bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultraviolet photoelectron spectroscopy of poly(pyridine-2,5-diyl), poly(2,2′-bipyridine-5,5′-diyl), and their K-doped states
Authors: Miyamae Takayuki , Yoshimura Daisuke , Ishii Hisao , Ouchi Yukio , Seki Kazuhiko , Miyazaki Takafumi , Koike Tsuneaki , Yamamoto Takakazu
Published: July 2002

Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π-conjugated polymers, poly(pyridine-2,5-diyl) (PPy) and poly(2,2′-bipyridine-5,5′-diyl) (PBPy) which exhibit n-type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π-conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K-doped PBPy. While K-doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped PBPy. The difference of the dependence on dopant concentration between K-doped PPy and K-doped PBPy is discussed based on the conformational difference between these polymers.
Dissociation energies of the monochlorides and dichlorides of Cr, Mn, Fe, Co, and Ni
Authors: Hildenbrand D. L.
Published: July 2002

The gaseous equilibria MCl+Ag=M+AgCl and M+MCl2=2 MCl, where M=Cr, Mn, Fe, Co, and Ni were studied by mass spectrometry in the region around 1500 K, and thermochemical data were derived by second and third law methods. Dissociation energies of the metal monochlorides were evaluated by reference to AgCl, leading to the values at 298 K in kcal/mol−1: CrCl, 90.3; MnCl, 80.9; FeCl, 78.8; CoCl, 80.7; NiCl, 90.1; all ±1.6 kcal mol−1. Entropies and enthalpies of sublimation of the solid dichlorides were derived from sublimation pressures measured by the torsion-effusion method. Sublimation enthalpies lead to MCl2 dissociation energies that are in close agreement with values derived from the mass spectrometric results; D(MCl2) values at 298 K are CrCl2, 179.3; MnCl2, 188.3; FeCl2, 190.1; CoCl2, 182.6; and NiCl2, 178.2; all ±1.0 kcal mol−1. Vapor entropies evaluated from the sublimation data show that the electronic energy levels of the doubly-charged free metal ions provide a reasonable representation of the electronic entropies of the metal dichlorides; in addition, the entropy data are consistent with a bent structure for CrCl2, but with linear structures for the other dichlorides. Judging from the second and third law agreement for the monochloride exchange reactions, the M+ free ion electronic levels also seem appropriate for the transition metal MCl species.
Characterization of the minimum energy paths and energetics for the reaction of vinylidene with acetylene
Authors: Walch Stephen P. , Taylor Peter R.
Published: July 2002

The reaction of vinylidene (CH2C) with acetylene may be an initiating reaction in soot formation. We report minimum energy paths and accurate energetics for a pathway leading to vinylacetylene and for a number of isomers of C4H4. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contacted configuration interaction (ICCI) and/or coupled cluster singles and doubles with a perturbational estimate of triple excitations [CCSD(T)] to determine the energetics. We find an entrance channel barrier of about 5 kcal/mol for the addition of vinylidene to acetylene, but no barriers above reactants for the reaction pathway leading to vinylacetylene.
An ab initio calculation of magnetic structure factors for Cs3CoCl5 including spin–orbit and finite magnetic field effects
Authors: Wolff Stephen K. , Jayatilaka Dylan , Chandler Graham S.
Published: July 2002

Spin–orbit interaction plays a significant role in determining the magnetic density in some transition metal complexes. We present a new ab initio technique, based on an extension of unrestricted Hartree–Fock theory, which includes nonperturbatively these spin–orbit effects, and simultaneously also the effects of a finite magnetic field. We also present a new and efficient method for calculating magnetic structure factors, based on the current density rather than magnetic dipole moment density, for a crystal composed of noninteracting molecular fragments. These structure factors are directly comparable to polarized neutron diffraction experiments. Results for the Cs3CoCl5 crystal are compared with experiment and previous studies. Without one-electron spin–orbit coupling terms, the magnitudes of the predicted structure factors are on average 10–15 % too low, whereas, with the spin–orbit terms, the magnitudes are 25–30% too high. Using an effective nuclear charge for Co in the spin–orbit term brings the results into much better agreement, and suggests that the two-electron spin–orbit shielding terms omitted in the present work are important. For over one quarter of the reflections studied, the magnetic contribution to the structure factors is more than 20% of the nuclear contribution.
Rotational excitations of a symmetric top in cubic orientational potentials: CH3D matrix-isolated in argon and krypton
Authors: Asmussen B. , Balszunat D. , Prager M. , Press W. , Carlile C. J. , Büttner H.
Published: July 2002

Measurements with inelastic neutron scattering of the rotational spectra of the symmetric top molecule CH3D as substitutional impurities in crystalline argon and krypton are presented. The energy of the J=1 doublet is shifted by 33% and 27% respectively, compared to the completely free rotor. Spin-rotational wave functions for the free CH3D rotor are constructed and used for the calculation of the dependence of the scattered intensities on momentum transfer. In contrast to the CH4 molecule, the center of mass does not coincide with the position of the carbon atom in the case of the CH3D molecule. However, energies and intensities can only be explained if this fact is disregarded. The spin conversion behavior has been studied on a CH3D/argon sample within the temperature range 1.5 K≤T≤5 K. Two different mechanisms for the conversion process (libron-phonon process and Raman process) are discussed.
Fermi resonances and local modes in stibine, SbH3: A Fourier interferometric and laser photoacoustic study of the overtone spectrum
Authors: Lummila Juha , Lukka Tuomas , Halonen Lauri , Bürger Hans , Polanz Oliver
Published: July 2002

The third stretching overtone region of a natural sample of stibine, SbH3, has been studied with high resolution infrared spectroscopy and the fifth and the sixth overtone region with Ti:Sapphire ring laser intracavity photoacoustic spectroscopy. The third overtone consists of a local mode pair of bands (400A1/E) which have been rotationally assigned both for 121SbH3 and 123SbH3 with a vibration-rotation model based on rectilinear normal coordinates. The vibrational dependencies of the model parameters are explained well with a simple block diagonal vibrational model. An extension of the standard vibration-rotation model is used to show that the upper state rotational energy level structures of both isotopic species are close to the rotational structure of an asymmetric rotor. High resolution laser spectrum of the fifth overtone consisting of a local mode pair of bands (600A1/E) shows severe perturbations in the upper state rotational structure. The (510A1/E) and (700A1/E) bands have been recorded with low resolution. All experimentally known vibration-rotation band origins of 121SbH3 have been reproduced well with a curvilinear internal valence coordinate system based Fermi resonance local mode model. The potential energy surface obtained agrees well with recent ab initio results.
Nearside–farside analysis of differential cross sections: Diffraction and rainbow scattering in atom–atom and atom–molecule rotationally inelastic sudden collisions
Authors: McCabe P. , Connor J. N. L.
Published: July 2002

Nearside–farside (NF) theory, as used to understand nuclear heavy-ion differential cross sections, is applied for the first time to the angular scattering of atom–atom and atom–diatom collisions. A NF decomposition of the partial wave series (PWS) for the scattering amplitude has the following advantages: (a) it is exact, (b) it uses PW scattering matrix elements (quantum or semiclassical) as calculated by standard computer programs, (c) it is easily incorporated into existing computer programs which calculate angular distributions, (d) semiclassical techniques, such as stationary phase or saddle point integration, are not invoked for the PWS, although the semiclassical picture is still evident. A disadvantage of a NF decomposition is that it is not unique. The Fuller and Hatchell NF decompositions are used to analyze the angular scattering of four collision systems whose PWS involve Legendre polynomials: (a) atom–atom He+Ne elastic diffraction scattering, (b) atom–atom H++Ar elastic rainbow scattering, (c) atom rigid-rotator Ne+D2(j=0) →Ne+D2(j) diffraction scattering under sudden conditions so that the infinite-order-sudden (IOS) approximation is valid, (d) atom rigid-rotator He+N2(j=0)→He+N2(j) rotational rainbow IOS scattering. The utility of these two NF decompositions is assessed by comparison with results from the semiclassical complex angular momentum (CAM) representation of the scattering amplitude. This is chosen because it allows an unambiguous separation of the scattering amplitude into nearside and farside subamplitudes under semiclassical conditions. The Fuller NF decomposition, unlike the Hatchell NF decomposition, provides a physically clear explanation of the angular scattering, which always agrees with the semiclassical CAM interpretation (except for scattering angles ≊180°). The Fuller NF decomposition is therefore recommended for applications to atomic and molecular collisions. The NF theory for the decomposition of Legendre polynomials is generalized to scattering amplitudes whose PWS involve associated Legendre functions or reduced rotation matrix elements.
A combined theoretical and experimental determination of the electronic spectrum of acetone
Authors: Merchán Manuela , Roos Björn O. , McDiarmid Ruth , Xing Xing
Published: July 2002

A combined ab initio and experimental investigation has been performed of the main features of the electronic spectrum of acetone. Vertical transition energies have been calculated from the ground to the ny→π*, π→π*, σ→π*, and the n=3 Rydberg states. In addition, the 1A1 energy surfaces have been studied as functions of the CO bond length. The 1A1 3p and 3d states were found to be heavily perturbed by the π→π* state. Resonant multiphoton ionization and polarization-selected photoacoustic spectra of acetone have been measured and observed transitions were assigned on internal criteria. The calculated vertical transition energies to the ny→π* and all Rydberg states were found to be in agreement with experiment. This includes the 3s-, all three 3p-, and the A1, B1, and B2 3d-Rydberg states. By contrast, there is little agreement between the calculated and experimental relative intensities of the A1 and B2 3d-Rydberg transitions. In addition, anomalously intense high vibrational overtone bands of one of the 3p-Rydberg transitions have been observed. These results confirm the strong perturbation of the 3p- and 3d-Rydberg states by the π→π* state found in the theoretical calculation and support the calculated position of this unobserved state.
Polymorphism of a glass forming plastic crystal: A kinetic investigation
Authors: Willart J. F. , Descamps M. , Benzakour N.
Published: July 2002

The glassy crystalline state designates the frozen state of the rotator phase of some molecular crystals. These systems are very suitable for investigating the vitrification and the crystallization processes as well as the interrelations between these two processes. This paper sheds light on this problem through a kinetic investigation of the glass forming plastic crystal (cyanoadamantane)1−x (chloroadamantane)x for x=0.25. A careful study of both the equilibrium phase diagram and the mode of transformation upon deep quenching conditions has been performed by time resolved x-ray diffraction and differential scanning calorimetry in a variety of thermal treatments. The results reveal a complex kinetic behavior corresponding to the imbrication of the kinetics toward two low temperature phases: (IV) and (III). Phase (IV) is found to be metastable with respect to phase (III) and only appears upon specific thermal treatments which are clearly established. The conditions in which the monotropic transition between the transient metastable phase (IV) and the undercooled rotator phase (I) can be seen are described.
Theory of domain boundary effects in a phase-separated mixture of polymer and liquid crystal
Authors: Yu Y.-K. , Wang X.-Y. , Taylor P. L.
Published: July 2002

We calculate the end results of a phase-separation process in which a nematic liquid crystal is expelled from solution in a polymerizing solvent. Our model combines the essential features of the Flory–Huggins, Maier–Saupe, and Hillert models, and contains terms related to the nature of the anchoring process at the polymer–liquid–crystal interface. We find that there is a minimum size of droplet below which no nematic ordering can occur, and that in the absence of the effects due to curvature of the interface, homeotropic anchoring gives rise to a lower surface free energy than does planar anchoring.
Intracavity laser absorption spectroscopy of HOCl overtones. I. The 3v1+2v2 band and numbers of vibrational states
Authors: Abel Bernd , Hamann Hilmar H. , Kachanov Alexander A. , Troe Jürgen
Published: July 2002

The near infrared high-resolution spectra of the a-type transitions of the weak 3v1+2v2 combination band of transient HO35(37)Cl at 12 600 cm−1 has been recorded in an ultrasensitive titanium:sapphire intracavity laser absorption spectrometer (ICLAS). We report line assignments, new and refined anharmonicity parameters, and the spectroscopic constants for the excited rovibrational states of 3v1+2v2. The Fermi resonance perturbations in this five quanta region, where the internal energy of the molecule is already more than 2/3 of the dissociation energy E0, remain localized and they are the exception, while the extent of intermode mixing and thus intramolecular vibrational energy distribution (IVR) seems to be still small. A Dunham expansion is used for band origin predictions and representations of vibrational states N(E) of HO35Cl up to the dissociation threshold. The results are compared with harmonic and anharmonic numbers of states from a recently proposed stretch–bend coupling model.
Solvent diffusion in ordered macrofluids: A stochastic simulation study of the obstruction effect
Authors: Jóhannesson Haukur , Halle Bertil
Published: July 2002

An off-lattice random-flight simulation procedure is described which accurately predicts the obstruction factors for self-diffusion of small molecules in macrofluids. The simulation procedure, employing a continuous step length distribution that ensures rapid and accurate convergence, was validated by comparison with exact results for cylindrical and spherical obstructions on 2D and 3D lattices. The exact results were computed with Rayleigh’s multipole method, which also was used to derive a new analytical formula for the obstruction factor of parallel cylinders on a hexagonal lattice, of much higher accuracy than the commonly used approximations. Random-flight simulations were used to assess the accuracy of existing mean-field approximations for the obstruction factors of orientationally ordered nonspherical objects. Due to a near-cancellation of errors, the mean-field result accurately describes the obstruction effect on the trace of the diffusion tensor, as measured in isotropic systems, up to moderately high volume fractions. In contrast, the diffusion anisotropy, a sensitive indicator of microstructure in anisotropic fluids, is accurately predicted by mean-field theory only at low volume fractions.
Condensation of a supersaturated vapor. XI. Stable operation of a thermal diffusion cloud chamber
Authors: Fisk Jeffery A. , Chakarov Vasil M. , Katz Joseph L.
Published: July 2002

A key approximation in the analysis of upward thermal diffusion cloud chambers is that both the heat flux and mass flux (in the center part of the chamber) are plane parallel. Procedures for determining when this approximation is valid are described. The key experimental parameter is the amount of heat added to the chamber walls to prevent vapor from condensing on them. Underheating the walls causes the experimental rates to be as much as 105 smaller than they would be when the plane parallel flux approximation is valid. Overheating the wall causes convection in the chamber center, also invalidating the plane parallel flux approximation. However, there does exist a range of wall heat values between these under and overheating limits over which the nucleation rate is negligibly affected. The overheating limit depends on the type and amount of noncondensible carrier gas present, whereas the underheating limit does not. Decreasing the molecular weight of the noncondensible gases, or decreasing the noncondensible gas pressure, increases the overheating limit.
Orientation and energy dependence of NO scattering from Pt(111)
Authors: Lahaye R. J. W. E. , Stolte S. , Holloway S. , Kleyn A. W.
Published: July 2002

A classical molecular dynamics study is applied to simulate the scattering of NO from Pt(111) in the energy range of 0.3–1 eV. The solid consists of a large number of crystal atoms that interact via an anharmonic nearest-neighbor potential. The NO–Pt(111) interaction potential is constructed as a pairwise additive potential with a well depth of 1 eV for the N end of the molecule towards the surface and purely repulsive for the O end. The in-plane scattering results obtained with this model potential are compared with recent experiments for NO–Pt(111). The angular intensity distributions, the final translational energy, as well as the rotational energy distributions with the corresponding alignment are in qualitative agreement with those experimental results. A detailed examination of the collision dynamics shows that multiple collisions with the surface results predominantly in superspecular scattering. The rotational angular momentum of the scattered molecules exhibits a preference for cartwheeling alignment and the rotational energy distributions for specular and normal exit angles can be described with a Boltzmann distribution, whereas for grazing exit angles they are distinctly non-Boltzmann. The latter structure results from a cutoff in the rotational excitation by the attraction of the well. The high rotational excitation clearly originates from molecules that initially are oriented with the O end towards the surface, whereas for the low rotational excitation this orientation preference disappears.
The Ã 1A
u state and the T2 potential surface of acetylene: Implications for triplet perturbations in the fluorescence spectra of the Ã state
Authors: Sherrill C. David , Vacek George , Yamaguchi Yukio , Schaefer Henry F. , Stanton John F. , Gauss Jürgen
Published: July 2002

The cis–trans isomerization reaction on the T2 surface of acetylene and the lowest excited singlet state of acetylene, Ã 1Au, are investigated by ab initio electronic structure theory. We report optimized geometries, dipole moments, and harmonic vibrational frequencies of stationary points and adiabatic energy differences between them using basis sets as large as triple-ζ plus double polarization with higher angular momentum functions, TZ(2df,2pd), and theoretical methods up to coupled-cluster singles and doubles with a perturbative triples correction [CCSD(T)] and the equation-of-motion coupled-cluster method (EOM-CCSD). Our theoretical predictions should aid the interpretation of observations from a series of recent spectroscopic studies involving excited electronic states of acetylene. In particular, the present theoretical results rule out several possible explanations for the anomalous sudden increase in detectable Zeeman anticrossings reported by Dupré et al. [Chem. Phys. 152, 293 (1991)].
Production of NH(ND) radicals in the reactions of N(22D) with H2(D2): Nascent vibrational distributions of NH(X 3Σ−) and ND(X 3Σ−)
Authors: Umemoto Hironobu , Matsumoto Ken-ichi
Published: July 2002

A mixture of NO and H2(D2) was irradiated with an intense laser pulse at 275.2 nm which dissociates NO to produce N(2D). Electronically ground state NH(ND) radicals could be detected as products of the N(2D)+H2(D2) reaction by laser-induced fluorescence. The nascent vibrational population ratios, NH(v″=1)/NH(v″=0) and ND(v″=1)/ND(v″=0), were determined to be 0.9 and 1.0, respectively.
Ordering in many component Widom–Rowlinson models
Authors: Sear Richard P.
Published: July 2002

We examine the changing immiscibility of Widom–Rowlinson-type model mixtures as the number of components is varied. In these mixtures, molecules interact with molecules of a different component via a hard-core interaction but do not interact with other molecules of the same component at all. The interaction between any two molecules of different components is the same, it is that between two parallel hard (hyper-)cubes. Theoretical calculations show that an equimolar mixture of less than 31 components demixes in the fluid phase but if there are 31 or more components demixing is preempted by solidification. These calculations are exact in the limit of infinite dimensionality. The switchover from demixing to solidification as the number of components increases is due to the high entropy cost incurred by a mixture of many components phase separating into a large number of pure phases. A Widom–Rowlinson mixture is shown to be (nearly) equivalent to the Zwanzig model of a liquid crystal at a dimensionality equal to the number of components in the mixture. Using the results obtained for the mixtures we speculate that there is an upper bound to the dimensionality at which liquid crystallinity can exist.
Calculation of the Herman–Wallis effect in Π–Σ vibrational overtone transitions in a linear molecule: Comparison with HCN experimental results
Authors: Romanini Daniele , Lehmann Kevin K.
Published: July 2002

The high sensitivity of cavity ring-down spectroscopy has allowed us to observe a few perpendicular vibrational overtone transitions of HCN in the visible. These transitions display a sizable Herman–Wallis effect, that is an asymmetry in the relative intensities of the R and P branch lines. We have developed a theory for the first-order Herman–Wallis effect based upon using variational vibrational wave functions but treating the vibration–rotation interaction by first-order perturbation theory. In the specific case of perpendicular transitions, the first-order effect is dominated by Coriolis mixing of Σ and Π overtone states. We used the empirical energy surface by Carter, Mills, and Handy [J. Chem. Phys. 99, 4379 (1993)] restricted to the stretching degrees of freedom. Bending was included by multiplication of these stretching wave functions by harmonic wave functions of the bend. Vibrational transition moments were calculated using a polynomial surface fit to ab initio CCSD(T) dipole moment points by Botschwina et al. [Chem. Phys. 190, 345 (1995) and private communication]. We expected that this treatment would be accurate but the calculated Herman–Wallis effect is about one order of magnitude too large. To gain further insight into the poor agreement between theory and experiment, we have calculated the sensitivity of the Herman–Wallis coefficient and of the transition moment to the dipole and energy surface parameters. From this, it appears that the dipole surface, while producing accurate band intensities, could at the same time be inadequate to account for the Herman–Wallis effect. A similar possibility stands for the energy surface, which however is highly constrained by the requirement to fit the observed band origins.
Bond selective infrared multiphoton excitation and dissociation of linear monodeuterated acetylene
Authors: Kaluža Matjaž , Muckerman James T.
Published: July 2002

Quantum mechanical simulations of vibrational excitation of monodeuterated linear acetylene (HCCD) with linearly polarized, frequency-swept, intense but nonionizing infrared laser pulses are performed. The aim is selective dissociation of either H or D atoms by optimal shaping of the laser pulses. We use a discrete variable representation and a compact (<400 states) bright-state expansion to represent the wave function during and after the pulse. Wave packet propagations in the bright-state expansion are at least an order of magnitude faster than discrete variable representation wave packet propagations. This enables optimal-control calculations to find the best parameters for the laser pulses. The dynamics of CH-bond breaking with infrared pulses are very different from the dynamics of CD-bond breaking. This is a direct consequence of CH being the highest-frequency mode in the molecule. Selective CH-bond breaking is possible with two synchronized pulses, the first being quasi-resonant with the Δv=1 transitions in the CH stretch between v=0 and v=8, and the second being quasiresonant with Δv=2 transitions at higher v. H-atom yields as high as 7.7%, with H to D yield ratio as high as 2.1, are demonstrated. Selective CD-bond breaking is possible using a single, subpicosecond, frequency-swept pulse. D-atom yields as high as 3%, or D to H atom yield ratios as high as 3.9, are calculated.
Theoretical studies of geometric structures of phenol-water clusters and their infrared absorption spectra in the O–H stretching region
Authors: Watanabe Hidekazu , Iwata Suehiro
Published: July 2002

The structures of the phenol-(H2O)n clusters (n≤4) are determined with ab initio molecular orbital methods, and their infrared spectra are calculated to analyze the experimental spectra reported in the preceding paper. The experimental infrared spectra of phenol-(H2O)n clusters for n≤4 have ‘‘window regions,’’ which are intervals of two types of OH stretching modes of the water molecules. The calculated IR spectra of isomers with a ring structure will reproduce these window regions. The ring is formed by a network of the hydration bonds of the −OH group of the phenol and water molecules. For n=4, two kinds of spectra are reported in the experiments. One spectrum has a window region similar to that of n≤3. It is, therefore, identified to the isomers of a ring structure. The other one has several bands in the window region. The calculations for several isomers and large clusters suggest that this spectra might be attributed either (i) to the mixture of several branched isomers, (ii) to the products of evaporation of large clusters, or (iii) to the product of the proton transfer reaction in phenol-(H2O)4 cluster.
Interaction between macroparticles in aqueous electrolytes
Authors: Kinoshita Masahiro , Iba Shin-ya , Harada Makoto
Published: July 2002

We report results of theoretical calculations for the interaction between two isolated, structureless spherical macroparticles immersed in aqueous electrolytes comprising waterlike molecules (hard spheres embedded with point dipoles and tetrahedral quadrupoles), 1:1 cations and anions (the diameter of cations is equal to that of anions). The reference hypernetted-chain (RHNC) theory with hard-sphere bridge functions is employed in the calculations. The fluid structure and the potential of mean force scaled by dS/(πdM) (dS and dM denote the solvent and macroparticle diameters, respectively) converge to limiting behaviors with increasing dM as long as the surface charge density of the macroparticle is kept constant. The qualitative aspects of the conclusions are not altered even when dM is set at 10dS. The interaction between neutral macroparticles in pure water is characterized by strong, short-range attraction. However, a relatively minor, short-range repulsive component is added to the interaction when ions are included in water. On the other hand, the presence of apolar particles in water at a trace concentration leads to considerable enhancement of the attraction. Effects of ionic sizes on the interaction between charged macroparticles are substantially large and opposite to those which would be observed using the primitive model. When the size of counterions is sufficiently large and the ionic concentration is sufficiently high, there is a regime where the interaction between highly like-charged macroparticles is strongly attractive.
Simulation of nondynamical correlation in density functional calculations by the optimized fractional orbital occupation approach: Application to the potential energy surfaces of O3 and SO2
Authors: Wang S. G. , Schwarz W. H. E.
Published: July 2002

If the energies of occupied and empty orbitals as functions of N geometric parameters vary in different manners, single determinantal approaches such as the density functional ones may violate the Aufbau principle and the noncrossing rule, i.e., states of the same symmetry may cross in an (N−1)-dimensional parameter subspace. If dominant configuration mixing is simulated by a mixed ensemble density with fractional occupation numbers of the orbitals at the Fermi level, the density functional approaches recover the avoided crossing, as first pointed out by Dunlap and Mei [J. Chem. Phys. 78, 4997 (1983)]. However, present density functional approaches do not recover the nonavoided crossing in (N−2)-dimensional subspaces. The density functional–fractional occupation number approach yields reasonable pathways for the transition from the cyclic to the open ground state of O3 and SO2 molecules.
The influence of morphology on the vibrational spectra of crystalline aerosols: SF6 and CO2
Authors: Gough T. E. , Wang T.
Published: July 2002

Intense infrared absorptions of small particles suspended in a dissimilar medium show effects that allow one to deduce, at least in principle, the morphology of the particles. These effects have their origins in the discontinuity in optical constants at the surface of the particle, and may be described analytically when the particles are spherical or ellipsoidal. This paper addresses the problems that arise when the particles under consideration are crystalline and have sharp edges and corners. Analytical descriptions are not available for such cases and it is shown that a numerical implementation the current standard electrostatic approach does not converge for cubes. However, convergence can be obtained by chamfering the edges and corners of the simulated particle. Experimental spectra are reported for suspended microcrystallites of sulfur hexafluoride and of carbon dioxide. The spectra were simulated using a variety of morphologies and it is concluded that the sulphur hexafluoride particles were cubes while the carbon dioxide particles were cuboctahedra.
Scaling and scale breaking in polyelectrolytes
Authors: Peterson Carsten , Sommelius Ola , Söderberg Bo
Published: July 2002

We consider the thermodynamics of a uniformly charged polyelectrolyte with harmonic bonds. For such a system there is at high temperatures an approximate scaling of global properties like the end-to-end distance and the interaction energy with the chain length divided by the temperature. This scaling is broken at low temperatures by the ultraviolet divergence of the Coulomb potential. By introducing a renormalization of the strength of the nearest-neighbor interaction the scaling is restored, making possible an efficient blocking method for emulating very large polyelectrolytes using small systems. The high temperature behavior is well reproduced by the analytical high-T expansions even for fairly low temperatures and system sizes. In addition, results from low-T expansions, where the coefficients have been computed numerically, are presented. These results approximate well the corresponding Monte Carlo results at realistic temperatures. A corresponding analysis of screened chains is performed. The situation here is complicated by the appearance of an additional parameter, the screening length. A window is found in parameter space, where scaling holds for conformational as well as thermodynamical properties. This window corresponds to situations where the range of the potential interpolates between the bond length and the size of the chain. This scaling behavior, which is verified by Monte Carlo results, is, for the end-to-end distance, consistent with Flory scaling. Also for the screened chain a blocking approach can be devised, that performs well for low temperatures, whereas the low-T expansion is inaccurate at realistic temperatures.
Cluster formation in sputtering: A molecular dynamics study using the MD/MC-corrected effective medium potential
Authors: Wucher A. , Garrison B. J.
Published: July 2002

We report on a molecular dynamics simulation of cluster emission during sputtering of metals using a new many-body potential developed by DePristo and co-workers. For the specific case of silver as a sample target material, it is shown that this potential allows a much more realistic description of small clusters than the EAM potential used in our previous work. While this has a relatively large effect on the relative abundance of clusters within the total flux of sputtered material, other cluster properties like kinetic energy distributions and internal excitation are found to be less affected. By comparison with corresponding experimental data, we conclude that the formation of sputtered silver clusters can now be almost quantitatively modeled by the simulation.
Electronically adiabatic reaction field approach to solvation. I. Theoretical formulation via multipole expansion in a fluctuating cavity
Authors: Kim Hyung J.
Published: July 2002

A theoretical framework for the solute electronic structure description under nonequilibrium solvation is developed via multipole expansions of a quantum dielectric continuum solvent formulation of Kim and Hynes [J. Chem. Phys. 96, 5088 (1992)]. By employing a spherical cavity for the solute and invoking a Born–Oppenheimer description for the solvent electronic polarization P⃗el, the cavity boundary effects on the solute electric and solvent polarization fields are taken into account exactly. The solute–solvent electronic correlation effects are also included within the dielectric continuum context in the fast P⃗el limit. Another novel feature of the theory includes the cavity size variation with the solute electronic charge distribution and its thermal fluctuations. This effectively accounts for, e.g., electrostriction, largely ignored in many solution-phase quantum chemistry calculations based on the reaction field methods. By employing a coherent state description for P⃗el, we obtain electronically adiabatic free energies as a function of the cavity radius variable that measures the fluctuating cavity size and the solvent coordinates that gauge the nonequilibrium solvent orientational polarization P⃗or. These define multidimensional electronic free energy surfaces, upon which nuclear dynamics occur. Their local structure near equilibrium, along with the solute polarizability effects on the force constant matrix, is analyzed. With a polaron description for the P⃗or kinetic energy, it is found that the frequency relevant for ultrafast inertial solvation dynamics decreases as the Pvec;or multipole character increases. This is in qualitative agreement with recent molecular solvation theory predictions. As for the cavity, the frequency associated with its symmetric breathing mode is examined by analyzing our previous molecular dynamics simulation results via the equipartition principle. It is found that the cavity frequency is comparable to that of P⃗or. The variation of the equilibrium cavity size with the solute charge distribution and its influence on free energetics are also studied. Model calculations in water show that the cavity size decreases with the increasing solute dipole moment. This results in a significant reduction of equilibrium free energy, compared to that obtained with the neglect of the electrostriction effect.
Condensation of supersaturated vapors of hydrogen bonding molecules: Ethylene glycol, propylene glycol, trimethylene glycol, and glycerol
Authors: Kane David , El-Shall M. Samy
Published: July 2002

The critical supersaturations required for the homogeneous nucleation (rate of 1 drop cm−3 s−1) of ethylene glycol, propylene glycol, trimethylene glycol and glycerol vapors have been measured over wide temperature ranges (e.g., 280–400 K) using an upward thermal diffusion cloud chamber. At lower temperatures the experimental nucleation rates are much higher than the predictions of the classical nucleation theory. Glycerol shows the best agreement between experiment and theory in the temperature range of 340–370 K. An apparent increase in the critical supersaturation of glycerol is observed with increasing carrier gas (helium) pressure and this effect is more pronounced at lower temperatures. The results from corresponding states and scaled nucleation models indicate that the nucleation behavior of glycerol is quite different from other glycols. Glycerol requires higher critical supersaturations compared to the other glycols at the same reduced temperatures. This implies quite small critical clusters for glycerol (20–50 molecules) in the temperature range 300–380 K. The discrepancy between experiment and theory at lower temperatures may be explained by considering that the surface tension of the critical clusters is lower than the bulk surface tension. It is, however, surprising that a Tolman type correction for the curvature dependent surface tension could be applicable for such small critical clusters. Further theoretical work is required in order to fully understand the observed higher nucleation rates at lower temperatures of glycols and glycerol.
Monte Carlo simulation study of recombination dynamics in solution
Authors: Kim Hyojoon , Shin Seokmin , Lee Sangyoub , Shin Kook Joe
Published: July 2002

A Monte Carlo (MC) simulation study of radical recombination dynamics in solution is presented. Random walks on a three-dimensional lattice are used to simulate the diffusion of radical pairs initially prepared by photolysis. The survival probability as a function of time is calculated by the dynamical interpretation of the MC methods. For the dynamics of radical pairs, the interplay of the geminate and the bulk recombinations is important at high concentrations. In most previous simulations only the dynamics of isolated geminate pairs were considered. In our simulations such interplay is clearly demonstrated by studying the intermediate to long time dynamics of many radical pairs. We compare our results with the predictions of the diffusion and the kinetic theories to examine the validity of their approximations. For low concentrations, results of the simulations show good agreement with theoretical predictions and provide a microscopic basis for understanding the recombination process. But notable deviations are found for high concentrations, which can be attributed to the approximations in theoretical studies.
Rotational spectra, structures, and dynamics of small Arm–(H2O)n clusters: The Ar2–H2O trimer
Authors: Arunan E. , Dykstra C. E. , Emilsson T. , Gutowsky H. S.
Published: July 2002

Rotational spectra have been observed for the Ar2–H2O trimer and several of its isotopomers with the Balle/Flygare pulsed nozzle, Fourier transform microwave spectrometer. Analysis gives a planar T-shaped structure with C2v symmetry and the bidentate protons pointed at the argons. Two sets of asymmetric top transitions were found for the species with H2O, D2O, or H2 18O. Several lines of evidence support assigning them to internal rotor states of the water, the upper set to Σ(000) and the lower to Σ(101) as in the Ar–H2O dimer. Support includes: Hyperfine interaction constants which differ for the two states of water; Systematic aspects of the rotational constants such as B’s that are little affected by isotopic substitution; and MMC calculations which indicate the importance of rovibrational coupling. The Ar–Ar distance for the Σ(000) state of the trimer is estimated to be 3.822 Å and the center of mass (Ar2) to center of mass (H2O) distance to be 3.173 Å. An intriguing result is finding the hyperfine interaction constants of the water in the trimer to be very nearly the same as those reported earlier for the dimer. This shows the two dynamic states of the water are the same in the dimer and trimer, as is the average projection of the water C2 axis onto the inertial frame.
An improved perturbation approach to the 2D Edwards polymer: Corrections to scaling
Authors: Shannon S. R. , Choy T. C. , Fleming R. J.
Published: July 2002

We present the results of a new perturbation calculation in polymer statistics which starts from a ground state that already correctly predicts the long chain length behaviour of the mean square end-to-end distance 〈R2N〉, namely, the solution to the two dimensional (2D) Edwards model. The 〈R2N〉 thus calculated is shown to be convergent in N, the number of steps in the chain, in contrast to previous methods which start from the free random walk solution. This allows us to calculate a new value for the leading correction-to-scaling exponent Δ. Writing 〈R2N〉=AN2ν(1+BN−Δ+CN−1+...), where ν=3/4 in 2D, our result shows that Δ=1/2. This value is also supported by an analysis of 2D self-avoiding walks on the continuum.
A study on the structure and vibrations of diphenylamine by resonance-enhanced multiphoton ionization spectroscopy and ab initio calculations
Authors: Boogaarts Maarten G. H. , von Helden Gert , Meijer Gerard
Published: July 2002

Laser-desorption jet-cooling has been applied in combination with mass-selective gas-phase spectroscopic techniques to study the structure and low-frequency vibrations of diphenylamine (DPA). Two-color (1+1′) resonance-enhanced multiphoton ionization has been used to measure the vibrationally resolved excitation spectrum of the S1←S0 transition in the 305–309 nm region. Ion-dip measurements have been performed to determine the vibrational structure in the electronic ground state. The electronic spectra of DPA are dominated by long progressions in low-frequency vibrations involving the motion of the phenyl rings as a whole. For the interpretation of the experimental data ab initio calculations have been performed at the Hartree–Fock level for the S0-state and using single-excitation configuration interaction for the S1-state. The DPA molecule is found to change from a pyramidal geometry around the N-atom with unequal torsional angles of the phenyl groups in the S0-state to a planar geometry with equal torsional angles in the S1-state. The two most prominent vibrational motions are the in-phase wagging and the in-phase torsion of the phenyl rings. In addition, the resonance-enhanced multiphoton ionization spectra of the S1←S0 transition in the DPA-Ar, DPA-Kr, and DPA-Xe van der Waals complexes have been measured. From these spectra it is inferred that there is a coupling between the van der Waals modes and the low-frequency intra-molecular modes of DPA.
Diabatic approach to the close-coupling wave packet method in reactive scattering
Authors: Caratzoulas Stavros , Jackson Bret
Published: July 2002

This article explores a diabatic formulation of electronically adiabatic quantum reactive scattering problems. In particular, a diabatic approach to the close-coupling wave packet method is developed which allows us to perform, efficiently, accurate calculations using natural coordinate systems for both the reactant and product configurations. The efficiency and accuracy of the method are investigated in terms of the size of the basis sets, as well as in terms of the symmetry of the diabatic coupling and demonstrated for a model H+H2 collinear reaction. Extension of the method to three-dimensional problems is discussed.
Further study on collisional quantum interference effect in energy transfer within CO singlet–triplet mixed states
Authors: Chen Xiangling , Sha Guohe , Jiang Bo , He Jinbao , Zhang Cunhao
Published: July 2002

In our previous study [G-H Sha, J-B. He, B. Jiang, and C-H. Zhang, J. Chem. Phys. 102, 2772 (1995)], a concept of interference phase angle (θST) has been formally introduced to define the coherence effect between singlet and triplet energy transfer channels for mixed states. In this contribution, we have measured θST for various monoatomic (He, Ne, Ar) and diatomic (H2, N2) collision partners at temperatures of 77 K and 470 K. Via a new data processing approach, θST is fitted more accurately than earlier approximations. Our experimental results show that θST increases with the polarizability of monoatomic collision partners, while for diatomic collision partners θST is significantly higher than that for monoatomic ones. θST at 470 K for He and Ne is higher than that of 77 K, while an adverse temperature effect on θST has been found for N2. The influence of intermolecular potential and possible complex formation between excited CO and N2 or H2 has been discussed.
On the shattering of clusters by surface impact heating
Authors: Raz T. , Levine R. D.
Published: July 2002

The onset of a shattering regime when a supersonic cluster undergoes an ultrafast heating by its impact at a surface, proposed on the basis of an information theoretic analysis, has now been demonstrated experimentally for molecular clusters. It is emphasized that the sudden onset of shattering as a function of impact velocity is a robust result depending essentially only on the multitude of possible isomers of larger clusters. There is one underlying assumption of the information theoretic approach—namely that there is a rather rapid thermalization of the translational degrees of freedom of the impact heated cluster so that mean energy is the only energetic constraint. When this is not necessarily the case, e.g., for ionic clusters at lower energies, there will not be extensive fragmentation.
A quantum chemical investigation of possible intermediates in the reaction of the amidogen and hydroperoxyl radicals
Authors: Sumathi R. , Engels Bernd , Peyerimhoff S. D.
Published: July 2002

The reaction NH2+HO2→products, a reaction of atmospheric interest, has been studied using ab initio molecular orbital calculations with extended basis sets [6-311++G** and 6-311++G(2df,2pd)], and a variety of methods accounting for electron correlation such as second order Mo/ller–Plesset perturbation theory (MP2) with all electrons, quadratic configuration interaction singles and doubles with triples correction [QCISD(T)], complete active space [CAS(8,8)], and multireference double excitation configuration interaction (MRDCI). Also, the performance of density functional (DFT) calculations has been investigated. In the present study, all stationary points on various potential energy surfaces giving rise to different products, HNO+H2O, NH2O+OH, NH3+O2, H2O2+NH, and HNOO+H2, have been optimized and characterized by their Hessian matrix. Amine oxide and dihydroxyamine have been found to be the precursors for HNO formation. In addition, the paper attempts to explain the experimental finding, nonobservation of OH⋅ during photolysis of ammonia, and it demands new experiments with spectroscopic identification of OH radicals.