bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics of chemical reactions in solution: Experimental picosecond transient spectra for I2 photodissociation
Authors: Bado Philippe , Dupuy Charles , Magde Douglas , Wilson Kent R. , Malley Michael M.
Published: February 2003

Picosecond transient electronic absorption spectroscopy is used to investigate the molecular dynamics of I2 reaction in solution following dissociative excitation into the A state. Our aim is to provide a sufficiently detailed set of experimental observations so that theoretical treatments of the molecular dynamics of a solution reaction can be rigorously tested. We report here measurements obtained in 13 solvents, pumping at 680 nm with four probe wavelengths of 840, 690, 630, and 595 nm. The time scale of the transient electronic spectral response varies considerably among the different solvents. For a given solvent, the response in general varies strongly with probe wavelength, being slower for higher probe photon energy. This indicates that the spectral response times are not due to recombination times, but are more likely due to the time for vibrational or perhaps electronic decay of already recombined molecules. This conclusion is reinforced by the presence of transient absorption in the near infrared, where vibrationally excited molecules would be expected to absorb. The connection to vibrational decay is supported by the correlation for the different solvents between observed transient response times and the presence of solvent vibrational modes in the frequency range needed to vibrationally relax hot I2.
Intramolecular vibrational relaxation in dimethyl ether
Authors: Stewart George , Ruoff Rodney , Kulp Thomas , McDonald J. D.
Published: February 2003

Intramolecular vibrational energy transfer in isolated gas phase molecules of dimethyl ether has been studied by spectrally resolving the IR laser induced fluorescence from C–H stretch fundamentals, combination bands, and overtones in the region from 2800 to 3050 cm−1. By comparing the experimentally observed relative energy content of the IR active modes with that calculated from a statistical model it is concluded that the C–H stretches are strongly mixed with nearby bath states. The observed energy content distribution is best described by a statistical model which assumes coupling over a 20 cm−1 range. Dilution factors for the various C–H stretch modes were observed which vary from 0.01 to 0.20. These values are dependent on the rotational state excited. The experiments are performed by exciting a molecule cooled in a nozzle expansion with light from an infrared optical parametric oscillator. The fluorescence spectra are recorded with a cryogenic Michelson interferometer.
Kinetic theory of the depolarized light scattering R parameter. II. Explicit results
Authors: Cole R. G. , Hoffman D. K. , Evans G. T.
Published: February 2003

An Enskog-like kinetic theory calculation of R for a neat fluid of hard smooth ellipsoids (or rotors) predicts that R is insensitive to molecular shape, to the density of the fluid (at high densities), and to the temperature, as is borne out by experiment. Calculated numerical values of R range from 0.5 to 0.65 as compared to the experimental values of 0.4.
Intersystem crossing in isolated molecules. Magnetic field effects on the fluorescence decay behavior of 1B3u pyrazine with ‘‘single’’ rovibronic level excitation
Authors: Matsumoto Yoshiyasu , Spangler Lee H. , Pratt David W.
Published: February 2003

Coherently prepared ‘‘single’’ rovibronic levels of electronically excited pyrazine typically exhibit a biexponential fluorescence decay whose preexponential factor ratio A+/A− is magnetic field dependent. Studies of this dependence for different J′ values show that complete spin decoupling from the molecular frame, leading to the intersystem crossing (ISC) selection rule ΔJ=0,±1, occurs at the anomalously low field of ∼100 G. The monoexponential decay of the J′=0 level is independent of the field. An analysis of these results suggests that Coriolis coupling plays an important role in the ISC process, a fact which explains the magnetic quenching behavior of many polyatomic molecules.
Effects of small distortions on the EPR of the Γ6 state (T
d symmetry) of Np(BH3CH3)4 diluted in Zr(BH3CH3)4
Authors: Gamp E. , Edelstein N.
Published: February 2003

The single crystal and powder EPR spectra of 237Np(BH3CH3)4 diluted in Zr(BH3CH3)4 are reported and interpreted in terms of an orthorhombic spin Hamiltonian with gx=1.7739(4), gy=1.8292(4), gz=1.7961(5), Ax=0.1079(2) cm−1, Ay=0.1153(2) cm−1, Az=0.1135(2) cm−1. A modified point charge calculation indicates qualitatively that very small deviations of the site symmetry from Td are enough to account for the observed splitting of the g values.
Far infrared and low frequency gas phase Raman spectra and conformational stability of the 1-halopropanes
Authors: Durig J. R. , Godbey S. E. , Sullivan J. F.
Published: February 2003

The far infrared (375–50 cm−1) and low frequency Raman (400–70 cm−1) spectra of the gaseous 1-halopropanes CH3CH2CH2F, CH3CH2CH2Cl, and CH3CH2CH2Br have been recorded and both the methyl and asymmetric torsional modes have been observed and assigned for both the gauche and trans conformers for all of these molecules. The asymmetric torsions for each molecule have several observed excited states which fall on the low frequency side of the fundamental. The asymmetric torsional potential functions have been calculated and, from these potential functions, the enthalpy differences between the high energy trans and low energy gauche conformers have been determined to be 122±10 cm−1 for the fluoride, 127±10 cm−1 for the chloride, and 37±10 cm−1 for the bromide. The trans and gauche methyl torsions have also been observed and assigned for the three 1-halopropanes. The resulting barriers in cm−1 are: 936±4 (trans), 986±9 (gauche) for 1-fluoropropane; 929±2 (trans), 1080±3 (gauche) for 1-chloropropane; and 841 (trans), 1016±8 (gauche) for 1-bromopropane. A complete vibrational assignment has also been made for the 1-fluoropropane molecule and, from the spectral data for the solid, it appears that there are two or more molecules per primitive cell. Attempts to obtain experimental values for the enthalpy differences in the gas phase were made and these results, as well as the determined potential functions, are discussed in relation to previous studies.
Photoassociative laser-induced fluorescence of XeCl* and kinetics of XeCl(B) and XeCl(C) in Xe
Authors: Inoue Gen , Ku J. K. , Setser D. W.
Published: February 2003

Laser induced fluorescence studies have been done with XeCl molecules, which demonstrate photoassociation (free–bound absorption) as well as conventional bound–bound absorption from the XeCl(X) van der Waals molecules. The XeCl(X) and Xe+Cl pairs were generated by a pulsed dc discharge through 1–6 Torr Xe/Cl2 mixtures. Experiments also were done with Xe/HCl mixtures. The time and wavelength resolved XeCl(B–X) and XeCl(C–A) excitation and fluorescence spectra are reported. Model calculations were done to demonstrate that the laser excitation spectra for XeCl(B,v=0−3) show laser-assisted photoassociation. The time resolved decay of the XeCl(B,v′=0) and XeCl(C,v′=0,1) states was used to measure the radiative lifetimes 11.1±0.2 and 131±10 ns, respectively, and the XeCl(B) and XeCl(C) coupling and quenching rate constants. The transfer and quenching rate constants for XeCl(B) by Xe are assigned as (11±1)×10−11 and (2.3±0.3)×10−11 cm3 molecule−1 s−1, respectively; although, the sum is known with greater certainty than the individual values. The quenching rate constants of XeCl(B,v′=0) by Cl2 and HCl are (4.3±0.2)×10−10 and (6.3±0.5)×10−10 cm3 molecule−1 s−1. Evidence is presented to show that the B–C coupling rate constants are comparable to the XeCl(B,C) vibrational relaxation rate constants for XeCl(B,v′=2) in collisions with Xe.
Relaxation oscillations in the revised Oregonator
Authors: Tyson John J.
Published: February 2003

The Oregonator, a simple but surprisingly successful model of the complex chemistry of the oscillating Belousov–Zhabotinskii reaction, has for years been criticized for the manner in which it accounts for the regeneration of bromide ions. In response to recent objections raised by Noszticzius, Farkas, and Schelly, Noyes has revised the Oregonator model so that the regeneration of Br− is now attributed to the oxidation of organic compounds by HOBr. In this paper the kinetic equations of the revised Oregonator are presented in dimensionless form, and it is shown that the equations admit limit cycle solutions of relaxation type as long as a certain dimensionless ratio of rate constants lies within a relatively narrow interval.
Bead-spring models for an adsorbed polymer molecule in a shear flow
Authors: Atkinson John , Goh C. J. , Phan-Thien Nhan
Published: February 2003

A bead-spring (Rouse-type) model is used to model an adsorbed fragment of a long-chain polymer attached to a surface in a steady shear flow. Analytic solution of the moment equations for the case of linear Hookean springs gives the mean configuration, wall traction, and power dissipation of the segment. A Monte Carlo method based on simulation of the stochastic differential equations of motion is used to deal with nonlinear finite-extension springs. The large increase in hydrodynamic thickness with shear rate found by some experimenters cannot be obtained with these models, although some increase can be obtained if there is an elongational component in the flow, due perhaps to surface nonuniformity or interference between segments.
Comment on the structures of the liquid–vapor interfaces of Na and Na–Cs alloys
Authors: Gryko Jan , Rice Stuart A.
Published: February 2003

We report the results of Monte Carlo simulations of the liquid–vapor interfaces of Na and of two Na–Cs alloys. The calculations are intended to test, on larger samples and using different geometries, the previously reported stratification of the liquid– vapor interface of a metal. Our results show that stratification of the liquid–vapor transition zone of a metal is independent of sample size and geometry. We also demonstrate how the segregation of one component of an alloy at the surface influences the structure of the liquid–vapor interface, and how the pair correlation function in the liquid–vapor interface of a free cluster depends on the radius of curvature of the cluster.
Crystal field effect and paramagnetic susceptibility of Na5Eu(MoO4)4 and Na5Eu(WO4)4
Authors: Huang Jinggen , Loriers Jean , Porcher Pierre , Teste de Sagey Geneviève , Caro Paul , Levy-Clement Claude
Published: February 2003

Fluorescence spectra have been analyzed at 4.2, 77, and 300 K and crystal field calculations have been performed on Na5Eu(MO4)4 (M=Mo, W) powders. The two sets of crystal field parameters are almost identical yielding a rms deviation of 5 cm−1. The values are (in cm−1): B20 =200 (180), B40 =−326 (−330), B44 =−863 (−852), B64 =−442 (−416), and B66 =−327 (−339) for Na4Eu(MoO4)4 and Na5Eu(WO4)4, respectively. Wave functions derived from the simulation were applied to the calculation of the temperature dependent paramagnetic susceptibility. Comparisons with experimental average paramagnetic susceptibilities, measured between 4.2 and 300 K, are very satisfactory.
Vibrational behavior of molecular constituents of organic superconductors: TMTSF, its radical cation and the sulphur analogs TMTTF and TMTTF+
Authors: Meneghetti Moreno , Bozio Renato , Zanon Ines , Pecile Cesare , Ricotta Carmelo , Zanetti Miranda
Published: February 2003

The results of an extensive vibrational analysis of tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene (TMTSF) and of their radical cations are presented. The polarized infrared absorption spectra of oriented crystalline samples of neutral TMTTF and TMTSF (4000–80 cm−1) are reported and compared with powder and solution spectra. The polarization data are used as a basis for the symmetry assignment of the infrared active fundamental modes. Powder and solution Raman spectra are presented and discussed considering the values of the depolarization ratios measured for some bands. Raman and infrared spectra of powder and solution samples of the (1:1) bromide and perchlorate salts of TMTTF and TMTSF are reported. Vibronic infrared absorptions originated by the coupling of the unpaired electrons to totally symmetric modes are identified and compared with those previously reported for unsubstituted tetrathiafulvalene (TTF) radical systems. The assignment of the normal modes of the radical cations is based on the measured Raman depolarization ratios and on the correlation with the neutral molecules and with neutral and ionized TTF. The results of a normal coordinate calculation using a modified valence force field for TMTTF, TMTSF, and their radical cations are presented and the calculated shape of the normal modes of the neutral molecules are reported. The frequency changes following ionization observed for the present case of TMTTF and TMTSF and previously reported for TTF are compared and discussed.
An NMR study of molecular reorientations and diffusion in solid LiBF4
Authors: Reynhardt E. C. , Lourens J. A. J.
Published: February 2003

The 19F second moment and the 19F, 11B, and 7Li spin-lattice relaxation times in a powdered sample of LiBF4 have been measured as a function of temperature. The results show that the BF−4 ion reorients isotropically while the Li+ ion remains stationary. The activation energy associated with the BF−4 motion is 8.5 kcal/mol. Above ∼385 K the BF−4 and Li+ diffuse through the lattice resulting in a 19F second moment which is almost zero. The activation energy of this process is 19.2 kcal/mol. A librational motion presumably influences the 19F T1ρ results just before diffusion sets in. The migration of Li+ around the BF−4 ion, which seems to be a strong possibility in the case of an isolated LiBF4 molecule, does not take place in the solid.
Quantum chemistry in Fock space. III. Particle-hole formalism
Authors: Kutzelnigg Werner
Published: February 2003

A particle-hole formalism is defined which is consistent with the tensor notation (and the index pairing) of the generators of the unitary group and their many-particle generalization. The corresponding generalized Wick theorem is formulated. The classification of basis operators as ‘‘closed,’’ ‘‘open,’’ ‘‘closed from above,’’ and ‘‘closed from below’’ given previously is extended to the present context. Both the perturbative and nonperturbative construction of effective Fock space Hamiltonians is discussed in the particle-hole formalism, especially for the case that the ‘‘physical vacuum’’ satisfies a Hartree–Fock condition. For the calculations of the correlation energy of the physical vacuum Cizek’s CP-MET is compared formally with the unitary variant, the UNICEP (unitary coupled electron pair) approximation. A simple derivation of Goldstone’s ‘‘linked-cluster expansion’’ is also given.
PNO-CEPA and MCSCF-SCEP calculations of transition probabilities in OH, HF+, and HCl+
Authors: Werner Hans-Joachim , Rosmus Pavel , Schätzl Wolfgang , Meyer Wilfried
Published: February 2003

Electronic transition moment functions for the A 2Σ+–X2Π transitions in OH, HF+, and HCl+ have been calculated using RHF, PNO-CI, PNO-CEPA, MCSCF, and MCSCF-SCEP wave functions. The vibrational band transition probabilities are obtained, and the resulting radiative lifetimes are compared with measured values. For OH and HCl+ the deviations are smaller than 10%, but the theoretical lifetimes for HF+ are larger by about 300% than the experimental values. For the electronic ground states of HF+ and HCl+ vibrational transition probabilities have been calculated from MCSCF-SCEP dipole moment functions. Both ions are predicted to be excellent absorbers and emitters in the infrared spectral region.
Accurate Hartree–Fock wave functions without exponent optimization
Authors: Davis C. L. , Jensen Hans-Jo/rgen Aa. , Monkhorst Hendrik J.
Published: February 2003

Basic functions with singularities matching those of the actual orbitals have been tested in analytical Hartree–Fock calculations. Such functions should provide the most rapidly convergent basis set expansions. Exponential singularities at r=∞, characterized by certain ‘‘asymptotic exponents,’’ have been identified by an asymptotic analysis of the Fock equation. Basis sets of Slater functions with these exponents give atomic energies and properties comparable to the most accurate existing analytical calculations, without significantly increasing the number of basis functions. No nonlinear optimizations were required. Calculations of the orbital moments 〈rn〉 show that only moments with n≤N, the number of Slater basis functions, can be evaluated with accuracy, whether or not the exponents are optimized. This effect appears to be caused by the neglect of certain irrational powers in asymptotic forms of the orbitals. The results for molecules suggest that basis functions which more adequately describe the nuclear cusp singularities are required to reproduce the accuracy of numerical Hartree–Fock calculations.
Study of an F center in molten KCl
Authors: Parrinello M. , Rahman A.
Published: February 2003

It is shown that a discretized version of Feynman’s path integral provides a convenient tool for the numerical investigation of the properties of an electron solvated in molten KCl. The binding energy, the magnetic susceptibility, and the pair correlation functions are calculated. The local structure around the solute electron appears to be different from that of an F center in the solid. The Feynman path of the electron dissolved in molten KCl is highly localized thus justifying the F center model. The effect of varying the e−-K+ pseudopotential is also reported.
Monte Carlo simulation of liquid ethane
Authors: Byrnes James M. , Sandler Stanley I.
Published: February 2003

Results of Monte Carlo simulations of liquid ethane are presented for two thermodynamic states: (1) T=105 K, ρ=0.630 g/cc; and (2) T =181 K, ρ=0.548 g/cc. Three types of interaction models were used. The first was a two-site model of ethane using exp-6 type interactions with various locations considered for the interaction sites and correlation sites. The second was an eight-site model considering all atoms locked in the staggered conformation, with geometric and potential parameters that have been derived from other data. The final model was similar to the second except that a hindered rotation of the methyl groups around the C–C bond was allowed. The Monte Carlo results demonstrate the importance of using a detailed molecular model on liquid structure. Further, the lack of importance of intramolecular rotation in liquid ethane is demonstrated.
Monte Carlo study of polyelectrolyte behavior. II.
Authors: Brender Chava , Lax Melvin , Windwer Stanley
Published: February 2003

A Monte Carlo simulation of an isolated polyelectrolyte chain in the presence of its counter ions without added salt is reported. Configurational properties such as the mean square end-to-end distance 〈R2N〉, the mean square radius of gyration 〈S2N〉, and the mean reduced electrostatic energy 〈UNI/NIkT〉, were studied as a function of the ionic strength of the solution, C; and the % charge of the chain. System behavior is classified into three regions: I: κ−1≪〈S2N〉1/2; II: κ−1∼1/2<S2N〉1/2; III: κ−1≫〈S2N 〉1/2, where κ−1 is the Debye–Hückel radius describing the scale of order of interionic interactions. In region I, the polyelectrolyte behaves configurationally as a random coil and the system is describable electrically as a simple electrolyte. In region II, the polyelectrolyte undergoes a conformational transition and the system electrically deviates notably from simple electrolyte behavior. In region III, the polyelectrolyte behaves configurationally as a rod and the system is characterized electrically by 〈UNI/NIkT〉≳0.
Theory of nematic systems of semiflexible polymers. II. Chains of finite length in the bulk
Authors: Ronca G. , Yoon D. Y.
Published: February 2003

The statistical theory of nematic systems of semiflexible polymers is extended to polymer chains of finite length in the bulk. Isotropic-nematic transition temperature, entropy change, orientational order, and conformational order at the transition are computed as a function of chain flexibility and chain length, using the new worm-like chain model with limiting curvature developed recently. The theory can cover a wide range of chain flexibility and its results converge to those of the theory of rod-like particles in the limit of completely rigid chains. Absolute stability of the nematic phase at all temperatures is predicted within suitable ranges of chain flexibility and chain length.
Configurational statistics of the worm-like chain with limiting curvature (WCLC)
Authors: Ronca G. , Yoon D. Y.
Published: February 2003

Configurational properties of the worm-like chain with limiting curvature (WCLC), developed recently to investigate nematic systems of semiflexible polymers, are analyzed in detail. Employing this WCLC model, expressions for the characteristic ratio, the distribution of the end-to-end chain vector, and the persistence length are derived. Specific numerical results are obtained for polymethylene chains of finite length, and compared with the corresponding results of the rotational isomeric state theory and the classical Kratky–Porod worm-like chain. Orientational correlations of chain segments with chain vector and the temperature dependence of the internal energy are also derived and compared with the predictions of other chain models.
Coexistence curve of polystyrene in methylcyclohexane. III. Asymptotic behavior of ternary system near the plait point
Authors: Dobashi T. , Nakata M. , Kaneko M.
Published: February 2003

Coexistence curve measurements were made on the systems of polystyrene (PS) in methylcyclohexane (MC) by using the refractive index method. The entire shape of the coexistence curve of the ternary system PS I(Mw=1.72×104)–PS II(Mw=7.19×105)–MC with ξ2=0.1704 is very different from those of the binary systems PS(I)–MC and PS(II)–MC, where ξ2 denotes a volume fraction of PS(II) to total polystyrene. The asymptotic shape of the ternary coexistence curve near the critical point was found to be characterized by the exponent βt=0.384±0.004, which is close to the fully renormalized critical exponent. The direction of tie lines near the critical point was found to be parallel to each other from the calculation with the classical empirical equation of the Gibbs free energy.
Spin-orbit coupling induced homogeneous broadening of the acetone ODMR transitions
Authors: Gehrtz M. , Bräuchle Chr. , Voitländer J.
Published: February 2003

The zero-field ODMR transitions of the acetone triplet state were observed to be homogeneously broadened. The linewidths (4 and 8 MHz for the Tx1 −Ty1 and Ty1 −Tz1 transitions, respectively) do not change upon deuteration. Thus broadening due to fluctuating (proton or deuteron) hyperfine fields has to be ruled out. Instead the homogeneous broadening is related to the spin-orbit contribution to the zero-field splitting of the acetone 3nπ* state which was evaluated previously. A straightforward calculation based on Zewail’s idea, that the spin-orbit coupling strength correlates with the homogeneous linewidth for 3nπ* states, is in fairly good agreement with the experimental results.
Translational motions of rare-gas atoms, rotation-translation motions of diatomic molecules, and host–guest interactions in β-quinol clathrate
Authors: Hirokawa Shoji
Published: February 2003

Dynamical states of Ne, Ar, Kr, Xe, N2, O2, NO, CO, and HCl moving in a rigid cage, which is assumed to be the same as that of H2S-β-quinol, are studied quantum mechanically. The host–guest interaction is considered to consist of 6-N atom–atom potentials and electrostatic multipolar interactions. Each theoretical far-infrared line fits to experiment at least within the range of the observed linewidth. Agreement of the adjusted value of the quadrupole moment referred to the center of mass with its gas-phase value is excellent for CO, and moderate for NO and HCl. It may be due to distortion of the cage structure that the fitted value of the atomic radius of oxygen atom for the clathrate of rare-gas atoms is different from the van der Waals radius. As to the constrained translation (rattling motion), it is suggested that Ne is different; its frequencies of the motion on the XY plane are imaginary in the harmonic approximation. As to the rotational motion, HCl is different; its molecular axis oscillates near the XY plane, rotating almost freely around the Z axis. Effects of the translation-rotation coupling for each diatomic molecule are discussed in detail.
Determination of the optimum number of bins to use in a histogrammic representation of a probability density function
Authors: Gislason Eric A. , Goldfield Evelyn M.
Published: February 2003

A method for obtaining the optimum number of bins to use in a histogrammic representation of a probability density function is presented. Common examples of these functions are final-state distributions determined in classical trajectory studies. The method determines the minimum number of bins consistent with the requirement that the trajectories are randomly distributed within each bin. The method has been tested against two model systems and works very well.
Molecular supersonic jet studies of aniline solvation by helium and methane
Authors: Bernstein E. R. , Law K. , Schauer Mark
Published: February 2003

The technique of two color resonant two photon ionization coupled with time of flight mass spectroscopy has been employed to study aniline–He (AnHex) and aniline–CH4 [An(CH4)x] van der Waals clusters generated in a supersonic molecular jet. This technique allows identification of spectroscopic transitions with clusters of known mass because no ion fragmentation is observed. Specific features in the optical fluorescence excitation and dispersed emission spectra can thereby be uniquely identified with a particular cluster. Cluster vibrations can be analyzed by a Morse potential to yield the An–He bond dissociation energy D0∼100±50 cm−1. Careful analysis of the dispersed emission from AnHex suggests 145<D0<155 cm−1. It is found that the van der Waals bond stretching frequency is nearly the same in the ground and excited states and that there is a strong propensity rule for ΔV=0(V=vdW bond mode) as expected in this case, although ΔV=±1 transitions can be observed. The AnHe1 and AnHe2 origins are slightly red shifted with respect to the An origins, while the AnHex (x≥3) origin is broad and nearly unshifted. This pattern is followed for An(CH4)x clusters; AnCH4 transitions are red shifted 80 cm−1 from the comparable An features and An(CH4)2 transitions are 160 cm−1 below their comparable An feature. The An(CH4)x (x≥3) transitions appear at ∼200–300 cm−1 below their comparable An mode. The binding energy for the An–CH4 bond is found to be 500<D0<700 cm−1 in the 1B2 state of aniline. Aniline has a strong preference for binding the solvent above and below the aromatic ring. Since the D0 is large for An–CH4 and the stretching mode is only ∼25 cm−1 the An(CH4)x system builds up a large density of states in the van der Waals degrees of freedom. This density of states allows intramolecular vibrational redistribution (IVR) to take place, if the An mode excited is lower in energy than the D0 value. The rate of IVR from 6a1 (O00+500 cm−1) is somewhat faster than the 5 ns fluorescence rate but much slower than the rate of vibrational predissociation (VP) from higher levels. Both the IVR process, due to the van der Waals vibrational density of states, and the limiting solvent red shift, at a value similar to that found for cryogenic solutions, are discussed in terms of these clusters as model solute/solvent systems.
Exciton kinetics in condensed rare gases
Authors: Hitachi Akira
Published: February 2003

An attempt has been made to explain large rate constants for Penning-type ionization in condensed rare gas mixtures. Excitons are treated as particles moving rapidly in condensed media with a mass similar to the electron mass. A formula for Penning ionization of atom collisions induced by a dipole–dipole interaction gives cross sections of the correct order of magnitude.
Laser induced vibrational energy transfer in iron pentacarbonyl
Authors: Langsam Yedidyah , Ronn A. M.
Published: February 2003

The internal kinetics of Fe(CO)5 as well as the kinetics between Fe(CO)5 and other nonreactive species were studied using the technique of laser induced fluorescence. The energy transfer behavior of this large polyatomic is discussed in terms of existing V–V and V-T/R theories and collisional energy transfer. Iron pentacarbonyl’s vibrational energy structure is treated by means of a simple three and four level energy transfer scheme. Subsequent to excitation of the 10 μ region by a CO2 laser, infrared fluorescence has been detected from the ∼16, ∼5, and ∼4 μ regions of Fe(CO)5. A single exponential decay rate of 13.6 ms−1 Torr−1 is observed from the ∼5 μ region, in good agreement with other decay rates established for smaller polyatomics possessing similar vibrational level structure. Under conditions of low fluence (∼30 mJ/cm2), this region is activated at a rate of 474 ms−1 Torr−1 suggesting a rapid near resonant collisional energy transfer. Under conditions of high fluence (∼5 J/cm2), the activation of the ∼5 μ region proceeds at a rate of 1250 ms−1 Torr−1 suggesting a different pathway for the determining step of the excitation process. The rare gas deactivation rates as well as those with Ni(CO)4, CO(CO)3No, and CO (as well as the reverse rate) and the crossover rate from excited Fe(CO)5 to CO in high rare gas dilution have also been determined.
Rate calculations from time-dependent wave packet methods: The relationship of the pure state and canonical total reaction probability
Authors: Agrawal P. M. , Agrawal N. C. , Viswanathan R. , Raff L. M.
Published: February 2003

Reaction rate coefficients for collinear A+BC → AB+C exchange reactions computed directly by the time evolution of an initial pure-state wave packet given by ψ(t=0)= φ(q1)Σn′n=0Aneiδn Xn(q2), where φ(q1) describes the distribution of translational states present in the collision, Xn(q2) is an nth-state vibrational wave function for the BC molecule, An is a weighting coefficient dependent upon the nth vibrational-state eigenvalue and temperature, and the δn are the relative phases of the initial-state vibrational wave functions, are compared with those obtained using the density operator for a canonical ensemble of noninteracting A+BC molecules. It is shown that the computed total reaction probability for the pure-state wave packet is independent of the δn and is the same as that obtained from the density operator for the full canonical ensemble provided certain integrals over the product configuration space vanish. Qualitative arguments are advanced that suggest these integrals may indeed vanish. Numerical computations of the reaction probability for the T+HD → TH+D exchange reaction at 1000 and 10 000 K indicate that the contribution of these integrals to the overall reaction probability is small. The immediate consequences of this result are that rate coefficients may be computed using pure-state wave packets that do not include phase factors, and a general theorem that orthogonal reactant states will evolve as orthogonal product states.
Atom anisotropies and effective charges in vibrational intensity analysisa)
Authors: Kim Kwan , King W. T.
Published: February 2003

In the analysis of infrared intensity data, it is shown that the value of the atom anisotropy relative to the effective atomic charge is a very useful parameter in characterizing chemical bonds in terms of atomic properties. The application of this parameter reveals that the anomalously large effective charges of hydrogen in HCCH and HCN in comparison with those in other hydrocarbons can be adequately explained by virtue of their rather ionically bonded protons.
Scaling relations for multiplicative quantum mechanical operators
Authors: Chang Britton , Eno Larry , Rabitz Herschel
Published: February 2003

Scaling relations are derived for the matrix elements of a multiplicative quantum mechanical operator. While the relations have been set down earlier, the present derivation is, we believe, more transparent. In order to satisfy certain attractive properties, the coefficients in these relations have previously been identified as the principal values of improper integrals. We show here, however, that the scaling relations can be rewritten in terms of scaling coefficients which are given as proper integrals and which satisfy the same attractive properties.
On the orientation function of the electric dichroism of DNA
Authors: Diekmann Stephan , Jung Manfred , Teubner Max
Published: February 2003

Various orientation functions based on the ‘‘saturating induced dipole model’’ are fitted to the electric dichroism data of DNA restriction fragments measured by Diekmann et al. All functions give acceptable to excellent fits. Orientation functions with an increasing number of fittable parameters are discussed. The DNA fragment length dependence of the fitted parameters as well as the error sum of the fit of some functions are presented. Simple orientation functions with three free parameters are suggested: the polarizability, the saturation field strength, and the limiting electric dichroism at infinite field strength.
NMR study of the thallium ions mobility in the pyrochlore TlNb2O5F
Authors: Man P. P. , Theveneau H. , Papon P. , Fourquet J. L.
Published: February 2003

Both cw and pulsed NMR measurements on the two naturally abundant isotopes of thallium 203Tl and 205Tl and on fluorine 19F have been made in TlNb2O5F as a function of temperature and frequency. The temperature dependence of the Tl resonance linewidth gives an Arrhenius activation energy of 0.08 eV. The wide line spectra indicate the presence of nucleus–electron–nucleus indirect interaction. T1 and T2 measurements of both isotopes demonstrate thermally activated motion of Tl+. The nonexponentiality of spin lattice relaxation leads us to introduce two relaxation times corresponding to two motions of Tl ions giving two activation energies—0.25 and 0.14 eV. A consistent interpretation of these results can be given in terms of an exchange interaction between the two isotopes of Tl associated with a jump from cavity to cavity, and a chemical shift interaction associated with a swing from site to site inside a same cavity of the rigid network NbO5F.
A pressure consistent BGY equation: Virial coefficients for rigid disks and spheres
Authors: Devore Jerald A.
Published: February 2003

A closure for the two-particle BGY equation is formulated by coupling the three-particle distribution functions for the superposition approximation and the exact expression for rigid rods through a density and angle dependent quantity f( ρ,θ)=Ψ( ρ)χ(θ). Given the angle dependence, one can determine Ψ( ρ) by requiring that the virial theorem and the compressibility relation yield the same equation of state (pressure consistency). Fourth through tenth virial coefficients for rigid disks and spheres are calculated using this closure.
Linear response theory of dye–polymer complex circular dichroism
Authors: Rabenold David A.
Published: February 2003

A linear response theory for the circular dichroism of dye–polymer complexes is presented. The time dependent Hartree approximation is employed and the effect of electrostatic fields is included in an approximate manner. Periodic boundary conditions are used to describe the polymer which may have several chromophores per repeating unit and any number of bound dye molecules. A partitioning technique is used to obtain the two components of the circular dichroism, that of the dye aggregate which is shifted due to dye–polymer interaction. The induced circular dichroism predicted by a less accurate scheme, a matrix approach, is shown for a typical case to be at least 24% less intense than that predicted by the time dependent Hartree scheme.
Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. I. Fundamental tones
Authors: Lalov I. J.
Published: February 2003

A theory is advanced of the vibrational circular dichroism (VCD) and of the vibrational optical rotatory dispersion (VORD) in molecular crystals for the frequency region of the intramolecular vibrations. The substantial role of the symmetry of the crystal lattice is shown. The contributions of the fundamental tones of nondegenerate and degenerate vibrations in VORD and VCD are discussed. The effect of the separate molecule and of the intermolecular interaction on the gyrotropy of the crystal is estimated. The most favorable conditions for experimental examination of VCD are commented upon, as well as the information obtained from the VCD spectra.
Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. III. Overtones of nondegenerate vibrations
Authors: Lalov I. J. , Svetogorsky D. A.
Published: February 2003

The contribution of overtones of nondegenerate intramolecular vibrations to the vibrational circular dichroism of molecular crystals with one and two molecules in an elementary cell is calculated. The dielectric permittivity and the gyration tensor are found in the region of the bound states of the two phonons and in the band of many particle states.
Scaling behavior in collinear atom–diatom collisions: Energy transfer from high vibrational states
Authors: Ramaswamy Ramakrishna , Bhargava Rajeev
Published: February 2003

Classical energy transfer in a simple collinear collision system is studied within the framework of a recently derived classical scaling formalism. In the present system which corresponds to collinearly colliding I2 (Morse oscillator) + rare gas (He, Ar), a simple polynomial function in the initial vibrational quantum number of I2 scales the first moment of the energy transfer (ET). Using the computed ET moment from only five states as input, it is possible to predict that from any other bound state in the vibrational manifold 0≤n≤110. There is correspondence—in a limiting case—between the classical scaling law and the (quantum) energy corrected sudden scaling theory given earlier; this allows for the interpretation of the classical scaling coefficients and indicates that the higher order terms are necessary in order to account for multiquantum vibrational transitions in the V-T process.
Charge transfer between plastocyanin and iron hexacyanide
Authors: Cho K. C. , Austin Robert H.
Published: February 2003

The electron exchange kinetics between plastocyanin and iron hexacyanide has been studied by laser temperature jump. A general kinetic scheme assuming weak binding of the iron hexacyanide to a distant (10 Å) site has been used to extract intrinsic exchange rates of 68±20 s−1 for Cu(2+) to Cu(1+) reduction, and 200±100 s−1 for the reverse reaction. These intrinsic rates have been compared to simple nonadiabatic tunneling theories and found to be in reasonable agreement.
HF rotational laser emission from the ClF/H2 reaction: Time evolution of the gain
Authors: Richmond Geraldine L. , Pimentel George C.
Published: February 2003

The time evolution of the HF rotational laser gain from the C1F/H2 reaction was measured for seven v=0 and v=1 high J transitions using the pulsed, tandem laser technique. Three of the seven transitions studied, v=0, J=30→29, 29→28, 28→27, show initial negative gains that become positive within about 1 μs and then, after 3 or 4 μs become negative again. This behavior and its concentration dependence shows that the gain at the earliest times is due to the nascent population created by the H+ClF→HF+Cl reaction. The change to positive gain must be due to a combination of collisionally induced R→T erosion pumping and V→R near-resonant energy transfer involving ClF as a collision partner. The subsequent shift to negative gain is caused by accumulating reaction products HF and HCl, which reduce gain through R→R′ energy transfer. The results show that the H+ClF reaction directly populates rotational states up to J=30 in v=0 and to J=32 in v=1 with g-weighted populations [nv, jg=nv, j/(2J+1)] such that n0,30g<n0,29g<n0,28gn0,21g; n1,32g<n1,31g; n1,30gn1,23g. They show, further, that collisional relaxation processes quickly become influential factors in the gain of a pulsed HF rotational laser.
Use of energy constraint for refinement of electron momentum distributions
Authors: Gadre Shridhar R. , Gejji Shridhar P.
Published: February 2003

A procedure based on the calculus of variations to refine a given electron momentum density with the constraints of the given number of electrons and a prescribed 〈p2〉 expectation value has been developed. This procedure has been tested with near-Hartree–Fock (NHF) electron momentum density data employed as initial distributions for the atoms beryllium through neon. The constraints of the corresponding CI-theoretical and experimental energy values have been imposed. In all cases, the values of the refined electron momentum density at p=0 and the peak value of the Compton profile J(0) are lower than the corresponding NHF ones. This lowering of J(0) value is generally in conformity with the corresponding results from CI and MC–SCF calculations. The present procedure is seen to simulate well the electron momentum densities and 〈p−1〉 as well as 〈p〉 expectation values obtained from correlated wave functions without ever doing extra quantum mechanical calculations.
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o calculation of the thermochemical data on the H3O++H2O=H5O+2 gas-phase clustering
Authors: Yamabe Shinichi , Minato Tsutomu , Hirao Kimihiko
Published: February 2003

Thermochemical data for the clustering H3O++H2O=H3O+⋅⋅⋅H2O have been estimated theoretically. Basis set dependence of the entropy change as well as of the enthalpy change has been examined. The double-zeta plus polarization quality is required to reproduce them satisfactorily. The stabilizing dispersion energy which corresponds to an intermolecular correlation correction is calculated to be about 3 kcal/mol. The role of components of enthalpy and entropy changes has been discussed.
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o calculations on negatively charged water clusters
Authors: Rao Bijan K. , Kestner Neil R.
Published: February 2003

Using a 4-31G basis set for water and a four term wave function for the trapped electron (two s and two p functions), the energy of clusters as large as (H2O)−n with n=8 are calculated. Large clusters (n>7 or 8) might be thermally stable, but still appear to have higher energies than neutral clusters. The excitation energy of the excess electron increases rapidly as a function of cluster size. Most of the stability of the clusters is obtained with the second coordination layer. Convergence of the ‘‘heat of solution’’ energy is quite slow as a function of the number of waters; the increases are about 7 kcal/mol per water in the second coordination shell and about 1–2 kcal/mol for molecules in the third coordination shell. Sixfold coordination in the first coordination layer about the electron leads to a species slightly lower in energy than fourfold coordination. Spin density at the hydrogens in all cases is negative in these SCF calculations, contrary to the experimental results of Kevan in alkaline glasses.
Effective core potential calculation using frozen orbitals applications on the fourth period main group elements
Authors: Strömberg Ann , Gropen Odd , Wahlgren Ulf
Published: February 2003

The degree of polarization of the 4d shell in fourth row main group elements in bond formation has been investigated for the systems TeO2 and I2 using frozen orbital techniques. Furthermore, the frozen orbital ECP method has been appplied on these systems and on the atoms In–Xe. It is concluded that effects on the 4d orbital in bond formations in fourth row main group elements are negligible and that FO–ECP can be used for quantitative investigations of systems of this type.
Thermodynamics of steady states: Is the entropy-production surface convex in the thermodynamic space of steady currents?
Authors: Tykodi R. J.
Published: February 2003

If the rate of entropy production Θ is a convex function of the steady currents Yi in a steady-state situation, then the quantities ∂2Θ/∂Yk ∂Yi and ∂Ωi/∂Yk (Ωi is the affinity conjugate to the steady current Yi) must meet certain requirements, and Θ itself must be an ‘‘inflationary’’ function of the steady currents Yi, i.e., Θ(λYi)≳Θ(Yi) for λ≳1 and {Yi}≠{0}. A heat-conduction counterexample that does not satisfy the convexity conditions is displayed.
Computer experiments on aqueous solutions. III. Monte Carlo calculation on the hydration of tertiary butyl alcohol in an infinitely dilute aqueous solution with a new water–butanol pair potential
Authors: Nakanishi Koichiro , Ikari Kenji , Okazaki Susumu , Touhara Hidekazu
Published: February 2003

Monte Carlo calculations have been carried out for an infinitely dilute aqueous solution of tertiary butylalcohol (TBA) at 298.15 K and ordinary density by the Metropolis scheme in an NVT ensemble. The total number of molecules is 216, one of which is TBA. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential is used, whereas a new pair potential is determined for water–TBA interaction from ab initio LCAO SCF MO calculations for more than 380 different dimeric configurations with an STO-3G basis set and subsequent multiparameter fitting of MO data thus obtained to an appropriate potential function with a nonlinear optimization method. In the Monte Carlo run, 3 600 000 configurations have been generated and final 2 000 000 configurations have been used to obtain energetic and structural information on the hydration structure of TBA. As in the case of the methanol solution studied previously, the potential energy and structure of water tend to be stabilized by the introduction of one TBA molecule, and two strong hydrogen bonds between TBA and the surrounding water molecules favor the formation of a bulky and stable hydration shell which includes cooperatively connected hydrophobic hydration structures. This is clearly shown in density diagrams for water around TBA which visualize remarkable structuration of water. The iceberglike structure around TBA is more pronounced than that of methanol.
Temperature dependence of positronium reactivities with charge transfer molecules in bilayer membranes
Authors: Jean Y. C. , Yu C. , Wang Y. Y. , Yeh Y. Y.
Published: February 2003

Rate constants for positronium atoms reacting chemically with charge-transfer molecules such as p-benzoquinone, nitrobenzene, and coenzyme Q-10 in a model bilayer membrane, dipalmitoylphosphatidylcholine (DPPC), have been measured at temperatures between 23 and 65 °C. A strong variation of the positronium chemical reactivities, kPs was observed in these systems: kPs increases with increasing temperature until the pretransition temperature of the membrane reaches a maximum value near the main transition temperature and decreases at temperatures higher than the main transition temperature. This variation is interpreted in terms of fluidity and permeability changes associated with the phase transitions of membranes and in terms of charge-transfer-complex formation between the solubilized molecules and the polar head of the membrane. These results demonstrate that positronium and its annihilation characteristics can be employed to investigate charge transport phenomena and microstructural changes of real biological membranes.