bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactive islands as essential mediators of unimolecular conformational isomerization: A dynamical study of 3-phospholene
Authors: Marston C. Clay , De Leon N.
Published: July 2002

In this paper we focus on the detailed nonlinear classical dynamics of conformational isomerization. In particular we concentrate on systems which admit phase space structures we call ‘‘reactive islands.’’ Our calculations are on a two degree of freedom model of the molecule 3-phospholene with an experimentally fit potential energy surface by Harthcock and Laane. The reactive islands (RIS) are embedded within and are part of chaotic regions of phase space. We find that the RIS are constructed from a linear stability analysis of the period 1 orbit at the transition state or approximated by a similar analysis on reactive periodic orbits. The two approaches converge as the order of the reactive periodic orbit increases. It is found that the fully constructed RIS have well defined regions of reactivity and thus mediate the process of conformational isomerization.The overlap areas of the RIS give important kinetic information such as probabilities for trapped to reactive motion, reactive to trapped motion, and primary back reaction. The implications of this work lie in precisely identifying the microscopic dynamical pathway for isomerization. The results of this work and its relation to the related work of Gray and Rice as well as its relation to contemporary work in modern dynamics is also discussed.
On the 1593 Å transition of CS2
Authors: McDiarmid Ruth , Doering J. P.
Published: July 2002

The optical absorption spectrum of the 1593 Å transition of static (room temperature) and jet-cooled CS2 has been remeasured to determine the temperature dependence of the spectrum and to better characterize the experimental band shapes and intensities. Based on these results and others, current assignment controversies are resolved and the separation between the singlet and triplet components of the excited state, the excited state vibrational frequencies, and electronic and vibronic properties of the excited state are determined.
Measurements of the diffusion coefficients of atomic bromine in rare gases
Authors: Hwang M.-D. , Jiang R.-C. , Su T.-M.
Published: July 2002

The laser-photolysis-recombination-luminescence-detection method was employed to measure the diffusion coefficients of Br–rare-gas systems. The measured values are DBr–He=0.66 ±0.11, DBr–Ne=0.25±0.04, DBr–Ar=0.16±0.03, DBr–Kr=0.12±0.02, and DBr–Xe =0.086±0.015 cm2/s, respectively, at 1 atm and 300 K. Also, the diffusion coefficient of Br–Br2 was measured to be 0.065±0.011 cm2/s. A comparison was made with the results calculated from the potential curves obtained by the crossed molecular beam experiments for the Br–Ar, Br–Kr, and Br–Xe systems, and with the datum of Br–Ar measured by the time resolved–mass spectrometry. The extent of the similarity of the diffusion constants between the Kr–rare-gas systems and the Br–rare-gas systems was discussed over the temperature range of 250K to 1050 K.
Accurate quantum thermal rate constants for the three-dimensional H+H2 reaction
Authors: Park Tae Jun , Light J. C.
Published: July 2002

The rate constants for the three-dimensional H+H2 reaction on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) surface are calculated using Pack–Parker hyperspherical (APH) coordinates and a C2v symmetry adapted direct product discrete variable representation (DVR). The C2v symmetry decomposition and the parity decoupling on the basis are performed for the internal coordinate χ. The symmetry decomposition results in a block diagonal representation of the flux and Hamiltonian operators. The multisurface flux is introduced to represent the multichannel reactive flux. The eigenvalues and eigenvectors of the J=0 internal Hamiltonian are obtained by sequential diagonalization and truncation. The individual symmetry blocks of the flux operator are propagated by the corresponding blocks of the Hamiltonian, and the J=0 rate constant k0(T) is obtained as a sum of the rate constants calculated for each block. k0(T) is compared with the exact k0(T) obtained from thermal averaging of the J=0 reaction probabilities; the errors are within 5%–20% up to T=1500 K. The sequential diagonalization–truncation method reduces the size of the Hamiltonian greatly, but the resulting Hamiltonian matrix still describes the time evolution very accurately. For the J≠0 rate constant calculations, the truncated internal Hamiltonian eigenvector basis is used to construct reduced (JKJ) blocks of the Hamiltonian. The individual (JKJ) blocks are diagonalized neglecting Coriolis coupling and treating the off-diagonal KJ±2 couplings by second order perturbation theory. The full wave function is parity decoupled. The rate constant is obtained as a sum over J of (2J+1)kJ(T). The time evolution of the flux for J≠0 is again very accurately described to give a well converged rate constant.
Periodic orbits, bifurcations, and quantum mechanical eigenfunctions and spectra
Authors: Founargiotakis M. , Farantos S. C. , Contopoulos G. , Polymilis C.
Published: July 2002

The influence of periodic orbits on quantum eigenfunctions and the dynamical evolution of wave packets is studied on a model 2D potential. The phase space structure is portrayed by constructing the diagram of characteristics of the most important families of periodic orbits. It is found that the topologies of eigenfunctions can be assigned to certain periodic orbits. Families bifurcating from the principal families, as well as irregular families which are not associated with the principal families, mark the eigenfunctions. The evolution of wave packets and the spectra which are produced from them show that quantum mechanically there is no communication among different resonance regions even at energies where chaos predominates in classical mechanics. This quantum regularity and localization is explained by testing the conjecture that the area of turnstiles (regions in the surfaces of section through which trajectories pass to get in or out the resonance zone) is less than Planck’s constant. It is demonstrated that the systematic study of phase space structure through the families of periodic orbits in association with the solution of the time dependent Schrödinger equation for wave packets localized initially on such orbits provide the numerical techniques for studying highly excited species.
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o test of the pairwise additivity assumption of semiempirical electronic structure: Spectator model of correlation contributions
Authors: Wang Xiao-Chuan , Freed Karl F.
Published: July 2002

Correlated, ab initio effective valence shell Hamiltonian (Hν) calculations of CH2 are used to test the pairwise additivity assumption of semiempirical electronic structure methods in which it is assumed that the one- and two-center semiempirical integrals in an atomic basis are independent on the molecular environment (except for the one-electron, one-center Coulomb integral). Computed ab initio Hν matrix elements are transformed into a molecule independent atomic basis and are analyzed as a function of bond length and bond angle. Large departures from the pairwise additivity assumption emerge for several classes of Hν matrix elements. The computed behavior is well represented by a spectator model for the influence of molecular environment on correlation contributions to individual Hν matrix elements. Tests are also made of the applicability of Mulliken type approximations to Hν integrals.
Photoinduced nucleation in supersaturated isoprene vapor: The mechanism of photopolymerization
Authors: El-Shall M. S. , Rabeony M. H. , Reiss H.
Published: July 2002

Homogeneous nucleation and photochemical polymerization in supersaturated isoprene vapor were investigated using a diffusion cloud chamber. The mechanism of photoinitiation appears to involve the photolysis of a volatile product which results from the primary photodecomposition process. The photonucleation results from direct propagation as well as recombination of polymer molecules. At very low supersaturation ratio (e.g., 1.2), recombinative termination becomes the only possible way to achieve the tunable size required for the nucleation. A nonsteady state experiment allows us to resolve the nucleation spectrum into contributions due to polymers resulting from recombination and those due to direct propagation.
Microwave spectrum, quadrupole coupling constants, and structure of N–bromodifluoromethanimine
Authors: Groner P. , Nanaie H. , Durig J. R. , DesMarteau D. D. , Bauknight C. W.
Published: July 2002

A continuous flow system has been used to measure the microwave spectrum (26.5 to 40.0 GHz) of rapidly decomposing N–bromodifluoromethanimine, CF2=NBr. The hyperfine structure of the spectrum caused by two nuclei with nuclear quadrupole moments (14N,79Br, or 81Br) has been analyzed and rotational, centrifugal distortion, and quadrupole coupling constants have been determined by a global least-squares fit for both bromine isotopic species. Identification and assignment of R transitions in the b-type spectrum have been facilitated considerably by the determination of the magnitude of the rotational constants from the hyperfine structure of the Q transitions alone. A structure compatible with the observed rotational constants has been determined for this planar molecule.
Spin–orbit distortion of the hyperfine structure in heavier molecules: Breakdown of the case (aβ) formalism in the B 3Φ–X 3Δ system of gaseous NbN
Authors: Azuma Y. , Barry J. A. , Lyne M. P. J. , Merer A. J. , Schröder J. O. , Féménias J.-L.
Published: July 2002

A detailed analysis of the rotational and hyperfine structure of the (0,0) band of the B 3Φ–X 3Δ electronic transition of NbN has been performed from sub-Doppler spectra taken with linewidths of about 50 MHz. The Nb hyperfine structure is impressively wide in both states, but particularly so in X 3Δ where one of the unpaired electrons occupies a σ orbital derived from the metal 5s orbital. The electron spin and hyperfine structures do not follow the expected case (aβ ) coupling because of extensive second order spin-orbit effects. It is shown that the asymmetry in the spin–orbit structure of X 3Δ is explained almost quantitatively by interaction with a 1 Δ state from the same electron configuration (which lies at 5197 cm−1); also cross terms between the spin–orbit and Fermi contact interactions in the matrix element 〈3Δ2‖H‖1Δ〉 produce a large correction to the apparent coefficient of the I⋅L magnetic hyperfine interaction in X 3Δ2. The hyperfine structure in a triplet state turns out to be extremely sensitive to the details of the electron spin coupling, and reversals in the sense of the hyperfine structure in the 3Φ4–3Δ3 and 3Φ2–3Δ1 subbands are shown to be consistent with the3Δ state being a regular spin–orbit multiplet (A>0). Particular care has been taken with the calibration, which has meant that extra terms have needed to be added to the magnetic hyperfine Hamiltonian to account for the spin–orbit distortions: instead of the usual three parameters needed in case (aβ ) coupling, the B 3Φ state has required four parameters and the X 3Δ state has required five. The model explains the data very well, and the standard deviation in the least-squares fit to more than 1000 hyperfine line frequencies was 0.000 58 cm−1 (17 MHz).
Surface-pressure-induced reversible color change of a polydiacetylene monolayer at a gas–water interface
Authors: Tomioka Yasushi , Tanaka Noaki , Imazeki Shuji
Published: July 2002

The photopolymerization properties and the surface pressure dependence of excitonic absorption of polydiacetylene (PDA), i.e., poly-(heptacosa-10,12-diynoic acid), are investigated by in situ reflection spectra of a monolayer at a N2 gas–water (pure water, CdCl2 aqueous solution) interface. The electronic structure of the initially appearing PDA is strongly dependent on the packing condition of diacetylene monomer, the blue- and red-form PDAs that initially appear in loose and close packing conditions, respectively. A reversible color change induced by a change in surface pressure is found for the first time in a partially polymerized PDA monolayer on pure water, in which blue and red form PDAs appear predominantly at low and high surface pressures, respectively. Further, two metastable phases, observed at wavelengths about 40 nm longer than those of the blue form, are thought to be induced by the rheological process of the substitutional sidegroups. Our results suggest that these color changes are closely related to a strain on the acetylenic backbone structure which is induced by the deformation of substitutional alkyl sidegroups.
The observation of the ν1 fundamental bands of HOSi+ and DOSi+
Authors: Warner H. E. , Fox A. , Amano T. , Bohme D. K.
Published: July 2002

The ν1 (H–O) stretches of HOSi+ and DOSi+ have been observed with the difference frequency laser system in the hollow cathode discharge. The ions were observed either by discharge of (CH3)3SiOH or (CH3)3SiOD in a buffer containing H2 (or D2) and He, or by discharging SiH4 and N2O in a buffer of H2 or H2 and He. Discharging SiH4 and H2O in buffers of H2 and H2 and He was not successful in producing HOSi+. The constants obtained were: ν0=3662.364 67(15) cm−1, B000=18 260.75(22) MHz, D000=20.44(31) kHz, B100=18 195.05(23) MHz, and D100=19.91(34) kHz for HOSi+. For DOSi+ we obtained: ν0=2716.558 13 (12) cm−1, B000=16 231.05(24) MHz, D000=15.69(38) kHz, B100=16 154.41(22) MHz, and D100=15.60(33) kHz. These values agree well with the theory. The combination of the two B0 values results in r0(H–O)=0.940 Å and r0(O–Si) =1.537 Å. The differences between these values and the calculated equilibrium values are similar to the corresponding results for HOC+.
Atomic jump lengths in surface diffusion: Re, Mo, Ir, and Rh on W(211)
Authors: Wang S. C. , Wrigley J. D. , Ehrlich Gert
Published: July 2002

Measurements with the field ion microscope have been made of the distribution function governing the displacements of a single metal atom diffusing on W(211). It is shown that by comparing such measurements with the predictions of random walk models, the contributions of jumps of different lengths to the overall diffusion can be ascertained, even if diffusion is dominated by transitions between adjacent sites. Extensive observations for rhenium, as well as for molybdenum atoms on W(211) are best represented by a symmetric random walk between nearest-neighbor sites, without any contribution from longer jumps. For iridium and rhodium atoms, however, double jumps do appear to participate in the diffusion, but amount to only ≊3% of the total.
Solvation shell effects and spectral diffusion: Photon echo and optical hole burning experiments on ionic dyes in ethanol glass
Authors: Pack Dee William , Narasimhan L. R. , Fayer M. D.
Published: July 2002

Results of picosecond photon echo and optical hole burning experiments are reported for four ionic dyes in ethanol glass. At low temperatures, the dephasing times deduced from the hole widths are as much as nine times shorter than those measured by the two-pulse echo because of the effect of spectral diffusion. The temperature dependences found are of the form aTα+b exp (−ΔE/kT) due to glass two level system dynamics (T<4 K) and a process that activates exponentially at higher temperatures, possibly from a pseudolocal mode or glass optical phonon. Comparing the ratios of echo to hole burning measured dephasing times for the four dyes suggests that the dephasing is influenced by the existence of distinct local ethanol solvation shells in addition to the dynamics of the bulk solvent. A theoretical description of solvent shell effects is achieved through the use of a two spatial domain model of the glass dynamics. Calculations of dynamic perturbations from distinct solvation shell and bulk solvent regions show that the observed differences between the dyes’ dephasing ratios can be explained if the ionic chromophores alter glass dynamics locally.
Mean field theory of transient fluorescence quenching in the frequency domain
Authors: Zhou Huan-Xiang , Szabo Attila
Published: July 2002

A dynamical mean field theory is developed for the frequency dependence of the modulation and phase angle that are monitored in frequency domain fluorometric studies of diffusion-influenced fluorescence quenching. These quantities are obtained directly in the frequency domain by solving an implicit equation involving the Laplace transform [k̂(z)] of the time dependent bimolecular rate coefficient [k(t)] that plays a central role in the Smoluchowski approach to this problem. For several models of the quenching process, the mean field results are compared with those obtained by Fourier transforming the time domain response calculated using the Smoluchowski approach. The agreement is good except at high quencher concentrations. For models where k̂(z) is known analytically, the mean field approach is computationally simpler than the Smoluchowski one. This is even more so when it is not possible to find k(t) analytically from k̂(z) (e.g., a model in which the nonradiative lifetime of a quencher-fluorophore pair is constant up to a critical separation but infinite thereafter).
Comment on ‘‘The dielectric relaxation studies on poly(vinyl acetate)’’
Authors: Murthy S. S. N.
Published: July 2002

It is shown that the abnormality in the relaxation frequency as reported by Nozaki and Mashimo [J. Chem. Phys. 84, 3575 (1986); 87, 2271 (1987)] in the case of poly(vinyl acetate) can be removed by the use of an equation of the type of a critical power law slowing down, which is believed to be fundamental to the cooperative process occurring within certain regions of the supercooled liquid which lead to the primary (α) relaxation process.
On the characterization of regions of avoided surface crossings using an analytic gradient based method
Authors: Yarkony David R.
Published: July 2002

The characterization of regions of avoided surface crossings in polyatomic systems using multiconfiguration self-consistent field/configuration interaction(CI) wave functions is considered. It is shown that when the difference density matrices ΔγIJ =γI−γJ and ΔΓIJ =ΓI−ΓJ are introduced construction of the energy difference gradient ∂/∂Rα [EI(R)−EJ(R) ] becomes formally equivalent to the construction of the energy gradient (∂/∂Rα)EI(R). Here EI(R) is electronic potential energy surface at the CI level and γI and ΓI are the standard one- and two-particle density matrices.

The formalism presented here provides a unified approach for the evaluation of three important classes of derivatives, energy gradients (using standard density matrices γI and ΓI ), energy difference gradients (using difference density matrices ΔγIJ and ΔΓIJ) and intersurface nonadiabatic derivative couplings (using transition density matrices γIJ and ΓIJ). The algorithms for the evaluation of the energy difference gradient permit efficient exploitation of the formal similarities which exist between the Newton–Raphson procedure for the minimization of EI(R) and that for ΔEIJ(R)2 =[EI(R)−EJ(R) ]2. These similarities can be used to advantage in the characterization of ΔEIJ(R)2 surfaces. The use of this formalism to analyze regions of avoided surface crossings is considered. In particular the methodology introduced here is used to locate an energetically accessible portion of a seam of avoided crossings for the two lowest potential energy surfaces of the Na–HCl system.

These surfaces describe the electronic quenching reaction Na(2P)+HCl→NaCl+H. This avoided crossing seam is the analog of a (C2v) allowed crossing seam in the Na(2P)+H2→Na(2S)+H2 nonreactive quenching problem.
NO(B 2Π) radiative lifetimes: v=0–6
Authors: Gadd G. E. , Slanger T. G.
Published: July 2002

The zero-pressure radiative lifetime of the NO(B 2Π) state has been measured over the vibrational level range v=0–6. Laser-induced fluorescence was the technique chosen for this study, using two different sources of ground state NO. In one case, photodissociation of NO2 at 193 nm was used to obtain a range of ground state vibrational levels, from which selected rotational levels were then pumped to the B 2Π state, while in the second case, NO in v=0 was directly pumped. The two methods of preparing the excited state gave identical lifetime results. The data show a linearly decreasing lifetime with increasing vibrational level and to a good approximation the lifetimes, are given by τ(μs)=2.00–0.193v. Recent calculations for the B–X system show excellent agreement with experiment at low v, and an increasing discrepancy with increasing vibrational level, the experimental lifetimes decreasing more rapidly than the calculated ones. The lifetime values fall within the 0.85–2.0 μs range for v=0–6, and are substantially different from the 2–3 μs values that are currently quoted. Comparison of the new values with data derived from absorption studies shows good agreement even for the high vibrational levels. Absolute Einstein A factors are presented for the v′=6 progression.
Experimental measurement of the relative stability of two stationary states in optically bistable ZnSe interference filters
Authors: Harding Robert H. , Ross John
Published: July 2002

We report the first measurement of the relative stability of two stable stationary states in a bistable system, that of an optically bistable ZnSe interference filter. A thin ZnSe rectangle is illuminated and bounded by nonilluminated regions at room temperature. This geometry allows the observation of a boundary between the upper and lower stationary state. We describe the inhomogeneous experiments, discuss relative stability in such systems, and compare them with the relative stability of the related homogeneous system. The right marginal stability point of the hysteresis loop is marked by the appearance of the upper stationary state. The homogeneous left marginal stability point is determined by a measurement of the transient relaxation in our inhomogeneous experiment. At the constraint of illumination corresponding to equistability of the two stable stationary states, the upper state becomes unstable for inhomogeneous conditions. We find close agreement between our experiments and the predictions of deterministic physical kinetics. In an accompanying article we discuss the issue of the relative stability of bistable states with thermodynamic fluctuations for inhomogeneous conditions and homogeneous conditions with noise that is independent of the state variable.
Optimized trial functions for quantum Monte Carlo
Authors: Huang Sheng-yu , Sun Zhiwei , Lester William A.
Published: July 2002

An algorithm to optimize trial functions for fixed-node quantum Monte Carlo calculations has been developed based on variational random walks. The approach is applied to wave functions that are products of a simple Slater determinant and correlation factor explicitly dependent on interelectronic distance, and is found to provide improved ground-state total energies. A modification of the method for ground-states that makes use of a projection operator technique is shown to make possible the calculation of more accurate excited-state energies. In this optimization method the Young tableaux of the permutation group is used to facilitate the treatment of fermion properties and multiplets. Application to ground states of H2, Li2, H3, H+3, and to the first-excited singlets of H2, H3, and H4 are presented and discussed.
The ground state infrared spectra of several isotopic forms of the CdH and ZnH radicals
Authors: Urban Rolf-Dieter , Magg Ulrich , Birk Helmut , Jones Harold
Published: July 2002

The infrared spectra of six isotopic forms of cadmium monohydride, [116CdH (7.6%), 114CdH (28.9%), 113CdH (12.3%), 112CdH (24.1%), 111CdH (12.7%), and 110CdH (12.4%)] and four isotopic forms of zinc hydride [68ZnH (18.6%), 67ZnH (4.1%), 66ZnH (27.8%), and 64ZnH (48.9%)] have been observed in natural abundance in their ground electronic state (2∑+) in the gas phase using a diode laser spectrometer. A number of transitions of 108CdH (0.9%) and 106CdH (1.2%) and 70ZnH (0.6%) were also observed, but too little data was accumulated to allow a good analysis. The hydrides were produced by reaction of hydrogen with metal vapor at elevated temperature in an electric discharge. The analysis of the experimental data was carried out in two ways. (a) A complete set of Dunham parameters and spin–rotation parameters (γ parameters) was determined for each isotopic species of the two radicals and (b) a set of mass-independent parameters were calculated for both ZnH and CdH. Since only information over the isotopic species of the heavy atom was produced in each case, effects arising from a breakdown of the Born–Oppenheimer approximation were negligible. The effects of the strong anharmonicity present in these two molecules on the values of the parameters are discussed.
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o study of the ground and low-lying states of FeH
Authors: Sodupe M. , Lluch J. M. , Oliva A. , Illas F. , Rubio J.
Published: July 2002

The electronic state of FeH dissociating into Fe (5D) and H (2S) have been studied at the restricted open Hartree–Fock (ROHF) and configuration interaction (CI) level of theory, using a relativistic pseudo-potential approach and including or not f functions in the basis set. The relative energies and spectroscopic properties obtained by us are in very good agreement with the experimental results found in the literature. In particular this work provides a new theoretical confirmation of a 4Δ electronic ground state for FeH. The effect of including or not f functions in the basis set is discussed.
Energy transduction between a concentration gradient and an alternating electric field
Authors: Markin Vladislav S. , Tsong Tian Y. , Astumian R. Dean , Robertson Baldwin
Published: July 2002

A four-state transport protein that has electric charges that move concomitantly with conformational changes can transduce energy in either direction between an alternating membrane potential and a concentration gradient. An alternating electric potential of very large amplitude is considered, and the rapid-equilibrium approximation is used for the conformational changes. This permits deriving simple expressions for the transport flux, the power supplied by the alternating potential, and the power applied to the concentration gradient.
Environmental dependence of preresonance Raman cross-section dispersions: Benzene vapor-phase excitation profiles
Authors: Harmon Paul A. , Asher Sanford A.
Published: July 2002

The vapor-phase Raman cross-section excitation profiles of the two totally symmetric modes of benzene have been measured between 514 and 220 nm. At wavelengths longer than 280 nm, the 992 cm−1 ν1 ring breathing-mode cross-section dispersion is not indicative of preresonance enhancement by the dipole-allowed E1u electronic transition near 185 nm. With excitation wavelengths shorter than 240 nm, however, the ν1 mode dispersion is dominated by enhancement from the B1u/E1u state. The ν2 3060 cm−1 carbon–hydrogen stretching vibration shows no enhancement by the E1u state in the 514–220 nm spectral region. The cross-section dispersion is indicative of enhancement by states centered near 100 000 cm−1. We compare the gas- and condensed-phase Raman cross-section dispersions of benzene, cyclohexane, and acetonitrile. Surprisingly, we do not observe the ‘‘extra’’ dispersion predicted to occur in the condensed phase which derives from the dispersion in the local electromagnetic field strength.
Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of CH2CO into CH2 and CO. II. Photofragment excitation spectra for vibrationally-excited fragments
Authors: Klippenstein Stephen J. , Marcus R. A.
Published: July 2002

Results on vibrationally-excited ketene photofragment excitation (PHOFEX) spectra of Moore and co-workers are interpreted in terms of a previously described variational implementation of Rice–Ramsberger–Kassel–Marcus (RRKM) theory. At subvibrational excitations, the predictions of this theory reduce to those of phase space theory (PST). However, for excess energies just above the threshold of excitation of a particular vibrational mode of the products, the present theory predicts a significantly greater probability for vibrational excitation, compared with PST, in closer agreement with the experimental results, and predicts an energy dependence of the PHOFEX spectrum that is closer to the observed one. A key feature, to which the present calculations lead, is a two-transition state (TS) description for each vibrational excitation of the products, the PST TS region dominating at the threshold for that excitation and an inner TS region dominating at somewhat higher (∼200 cm−1) energies. The behavior contrasts partly with that of the unimolecular dissociation rate constant kEJ (except at the threshold for kEJ), because of the different focus of the two types of measurements. The theory provides a consistent interpretation of both properties.
Collective dynamics of polymer solutions
Authors: Fredrickson Glenn H. , Helfand Eugene
Published: July 2002

We present a field-theoretic treatment of polymer solution dynamics. The method involves the construction of a Martin–Siggia–Rose generating functional for flexible polymer chains dissolved in a low molecular weight solvent. This formalism is shown to be particularly convenient for investigating the dynamics of collective field variables, such as polymer concentration or elastic stress. Moreover, it provides an extension to nonequilibrium phenomena of Edwards’ conjugate field approach to the static properties of semidilute polymer solutions. The theory is amenable to conventional field-theoretic approximation methods. We explicitly investigate a Gaussian approximation, which gives concentration correlation and response functions identical to those obtained from the dynamical random phase approximation. However, the method also demonstrates screening of hydrodynamic disturbances, the crossover to macroscopic hydrodynamics, and provides information on elastic stress dynamics. We expect that the advantages of the present formalism will become most apparent far from equilibrium, such as in the description of flow-induced fluctuations and phase separation of polymer solutions.
The electronic structure and stability of NF5 and PF5
Authors: Michels H. H. , Montgomery J. A.
Published: July 2002

An ab initio study of the electronic structure of NF5 and PF5 has been carried out using Mo/ller–Plesset perturbation theory. Optimized geometries were calculated at the SCF and MP2 levels of theory using several basis sets, ranging from 6-31G to 6-311G(2df). A vibrational analysis indicates that NF5, in D3h symmetry, has all real frequencies, even if d orbital contributions are set to zero. Dissimilar N–F bond lengths (req=1.38 Å, rax=1.58 Å) are found, in contrast to PF5, which exhibits nearly equal equatorial and axial bond lengths. A topological analysis of the calculated charge distribution in NF5 indicates true pentavalent coordination. Spectroscopic data for NF5 have been calculated using a scaled MP2 force field based on the known NF3 molecule. A comparison of calculated thermochemistry between NF3 and NF5 yields a predicted standard state heat of formation for NF5(g) of ΔH0f (0 K)=+25.1 kJ/mol. Our results suggest that synthesis of NF5 and other hypervalent compounds of first row atoms may be possible.
Monte Carlo reptation model of gel electrophoresis: Response to field pulses
Authors: Duke T. A. J.
Published: July 2002

We study the transient response of DNA molecules to sudden changes in the external field during gel electrophoresis, using a reptation model that includes longitudinal fluctuations of the chain in the tube. When the field is suddenly applied, there is an overshoot in the molecular orientation before the steady state is reached, which is related to the formation of U-shaped tube conformations in which the chain is stretched. A corresponding undershoot, when the field is instantaneously inverted, is caused by the reversal of an asymmetry in the typical steady-state configuration. When the field is switched off, the orientation decays with fast and slow components, linked with the relaxation of chain stretching and reptation out of the oriented tube, respectively. The connection between the these transient effects and the mobility of DNA in field-inversion gel electrophoresis (FIGE) is discussed. In particular, the pulsing time at which the dip in mobility occurs is related to the transient orientation time, since the reduction of mobility is caused by the frequent trapping of the molecule in U-shaped conformations.
Rotationally resolved vibrational overtone spectroscopy of hydrogen peroxide at chemically significant energies
Authors: Luo X. , Rizzo T. R.
Published: July 2002

An infrared–optical double resonance scheme simplifies the room temperature 6νOH vibrational overtone spectrum of hydrogen peroxide and prepares highly excited reactant molecules in single rotational states for unimolecular reaction studies. First, an optical parametric oscillator excites the OH asymmetric stretch (ν5) and selects a single or small subset of rotational states. A visible dye laser pulse then promotes molecules from vOH=1 to vOH=6 where they subsequently dissociate to produce two OH fragments. A third laser detects the dissociation products via laser induced fluorescence. The rotationally resolved vibrational overtone spectra of hydrogen peroxide generated by scanning the visible dye laser frequency are assignable to a parallel band of a near prolate symmetric top. Linewidths of the individual rovibrational features range from 1–3 cm−1 but show no systematic dependence upon the rotational quantum numbers and are attributed predominantly to anharmonic coupling of the zeroth-order bright state to dark background states. The assignability of the double-resonance vibrational overtone spectra to J and K quantum numbers implies that K is conserved for at least a time determined by the linewidth of a single zeroth-order rovibrational feature.
Alloy structures in binary mixtures of highly deionized colloids at sedimentation equilibrium
Authors: Okubo Tsuneo
Published: July 2002

Alloy structures for binary mixtures of different sizes of monodisperse polystyrene spheres at sedimentation equilibrium and in deionized suspensions are studied by the reflection spectrum method and ultramicroscopic observation. Substitutional solid solution (sss)-like alloy structures form in mixtures of D1C25(85 nm in diameter) and D1C27 (91 nm). When D1C25 and D1B76 (109 nm) are mixed, the structures are eutectic mixtures of sss-type lattices and the MgCu2-type super lattices. The alloy structures of D1C25 and D1B72 (137 nm) spheres are the eutectic mixtures of (1) sss-like lattices, (2) MgCu2-type super lattices, (3) fcc crystals of D1B72 spheres, and (4) crystals of D1C25 spheres. D1C25 and D1B41 (173 nm) suspensions are also eutectic mixtures of (1) NaZn13-type super lattices, (2) D1C25 lattices, and (3) lattices of D1B41 spheres. These dramatic changes in the alloy structures depending on the combination of the spheres are explained reasonably by the changes in the effective size ratio (effective size of small spheres containing the electrical double layer thickness against that of large spheres) and the segregation effect. The intersphere distances in the alloy structures agree with the effective sizes of spheres including the Debye-screening length. The elastic moduli of the alloy structures are between 18 and 108 Pa. These results are consistent with the significant role played by the electrical double layers under the influence of purely electrostatic repulsion in the effective hard-sphere model.
Microwave spectrum, structure, barrier to internal rotation, dipole moment, and deuterium quadupole coupling constants of the ethylene–sulfur dioxide complex
Authors: Andrews Anne M. , Taleb-Bendiab Amine , LaBarge Marabeth S. , Hillig Kurt W. , Kuczkowski Robert L.
Published: July 2002

The microwave spectra of the complex between ethylene and sulfur dioxide and nine of its isotopic species have been observed in a Fourier transform microwave spectrometer. The spectra exhibit a and c dipole selection rules; transitions of the normal species and several of the isotopically substituted species occur as tunneling doublets. The complex has a stacked structure with Cs symmetry; the C2H4 and SO2 moieties both straddle the mirror plane with the C2 axis of SO2 crossed at 90 ° to the carbon–carbon bond axis (i.e., only the S atom lies in the symmetry plane). The distance between the centers of mass (Rcm) of C2H4 and SO2 is 3.504(1) Å and the deviation of their planes from perpendicular to Rcm is 21(2) ° and 12(2) °, respectively. The tunneling splittings arise from a rotation of the ethylene subunit in its molecular plane. The barrier to internal rotation is 30(2) cm−1. The dipole moment of the complex is 1.650(3)D. The deuterium nuclear quadrupole coupling constants for C2H3D⋅SO2 are χaa=−0.119(1) MHz, χbb=0.010(1) MHz, and χcc=0.109(1) MHz. The binding energy is estimated to be 490 cm−1 from the pseudo-diatomic approximation. A distributed multipole electrostatic model is explored to rationalize the structure and binding energies.
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o calculation of the X 2Σ+ and A 2Π states of CF++
Authors: Senekowitsch Jörg , ONeil Stephen V. , Werner Hans-Joachim , Knowles Peter J.
Published: July 2002

The potential energy, dipole moment, and electronic transition moment curves of the X 2 Σ+ and A 2 Π states of the doubly charged CF++ ion have been calculated from internally contracted multireference CI wave functions with full valence complete active space self-consistent field reference wave functions and large Gaussian basis sets. Both states are predicted to be metastable with equilibrium geometries Re(X 2 Σ+ )=2.139 Bohr and Re(A 2 Π)=3.073 Bohr. These minima lie above and are separated from their asymptotes by broad barriers with a height of 4.99 eV (X 2 Σ+ ) and 1.04 eV (A 2 Π), respectively. Similar to the isoelectronic NO++, the Π state intersects the barrier of the X 2 Σ+ state. Radiative and nonradiative (tunneling) lifetimes of all vibrational levels have been calculated by standard quantum mechanical scattering techniques.
Anisotropic molecular reorientation of liquid benzene revisited. A study using 13C magnetic relaxation through chemical shift anisotropy and spin rotation
Authors: Dölle Andreas , Suhm Martin A. , Weingärtner Hermann
Published: July 2002

We report on 13C magnetic relaxation in liquid benzene (C6H6 and C6D6) at 293 K at resonance frequencies of 22.63 and 100.6 MHz. The total spin–lattice relaxation rates are separated into contributions from 13C–1H and 13C–2D dipolar, spin–rotation (SR), and chemical shift anisotropy (CSA) interactions. As the principal axis of the 13C chemical shift tensor is collinear with the sixfold axis of benzene, CSA relaxation provides information on the tumbling motion of benzene, hitherto not available from other NMR experiments. The resulting correlation time τ⊥ = 1.63 ps at 20 °C indicates a faster tumbling motion, and hence, a smaller anisotropy of benzene reorientation than usually quoted in the literature. This result is qualitatively confirmed by an analysis of 13C relaxation by SR interaction, and is in fair agreement with recent results obtained in Raman line shape measurements. It is, however, at variance with the figures deduced from Rayleigh scattering experiments. The activation energy for the tumbling motion is found to be 9.8 kJ mol−1, which is markedly larger than the activation energy of 7.7 kJ mol−1 obtained from 2D relaxation data for the motion of in-plane axes.
Models for self-diffusion in the square well fluid
Authors: Dufty James W. , Mo Kingtse C. , Gubbins Keith E.
Published: July 2002

Transport properties of moderately dense fluids reflect several effects not present at the Boltzmann level: excluded volumes, finite collision times, and bound states. The description and relative importance of these effects is illustrated here by a discussion of the self-diffusion coefficient for the square well fluid. At high temperatures the particles behave as hard spheres and the Enskog kinetic theory describing excluded volume effects is adequate. Generalizations of the Enskog theory for application at lower temperatures are discussed critically. One approach assumes partial (uncorrelated) collisions for initial conditions modified to account for excluded volume effects. Another describes completed binary collisions at the core and edges of the well for the same modified initial conditions. Although both reduce to the Enskog theory at high temperatures, neither is adequate outside this limit for prediction of either the temperature or the density dependence. The proper treatment of both temperature and density effects is found to require a more detailed description of interrupted binary collisions and bound states. Such a treatment is described and shown to be in good agreement with computer simulation results. We conclude that generalization of the hard-sphere Enskog theory to the square well, and probably other potential models, is more subtle than might be expected and requires dynamical as well as static many-body effects.
Tunable far infrared laser spectroscopy of a ternary van der Waals cluster Ar2HCl: A sensitive probe of three-body forces
Authors: Elrod M. J. , Steyert D. W. , Saykally R. J.
Published: July 2002

The first far infrared intermolecular vibration–rotation spectrum of the ternary van der Waals cluster has been measured near 39.5 cm−1 and assigned to an a-type ∑ bending vibration of Ar2HCl. Spectra of both chlorine isotopes were observed and nuclear quadrupole hyperfine structure was resolved. Values of the fitted constants (rotational constants, hyperfine projections) evidence large amplitude out-of-plane motion, and demonstrate the sensitivity of spectroscopic observables to the three body forces operative in the Ar2HCl system. Spectroscopic predictions calculated by Hutson et al. from pairwise-additive and ‘‘three-body’’ corrected potential energy surfaces [J. Chem. Phys. 90, 1337 (1989)] are compared to experimental results.
Hyperpolarizability determined from the intermediate neglect of differential overlap model
Authors: Parkinson William A. , Zerner Michael C.
Published: July 2002

An algorithm for the calculation of molecular polarizability is developed for the determination of the first hyperpolarizability. The first-order singly excited configuration interaction response function, which is determined during the calculation of the second-order energy, is used to directly evaluate the third-order expression. The resulting computational effort is on the order of the self-consistent field (SCF) step. The method is therefore more efficient than a finite-field (FF) calculation, which involves several SCF points. The method is tested using the intermediate neglect of differential overlap/spectroscopic model Hamiltonian to calculate hyperpolarizabilities. These values are compared to a previous complete neglect of differential overlap (CNDO/2) FF study, and to ab initio calculations performed with various basis sets and levels of theory. The effects of the semiempirical basis are also studied with calculations that contain polarization functions added to hydrogen centers.
Fragment energy and vector correlations in the overtone-pumped dissociation of HN3 X̃ 1A′
Authors: Casassa Michael P. , Foy Bernard R. , Stephenson John C. , King David S.
Published: July 2002

NH stretching overtone and combination states in HN3 X̃ 1A′ were excited by IR–visible double resonance pumping and by direct overtone pumping in the range 6ν1 (17 670 cm−1) to 7ν1 (20 070 cm−1). NH fragments in the a 1Δ and X 3Σ− states were detected by laser induced fluorescence with sub-Doppler resolution to determine branching ratios, correlated fragment rotational state and kinetic energy distributions, and fragment vector correlations. The spin-forbidden triplet channel was accessible to all states excited, while the threshold for the singlet channel was determined to lie in the range 18 190 to 18 755 cm−1. The measured energy release places limits on the HN–NN bond energy, and the heights of barriers to reaction. The barrier in the singlet exit channel is at least 540 cm−1. The singlet channel accessed by 7ν1 dissociation is characterized by a Boltzmann-like NH rotational distribution (〈J NH〉≊3.5), highly excited N2 rotations (〈JN2〉 ≥ 20), and total translational energy release peaked away from zero (〈ET〉≊1350 cm−1). Vector correlations and Λ-doublet propensities indicate that nonplanar dissociation processes influence the NH rotations, but become less important for higher NH rotational states. The principal correlations are a strong positive recoil anisotropy (β≊0.6), a weak positive v–J correlation (βvJ≊0.17), and a JNH-dependent Λ-doublet propensity. A model using parent vibrational motion projected onto fragment rotation is suggested to explain these observations. The triplet channel exhibits similar NH and N2 rotational state distributions, with most of the available energy (substantially greater than in the singlet channel) appearing as fragment kinetic energy.
Electron temperatures in a pulsed electric discharge and the associated N2 electron excitation rate coefficients
Authors: Morrill Jeff S. , Benesch William M. , Widing Kenneth G.
Published: July 2002

Investigations of excitation processes of molecular nitrogen in pulsed electric discharges have been focused on the N2 first positive system (1PG) which corresponds to the transition, N2(B 3Πg→A 3Σ+u). Our observations indicate that collisions during the afterglow which follows the 4 μs current pulse couple the vibrational levels of the B 3Πg state with those of the overlapping triplet states (A 3Σ+u,W 3Δu, B′ 3Σ−u) by the intersystem collisional transfer of excitation (ICT). In order to model the effect of this process we must first estimate the relative direct electron excitation rates of these overlapping states. This, in turn, requires knowledge of the electron energies during the course of the electric discharge. We have used the Balmer series Hα/Hβ intensity ratio to measure the varying electron temperatures that occur during the pulse for a number of experimental conditions. The electron temperatures which we observe range from 5 000 to 11 000 K. This is below the temperatures for which electron excitation rate coefficients for N2 excited states have been tabulated previously, so we also present these coefficients for electron temperatures from 1 000 to 15 000 K.