bluuebunny/crossref_metadata_2025 huggingface dataset:
Classical theory for real-time femtosecond probing of the NaI* photodissociation
Authors: Lee Soo-Y. , Pollard W. Thomas , Mathies Richard A.
Published: July 2002

Femtosecond pump-probe experiments [Rosker et al., Chem. Phys. Lett. 146, 175 (1988)] on the dissociation of NaI* are modelled classically to obtain the absorption transients as a function of the pump and probe wavelengths. The initial ground state, the optically pumped predissociating state, and the third surface that is coupled by the probe pulse are explicitly included. The classical model can almost quantitatively explain all the features of the experimental results. The oscillations in the transients are shown to be due to molecules trapped in the adiabatic well, with most of the intensity coming from the covalent region of the well before the crossing point rx. The rising troughs in the transients are due to successive leaks out of the well into the covalent region after the crossing point rx, leading to dissociation. The difference in the absorption transient between on-resonance 589 nm probing as compared to off-resonance 612 nm and 580 nm probing is shown to arise from the difference in Lorentzian absorption widths between the trapped (500 cm−1) and dissociating (100 cm−1) covalent molecules, with the transition dipole moment to the third surface having about the same magnitude over the range of the covalent potential.
Measurement of vibrational frequencies of the H3 molecule using two-step photoionization
Authors: Lembo L. J. , Helm H. , Huestis D. L.
Published: July 2002

A two-step photoionization scheme is used to determine vibrational frequencies of the n=3 Rydberg states of triatomic hydrogen. Symmetric-stretch frequencies in orthotrihydrogen of 3212.1 and 3168 cm−1 are measured for the N=1 levels of the states 3s 2A′1 and 3d 2E″, respectively. Transitions to bending-mode excited levels in the 3d states and overtone levels of the 3p 2 E′ state are observed also.
Nonresonant hyper-Raman and hyper-Rayleigh scattering in benzene and pyridine
Authors: Neddersen John P. , Mounter Sarah A. , Bostick James M. , Johnson Carey K.
Published: July 2002

Nonresonant hyper-Raman and hyper-Rayleigh spectra excited at 1064 nm are reported for neat benzene and pyridine. The theory of Herzberg–Teller vibronic coupling in nonresonant and preresonant hyper-Raman scattering is developed. Nonresonant hyper-Raman scattering is shown to be vibronically induced by modes that efficiently couple strongly allowed one-photon and two-photon transitions. A weak and broad (55 cm−1) hyper-Rayleigh band was observed in benzene and attributed to collective scattering, while in pyridine, a much more intense and much narrower hyper-Rayleigh band was observed. Only the a2u vibration (ν11) was observed in the hyper-Raman spectrum of benzene, while several strong bands were observed in pyridine. Possible vibronic-coupling pathways are discussed for these modes. In addition, the observed hyper-Raman spectrum of pyridine is compared to a recent calculation.
The time-dependent Schrödinger equation: Application of absorbing boundary conditions
Authors: Neuhasuer Daniel , Baer Michael
Published: July 2002

In this work the time-dependent Schrödinger equation, which is solved by employing absorbing boundary conditions, is considered. To achieve this, a negative imaginary short-range potential was added to the potential at the asymptotic regions. The study concentrated on deriving the necessary conditions for the absorbing potential to be almost perfect. The theoretical findings are supported by a numerical study carried out for a one-dimensional (reactive) model.
The quantum mechanics of cluster melting
Authors: Beck Thomas L. , Doll J. D. , Freeman David L.
Published: July 2002

We present here prototype studies of the effects of quantum mechanics on the melting of clusters. Using equilibrium path integral methods, we examine the melting transition for small rare gas clusters. Argon and neon clusters are considered. We find the quantum-mechanical effects on the melting and coexistence properties of small neon clusters to be appreciable.
Post-attachment interactions in K(n
d)–CF3I collisions at intermediate n
Authors: Kalamarides A. , Walter C. W. , Lindsay B. G. , Smith K. A. , Dunning F. B.
Published: July 2002

A kinematic study of the velocities and angular distribution of I− ions detected following Rydberg electron transfer in K(nd)/CF3I collisions is reported. The data show that post-attachment interactions between the product ions are important at intermediate n (9≲n≲20) even though the I− ions are formed with appreciable kinetic energies.
A theoretical rotationally resolved infrared spectrum for H2O+ (X 2B1)
Authors: Weis B. , Carter S. , Rosmus P. , Werner H.-J. , Knowles P. J.
Published: July 2002

Three-dimensional potential energy and electric dipole moment functions for the electronic ground state of H2O+ have been calculated from highly correlated multiconfiguration reference configuration interaction (MRCI) electronic wave functions. The analytic representations of these functions have been used in vibrational and perturbational calculations of the rovibrational absorption spectrum of H2O+. The quartic force fields in normal coordinates have been employed in the evaluation of the equilibrium spectroscopic constants in H2O+, D2O+, and HDO+ by perturbation theory. The equilibrium structure, vibrational band origins, centrifugal distortion constants and rotational energy levels agree very well with the available experimental data. Absolute vibrational band intensities have been calculated from the dipole moment functions and are compared with theoretical integrated band intensities. The radiative lifetimes of excited vibrational states exhibit mode specific variations. The rotationally resolved room temperature absorption spectra have been evaluated ab initio for the pure rotational and the ν2, 2ν2, ν1, ν3, and 3ν2 transitions. The rovibrational electric dipole transition matrix elements and absolute line intensities are given for the most intense transitions. These data take full account of anharmonicity effects and vibration–rotation coupling.
Theoretical study of the C–H bond dissociation energy of acetylene
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

A systematic study of the energies of acetylene and ethynyl radical and cation using high levels of ab initio molecular orbital theory, including correlation effects beyond fourth order perturbation theory, is presented. The bond dissociation energy of acetylene D0(HCC–H), is calculated to be 5.79 eV with an expected accuracy of ±0.1 eV. The 3Π ground state of C2H+ is calculated to be 0.36 eV more stable than the 3Σ− state. The ionization potential of ethynyl radical (C2H) is calculated to be 11.65 eV.
Molecular motions in n-nonadecane C19H4: An incoherent neutron scattering study
Authors: Guillaume F. , Doucet J. , Sourisseau C. , Dianoux A. J.
Published: July 2002

Incoherent neutron scattering (INS) experiments were performed on semioriented samples of n-nonadecane C19H40 and of the selectivity deuterated derivatives CD3–C17H34–CD3 and C2D5–C15H30–C2D5 over a wide temperature range, in particular, over the domain of existence (295–305 K) of the disordered ‘‘rotator phase’’ RI. From analyses of the inelastic and quasielastic parts of the spectra, we were able to discriminate between motions parallel or perpendicular to the main long-chain axis. In the low frequency region (0–300 cm−1), numerous lattice and internal vibrational modes, showing pronounced polarization effects, were assigned. In the quasielastic regions, large broadenings were observed in the RI phase, providing clear evidences of both fast restricted translational motions along the chain axis and uniaxial rotational diffusions. For the rotational process, we have elaborated and made use of a model of diffusion for a particle in a twofold potential. Finally, the INS spectra of C19H40 in the RI phase and of C23H48 and C24H50 in their RII phases were compared and nicely interpreted within an unique model of ‘‘effective potential’’ by fitting the barrier heights against rotational diffusion. All these results are discussed and compared to literature data.
Microwave spectroscopic detection of dichlorosilylene SiCl2 in the ground state
Authors: Tanimoto Mitsutoshi , Takeo Harutoshi , Matsumura Chi , Fujitake Masaharu , Hirota Eizi
Published: July 2002

The microwave spectrum of dichlorosilylene SiCl2 has been observed to characterize this molecule of chemical interest. The molecule was generated by the thermal reaction between silicon powder and tetrachlorosilane at about 1000 °C. The rotational constants and the centrifugal distortion constants were determined for the three isotopic species Si35Cl2, Si35Cl37Cl, and Si37Cl2. The nuclear quadrupole coupling constants were determined from triplet hyperfine splittings observed for several transitions. The asymmetry of the Cl nuclear quadrupole coupling tensor was found to be very large and was accounted for by π electron backdonation from Cl to Si.
Thermal diffusion in helium–hydrocarbon and helium–fluorocarbon systems at 300 K
Authors: Dunlop Peter J. , Bignell C. M.
Published: July 2002

Thermal diffusion factors are reported for a series of helium–hydrocarbon and helium–fluorocarbon systems. The data are used to test the first-order corresponding states theory: An amusing empirical correlation is also reported.
Crossed molecular beam study of the reaction O(3P)+C2H4
Authors: Schmoltner A. M. , Chu P. M. , Brudzynski R. J. , Lee Y. T.
Published: July 2002

The reaction between ground state atomic oxygen and ethylene was studied under single collision conditions using the crossed molecular beam method. At an average collision energy of about 6 kcal/mol, the two major primary reaction channels are (a) the formation of CH3 and CHO and (b) the formation of C2H3O and H. Product angular distributions and time-of-flight spectra were measured and the translational energy release was determined for each channel. The observed results and calculated potential energy surfaces suggest that after the addition of O(3P) to ethylene forming a triplet diradical, channel (a) occurs by way of intersystem crossing to the singlet state, 1,2-H migration and subsequent C–C bond rupture, whereas channel (b) proceeds mostly through the direct dissociation of the intermediate triplet diradical, except for a small contribution from H atom elimination of the singlet acetaldehyde intermediate.
Solution of atomic Hartree–Fock equations with the P version of the finite element method
Authors: Flores J. R. , Clementi E. , Sonnad V.
Published: July 2002

The Hartree–Fock equations for atoms are solved with the p version of the finite element method, which differs from the traditional finite element method in using high order, hierarchic polynomials as basis functions. Recursion formulas are developed for the analytical evaluation of integrals, which are crucial in reducing the computation time and maintaining the accuracy of the solution. A hierarchic computational approach is used where the solution at a certain level is used to start the calculation at the next level. Results are presented for closed and open shell atoms taken from various columns of the periodic table that show excellent agreement with accurate numerical calculations.
Structural and dielectric properties of dipolar hard sphere mixtures: Reference hypernetted chain and pertubation theory results
Authors: Lee Pil H. , Ladanyi Branka M.
Published: July 2002

Reference hypernetted chain (RHNC) equations for mixtures of molecules with spherical cores and axial multipoles are presented and solved for binary mixtures of dipolar hard spheres. The dielectric constant of dipolar hard sphere mixtures is also obtained by perturbation theory. The composition dependence of the pair correlation functions of binary dipolar mixtures is studied. The RHNC theory predicts that contacts between molecules of the more polar component are strongly favored, especially when the concentration of this component is low. The dependence of dielectric constant, the internal energy, the pressure, and the Kirkwood orientational pair correlation factors on the relative sizes and dipole moments of the two components is examined. Of these quantities, the pressure is found to be the most sensitive to size disparity and to other aspects of short range structure. The RHNC and perturbation theory (PT) results for the dielectric constant, as well as the RHNC results for the internal energy, are found to be in very good agreement with Monte Carlo simulation data.
A reduced dimensionality quantum reactive scattering study of the insertion reaction O(1D)+H2→OH+H
Authors: Badenhoop Jay K. , Koizumi Hiroyasu , Schatz George C.
Published: July 2002

This paper presents a two degree of freedom model for describing the quantum dynamics of the insertion reaction O(1 D) + H2 in which bend motions are treated with a sudden approximation. Comparison of product state vibrational distributions from a classical version of this model with three dimensional trajectory results indicates that the model is realistic. Quantum/classical comparisons for the model Hamiltonian indicate that recrossing is more important in the quantum dynamics, and as a result, the quantum reaction probability from ground state reagents is lower by as much as 40%. In addition, the quantum vibrational state distribution shows higher excitation than its classical counterpart. This difference in excitation is due to trajectories that produce vibrationally cold products, and it is found that these trajectories always cross the deepest part of the H2O well.
The topology of the molecular charge distribution and the electric field gradient at the N atom in nitriles
Authors: Aray Yosslen , Murgich Juan
Published: July 2002

A direct relationship between the N valence shell charge topology reflected in its Laplacian and the components of the electric field gradient (EFG) at the N nucleus in nitriles was found. The largest diagonal component of the N EFG tensor was showed to be determined by the combined effect of the bonded and nonbonded maxima of the Laplacian of the valence shell of the N atom along the C–N direction. In nonlinear nitriles, the asymmetry of the N field gradient around the C–N direction was showed to be determined by the difference in the values of the maxima of the Laplacian of the valence shell obtained in two mutually perpendicular planes that are also normal to the C–N direction and contained the N nucleus.
A theoretical model of phospholipid dynamics in membranes
Authors: Ferrarini Alberta , Nordio Pier Luigi , Moro Giorgio J. , Crepeau Richard H. , Freed Jack H.
Published: July 2002

Static and dynamic properties related to the internal configurational motions have been calculated for the alkyl chains of phospholipid molecules in a membrane environment in the liquid crystal phase. The calculations have been performed for the chain 1 of 1,2-dipalmitoyl 3-sn-phosphatidylcholine (DPPC), a typical constituent of phospholipid membranes. Under the assumption of fixed bond lengths and bond angles, the internal dynamics of the chain is described in terms of 15 dihedral angles. The time evolution of the angular variables is assumed to be diffusional in character, and a master equation for transitions among the stable conformers is constructed from the energetics and hydrodynamics of the chain. This method is an extension to the time domain of the rotational isomeric state (RIS) approximation, which has been widely used to compute static properties of the chains. After calculation of the suitable correlation functions, effective rate constants relevant for spectroscopic and kinetic observables have been computed, and the results have been compared with those obtained by recent Brownian dynamics (BD) calculations. The position dependence of the rate constants along the chain has been examined with special reference to understanding the effects resulting from cooperativity in the conformational transitions. The overall spinning and tumbling of the chain has also been described by a diffusive model. The calculated spectral densities for the composite motional process have been used to rationalize the behavior of the relaxation times T1, T2, and T1ρ measured in deuterium nuclear magnetic resonance (NMR) experiments.
Hyperfine structure of the Na2 b 3Πu state
Authors: Katô Hajime , Otani Mitsuhiko , Baba Masaaki
Published: July 2002

The energy shifts and the hyperfine structures of the strongly perturbed levels between the A 1Σ+u (v=7) and b 3Π0u (v=13) states and between the A 1Σ+u (v=8) and b 3Π0u (v=14) states are studied. The hyperfine splitting of the Na2 b 3Πu state can be attributed predominantly to the Fermi contact interaction.
Viscosity of aqueous solutions of monohydric alcohols in the normal and supercooled states
Authors: Bulone D. , Spinnato C. , Madonia F. , Palma M. U.
Published: July 2002

Viscosities of solutions of Met−, Et−, and n-PropOH were measured by quasielastic light scattering of polystyrene lattice spheres of 800 Å diameter, in the low-concentration, low-temperature ranges where effects of alcohols on the structure and properties of liquid water are most pronounced. Raw data already indicate that alcohols promote the formation of clathrate-like structures of H bonds which add to those occurring naturally in the pure solvent. Evaluation of the number of water molecules taking part in longer-lived structures further indicates that this promotion is most effective in the infinite-dilution limit. Available thermodynamic data agree with this evaluation. Mismatches among solute-promoted clathrate-like cages are evidenced. The known disruptive effects of alcohols on the anomalous properties of cold and supercooled water are thus understood in terms of limitations to the correlation length of density fluctuations, set by mismatches. A contribution of OH groups to cage promotion is also evidenced at least at low temperature. Constraints or hindrances to the motion of water molecules, irrespective of their nature, appear to be effective in favoring structures of stabler, longer-lived H bonds, corresponding to water molecules of lower mobility. This offers a unified view of hydrophobic and hydrophilic interactions which agrees with earlier work and with more recent computer experiments. The possibility here evidenced for structures of H bonds promoted in solvent water of being conflictual or synergistic, adds to the microscopic understanding of solvent-mediated interactions, e.g., of biosolutes.
The derivation of the rotational potential function from atom–atom potentials. IV. The symmetric tops NH3D+ and NHD+3 and the asymmetric top NH2D+2
Authors: Smith David
Published: July 2002

A procedure is presented for computing the hindered rotational energy levels of symmetric and asymmetric tops in tetrahedral crystalline fields. By considering the equations used to derive the rotational potential function from atom–atom potentials, the change required in the potential function on going from spherical tops to symmetric and asymmetric tops has been determined. The rotational potential function of NH3D+ and NHD+3 in NH4Cl, (NH4)2SnCl6, (NH4)2SiF6, and NH4F have been derived from atom–atom potentials. The rotational potential functions of NH2D+2 in the first three compounds listed above have also been derived from atom–atom potentials. Only the J=4 term in the rotational potential functions has been found to be significantly different for symmetric and asymmetric tops than for spherical tops. The librational frequencies have been computed for the isotopic forms of the solids listed above. For some of solids that have been studied, tunneling frequencies have also been computed. For the symmetric tops NH3D+ and NHD+3, there are two librational frequencies, one with A symmetry and the other with E symmetry. The difference in energies of these frequencies is due to the changes in both the kinetic energy term and potential energy term in the Hamiltonian on going from a spherical top to a symmetric top. The contributions of each of these terms to the splitting in the librational frequency has been determined for the compounds under study. The tunneling frequencies of CH3D have been computed in phase II for both the methane molecules on the D2d and Oh sites. For the molecules on the D2d sites, only the molecular field is considered. The potential function is due to the crystalline field for molecules on the Oh sites. The computed tunneling frequencies of the molecules on both sites are in good agreement with the observed results of the INS of CH3D in CH4.
Quantum and classical descriptions of position and phase-space densities of a model Hamiltonian
Authors: Anchell James L.
Published: July 2002

Excited states of a model Hamiltonian with strongly coupled modes are investigated. Comparisons are made between the classical Poincaré surfaces of section and the Husimi function, which is a quantum mechanical phase-space density function. It is found that for the majority of states the Husimi function exhibits local maxima which resemble in shape and location the invariant tori of Poincaré surfaces of section. For certain anomalous states the maxima in the Husimi function bears no such resemblance, and these states are considered as quantum chaotic.
Phase separation dynamics of polycarbonate/polymethyl methacrylate blends. II. Temperature jumps above an immiscibility loop
Authors: Kyu Thein , Lim Dong-Soo
Published: July 2002

Blends of polycarbonate (PC) and moderately low molecular weight polymethyl methacrylate (PMMA) reveal an immiscibility loop in which an upper critical solution temperature (UCST) is located above a lower critical solution temperature (LCST). Temperature jump (T jump) experiments were conducted on a 40/60 PC/PMMA composition from a single phase (150 °C) to two-phase temperatures above the immiscibility loop (205–241 °C). Phase separation first occurs, then is accompanied by phase dissolution. The early stage of phase separation is explicable in terms of the linearized Cahn–Hilliard theory. The late stage of SD follows the power-law relationship with the exponents of −1/3 and 1 as predicted by the cluster theory of Binder and Stauffer. The universal curve was established with dimensionless variables of Qm and τ. The late stage data were analyzed in accordance with the recent scaling laws of Furukawa. Time evolution of structure function exhibits universality in the late stages of spinodal decomposition. The shape of the structure function can be scaled with slopes of 2 and −6 for qqm regions, respectively. At a later time, phase dissolution takes place and the system returns to a single phase.
The application of density functional theory to the optimization of transition state structures. I. Organic migration reactions
Authors: Fan Liangyou , Ziegler Tom
Published: July 2002

We present here the first determination of transition state structures based on Hartree–Fock–Slater (HFS) density functional calculations. The optimization of the transition state structures utilized the analytical energy gradients evaluated by a recently developed method. The transition state structures obtained by the HFS method are similar to those determined by ab initio methods in which electron correlation is partly taken into account. The energy barrier for the CH3NC→CH3CN isomerization process, corrected for the zero-point energy, is 40.1 kcal mol−1 which is in good agreement with the experimental value of 38.4 kcal mol−1. Experimental barriers are not available for the HNC→HCN and N2H2(trans)→N2H2(cis) isomerization processes. However, the barriers calculated by the HFS method are in good accordance with the barriers obtained from electron-correlation calculations. Furthermore, the vibrational frequencies calculated by the HFS method are in good agreement either with experiments in the case of stable molecules or with electron-correlation calculations in the case of transition states. The HFS method seems in general to provide better estimates of vibrational frequencies and activation barriers than the Hartree–Fock method in which electron correlation is lacking. The later method generally afford too high values for both frequencies and activation barriers.
Comment on: ‘‘Lifetime of the A 2Σ+, v′=0 state of HS’’
Authors: Ubachs Wim , ter Meulen J. J.
Published: July 2002

Determination of the lifetime τ=0.17–0.3 ns of SH(A 2Σ+, v′=0) by Loge and Tiee [J. Chem. Phys. 89, 7167 (1988)] using the Hanle effect, is in clear contradiction to our LIF–molecular beam results, where single hyperfine components in the A 2Σ+–X 2Π system were measured with a linewidth as low as 54 MHz.
Phase space bifurcation structure and the generalized local-to-normal transition in resonantly coupled vibrations
Authors: Li Zhiming , Xiao Lin , Kellman Michael E.
Published: July 2002

The generalization of the local-to-normal transition seen in symmetric triatomics is considered for nonsymmetric molecules and 2:1 Fermi resonance systems. A straightforward generalization based on a division of phase space into local and normal regions is not possible. Instead, classification of the phase space bifurcation structure is presented as the complete generalization of the local–normal concept for all spectroscopically relevant systems of two vibrations interacting via a single nonlinear resonance. The polyad phase sphere (PPS) is shown to be the natural arena to analyze the bifurcation structure for resonances of arbitrary order. For 1:1 and 2:1 resonances, the bifurcation problem is reduced to one or two great circles on the phase sphere. All bifurcations are shown to be examples of elementary bifurcations of vector fields in one dimension. The classification of the bifurcation structure is therefore governed and greatly simplified by the theory of the universal unfolding and codimension of elementary bifurcations. The implications for large-scale bifurcation structure and transport in molecules with chaotic motion are briefly discussed.
Rotational energy transfer in excited states of halogen molecules. I. Transfer from v′=6, J′=72 in IF B 3Π(0+)
Authors: Davis S. J. , Holtzclaw K. W.
Published: July 2002

A cw laser-induced fluorescence experiment is described in which state-to-state rate coefficients for rotation-to-translation energy transfer have been determined from J′=72, v′=6 in IF B 3Π(0+). Using the collision partners He, Ne, Ar, Kr, Xe, N2, and CF4, nearly 500 individual rate coefficients have been measured. The rare-gas atoms display a characteristic behavior with the heavier collision partners more efficiently exchanging large amounts of angular momentum in a single collision.
Infinitesimal differential diffusion quantum Monte Carlo: Diatomic molecular properties
Authors: Vrbik Jan , Legare Daniel A. , Rothstein Stuart M.
Published: July 2002

We show how to estimate, for a given molecule, the first and higher derivatives of the expected value of an operator with respect to one or more physical parameters. This is done with high accuracy achieved by sampling to within a certain approximation from the exact electron distribution, compatible with the Hellmann–Feynman theorem. Finite difference approximations are avoided. The required derivatives of the unknown exact wave function are determined by averaging expressions involving only the total serial correlation of known quantities. The operator is not restricted to the case of the molecular Hamiltonian. This allows for computation of virtually all ground-state properties of a molecule by a single, relatively trivial computer program. Our formulas are presented and applied in the context of a diatomic molecule (LiH), but they can be readily extended to polyatomics.
Theoretical and experimental studies of optical nonlinearities of haloforms CHX3, X=F, Cl, Br, I
Authors: Karna Shashi P. , Dupuis Michel , Perrin Eric , Prasad Paras N.
Published: July 2002

In this paper, we present ab initio calculations of static polarizability α and static first and second hyperpolarizabilities β and γ for the haloform series CHX3, where X=F, Cl, Br, and I using the effective core potential (ECP) approach. The microscopic optical nonlinearities α, β, and γ are calculated as the derivatives of the energy with respect to the electric field, with the energy determined by means of the self-consistent-field approach (SCF), and nonlinearities calculated by means of the coupled perturbed Hartree–Fock (CPHF) formalism. To test the approximation introduced by the ECP method, nonlinear optical responses for the lighter members of the series CHF3 and CHCl3 are compared with all electron calculations. The effects due to basis set size and inclusion of diffuse and polarization functions of d and f type are examined. The ECP technique is then used to calculate optical nonlinearities for CHBr3 and CHI3. Although very good agreement is found between calculated and experimental polarizabilities α for the haloform series, the agreement is not as good for the higher order polarizabilities. Possible causes for this discrepancy are discussed.
Quantum Monte Carlo calculations of three-body corrections in the interaction of three helium atoms
Authors: Mohan V. , Anderson J. B.
Published: July 2002

We have used the quantum Monte Carlo (random walk) method to calculate accurate total electronic energies for the helium trimer in several nuclear configurations to obtain potential energies of interaction and determine three-body corrections to pairwise-additive potential energy expressions. For equilateral triangle configurations with side length 2.5 to 6.5 a.u. the three-body correction ranges from −5930 K at 2.5 a.u. to approximately 0 K at 6.5 a.u. Comparisons of results from simple analytic variational calculations of the same correction show the simple calculations to be remarkably accurate. The Axilrod–Teller–Muto predictions fail for side lengths less than 5.6 a.u.
Excited-state dipole moments of SO2
Authors: von Moers F. , Schmalzigaug K. , Schmoldt A. , Heitz S. , Hese A.
Published: July 2002

High resolution ultraviolet spectroscopy of SO2 in a cold supersonic beam is reported. The behavior of single rovibronic lines of the E band (305 nm) and the 322 nm band was investigated in an electric field of up to 12 kV/cm. The dipole moment of the 1 A1 state was determined to be 1.85 (0.03) D. Problems of the dipole determination in SO2 are discussed.
Spontaneous development of optical activity during freezing. A kinetic lattice model
Authors: Harris J. G. , Stillinger F. H.
Published: July 2002

Spontaneous optical resolution, a process by which a racemic mixture develops optical activity without the intervention of any external optically active reagent or field, has been observed during the freezing of certain organic compounds, i.e., 1,1′ binaphthyl. We develop a two-dimensional lattice model with simple triatomic molecules to investigate this phenomenon. Simulations verify that this model has a first-order phase transition from a racemic fluid phase to a chiral crystal. At high density and deep undercooling this phase transition proceeds by rapid nucleation of small clusters followed by a much slower flocculation of the clusters through the motion of grain boundaries. At low density homogeneous nucleation and growth by the addition of single particles or very small clusters is capable of producing crystals with close to the maximum possible optical activity from a racemic mixture. A requirement for spontaneous optical resolution is that the two optical isomers can interconvert within the time scale of the observed crystal growth. Even though the thermodynamic stability of the racemic liquid and the chiral solid are independent of the interconversion rate, when it is zero or extremely low the optical activity is fixed at its initial value and crystallization is inhibited.
Comparison of perturbatively corrected energy results from multiple reference double-excitation configuration-interaction method calculations with exact full configuration-interaction benchmark values
Authors: Knowles David B. , Alvarez-Collado José Ramon , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

Perturbative corrections of various types are employed in conjunction with variational configuration-interaction (CI) treatments in order to obtain estimates of the energy eigenvalues of a large series of full CI benchmark calculations recently reported by Bauschlicher et al. It is found that the best agreement is generally obtained when an extrapolation procedure based on the results of truncated secular equations is employed to estimate the zero-threshold multiple reference double-excitation configuration-interaction (MRD-CI) energy values, after which these intermediate results are then corrected for effects of higher excitations by means of a multireference analogue of the Davidson–Langhoff correction D1. A root-mean-square (rms) deviation of 0.94 kcal mol−1 relative to the full CI energy results for 66 cases is obtained with this perturbative method, which has been in general use now for most of the last decade. By comparison the T=0 variational MRD-CI energy values give a higher rms deviation of 3.19 kcal mol−1. This behavior is shown to result from a systematic cancellation of errors in the two types of perturbative corrections employed in the above scheme. The advantages of employing a multireference set of configurations in generating singles and doubles CI spaces are also underscored by the present results, and the advisability of employing a nearly uniform value of ∑mpc2p for all states and nuclear conformations of a given atomic or molecular system is stressed.
Molecular dynamics model of absorption of water in polymers
Authors: Vallés J. L. , Halley J. W.
Published: July 2002

We report a molecular dynamics simulation of the absorption of water in polymers. We embed a relatively large repeat unit containing several characteristic chemical groups in a molecular dynamics model of water which includes the internal dynamics of the water molecule. Potentials describing the interaction of the water with the repeat unit were taken from the work of Jorgensen and co-workers. We calculated the density of water around oxygen in ester, ether and alcohol groups in the repeat unit. We find in this model that the water is only bound by the alcohol group in agreement with recent experimental results. We have computed the infrared absorption spectrum of the bound and unbound water in this model. We find shifts which should be observable in the position of the stretch and librational frequencies due to binding of the water. We discuss implications for the dual diffusion model of water in polymers.
The isotope exchange reaction of fast hydrogen atoms with deuterated alkynes and alkenes
Authors: Johnston Grace W. , Satyapal Sunita , Bersohn Richard , Katz Benjamin
Published: July 2002

The exchange reaction H(1 eV)+RD→RH+D, where RD was deuterated acetylene, methylacetylene, ethylene, and propylene was studied by laser induced fluorescence detection of the hydrogen and deuterium atoms. The reaction cross sections were 1.69±0.22, 0.50±0.15, 1.85±0.20, and 1.10±0.14 Å2, respectively. The fraction of the incident relative kinetic energy released as translation was 0.42, 0.34, 0.40, and 0.34. That the addition of a methyl group only slightly reduces the energy released as translation is regarded as proof that the reaction at this energy does not involve a long lived intermediate complex, but is a direct localized displacement. Further confirmation comes from the observation that the velocity of the D atom product of H+C2D2 is correlated with the velocity of the H atom reactant.
Theory of reversible diffusion-influenced reactions
Authors: Agmon Noam , Szabo Attila
Published: July 2002

A unified theory of reversible diffusion-influenced geminate and pseudo- first-order reactions is developed. Explicit results are presented for the time dependence of the fraction of molecules that are dissociated at time t for a variety of initial conditions. To introduce the basic ideas of our approach, an elementary and rather complete treatment of the irreversible reaction between a pair of interacting, spherically symmetric particles is presented. The focus is on deriving relations among survival probabilities and bimolecular time-dependent rate coefficients for the radiation and absorbing boundary conditions and the asymptotic behavior of these quantities. These relations are then generalized to reversible geminate reactions. For example, it is shown that the separation probability for an initially bound pair satisfies a simple convolution relation involving the survival probability of an irreversibly reacting geminate pair initially at contact. An analytic expression is obtained for this separation probability that is exact for free diffusion and is an accurate approximation for interacting particles. Finally, the Smoluchowski approach to irreversible pseudo-first-order reactions is extended to reversible reactions. The analysis is based on the generalization of the convolution relations that are rigorously valid for isolated pairs.
Ultraviolet photoemission study of oligothiophenes: π-band evolution and geometries
Authors: Fujimoto H. , Nagashima U. , Inokuchi H. , Seki K. , Cao Y. , Nakahara H. , Nakayama J. , Hoshino M. , Fukuda K.
Published: July 2002

Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4–8 thiophene rings. In a series of α-linked oligomers (αn with n being the number of rings), a systematic evolution of the π band is observed. Several peaks which correspond to the π band are observed in the region of 0.7–3 eV below the Fermi level (EF), and the bandwidth becomes broader with increasing n. The nonbonding π band is observed at 3.5 eV below EF and its energy is almost independent of the number of thiophene units. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compounds of the polymer. The ionization threshold energy of α7 and polythiophene was observed to be 5.3 eV. The effect of irregularity on the π-electron system was also studied by using oligomers which contain a β linkage or a vinylene group at the middle of the molecule. The UPS spectra showed that the β linkages significantly affect the electronic structure of polythiophene, while the vinylene group does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometries of these oligomers are calculated by the semiempirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consistent-field molecular orbital) method. Theoretically simulated spectra of these oligothiophenes derived from the obtained orbital energies by Gaussian broadening are compared with the observed ones. The agreement between the observed and calculated spectra is very good, particularly in the π region. It is shown from the optimized geometry that (1) αn ’s have planar structure and π electrons are delocalized, (2) the oligomer with β linkages has nonplanar structure leading to limited delocalization of π electrons, and (3) the oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small.
Laser vaporization/rare gas matrix-isolation electron spin resonance studies of PdH+2, PdD+2, PdH, and PdD: Theoretical investigation of PdH+2
Authors: Knight Lon B. , Cobranchi S. T. , Herlong John , Kirk Thomas , Balasubramanian K. , Das K. K.
Published: July 2002

The reactive laser vaporization of palladium metal in the presence of H2(g) has been used to generate and trap neutral and charged palladium hydride radicals in neon and argon matrices for electron spin resonance investigations. The radicals identified are: 0PdH+2, 105PdH+2, 0PdD+2, 105PdD+2, 0PdH, 0PdD, 105PdH, and 105PdD. The electron spin resonance results prove that the H atoms are equivalent in PdH+2 but it cannot be determined for certain whether a highly bent 2A1 ground state complex or a linear 2Σ state is being observed, although the evidence seems to favor the 2A1 state. The observed nuclear hyperfine properties (A and g tensors) are best accounted for by assigning the unpaired electron predominantly to 4d2z/5s orbitals on the metal in PdH+2 with only about 3% of the spin density on each H atom. Electron spin resonance results for PdH constitute the first observations for a diatomic metal hydride radical trapped in a neon matrix. The magnetic parameters (MHz) for 105PdH+2 in a neon matrix at 4 K are: g⊥=2.568(2); g∥≂1.92; A⊥(105Pd)=−713(2); A∥(105Pd)=−688(15); A⊥(H)=45(1). Multireference SCF (MCSCF) and configuration interaction (CI) calculations were conducted for PdH+2 to calculate the energies and gross population distributions in the lowest-lying electronic states. The lowest of these, the 2A1 state, has a calculated stabilization energy of 9.3 kcal/mol relative to Pd+(4d9)+H2.
Classical trajectory calculation of transport and relaxation properties for N2–Ne mixtures
Authors: Wong Clement C. K. , McCourt Frederick R. W. , Casavecchia Piergiorgio
Published: July 2002

A detailed comparison of the predictive powers of two recently determined potential energy surfaces [J. Chem. Phys. 88, 5465 (1988); 89, 3505 (1988)] for the N2–Ne interaction has been carried out. In particular, the following has been tested: calculations using these two surfaces against experimental values of the total differential scattering cross section at 75.8 meV, the temperature dependence of the interaction second virial coefficient over the range 90 K to 323 K, the temperature dependence of the binary diffusion coefficient and the mixture viscosity over the range 280 K to 973 K, the mixture thermal conductivity at 300 K, and viscosity and thermal conductivity field-effects, rotational relaxation, and collision-broadening of the depolarized Rayleigh line over a restricted temperature range. Forty-five effective cross sections that determine the bulk transport and relaxation phenomena have been calculated by classical trajectory methods for temperatures varying from 77.5 K to 973 K. Second-approximation calculations of the mixture transport phenomena using these calculated cross sections give impressive agreement with the experimental results over a wide temperature range for both potential surfaces. While one potential gives better agreement with the scattering data, the second virial coefficient data, the bulk transport data, and the depolarized Rayleigh collision-broadening data than does the other potential, the opposite is true for the rotational relaxation and field-effect data.
Configuration interaction spectroscopic properties of X 2Σ+ HNC+ and X 2Π HCN+
Authors: Peterson Kirk A. , Mayrhofer Rudolph C. , Woods R. Claude
Published: July 2002

Stretching potential energy and dipole moment functions have been calculated for the ground electronic states of the HNC+ (X 2Σ+) and HCN+ (X 2Π) radical ions by singles and doubles configuration interaction (CI-SD) with a basis set of 74 contracted Gaussian-type orbitals. By comparison to analogous calculations on the CN radical, the ground state rotational constants B0 of HNC+ and HCN+ are predicted to be 47, 122±70 MHz and 40, 185±70 MHz, respectively, by CI-SD with the addition of a size consistency correction, designated CI-SD(s). Stretching band origins and intensities have been obtained from the analytical potential energy and dipole moment functions using variational methods. The stretching fundamental band origins, ν1 and ν3, and their intensities are predicted to be 3464 cm−1/2507 cm−2 atm−1 and 2212 cm−1/2.3 cm−2 atm−1, respectively, for HNC+ and 3099 cm−1/1189 cm−2 atm−1 and 1806 cm−1/70.3 cm−2 atm−1 for HCN+. The equilibrium dipole moment of HNC+ calculated by CI-SD is just 0.66 D, while that of HCN+ is 3.63 D. In agreement with earlier studies the HNC+ isomer is predicted to be lower in energy than HCN+, and our calculated energy of isomerization, 18.91 kcal/mol, is in good agreement with this previous work. The Renner–Teller effect in the HCN+ isomer is predicted within the harmonic approximation by CI-SD(s) to be small, with the Renner parameter ε and the Renner–Teller constant εω2 being −0.056 and −39.2 cm−1, respectively (ω2=696 cm−1). Quadrupole coupling constants have also been computed for HNC+ and HCN+, as well as for the neutrals HCN and HNC. The proton affinity at zero temperature PA0 of CN is 150.6 kcal/mol by CI-SD(s), and the adiabatic ionization potentials of HNC and HCN are 273.3 and 307.7 kcal/mol.
The multiphoton ionization spectrum of HBr. I. 74 000 to 85 000 cm−1
Authors: Callaghan Robert , Gordon Robert J.
Published: July 2002

The technique of resonantly enhanced multiphoton ionization with time-of-flight mass resolution has been used to measure the two-photon absorption spectrum of HBr. We have analyzed 37 bands and assigned them to transitions to the valence, ion-pair state and to Rydberg states belonging to the sσ, pσ, dσ, pπ, dπ, and dδ manifolds. Our results are in excellent agreement with the earlier work of Ginter, Ginter, and Tilford. In addition, we have analyzed twelve new electronic states as well as numerous bands of vibrationally excited levels of previously known electronic states. Strong mixing was observed among the 0+ states.
A generic test of gas phase isolated binary collision theories for vibrational relaxation at liquid state densities based on the rescaling properties of collision frequencies
Authors: Paige M. E. , Harris C. B.
Published: July 2002

The X state vibrational relaxation of geminately recombined I2 in liquid Xe is monitored as a function of solvent density by means of transient absorption spectroscopy. Plots of vibrational energy vs time at different solvent density can be exactly overlapped by linearly scaling the time axis. This linear scaling behavior indicates that the isolated binary collision (IBC) model’s assumption regarding the density independence of the probability of relaxation per collision is valid at liquid densities, even for a low frequency oscillator (≊200 cm−1). This new method of testing the IBC model’s validity at liquid density is independent of the explicit form of the intermolecular potential and is independent of a determination of collision frequency and thus, eliminates the ambiguities associated with evaluating this quantity.
A laser-induced fluorescence determination of the complete internal state distribution of OH produced in the reaction: H+NO2→OH+NO
Authors: Irvine Alistair M. L. , Smith Ian W. M. , Tuckett Richard P. , Yang Xue-Feng
Published: July 2002

Laser-induced fluorescence (LIF) spectra have been recorded of OH radicals produced when H atoms and NO2 react in thermal energy collisions in the region where two uncollimated beams containing the reagents intersect. Spectra of the (0,0) and (1,1), (0,1) and (1,2), and (0,2) and (1,3) bands of the A 2Σ+−X 2Π system have been observed. Distributions of OH over the whole energetically accessible range of rovibrational levels have been determined using surprisal analysis to deduce complete rotational distributions from the observed distributions over high rotational levels (N≥10). Both the vibration and rotation of OH are more excited than expected on a purely statistical basis, the average fractional yields of energy being 〈fvib〉OH=0.23 and 〈frot〉 OH=0.29. Mild preferences are also found for the lower 2Π3/2 spin–orbit component and for the Π(A′) Λ-doublet levels. The last finding is consistent with reaction via a short-lived HONO complex with some consequent constraint to planarity. The paper immediately following this one describes experiments on the energy disposal into the NO product of the H+NO2 reaction and the collision dynamics of this radical-radical reaction are discussed in that paper.