Volume 103, Issue 20, 22 November 1995
Index of content:

Excitonic interactions and excimer formation in pure and mixed cluster isotopomers of naphthalene
View Description Hide DescriptionThe nature of electronic excitation and subsequent excited‐state dynamics in small naphthalene clusters has been elucidated using isotope labeling techniques. The S _{1}←S _{0} spectra of tetramer isotopomers are characterized by structurally inequivalent site splitting which is induced by an exciton interaction in the S _{2} state through vibronic coupling. It is suggested that the tetramer involves a pair of nearly overlapped chromophores, which is responsible for the excitonic interaction, and other two chromophores acting as solvents. An excitonic state originating in this pair is proposed to correlate with the lowest excimer state of the cluster as the interplanar separation and dihedral angle are reduced. In contrast, the spectra of trimer isotopomers are analyzed by invoking a symmetric geometry in which three chromophores are less overlapped and thus give rise to weak excitonic effects. The isomerizationdynamics of these clusters is discussed in terms of a vibrationally activated process which makes the cluster framework wobble, allowing for stronger exciton interaction.

Electronic absorption spectra of linear carbon chains in neon matrices. III. HC_{2n+1}H
View Description Hide DescriptionThe electronic absorption spectra of HC_{2n+1}H and HC_{2n+1}H^{+} (n=2−7) in neon matrices have been obtained. The species were prepared by codeposition of mass selected HC_{2n+1}H^{+} ions with neon at 5 K. For each species the 0^{0} _{0} band of the electronic transition as well as frequencies of fundamental vibrations in the excited state have been determined. The wavelength of the origin band in this homologous series is linearly dependent on the number of carbon atoms. Isotopic labeling supports the vibrational assignment and confirms the presence of two equivalent hydrogen atoms for HC_{2n+1}H and HC_{2n+1}H^{+}. Based on spectroscopic considerations, a linear geometry is assumed with one hydrogen atom located at each end, and that the observed spectra of HC_{2n+1}H are their ^{3}Σ_{ u } ^{−}←X ^{3}Σ_{ g } ^{−} transition.

Xe_{ n } clusters in the alpha cages of zeolite KA
View Description Hide DescriptionWe have observed the individual signals of the Xe_{ n } clusters (n=1–5) trapped in the alpha cages of zeolite KA. The ^{129}Xe NMRchemical shift of each cluster in zeolite KA is larger than that of the corresponding Xe_{ n } cluster in zeoliteNaA. The temperature dependence of the chemical shifts of the clusters vary systematically with cluster size as they do in NaA, but the change of the temperature coefficients with n is somewhat more pronounced for Xe_{ n } in the cages of KA than in NaA. The Xe_{ n }chemical shifts and their variation with temperature are reproduced by the grand canonical Monte Carlo (GCMC) simulations. GCMC simulations of the distribution of the Xe atoms among the alpha cages in KA provide the fractions of cages containing n Xe atoms which agree reasonably well with the observed equilibrium distributions. The characteristics of Xe distribution and chemical shifts in KA are compared with that in NaA.

Observation of characteristic, polarity‐dependent, Doppler shifts from neutral species in the positive column of a discharge plasma
View Description Hide DescriptionDoppler shifts of emissions from neutral species have been observed for the first time in a positive column of a dc glow discharge. For a detailed study, we have chosen emissions of H_{2} and its isotopomers in a hydrogen discharge using a high resolution Fourier‐transform spectrometer. Experimetal evidence supports the conclusion that the observed shifts derive from momentum transfer during electron‐molecule collisions that produce the emissive excited states. A theoreticalmodel has been formulated to relate Doppler shifts to the drift velocity of electrons in the discharge as well as to the momentum transfer of electron‐molecule collisions. Predictions based on the model are consistent with the experimental observations.

High resolution infrared molecular beam spectroscopy of cyanoacetylene clusters
View Description Hide DescriptionHigh resolution infrared optothermal spectroscopy has been used to study small clusters of cyanoacetylene, (HCCCN)_{ n }, containing three or more monomer units. For the linear HCCCN trimer the fundamental of the free C–H stretch vibration has been rotationally resolved and analyzed, yielding a ground vibrational staterotational constant (B _{0}) of 94.031(13) MHz. In addition, an inner C–H stretch fundamental band of the linear trimer (rotationally unresolved) has been identified by observing the effect of large electric fields on the band shape. This assignment is also consistent with spectral intensity measurements as a function of molecular beam stagnation pressure. Predissociation lifetimes of the upper states of these two bands were determined from the observed homogeneous linewidths. Several other rotationally unresolved vibrational bands have also been observed. The stagnation pressure and electric field dependence of these spectra are used to estimate the cluster sizes and to make tentative structural assignments.

Photoinduced electron transfer reactions in mixed films of π‐conjugated polymers and a homologous series of tetracyano‐p‐quinodimethane derivatives
View Description Hide DescriptionNear‐steady‐state photoinduced absorption (PIA) and photoluminescence studies are presented on photoinduced electron transferreactions from poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylene vinylene] (MEH‐PPV) and poly[3‐(2‐(3‐methylbutoxy)ethyl)thiophene] (P3MBET) as donors (D) onto a homologous series of tetracyano‐p‐quinodimethane (TCNQ) derivatives containing fused aromatic rings as acceptors (A) to systematically study the effect of acceptor electron affinity. We observe that composite films of these D/A couples give rise to long‐lived charge separated states upon photoexcitation, as evidenced from the formation of polaron bands (radical cation absorption) in the PIA and the concomitant loss of the metastable triplet PIA and photoluminescence that are observed in pristine MEH‐PPV and P3MBET. We find that the efficiency of the photoinduced electron transferreaction correlates with the reduction potential of the acceptors.

Rotational analysis of the ν_{1} band of trichlorofluoromethane from high resolution Fourier transform and diode laser spectra of supersonic jets and isotopically enriched samples
View Description Hide DescriptionThe spectrum of CCl_{3}F (trichlorofluoromethane, CFC 11) has been measured in the region of the ν_{1} fundamental (1050–1120 cm^{−1}) by high resolution interferometric Fourier‐transform spectroscopy [0.004 cm^{−1} bandwidth full width at half maximum, apodized] and by diode laser spectroscopy (bandwidth 0.0008–0.0020 cm^{−1} FWHM) at room temperature, in cold cells and in supersonic jet expansions. Fourier‐transform infrared and diode laserspectra of isotopically pure C^{35}Cl_{3}F have been recorded at room temperature in static cells. The C^{35}Cl_{3}F spectra allowed an analysis of the rotational structure to be started successfully for the first time. The rotational analysis of the isotopic species C^{35}Cl_{2} ^{37}ClF and C^{35}Cl^{37}Cl_{2}F, which are asymmetric rotors, was initiated from diode laserspectra of natural CCl_{3}F with a rotational temperature of about 20 K (5% seeded in He) resulting from expansion in a supersonic pulsed slit jet. The rotational analysis yielded effective Hamiltonian constants including accurate band centers for the three most abundant isotopomers C^{35}Cl_{3}F (ν_{1}=1081.2801 cm^{−1}), C^{35}Cl_{2} ^{37}ClF (ν_{1}=1080.7330 cm^{−1}), and C^{35}Cl^{37}Cl_{2}F (ν_{1}=1080.0663 cm^{−1}), excited state rotational constants, and for C^{35}Cl_{3}F, quartic centrifugal distortion constants. The results are important for and discussed in relation to simulations of atmospheric absorption, line coincidences with CO_{2} lasers for sub‐Doppler spectroscopy, as well as IR multiphoton excitation and infrared laser chemistry.

Computer simulation study of Rayleigh and Raman spectra and spectral moments of fluid C_{2}H_{6}
View Description Hide DescriptionDepolarized Rayleigh and Raman scattering from fluids is composed of orientational (OR), collision induced (CI), and cross (X) contributions. In order to investigate the influence of the interaction induced terms (CI and X) on allowed spectra (OR), C_{2}H_{6}, a molecule with a small ratio of the anisotropic part γ to the isotropic part α of the polarizability is considered. Both Rayleigh and ν_{3} Raman correlation functions were determined by molecular dynamics (MD) computer simulation and compared with experiment. The investigations extend over a wide range of densities (0.67ρ_{ c }–2.85ρ_{ c }, where ρ_{ c } is the critical density, ρ_{ c }=0.206 g/cm^{3}). The different contributions to the time correlation functions (TCFs) were simulated within the frame of a first‐order center–center dipole–induced‐dipole polarizability model. The potential model used was a standard two‐site Lennard‐Jones potential. Details concerning partial OR, CI, and Xcorrelation functions and second moments for Rayleigh and Raman scattering are presented. At the lower densities a large CI and a smaller X contribution to the total Rayleigh TCF is found. At higher densities the X and CI contributions increase but tend to cancel each other so that the OR contribution becomes more important. Concerning the Rayleigh second spectral moments an explanation of the minimum of the experimental second moment observed at intermediate densities and of the increase of the second moment with density for higher densities is presented. Both the Raman TCFs and second moments are essentially orientational due to the large polarizabilityanisotropy derivative γ′.

Photoinduced electron transfer and geminate recombination in liquids: Analytical theory and Monte Carlo simulations
View Description Hide DescriptionPhotoinduced electron transfer and geminate recombination in liquid solution are addressed with analytical theory and Monte Carlo simulations. The time‐dependent probabilities of the donor being excited and of an ion pair existing are obtained for a system of a donor and many acceptors undergoing diffusive motion. Multiparticle simulations are modeled as a Markov chain and are shown to agree with the analytical formalism presented previously. The calculations are performed using both a simple exponential form of the distance dependence of the transfer rate and using the more general Marcus distance‐dependent transfer rate. For a static donor, in the absence of acceptor–acceptor excluded volume, theory and simulations provide identical results, confirming the accuracy of the analytical method. For the calculation of properties of real systems in which both the donor and acceptors diffuse, to make the mathematics tractable, the donor is held static and each acceptor is given a Fick diffusion constant equal to the sum of the diffusion constants of the donor and acceptor, D=D _{ d }+D _{ a }. The validity of this approximation is examined in the absence of acceptor–acceptor excluded volume and found to work extremely well under all conditions. It is also examined with acceptor–acceptor excluded volume. In this case, the static donor approximation is found to work generally well up to moderately high acceptor concentrations, <5% packing fraction. However, the results suggest that at even higher packing fractions, the static donor approximation loses its validity.

Rate expressions for excitation transfer. IV. Energy migration and superexchange phenomena
View Description Hide DescriptionGeneral microscopic mechanisms of electronic excitation (energy) transfer (EET) in multichromophoric assemblies are investigated. Aspects of superexchange‐mediated EET and energy migration (EM) and their contribution to the efficiency of donor‐to‐trap energytransport processes in macromolecules are discussed from a quantum mechanical viewpoint. The possibility of superexchange pathways for EM via higher excited states of the intermediate chromophores is introduced. The role of quasicoherent EM pathways, and how they are manifested in the quantum mechanical rate expression, is investigated and the significance of contributions to the rate arising through quantum mechanical interference between pathways is elucidated. The theory indicates conditions under which coherent EM pathways may significantly increase the efficiency of energy transport and trapping and the applications to natural and synthetic light‐harvesting systems are outlined.

Theoretical study of the effect of reagent rotation on the reaction of F+H_{2}(ν=0,J)
View Description Hide DescriptionQuasiclassical calculations on the Muckerman V potential energy surface were carried out on the reaction F+H_{2}(ν=0, J)→FH+H at a relative energy of 2 kcal/mol for J=0 to 10. This surface is, to use the classification of Levine and co‐workers, very oblate; for a given distance R _{c.m.} from F to the center of mass of H_{2} the potential energy is much lower for the collinear configuration (χ=0) than for the perpendicular configuration (χ=90 deg). The goal of the work was to understand the effect of molecular rotation on such an extremely oblate surface. It proved useful to decompose the reactive cross section Q _{ R }(J) into the product of a hitting cross section Q ^{≠} _{hit}(J) for F hitting H_{2} times the probability P _{ R }(J) of reaction occurring once F hits H_{2}. Both Q ^{≠} _{hit}(J) and P _{ R }(J) go through minima at J≊4–5. We determined that Q ^{≠} _{hit}(J=0) is increased by about a factor of 2 by ‘‘reorientation’’ of the H_{2} molecule towards a linear configuration by the F atom as it approaches. For J≳0 Q ^{≠} _{hit}(J) declines due both to loss of this reorientation effect as well as to the more oblique approach of the trajectory to the reactant valley. Many trajectories bounce off the repulsive wall near χ=90 deg before the F atom can hit H_{2}; this effect has been discussed by other authors. The initial decline of P _{ R }(J) with J is due to a relatively unusual feature of the potential surface, whereby rotation of the H_{2} molecule away from a linear F–H–H configuration can easily switch the system from the product region back to the reactant region of the system. Both Q ^{≠} _{hit}(J) and P _{ R }(J) increase above J=5 because the H_{2} molecule now has enough rotational energy to rotate through the barrier at χ=90 deg rather than bounce off it.

Comparison of quantum and semiclassical variational transition state models for the HO_{2}→H+O_{2} microcanonical rate constant
View Description Hide DescriptionComparisons, for J=0, are made between HO_{2}→H+O_{2}reaction path anharmonic energy levels, variational transition states, and unimolecular rate constants determined by three different semiclassical models and a quantum mechanical model. The semiclassical models are based on the reaction path Hamiltonian. However, to determine anharmonic energy levels, the harmonic potential of this Hamiltonian is replaced by the actual anharmonic DMBE IV potential for the HO_{2} system. Two of the semiclassical models use Einstein–Brillouin–Keller (EBK) quantization to determine energy levels for motion orthogonal to the reaction path; i.e., one model neglects anharmonic coupling between modes, while the other retains all the coupling. The third semiclassical model is based on a quartic expansion of the potential and second‐order perturbation theory to determine the energy levels. A comparison of the results of these three semiclassical models shows that anharmonic coupling between modes orthogonal to the reaction path is unimportant for HO_{2}dissociation. The separable EBK model gives a RRKM rate constant versus energy in very good agreement with that obtained from a quantum mechanical calculation which retains full coupling between modes in determining the reaction path energy levels. If anharmonicity is treated, the reaction path Hamiltonian and its vibrator transition state give accurate RRKM rate constants for HO_{2}dissociation.Rate constants calculated with the flexible transition states model are in very good agreement with those of the semiclassical and quantum vibrator transition state models, if the O_{2} stretch conserved mode is treated as an anharmonic oscillator in the flexible model. However, in contrast to the vibrator transition state models, ‘‘steps’’ are not observed in the rate constants for the flexible model, since the transitional mode is treated classically. Harmonic and anharmonic rate constants are compared for both the vibrator and flexible transition state models.

Reaction dynamics of O(^{1} D) with HCN(0 0^{0} v _{3})
View Description Hide DescriptionThe quantum state resolved reaction dynamics of HCN(0 0^{0} v _{3}) with O(^{1} D) atoms were investigated by analyzing the complete product state distributions of OH(X ^{2}Π_{Ω},v,J) and CN(X ^{2}Σ^{+},v,J) using laser induced fluorescence(LIF). The influence of the CH‐stretching mode on the reaction dynamics and different branching ratios was inspected by exciting HCN to its first overtone band of the ν_{3} CH stretch in the 1.5 μm region. The oxygen atom in the ^{1} D state was generated in a laser photolysis of ozone at a wavelength of 266 nm. The measured rotational and vibrational distributions of the products were compared with statistical results from phase space theory (PST). Nearly statistical rotational and vibrational distributions are obtained for CN(X ^{2}Σ^{+}) in v=0–3. The rotational and vibrational distributions of OH(X ^{2}Π_{Ω}) are colder than statistically expected. Insertion of O into the CN bond with subsequent hydrogen migration seems to be a better characterization of the the reaction mechanism than an insertion of the oxygen atom into the CH bond. Direct abstraction of hydrogen to form OH is improbable to describe the molecular process.

Activated rate processes: Anharmonic corrections to the quantum rate
View Description Hide DescriptionEscape of a particle from a metastable potential, whose motion is governed by the generalized Langevin equation, is a common model of many chemical and physical activated processes in condensed phase. In the intermediate‐to‐strong damping regime the rate of escape is controlled by the particle dynamics near the barrier top. Since Kramers, the parabolic barrier approximation is commonly used to get the expression for the rate in this regime. We consider the influence of anharmonic corrections to the potential barrier on the quantum rate and get leading order corrections in terms of the inverse barrier height. New terms appearing in the quantum expression for the rate are associated with tunneling through the barrier and become important at low temperatures. The analytic theory is compared with recent numerically exact quantum simulations [M. Topaler and N. Makri, J. Chem. Phys. 101, 7500 (1994)].

Internal rotational barriers of ClOOCl
View Description Hide DescriptionThe rotational barrier for ClOOCl has been determined. The barrier for the cis and trans configurations is 8.8 kcal mol^{−1} and 4.9 kcal mol^{−1}, respectively. This is in disagreement with the experimental result of Birk et al. [J. Chem. Phys. 91, 6588 (1989)] and the discrepancy is discussed.

Three‐body effects on molecular properties in the water trimer
View Description Hide DescriptionWe report an application of diffusion Monte Carlo to investigate the importance of three‐body forces on the properties of the water trimer. The potential energy surface used is due to Millot and Stone and is based on intermolecular perturbation theory to which three‐body induction and dispersion energies are added. The effects of the three‐body forces are considered by comparison with the same potential containing only pairwise water interactions. We have calculated minimum energy structures, vibrationally averaged structures, zero‐point energies,rotational constants, cluster dissociation energies, and tunneling splittings, with and without the three‐body forces. The values obtained for the vibrationally averaged rotational constants with the three‐body potential are fairly close to the experimental values. Whereas the rotational constants are shown to have a significant dependence, the tunneling splittings are changed little by the three‐body forces. Based on the calculated difference in anharmonic zero‐point energies in water dimer and trimer, we predict that vibrational excitation of a stretching mode will cause predissociation in (H_{2}O)_{3} but probably not in (D_{2}O)_{3}.

Many‐body methods for excited state potential energy surfaces. II. Analytic second derivatives for excited state energies in the equation‐of‐motion coupled cluster method
View Description Hide DescriptionTwo distinct theoretical formalisms are developed for evaluating second derivatives of the energy analytically within the equation‐of‐motion coupled cluster approximation for excited electronic states (EOMEE‐CC). In the first approach, both perturbations are treated equivalently. In the alternative formulation, the final operator expression is not symmetric with respect to interchange of the perturbations, and calculation of the second derivative requires that four systems of linear equations be solved for the first‐order response of wave function parameters. However, only two systems need to be solved when the symmetric strategy is followed. While the symmetric approach superficially appears to be both more elegant and better suited for practical calculations, analysis shows that the former assertion is open to question and the latter only conditionally true. In particular, the asymmetric formulation is shown to be the preferred choice for all cases in which a large number of perturbations is involved. This is a rather important conclusion that holds not only for the EOMEE‐CC method, but also for CC treatments of the electronic ground state and their finite‐order many‐body perturbation theory approximations.

Multireference configuration interaction studies on metastable states of the dication BN^{2+}
View Description Hide DescriptionMultireference CI calculations predict fourteen states of BN^{2+} to be quasibound, an uncommonly large number for a first‐row diatomic, confirming the important role played by the electropositive B atom in stabilizing molecular dications. About two‐thirds of the metastable potentials have dissociation barriers (D _{eff}) between 0.59 and 2.25 eV, accommodating several vibrational levels and therefore being kinetically stable. The ground stateX ^{3}Σ^{−}, however, might be difficult to study experimentally because it is only weakly bound (D _{eff}=0.23 eV), and its formation via ionization of BN or BN^{+} has unfavorable Franck–Condon factors (FCF). The 1 ^{5}Σ^{−} state is the best candidate for detecting BN^{2+}: Its barrier is the highest (D _{eff}=2.25 eV), sustaining about 30 vibrational levels (with tunneling lifetimes τ^{ T } _{υ}≊∞ for υ<20), and the ionization FCFs are favorable. The 1 ^{1}Δ state might also be detectable since its properties are similar to those of 1 ^{5}Σ^{−}. Near equilibrium, most metastable states are described by the configurations 3σ^{2}4σ5σ1π^{2} and 3σ^{2}4σ1π^{3}, both having charge distributions B^{2+}N. The adiabatic ionization potential into BN^{2+} (X ^{3}Σ^{−}) is 21.4 eV for ionization from BN^{+} (X ^{4}Σ^{−}) and 32.9 eV from BN(X ^{3}Π). The corresponding IPs into 1 ^{5}Σ^{−} lie 1.12 eV higher.

Vibrational analyses of trans,trans‐1,3,5,7‐octatetraene and all‐trans‐1,3,5,7,9‐decapentaene based on ab initio molecular orbital calculations and observed infrared and Raman spectra
View Description Hide DescriptionNormal coordinate analyses are performed for trans,trans‐1,3,5,7‐octatetraene and all‐trans‐1,3,5,7,9‐decapentaene on the basis of ab initio molecular orbital (MO) calculations and observed infrared and Raman spectra. The infrared and Raman spectra of all‐trans‐1,3,5,7,9‐decapentaene are measured in this study. Ab initioMO calculations are carried out at the second‐order Mo/ller–Plesset perturbation (MP2) level with the 6‐31G* basis set. The calculated force fields are scaled by using the scale factors determined to obtain good fits between the calculated and observed frequencies of trans‐1,3,5‐hexatriene. The calculated frequencies are in satisfactory agreement with the observed for all the modes of both octatetraene and decapentaene including the in‐phase C=C stretches, for which the calculated frequencies at the Hartree–Fock (HF) level deviate considerably from the observed. It is concluded that good force fields of all‐trans oligoenes can be obtained from ab initio MP2 calculations by use of the scale factors derived in this study. From the comparison of the force fields at the MP2/6‐31G* and HF/6‐31G* levels, it is clear that electron correlation has large effects not only on the absolute values of diagonal and off‐diagonal force constants of the C=C and C–C stretches but also on the chain‐length dependence of these force constants. Important features of the vibrational modes are also discussed.

Vibrational analyses of trans‐polyacetylene based on ab initio second‐order Mo/ller–Plesset perturbation calculations of trans‐oligoenes
View Description Hide DescriptionStructures and vibrational force fields of all‐trans‐oligoenes of various chain lengths are studied by ab initio molecular orbital (MO) calculations at the second‐order Mo/ller–Plesset perturbation (MP2) level with the 3‐21G and 6‐31G* basis sets. Dependencies of the structure parameters and the force constants of trans‐oligoenes on the chain length and the position in the chain are analyzed quantitatively. The structure and the force field of trans‐polyacetylene are extrapolated from those of trans‐oligoenes. Normal coordinate calculations are performed for trans‐polyacetylene on this basis. The following results show the reliability of the structure and the vibrational force field of trans‐polyacetylene obtained in the present study. (1) The calculated C=C and C–C bond lengths of trans‐polyacetylene are in satisfactory agreement with the observed. (2) The calculated δ=0 (null phase difference) frequencies are in good agreement with the observed infrared and Raman frequencies. (3) The calculated phonon dispersion curves are consistent with some criteria obtained experimentally. (4) Most of the observed infrared bands which do not correspond to the δ=0 frequencies of trans‐polyacetylene are assignable to peaks in the profile of the calculated density of vibrational states. (5) The calculated hydrogen‐amplitude‐weighted density of states is in reasonable agreement with the observed inelastic neutron scatteringspectrum.