Volume 117, Issue 22, 08 December 2002
Index of content:
- Theoretical Methods and Algorithms
117(2002); http://dx.doi.org/10.1063/1.1520133View Description Hide Description
An improved valence bond model, in the form of an extended Heisenberg model including next-nearest-neighbor transpositions and six-cyclic permutations, is developed to describe the low-lying “covalent” portion of the Hubbard model eigenspectrum (at half filling). The use of a cluster expansion including all diameter 1 and 2 subgraphs plus the diameter 3 six-membered ring allows the coefficients in the extended Heisenberg model to be determined from the results of accurate calculations of the Hubbard spectrum of benzene. The model is found to predict the lower energy states of two test molecules, styrene and naphthalene, rather accurately, and is thus expected to be generally applicable to benzenoid aromatic hydrocarbons.
Orbital nonrelaxed coupled cluster singles and doubles with perturbative triples corrections calculations of first-order one-electron properties117(2002); http://dx.doi.org/10.1063/1.1517991View Description Hide Description
The calculation of first-order one-electron molecular properties is discussed for an orbital nonrelaxed CCSD(T) (coupled cluster singles and doubles with perturbative triples corrections) wave function model. The conventional CCSD(T) triples amplitude equations have been generalized to contain terms that depend explicitly on the perturbation to compensate for the fact that the Hartree–Fock molecular orbitals are not allowed to relax. Results of sample calculations are presented, including the molecular electric quadrupole moment of benzene in the d-aug-cc-pVTZ basis set, which contains 564 contracted basis functions.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
The Jahn–Teller effect in the lower electronic states of benzene cation. I. Calculation of linear parameters for the modes117(2002); http://dx.doi.org/10.1063/1.1519006View Description Hide Description
Jahn–Teller (JT) coupling parameters can be simply derived from features of the potential energy surfaces of the JT active vibrational modes of a molecule subject to that effect. Potential energy curves representing cuts of symmetry have been calculated using density functional theory for each of the four active modes of benzene cation in each of the lowest three degenerate electronic states. From these curves and the vibrational frequencies, the JT coupling parameters can be found (vibrational numbering follows Wilson’s notation, coupling parameters are in Moffit’s notation). For the state the largest JT coupling parameter is provided by mode 6 followed closely by mode 8 (0.30) and then mode 9 (0.10). Mode 7 provides almost no stabilization. For the state, calculated coupling parameters are very large, particularly for modes 8 (1.36) and 6 (0.93). Modes 7 (0.10) and 9 (0.07) are smaller but finite. For the state, mode 6 has an impressive linear coupling parameter of 4.12, able to support several vibrations below the JT cusp. Indications of a substantial quadratic stabilization for that mode result in the possibility of a completely static distortion in that state. Similar, but not identical, results are obtained for
The Jahn–Teller effect in the lower electronic states of benzene cation. II. Vibrational analysis and coupling constants of the state117(2002); http://dx.doi.org/10.1063/1.1519007View Description Hide Description
The vibrational structure seen in photoinduced Rydbergionization spectra of the state of benzene cation and has been analyzed by fitting the vibrational patterns to energy levels derived from multimode Jahn–Teller calculations. Most of the structure can be ascribed to various combinations of modes 6 and 16, with minor contributions from 4, 17, and 18 (using Wilson’s numbering convention). In qualitative agreement with parameters derived from electronic structure calculations, the linear coupling parameter for mode 6 is very large in and 1.28 in This raises questions about certain angular momentum selection rules in the classical Jahn–Teller model.
An L-shaped equilibrium geometry for germanium dicarbide Interesting effects of zero-point vibration, scalar relativity, and core–valence correlation117(2002); http://dx.doi.org/10.1063/1.1518966View Description Hide Description
The ground statepotential energy surface of the molecule has been investigated at highly correlated coupled cluster levels of theory. Large basis sets including diffuse functions and functions to describe core correlation effects were employed in order to predict the true equilibrium geometry for Like the much-studied valence isoelectronic the linear L-shaped and T-shaped structures must be investigated. The L-shaped geometry is found to have real harmonic vibrational frequencies along every internal coordinate, and the linear stationary point has an imaginary vibrational frequency along the bending mode at every level of theory employed. The T-shaped geometry is found to have an imaginary vibrational frequency along the asymmetric stretching mode. At the coupled cluster with single and double excitations and perturbative triple excitations [CCSD(T)]/correlation consistent polarized valence quadrupole-ζ (cc-pVQZ) level, the nonrelativistic classical relative energies of the T-shaped and linear structures with respect to the L-shaped minimum are 0.1 and 2.8 kcal/mol, respectively. Including zero-point vibrational energy, scalar relativistic, and core-valence corrections, the T-L energy separation is shifted to 0.4 kcal/mol and the relative energy between the L-shaped and linear structures is still 2.8 kcal/mol. All nonrelativistic and relativistic computations predict that the L-shaped structure is most favored for the ground state. The linear structure is predicted to be a transition state, as the case of
117(2002); http://dx.doi.org/10.1063/1.1519001View Description Hide Description
Quantum mechanical calculations on the vibrational predissociationdynamics of are performed using an ab initio(coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations)potential energy surface. Energy positions, lifetimes, and final rotational state distributions are determined for vibrational predissociation from the two lowest linear and T-shaped van der Waals levels of Comparison with the experimental assumption as regards the energy transfer to rotation provides information about the type of isomer involved in the experimental vibrational predissociation process, suggesting that it was the linear one.
Relativistic effects on interaction-induced electric properties of weakly interacting systems: The HF…AuH dimer117(2002); http://dx.doi.org/10.1063/1.1520131View Description Hide Description
The relativistic effect on interaction-induceddipole moment and polarizability in the HF…AuH dimer is studied in the framework of the long-range perturbation theorymodel and by using the supermolecular approach. The perturbation treatment is limited to the lowest multipole-expanded form known as the dipole–induced-dipole (DID) approximation. Although the long-range DID model may be useful in elucidating the origin of the relativistic effect on interaction-inducedelectric properties, it fails to predict the correct magnitude of this effect for all but very large separations between the two subsystems. At intermediate monomer separations the supermolecular model predicts a substantial relativistic contribution to interaction-induceddipole moments and a rather moderate relativistic effect on induced polarizabilities. It has also been found that the vibrational contribution to the interaction-inducedpolarizability estimated in the double harmonic approximation may dominate over the changes in the pure electronic term. However, sufficiently accurate calculations of the vibrational contribution to electric properties of a weakly bound dimer, which would confirm this finding, appear to be unlikely at present.
117(2002); http://dx.doi.org/10.1063/1.1518004View Description Hide Description
The kinetic energy dependences of the reactions of with COS and are studied in a guided-ion beam tandem mass-spectrometer. The main products arise from sulfur transfer and subsequent losses of Fe atoms. In the case of this reactant also formally replaces one Fe atom of the cluster to form with losses of further Fe atoms at elevated energies. In addition, the kinetic energy dependences of the reactions of with Xe and are studied. The former system yields collision-induced dissociations, whereas the latter reagent effects sulfur transfer accompanied by subsequent losses of Fe atoms. Analyses of the cross sections for endothermic reactions yield the bond energies and as well as the ionization energy These values are derived with explicit consideration of the lifetimes of the energized reaction intermediates. The binding between sulfur and the cluster core strengthens as the cluster size increases, which is rationalized by simple structural arguments.
Vibrational structures of predissociating methylamines and in states: Free internal rotation of with respect to117(2002); http://dx.doi.org/10.1063/1.1518005View Description Hide Description
Resonantly-enhanced one-color two-photon (1+1) ionizationspectra of jet-cooled methylamines and reveal the vibrational structures of these molecules in predissociative states. Rotational fine structure is clearly resolved for at the origin and first wagging vibrational level in the excited state. The spectral linewidth becomes homogeneously broadened to give only vibrationally resolved spectral features for the higher vibrational energy levels of From the spectral analysis of the transition of it is found that the methyl moiety rotates nearly freely about the C–N axis with respect to the amino group in the state, indicating that the removal of an electron from the nonbonding orbital of N is responsible for the free internal rotation. Vibrational levels are only barely resolved in the excitation spectrum of due to severe homogeneous line-broadening, indicating ultrashort lifetimes of ∼0.4 ps for predissociating molecules in the state. Spectral interpretation of the excitation spectrum of is carried out by the comparison with that of giving the confirmative vibrational assignment of methylamines in states for the first time. The dramatic difference of and in their lifetimes in states suggests that the major dissociation channel of the excited methylamine may be the N–H (or D) bond dissociation.
117(2002); http://dx.doi.org/10.1063/1.1518683View Description Hide Description
A revised absolute magnetic shielding scale for oxygen is established based on a recently reported highly precise experimental measurement of the spin-rotation constant in carbon monoxide. The isotropic oxygen magnetic shielding constant for at the equilibrium geometry, is found to be The experimental rovibrationally averaged value of the shielding constant at 300 K, is in excellent agreement with the ab initio value reported by Vaara et al. [J. Chem. Phys. 109, 8388 (1998)]. Based on the revised scale and on experimentally known oxygen chemical shifts, is and is
Accuracy of the centrifugal sudden approximation in the reaction and accurate integral cross sections for the abstraction reaction117(2002); http://dx.doi.org/10.1063/1.1519009View Description Hide Description
The initial state selected time-dependent wave packet method has been extended to calculate the total reaction probability for atom-triatom reactions with total angular momentum by treating both bonds in the triatom reagent reactively. The total exchange and abstraction reaction probabilities for the title reaction with calculated with 2 -blocks (the projection of the total angular momentum on the body-fixed axis) show that one has to treat both OH bonds in the reagent reactively for the exchange reaction, but for the abstraction reaction one can treat one OH bond as a spectator bond to get accurate results. This is in accord with what had been found for the total reaction probabilities for [Phys. Rev. Lett. 89, 103201 (2002)]. The reaction probabilities also show that the CS (centrifugal sudden) approximation is inadequate for the title reaction, in particular for the abstraction reaction. The integral cross sections for the abstraction reaction, calculated without the CS approximation but by freezing one OH bond in the reagent for the ground and first vibrationally excited states, are found to be substantially larger than the corresponding CS integral cross sections. However, the integral cross section for the ground vibrational state is still much smaller than the existing experimental result. Since the treatment of freezing OH bond is not expected to introduce any noticeable error, it is conceivable that the experiments overestimated the integral cross section for the abstraction reaction.
Study of the stretching vibrational band intensities of molecules employing four-dimensional ab initio and Sn) and effective and Si) dipole moment surfaces117(2002); http://dx.doi.org/10.1063/1.1520130View Description Hide Description
Stretching vibrational band intensities of molecules were investigated employing four-dimensional ab initio and Sn) and effective and Si) dipole momentsurfaces (DMS) in combination with the local mode potential energy surfaces. The ab initio DMS of and calculated at the coupled cluster CCSD(T) level of theory reproduced most of the observed intensities within a factor of 1.5. The effective DMS of and were obtained by adjusting some selected high-order terms in the ab initio DMS to fit the observed intensities. They were applied to the corresponding deuterated isotopomers yielding better results than the ab initio DMS. The intensities of the combination bands are mainly due to the interbond cross terms in the DMS for and while for both diagonal and cross terms are important. The relatively strong combination band that has comparable intensity with the pure overtone was predicted at the fourth local mode manifold for
117(2002); http://dx.doi.org/10.1063/1.1520532View Description Hide Description
A new instanton theory for decay rate problem at zero temperature is presented. The canonically invariant expression for the lifetime of metastable state is derived. The theory is fully implemented by an effective numerical recipe to find the instanton trajectory and is applicable to any high dimensional systems.
117(2002); http://dx.doi.org/10.1063/1.1520533View Description Hide Description
We propose a very simple and efficient way to stabilize ions issued from a collision complex through a femtosecond coherently controlled pump/probe process. Starting from a van der Waals complex, one can initiate a collision at a well-defined time and with a restricted impact parameter. Formation of stable ionic complex can be achieved by ionizing the collision complex at the “right time.” We present in this paper its application to the system. ion formation is coherently controlled by ionization of colliding Na atom on molecules issued from the dissociation of NaI within Classical mechanic calculations using simple ionization/dissociation conditions can reproduce the experimental data and give an insight into the control of such a reaction.
Dynamics of OH formation in the dissociation of acrylic acid in its and transitions excited at 248 and 193 nm117(2002); http://dx.doi.org/10.1063/1.1520534View Description Hide Description
The and transitions in acrylic acid are excited by KrF (248 nm) and ArF (193 nm) laser pulses, respectively, and the dynamics of its photodissociation to give OH fragments is studied using laser induced fluorescence technique. At both the photolysis wavelengths, the OH fragments produced are vibrationally cold, but have different rotational state distributions. To get an insight into the potential energy surface involved in the dissociation process, spin–orbit and Λ-doublets ratios are also measured. Average relative translational energy partitioned into the photofragments is determined using linewidth of the Doppler profiles to be and at 193 and 248 nm excitations, respectively. High percentage of translational energy released into the photofragments suggests the presence of an exit barrier for the dissociation. On 248 nm excitation, the OH radicals are formed instantaneously during the laser pulse, while on 193 nm excitation, a risetime of ∼2 μs is seen. Another difference between the photodissociation at 193 nm and 248 nm is the observation of an intense fluorescence in UV–visible region at the former, and no fluorescence at the later wavelength. Our experimental results are compared with those obtained by recent ab initio calculations by Fang and Liu. It is concluded that when transition of acrylic acid is excited at 193 nm, the initially prepared state undergoes nonradiative transitions to and states, and from where the molecule subsequently dissociates, while excitation to transition at 248 nm leads to dissociation solely from the initially prepared state.
A new model of quantum yield in the UV photolysis of mixtures: Contributions of electronically excited and117(2002); http://dx.doi.org/10.1063/1.1516795View Description Hide Description
Electronically excited and dimer are proposed as contributors to the quantum yield, following UVphotolysis of in mixture. At 100 to 900 torr pressures, is dominated by the electronically excited In this pressure regime in the 310⩽λ⩽340 nm region could, potentially, exceed in the λ<310 nm region. The “classical” association predominates above 10 atm. The may dominate at high pressures (≳500 atm) if the temperature is very low (≲50 K). The atmospheric importance of production via the classical mechanism is well known to be insignificant. In contrast, the production from excited appears to have the potential to significantly affect our current understandings of stratospheric coupled chemistry and the climatologically important source-sink budget. It is therefore critical to determine the wavelength variation of in the 310⩽λ⩽340 nm region by gas phase experiments. Theoretical studies are needed to understand, at the quantum chemistry level, the mechanism of the suggested formation from ultra-short-lived electronically excited
117(2002); http://dx.doi.org/10.1063/1.1518026View Description Hide Description
We present a combined theoretical and experimental study of intramultiplet transitions in collisions of and with He. Relaxation rate constants have been measured using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) technique at 15 K for and over the 15–49 K temperature range for Three sets of interaction potentials have been generated for the and electronic states of CHe and SiHe systems. Quantum-mechanical calculations have been performed on these potential curves for the spin–orbit relaxation and excitation. Cross sections and rate constants are very sensitive to the interaction potential. For the system, an overall good agreement between the theoretical and experimental rate constants is found with the best quality interaction potential, while for the system experimental rate constants are much smaller than the theoretical ones.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
117(2002); http://dx.doi.org/10.1063/1.1519862View Description Hide Description
We show that the increase of the interfacefree energy with deviation from equilibrium seen in recent Monte Carlo simulations [S. Auer and D. Frenkel, Nature (London) 413, 711 (2001)] can be recovered if the molecular scale diffuseness of the crystal–liquid interface is considered. We compare two models, Gránásy’s phenomenological diffuse interfacetheory, and a density functional theory that relies on the type of Ginzburg–Landau expansion for fcc nucleation, that Shih et al. introduced for bcc crystal. It is shown that, in the range of Monte Carlo simulations, the nucleation rate of the stable fcc phase is by several orders of magnitude higher than for the metastable bcc phase, seen to nucleate first in other fcc systems. The nucleation barrier that the diffuse interfacetheories predict for small deviations from equilibrium is in far better agreement with the simulations than the classical droplet model. The behavior expected at high densities is model dependent. Gránásy’s phenomenological diffuse interfacetheory indicates a spinodal point close to glass transition, while a nonsingular behavior is predicted by the density functional theory with constant Ginzburg–Landau coefficients. Remarkably, a minimum of the nucleation barrier, similar to the one seen in polydisperse systems, occurs if the known density dependence of the Ginzburg–Landau coefficients is considered.
Effects of anharmonicity and electronic coupling on photoinduced electron transfer in mixed valence compounds117(2002); http://dx.doi.org/10.1063/1.1519258View Description Hide Description
We develop a semigroup model of electron transfer(ET) dynamics in mixed valence compounds. This model is useful for investigating the effects of anharmonicity in inner sphere nuclear modes, as well as the dependence of the electronic dynamics on the nature of the electronic coupling. Two effective “subsystem” nuclear vibrations are treated explicitly in the model, to account for the rapid electronic energy gap fluctuations induced by the inner sphere vibrations. The essentially Markovian effects of the remaining “bath” modes are approximated by semigroups. We find that including the anharmonicity in inner sphere vibrations leads to a very small increase in the rate of ET. This effect is due to the change in reactant and product vibronic states when anharmonicity is included, as well as the rapid electronic dephasing induced by the bath. An assumption of strong electronic coupling is found to be sufficient to explain experimentally observed ET rates, but the possible role of conical intersections in ultrafast ET reactions is also noted.
117(2002); http://dx.doi.org/10.1063/1.1520142View Description Hide Description
X-band electron spin resonance(ESR)spectroscopy has been applied to the study of molecular rotation of molecules in isotopic solid hydrogen, and HD. ESR signal of the molecules in hindered rotational states has been observed, and its pressure dependence has been measured up to 19 MPa. Although molar volume of solid hydrogen decreases, the rotation has become less hindered with the increase in pressure and isotope substitution from HD to These effects of pressure and isotope substitution suggest that the potential barrier for the rotation is mainly produced by distortion of cages in and HD. Since solid hydrogen becomes less compressible with the increase in pressure and the isotope substitution from HD to the rotation becomes less hindered in the less distorted cages.