempirical potential energy surface. In particular, we report state-to-state integral and differential cross sections in the vicinity of a transition state resonance that occurs at a total energy E of 0.642 eV. This resonance, which is labeled by the transition state quantum numbers (0,0,2), strongly perturbs the cross sections for the initial rovibrational state HCl(v=1, j=5), which was therefore considered in all our calculations. For E≥0.680 eV, which is well removed from the resonance energy, the QCT and CSH results are in good agreement, but for E near the resonance energy, important quantum effects are found in the integral cross sections, product state distributions, and differential cross sections. The CSH integral cross sections show smooth steplike increases for E≊0.642 eV, which are not seen in the QCT results. Associated with these steps are increased branching to the v’=0 product HCl vibrational state, and a strong propensity for the production of rotational states with j’=15 and 16 for v’=0. These features of the product energy partitioning are not present in the QCT results, although the correct rotational distributions are approximately recovered if the final vibrational action is constrained to match its quantum value. The CSH differential cross sections show a sudden shift from backward to sideward scattering between 0.642 and 0.660 eV, while the QCT cross sections remain backward peaked. An analysis of the ‘‘number of atom–diatom encounters,’’ during the course of a reactive collision, shows that there are chattering trajectories. These are associated with sideward scattering, but their probability is low and as a result they do not produce distinct features in the angular distributions. However... [ABSTRACT FROM AUTHOR]

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time-of-flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm. [ABSTRACT FROM AUTHOR]

Calculations of the total and partial decay widths are presented for the vibrational predissociation of H2HF. The calculation is done for the total angular momentum J=0 of the molecule and for the dissociation process H2HF(v=1)→H2(v=0)+HF(v=0). A time-dependent golden rule wave packet method is employed in the numerical calculation for the decay widths. The lifetime of the complex is determined from our calculation to be about 600 ns for para- and 1600 ns for ortho-H2HF. These values are much larger than the experimentally measured value of 27 ns for ortho-H2HF. The large discrepancy in lifetime for H2HF is in sharp contrast to the excellent agreement in lifetime for D2HF between theory and experiment, though the same potential energy surface is used in both calculations. We also present the rotational state distribution of the fragments H2 and HF following the vibrational predissociation of H2HF. It is found that about 58% of the final rotational population is in j=4 states of H2 for para-H2HF and about 48% is in j=5 states of H2 for ortho-H2HF. [ABSTRACT FROM AUTHOR]

A quantum mechanical method is described for calculating state-selected cross sections and rate constants for four-atom reactions of the general form AB+CD→ABC+D with nonlinear geometry. The method involves using hyperspherical coordinates to describe the BC and CD bonds, accounting for both the rotation of the AB molecule and the bending mode of the ABC molecule with a spherical harmonic basis set, holding the AB spectator bond length fixed and applying a version of the bending-corrected-rotating line approximation to treat the rotation of the CD molecule. The method is applied to the OH(j)+H2(v)→H2O(n,m)+H reaction, and its reverse reaction, where v and j are vibrational and rotational quantum numbers, and n and m label bending and local OH-stretching vibrational states of the H2O molecule. A modified potential energy surface based on a fit to ab initio data is used. Comparisons of the calculated cross sections are made with quasiclassical trajectory calculations. The effect of the bending and stretching modes of the H2O molecule on the reaction is examined in detail. The calculated rate constants for the OH+H2 reaction agree very well with experiment over the whole temperature range of 250–1050 K. [ABSTRACT FROM AUTHOR]

Resonances in the quasistable A 1A‘ state of trans HONO are probed through photodissociation experiments. The OH fragment ejected by HONO excited to specific –N[larger_closed_square]O stretching vibrational levels of the A state is probed by high-resolution sub-Doppler spectroscopy of the R2(1) (1,0) A 2Σ+–X 2Π transition. The Doppler profiles provide information on the vibrational state dependence of HONO’s subpicosecond fragmentation time and OH fragment’s kinetic energy. The latter is used to deduce NO sister fragment’s translational and vibrational energy content. These provide quantitative probes for (a) V→T ‘‘energy transfer’’ from the photoselected N[larger_closed_square]O vibrational coordinate to the recoil O–N coordinate and (b) N[larger_closed_square]O ‘‘vibrational relaxation’’ during dissociation. The present results and our previous measurements on the HONO A–X linewidths are discussed in terms of an excited-state potential energy surface in which (a) the –N[larger_closed_square]O coordinate is coupled to the reaction coordinate (O–N) and (b) there is a bottleneck in the exit channel. [ABSTRACT FROM AUTHOR]

The potential energy surface for H(1 2S)+SO2 has been investigated computationally in order to study the catalytic removal of atomic hydrogen in flames by sulfur dioxide. HF/3–21G(*) and MP2/3–21G(*) levels of theory were employed to locate stationary points, which were then characterized by calculation of the vibrational frequencies. Some geometries were also optimized with the 6–31G* basis set. Two adducts HOSO and HSO2, with H bonded to O or S, respectively, were studied. Energies were estimated at the optimized geometries using spin-projected MP4/6–31G* calculations, which show that planar cis HOSO is more stable than Cs HSO2. An H–OSO bond energy of 109 kJ mol-1 is predicted. By contrast HSO2 is predicted to be 25 kJ mol-1 endothermic with respect to H+SO2, and is insufficiently stable to be significant in combustion chemistry. Transition states were located and the information used to derive the kinetics of H+SO2+Ar⇄HOSO+Ar from 298 to 2000 K. An unusually large energy barrier to recombination, of about 95 kJ mol-1 relative to H+SO2, is proposed. The results are compared with available kinetic measurements. Other potential decomposition channels for HOSO, to SO+OH and isomerization to HSO2, were also analyzed. [ABSTRACT FROM AUTHOR]

We have measured rotational distributions for H2 and D2 thermally desorbed from Si(100) surfaces using resonance-enhanced multiphoton ionization (REMPI) for detection. These distributions are described by average rotational energies that are significantly lower than kTs (Ts=surface temperature) and exhibit slight, if any, isotopic dependence, i.e, =368±67 K and =348±65 K. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high-impact-parameter collisions. The rotational distributions indicate that some dynamical constraint causes very little torque to be applied to molecular hydrogen during recombination. Our data may be interpreted as resulting from an exaggerated preference for reactive trajectories that are characterized by low-impact parameters and/or a high degree of symmetry of the bond axis relative to the potential energy surface, followed by prompt desorption of the newly formed molecular hydrogen from Si(100). [ABSTRACT FROM AUTHOR]

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry, as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H4, using a multiple reference (single and) double excitation configuration interaction (MRD-CI) program. Both systematic and ‘‘random’’ errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self-consistent way ab initio energies calculated by Schwenke, bringing the number of H4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H3; adding ab initio energies calculated by other authors yielded a total of 772 conformations of H3. (The H3 results, and an improved analytic PES for H3, are reported elsewhere.) Ab initio energies are tabulated in this paper only for a sample of H4 conformations; a full list of all 6101 conformations of H4 (and 772 conformations of H3 ) is available from Physics Auxiliary Publication Service (PAPS), or from the authors.The best existing analytic PESs for H4 are shown to be accurate only for pairs of H2 molecules with intermolecular separations greater than about 3 bohr (1.6 Å). High energy collisions (such as might lead to direct collisional dissociation) cannot be well represented by such surfaces. A more general analytic PES for H4 is required, which will be accurate for compact (high-energy) conformations and for conformations that cannot be subdivided into a pair of H2 molecules. Work in progress on devising such a surface (fitted to the 6101 conformations of this work) will be reported in a... [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the OH+CO reaction using a potential surface that has been derived from ab initio calculations. Among quantities that have been studied are cross sections for reaction and for HOCO complex formation, cross sections associated with reaction from excited vibrational and rotational states, product energy partitioning and CO2 vibrational-state distributions, HOCO lifetime distributions, and thermal and state-resolved rate constants. We also present the results of Rice–Ramsberger–Kassel–Marcus (RRKM) calculations, using the same potential-energy surface, of HOCO lifetimes and of reactive and complex formation rate constants. The trajectory results indicate that the dominant mechanism for reaction involves complex formation at low energies. However, a direct reaction mechanism is responsible for half the reactive cross section at higher energies. This leads to a rate constant that is weakly temperature dependent at low temperatures, and becomes strongly temperature dependent at high temperature. Our trajectory results agree with measured rates over a wide range of temperatures, but the trajectory results at low temperatures are dominated by classical ‘‘leak’’ through zero-point barriers, so this agreement may be somewhat fortuitous.Rate constants for nonreactive processes such as OH(v=1) deactivation by CO that are controlled by the HOCO formation step are well above experiment (factor of 6), while rate constants for processes such as CO(v=1) reaction with OH that are controlled by decay of HOCO into H+CO2 are much closer (factor of 2). This suggests that the entrance channel barrier on our surface is too loose while the exit barrier is accurate. The error in the entrance channel barrier is studied using RRKM, and it is found to be due to an incorrect out-of-plane bend potential in the analytical surface used. Modifying the potential so that it is more consistent with ab... [ABSTRACT FROM AUTHOR]

The potential-energy surface for the inorganic free-radical, HIF, known only from molecular-beam studies, is investigated using fourth-order many-body perturbation theory [MBPT(4)]. All-electron calculations with three different basis sets are compared with results using relativistic and nonrelativistic effective potentials on I. Two local minima are found with an H–I–F angle α equal to 137.5° and 82°, with the energy difference ranging from 2.3 to 5.1 kcal/mol depending upon the calculation. The barrier to internal conversion is 1.1 kcal/mol. From the lowest minimum our MBPT(4) calculations predict HIF to be bound to dissociation (De) to IF+H by 27.0 to 32.6 kcal/mol, and to HI+F dissociation by 22.1 to 30.9 kcal/mol, in good agreement with experiment. However, very large superposition errors are shown to be possible in all-electron calculations because of inevitable basis-set deficiencies for the I atom. Explanations for the double minima in HIF are presented. [ABSTRACT FROM AUTHOR]

The adiabatically adjusting principal-axis hyperspherical (APH) formulation of Pack and Parker for quantum reactive scattering in three dimensions (3D) is used to obtain converged results for the reaction of helium with H+2 (v=1–4) for total angular momentum J=0. The ab initio potential energy surface computed by McLaughlin and Thompson and fitted by Joseph and Sathyamurthy is utilized for the HeH+2 interaction potential. The predicted energy dependence of the accurate 3D state-to-state reaction probabilities show clear evidence for quantum resonances. These resonances are even more numerous than those reported earlier for reduced dimensionality studies of this reaction. The calculated time delays for several of these resonances are found to be over 1 ps. Bending corrected rotating linear model (BCRLM) studies of this same reaction are also reported. These results provide useful insight in sorting out the nature and contribution of the resonances found in the 3D studies. [ABSTRACT FROM AUTHOR]

The singlet potential energy surface of protonated disilyne, Si2H+3, has been investigated theoretically using the self-consistent field (SCF) and configuration interaction (CI) methods. Complete geometry optimizations and harmonic vibrational frequency analyses were performed using a double-ζ plus polarization (DZP) basis set at both the SCF and CI levels of theory. Five minima were located, with the global minimum being a highly symmetric tribridged structure. There are three minima clustered at 11–15 kcal/mol higher in energy. One of these has two bridging hydrogen bonds, another has only one and the third has none at all. A trans-bridged isomer is a higher-lying minimum. Structures corresponding to the classical and nonclassical forms of the vinyl cation, C2H+3, are shown not to be minima. Final energetic predictions are made using several extended basis sets which are at least triple-ζ plus double polarization (TZ2P) in quality. Inclusion of electron correlation is found to be very important for energetic comparisons, geometries and characterization of the stationary points. The theoretical infrared intensities may be useful in the spectroscopic identification of Si2H+3. [ABSTRACT FROM AUTHOR]

The room temperature rate constants for molecular angular momentum reorientation of H2, HD, and D2 (B 1∑+u, v’=0, J’=1, MJ’=0) in collisions with He, Ne, Ar and H2 (X 1∑+g) have been measured. The effective cross sections for changing MJ’ in collisions of H2, HD, D2 with He and Ne were found to be about 30 Å2 and were nearly the same for each isotope and with He and Ne as collision partners. The measured He–H2(B) reorientation cross section is about 50% larger than the cross section calculated with a simple semiclassical model using a potential that approximates the ab initio data for the H2(B)–He potential energy surface. The cross sections for reorientation of HD and D2 in collisions with Ar were found to be 10.6±2.0 and 13.9±3.0 Å2, respectively. The smaller cross section is due to the dominant role played by quenching of the electronic energy of molecular hydrogen in collisions with Ar. The reorientation of D2(B) in collisions with room temperature H2(X) occurs with a 7.6±3.4 Å2 cross section. The small cross section for reorientation of the angular momentum is again due to the dominance of quenching in the collision dynamics. [ABSTRACT FROM AUTHOR]

Molecular beam techniques have been utilized to measure the dissociative chemisorption probability at zero surface coverage S0 for D2(H2) on Pt(111) as a function of initial energy Ei, angle of incidence θi, surface temperature Ts, isotopic mass and nozzle temperature Tn. S0 shows a large increase with translational energy, but no threshold in Ei, a peaking at θi=0°, and an independence with Ts, isotope and Tn. These results are interpreted in terms of direct dissociative chemisorption on the Pt(111) terraces. The dynamical picture that emerges is that although there is no significant barrier to dissociation along the minimum energy path, barriers do exist along nonoptimal reactive trajectories. Thus, the ‘‘translational activation’’ and other dynamical observations are intimately related to the multidimensional aspects of the dissociative potential energy surface. Some aspects of the dissociative chemisorption, however, still seem somewhat surprising within this general description. [ABSTRACT FROM AUTHOR]

A quantum wave packet calculation for the activated dissociative adsorption of H2 is presented. Restricting the motion of the molecule to lie within a plane normal to the surface we have treated all four molecular degrees of freedom exactly. We compare results obtained using two-, three-, and four-dimensional simulations on the same potential and show that by restricting the molecular orientation, important dynamical effects are lost. The potential employed in the calculations has been obtained using the effective medium approximation. In the simulations it has been possible to treat dissociation, rotations and diffraction on an equal footing. By including a rotational degree of freedom, it is seen that strong orientational effects occur near to the transition state and result in an anisotropic selectivity in the dissociation. By examining the state-to-state scattering probabilities, it is possible to use the nonreacting (scattered) fraction to provide information on the reactive potential energy surface. [ABSTRACT FROM AUTHOR]

The photoelectron spectra of the ions BrHI-, ClHI-, and FHI-, along with their deuterated counterparts, are presented. These spectra provide information on the transition state region of the potential energy surfaces describing the exothermic neutral reactions X+HI→HX+I(X=Br, Cl, F). Vibrational structure is observed in the BrHI- and ClHI- spectra that corresponds to hydrogen atom motion in the dissociating neutral complex. Transitions to electronically excited potential energy surfaces that correlate to HX+I(2P3/2,2P1/2) products are also observed. A one-dimensional analysis is used to understand the appearance of each spectrum. The BrHI- spectrum is compared to a two-dimensional simulation performed using time-dependent wave packet propagation on a model Br+HI potential energy surface. [ABSTRACT FROM AUTHOR]

This paper presents the results of a detailed study of dissociation and recombination of H2 over the temperature range 1000 to 5000 K. The chemical processes are modeled by solving the master equation for the concentrations of the full set of rovibration states. All of the state-to-state energy transfer rate coefficients required by the master equation are evaluated by the quasiclassical trajectory method using a potential energy surface which is a fit to ab initio electronic structure calculations. The analysis of the results of the master equation to obtain the phenomenological rate coefficients was carried out using several techniques. This is required because there is a mixture of third bodies and their concentrations change as the reaction proceeds. The methods based on one-way fluxes are not reliable, while the methods based upon an eigenvalue solution of the linearized master equation are in reasonable agreement with our preferred method, which is based on fitting the concentrations from a two component master equation. The results from various methods of determining the phenomenological rate coefficient for H2 as a third body are in good agreement with experimental estimates. The recombination rate coefficients are most sensitive to the collision induced dissociation rates of initial states with moderate values of vibrational and rotational quantum numbers. A comparison of the orbiting resonance theory to the accurate results shows that this simple model is not valid under the conditions of the present study. [ABSTRACT FROM AUTHOR]

Accurate 3D coupled channel calculations for total angular momentum J=0 for the reaction F+H2→HF+H using a realistic potential energy surface are analyzed. The reactive scattering is formulated using the hyperspherical (APH) coordinates of Pack and Parker. The adiabatic basis functions are generated quite efficiently using the discrete variable representation method. Reaction probabilities for relative collision energies of up to 17.4 kcal/mol are presented. To aid in the interpretation of the resonances and quantum structure observed in the calculated reaction probabilities, we analyze the phases of the S matrix transition elements, Argand diagrams, time delays and eigenlifetimes of the collision lifetime matrix. Collinear (1D) and reduced dimensional 3D bending corrected rotating linear model (BCRLM) calculations are presented and compared with the accurate 3D calculations. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory calculations for the title reaction have been carried out using the recent double many body expansion III potential-energy surface by Varandas et al. (1988). Recrossing factors are calculated and found to depend strongly on how the vibrational zero-point energy is treated. Detailed and thermal rate constants for the temperature range 20–500 K are presented. Comparisons with experiments, trajectory calculations by Miller (1986), and by Quintales et al. (1988), and a previously proposed extended Langevin model are made. It is noted that the coupling of the electronic and nuclear motion in OH may have a large effect on the thermal rate constant below 200 K. Close complex formation rate constants are calculated and found to agree well with the experimental rate constants. [ABSTRACT FROM AUTHOR]

We present calculations for the dissociative adsorption of hydrogen molecules on a Cu surface as a function of initial translational energy and vibrational quantum state. Classical, semiclassical, and fully quantum calculations are performed and the results compared. The potential energy surface was based upon a total energy calculation for H2 on a small Cu cluster and has been previously employed in dynamical simulations. Our results show that for low primary beam energies, dissociation occurs primarily via tunneling through the activation barrier in the vibrational coordinate. Populating the initial vibrational states is shown to enhance reactivity, but not simply by a total energy shift. By changing the hydrogen isotope it is shown that tunneling effects can persist up to quite high molecular masses. This occurs because the activation barrier lies in the vibrational coordinate, where the reduced mass of the molecule determines the dynamics. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory calculations have been performed for motion on an ab initio potential energy surface to investigate energy transfer from N2(A 3∑+u, v=1, 3 and 6) to H2 and D2. Because of the unusual features of the surface, both vibrational relaxation and electronic quenching of N2(A) are observed, the latter process resulting in dissociation of the hydrogen molecule. It is deduced that coupling of the vibrational motions of the N2 and H2 molecules initiates the energy transfer process. The results are compared with experimental information on the quenching of N2(A) by σ-bonded molecules. [ABSTRACT FROM AUTHOR]

The dynamics of chemisorption and decomposition of SiH2 on Si(111)–(1×1) and recontructed Si(111)–(7×7) surfaces have been investigated using classical trajectories on a previously described [Surf. Sci. 195, 283 (1988)] potential-energy surface modified to yield the experimental bending frequencies for chemisorbed hydrogen atoms and to incorporate the results of ab initio calculations of the repulsive interaction between SiH2 and closed-shell lattice atoms. The Binnig et al. model is employed for the (7×7) reconstruction. Sticking probabilities are found to be unity on the (1×1) surface and near unity on Si(111)–(7×7). The major mode of surface decomposition on the (7×7) surface is by direct molecular elimination of H2 into the gas phase. Hydrogen atom dissociation to adjacent lattice sites is a much slower process and the chemisorbed hydrogen atoms thus formed exhibit very short lifetimes on the order of (1.13–10.6)×10-13 s. The calculated rate coefficients for these two decomposition modes are 3.4×1010 and 0.79×1010 s-1 , respectively. The rate coefficients for the corresponding reactions on the (1×1) surface are 6.6×1010 and 5.3×1010 s-1 , respectively. The rates on the (1×1) surface are faster due to the increased exothermicity released by the formation of two tetrahedral Si–Si bonds upon chemisorption compared to a single Si–Si bond on the (7×7) surface. Molecular beam deposition/decomposition experiments of SiH4 on Si(111)– (7×7) surfaces reported by Farnaam and Olander [Surf. Sci. 145, 390 (1984)] indicate that chemisorbed hydrogen atoms are not formed in the SiH4 decomposition process whereas the present calculations suggest that such a reaction, although slow, does occur subsequent to SiH2 chemisorption. It is suggested that energetic differences between SiH4 and SiH2 chemisorption are responsible for these differences. [ABSTRACT FROM AUTHOR]

The near-infrared spectrum of ArHF prepared in a slit supersonic expansion is recorded with a difference frequency infrared laser spectrometer. By virtue of the high sensitivity of the technique, and the lack of appreciable spectral congestion at the 10 K jet temperature, we observe 9 of the 11 vibrational states with energies below the Ar+HF(v=1, j=0) dissociation limit. These include (1000), the lowest bound HF (v=1) state, the singly, doubly, and quadruply van der Waals stretch excited states (1001) (1002), and (1004), both the Σ bend (1200) and Π bend (111e,f 0), and the multiply excited, Π bend plus van der Waals stretch (111e,f 1). Two Ar+HF(v=0) states, (0000) and (0001), are also characterized. This spectroscopic information is quite sensitive to the Ar+HF potential energy surface away from the equilibrium configuration, and thus provides a rigorous test of trial potential energy surfaces. Excellent agreement is obtained between experiment and the predictions of a recently reported Ar+HF(v=1) potential. [ABSTRACT FROM AUTHOR]

We compare the results of classical simulations to accurate quantum studies for the reaction D+H2(v=0, j=1)→HD(v’, j’)+H (where v and v’ are vibrational quantum numbers and j and j’ are rotational quantum numbers) on a double many-body expansion (DMBE) potential energy surface. The quantal results presented here are converged to three significant figures. We consider orbital angular momenta 0 and 1 and total angular momenta 0–2. The total energies considered, Etot=0.983–1.333 eV, correspond to relative translational energy Erel=0.70–1.05 eV for the v=0, j=1 vibrational–rotational state. For J=1 and odd parity, the number of channels included is up to 453 and the number of open channels is up to 177. For J=2 and even parity, the number of channels is 630 and of open channels is 147. [ABSTRACT FROM AUTHOR]

Algorithms for the effective calculation of reactive scattering probabilities are developed and tested on the hydrogenic atom–diatom system described by the Siegbahn–Liu–Truhlar–Horowitz potential energy surface. A three-dimensional finite element procedure is designed from a description in terms of hyperspherical coordinates. The Wigner–Eisenbud R-matrix theory is used for a recursive procedure which admits control with limits on the hyperradial propagation inward from an asymptotic region and for a symmetry preserving transformation to arrangement channel Jacobi coordinates. [ABSTRACT FROM AUTHOR]

The dynamics of activated associative desorption is discussed with particular reference to the system H2–Cu and to the partitioning of the energy released among the various product degrees of freedom. It is argued that a simple theory based on transition-state concepts should hold for this system because the potential energy surface (PES) divides naturally into reactant and product regions, separated by a ‘‘seam’’ or ‘‘ridge’’ at which it is reasonable to assume a thermal distribution of desorbing trajectories. Using a PES constructed in accordance with available electronic structure calculations we consider the angular distributions and translational, vibrational, and rotational energy distributions of the desorbing molecules. It is shown that, whereas the rotational energy reflects the surface temperature, the vibrational energy is markedly enhanced because the energetically low-lying regions of the ridge in the PES correspond to an H–H bond distance that is distended as compared with the gas-phase equilibrium separation. The enhancement is found to be a strong function of the surface temperature. The translational energy, however, is found to be only very weakly dependent on the temperature. These results are discussed in connection with available data. [ABSTRACT FROM AUTHOR]

The unimolecular decomposition dynamics of Si2H4 have been investigated using classical trajectory methods on a global potential-energy surface fitted to the results of ab initio calculations and the available experimental data. The required phase-space averages are computed using Metropolis sampling techniques. It is found that unless the parameters of the Markov walk are adjusted for each different type of atom present, extremely long Markov walks are required to adequately cover the phase space of the system. Microcanonical rate coefficients for the decomposition of Si2H4 into all open channels are reported at energies in the range 5.0

Some theoretical and experimental aspects of the interaction of H2 with metal surfaces are reviewed. A variety of model potential energy surfaces based on simple extensions to the Lennard-Jones model are presented. These potentials include activation barriers which vary across the surface unit cell giving rise to a variety of physically interesting and realistic topologies. The quantum-mechanical scattering of hydrogen and its isotopes from these potentials illustrate that it is possible, in a very simple way, to account for simultaneous sticking and diffraction at a single beam energy. The inclusion of tunneling in a realistic description of initial sticking coefficients is shown to be essential for these light gases. Finally, it is proposed that by inverting diffraction data, it should be possible to obtain key information regarding the form of the potential energy surface. [ABSTRACT FROM AUTHOR]

The diffusion of hydrogen atoms on a partially hydrogen-covered Si(111) surface has been studied by using Monte Carlo techniques with a potential-energy surface based on the available ab initio results and experimental data. The potential describes two kinds of binding sites, a covalent Si–H bond (top site) and an interstitial threefold bonding site (open site). Classical jump frequencies between the top and open sites were calculated using Monte Carlo variation phase-space theory with importance sampling at 300, 600, 900, and 1200 K. A new approach for treating tunneling through two-dimensional diffusional barriers is presented and used to calculate the phonon-assisted tunneling rates. This method assumes continuum-to-continuum WKB tunneling with classical Monte Carlo phase space averaging. Thermal diffusion coefficients are calculated using the jump frequencies. The diffusional barriers between the two binding sites on the equilibrium surface are 2.79 and 0.65 eV for top-to-open site and open-to-top site jumps, respectively. The calculated classical jump frequencies give Arrhenius parameters of A=1.3×1014 and 9.9×1013 s-1 Ea=2.72 and 0.59 eV for top-to-open and open-to-top site jumps, respectively. Monte Carlo techniques were used to compute the minimum energy path; the dynamical barrier is 2.64 eV for top-to-open site jumps. Tunneling rates were calculated at 300 K and estimated at higher temperatures. Due, in part, to the small width of the barrier, the tunneling rate at 300 K is 257 times larger than the classical value. Tunneling is important at room temperature, but its importance relative to the classical rate decreases with increasing temperature. The results indicate that surface phonons significantly enhance the tunneling rate. [ABSTRACT FROM AUTHOR]

The recombination dynamics for the SiH2+SiH2→H2Si=SiH2 reaction are studied by quasiclassical trajectory methods using a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 18 many-body terms whose functional forms are motivated by chemical and physical considerations. The surface contains 41 parameters which are fitted to calculated geometries, fundamental vibrational frequencies, and energies for H2Si=SiH2, H2Si=SiH, H2Si=Si, HSi=Si, Si2, H2, and SiH2, and to various calculated and/or measured reaction barrier heights and activation energies. In general, the equilibrium bond lengths and angles given by the global surface are in agreement with abinitio results to within 0.03 Å and 0.5°, respectively. The calculated exothermicities for various reactions involving silicon and hydrogen atoms are in excellent agreement with previous MP4 calculations and with experimental data. The average absolute error is 1.90 kcal/mol. The average absolute deviation of the predicted fundamental vibrational frequencies for H2Si=SiH2, H2Si=SiH, H2Si=Si, and SiH2 from the results reported by Ho et al. is 52.9 cm-1. The calculated barrier height for molecular hydrogen elimination from SiH2 is 34.27 kcal/mol with a backreaction barrier of 0.63 kcal/mol. The barrier for 1,2 elimination of H2 from H2Si=SiH2 is 115.3 kcal/mol with a backreaction barrier of 30.7 kcal/mol. The formation cross sections for H2Si=SiH2 decrease with both relative translational energy and internal SiH2 energy with translational energy being the more effective in reducing the cross sections. Thermally averaged formation cross sections vary from 66.3 Å2 at 300 K to 28.7 Å2 at 1500 K. The corresponding thermal rate coefficients lie in the range 2–4×1014 cm3/mol s over this temperature range and exhibit a maximum at an intermediate temperature. The... [ABSTRACT FROM AUTHOR]

This paper presents the first ab initio attempt to reconstruct the observed band profile of the stretching fundamental vFX (X=H,D) in the FX···O(CH3)2 hydrogen-bonded system. The two-dimensional potential energy surface V(rFH,RF···O) is evaluated by means of large basis set SCF calculations. The related force constants up to the fourth order are obtained via the analytical fit to a polynomial expansion. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The side bands of the stretching fundamental vFX are described in terms of the vFX ±nvFX···O combination bands in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

We have examined the photofragmentation HNCO→NH(a 1Δ)+CO using radiation at wavelengths shorter than 230 nm. Nascent NH(a 1Δ) shows relatively little rotational excitation, accounting for less than 12% of the energy in excess of the dissociation energy. The rotational state distributions evidence less population in high rotational states than predicted by statistical theories but more than expected on the basis of a simple impulsive dissociation. A semiclassical impulsive model that describes photoproduct rotation as developing during fragmentation successfully describes the rotational state distributions of NH(a 1Δ) produced by HNCO photodissociation over a wide range of wavelengths. The success of this model in describing the NH rotational state distributions and previously measured CO rotational state distributions suggests that the excited state potential energy surface may be repulsive with minima in HNC and NCO bond angles each near 120°. [ABSTRACT FROM AUTHOR]

The R(0) and R(1) line shapes of HD are of interest because observation of these lines in the atmosphere of the outer planets should lead to unambiguous values of the D/H ratio. The electronic potential energy surfaces used to calculate HD–He and HD–H2 line shape cross sections have been well validated in previous publications of this series. The results of two additional tests of the electronic potential energy surface of hydrogen-like systems are reported here: the second virial coefficient of para-hydrogen at low temperatures and HD–D2 elastic and inelastic differential scattering cross sections. Formulas are given for Dicke narrowing as well as for the line broadening and shift cross sections calculated here. Close coupling calculations were carried out for the R(0) and R(1) transitions of HD immersed in He and for the R(0) transition of HD immersed in H2. These cross sections are anticipated to be quite reliable at temperatures above 50 K, i.e., where clustering can be ignored. [ABSTRACT FROM AUTHOR]

A set of large-scale ab initio molecular orbital calculations on the CH+5 potential energy surface is presented. The final calculations were done with very large polarized basis sets and correlation corrections were included at the CI(SDQ) level. The geometries were gradient optimized, while the harmonic force fields and infrared intensities were also determined analytically at the SCF level. None of the modes are predicted to have an extremely large intensity. The frequency of the torsional mode for rotation of the H2 about the pseudo threefold axis of the CH+3 group is found to be extremely low. This makes the exact energy content of the ion difficult to define precisely. The proton affinity of CH4 has been calculated at the CI(SDQ) level including zero point energy and thermodynamic corrections. The calculated value is 129.0 kcal/mol as compared to an experimental value of 132.4 kcal/mol. It is suggested that the experimental value is somewhat too high. [ABSTRACT FROM AUTHOR]

The dissociative chemisorption and scattering of H2 on an unreconstructed Si(111) surface has been investigated using classical trajectories on a potential-energy surface previously used to study H2 recombination/desorption from Si(111) [J. Chem. Phys. 85, 3081 (1986)]. The results show H2 scattering from Si(111) to be elastic and predominately specular. The scattered translational energy distributions are broadened but there is virtually no change in the average values. There is very little energy transfer to or from the internal H2 modes. Lower limits to sticking probabilities are obtained as a function of the temperature. Adsorption of H2 is found to always involve dissociation and subsequent chemisorption of both hydrogen atoms. The reaction exothermicity for chemisorption is primarily deposited into vibrational motion of the newly formed Si–H bonds. Energy transfer out of these bonds into the phonon modes of the lattice is a rapid, first-order process whose rate decreases with increasing surface temperature. Hydrogen atom mobilities on the surface computed from the velocity autocorrelation function are found to decrease rapidly with increasing residence time on the surface due to energy transfer to the lattice. Thermal diffusion of hydrogen atoms on the Si(111) surface is a slow process with a high activation energy. [ABSTRACT FROM AUTHOR]