bluuebunny/crossref_metadata_2025 huggingface dataset:
Heteronuclear decoupling in rotating solids
Authors: Bennett Andrew E. , Rienstra Chad M. , Auger Michèle , Lakshmi K. V. , Griffin Robert G.
Published: July 2002

A simple two pulse phase modulation (TPPM) scheme greatly reduces the residual linewidths arising from insufficient proton decoupling power in double resonance magic angle spinning (MAS) experiments. Optimization of pulse lengths and phases in the sequence produces substantial improvements in both the resolution and sensitivity of dilute spins (e.g., 13C) over a broad range of spinning speeds at high magnetic field. The theoretical complications introduced by large homo- and heteronuclear interactions among the spins, as well as the amplitude modulation imposed by MAS, are explored analytically and numerically. To our knowledge, this method is the first phase-switched sequence to exhibit improvement over continuous-wave (cw) decoupling in a strongly coupled homogeneous spin system undergoing sample spinning.
A multichannel quantum defect fit to the n*=6–8 core-penetrating s∼p∼d
supercomplexes of CaF
Authors: Murphy James E. , Friedman-Hill Ernest , Field Robert W.
Published: July 2002

The spectrum of the core-penetrating s, p, and d Rydberg series of calcium monofluoride between n*=6 and 8 have been analyzed using multichannel quantum defect theory (MQDT). From least-squares fits of the n*=6, 7, and 8 (−) Kronig symmetry p∼d supercomplexes we have determined B+ (the rotational constant of CaF+) to be 0.3729 cm−1, the mixing coefficients between the pπ and dπ series (the 0.98 Π series is 41% p and 59% d while the 0.35 Π series is 41% d and 59% p), and the eigenchannel (rotationless) quantum defects of the 0.98 Π, 0.35 Π, and 0.14 Δ series. A similar MQDT fit was attempted for the (+) Kronig symmetry s∼p∼d supercomplexes, but the mixing coefficients for the σ series could not be determined even though the mixing coefficients for the π series were as well determined as in the fit to the (−) symmetry series. The failure of the s∼p∼d model to fit the (+) symmetry series, particularly the σ series, is an indication that, for the (+) symmetry series, mixing of the sσ, pσ, and dσ series with the fσ series must be considered. Experiments are currently under way to observe and characterize the f σ, π, δ, γ series so that an s∼p∼d∼f supercomplex model can be developed for the (+) symmetry series.
Aggregation-assisted induced electric order in the nematic phase of a metallorganic complex
Authors: Ionescu A. Th. , Pucci D. , Scaramuzza N. , Versace C. , Petrov A. G. , Bartolino R.
Published: July 2002

The family of nematic liquid crystals has been recently enlarged with the class of metallorganic molecules. Among them, AZPAC [4,4′-bis(hexiloxy)-azoxybenzene–palladium complex] presents some unusual behavior as a transitory ‘‘ferroelectric’’ state, which we explain by considering bound multimers of AZPAC of peculiar steric conformation.
Vibronic coupling and energy transfer in bichromophoric molecules: The effect of symmetry
Authors: van Dantzig Niels A. , Levy Donald H. , Vigo Craig , Piotrowiak Piotr
Published: July 2002

The fluorescence spectra of a series of bichromophoric molecules consisting of covalently linked fluorene units were investigated in a supersonic jet. In three of the systems (spirobifluorene, d8h8-spirobifluorene and 1-methyl spirobifluorene) no electronic coupling and no corresponding exciton splitting were detected in the zero-point level of the S1 state. Only 9,9′-bifluorene exhibited an exciton splitting in the v=0 state. The lack of coupling was attributed to symmetry; in the spirobifluorenes the planes of the fluorene moieties and the S1←S0 transition moments are perpendicular. When low vibrational levels were excited, state mixing, and energy transfer between the chromophores was observed. This behavior is characteristic of the ‘‘small molecule’’ regime of radiationless transition theory. When higher vibrational levels were excited, the systems exhibited typical ‘‘large molecule’’ behavior. In this limit, both electronic energy transfer, as well as intramolecular vibrational relaxation contribute to the decay of the initially excited state. Intramolecular dispersive interactions were also investigated by comparing the bifluorenes with a series of reference compounds.
Perturbation theory and computer simulations for linear and ring model polymers
Authors: Johnson J. Karl
Published: July 2002

Theory and computer simulations of model polymers are presented. Polymers are modeled as freely-jointed beads, with the nonbonded bead–bead interactions given by the Lennard-Jones potential; a harmonic spring potential is used for the bonding interactions. Simulation results for linear chains containing 200 beads are presented. A thermodynamic perturbation theory for polymerization is compared to simulation data for chains containing from two to 200 beads, over a range of temperatures and densities. Two variations of the theory are investigated, one utilizing a reference fluid of monomers (TPT1-M), and another employing a dimer reference fluid (TPT1-D). It is found that TPT1-D is far more accurate for predicting the pressures of linear flexible chains than TPT1-M. At low densities TPT1-M predicts internal energies that are too high compared to simulation data. This is because TPT1-M neglects intramolecular contributions to the configurational energy. TPT1-D gives a more accurate description of the low density energies of flexible chains by incorporating structural information about the dimer fluid into the reference term. Computer simulations of ring polymers are presented. Noninterlocking flexible rings with 3, 8, and 20 beads are modeled. Simulations of rigid planar rings containing 3 and 8 beads are also presented. Pressures and energies for rigid and flexible 3-mer rings are virtually identical, even though the flexible model includes bond vibrations which are absent in the rigid ring model. In contrast, the pressure of the rigid 8-mer ring fluid is always higher than the pressure of flexible ring fluids at the same temperature and density. Extensions of TPT1-M and TPT1-D for ring polymers are compared with simulation results for flexible and rigid rings. The monomer reference theory predicts pressures that are too high for flexible rings but too low for rigid 8-mer rings at high densities. TPT1-D for rings gives good agreement for pressures and energies of flexible rings at high densities, but incorrectly predicts a two-phase region for ring polymers at supercritical temperatures.
Femtosecond transient photoinduced transmission measurements on a novel conjugated zinc porphyrin system
Authors: O’Keefe G. E. , Denton G. J. , Harvey E. J. , Phillips R. T. , Friend R. H. , Anderson H. L.
Published: July 2002

We present a comparative study on the ultrafast nonlinear optical response of a novel conjugated zinc porphyrin system. The linear optical absorption spectra of these molecules all show the Q-band and B-band transitions of the basic porphyrin unit. We have taken spectrally resolved ultrafast pump–probe measurements on monomer, dimer, and polymer solutions, which allows us to compare their excited state dynamics and relate these to their linear optical absorption. The spectra show several common features, but these features have markedly different decay dynamics. The bleaching is preferential in the Q band for the polymer and the B band for the monomer. The polymer Q-band bleaching shows a two-component decay, of approximately 700 fs and 170±50 ps time constants in a biexponential fit, which we attribute to both exciton–exciton annihilation and exciton diffusion to recombination centers on the polymer chain. The Q band of the dimer also has a two-component decay with 13±5 and 1250±70 ps time constants which we attribute to rotational diffusion of the excited molecule in solution, and decay to the ground state, respectively. The B-band bleaching in the monomer is long lived and has a decay constant of approximately 3.5±0.5 ns; from the absorption recovery of the B band we estimate a triplet yield of 0.8. All molecules exhibit broad π*−π* absorptions in the visible spectral region (between the Q band and the B band). In particular, we show that the monomer has potential as a broadband optical limiter in the visible region from 455 (2.72 eV) to 620 nm (2.00 eV); we estimate that its excited state absorption cross section is 8.5 times that of its ground state cross section at 532 nm (2.33 eV). There is also clear evidence of triplet transitions in the dimer and monomer; the triplet absorption feature ranging from 940 (1.32 eV) to 1000 nm (1.24 eV) rises 10 ps after excitation in the dimer which suggests a short intersystem-crossing time.
Intermolecular spectral densities of liquids: A quantitative comparison of time-domain and frequency-domain techniques
Authors: Cong Peijun , Simon John D. , She C. Y.
Published: July 2002

Intermolecular spectral densities measured by two independent experimental techniques, time-domain optical-heterodyne-detected Raman-induced-Kerr-effect spectroscopy (OHD-RIKES) and frequency-domain stimulated gain spectroscopy (SGS), for three molecular liquids (benzene, carbon disulfide, and chloroform) are compared. The agreement in the high-frequency (interaction-induced) portion of the spectrum (20–150 cm−1) is quantitative. The relative merits between time-domain and frequency-domain coherent light-scattering techniques are discussed.
Equation of state for hard n-alkane models: Long chains
Authors: Vega C. , MacDowell L. G. , Padilla P.
Published: July 2002

An equation of state (EOS) for hard n-alkane models is proposed. This equation requires a previous knowledge of the second virial coefficient of the hard n-alkane model. Since the numerical determination of the second virial coefficient of chain molecules is computationally expensive, a new method for estimating the second virial coefficient of hard polymer models is proposed. This method yields predictions for the second virial coefficients in very good agreement with those determined numerically. In order to test the proposed equation of state, molecular dynamics simulations for repulsive n-alkane chains were performed. Excellent agreement was found between theoretical and simulated pressures for n-alkanes with up to 100 monomer units. The effect of changes in the torsional potential, bond angle, and bond length, on the equation of state of hard n-alkane models is analyzed. The equation of state is also extended to mixtures of hard n-alkane models. The proposed methodology provides an accurate equation of state for realistic models of hard n-alkane molecules. An empirical formula describing the EOS of repulsive n-alkane chains is given.
Interaction of D(H) atoms with physisorbed benzene and (1,4)-dimethyl-cyclohexane: Hydrogenation and H abstraction
Authors: Lutterloh C. , Biener J. , Schenk A. , Küppers J.
Published: July 2002

Benzene and (1,4)-dimethyl-cyclohexane monolayers were physisorbed on graphite covered Pt(111) surfaces. Exposure of benzene monolayers at 125 K to D atoms (1700 K) initially hydrogenates sp2 hybridized C atoms with a cross section of ca. 8 Å2 producing C6H6D intermediates. Additional D atom reactions either transform this intermediate via a second hydrogenation reaction to cyclohexadiene-d2, C6H6D2, or restore benzene, C6H5D, via H abstraction. Once the aromaticity is broken, successive hydrogenation of the diene occurs rapidly generating the saturated cyclohexane-d6, C6H6D6. The C6H5D reaction product can undergo further H/D exchange reactions and, at any level of deuteration, the benzene species might get hydrogenated. Monolayers of the saturated hydrocarbon (1,4)-dimethyl-cyclohexane (DMCH) that are exposed to D atoms produce deuterated DMCH via successive abstraction/hydrogenation reactions. Thermal desorption mass spectra revealed that H atoms at the ring were exchanged with an apparent cross section of 1.7 Å2. Methyl groups H atoms were exchanged much more slowly than ring H atoms. It was also observed that D exposed molecules/radicals exhibit a tendency to desorb from the surface, which is ascribed to the exothermicity of the reactions which lead to these species.
Induced infrared absorption of nitrogen physisorbed on NaCl films
Authors: Dai David J.
Published: July 2002

The induced infrared absorption of N2 physisorbed on annealed NaCl films is observed. Analysis of spectroscopic and thermodynamic data is consistent with molecules adsorption on defect sites and on Na+ sites of smooth NaCl(100) faces of the film crystallites. Isotherms of N2 on the smooth face at 74 and 77 K are reported with a heat of adsorption of −11.1±1.0 kJ/mol.
Structure and hyperfine parameters of cyclopropyl and bicyclobutyl radicals from post-Hartree–Fock computations
Authors: Barone Vincenzo , Subra Robert
Published: July 2002

Extensive post-Hartree–Fock calculations are reported for the geometrical structures and hyperfine parameters of cyclopropyl and bicyclobutyl radicals. Computations for the parent molecules, whose structures are experimentally well characterized, show that reliable geometrical parameters are obtained, especially for bicyclobutane, only when using sufficiently flexible basis sets including f functions on carbon. Isotropic hyperfine splittings obtained by purposely tailored basis sets, proper treatment of correlation, and inclusion of vibrational averaging effects are in remarkable agreement with experiment. Our results suggest a revision of the accepted assignment for bicyclobtyl radical and suggest that long-range couplings are not governed by the well-known W rule but rather by a syn rule.
An effective Hamiltonian-based method for mixed quantum-classical dynamics on coupled electronic surfaces
Authors: Fang Jian-Yun , Martens Craig C.
Published: July 2002

We describe an approximate method for treating the mixed quantum-classical (QC) dynamics of many-body systems on N coupled electronic surfaces. The approach is based on calculating N×N reduced Hamiltonian matrices for the classical and quantal degrees of freedom by partial averaging, and then solving the appropriate equations of motion—Hamilton’s equations or the Schrödinger equation—self-consistently. The degrees of freedom requiring a quantum mechanical description are treated using a multistate Schrödinger equation with classically averaged effective time-dependent Hamiltonians and off-diagonal couplings. The classical degrees of freedom are treated by propagating N ensembles of trajectories, one on each electronic surface, using N reduced classical Hamiltonians defined in terms of the expectation value of the full Hamiltonian calculated using the evolving quantum wave functions. An ansatz is proposed to approximately estimate classical off-diagonal density matrix elements required for calculating the classically averaged interactions that couple quantum wave functions on different electronic states. We present the theory and then test it for a simple two-dimensional and two-state model system. Exact quantum and multiconfiguration time-dependent self-consistent-field (MCTDSCF) calculations are carried out to evaluate the QC performance. Good agreement between the MCTDSCF and QC results is obtained for the model considered.
Dynamic aspects of electronic predissociation
Authors: Gro/nager Michael , Henriksen Niels Engholm
Published: July 2002

We consider electronic excitation induced with a continuous wave laser to an excited bound state which can predissociate due to a radiationless transition to a dissociative state. The conditions for a separation of the process into the preparation of a vibrational eigenstate which subsequently dissociates due to a radiationless transition are established. We point out that the probability of the radiationless transition can be calculated from a time-dependent nuclear autocorrelation function, an expression which nicely reflects the pictorial aspect of the Franck–Condon principle.
Interference effects in the two-photon ionization of nitric oxide
Authors: Pratt S. T.
Published: July 2002

An experimental demonstration of the interference between two different resonant two-photon, two-color ionization processes from the A 2Σ+, v′=1 state of nitric oxide is presented. This interference effect does not require a well-defined phase relationship between the two ionizing lasers and should, in principle, allow the control of the overall ionization process by small detunings of either or both of the two resonant ionization pathways. This interference effect is quite general, and the potential of this scheme for the control of photoionization and photodissociation partial cross sections is discussed.
Symmetries in proteins: A knot theory approach
Authors: Chen Shi-Jie , Dill Ken A.
Published: July 2002

Whereas the symmetries of small molecules are described by the methods of group theory, there is no corresponding way to describe the complex symmetries in proteins. We develop a quantitative method to define and classify symmetries in compact polymers, based on the mathematical theory of graphs and knots. We represent different chain folds by their ‘‘polymer graphs,’’ equivalent to contact maps. We transform those graphs into mathematical knots to give a parsing of different possible chain folds into conformational taxonomies. We use Alexander–Conway knot polynomials to characterize the knots. We find that different protein structures with the same tertiary fold, e.g., a βαβ motif with different lengths of α helix and β sheet, can be described in terms of the different powers of the propagation matrices of the knot polynomial. This identifies a fundamental type of topological length invariance in proteins, ‘‘elongatable’’ symmetries. For example, ‘‘helix,’’ ‘‘sheet,’’ ‘‘helix-turn-helix,’’ and other secondary, supersecondary, and tertiary structures define structures of any chain length. Possibly the nine superfolds identified by Thornton et al. have elongatable symmetries.
An ab initio study of TiC: A comparison of different levels of theory including density functional methods
Authors: Hack Michael D. , Maclagan Robert G. A. R. , Scuseria Gustavo E. , Gordon Mark S.
Published: July 2002

A comparison is made of various ab initio methods, including density functional methods for the 3Σ+ and 1Σ+ states of TiC. The calculated properties are sensitive to the level of theory. The results with the LSDA and BPW91 density functional methods are in poor agreement with higher level calculations obtained at the MRCI level of theory, but there is better agreement with the B3LYP method. The calculations confirm that the ground state of TiC is the 3Σ+ state.
Small angle x-ray scattering of an ordering aggregation
Authors: Zhu Peng Wei , White John W.
Published: July 2002

We present a new correlation function in the form of γ(r)=r(Df−D) exp(−r/a0)cos(br) to describe the essential features of aggregate structure. The scattered intensity distribution corresponding to this correlation function was obtained. Compared with existing models, the new scattering formula can predict a scattering peak in a finite wavevector and control the scattered intensity decay in larger wave vectors. The experimental scattering patterns obtained from polydimethylsiloxane/paraffin liquid mixtures were tested well by the new model.
The interaction representation and nonadiabatic corrections to adiabatic evolution operators. II. Nonlinear quantum systems
Authors: Schwartz Steven D.
Published: July 2002

This paper reports further applications of the recently developed interaction representation form of infinite order operator corrections to adiabatic evolution operators. Previous work derived the form of the correction, and applied the methodology to a bilinearly coupled system bath model. In this paper we present results on coupled quantum systems in which the coupling is highly nonlinear. The method is both easy to implement and numerically accurate.
Theory of Raman scattering with pulses: Application to continuum Raman spectroscopy
Authors: Lu J. , Lee Soo-Y.
Published: July 2002

A theory of real-time dependence of Raman scattering for a pulse-mode laser is developed within second-order perturbation theory and using the wavepacket terminology. The rate of spontaneous Raman emission with a pulse correctly reduces to the dynamical equivalent of the Kramers-Heisenberg-Dirac expression in the monochromatic limit. We apply the theory to continuum Raman scattering for short and long pulses and varying pulse carrier frequency. The rate of Raman emission as a function of time and pulse carrier frequency, from an initial ground vibrational state to various final vibrational states, is shown to be structureless for all pulses, and for pulses that are longer than the dissociation time the rate also rises and decays with the pulses. This is contrary to recent reports of recurring resonance fluorescence-type structures at long times after the pulse has vanished. We explain why such structures are unphysical for continuum Raman scattering. Results are also presented for excitation from an initial first excited vibrational state.
Infrared laser jet spectroscopy of transition metal hexacarbonyl-rare gas dimers
Authors: Hansford G. M. , Davies P. B.
Published: July 2002

High resolution infrared absorption spectra of nine van der Waals complexes M(CO)6⋅Rg (M=Cr, Mo, W; Rg=Ar, Kr, Xe) formed in a supersonic jet expansion have been recorded near the 5 μm carbonyl stretching fundamental bands of the hexacarbonyl monomers. In each case a single red-shifted perpendicular band was observed. It is shown that the spectral results are only consistent with a C3v symmetric top structure for each dimer; no effects due to internal motions are seen in the spectra. The M–Rg separations deduced from analysing the spectra are slightly larger than the separations calculated from the van der Waals radii. Red-shifts of the band origin are partly explained by a simple vibrational dipole-induced dipole model.
Mean field theory for long chain molecules
Authors: Pereira Gerald G.
Published: July 2002

We provide a mathematical formalism for a self-consistent mean field treatment of long chain molecules. The formalism is applied to the case of a neutral polymer under the excluded volume interaction. Upon scaling the problem in the N→∞ limit we find the natural scaling length RN, of the polymer, which is made up of (N+1) monomers or beads, is RN∼N3/5, the well known Flory result. The asymptotics of the problem is dominated by the neighborhood of the turning point, so that a uniformly valid Green’s function solution of the differential equations is necessary. In the neighborhood of a point y* the scaled polymer density fN(x), is found to decay sharply. If we let x denote the scaled distance from one end of the chain to a point in space we obtain, for y*−x≳O(N−2/15), a closed form expression for the polymer density viz., fN(x)∼{1/2x2[fN(x)−fN(y*)]1/2} while for x−y*≳O(N−2/15) the density is shown to be, to leading order, zero. Although our results imply the rate of decay of the density at y* is O(N1/5) we are unable to verify this explicitly by calculating fN′(y*). We believe this is due to the inability of the WKB theory to correctly approximate solutions in regions of rapid variation. We suggest remedies for this, so that a complete self-consistent solution may be obtained.
The anomalous behavior of the Zeeman anticrossing spectra of Ã 1Au acetylene: Theoretical considerations
Authors: Vacek George , Sherrill C. David , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

P. Dupré, R. Jost, M. Lombardi, P. G. Green, E. Abramson, and R. W. Field have observed anomalous behavior of the anticrossing density in the Zeeman anticrossing (ZAC) spectra of gas phase Ã 1Au acetylene in the 42 200 to 45 300 cm−1 energy range. To best explain this result, they hypothesize a large singlet–triplet coupling due to the existence of a linear isomerization barrier connecting a triplet-excited cis- and trans-acetylene in the vicinity of the studied energy range (∼45 500 cm−1). Theoretically such a linear stationary point, however, must have two different degenerate bending vibrational frequencies which are either imaginary or exactly zero. Neither case has yet been experimentally detected. Here, we have studied the two lowest-lying linear triplet-excited-state stationary points of acetylene, 3Σ+u and 3Δu, to see if they fit Dupré et al.’s hypothesis. We have completed geometry optimization and harmonic vibrational frequency analysis using complete-active-space self-consistent field (CASSCF) wave functions as well as determined energy points at those geometries using the second-order configuration interaction (SOCI) method. Harmonic vibrational analyses of both stationary points reveal two different doubly degenerate vibrational modes with imaginary vibrational frequencies (or negative force constants) indicating that they are indeed saddle points with a Hessian index of four. At the DZP SOCI//CASSCF level of theory with zero-point vibrational energy (ZPVE) correction, the 3Σ+u stationary point lies 35 840 cm−1 above the ground state of acetylene. This is much too low in energy to contribute to the ZAC spectral anomaly. At the same level of theory with ZPVE correction, the 3Δu stationary point lies 44 940 cm−1 above the ground state consistent with Dupré et al.’s hypothesis. Several solutions to the anomalous ZAC spectra are discussed. We propose that the anomaly may also be due to coupling with a nearly linear structure on the T3 surface of acetylene.
Vibrational structure of the N+2 ground state observed by threshold photoelectron spectroscopy
Authors: Morioka Y. , Lu Y. , Matsui T. , Tanaka T. , Yoshii H. , Hayaishi T. , Hall R. I.
Published: July 2002

The long vibrational progression of the ground state of N+2 was observed in a high resolution threshold photoelectron spectrum obtained using the penetrating field technique and synchrotron radiation. Vibrational states were observed up to v′=67 for the first time and the final vibrational level was deduced to be v′=77 from extrapolation. The complete molecular constants of this state were obtained and the entire potential curve was then drawn.
Exciton-to-biexciton transition in quasi-one-dimensional organics
Authors: Mazumdar S. , Guo F. , Meissner K. , Fluegel B. , Peyghambarian N.
Published: July 2002

In the previous paper we demonstrated novel multiexcitons in a neutral mixed-stack charge-transfer solid. The lowest multiexciton, the biexciton, has recently been of interest also in the context of quasi-one-dimensional organic materials that are different from the mixed-stack solids. The nature and strength of the optical transition from the exciton to the two-exciton states is of importance in understanding photoinduced absorption as well as two-photon absorption. We show that within the diverse theoretical models that describe these different classes of materials, the excited state absorption from the optical exciton to the two-exciton states changes in a fundamental way upon the formation of the biexciton. The identical nature of the exciton absorption within these models is a consequence of one dimensionality.
Curve crossing problem with dissipation: Uniform rate expression in diabatic representation
Authors: Rips Ilya
Published: July 2002

The approach based on identification of the quasi-ballistic collective mode using the variational transition state theory (VTST) is applied to evaluation of the radiationless transition rate in the diabatic representation. The results for the rate match smoothly with the corresponding results derived in the adiabatic representation. This implies that the uniform expression for the rate constitutes a good approximation to the exact result. Analytic expression for the renormalized barrier frequency for Ohmic dissipation in the high barrier limit is derived. The result for the adiabaticity parameter in the strong damping regime reduces to Zusman’s result. The effect of non-Ohmic dissipation on the electron transfer kinetics in polar solvents is explored. A new procedure for the determination of the quasi-ballistic mode, based on variation of the total flux, is suggested. The procedure reduces to the VTST procedure in the adiabatic limit. In the nonadiabatic limit the quasi-ballistic mode coincides with the original reaction coordinate independently of the dissipation strength.
Conformation of confined alkanes: n-butane in zeolite Y
Authors: Bandyopadhyay Sanjoy , Yashonath Subramanian
Published: July 2002

Configurational-bias Monte Carlo simulation of n-butane in NaY zeolite has been reported at different temperatures and loadings. In contrast to n-butane in silicalite, the gauche conformer population is always found to be higher in zeolite NaY than in pure fluid at the same density. Normally, an increase in adsorbate concentration in gases such as methane, rare gases, etc. leads to an increase in population near the cage centre. In contrast, we find that an increase in n-butane concentration gives rise to an increase in population near the periphery of the cage. Enhancement in gauche conformer population observed for NaY zeolite is attributed to the surface curvature of the α-cavity.
Experimental and theoretical studies of the decomposition of N2O catalyzed by chlorine
Authors: Lesar Antonija , Hodošček Milan , Senegačnik Marjan
Published: July 2002

Kinetic isotope effects (KIEs) for the thermal decomposition of N2O catalyzed by chlorine were experimentally determined in the temperature range 773–923 K, and may be expressed as follows: KIEt(15N)=(4100/T−1.90)±0.15, KIEp(15N)=(3940/T−2.35)±0.10 and KIE(18O)=(6990/T−3.60)±0.25. An Arrhenius fit to the measured rate constants resulted in an activation energy of 136±8 kJ mol−1 and a preexponential factor of 7.7×107±0.1 m3 mol−1 s−1. The KIEs were interpreted according to the Bigeleisen formalism. Furthermore, we calculated the activation energy following the Sanderson bond-energy–bond-order relationship, and the preexponential factor from transition state theory and compared them to experimental values. Additionally, ab initio molecular theory was employed to study parts of the potential energy surface of the elementary bimolecular reaction between a N2O molecule with a Cl atom. Equilibrium geometries, energies and harmonic vibrational frequencies were calculated at the HF/6-31G* and MP2/6-31G* level for some distinct stationary points on the potential energy surface, with energy refinements at the MP2/6-311G* level. In our study the transition state was located by the eigenvalue-following method. The ab initio properties of the transition state and reactants were also used for an evaluation of the kinetic isotope effects.
The B(1/2 2P3/2)→X(1/2 2Σ+) transition in XeI
Authors: Radzykewycz Dan T. , Tellinghuisen Joel
Published: July 2002

The B(1/2 2P3/2)→X(1/2 2Σ+) transition in XeI (2385–2490 Å) is recorded at high resolution for the single isotopomer 136Xe127I, using a Tesla discharge source and a CCD array detector. The high signal-to-noise capabilities of the detector make it possible to measure the discrete vibrational structure in this system for the first time. The assignments consist of 86 v′–v″ bands spanning 15 upper-state levels (assigned as v′=3–17) and 17 lower-state levels (tentatively assigned as v″=0–16). A vibrational analysis yields the following spectroscopic constants (cm−1): ΔTe=40 047.8, ωe′=110.6, ωexe′=0.217, ωe″=24.0, ωexe″=0.66. From a near-dissociation analysis, the X state has a dissociation energy (𝒟e) of 267±5 cm−1 and supports 28 bound vibrational levels. However, it is likely that our lowest observed v″ level is not v″=0, so these values should be considered lower limits. The potential energy curves are modeled by a Morse–RKR function for the X state and a Rittner function for the B state. Trial-and-error Franck–Condon calculations are used to locate the B- and X-state potential curves relative to each other, fixing the X-state internuclear distance Re at a value ∼0.7 Å larger than that for the B state. Bound–free simulations of the low-resolution spectrum indicate that the X potential must be significantly steeper in the Franck–Condon region than found from previous scattering studies.
Electronic excitation transfer in chains modulated by conformational dynamic disorder
Authors: Pálszegi T. , Kauffmann H. F.
Published: July 2002

Electronic excitations along sites that undergo spatial and temporal fluctuations due to conformational chain motion have been studied in the picture of the stochastic master equation by means of the dynamic Monte Carlo (DMC) and the cumulant expansion (CE) approach. An incoherent site-to-site hopping which is adiabatic relative to the changes of conformational site coordinates has been assumed. The elementary act of conformational change has been considered to be fast, whereas the electronic transfer during the time period of the conformational event has been assumed to be negligibly small. The time evolution of electronic intersite coupling is thus controlled by chromophore sites that, in particular, correspond to the conformational minima of the potential energy landscape. The generalized equations of motion adapted for both the DMC and the CE analysis have been reduced to formulate donor site excitation probabilities 〈Piexc(t)〉 and donor excitation survival functions 〈PD(t)〉 for a simplified chain. In this polymer model, (i) specific nearest-neighbor electronic coupling occurs with two distinct transfer rates W1 and W2 corresponding to two different spatial arrangements of the pendant sites in the pair and (ii) transitions between two definite conformational states occur both in the correlated and in the uncorrelated regime. For short chains and a moderate number of sites in the rotational dyads the whole range from the dynamic to the static limit in the interplay between excitation transfer and correlated conformational motion has been calculated by the DMC method. By means of the cumulant technique well-behaved solutions could be obtained only in the fast conformational transition regime which allows a direct comparison with the DMC results. For longer chains up to 100 sites, in the limit case of uncorrelated conformational motion, preliminary cumulant approaches have been given which, for very rapid conformational rates, agree well with the dynamic effective medium approximation (DEMA) solutions.
Symmetry and native structure in lattice proteins
Authors: Kellman Michael E.
Published: July 2002

A major unsolved problem in protein dynamics is the connection between the amino acid sequence and the propensity to fold to a unique, ordered structure. Many globular proteins fold to native states with great regularity in their tertiary structure, reminiscent of symmetry. This paper considers the relation between sequence, regular folded structure, and formal group theoretical symmetry in lattice protein models. In computer simulations of model proteins consisting of a chain of hydrophobic (H) and polar (P) units on a lattice [K. Yue and K. A. Dill, Proc. Natl. Acad. Sci. USA 92, 146 (1995)] it was observed that some chains fold to regular, ‘‘symmetric’’ structures. These structures have a small number of native states, i.e., the conformations of minimum energy have low degeneracy in this model, in which the energy depends only on the number of H–H contacts. The present work is concerned with the connection between the symmetry properties of the H–P sequence and those of the folded lattice protein. The meaning of symmetry in lattice models is discussed in terms of mathematical groups. The concepts of the lattice symmetry, the hydrophobic lattice core and its geometrical symmetry, and the core sequence and its permutation symmetry, are defined and related. With the help of a simple rescaling of the energy, the folded lattice protein is seen to have the symmetry of a microcrystal of H and P ‘‘atoms’’ arranged on a lattice. Geometrical symmetry of the folded structure is related to formal permutation symmetry properties of the H–P sequence, in order to establish the connection between sequence and fold. How these relations among symmetry, structure, and sequence might relate to folding in real proteins, including the design of sequences to fold into desired structures, is discussed.
Scaling of the interfacial tension of microemulsions: A phenomenological description
Authors: Leitão H. , Somoza A. M. , Telo da Gama M. M. , Sottmann T. , Strey R.
Published: July 2002

Recent experiments revealed a striking scaling behavior of the low and ultralow interfacial tension of microemulsions. A description of this behavior based on the Helfrich elastic free energy, which is symmetric in the principal curvatures c1 and c2, appears to be inconsistent. We show that, within the phenomenological theory of membrane bending elasticity, symmetry breaking between the two principal curvatures seems to be required in order to explain the low, but nonzero, values of the interfacial tension and its temperature dependence. We propose two simple generalizations of the Helfrich free energy which describe the experimental results. The first considers a quadratic elastic free energy and anisotropy in the membrane which breaks the symmetry between the two principal curvatures. In the second, which is applicable to systems with positive saddle-splay rigidities, the symmetry between the two principal curvatures is spontaneously broken by inclusion of higher-order terms in the curvatures in order to stabilize the free energy of the system. This analysis provides a straightforward method to obtain estimates of the bending elastic constants from interfacial tension measurements. Experiments confirming the theoretical picture are presented and values for κ and κ̄, for a variety of systems, are obtained.
Vibrational energy transfer from four levels below 410 cm−1 in S1 p-difluorobenzene. II. A search for vibration to rotation transfer
Authors: Mudjijono  , Lawrance Warren D.
Published: July 2002

Collision-induced vibrational energy transfer has been studied in S1 p-difluorobenzene in a supersonic free jet expansion at ∼30–40 K with the diatomic partners H2, D2, and N2. Transfer has been studied from the initial levels 302 (Evib=240 cm−1), 82 (Evib=361 cm−1), 271 (Evib=403 cm−1) and 61 (Evib=410 cm−1). The diatomic partners provide the possibility for transfer of vibrational motion in p-difluorobenzene to rotational motion in the diatomic (vibration to rotation transfer) in addition to the ubiquitous transfer of vibrational to translational motion. No compelling evidence is found for vibration to rotation transfer. Consequently, the diatomics are expected to behave analogously to monatomics, for which it has previously been observed that across the He to Kr series there is a substantial increase in multiple quanta (|Δυ|≳1) transfer. The results for the diatomics are qualitatively in accord with this trend, with increased multiple quanta transfer from H2 to D2 to N2. However, the diatomics do not always slot into the monatomic series where expected. Furthermore, the particular channels observed to be prominent in the |Δυ|=2 transfer for the diatomics are in a number of instances different to those seen for the monatomics. There are subtle but clear differences between these two classes of collision partner. The behavior of D2 is particularly unusual, and at this stage unexplained. This collision partner has an unexpected preference for transfers involving multiple changes in vibrational quanta. The state-to-state branching ratios for transfer from 271 and 61 are very similar, suggesting that the initial vibrational motion and its symmetry play little role in determining the favored destination levels.
Reaction mechanisms and energy disposal in the [C2H2:OCS]+ system: A mode-selective differential cross section study
Authors: Chiu Yu-hui , Fu Hungshin , Huang Jui-tsen , Anderson Scott L.
Published: July 2002

Charge transfer and S-atom transfer have been studied for reaction of both charge states of the [C2H2:OCS]+ system. Reactions have been studied as a function of six different modes of reactant nuclear motion, including relative motion and nine levels of five vibrational modes for the two reactant ions. Integral cross section measurements provide information on how total reactivity and product branching are affected by different forms of reactant excitation. Detailed insight into the reaction mechanism is obtained from differential scattering measurements—the first ever for mode-selectively excited reactants. The S-transfer reaction is found to be nearly identical for the two reactant charge states, which appear to be coupled by charge transfer in the collision entrance channel. In both charge states S transfer is dominated by two distinct direct scattering mechanisms, rebound and glancing/stripping, each of which are affected by collision energy and vibrational state. At collision energies below 0.5 eV, complex-mediated scattering becomes an important mechanism as well. It appears that the cis-trans symmetry of the C2H+2 bending vibration strongly affects formation of [C2H2:OCS]+ complexes. Charge transfer goes by very different mechanisms in the two charge states. For C2H+2+OCS, long-range electron hopping is the dominant mechanism, while for OCS++C2H2 the dominant mechanism requires impulsive collisions.
The effect of chain correlations on the size of polymer coils in binary polymer blends
Authors: Garas George E. , Kosmas Marios K.
Published: July 2002

In order to study the size of the chains in binary polymer blends we determine the mean-square end-to-end distance of the polymer coils of the A or B kind. Chains A and B interact generating the one-loop diagrams, which are summed up to all orders of concentration and interaction. Interactions that do not favor mixing are found to reduce the size of the macromolecules producing a deviation from the unperturbed dimensions of the order of 1/√N. The effect is stronger on chains of the smaller concentration whereas chains of large concentration and of large molecular weight, N→∞, tend to reach an ideal Gaussian behavior. Chains A are found to be more contracted when chains B are small acting as a bad solvent. As the system comes close to the phase separation limit, chains shrink obtaining a smaller size on the spinodal, where a second-order phase transition is found. Our results are in accord with relevant SANS small angle neutron scattering and Monte Carlo experiments.
Femtosecond reaction dynamics in the gas-to-liquid transition region: Observation of a three-phase density dependence
Authors: Liu Qianli , Wan Chaozhi , Zewail Ahmed H.
Published: July 2002

We report the observation of a striking density dependence in the coherence dynamics of an elementary reaction, solute iodine in solvent rare gases (density from 0 to 50 mol/l). With the help of MD simulations, the time scales of slow and fast solvent-force fluctuations are resolved and the underlying mechanism is revealed.
Spectroscopy of mass-selected niobium trimers in argon matrices
Authors: Wang Huaiming , Craig Robert , Haouari Hanae , Liu Yifei , Lombardi John R. , Lindsay D. M.
Published: July 2002

The absorption (scattering depletion) spectrum and Raman spectra for Nb3 in an argon matrix prepared by the mass selected ion deposition technique have been obtained. The absorption spectrum in the visible region shows three overlapping transitions, centered at 20 300 cm−1 (A), 18 800 cm−1 (B), and 17 000 cm−1 (C), respectively. Resonance Raman spectra obtained with excitation into these bands display two distinct fundamental frequencies at 227.4±2.9(e′) and 334.9±2.8(a1′) cm−1 which indicate that the ground state of the Nb3 molecule has a nearly equilateral triangular geometry (D3h). The fr (bond stretch) and frr (stretch–stretch interaction) force constants for Nb3 are determined to be 1.95 and 0.05 mdyn/Å, respectively. The Raman excitation profiles bear a strong resemblance to the triniobium absorption spectrum, but the peaks seem more closely spaced than the absorption spectrum. Both a1′ and e′ vibration are observed with the same intensity near the peak A, while the a1′ vibration dominates the profile at the peak B. Neither displays much intensity near the peak C.
Nucleation of Lennard-Jones fluids: A density functional approach
Authors: Shen Yu Chen , Oxtoby David W.
Published: July 2002

We have examined the critical nucleus through density functional calculations of crystalline fluctuations in a Lennard-Jones fluid. The free energy functional we employ has a square-gradient form, with the parameters for a Lennard-Jones interaction potential determined by a modified weighted density approximation applied locally through the liquid–solid interface. We have found that the interface and the center of the critical nucleus behave differently at large undercoolings and at large superheatings. At large undercoolings, the interface of the critical nucleus sharpens, in contrast to the critical nucleus near the spinodal (at large superheatings) that is wide in extent and small in amplitude. We compare the radius and work of formation from classical nucleation theory to that from density functional calculations.
Experimental investigation of the rotational- and vibrational-state dependence of the HF–Rg interactions
Authors: van Duijn E. J. , Nokhai R. N. , Hermans L. J. F.
Published: July 2002

A systematic investigation of the rotational- and vibrational-state dependence of the HF-Rg (Rg = He, Ar, Kr, Xe) intermolecular interactions has been performed using the technique of light-induced drift. Data are presented of the relative change in collision rate upon excitation Δν/ν≡(νe−νg)/νe of HF with respect to Rg. We studied the P- and R-branch of the fundamental vibrational band (v=0→1) with the rotational quantum number J ranging from J=0 through 6 using a continuously tunable F-Center Laser (λ≊2.5 μm). The results indicate that v and J have independent additive influences upon the collision rate ν. This allows one to determine the v- and J-dependence of ν separately. It is found that, upon vibrational excitation v = 0 → 1, ν increases by ≊0.15% for HF-He; ≊1.0% for HF-Ar, Kr and ≊1.5% for HF-Xe. A remarkable J-dependence of ν is observed: for HF-Ar, Kr and Xe, the collision rate ν first decreases by ≊5% for J=0→1, subsequently reaches a minimum for J=2 and then increases again for higher J. By contrast, for HF-He, ν decreases monotonically for J=0 through 5 by ≊ 4%. The influence of the optically induced alignment of J is found to produce a minor contribution only.
Laser excitation spectroscopy of the B 1Σ+u(O+u)←a 3Πg(O+g) transition of Cd2: Vibrational analysis and rotational structure
Authors: Tran H. C. , Eden J. G.
Published: July 2002

The B 1Σ+u←a 3Πg transition of Cd2 (natural abundance) and 114Cd2 has been observed by laser excitation spectroscopy and analyzed. By exciting the Cd2 B←a transition in the visible (560≤λ≤730 nm) while monitoring B 1Σ+u→X 1Σ+g (bound→free) emission in the ultraviolet (∼270–310 nm), more than 40 red-degraded vibrational bands were recorded. Analysis of the spectrum has yielded vibrational constants for both the a 3Πg and B 1Σ+u states: ωe″=153.6±4.0 cm−1, ωe″xe″=0.52±0.06 cm−1, ωe′=105.3±1.0 cm−1, and ωe′xe′=0.44±0.03 cm−1. In addition, ΔRe≡ReB−Rea was determined to be 0.31±0.03 Å. Rotational structure has been partially resolved for 114Cd2 bands in the 620–655 nm and 719–723 nm regions, and the spontaneous emission lifetime of the a 3Πg state and the rate constant for quenching of Cd2 (a 3Πg) by collisions with background Cd atoms have been determined to be 8.6±2.5 μs and (2.2±0.3)×10−13 cm3 s−1, respectively. Also, analysis of the B→X emission (Condon internal diffraction) spectra produced when specific B 1Σ+u vibrational levels are populated has yielded ReX−ReB=0.95±0.02 Å.
Effect of interaction potentials in diffusion-influenced reactions with small reactive regions
Authors: Zhou Huan-Xiang
Published: July 2002

The effect of interaction potentials on the rate coefficients of irreversible diffusion-influenced bimolecular reactions is studied. If the region of configurational space where reaction can occur is small, the rate coefficient k in the presence of an interaction potential U is shown to be simply given by k0, the rate coefficient in the absence of the interaction potential, scaled by 〈exp(−βU)〉, the average Boltzmann factor in the reaction region. For a finite size of the reactive region, this relation is generally more accurate for potentials with smaller values of 〈exp(−βU)〉 and, at a given value of 〈exp(−βU)〉, more accurate for potentials that vary less significantly around the reactive region.