Index of content:
Volume 105, Issue 24, 22 December 1996

Vibrational and geometric structures of Nb_{3}C_{2} and Nb_{3}C^{+} _{2} from pulsed field ionization‐zero electron kinetic energy photoelectron spectra and density functional calculations
View Description Hide DescriptionVibrational frequencies of three niobiumnormal modes of triniobium dicarbide neutral and cation have been determined from pulsed field ionization‐zero electron kinetic energy photoelectron spectra. The niobium stretching mode has a frequency of 326 cm^{−1} in the neutral and 339 cm^{−1} in the ion. The two deformation modes have frequencies of 238 and 82 cm^{−1} in the neutral and a degenerate frequency of 258 cm^{−1} in the ion. The geometry of the triniobium dicarbide has been established by comparing the experimental spectra with theoretical calculations. The cluster has a trigonal bipyramid geometry with carbon atoms capping on each face of the metal frame. The cation cluster has D _{3h } symmetry whereas the neutral cluster has lower symmetry resulting from a Jahn–Teller distortion. A second low‐lying structure with doubly bridging carbon atoms has been identified by the calculations but has not yet been observed.

Nuclear magnetic resonance polarization and coherence echoes in static and rotating solids
View Description Hide DescriptionThe mechanisms of defocusing and refocusing of spin order in extended dipolar coupled nuclear spin systems are investigated by experiments on static and on rotating solids. It is demonstrated that polarization or coherence echoes are possible also under magic‐angle sample spinning. The dipolar interactions, averaged by the spinning, are recovered by rotor‐synchronized multiple‐pulse sequences. By a simple modification of the pulse sequences, it is possible to reverse the sign of the effective dipolar Hamiltonian and to induce the refocusing of polarization or coherence. The creation of multiple‐spin order in the course of polarization evolution or free precession is monitored by a modified echo experiment. Experimental results for a polycrystalline sample of calcium formate are presented.

Vibrational wave functions and energy levels of large anharmonic clusters: A vibrational SCF study of (Ar)_{13}
View Description Hide DescriptionThe vibrational ground state and the fundamental excited states of (Ar)_{13} were studied by vibrational self‐consistent field (VSCF) calculations. These calculations treat the interaction between different modes through a mean potential approximation, and incorporate anharmonicity in full. The good accuracy of VSCF for such systems was demonstrated by test calculations for (Ar)_{3} and other clusters. The study of (Ar)_{13} focused on the properties of the wave functions and the excitation energies, on the role of the coupling between the modes and on the deviation from the harmonic approximation. It was found that SCF excitation energies for the fundamental transitions differ from the harmonic values by about 25% for the softest modes, and by about 10% for the stiffest modes. Coupling between the modes, treated by SCF, was found to be much more important than the intrinsic anharmonicity of the individual modes. For the ground state, the harmonic wave function compares well with VSCF, but for the fundamental excited states appreciable differences were found. The results for a potential field expanded to fourth‐order polynomial in the normal mode displacements are found to be valid, almost indentical with those for a more elaborate sixth‐order polynomial expansion. The fundamental excitation frequencies computed using the Aziz–Slaman Ar–Ar pair potential are very similar, with some quantitative deviations, to the values obtained with a Lennard‐Jones potential. The differences are larger for certain specific modes, and very small for the others. These calculations demonstrate the computational power of VSCF as a tool for quantum‐mechanical calculations for large clusters, at the level of specific wave functions.

Nonadiabatic corrections to the lowest EF ^{1}Σ_{ g } and I ^{1}Π_{ g } ^{−} vibrational levels of the hydrogen molecule
View Description Hide DescriptionEF ^{1}Σ_{ g } and I ^{1}Π_{ g } ^{−} electronic states of molecular hydrogen are investigated and nonadiabaticab initioenergies of lower lying vibrational levels of H_{2}, D_{2}, and T_{2} are computed. The resulting term values are compared with experiment and existing theoretical results. For the I‐state and the EF‐state levels, with energies below −0.675 a.u., the remaining discrepancies with experiment do not exceed ±0.5 cm ^{−1}. The origin of the residual errors is briefly discussed.

Autodetachment spectroscopy and dynamics of dipole bound states of negative ions: ^{2} A _{1}–^{2} B _{1} transitions of H_{2}CCC^{−}
View Description Hide DescriptionThe H_{2}CCC^{−} ion is studied by autodetachment spectroscopy in a coaxial laser‐ion beamspectrometer. Sharp resonances at photon energies near the photodetachment threshold energy are ascribed to a ^{2} A _{1}←^{2} B _{1} transition followed by autodetachment of the dipole‐bound state (DBS). Some 2500 rotational transitions are assigned and the band origin is determined to be 14 284.420(5) cm^{−1}. The observation of individual rotational lines allowed determination of the rotational spectroscopic constants as A=9.651 53(4) cm^{−1} and (B+C)/2=0.346 461(3) cm^{−1} for the DBS as well as the spin‐rotation coupling constant ε_{ aa }=2.17(6)×10^{−3} cm^{−1}. Based on an electron affinity of 14 469±64 cm^{−1}, the binding energy of the electron in the DBS is 170±50 cm^{−1}. Anomalous rotational line positions are found in the vicinity of K _{ a }=7–10 in the DBS and have been attributed to the centrifugal distortion couplings caused by mixing with the CCC out‐of‐plane bending mode (ν_{6}) and the CCC in‐plane bending mode (ν_{9}). The linewidths provide information about autodetachment rates that make it possible to obtain electron detachment dynamics for individual rotational states.

Autodetachment spectroscopy and dynamics of vibrationally excited dipole‐bound states of H_{2}CCC^{−}
View Description Hide DescriptionDirect observation of the rotational fine structure levels of a vibrationally excited negative ion dipole‐bound state (DBS) is reported. Autodetachment resonances of H_{2}CCC^{−} were observed for the ^{2} A _{1}−^{2} B _{1} transition in one quanta of ν_{6}, ν_{4}, and ν_{2} and two quanta of ν_{6} vibrational modes of the DBS. Rotational assignments for both the electronic ground state and the DBS were performed. Strong type (a) Coriolis coupling between ν_{6} and ν_{9} in both the electronic ground and excited states was observed, and coupling constants were determined. QCISD ab initio calculations were performed for the ground state, the negative ion, and the neutral state of H_{2}CCC. The calculations on the neutral agree well with measured vibrational frequencies of the dipole‐bound state. The autodetachment resonances contain information about the detachment dynamics via the observed linewidths, showing, e.g., that motions along the dipole moment axis significantly enhance autodetachment, indicating that the DBS is unstable with respect to neutral core motion which modulates the magnitude value of the dipole moment.

Site‐selective participator decay of core‐excited butadiene
View Description Hide DescriptionThe decay of core‐excited electronic states in free 1,3 trans butadiene molecules has been studied using high‐resolution synchrotron radiation and electron spectrometry. The core‐level energy shift between the terminal and central carbon atoms is 0.64 eV making selective excitation of core electrons from these atoms possible. Resonant excitation to the a _{ u }(π*) valence state leads to autoionizing decay channels which proceed according to the atomic site in the molecule. The radiationless decay is localized, and certain molecular orbitals are excluded from the decay depending upon the site of the core hole. This phenomenon is confirmed by semiempirical INDO calculations based upon the equivalent core approximation. The vibrational structure of the resonances below the carbonK edge has been measured and fit to extract vibrational energies and intensities, chemical shifts, and the lifetimes of the centrally and terminally excited states. The C 1s spectrum is also measured with vibrational resolution and the energies of the normal vibrational modes are extracted. The bond lengths are derived by application of a linear coupling analysis.

Molecular beam infrared spectroscopy of the HCCCN–HCCH and HCN–HCCCCH van der Waals complexes
View Description Hide DescriptionHigh resolution IR spectra of the linear HCCCN–HCCH and HCN–HCCCCH hydrogen bonded complexes have been obtained using optothermal detection molecular beam techniques. Two vibrational bands have been observed for each complex, which correspond to the terminal ‘‘free’’ C–H stretch vibrations (ν_{2}) of the cyano units and the hydrogen bonded vibrations (ν_{3}) of the acetylenic CH stretches. For both complexes, accurate molecular constants have been obtained. Furthermore, predissociation lifetimes for the ν_{3}=1 states of the both complexes have been determined. The results are compared with those of the linear HCN–HCCH complex obtained by Block et al. [Chem. Phys. 139, 15 (1989)].

Size dependence of the photoabsorption spectra of Ar^{+} _{ n }, n=4–25: A solvation effect on the Ar^{+} _{3} chromophore
View Description Hide DescriptionThe photoabsorptionspectra of argon cluster ions, Ar^{+} _{ n } are calculated for n=4 to 25. The internal motion of the cluster is accounted for by the molecular dynamics method. The diatomics‐ in‐molecules (DIM) potential energy surface is used for the calculation. There are basically two peaks in the spectra. At a low internal energy, the primary peak shifts from 510 to 550 nm at n≂10, and then shifts abruptly to 600 nm at n≂14. As the internal energy increases, the spectra become broad and the clear transition disappears. The spectral shift is explained by the solvation of the ion core in the cluster, with the rest of the constituent atoms acting as solvent atoms. The first red shift is due to the crossover of the energy levels between the ion core and the solvent shell. The second one takes place because the solvation energy is increased abruptly, which is explained in terms of the structural change in the solvation shell.

Bending dynamics from acetylene spectra: Normal, local, and precessional modes
View Description Hide DescriptionThe bending dynamics of acetylene are analyzed starting from spectroscopic fitting Hamiltonians used to fit experimental spectra. The possibility is considered of a transformation in the dynamics from normal to local bending modes, as well as a new kind of correlated bending motion called precessional modes. The spectroscopic fitting Hamiltonian of C_{2}H_{2} is discussed with particular attention to the coupling interactions present due to Fermi and Darling–Dennison resonances. It is argued that for analysis of experiments in which the energy is initially placed in the bends, many couplings can be neglected. Of the remaining couplings, that responsible for the primary pathway of energy transfer out of the bends is a single Darling–Dennison coupling between the bends. A Hamiltonian containing this coupling alone is analyzed to isolate the bending dynamics involved in the primary energy transfer pathway. The anharmonic modes born in bifurcations from the low‐energy normal modes are determined from analysis of the classical form of the Hamiltonian. In addition to the usual normal modes, local and precessional modes are found. Precessional modes have relative phases of π/2 or 3π/2, with one local bend fully extended while the other has maximal velocity. Sets of levels or ‘‘polyads’’ with the same total number of bend quanta are plotted in phase space on the polyad phase sphere, allowing a determination of the normal, local, or precessional character of a given quantum state. It is determined that local modes are found in the experimentally observed bend polyads with P≥14, and precessional modes are found in the polyads P≥20. Polyads are classified on the molecular catastrophe map according to their structure of normal, local, and precessional modes. Energy level spacing patterns within a polyad, shown previously to be characteristic of phase space bifurcation structure, are determined and correlated with the phase sphere. A diabatic correlation diagram analysis, previously applied to H_{2}O, is suggested to extend the analysis here of normal, local, and precessional bending states to the full multiresonance, chaotic spectral fitting Hamiltonian.

Vibrational relaxation rates of a polar molecule in polar liquids
View Description Hide DescriptionBoth the vibrational energy relaxation and pure dephasing of a polar solute in polar liquids are theoretically studied and particularly the role of the long‐range Coulomb interaction between the solute and the polarization modes of the polar liquid is focused on in this paper. If the linear coupling potential between the vibrational displacement and the solventpolarization mode is taken into account and assuming that the anharmonic contribution to the pure dephasing is the dominant mechanism, the vibrational relaxation rates are shown to be related to the dielectricfriction. However the pure dephasing of a perfect harmonic oscillator is an effect arising from the nonlinear (with respect to the vibrational coordinate) vibrational coupling potential, and the relationship between the pure dephasing rate and the frequency‐dependent friction is no longer valid. By expanding the bare electric field of the polar solute in terms of the multipoles, the vibrational coupling potential is obtained as a power series summation with respect to the vibrational displacement. Each expansion coefficient is found to be associated with the interaction of the projected multipole fields with the solventpolarization. A few simple cases are discussed in detail with an emphasis on the connection between the vibrational relaxation rates and solvation dynamics of static multipoles in polar liquids.

Computer simulation study of the Rayleigh light scattering in the isotropic phase of PCH5
View Description Hide DescriptionConfigurations of 200 p‐n‐pentyl‐(p‐cyanophenyl)‐cyclohexane (PCH5) molecules from molecular dynamics simulations were used to calculate the Rayleigh light scatteringspectra of the isotropic phase. The interaction induced contribution is calculated to the first order dipole–induced dipole terms in the point molecular polarizability approximation. The effect of the size of the simulated system is checked from different points of view. Molecular flexibility is explicitly taken into account and comparisons in terms of time correlation functions are performed. In contrast to systems of small‐sized molecules no cancellation effect is observed for the interaction induced contribution to the scattered light intensities of this mesogen. Studies of the different contributions induced by the isotropic and the anisotropic part of the molecular polarizability, evidenced the negligibility of the second one and showed the importance of orientational‐translational intermolecular correlations for spectra formation in mesogen systems. The problem of comparing anisotropic and isotropic components of Rayleigh spectra is outlined.

Nuclear quadrupole resonance line shape study of the orientationally disordered phase of p‐chloro nitrobenzene
View Description Hide DescriptionAn experimental study of the nuclear quadrupole resonance (NQR) line shape of p‐chloro nitrobenzene in the temperature range 77–150 K is reported. The molecules of p‐ClC_{6}H_{4}NO_{2} in the solid phase are arranged following an orientationally disordered pattern. The experimental NQR spectra at several temperatures are unusually broad for a molecular crystal and show a structure of peaks. Our measurements suggested that the line broadening is due to a static distribution of disordered molecules. In order to explain the nontypical resonance frequency dispersion, a simulation of the NQR line shapes is carried on. The model for the simulated crystal assumes that each molecule is an electric dipole and that the crystal is a disordered binary alloy of dipoles having two possible orientations. By calculating the intermolecular contribution to the electric field gradient (EFG) at the resonant nuclei sites, we get the expected EFG distribution in the sample. The approach we propose here explicitly involves the symmetry of the lattice.

Quantum mechanical analysis of photofragment alignment near asymmetric resonances
View Description Hide DescriptionQuantum mechanical analysis is presented for the alignment of the oxygen atoms produced from the photodissociation of OH. The alignment parameters are predicted to be independent of energy across the isolated Lorentzian resonances, when only one channel contributes to indirect dissociation. When more than one channel interferes with one another, they may change very slowly. Across the asymmetric resonances, the alignments exhibit rapid variations due to the quantum interference between the indirect and the direct dissociation pathways. The alignments of O(^{3} P _{2}) and O(^{3} P _{1}) exhibit different variations, both of which are asymmetric across the asymmetric resonances. It is also shown that photoexcitation to repulsive states, coupled with bound electronic states, can give asymmetric resonances and sharp variations of the alignment, suggesting that the analysis of the dynamics of direct photodissociation by measurements of vector properties could be complicated by the effects of quantum interference.

Vibrational relaxation and geminate recombination in the femtosecond‐photodissociation of triiodide in solution
View Description Hide DescriptionThe dynamics of product vibrational deactivation and subsequent geminate recombination of diiodide ions with atomic iodine following 400‐nm photolysis of triiodide in ethanol solution has been studied using femtosecond transient absorption spectroscopy. The excess vibrational energy of the diatomic product was found to decay on two distinct time scales. An ultrafast subpicosecond component, which accounts for the dissipation of most of the energy that is initially deposited into fragment vibrations, is followed by thermalization near the bottom of the I^{−} _{2} potential on a time scale of several picoseconds. The former process is associated with recoil of the fragments in the exit channel of the potential energy surface relevant to bond breakage whereas the latter process represents relaxation in the asymptotic limit where interaction between the atom–diatom fragments becomes negligible. Transient product vibrational distributions are determined for delay times larger than the dephasing time of nuclear coherences in the diiodide product ions, thereby providing new information about the mechanism for bond fission. These product distributions are translated into energy‐time profiles which are analyzed by a master‐equation approach using various model functions for the power spectrum of solvent forces acting on the I^{−} _{2} vibrational coordinate. The dynamics of geminate recombination are found to exhibit a strongly nonexponential character and are interpreted with a simple diffusion model that takes the initial stages of bond breakage and recoil of the fragments into account.

Translational spectroscopy studies of the photodissociation dynamics of O^{−} _{4}
View Description Hide DescriptionAn investigation of the photodissociation dynamics of the dimer anion O^{−} _{4} at 523.6, 349.0, and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation (O^{−} _{4}+hν→O_{2}+O^{−} _{2}) is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O_{2}(a ^{1}Δ_{ g }) and ground state O^{−} _{2}(X ^{2}Π_{ g }), indicating that O^{−} _{4} is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.

A position dependent friction model for solution reactions in the high friction regime: Proton transfer in triosephosphate isomerase (TIM)
View Description Hide DescriptionA position dependent friction model based on Grote–Hynes theory is developed to describe activated rate processes in the high friction regime. The model is employed to determine the transmission coefficient, which corrects the transition state theoryrate constant for recrossing of the transition state. A simple expression is derived for the transmission coefficient in the limit of a slow response of the thermal bath. The model is applied to the initial proton transfer step in the reaction catalyzed by triosephosphate isomerase, for which the standard Grote–Hynes theory was found to be inappropriate. The predictions of the position dependent friction model are in a good agreement with results of detailed molecular dynamics simulations. The method used to determine the transmission coefficient should be generally applicable to reactions that are strongly coupled to a slow thermal bath.

Tunneling currents in electron transfer reaction in proteins. II. Calculation of electronic superexchange matrix element and tunneling currents using nonorthogonal basis sets
View Description Hide DescriptionIn this paper we further develop the concept of interatomic tunneling currents [A.A. Stuchebrukhov, J. Chem. Phys. 104, 8424 (1996)] for the description of long‐range electron tunneling in proteins. Here we discuss a formulation of the theory for the case when nonorthogonality of the atomic basis set of the medium propagating electron is explicitly taken into account. This method provides an effective computational scheme for an exact, i.e., nonperturbative, evaluation (in one‐electron approximation) of the superexchange electron tunneling matrix element, and allows one to determine which regions in the protein matrix are important for the tunneling process. The theory is applied for calculation of tunneling currents and the electronic matrix element in His126‐Ru‐modified blue copper protein azurin from a recent experimental work of Gray and co‐workers. Analysis of interatomic currents reveals a nontrivial structure of the tunneling flow between donor and acceptor in the intervening protein medium in this system.

Transitions in two‐dimensional patterns in a ferrocyanide–iodate–sulfite reaction
View Description Hide DescriptionTransitions in two‐dimensional (2D) spatial patterns were investigated in a ferrocyanide–iodate–sulfite (FIS) reaction in a circular thin gel reactor. The state of the gel reactor was maintained by contact of one side of the gel with a continuously refreshed well‐stirred reservoir. For long residence times of the chemicals in the reservoir, the gel reactor was in a spatially uniform state of low pH (about 4), while at short reservoir residence times the reactor was in a uniform state of high pH (about 7). At intermediate residence times the spatiotemporal 2D structures observed include a large low pH oscillating spot, small metastable high pH oscillating spots, shrinking rings, spirals that formed when the axisymmetry of shrinking rings was broken, self‐replicating spots that either grew and divided or died from overcrowding, and highly irregular, stationary lamellae. Transitions among the different patterns were examined as a function of gel thickness (0.2–0.6 mm), reservoir residence time (0.6–4 min), and ferrocyanide concentration (12–80 mM). Iodate and sulfite concentrations were held fixed at 75.0 and 89.0 mM, respectively. Several transitions were examined in detail: from a stationary spot to an oscillating spot; from an oscillating spot to a shrinking ring or spirals; the onset of replicating spots; and the transition from a homogeneous state to lamellar patterns. The observed phenomena can all be described in terms of a parity‐breaking front bifurcation (nonequilibrium Ising‐Bloch bifurcation).

Study on ‘‘regularity’’ of barrier recrossing motion
View Description Hide DescriptionA method to scrutinize ‘‘regularity’’ of barrier recrossing dynamics of chemical reactions in the vicinity of the transition state is developed by using Lie canonical perturbation theory (LCPT). As an example, the recrossing dynamics of a four‐degrees of freedom Hamiltonian regarded as a model of proton transferreaction of malonaldehyde is investigated. It is shown that the second order LCPT is essential to describe frequent saddle recrossings whose total number of crossings is greater than three, and reproduces the time‐dependent transmission coefficient. It is found that the local recrossing dynamics can be regarded as quasiperiodic and a well‐defined reaction coordinate along which no barrier recrossings occur, can be extracted in the phase space by using the second order LCPT Hamiltonian. We also formulate a new transition state theory which allows us to estimate the reaction rate constant taking account of the barrier recrossing effect if the recrossings are near‐integrable in the short time but long enough to determine the final state of the recrossing dynamics.