Volume 111, Issue 15, 15 October 1999
 COMMUNICATIONS


Substituent effects and the noncrossing rule: The importance of reduced symmetry subspaces. I. The quenching of by
View Description Hide DescriptionThe effects of substituent substitution on the locus of a seam of conical intersection and the importance of conical intersections in the associated low symmetry subspaces are considered. For molecules with more than three atoms and with some symmetry the seam of conical intersection may well include an accidental symmetryallowed portion involving two states of different symmetry. However, in regions of reduced point group symmetry, conical intersections involving two states of the same symmetry may exist. This later class of conical intersections is rarely considered although it could significantly alter the predicted outcome of a nonadiabatic process. The efficient quenching of by a consequence of conical intersections, is particularly compelling in this regard. Previous analyses have considered only the accidental symmetryallowed portion of the seam of conical intersection. It is demonstrated that when intersections of states of the same symmetry are considered conical intersections with symmetry are found that are likely to play an important role in the quenching process.

A quantummechanical study of the dynamics of the reaction
View Description Hide DescriptionWe have studied the low energy quantum dynamics of the reaction. We use the hyperspherical method and a recently published ab initiopotential energy surface. We find a forward–backward symmetry in the differential cross sections which is characteristic of a complex formation. We also present rotational and vibrational integral cross sections.

Surface color centers as novel hydrogen bond acceptors
View Description Hide DescriptionIn recent combined electron paramagnetic resonance and infrared spectroscopic studies a very specific interaction between surface OH groups and surface color centers was observed on nanometersized MgO particles. The positions of the respective OH stretching bands point to hydrogen bondlike interactions. On the other hand, the bandwidths indicate that the color center electron as proton acceptor is strongly localized.
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 THEORETICAL METHODS AND ALGORITHMS


Critical analysis of the calculated frequency shifts of hydrogenbonded complexes
View Description Hide DescriptionThe frequency shift of the proton donor in hydrogen bonded complexes is an important quantity which enables to discuss the nature of the hydrogen bond. Calculations of frequency shifts by quantum chemical methods are usually performed within the harmonic approximation and therefore the comparison with experimental data is biased. We have investigated the importance of anharmonic corrections in the case of twelve complexes in which either FH or ClH is the proton donor. Hartree–Fock, Mo/ller–Plesset second and third order (MP2, MP3), density functional theory(DFT), and hybrid Hartree–Fock/DFT methods have been used for the calculations. It is shown that the anharmonic contribution to the frequency shift is rather method dependent. Its magnitude is usually 10%–20% of the total shift though it can be as large of 50% for Once anharmonicity is taken into account, most methods tend to noticeably overestimate the frequency shifts. In the case of DFT related approaches this trend is interpreted in terms of a poor description of the exchange in the intermolecular region.

Coupledcluster methods with internal and semiinternal triply excited clusters: Vibrational spectrum of the HF molecule
View Description Hide DescriptionThe recently proposed, singlereference, coupledcluster (CC) methods with singly, doubly, and triply excited clusters, in which triexcited clusters are restricted to internal and semiinternal components defined through active orbitals, designated as the and CCSDt approaches [P. Piecuch, S. A. Kucharski, and R. J. Bartlett, J. Chem. Phys. 110, 6103 (1999)] have been used to obtain the potential energy function and the vibrational spectrum of the HF molecule, as described by the large, augccpvtz, basis set. A comparison has been made with the vibrational term values obtained at the very high, full CCSDT (CC singles, doubles, and triples), level and with the experimental (RKR) data. In spite of using the restricted Hartree–Fock reference, the calculated CCSD{t^{′}} and CCSDt vibrational term values have been found to be in much better agreement with the full CCSDT and RKR data than the results of conventional CCSD (CC singles and doubles) calculations, which produce errors of an order of for highlying states near dissociation. It has been demonstrated that the CCSDt approach is capable of providing the vibrational spectrum of the full CCSDT quality.
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 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Reaction dynamics of chlorine atom with methane: Duallevel ab initio analytic potential energy surface and isotope effects
View Description Hide DescriptionAn analytic potential energy surface for the reaction in symmetry has been obtained by fitting to 1136 energy points from a duallevel MP2/SAC (Mo/ller–Plesset second order perturbation/scaling all correlation) calculation using the basis set. A zeropoint energy correction is made to account for all modes not explicitly treated with the timeindependent quantum scattering rotating line umbrella (RLU) model, which is used for the dynamics calculations. The effective potential gives a vibrationally adiabatic groundstate barrier height of 3.36 kcal/mol and an endothermicity (0 K) of 1.19 kcal/mol for the reaction, and 4.43 kcal/mol and 2.29 kcal/mol, respectively, for Thermal rate constants, tunneling and kinetic isotope effects have been investigated in detail. Calculated differential cross sections for with reactants and products in their vibrational ground states, show that the DCl product is strongly backward scattered. Further, ground state reacts to give the product predominantly unexcited at collision energies ranging from 0.15 eV to 0.25 eV. Generally, good agreement with experimental measurements and previous theoretical work is obtained.

Split operator method in hyperspherical coordinates: Application to and OClO
View Description Hide Description3D wave packet calculations on the and OClO molecules have been performed for a total angular momentum equal to zero. The initial wave function is found by solving the time independent Schrödinger equation in internal bond coordinates. The split operator method and the fast Fourier transform in hyperspherical coordinates are used in order to follow the quantum dynamics. An absorptionspectrum of is obtained and compared with a previous 2D calculation. A Raman spectrum for the molecule at 355 nm is calculated and compared with experimental results. The absorptionspectrum for the transition of the OClO molecule is calculated using the same method as for Good agreement with experiment is obtained.

Transition state spectroscopy via infrared excitation of Li⋯HF and Li⋯DF van der Waals precursors
View Description Hide DescriptionThe photoinitiated reactions after infrared excitation from the LiHF and LiDF complexes in the reactant valley are studied as an extension of a recent communication by Paniagua et al. [J. Chem. Phys. 109, 2971 (1998)]. For LiHF two broad bands, associated to and 2 transitions, are obtained at which the probability of forming LiF products is very high, >90%. For LiDF the band consists of several narrow resonances, and some of them are supported by the barrier separating reactant and product valleys. Even at these resonances the reaction probability is relatively high, starting at a value about 30% and increasing rapidly to >90% with increasing energy. This implies the tunneling through the barrier. The reason for the high efficiency in the photoinitiated reaction is that the main excitation corresponds to the HF (or DF) stretch within the complex, which is the “active” mode for the reaction in agreement with the presence of a late barrier. These results are very different from those obtained in Li+HF or Li+DF collisions at the same total energies, the reaction probabilities being much lower in these latter since the excitation of the HF (DF) mode is unlikely to occur during the collision.

Vibrationally resolved cross sections for singlephoton ionization of LiH
View Description Hide DescriptionAb initio studies of the singlephoton ionization of leading to are reported. The process is studied as function of the internuclear distance (2 a.u.–6 a.u.) and the kinetic energy of the electron (0.05 eV–10 eV). The calculations are based on the socalled iterative Schwinger approach which is implemented within the frozencore single center approximation. By comparing vibrationally resolved cross sections calculated beyond and within the Franck–Condon principle we find, that the Franck–Condon principle applies relatively well to the present photoionization process. The vibrational wave functions needed for these calculations are obtained from very accurate CIpotentials, also reported in this paper. For LiH the first 10 vibrational spacings are found to agree with the experimental data within 0.2%. Photoelectron spectra (PES) calculated under simulated experimental conditions suggest that photoionization experiments on LiH may yield information on the vibrational structure of The calculated permanent dipole moment of the LiH ground state shows strong influence of the ionpair channel

Vector signatures of adiabatic and diabatic dynamics in the photodissociation of ICN
View Description Hide DescriptionNascent Doppler profiles of CN fragments from the band photodissociation of room temperature ICN have been measured using highresolution transient frequency modulated absorption spectroscopy. Results for dissociation at 222 nm, 248 nm, 266 nm, and 308 nm are presented. From the Doppler profiles of multiple CN states, we determine branching ratios of the coincident atomic iodine states, and bipolar moments characterizing the CN velocity and angular momentumanisotropy. The measurements provide sensitive tests of the strengths of optical coupling to the excited states contributing to the band continuum, and the adiabatic and diabatic dynamics leading to the observed product states. Precise velocity measurements resolve differences in the average energy of the ICN molecules leading to selected fragment channels. We find a bond energy for ICN of somewhat higher than previous literature values.

Rotational spectrum and theoretical structure of the carbene
View Description Hide DescriptionFollowing a highlevel coupled cluster calculation, the rotational spectrum of the bent singlet carbene was detected in a supersonic molecular beam by Fourier transformmicrowave spectroscopy. The three rotational constants, the leading centrifugal distortion constants, and two nitrogen hyperfine coupling constants were determined to high accuracy. The rotational constants agree with those calculated ab initio to better than 0.5%. Like the isoelectronic carbene of similar structure, was found to have fairly large centrifugal distortion and a large inertial defect. The calculated dipole moment of is 2.95 D.

Quantum Monte Carlo calculations of molecular electron affinities: Firstrow hydrides
View Description Hide DescriptionVery accurate energies can be computed by the fixednode diffusionMonte Carlo method. They are affected only by the nodal error due to the approximate description of the nodal surfaces by the trial wave function. We examine the cancellation of nodal errors in molecular electron affinity calculations. Ground state energies of the anions of firstrow hydrides AH have been computed using the fixednode diffusionMonte Carlo method with a determinant times a correlation factor as the trial wave function. The energies are among the lowest to date. Using the energy values for the neutral molecules computed by Luchow and Anderson [A. Luchow and J. B. Anderson, J. Chem. Phys. 105, 7573 (1996)] we computed adiabatic electron affinity values and found them in agreement with the experimental data. As a consequence, the values of the anion dissociation energies are also correctly evaluated.

Unimolecular decay of the thiomethoxy cation, A computational study on the detailed mechanistic aspects
View Description Hide DescriptionThe unimolecular decay of the triplet thiomethoxy cation ion 1, has been investigated by density functional theory,ab initio, and Phase–space/Rice Ramsperger Kassel Marcus (PST/RRKM) calculations. We have first located on the singlet and triplet potential energy surfaces the energy minima and transition structures involved in the lowest energy decompositions of 1, including the loss of H, and S. We have subsequently located the minimum energy points lying on the hyperline of intersection between the singlet and triplet surfaces, using a recently described steepest descentbased method [Theor. Chem. Acc. 99, 95 (1998)]. The total energies of all these species were refined by CCSD(T)/ccpVTZ singlepoint calculations. The obtained potential energy surface has been used to outline the full kinetic scheme for the unimolecular decay of ion 1. The rate constants of the various elementary steps have been calculated by the PST and the RRKM theory. We used a nonadiabatic version of the latter to evaluate the rate constants of the elementary steps which involve a change in the total spin multiplicity. We found that the two kinetically favored decomposition channels are the loss of atomic hydrogen, with formation of and molecular hydrogen, with formation of The former process is predicted to prevail for ions 1 in the lowest rotational states and with an internal energy content of at least 60 kcal mol^{−1}. The loss of was found to be by far the prevailing process in the time scale of ca. to ca. from the formation of 1. This is fully consistent with the experimentally observed exclusive loss of by the ions which decompose in the “metastable” time window of the mass spectrometer. The loss of from ion 1 with formation of may occur by two distinct “spinforbidden” paths, i.e., a simple concerted elimination or a 1,2 H shift followed by a elimination from the singlet mercaptomethyl ion 2. In the metastable time window, these two mechanisms may occur alternatively, depending on the degree of rotational excitation of 1.

Oneparticle resonances in lowenergy electron scattering from
View Description Hide DescriptionThe quantum dynamics of lowenergy electron scattering from molecules is carried out using the full anisotropicinteraction between the molecule at its equilibrium geometry and the impinging electron. The interaction is constructed using a nonempirical model potential for the shortrange dynamical correlation between the scattered electron and the bound electrons, while both the static interaction and the nonlocal exchange interaction are treated correctly. The fairly large size of the carbon cage, and the hollow structure which is created inside it, are unique features of the molecule that are shown here to play a very important role in forming a broad variety of metastable electronattachment states without electronic excitation being involved. In spite of various aspects of the resonant process which are not considered in the present treatment, the calculations show very clearly the occurrence of several trapping processes for the scattered electrons which could be amenable to experimental observation.

First principles calculations of Si doped fullerenes: Structural and electronic localization properties in and
View Description Hide DescriptionSidoped heterofullerenes and obtained from by replacing one and two C atoms with Si atoms, are investigated via first principles calculations. Static geometry optimizations show that structural deformations occur in the vicinity of the dopant atoms and give rise to Si–C bonds significantly larger than the ordinary bonds of the fullerene cage. In the case of the lowest energy isomer has two Si atoms located at distances corresponding to third nearest neighbors. The electronic structure of these heterofullerenes, although globally close to that of is characterized by a strong localization of both the HOMO’s and the LUMO’s on the Si sites. Charge transfer occurs from the dopant atoms to the nearest neighbor C atoms, contributing to the formation of polar Si–C bonds. A detailed analysis of the charge localization, based on the electron localization function and maximally localized Wannier function approaches, reveals that the bonding of Si in the fullerene cage consists of two single and one weak double bond, thus preserving the conjugation pattern of the undoped Beside the charge localization along the bonds, we observe a peculiar region of charge localization outside the cage above each Si atom. These features are discussed in comparison with the corresponding patterns exhibited by the system.

Hexapole stateselection and orientation of asymmetric top molecules:
View Description Hide DescriptionMolecular beams of the btype asymmetric top molecule methylenefluoride were focused and rotationally stateselected with an electrostatic hexapole. The focusing behavior is mediated by the dependence of rotational energy on electric field strength, the Stark effect. The matrix quantum theory needed to calculate the rotational energies of asymmetric top molecules within an electric field are summarized. These Stark energies were calculated and parameterized for the lowest rotational levels of Excellent agreement was found between classical trajectory simulations incorporating these calculated energies and experimentally measured hexapole focusing spectra. Based on this agreement, the rotational state distribution transmitted by the hexapole as a function of hexapole voltage has been ascertained. Comparisons are made with simulated focusing spectra of the atype rotor, formaldehyde The theoretical formalism needed to describe the orientational probability distribution functions (opdf's) of hexapoleselected asymmetric top wave functions is developed and applied to the experimentally selected states of Calculated opdf's demonstrate the remarkable control we have over the orientation by varying the “orienting” field strength in the region following the hexapole selector.

Vibrational analysis of HOCl up to 98% of the dissociation energy with a Fermi resonance Hamiltonian
View Description Hide DescriptionWe have analyzed the vibrational energies and wave functions of HOCl obtained from previous ab initio calculations [J. Chem. Phys. 109, 2662 (1998); 109, 10273 (1998)]. Up to approximately 13 000 cm^{−1}, the normal modes are nearly decoupled, so that the analysis is straightforward with a Dunham model. In contrast, above 13 000 cm^{−1} the Dunham model is no longer valid for the levels with no quanta in the OH stretch In addition to these levels can only be assigned a socalled polyad quantum number where 2 and 3 denote, respectively, the bending and OCl stretching normal modes. In contrast, the levels with remain assignable with three quantum numbers up to the dissociation The interaction between the bending and the OCl stretch is well described with a simple, fitted Fermi resonance Hamiltonian. The energies and wave functions of this model Hamiltonian are compared with those obtained from ab initio calculations, which in turn enables the assignment of many additional ab initio vibrational levels. Globally, among the 809 bound levels calculated below dissociation, 790 have been assigned, the lowest unassigned level, No. 736, being located at 18 885 cm^{−1} above the (0,0,0) ground level, that is, at about 98% of In addition, 84 “resonances” located above have also been assigned. Our best Fermi resonance Hamiltonian has 29 parameters fitted with 725 ab initio levels, the rms deviation being of 5.3 cm^{−1}. This set of 725 fitted levels includes the full set of levels up to No. 702 at 18 650 cm^{−1}. The ab initio levels, which are assigned but not included in the fit, are reasonably predicted by the model Hamiltonian, but with a typical error of the order of 20 cm^{−1}. The classical analysis of the periodic orbits of this Hamiltonian shows that two bifurcations occur at 13 135 and 14 059 cm^{−1} for levels with Above each of these bifurcations two new families of periodic orbits are created. The quantum counterpart of periodic orbits are wave functions with “pearls” aligned along the classical periodic orbits. The complicated sequence of ab initiowave functions observed within each polyad is nicely reproduced by the wave functions of the Fermi resonance Hamiltonian and by the corresponding shapes of periodic orbits. We also present a comparison between calculated and measured energies and rotational constants for 25 levels, leading to a secure vibrational assignment for these levels. The largest difference between experimental and calculated energies reaches 22 cm^{−1} close to

Dynamics of collisional alignment in supersonic expansions: Trajectory studies of and
View Description Hide DescriptionClassical trajectory calculations have been performed on experimentally determined intermolecular potentials for He–CO, and in order to simulate the collisional formation of rotationally aligned molecular distributions in a supersonic expansion. These calculations verify that multiple collisions between the light “diluent” gas and heavier “seed” rotor molecules result in a distribution of rotor molecules with negative alignment i.e., a preference for j perpendicular to the expansion axis. These rotational alignment effects are found to be robustly insensitive to collision energy and qualitatively similar for all three collision systems, thereby providing a useful basis for comparison with experimental studies. The asymptotic alignment is observed to depend strongly on the angular momentum, increasing monotonically with j. When analyzed on a collisionbycollision basis, this j dependence can be traced to gyroscopic stability, i.e., higher j states are classically more resistant to the collisional loss of alignment. In addition, collisional formation of the alignment is found to reflect comparable contributions from both elasticchanging) and inelastic (jchanging) collisions. Finally, the calculations indicate that molecules with j aligned parallel to the expansion axis are correlated with faster average velocities than molecules with j perpendicular to the axis, which is consistent with the experimental studies of Harich and Wodtke [J. Chem. Phys. 107, 5983 (1997)], as well as the drift tube studies of Anthony et al. [J. Chem. Phys. 106, 5413 (1997)].

Rydberg transitions in Xray absorption spectroscopy of alkanes: The importance of matrix effects
View Description Hide DescriptionPrecise ab initio electronic structure calculations on isolated alkanes (propane, butane) and on propane embedded in a matrix of several propane molecules were used in order to characterize the nearedge xray absorption fine structure(NEXAFS)resonances of condensed saturated hydrocarbons. The results demonstrate that upon condensation the dominating NEXAFS spectral features, i.e., the Rydbergresonances located between 287.4 and 288.1 eV, have a significant blue shift from the corresponding transition energies of an isolated alkane molecule. Furthermore, the theoretical results confirm the excitonic character of the final state. Additional calculations carried out for different conformations (trans/gauche) of isolated butane demonstrate that the transition energies of the corresponding NEXAFSresonances do not differ significantly. Previously observed changes in the electronic structure of disordered thin organic films of calciumarachidate are thus attributed to changes in the intermolecular spacing of the molecules rather than to changes in the electronic structure of the different, trans and gauche, conformations of the alkane chain.

An extreme problem revisited: The hyperpolarizability of the open and cyclic forms of ozone
View Description Hide DescriptionSingle and double excitation coupledcluster [CCSD(T)] theory including an estimate of connected triple excitations by a perturbational treatment, calculations with a specially optimized basis set, consisting of 168 contracted Gaussiantype functions, yield mean values and for the first and second dipole hyperpolarizability of the open form of ozone. Due to the higher predictive capability of the CCSD(T) method and the improved completeness of the basis set, we expect these values to be more accurate than those obtained with the method and a smaller basis set [G. Maroulis, J. Chem. Phys. 101, 4949 (1994)]. The hyperpolarizability of the cyclic form of ozone, calculated with a basis set optimized for the ring structure, is and In both symmetries, electron correlation changes drastically the selfconsistent field picture of the hyperpolarizability tensor. Fourthorder Møller–Plesset perturbation theory is seen to predict very reliable values for the hyperpolarizability of the cyclic form. This is in marked contrast to the state of things brought forth by the study of the open form.
