Volume 117, Issue 5, 01 August 2002
 COMMUNICATIONS


Atomic layer deposition of hafnium oxide: A detailed reaction mechanism from first principles
View Description Hide DescriptionAtomic layer deposition(ALD) of hafnium oxide using and as precursors is studied using density functional theory. The mechanism consists of two deposition halfreactions: (1) with HfOH sites, and (2) with HfCl sites. Both halfreactions exhibit stable intermediates with energies lower than those of the final products. We show that increasing the temperature reduces the stability of the complex. However, increasing temperature also increases the dissociationfreeenergy barrier, which in turn results in increased desorption of adsorbed precursors. Both halfreactions are qualitatively similar to the corresponding reactions of ALD using and

Pulsed field gradient nuclear magnetic resonance study of long–range diffusion in beds of NaX zeolite: Evidence for different apparent tortuosity factors in the Knudsen and bulk regimes
View Description Hide DescriptionThe pulsed field gradient nuclear magnetic resonance method is applied to study selfdiffusion of ethane in beds of zeolite NaX for displacements which are orders of magnitude larger than the size of individual crystals. Comparison of the measured diffusivities with those calculated using simple gas kinetic theory indicates that for the same bed of NaX crystals the apparent tortuosity factor in the Knudsen regime is significantly larger than that in the bulk regime. This finding is tentatively attributed to the more pronounced geometrical trapping by surface imperfections in the Knudsen than in the bulk regime. Tortuosity factors, which are much larger in the Knudsen regime than in the bulk regime, were also recently obtained by dynamic Monte Carlo simulation of gas diffusion in various porous systems.

The ionization energy of methylene from a rotationally resolved photoelectron spectrum and its thermochemical implications
View Description Hide DescriptionThe adiabatic ionization potential of methylene has been determined to be from a rotationally resolved photoelectron spectroscopic study of the transition. This value was used to determine thermochemical quantities such as the 0 K dissociation energy of the ketene cation in CO and the 0 K dissociation energy of the methyl radical the 0 K dissociation threshold of methane in and and the 0 K enthalpy of formation of
 Top

 ARTICLES

 Theoretical Methods and Algorithms

Energy versus amplitudecorrected coupledcluster approaches. III. Accurate computation of spectroscopic data exemplified on the HF molecule
View Description Hide DescriptionThe socalled energycorrectedcoupledcluster method with singles and doubles (CCSD), which is closely related to the recently proposed method of moments of Kowalski and Piecuch, accounts for the nondynamic correlation effects, which are missing in the standard singlereference CCSD, by projecting onto the modestsize multireference (MR) configurationinteraction SD (CISD) wave function when evaluating the energy, rather than onto the singledeterminantal CCSD reference. The ability of this approach to generate reliable potential energy curves (PECs) or surfaces over a wide range of geometries is examined using a realistic ab initio model of the HF molecule. The PECs obtained with the correlationconsistent polarized valence double (triple, quadruple) zeta (T,Q) basis sets are used, together with the PEC extrapolated to the complete basis set limit, to compute the rotational and vibrational levels, which are then compared with the experimentally determined values as well as with the directly measured vibrational frequencies. A comparison is also made with the earlier results [X. Li, J. Mol. Struct.: THEOCHEM547, 69 (2001)] that were obtained with the amplitudecorrected CCSD method, namely the socalled reduced MR CCSD that exploits the same MR CISD wave function, as well as with recently published renormalized CCSD(T) results [P. Piecuch et al., J. Chem. Phys. 115, 5796 (2001)]. It is shown that both the amplitude and energycorrected CCSD PECs may be used to generate highly accurate and reliable spectroscopic data once the basis set limit is achieved.

Estimating electron correlation energies of from and ionic units with a simple scaling approach
View Description Hide DescriptionElectron pair correlation contribution from individual electron pairs of the molecular orbitals is defined and used in analyzing the correlation patterns of and isoelectronic covalent systems. Based on our simple strategy socalled “separating large system into smaller ones” for estimating the correlation energies by investigating both the ionic and the diradical partitioning schemes for covalent systems, a simple scaling scheme is presented for estimating the total correlation energy. It is achieved by summing the scaled correlation energies of its ionic fragments derived from the ionic partitioning scheme rather than from the diradical partitioning scheme. Of the three estimated results, the absolute deviations are less than ±0.29 kcal/mol, however, using this simple scaling approach, at least 90% of computational work can be saved. At the present condition with the computational demand for calculating the electron correlation energies of large covalent molecules, it is hopeful that this simple scaling approach could be useful to estimate the correlation energies of large CHcontaining alkaline compounds.

The optimal tunneling path for the proton transfer in malonaldehyde
View Description Hide DescriptionThe protontunnelingreaction in malonaldehyde at low temperatures is investigated. The principal aim of this study is to find the optimal tunneling path at 0 K in the framework of the semiclassical theory with a global optimization method. An amount of 11366 ab inito points was determined in the reaction swath (i.e., the conformational space enclosed by the minima and the transition state) of malonaldehyde. With a simulated annealing approach, the path with the smallest integral of the imaginary action through the swath from minimum to minimum was determined. Surprisingly the optimal tunneling path was found to be quite far off the large curvature tunneling path [i.e., the straight connection of the two minima largecurrent tunneling (LCT path)]. At the beginning, it is following the minimum energy path (MEP) (i.e. the path with the lowest energy connecting the two minima and passing through the transition state), and then it is describing a curved path through the reaction swath. This curve was determined several times with different annealing schemes, which ended up with the same result—the tunneling path is proceeding close to the MEP rather than to the LCT path. Along the optimal tunneling path, the groundstatetunneling splitting was calculated with a new semiclassical method introduced in an accompanying study [C. S. Tautermann, A. F. Voegele, T. Loerting, and K. R. Liedl, J. Chem. Phys. 117, 1967 (2002), following paper]. Another focus of investigation was the influence of deformation of the tunneling paths and a general scheme of determining an approximated optimal tunneling path at 0 K is introduced.

An accurate semiclassical method to predict groundstate tunneling splittings
View Description Hide DescriptionA new method for calculating the groundstatetunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The densityfunctional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the lowlevel energy surface is mapped onto the highlevel points to get a good quantitative description of the highlevel PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the highlevel PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional consideration of zeropoint vibrational effects. The method is applied to three molecular systems, namely hydrofluoric acid dimer, malonaldehyde, and tropolone. These systems were chosen because their energy barriers differ strongly (1 kcal/mol–7 kcal/mol). The predicted tunneling splittings agree very well with the experimental ones, therefore, we expect our method to be generally applicable, independent of the magnitude of the energy barrier.

Dynamics of a local algorithm for simulating Coulomb interactions
View Description Hide DescriptionCharged systems interacting via Coulomb forces can be efficiently simulated by introducing a local, diffusing degree of freedom for the electric field. This paper formulates the continuum electrodynamic equations corresponding to the algorithm and studies the spectrum of fluctuations when these equations are coupled to mobile charges. I compare the calculations with simulations of a charged lattice gas, and study the dynamics of charge and density fluctuations. The algorithm can be understood as a realization of a mechanical model of the ether.

Local weakpairs pseudospectral multireference configuration interaction
View Description Hide DescriptionWe present a new reduced scaling multireference singles and doubles configuration interaction (MRSDCI) algorithm based upon the combination of local correlation and pseudospectral methods. Taking advantage of the locality of the Coulomb potential, the weakpairs approximation of Saebo/ and Pulay is employed to eliminate configurations having simultaneous excitations out of pairs of distant, weakly interacting orbitals. In conjunction with this, the pseudospectral approximation is used to break down the most timeconsuming twoelectron integrals into a product of intermediate quantities depending on no more than two orbital indices. The resulting intermediate quantities are then used directly in the CI equations to substantially reduce the number of floating point operations required for diagonalization of the Hamiltonian. Additionally, our CI algorithm is based upon the symmetric group graphical approach CI (SGGACI) of Duch and Karwowski. For the purpose of developing reduced scaling CI algorithms, this approach has some important advantages. The most important of these advantages are the onthefly calculation of integral coupling coefficients and the separation of the spin and spatial parts of the wave function, which simplifies implementation of local correlation approximations. We apply the method to determine a series of binding energies in hydrocarbons and show that the approximate method predicts binding energies that are within a few kcal/mol of those predicted by the analytic nonlocal method. For large molecules, the local pseudospectral method was shown to be over 7 times as fast as the analytic nonlocal method. We also carry out a systematic study on the performance of different basis sets in the weakpairs method. It was determined that tripleζ basis sets were capable of recovering only 99.0% of the correlation energy, whereas doubleζ basis sets recovered 99.9% of the correlation energy.

Relationship between magnetic shielding and magnetizability
View Description Hide DescriptionA simple relation between magnetic shielding and magnetizability has been found. Its validity has been first shown by numerical calculations and than proven analytically. This fundamental relation shows that the magnetic shieldingtensor integrated over a whole space is simply proportional to the magnetizability tensor with a constant factor equal to twothirds of the vacuum permeability with a negative sign.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Water hexamer clusters: Structures, energies, and predicted midinfrared spectra
View Description Hide DescriptionWe present an ab initio theoretical study of five lowenergy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies and dissociation energies Møller–Plesset second order perturbation calculations using the augccpVTZ basis set at augccpVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the and levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O–H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets,liquidhelium droplets and solid parahydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8–26 K, and Book above 26 K. Several of the lowfrequency vibrational modes are identified as lowamplitude precursors of the Chair↔Book↔Cage isomerization pathways.

Atomradical reaction dynamics of Nascent rovibrational state distributions of product OH
View Description Hide DescriptionThe reaction dynamics of groundstate atomic oxygen with allyl radicals has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants and were produced by the photodissociation of and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of was observed and the nascent internal state distributions of the product OH showed substantial bimodal internal excitations of the low and high components without Λdoublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBSQB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of with is the formation of acrolein which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reactionenthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atomradical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high components implies that some fraction of reactants is sampled to proceed through the indirect shortlived additioncomplex forming process.

An exact variational method to calculate vibrational energies of five atom molecules beyond the normal mode approach
View Description Hide DescriptionA full dimensional variational algorithm to calculate vibrational energies of pentaatomic molecules is presented. The quantum mechanical Hamiltonian of the system for is derived in a set of orthogonal polyspherical coordinates in the bodyfixed frame without any dynamical approximation. The vibrational Hamiltonian has been obtained in an explicitly Hermitian form. Variational calculations are performed in a direct product discrete variable representation basis set. The sine functions are used for the radial coordinates, whereas the Legendre polynomials are employed for the polar angles. For the azimuthal angles, the symmetrically adapted Fourier–Chebyshev basis functions are utilized. The eigenvalue problem is solved by a Lanczos iterative diagonalization algorithm. The preliminary application to methane is given. A comparison with previous results is made.

Photodissociation of and related iodides in the ultraviolet
View Description Hide DescriptionThe quantum yields of production from iodobenzene and pentafluoroiodobenzene at five different dissociation wavelengths of 222, 236, 266, 280, and ∼305 nm are presented and compared with those obtained from nonaromatic cyclic iodides (i.e., cyclohexyl iodide and adamantyl iodide). The and atoms generated in the photolysis of the above iodides were monitored using a twophoton laserinduced fluorescence technique. From the measured I^{*} quantum yields, two general observations are made for aryl iodides. They are that (i) the I^{*} yield is influenced by the as well as π^{*}←π transitions at all photolysis wavelengths within the A band and (ii) there is a clear indication of a fluorine substitution effect on the dynamics of I^{*} production. The contribution from the benzene type π^{*}←π transition varies with excitation wavelength. Fluorine substitution in aryl iodides is found to increase the I^{*} quantum yield similar to what is reported in alkyl iodides. The effect of fluorine substitution is more pronounced at the red edge of the Aband excitation than at any other wavelengths. This is explained by invoking the presence of a chargetransfer band arising due to the transition of a nonbonding iodine electron to the π^{*} molecular orbital near the red edge of the A band. This chargetransfer state is coupled more strongly to the state of the transition in pentafluoroiodobenzene than in iodobenzene. The dynamics of I^{*} formation is found to be unaltered by ring strain in cyclic iodides except at the blue wing excitation. At the blue wing, Bband transitions affect the dynamics of I^{*} production in cyclic iodides, leading to the formation of more I^{*} from adamantyl iodide.

Excited and ionized states of aniline: Symmetry adapted cluster configuration interaction theoretical study
View Description Hide DescriptionSinglet excited states and ionized states of aniline are studied by the symmetry adapted cluster/configuration interaction method. Absorption bands of states that have mainly nature are assigned as in increasingenergy order. An Rydberg state is predicted to lie between the first and second valence states, in agreement with recent experimental results. The lowest band has a chargeresonance character with a slight chargetransfer (CT) character (CT is defined as third and fifth valence bands have backCT (BCT) nature, and second and fourth are local excitations within the benzene ring. The extent of CT of excited states depends on amino group conformation. In the planar form, CT characters of several states were altered; however, spectral shapes are very similar to that of the equilibrium form. On the other hand, amino group twisting altered both the spectrum and nature of excited states. Third and fourth lowest valence states exhibited strong CT character, while fifth to eighth states are of the strong BCT type, implying that the CT nature of excited states of aniline can be changed by amino group twisting. For ionized states, the lowest three states are assigned to in increasingenergy order, all being πionizations. The sixth one is also due to πionization and the others are σionizations. Ordering was the same as Koopmans’ case.

Quantum dynamics study on predissociation of Rydberg states: Importance of indirect mechanism
View Description Hide DescriptionPredissociation of Rydberg states was investigated using the effective Hamiltonian which describes the vibronic transitions among the Rydberg states as well as the predissociation through the vibrationally excited states. The motion of a Rydberg electron and the vibrations of the ion core was treated simultaneously without assuming the Born–Oppenheimer approximation. We developed the effective potential for a Rydberg electron, which contains the Coulomb potential and the exchange effect. The energies and predissociation lifetimes of Rydberg states were obtained by analyzing the effective Hamiltonian and compared with the available experimental values. The s and pRydberg states with lower vibrational excitation have lifetimes between a few ps to 1 ns and show an irregular lifetime distribution with respect to the principal quantum number. In contrast, d and fRydberg states have longer lifetime, 10 ns for example. The energy level spacings of the Rydberg states obey the distribution close to the Poisson one and thus indicates these states being regular. The route of predissociation was investigated by propagating a wave packet as well as analyzing the eigenvectors of the effective Hamiltonian. We found that the energy level matching between nearby states play an important role for efficient predissociation. The present results suggest that the predissociation of the molecule and the dissociative recombination of the ion might be efficient under rotational excitation through inclusion of additional energy levels.

Exotic isomers of dicyanoacetylene: A density functional theory and ab initio study
View Description Hide DescriptionProspects for the existence and detection of yet unknown dicyanoacetylene (NCCCCN) isomers are discussed, based on quantumchemical calculations for linear, hexagonal and branched structural variants. It is concluded that apart from dicyanoacetylene itself and its two already discovered isomers, NCCCNC and CNCCNC, at least two other species are of importance: linear CCCNCN and Yshaped CC(CN)CN (dicyanovinylidene). Combined CCSD(T) and MP4 calculations predict CC(CN)CN and CCCNCN to be 57 kcal/mol and 66 kcal/mol less stable than dicyanoacetylene, respectively. The height of the energy barrier for isomerization is about 5 kcal/mol. Density functional theory calculations indicate that CCCNCN should give rise to prominent IR absorption bands, two orders of magnitude stronger than those of dicyanoacetylene.

Laser induced reactions in a 22pole ion trap:
View Description Hide DescriptionA sensitive experimental method for ion spectroscopy and state specific reaction dynamics is described, briefly called laser induced reactions (LIR). The technique is based on (i) trapping ions over a long time in a cold 22pole rf ion trap followed by mass spectrometric detection, (ii) providing a suitable low density gas environment for collisions, (iii) modifying the low temperaturechemical kinetics using selective excitation via a tunable radiation source. In this paper, the Hatom transfer reaction is used to monitor the infrared excitation of acetylene ions. Rotationally resolved spectra are presented for the antisymmetric C–H stretching vibration. For recording a spectrum, it is sufficient to fill the trap with a few thousand parent ions. Differences with respect to conventional IR spectroscopy are discussed, especially the processes which influence the LIR signal. From the measured intensities and their dependence on parameters such as storage time, laser fluence and target gas density, information on state specific rate coefficients has been obtained at an ambient temperature of 90 K. Based on a model simulating the kinetics, rate coefficients for various inelastic and reactive collisions are derived. Vibrational excitation of increases the rate of the title reaction by more than three orders of magnitude, while rotation hinders the reaction. The finestructure state of the parent ion does not affect its reactivity. Ways are pointed out to apply the method to various classes of molecular ions.

Potential energy surface for the hydrogen abstraction reaction: Thermal and stateselected rate constants, and kinetic isotope effects
View Description Hide DescriptionThe gasphase hydrogen abstraction title reaction was carefully investigated. First, ab initio molecular orbital theory was used to study the stationary points along the reaction path: reactants, hydrogenbonded complex, saddle point, and products. Optimized geometries and harmonic vibrational frequencies were calculated at the secondorder Møller–Plesset perturbation theory level, and then singlepoint calculations were performed at a higher level of calculation: coupledcluster with triplezeta basis set. The effects of the level of calculation, zeropoint energy (ZPE), thermal corrections [TC (298.15 K)], spin–orbit coupling, and basis set superposition error (BSSE) on the energy changes were analyzed. It was concluded that at room temperature (i.e., with ZPE and TC), when the BSSE was included, the complex disappears and the activation enthalpy is +0.39 kcal mol^{−1} above the reactants. Second, an analytical potential energy surface was constructed with suitable functional forms to represent vibrational modes, and was calibrated by using experimental and theoretical stationary point properties and the tendency of the kinetic isotope effects. On this surface, the forward and reverse thermal rate constants were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range. In both cases, we found a direct dependence on temperature and, therefore, positive activation energies. The influence of the tunneling factor was very small due to the flattening of the surface in the entrance valley. This surface was also used to analyze dynamical features, such as reactionpath curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate constants. It was found that excitation of the BrH stretching mode enhances the forward reaction, whereas the excitation of the umbrella mode has the opposite effect.

Ion imaging studies of fragments arising in the visible photolysis of BrCl: Measurement of orientation, alignment, and alignmentfree anisotropy parameters
View Description Hide DescriptionThe photodissociation dynamics of jetcooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by highresolution velocity map ion imaging. Four images of the atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual selfnormalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters and and determination of the “alignmentfree” recoil anisotropy parameter, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the and states and excitations to the components of the C state contributing to the latter. The deduced values of the alignmentfree β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorptionspectrum that are currently available, and with determinations of the orientation parameter obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis.