Index of content:
Volume 106, Issue 1, 01 January 1997

Quantum mechanical exchange in a transition metal hydride complex: NMR data for [cp(PPh_{3})IrH_{3}]^{+} fitted by a twodimensional model
View Description Hide DescriptionA twodimensional model for hydrogen pair exchange in transition metal trihydrides is used to interpret NMR data observed for [cp(PPh_{3})IrH_{3}]^{+}. Inspired by quantum chemical results for [cp(PH_{3})IrH_{3}]^{+}, the model describes a combined process of rotational tunneling and IrH_{2} bending that merges into an H_{2} “liftoff’’ motion at a small proton–proton distance. The condensed environment with which the tunneling system interacts is represented by a heat bath. A secondorder perturbation treatment yields a master equation for the populations of the vibrational states within each of the rotational symmetry species A and B and for the respective AB coherences. A theoretical basis is provided for the evolution of the tunneling(AB)coherence as a damped oscillation in agreement with an independent treatment very recently published by Szymanski [J. Chem. Phys. 104, 8216 (1996)]. A simplified model assumption, containing one adjustable parameter, is made for the system–bath interaction. The temperaturedependent frequency of the tunneling process is found to be close to the Boltzmann average of the tunnel frequencies in the individual vibrational states. Both the calculated temperaturedependent coherence dampingrate constant and the tunnel frequency fit the experimental data after adjustment of three parameters describing the potential energy surface and of the parameter representing the system–bath interaction strength.

Comparative study of partially coherent antiStokes Raman scattering (PCARS) and Rayleigh scattering: Concentration and temperature dependence of scattered intensities
View Description Hide DescriptionThe concentration dependence of PCARS intensity has been measured for the 992 cm^{−1} band of benzene in five different solvents and compared with that of Rayleigh scattering intensity. The temperature dependence of PCARS and Rayleigh scattering has also been studied for neat benzene. The concentration dependence of PCARS and Rayleigh scattering shows a similar correlation to the refractive index difference between benzene and the solvents. It is shown that PCARS cannot be considered as the Rayleigh scattering of the (phase mismatched) CARS radiation, by comparing the concentration and temperature dependence of their intensities. The fluctuation of the thirdorder nonlinear susceptibility itself plays an important role in PCARS. This means that PCARS may bear unique information on the microscopic properties of liquids that Rayleigh scattering can not provide.

Auger decay of the molecular field split core excited states in HS radical
View Description Hide DescriptionUsing synchrotron radiation, very high resolution electron spectra at the and resonances of have been recorded. Auger transitions from molecular field split S hole states of fragment, created by fast dissociation of the core excited , are found to reveal different partial rates depending on the symmetry of the final state. The experiments have made it possible to determine, for the first time, the molecular field splitting of the core levels in a free radical. The molecular field splitting between the and levels in the fragment is found to be 90(5) meV, which significantly differs from the molecular field splitting of the “S ’’ levels in the molecule. Resonant Auger decay in the molecule before dissociation is also observed.

Nonlinear optical and vibrational properties of conjugated polyaromatic molecules
View Description Hide DescriptionRaman spectra of oligopphenylenes, oligorylenes, and oligoacenes of different chain lengths have been obtained in the solid state and in solution. Among the properties studied, particular attention is devoted to frequency and intensity dispersion of the Raman bands with increasing conjugation length and to the vibrational second order hyperpolarizability γ ^{r} . The results obtained are compared with those relative to polyenic systems. The behavior of the various classes of molecules studied is in some cases different both in absolute values and trends. This fact is discussed in order to clarify the influence of the topology of the πelectron system on the properties of conjugated materials and to determine whether the presence of aromatic rings in the main chain can confine π electrons and so reduce delocalization. Oligorylenes turn out to be the compounds with the largest vibrational γ ^{r} . The results also indicate that absolute Raman intensity shows strong intensity dispersion with conjugation length and can be used as a powerful tool in characterizing conjugated compounds.

Innershell excitation of monocyanoethylene, transdicyanoethylene, and allylcyanide by electron energy loss spectroscopy
View Description Hide DescriptionInnershell excitationspectra of gaseous monocyanoethylene, transdicyanoethylene, and allylcyanide have been measured at C and N edges using 2 keV electron collisions in quasidipolar excitation conditions and the electron energy loss spectroscopy. The energy resolution has allowed the observation of transitions from different carbon sites. Ab initio calculations have been made to help the assignment of the experimental features. The spectra below the coreelectron ionization limit have been interpreted in terms of transitions to the lowestenergy valence unoccupied molecular orbitals shown to be of π^{*} type in each molecule. The effects of the conjugation between the multiple bonds on the spectra and on the splitting of the π^{*} type molecular orbitals have been discussed. Strong electronic relaxation effects have been evidenced.

Instantaneous normal mode theory of quantum time correlation functions: Raman spectrum of liquid CS_{2}
View Description Hide DescriptionAn instantaneous normal mode (INM) theory is presented for quantum time correlation functions. It is argued that the INM formalism for classical correlation functions is particularly amenable to quantum correction. The intermolecular (Rayleigh) and allowed vibrational Raman spectra of liquid CS_{2} are calculated as an illustration. The Applequist–Quicksall polarizability model is employed, yielding the correct values for both the molecular polarizability and its derivatives with respect to the normal coordinates. Agreement with experiment is reasonable for the intermolecular Raman and for some aspects of the allowed Raman, but not for the linewidth. A brief discussion is given regarding the future developments which will be needed for an accurate INM theory of vibrational line shapes.

Dynamics of dissipation processes in the Ag–Xe complex
View Description Hide Description2ω–2ω bleaching measurements of the ground stateabsorption and 2ω–1ω fluorescence dip measurements were carried out on the s–p transitions of Ag atoms in Xe matrices by ps laser pulses with 2ω corresponding to about 320 nm and 1ω to 640 nm. The absorptionspectrum is analyzed in terms of a dynamic Jahn–Teller (JT) effect with a depth of the bound −JT state of about 30 meV and a pseudorotation frequency of about 20 ps. The −JT state seems to be rather long lived with a depopulation time of 3(±1) ps and a small energy dissipation rate of about 1 phonon per ps. Fluorescence occurs after a significant static lattice deformation accompanied by an energy relaxation of 0.4 eV. This static deformation and the energy relaxation to the emitting states proceed on a similar time scale of 3 ps which is very fast if the involved energies and the dissipation rate of about 30 phonons per ps are considered. The different rates are related to the different coupling to the lattice in the pseudorotating and the statically deformed geometry. Transient absorption is only observed in the relaxed state with σ=1.6(±0.5)⋅10^{−16} cm^{−2}.

Vector and scalar correlations in statistical dissociation: The photodissociation of NCCN at 193 nm
View Description Hide DescriptionNascent Doppler profiles of CN (X ^{2}Σ^{+}) fragments from the 193 nm photodissociation of NCCN have been measured using highresolution transient frequency modulated (FM)absorption spectroscopy. This new method is highly suited for Doppler spectroscopy of nascent photoproducts. The experimental line shapes suggest an asymptotic available energy of 5300±100 cm^{−1} and are well described by a model in which the available energy is partitioned between a statistical reservoir (4700 cm^{−1}) and a modest exit barrier (600 cm^{−1}). We have determined state dependent v⋅jcorrelations. A trend of j becoming increasingly perpendicular to v for the higher rotational states is in accord with phase space theory, although the observed correlations are more than twice as strong. The v⋅jcorrelations can be quantitatively modeled by further restricting the phase space model with an approximate conservation of the Kquantum number, the projection of total angular momentum about the linear axis of NCCN. Global rotational and vibrational product distributions have also been measured. The highest accessible rotational states are underpopulated, compared to a phase space calculation. The global vibrational distribution is substantially colder than the phase space theory predictions. Vibrational branching ratios for coincident fragments have been measured as a function of the detected CN state from a close analysis of high signaltonoise Doppler profiles. The correlated vibrational distribution, P(v_{1},v_{2}), shows an excess of vibrationless coincident fragments, at the expense of dissociation to give one ground state and one vibrationally excited CN fragment. The correlated formation of two vibrationally excited CN fragments is as likely as the phase space prediction, yet the formation of v=2 is strongly suppressed. The fragment vector and scalar correlations provide a highly detailed view of the loose transition state typical for reactions well described by statistical reactiontheories.

Photofragmentation dynamics of Mg_{2}H_{2}O^{+}
View Description Hide DescriptionWe report studies of the structure and dissociationdynamics of the Mg_{2}H_{2}O^{+} ion–molecule complex. The weakly bound clusters are formed in a supersonic molecular beam equipped with a laser vaporizationsource, massselected, and studied by laser photofragmentation spectroscopy in a tandem timeofflight mass spectrometer. Broad structureless molecular absorption bands are observed in the red (610–745 nm), the green (515–595 nm), and the near UV (335–390 nm) spectral regions, and are assigned to transitions localized on the Mg_{2} ^{+} chromophore. Three daughter ions (Mg^{+}, Mg, and MgH_{2} ^{+}) have been observed. We have studied the competitive branching into accessible product channels as a function of photolysis wavelength in these bands. In order to understand the dissociation mechanisms we have carried out ab initio calculations of the ground and low lying excited states of the Mg_{2}H_{2}O^{+} complex. These studies give insight into the dissociationdynamics. Results from these experiments are compared and contrasted with previous work on the photodissociation of Mg_{2}CO.

A 193nmlaser photofragmentation timeofflight mass spectrometric study of dimethylsulfoxide
View Description Hide DescriptionThe photodissociation of dimethylsulfoxide [(CH_{3})_{2}SO] at 193.3 nm has been investigated using the molecular beamtimeofflight(TOF) mass spectrometric technique. In addition to CH_{3} and SO, CH_{3}SO is also observed as a stable primary product, indicating that CH_{3}SO+CH_{3} is an important product channel for the 193.3 nm photodissociation of (CH_{3})_{2}SO. The analysis of the TOF data provides evidence that SO is formed via a stepwise mechanism: (CH_{3})_{2}SO+hν (193.3 nm)→CH_{3}SO+CH_{3}→2CH_{3}+SO. The analysis also indicates that ≈53% of the primary CH_{3}SO radicals undergo further dissociation to produce CH_{3}+SO, yielding a quantum yield of ≈1.53 for CH_{3}. Within the sensitivity of our experiment, the product channel of CH_{3}SCH_{3}+O is not found. The angular distribution for the formation of CH_{3}SO+CH_{3} is found to be isotropic, an observation consistent with a predissociation mechanism, in which the dissociation of photoexcited (CH_{3})_{2}SO is slow compared to its rotational period. The energetics for selected dissociationreactions of (CH_{3})_{2}SO have also been investigated by ab initio calculations at the G2(MP2) level of theory. The experimental dissociation energy at 0 K (53±2 kcal/mol) for the CH_{3}–SOCH_{3} bond obtained here is in excellent agreement with the theoretical prediction of 52.6 kcal/mol.

A Fermi Golden Rule, Liouvillespace approach to the study of intramolecular electron transfer rate in solution
View Description Hide DescriptionWe consider the problem of the calculation of the intramolecular electron transfer(ET) rate for molecules in solution and focus on the case of rapid, almost activationless processes. We assume that the weakcoupling, nonadiabatic limit holds and utilize the Fermi golden rule expression for the ET rate, avoiding the introduction of phenomenological data. The Fermi Golden Rule is elaborated in the Liouville space formalism taking into account at second order the coupling to the bath of instantaneous normal modes (INM) of the solvent as well as to the intramolecular bath responsible for relaxation in the isolated molecule. The couplings among the principal modes (the ones more directly involved in the ET process), mainly intramolecular, are taken into account exactly. The main inputs are weighted densities of states which can be, at least in principle, calculated. For those concerning the solvent we take advantage from the recent progresses in the INM approach to the description of the short time dynamics. We compute the ET line shape (i.e., the ET rate as a function of the electronic energy gapE) for some model cases, with one, two or more principal modes, investigating the influence of the solvent and of the temperature. The ET rates show a complex, but not dramatic, dependence on the solvent and are quite sensitive to the energy gapE. The temperature dependence is generally weak. The results seem to be in general agreement with recent experimental data on molecular systems exhibiting rapid ET.

Vibrational excitation of ammonia clusters by helium atom scattering
View Description Hide DescriptionIn a crossed molecular beam arrangement helium atoms are scattered from ammonia clusters of the averaged sizes , and 1040 which are generated by isentropic expansions with conical nozzles. The inelastic energy transfer is detected by timeofflight analysis of the scattered helium atoms with a resolution of less than 5 meV at a collision energy of 95 meV. The energy transfer increases with increasing deflection angle and extends to 65 meV. Intensity maxima are observed between 11 and 16 meV, around 27 meV, and at 33 meV. The one in the middle is attributed to the vibration of a specific cluster network, while the other two occur in the energy regime of the translational or librational modes of the solid with a preference for small or large clusters, respectively.

Quantum energy gap law of outersphere electron transfer reactions: A molecular dynamics study on aqueous solution
View Description Hide DescriptionThe quantum energy gap law for electron transfer(ET)reactions in water is examined. Molecular dynamics (MD) simulation analysis is carried out to obtain the solvent reorganization energies, time correlation functions (TCF), spectral density functions, and quantum rate constants. Their dependence on the reactionfree energy and on the donor–acceptor distance is explored along with the solvent isotope effects. Properties of the imaginarytime saddlepoint for the TCF expression of the ET rate formula are also examined. The highfrequency intramolecular vibrational modes of the solvent water are found to present marked quantum effects on the ET rate, while their contribution to the static reorganization energy is small (less than 6%). The energy gap dependence of the quantum activation free energy is shown to become nearly independent of the donor–acceptor distance when renormalized by the reorganization energy. Approximations to compute quantum rate constants from MD simulation data are briefly discussed in light of the present results.

Inelastic collision dynamics of vibrationally excited
View Description Hide DescriptionRotational and vibrational energy transfer rate constants have been measured for excited rovibrational levels of . Stimulated emissionpumping was used to excite the levels , , and , via the – transition. Laser induced fluorescence from the – system was used to follow the collision dynamics. Energy transfer processes induced by collisions with , and were investigated. Rotational energy transfer was found to be efficient for all collision partners. In accordance with classical models, the total rotational transfer rate constants were proportional to the collision momentum (except for . The total transfer rate constants and the distributions of rotational levels populated by collisions were not dependent on the initial vibrational state. For colliders that are not good quenchers of , the rotational energy transferdynamics of the and states were found to be very similar. For colliders that are good quenchers, comparisons of the and state dynamics show that quenching competes with rotational energy transfer in the state. Vibrational energy transfer was characterized for all collision partners with the exception of , which appears to have a low vibrational transfer efficiency. Vibrational transfer was dominated by steps. Multiquantum vibrational transfer was not observed. The dependence of the vibrational transfer rate constants on the initial vibrational state appeared to be weaker than the linear scaling predicted by the Landau–Teller model. Vibrational deactivation of plays an important role in chemically driven oxygen–iodine lasers. Effective deactivation rate constants have been derived from the vibrational transfer rate constants. Estimates for the deactivation rate constants for and differ from those currently in use by almost an order of magnitude.

On the “direct” calculation of thermal rate constants. II. The fluxflux autocorrelation function with absorbing potentials, with application to the O+HCl→OH+Cl reaction
View Description Hide DescriptionWe present a method for obtaining the thermal rate constant directly (i.e., without first solving the statetostate reactive scattering problem) from the time integral of the fluxflux autocorrelation function, . The quantum mechanical trace involved in calculating is efficiently evaluated by taking advantage of the low rank of the Boltzmannized flux operator. The time propagation is carried out with a Hamiltonian which includes imaginary absorbing potentials in the reactant and product exit channels. These potentials eliminate reflection from the edge of the finite basis and ensure that goes to zero at long times. In addition, the basis can then be contracted to represent a smaller area around the interaction region. We present results of this method applied to the O+HCl reaction using the shifting and helicity conserving approximations to include nonzero total angular momentum. The calculated rate constants are compared to experimental and previous theoretical results. Finally, the effect of deuteration (the O+DCl reaction) on the rate constant is examined.

Diazasiline (SiNN): Is there a conflict between experiment and theory?
View Description Hide DescriptionThe molecule SiNN was investigated with the highly correlated coupled cluster method with single and double excitations and corrections for the connected triples [CCSD(T)], and with the multireference single and double excitations configuration interaction approach (MRCI). A series of basis sets ranging from the simple 631G^{*}, 6311G^{*}, and Dunning’s doublezeta (DZ) plus polarizationbasis sets to the more extended correlated consistent ccpVTZ and ccpVQZ ones were employed to check the dependence of the geometry optimization and of the frequency evaluation on the basis sets. Our highest level result of 1859 cm^{−1} for the harmonic stretching frequency of the NN bond differs considerably from a previous CCSD(T)/DZ prediction of 1726 cm^{−1}. In fact, most of the results analyzed in this study point to a frequency value greater than 1810 cm^{−1}. Reexamining the scant experimental evidence, we estimate the harmonic frequency to be very close to 1830 cm^{−1}. Surprisingly, an extended internally contracted MRCI calculation with the ccpVTZ basis predicts a frequency 144 cm^{−1} higher than the corresponding CCSD(T)/ccpVTZ result. Comparisons with existing density functional studies and with a previous MRCI calculation are also carried out. At the CCSD(T)/ccpVTZ level, other isomers of SiN_{2} are further investigated for the first time. One symmetric linear (^{3}∑), and one symmetric bent (^{1} A _{1}) structure are found to lie relatively high in energy: 84.60 and 102.23 kcal/mol, respectively, relative to SiNN (^{3}Σ^{−}). However, one cyclic ^{1} A _{1} and the singlet asymmetric isomer SiNN (^{1}Σ^{+}) are only 6.09 and 15.81 kcal/mol above the global minimum. Although the higher frequencies of the former isomers do not fall in the region of relevance to the experimental assignment discussed in this work, that of the lowest lying ^{1} A _{1} state (1850 cm^{−1}) is practically identical to the CCSD(T)/ccpVTZ frequency computed for the ^{3}Σ^{−} state.

Theoretical investigation of structural and thermodynamic properties of lanthanum carbides
View Description Hide DescriptionTheoretical studies of monolanthanum carbides, for n=2–6, are presented. The fan structures were found as ground states in most cases studied. The computed enthalpies of formation of and atomization energies of these species are close to the corresponding experimental data. The agreement is even closer when experimental Gibbs energy functions are corrected using theoretical ground state structures and partition functions. The La–C bond is strongly ionic due to electronic charge transfer from lanthanum to carbon atoms.

Orbital hardness matrix and Fukui indices, their direct selfconsistentfield calculations, and a derivation of localized Kohn–Sham orbitals
View Description Hide DescriptionFormulas governing fixed orbital hardnesses and their relation to the hardness kernel are derived. It is shown how the orbital hardness matrix and its inverse matrix, the orbital softness matrix, may thus be directly calculated, and then the total chemical hardness, softness, and electronegativity of a molecular species. These quantities are calculated for the molecule HCN, using Dirac exchange and von Barth–Hedin correlation in the local density form of Kohn–Sham theory. The result complies with the frontier orbital theory. As quantitative indicators of orbital reactivity, the frontier orbital softness and Fukui indices generally have larger values than inner electron orbitals. The relation of orbital hardness matrix elements to the twoelectron orbital integrals in a typical molecular orbital calculation is discussed, and it is demonstrated that diagonalization of the orbital hardness matrix leads to orbitals more localized than conventional Kohn–Sham orbitals.

Ab initio potential energy surfaces and nonadiabatic couplings involved in Be^{4+}+H_{2} electron rearrangement
View Description Hide DescriptionWe present the main characteristics of the energy and coupling surfaces for the BeHquasimolecule, that are relevant to the dynamics of electron capture in Be^{4+}+H_{2} collisions in the 50 eV amu^{−1}<E<1 keV amu^{−1} energy range. To construct the wave functions, we implemented a blockdiagonalization method using the many electron description standard (MELD) program, which was recently modified to calculate nonadiabatic couplings.

Calculation of twocenter integrals involving a rapidly oscillating free electron wave function
View Description Hide DescriptionOptical potentials are used in a quantum mechanical treatment of loss processes, e.g., ionization, where the loss of flux is described by the imaginary part. We present a numerical method for calculating twocenter twoelectron integrals necessary to construct the imaginary part of the optical potential. By introducing Slatertype orbitals with complexvalued exponents (CSTOs), we are able to represent the free electron wave with a limited number of CSTOs. For the representation of free electron wave functions with many oscillations, i.e., in a large r range or for a high kinetic energy, these new CSTOs form a more natural set of basis functions. The introduction of CSTOs is inevitable for the calculation of integrals concerning collisions in the mK energy range, where the interaction acts over large internuclear distances. Extensive numerical checks show that the final imaginary part of the optical potentials can be calculated with an accuracy better than 2%.