bluuebunny/crossref_metadata_2025 huggingface dataset:
The extended Koopmans’ theorem and its exactness
Authors: Morrison Robert C.
Published: July 2002

The extended Koopmans’ theorem (EKT) is shown to give accurate and potentially exact values for the lowest ionization potential (IP). Accurate results are reported for LiH, H+5, He2, and Li2. Results obtained for the lowest IP’s of LiH and He2 are nearly identical with those obtained by taking the difference of the total energies from separate, full configuration interaction calculations on the N-electron and the (N−1)-electron systems. The differences between the two IP calculations for a series of increasingly larger basis sets are 0.0017, 0.0004, and 0.0001 eV for LiH and are 0.0154, 0.0018, and 0.0007 eV for He2. The implication is that the lowest IP can be calculated to arbitrary accuracy. The EKT method is easily implemented as part of a multiconfigurational self-consistent-field calculation by diagonalizing the matrix of Lagrange multipliers with the first-order reduced density matrix as the metric.
The absorption spectrum of an electron solvated in sodalite
Authors: Haug Kenneth , Srdanov Voislav , Stucky Galen , Metiu Horia
Published: July 2002

We use a simple model to study the color change taking place when sodium atoms are absorbed in the zeolite sodalite. The Hamiltonian is that of an electron moving in the electrostatic field created by the ions in the zeolite framework and by the alkali ion core. We examine the sensitivity of the absorption spectrum on the magnitude of framework charges, the orientation of the Na4 cluster in the sodalite cells, the localization of the electron, the nature of the alkali impurity (Li, Na, K), and the laser polarization. Comparisons with the experiment help decide which framework charge models are consistent with the absorption spectrum.
The known and unknown group 13 hydride molecules M2H6: Diborane(6), dialane(6), and digallane(6)
Authors: Shen Mingzuo , Schaefer Henry F.
Published: July 2002

Molecular structures, harmonic vibrational frequencies, and infrared intensities for diborane B2H6, dialane Al2H6, and digallane Ga2H6 have been determined using high level ab initio quantum mechanical methods. The highest level of theory employed in this study is the single and double excitation coupled cluster (CCSD) method with double-zeta plus polarization (DZP) basis sets. This study serves to assess the reliabilities of various theoretical methods, laying the foundation for future theoretical studies of larger boron, aluminum, and gallium molecular systems. It appears that the theoretical methods employed here are adequately reliable when compared with existing experiments. Several shortcomings of previous ab initio quantum mechanical studies on diborane(6) and its analogs have been addressed.
A new functional form to obtain analytical potentials of triatomic molecules
Authors: Aguado Alfredo , Paniagua Miguel
Published: July 2002

A new way of fitting ab initio values of the potential energy of triatomic molecules is discussed. The new functional form proposed for the potential energy function satisfies several criteria for use in scattering calculations and it is so stable that, when increasing the number of parameters in the fit, it maintains a good behavior at short, intermediate, and long range of the whole potential. Applications of the fitting technique to obtain the potential energy functions for the ground states of H3, H2He+, and LiHF are presented. For the H3 system, we fit all 267 of Liu and Siegbahn’s ab initio points. With one nonlinear parameter and 23 linear parameters, the root-mean-square error was 1.21 kcal/mol and the maximum absolute deviation 7.94 kcal/mol; these values are lower than previous global fittings with a similar number of parameters except for the Truhlar and Horowitz fit of the Liu and Siegbahn data, LSTH potential, but when the number of linear parameters was increased to 71, the root-mean-square error was reduced to 0.07 kcal/mol and the maximum absolute deviation to 0.25 kcal/mol (approximately one half the corresponding values of the LSTH ones). The new global fitting procedure is then applied to obtain potential energy functions for the H2He+ and the LiHF systems within the chemical accuracy of 1 kcal/mol.
Mixtures of rigid and flexible nematogenic polymers
Authors: Hol/yst Robert , Schick M.
Published: July 2002

We consider three systems containing rigid and/or flexible main-chain nematogenic polymers: mixtures of a rigid and a flexible polymer, mixtures of two different rigid polymers, and a system of diblocks of rigid and flexible pieces. The flexible components are modeled as chains with freely rotating bonds, while the rigid components are modeled as rods of negligible thickness. The Landau–de Gennes expansion of the free energy is derived, and the form of the phase diagrams is examined. Conditions for the existence of generic diagrams is given in terms of the Flory–Huggins and Maier–Saupe interaction parameters. The location of the azeotrope in the rigid–rigid mixture is calculated analytically. The nematic–isotropic phase transition in the rigid–flexible diblock (in general, n-block) copolymer system is also studied. In particular, the change in the transition temperature with the number of monomers in a rigid block is determined.
Anisotropic diffusion of n-butane on a stepped Ru(001) surface
Authors: Arena M. V. , Westre E. D. , George S. M.
Published: July 2002

The surface diffusion of n-butane on a stepped ruthenium {Ru(S)-[15(001)×2(100)]} surface was studied using laser-induced thermal desorption (LITD) techniques. Large aspect ratio laser desorption areas were used to measure the anisotropy of n-butane diffusion on the stepped Ru(001) surface. Surface diffusion coefficients at Θ=Θsat for diffusion parallel (D∥) or perpendicular (D⊥) to the steps were measured by orienting these desorption areas either perpendicular or parallel to the step edges. At 115 K, the surface diffusion coefficient was much larger in the direction parallel to the step edges and D∥/D⊥≊26. Surface diffusion coefficients for diffusion on a terrace (Dt) or over a step (Ds) were deconvoluted from the measured D∥ and D⊥ values. The temperature-dependent D∥ and D⊥ values yielded an activation barrier for n-butane diffusion on the Ru(001) terrace at Θ=Θsat of Et=2.8±0.4 kcal/mol and a diffusion preexponential of D0,t=1.6×10−2±0.2 cm2/s. For diffusion over a step at Θ=Θsat, the surface diffusion barrier was Es=4.8±0.5 kcal/mol and the diffusion preexponential was D0,s=6.1×10−1±0.3 cm2/s. The measured corrugation ratio for n-butane on the Ru(S)-[15(001)×2(100)] surface was Ω≡Edif/Edes=0.24 for diffusion on the terrace and Ω=0.41 for diffusion over a step. The surface corrugation ratio on the terrace of Ru(S)-[15(001)×2(100)] was similar to Ω=0.29 measured previously for n-butane on Ru(001). This similarity indicated that step defects did not dominate previous LITD measurements of alkanes on Ru(001).
The 5f→6d absorption spectrum of Pa4+/Cs2ZrCl6
Authors: Edelstein N. , Kot Wing K. , Krupa J.-C.
Published: July 2002

The 5f1→6d1 absorption spectrum of 231Pa4+ diluted in a single crystal of Cs2ZrCl6 has been measured at 4.2 K. Three bands corresponding to the 6d(Γ8g, Γ7g, and Γ′8g ) levels are assigned. Extensive vibronic structure has been observed for the lowest 5f→6d transition and this structure is compared to that recently reported for the 6d1→5f1 emission spectra in the same system.
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o study of aqueous Fe2+–Fe3+ electron transfer
Authors: Karlström Gunnar , Malmqvist Per-Åke
Published: July 2002

The tunneling rate for the transfer of an electron from an Fe2+ to an Fe3+ ion is determined by a Hamiltonian matrix element, which has been calculated for interionic distances in the range 5.10–7.22 Å. The ions were either alone, or surrounded by water molecules. The calculations were made by computing the electronic wave function of the system by the complete active space self-consistent field (CASSCF) method, and then evaluating the relevant matrix element for this wave function interacting with a symmetry image of itself, representing the final state. Basis set dependence, and in particular the effect of bond functions, was investigated, as well as the effect of removing some water molecules or replacing them with point charges. The results seem to indicate that earlier calculated values, which are frequently used, of the tunneling rate may be too large by as much as a factor of 10. If so, then the agreement between earlier calculated electron transfer rates and experimental data may be explained by compensating deficiencies in the assumed statistical distribution of interionic distances.
Photodissociation dynamics of doubly excited Rydberg states of molecular hydrogen
Authors: Buntine Mark A. , Baldwin David P. , Chandler David W.
Published: July 2002

We have applied photofragment ion imaging to investigate the dissociation dynamics of low-lying, doubly excited states of molecular hydrogen. A doubly excited electronic state is one in which both of the hydrogen electrons reside in excited molecular orbitals. Two-step, two-color multiphoton excitation of H2, first via 201.8 nm, two-photon excitation into the E, F 1Σ+g(vE=0, J=1) state, followed by ∼563 nm, 1+m (m=1, 2) excitation through the B″ 1Σ+u(v=0, J=0, 2), D 1Πu(v=2, J=1, 2), and B′ 1Σ+u(v=4, J=0, 2) states provides a ready means of populating several low-lying doubly excited states of H2 at increasing internuclear separations. From these doubly excited repulsive states, both dissociation and autoionization processes are possible. Because the excitation energy remains relatively constant as each intermediate state is accessed, differences in the photodissociation dynamics via each state can be ascribed directly to the effects of changing internuclear separation and electronic symmetry of the intermediate and dissociative states. H+ fragments detected from each photodissociation pathway are distinguished by their differing velocities, determined from an ion image.
Structural and orientational relaxation in supercooled liquid triphenylphosphite
Authors: Silence Scott M. , Duggal Anil R. , Dhar Lisa , Nelson Keith A.
Published: July 2002

Structural and orientational relaxation processes in the glass-forming liquid triphenylphosphite (TPP) were studied by impulsive stimulated scattering, a time-resolved four-wave mixing technique. The α structural relaxation processes which can couple to the longitudinal and shear acoustic modes are analyzed phenomenologically in terms of a distribution of relaxation times fL(τL,T) or fS(τS,T), respectively, in the temperature range of 270–240 K. The two distributions appear to be identical, and undergo marked broadening as the sample is cooled. They are best described by a stretched exponential relaxation function whose exponent decreases with temperature. The molecular orientational relaxation times τor follow an Arrhenius temperature dependence and are longer than the average relaxation times 〈τL〉 and 〈τS〉 at high temperature, but converge as the temperature is lowered. We conclude that the observed orientational relaxation is decoupled from the α structural relaxation process and is a manifestation of the β orientational relaxation process. The low frequency intramolecular mode observed in the experiment was found to be essentially independent of temperature. A picture of structural and orientational relaxation processes in supercooled TPP consistent with recent theories of structural relaxation processes in supercooled liquids is proposed.
Infrared diode laser spectroscopy of the allyl radical. The ν11 band
Authors: Hirota Eizi , Yamada Chikashi , Okunishi Misaki
Published: July 2002

Allyl radicals were generated by the photolysis of 1,5-hexadiene at 193 nm and were detected by observing the ν11, i.e., CH2 symmetric wagging band by infrared diode laser kinetic spectroscopy. The observed spectrum showed clearly the effect of nuclear-spin statistical weights, establishing the presence of a Cb2 axis in the molecule. The spin–rotation splitting was not resolved; only some high-Ka lines were found broader than others, placing an upper limit for the εaa constant at about 200 MHz or less. The rotational constants derived from the observed spectrum indicate that the molecule is essentially planar. The C–C bond length and the CCC bond angle were calculated from the ground-state rotational constants to be 1.3869 Å and 123.96°, respectively, where structural parameters involving hydrogens were fixed to those of ab initio values.
Comment on: Interchain dispersion and second hyperpolarizability of conjugated polymers
Authors: Guo Dandan , Mazumdar S.
Published: July 2002

From calculations of the third harmonic generation (THG) spectra of finite polyene chains McWilliams and Soos [J. Chem. Phys. 95, 2127 (1991)] have attempted to explain the experimental THG spectrum of trans-polyacetylene, the infinite polyene chain. According to the authors, the frequency of the two-photon resonance, as well as its intensity, are too large, and substantial interchain Coulomb interactions are necessary to explain experimental data. We point out here that these results are finite size effects. Determination of correlation effects requires comparison of correlated and uncorrelated chains of same lengths. Comparison of short correlated chains with long uncorrelated chains is not the ideal approach, as results assigned to correlation effects can in reality be finite size effects.
Inversion of experimental data and ab initio studies of a pseudo-atom–diatom model for the vibrational dynamics of HCN–HF
Authors: Quiñones A. , Bandarage G. , Bevan J. W. , Lucchese R. R.
Published: July 2002

Spectroscopic data was inverted to generate simplified atom–diatom intermolecular potentials for the hydrogen bonded dimer HCN–HF. Both the HF ν1=0 and ν1=1 adiabatic surfaces of this complex involving the hydrogen bond stretch (ν4) and the high frequency HF bend (ν6) degrees of freedom were considered. Adiabatic separation of angular and radial degrees of freedom allowed a modified vibrational Rydberg–Klein–Rees (RKR) inversion to give effective radial potentials for each bending state. The final potential at a sequence of intermolecular separations was obtained by requiring agreement between the eigenvalues of the angular Hamiltonian and the effective radial potentials which were obtained from the RKR inversion. Potentials obtained from the inversion procedure were tested by a variational calculation with a basis set that consisted of products of preoptimized radial and angular eigenfunctions. Transition frequencies included in the inversion procedure were reproduced to better than 1 cm−1, respective rotational constants were predicted within 0.15% of the experimental value and predicted intensities were in qualitative agreement with experimental results. Ab initio potentials were also calculated using second order Mo/ller–Plesset perturbation theory to treat electron correlation effects and with a triple-zeta-valence basis set plus two sets of polarization functions. The energies were computed at geometries where the potential had been determined by the inversion procedure. A total of 264 geometries were considered. A correction for basis set superposition errors led to good agreement between ab initio and experimental values of the well depth. We calculated the bound states of the ab initio surfaces assuming the adiabatic separation of the ν1 mode from the ν4 and the ν6 modes. The transition frequencies calculated from the ab initio surface differed from the experimental energies by less than 20 cm−1 even for highly excited overtones. Both the potentials obtained from the inversion procedure and from the ab initio calculations were modified to predict the spectrum of HCN–DF.
On the large entropic contribution to the effective interaction parameter of polystyrene–poly(methyl methacrylate) diblock copolymer systems
Authors: Freed Karl F. , Dudowicz Jacek
Published: July 2002

The lattice cluster theory with structured monomers is used to analyze the small angle neutron scattering experiments of Russell et al. on the polystyrene–b-poly(methyl methacrylate) diblock copolymer system in order to gain insight into the observed large entropic contribution to the effective interaction parameter χeff. Predictions are made of the composition and molecular weight dependence of χeff and of the pressure and composition dependence of the microphase transition temperature to the bcc microphase. General predictions are also compared with experimental data for poly(ethylenepropylene)–poly(ethylethylene) diblock copolymers.
Subpicosecond OH production from photoexcited CO2–HI complexes
Authors: Ionov S. I. , Brucker G. A. , Jaques C. , Valachovic L. , Wittig C.
Published: July 2002

The subpicosecond resolution pump–probe method is used to obtain rates for OH production from CO2–HI complexes photoexcited in the HI ultraviolet continuum. Production lifetimes between 300 and 600 fs are reported for wavelengths between 255 and 235 nm, respectively. These lifetimes are close to the RRKM predictions of HOCO° unimolecular decomposition under single collision conditions.
A perturbation density-functional theory for polyatomic fluids. I. Rigid molecules
Authors: Kierlik E. , Rosinberg M. L.
Published: July 2002

We present a density-functional theory of nonuniform fluids composed of polyatomic species. It is derived from Wertheim’s first-order perturbation theory of polymerization by taking the limit of complete association. In the case of the uniform fluid, an explicit expression of the complete angle-dependent pair distribution function is obtained which shows that the theory yields nontrivial results for the intermolecular structure. However, features of the long-wavelength structure, such as the existence of an orientational order, are not described. Illustrative applications of the theory are given in the case of diatomic and linear tetra-atomic molecules formed by tangent hard spheres of the same diameter.
Multidimensional path integral calculations with quasiadiabatic propagators: Quantum dynamics of vibrational relaxation in linear hydrocarbon chains
Authors: Topaler Maria , Makri Nancy
Published: July 2002

This paper presents the first application of a new method for multidimensional real time quantum dynamics described in a previous Letter [Chem. Phys. Lett. 193, 435 (1992)]. The key feature of the method is the use of an improved zeroth order representation in the Feynman propagator, which allows large time steps in the path integral. Use of the adiabatic approximation in the case of a system coupled to a harmonic bath leads to a path integral over the system coordinate with a one-dimensional propagator which is constructed numerically and which corresponds to dynamics along the adiabatic path, and with a nonlocal influence functional that accounts for nonadiabatic effects. We have performed accurate quantum mechanical calculations on the dynamics of CH overtone relaxation in linear hydrocarbon chains by direct numerical evaluation of the path integral in the quasiadiabatic representation. Converged results for the survival probability of the υ=5 and υ=8 states of HC6 are reported up to five vibrational periods of the CH stretch and compared to those obtained from standard classical and semiclassical simulations.
The remarkable monobridged structure of Si2H2
Authors: Grev Roger S. , Schaefer Henry F.
Published: July 2002

Inspired by the observation of a monobridged structure of Si2H2 by Cordonnier et al. via microwave spectroscopy (see the following paper), we have reinvestigated the Si2H2 singlet state potential energy surface using large basis sets and extensively correlated wave functions. Coupled-cluster single, double, and (perturbative) triple excitation methods [CCSD(T)] in conjunction with a triple-zeta 2df (TZ2df ) basis set on silicon and a triple zeta with two sets of polarization (TZ2P) basis set on hydrogen predict that the monobridged Si(H)SiH structure is indeed a minimum; in fact, Si(H)SiH is the second most stable Si2H2 isomer, as suggested by a recent theoretical study [B. T. Colegrove and H. F. Schaefer, J. Phys. Chem. 94, 5593 (1990)]. The predicted Si(H)SiH geometrical structure—which exhibits the shortest SiSi bond distance of any molecule characterized to date—and hence the rotational constants, as well as the quartic centrifugal distortion constants are in good agreement with the experimental data. We have located transition states between these pairs of minima—disilavinylidene H2SiSi and monobridged Si(H)SiH; monobridged and dibridged Si(H2)Si; trans-HSiSiH and monobridged. We predict Si(H)SiH to lie 8.7 kcal mol−1 above Si(H2)Si, with the transition state between them 3.7 kcal mol−1 higher. H2SiSi is predicted to lie 11.6 kcal mol−1 above Si(H2)Si and the transition state barrier between H2SiSi and Si(H)SiH is 2.4 kcal mol−1 above H2SiSi. Predictions of absolute 0 K heats of formation for the various structures are presented.
Theoretical search for large Rydberg molecules: NH3CH3, NH2(CH3)2, NH(CH3)3, and N(CH3)4
Authors: Boldyrev Alexander I. , Simons Jack
Published: July 2002

On the basis of ab initio calculations, four new large metastable Rydberg molecules: NH3CH3, NH2(CH3)2, NH(CH3)3, and N(CH3)4 are predicted to exist. All of these neutral species possess high-symmetry local minima but are not thermodynamically stable with respect to loss of H or CH3. Because they have pronounced barriers on the dissociation pathways, they should be metastable. The dissociation barriers for NHn(CH3)4−n decomposing to NHn−1(CH3)4−n+H are smaller than for dissociation into NHn(CH3)4−n−1+CH3. Thus, the lifetime of N(CH3)4 should be much longer than that of NH4 due to the larger dissociation barrier and the heavier mass of the departing CH3 species.
Comparison and assessment of different forms of open shell perturbation theory
Authors: Murray Christopher W. , Handy Nicholas C.
Published: July 2002

Existing methods for the calculation of the correlation energy of open shell atoms and molecules using Mo/ller–Plesset perturbation theory are discussed with special emphasis on recent advances in methods based on the restricted Hartree–Fock wave function. The convergence of the Mo/ller–Plesset series is examined to high order for a number of small systems and it is shown that some of the methods offer significant advantage over traditional unrestricted Mo/ller–Plesset theory. The optimized bond length and vibrational frequencies of CN, NO, and O2 are also compared for second and fourth order Mo/ller–Plesset theory, and again it is found that the same methods give much better results than the corresponding unrestricted results. Further supporting calculations are presented on the electron affinity of CN and the dissociation energy of HCN. The new methods involve relatively minor adaptation of existing unrestricted Mo/ller–Plesset (UMP) codes, and in the light of their superior performance are recommended instead of UMP theory for open shell molecules.
Quantum functional sensitivity analysis within the log-derivative Kohn variational method for reactive scattering
Authors: Chang Johnny , Brown Nancy J. , D’Mello Michael , Wyatt Robert E. , Rabitz Herschel
Published: July 2002

A new approach to calculating quantum functional sensitivity maps of transition probabilities is described in this paper. This approach is based on the log-derivative version of the Kohn variational principle and is applied here to the collinear H+H2 hydrogen exchange reaction. The sensitivity maps provide detailed quantitative information about how variations in the potential energy surface affect the state-to-state transition probabilities. The key issues investigated are (i) the evolution of sensitivity structure in the 0.30–1.50 eV range of total energy; (ii) the comparison of sensitivity structure on the Porter–Karplus, the Liu–Siegbahn–Truhlar–Horowitz, and the double-many-body-expansion potential energy surfaces; and (iii) the range of linearity for first order sensitivity predictions.
Determination of solvation free energy using molecular dynamics with solute Cartesian mapping: An application to the solvation of 18-crown-6
Authors: Sun Yaxiong , Kollman Peter A.
Published: July 2002

The determination of relative stabilities of conformations of a molecule in solution has received much interest due to its relevance to chemical and biological properties in solution. Free energy perturbation (FEP) methodologies using molecular dynamics simulations have been used for the evaluation of free energy differences between two similar molecular systems. We present here a FEP method for the evaluation of the solvation free energy difference between two conformations. The simulations are carried out by fixing the Cartesian coordinates of a solute molecule. The molecule is switched from one conformer to another through a linear couping of the Cartesian coordinates of the two conformers. The solvation free energy is calculated from the interactions between the solvent and the solute using the standard FEP approach. This method is expected to be particularly useful for molecules with complicated conformational changes involving a large number of dihedral angle rotations. A crown ether, 18-crown-6, a cyclic molecule with 18 rotatable bonds, is used as a test case to demonstrate the method. The solvation free energy differences between four selected conformations of 18-crown-6 have been determined. The results are in agreement with available experiments and consistent with other theoretical calculations.
Influence of broken conformational symmetry on the surface enrichment of polymer blends
Authors: Fredrickson Glenn H. , Donley James P.
Published: July 2002

We construct an expression for the free energy of an incompressible binary blend of homopolymers in the vicinity of a hard reflecting surface. Expressed as a functional of the concentration profile away from the surface, we find a contribution to the free energy that is linear in the gradient of the concentration and with a range that extends from the surface a distance of order of the radius of gyration. This term, which was overlooked in previous investigations of wetting phenomena in polymer blends, derives from conformational restrictions on the polymers in the surface layer. An important feature of this nonlocal surface term is to favor an excess of the more flexible component at the surface. In addition, it is capable of influencing the detailed shape of the concentration profile and modifying wetting and prewetting behavior.
Multichannel quantum defect analysis of preionizing Rydberg states of Li2 including rovibronic interactions
Authors: Roche A. L. , Jungen Ch.
Published: July 2002

The multichannel quantum defect analysis of the preionization spectrum of Li2 published recently by Schwarz et al.1 is reconsidered and unified. In addition to the rotational–electronic interactions taken into account in Ref. 1, nuclear vibrational motion and vibrational–electronic interactions between Rydberg l complexes associated with different vibrational limits are included. A quantum defect matrix is constructed which depends linearly on the internuclear distance and whose elements are determined from the experimental data by a nonlinear least-squares fitting procedure. The resulting R-dependent quantum defect matrix is used to calculate the preionization profiles of the resonance peaks.
Reactions of pulsed-laser evaporated uranium atoms with molecular oxygen: Infrared spectra of UO, UO2, UO3, UO2+, UO22+, and UO3–O2 in solid argon
Authors: Hunt Rodney D. , Andrews Lester
Published: July 2002

Uranium atoms from the Nd:YAG laser ablation of a uranium target were codeposited with molecular oxygen and excess argon at 12 K. Infrared spectra following the U+O2 reaction revealed a wide range of reaction products. The 776.0 cm−1 band due to UO2 was the strongest product absorption, strong UO3 bands were observed at 852.5 and 745.5 cm−1, and a weak UO absorption appeared at 819.8 cm−1. These product absorptions are in agreement with earlier work, which evaporated UO2 from a tungsten Knudsen cell at 2000 °C. The 16O2/18O2 reaction gave only U 16O2 and U 18O2, which verified an insertion mechanism. New product absorptions were observed at 952.3, 892.3, and 842.4 cm−1. The 842.4 cm−1 absorption due to the UO3–O2 complex and the 892.3 cm−1 band assigned to the charge-transfer complex (UO2+)(O2−) grew markedly at the expense of the other uranium oxides during annealing the matrix to allow diffusion and reaction of O2. With 25% 16O2, 50% 16O18O, and 25% 18O2 samples, the 952.3 cm−1 band became a sharp triplet at 952.3, 936.5, and 904.5 cm−1 and exhibited an isotopic ratio appropriate for a linear OUO species. Agreement of this band with uranyl ion spectra suggests assignment to a (UO22+) complex. Mechanisms of formation of charged species are discussed.
The microwave spectrum, structure, and harmonic force field of the van der Waals trimer Ar2–OCS
Authors: Xu Yunjie , Gerry M. C. L. , Connelly J. P. , Howard B. J.
Published: July 2002

The microwave rotational spectra of the van der Waals trimers Ar2–OCS, Ar2–OC34S, and Ar2–18OCS have been observed between 7.0 and 18.0 GHz using pulsed molecular beam microwave Fourier transform (MWFT) spectroscopy. Sixty-five transitions, both a and c type, have been assigned for Ar2–OCS, as well as 37 transitions and 19 transitions for Ar2–OC34S and Ar2–18OCS, respectively. The rotational constants and quartic centrifugal distortion constants have been determined. The trimer has been found to be a distorted tetrahedrally shaped highly asymmetric top (κ=∼0.377) with the OCS subunit lying in the Cs symmetry plane. An effective ground state structure has been determined. The Ar–Ar distance and the two ‘‘Ar–center of mass of OCS’’ distances are slightly longer than those in the corresponding dimers Ar2 and Ar–OCS, respectively. A harmonic force field analysis has been performed on the basis of the centrifugal distortion constants, to predict the van der Waals force constants and the corresponding vibrational frequencies. The Ar–Ar stretch force constant of the trimer is 0.008 42 mdyn Å−1, which compares favorably with 0.0078 mdyn Å−1 reported for the free dimer Ar2. A comparison of the geometry and spectroscopic properties of Ar2–OCS with those of Ar2 and of the Ar2–HX (X=F, Cl, CN) trimers is given.
Protein structure and polymer collapse
Authors: Dewey T. Gregory
Published: July 2002

Scaling relationships and a self-consistent field (SCF) theory of the structure of globular proteins are presented. A large data base is examined which shows that the radius of gyration of a protein Rg scales as Nν, where ν is approximately 1/3. This scaling behavior is predicted for collapsed polymers. Additional justification for polymer collapse conditions is given by an analysis of the fractal dimension of protein backbones. A collection of data also shows that the surface area of proteins S scales as S ∼ Rdsg, where ds equals 2.1. This property is also shown to be consistent with polymer collapse. A detailed SCF theory is presented for a collapsed polymer in which two- and three-body interactions between the segments are considered. The problem is exactly solvable within the SCF level of approximation and analytic solutions are obtained for the configurational distribution function. The distribution function is used to calculate scaling relationships and the free energy of the polymer.
Vibrational predissociation of methylnitrite using phase-locked ultrashort laser pulses
Authors: Dateo Christopher E. , Metiu Horia
Published: July 2002

We solve numerically the time-dependent Schrödinger equation to study the behavior of a molecule interacting with two phase-locked ultrashort laser pulses. The two-dimensional model used in the calculations mimics the properties of the CH3ONO molecule. The two pulses are identical except for their relative phase and are tuned to excite an upper electronic state of the molecule. After excitation the molecule predissociates and we calculate the dependence of the NO yield and of the NO vibrational population on the delay time between the pulses. Because the molecular wave functions representing the excited states created by each pulse interfere, the total product population created by the joint action of the pulses differs from the total population obtained when the two pulses act independently on the molecule. This type of experiment provides a means of using quantum interference for a modest control of photodissociation.
Wave vector dependent dielectric relaxation in hydrogen-bonding liquids: A molecular dynamics study of methanol
Authors: Skaf Munir S. , Fonseca Teresa , Ladanyi Branka M.
Published: July 2002

Molecular dynamics simulation is used to study ε(k,ω), the frequency and wave vector dependent dielectric permittivity of a three-site model of methanol in which the methyl group is represented as a single site. The effects of induced dipoles are taken into account using perturbation theory and a three-site molecular polarizability model. The data are analyzed in terms of projected variables which allow us to distinguish between local field factors which renormalize the permanent-dipole contribution to the permittivity from the ‘‘collision induced’’ dipole relaxation. We find that induced dipoles significantly enhance the static permittivity, mainly through the local field factors. The time correlation functions for the longitudinal and transverse components of the collective dipole moments, evaluated at several of the smallest wave vectors in the system, present rapid oscillations at short times, followed by a nonexponential relaxation regime at intermediate times. At long times, for which the longitudinal component has essentially decayed to zero, the transverse component decays exponentially. Collision-induced dipole fluctuations are a major contributor to the longitudinal collective dipole relaxation and a much smaller one to transverse component dynamics. The far-infrared absorption spectrum, evaluated from the smallest k transverse component, exhibits three well resolved absorption bands, with maxima located at frequencies 60, 150, and 645 cm−1, in qualitative agreement with recent experimental measurements. We find that the fluctuations of the induced dipoles contribute in a nontrivial way to the shape of the spectra at frequencies above 50 cm−1. The permanent dipole contribution to ε(k,ω) was obtained also for partially deuterated methanol, CH3OD, in order to aid in the analysis of the hydrogen-bond dynamics. In addition, we briefly describe an approach to the study of solvation dynamics using the MD results for ε(k,ω) obtained here.
Integral equation theory of polymer blends: Numerical investigation of molecular closure approximations
Authors: Yethiraj Arun , Schweizer Kenneth S.
Published: July 2002

The thermodynamics of symmetric polymer blends is investigated using the polymer reference interaction site model integral equation theory with the new molecular closures presented in the previous paper. In contrast to the atomic mean spherical approximation reported earlier by Schweizer and Curro [J. Chem. Phys. 91, 5059 (1989); Chem. Phys. 149, 105 (1990)] (in which the critical temperature is proportional to the square root of the degree of polymerization), the molecular closures predict a linear dependence of the critical temperature on the degree of polymerization, in agreement with classical mean field theory. Detailed numerical calculations using the reference molecular mean spherical approximation (R-MMSA) and the reference molecular Percus–Yevick (R-MPY) closures are presented for the intermolecular structure and effective chi parameter in symmetric blends of semiflexible chains. For the symmetric blend, the R-MMSA closure is almost an integral equation realization of mean field theory, consistent with the analytical results presented in the previous paper. With the R-MPY closure, at low densities, the effective chi parameter is significantly renormalized down from its mean field value and displays a strong composition dependence. As the density is increased, both the renormalization of the effective chi parameter and its composition dependence become weaker. These trends are consistent with recent computer simulations. The influence of chain aspect ratio and the precise choice of intermolecular potentials on blend thermodynamics and phase separation are also explored. With the exception of the composition dependence of the effective chi parameter in the R-MPY theory, the analytical thread calculations are shown to be in qualitative, and sometimes quantitative, agreement with all the numerical results for symmetric blends.
Trajectory studies of SN2 nucleophilic substitution. III. Dynamical stereochemistry and energy transfer pathways for the Cl−+CH3Cl association and direct substitution reactions
Authors: Hase William L. , Cho Young June
Published: July 2002

Classical trajectory calculations are performed to determine differences in the microscopic dynamics for two fundamental processes for the Cla−+CH3Clb→ClaCH3+Clb− reactive system: Cla−–CH3Clb complex formation and directly attaining the [Cla–CH3–Clb]− central barrier without first forming the complex. This latter process becomes important when the C–Clb stretch mode is excited in the CH3Clb reactant. The total cross section for complex formation and directly attaining the central barrier increases as nC–Clb is increased. The value for the Cla−—C–Clb angle θ as the reactants interact, the dynamical stereochemistry, is found to be a very important property for distinguishing between the mechanisms for the two fundamental processes. Directly attaining the central barrier requires oriented reactants with θ≊π, while orientation suppresses complex formation. Substantial reactant orientation only occurs for CH3Clb rotational temperatures less than 300 K. The complex is formed by a T→R energy transfer process which involves coupling between the reactant orbital angular momentum and CH3Clb rotational angular momentum. Complex formation does not involve energy transfer to the CH3Clb vibrational modes, which is consistent with an earlier finding that the CH3Clb intramolecular modes are inactive during decomposition of the Cla−–CH3Clb complex. Orientation of CH3Clb enhances coupling between the Cla−+CH3Clb radial motion and the C–Clb stretch mode. This coupling leads to the above direct substitution process and extensive deactivation of the excited C–Clb stretch during direct unreactive collisions. Considerably less deactivation results from Cla−–CH3Clb complex formation followed by dissociation to the reactants. Rotationally exciting CH3Clb eliminates orientation and, thus, suppresses deactivation of the C–Clb stretch.
Theoretical study of thiophene oligomers: Electronic excitations, relaxation energies, and nonlinear optical properties
Authors: Beljonne D. , Shuai Z. , Brédas J. L.
Published: July 2002

Using Hartree–Fock ab initio or semiempirical Austin model 1 (AM1) techniques to estimate geometries and an intermediate neglect of differential overlap/configuration interaction (INDO/CI) technique to calculate electronic transition energies, we examine the nature of the lowest singlet excited states in thiophene oligomers containing up to eight rings. The evolution of the ordering of the lowest two (1B and 2A) excited states is investigated as a function of chain length. The geometry relaxation phenomena in the 1B and 2A states are modeled on the basis of either bond-order/bond-length relationships or the formation of bipolaron-type defects. The chain-length dependence of the third-order polarizability γ is evaluated from a sum-over-states approach. All the results are discussed in comparison to those obtained in polyenes of similar sizes.
A joint experimental and theoretical study of A 2Π→X 2Σ+ electronic energy transfer in the CN molecule induced by collisions with helium
Authors: Dagdigian Paul J. , Patel-Misra Dipti , Berning Andreas , Werner Hans-Joachim , Alexander Millard H.
Published: July 2002

In this paper, we report an experimental and theoretical reinvestigation of the electronic quenching of CN(A 2Π) induced by collisions with He, concentrating on transitions from individual rotational-fine-structure levels of the vA=7 manifold into individual levels of the energetically nearby vX=11 manifold. Experimental cross sections are obtained by an optical–optical double resonance (OODR) technique. Simultaneously, for the first time, the small non-Born–Oppenheimer mixing between the A 2Π and X 2Σ+ states of the isolated CN molecule is included in the quantum scattering calculations. The presence of these terms results in a significant enhancement in the electronically inelastic cross sections, but only in the vicinity of the electronic ‘‘gateways,’’ where the perturbational mixing is largest. There is reasonable agreement between the experimental and calculated final state distributions.
Dynamics of the reactions of O(1D) with HCl, DCl, and Cl2
Authors: Matsumi Yutaka , Tonokura Kenichi , Kawasaki Masahiro , Tsuji Kazuhide , Obi Kinichi
Published: July 2002

The reactions O(1D)+HCl→OH+Cl (1a) and OCl+H (1b), O(1D)+DCl→OD+Cl (2a) and OCl+D (2b), and O(1D)+Cl2→OCl+Cl (3) are studied at an average collision energy of 7.6, 7.7, and 8.8 kcal/mol for (1), (2), and (3), respectively. H, D, and Cl atoms are detected by the resonance-enhanced multiphoton ionization technique. The average kinetic energies released to the products are estimated from Doppler profile measurements of the product atoms. The relative yields [OCl+H]/[OH+Cl] and [OCl+D]/[OD+Cl] are directly measured, and a strong isotope effect (H/D) on the relative yields is found. The fine-structure branding ratios [Cl(2P1/2]/[Cl(2P3/2)] of the reaction products are also measured. The results suggest that nonadiabatic couplings take place at the exit channels of the reactions (1a) and (2a), while the reaction (3) is totally adiabatic.
Temperature-dependent ultrafast solvation dynamics in a completely nonpolar system
Authors: Fourkas John T. , Berg Mark
Published: July 2002

Transient hole burning measurements on dimethyl-s-tetrazine in n-butylbenzene are reported from the low-viscosity room-temperature liquid down to the low-temperature glass. The results give a detailed picture of the solvation of a nonpolar solute in a nonpolar solvent. The dynamics separate into a phonon modulated and a structural component, as was found previously for polar solvents. The structural component is frozen in the glass, but its relaxation rate increases into the subpicosecond range with increasing temperature. The time decay of the structural relaxation is highly nonexponential. The coupling of the solute electronic state to the structural coordinates is close to linear, but the coupling to the phonon coordinates cannot be accounted for by simple linear or quadratic coupling models. Effects are also found that are attributed to changes in coupling constants with changing density.
Acceleration of intramolecular vibrational redistribution by methyl internal rotation. A chemical timing study of p-fluorotoluene and p-fluorotoluene-d3
Authors: Moss David B. , Parmenter Charles S.
Published: July 2002

Time-integrated, frequency-resolved fluorescence spectroscopy has been used to determine rates of intramolecular vibrational energy redistribution (IVR) from the vibrational levels 31 (εvib≊1200 cm−1) and 3151 (εvib≊2000 cm−1) in both p-fluorotoluene and p-fluorotoluene-d3 for comparison with each other and with comparable levels in p-difluorobenzene. Methyl substitution increases the rate of IVR by roughly two orders of magnitude, while deuteration of the methyl rotor produces at most a small (two- to fourfold) further increase in the rate of IVR. It is argued that the IVR response to methyl substitution is a consequence of the methyl internal rotation without significant influence from the methyl vibrations. The increased IVR rate is predominantly a reflection of the large number of additional states that can couple through the exchange of energy between ring vibration and internal rotation. The difference, if any, between the protonated and deuterated methyl rotor species probably arises from subtle differences in the level structures and coupling strengths of the two systems. Fermi golden rule modeling of the relative IVR rates is built on these propositions. It accounts for much of the IVR rate increase associated with the methyl substitution as well as for the near equivalence of the –CH3 and –CD3 IVR rates.
Matrix infrared spectra of NUN formed by the insertion of uranium atoms into molecular nitrogen
Authors: Hunt Rodney D. , Yustein Jason T. , Andrews Lester
Published: July 2002

Pulsed-laser ablated uranium atoms were codeposited with 14N2(15N2) and excess Ar at 12 K. The Fourier transform infrared (FTIR) spectrum revealed a single product, UN2, which exhibited a ν3 absorption at 1051.0 cm−1. Ultraviolet (UV) photolysis increased the yield of UN2 by threefold and showed that electronic excitation facilitated the insertion reaction. N2 perturbed UN2 bands at 1041.3 and 1031.5 cm−1 grew sharply during matrix annealings. In 14N15N experiments the ν1 and ν3 modes of 14NU15N were observed at 987.2 and 1040.7 cm−1, respectively; FG matrix calculations were performed to determine Fr=8.27 mdyn/Å and Frr=0.12 mdyn/Å and to estimate the IR-inactive ν1 modes of U14N2 and U15N2 at 1008.3 and 985.7 cm−1, respectively. Energetic considerations suggest that the U+N2 insertion reaction has little exothermicity and that the activation energy for this reaction may be provided by hypothermal uranium atoms.
A comparison of calculated and experimental hyperpolarizabilities for acetonitrile in gas and liquid phases
Authors: Stähelin M. , Moylan C. R. , Burland D. M. , Willetts A. , Rice J. E. , Shelton D. P. , Donley E. A.
Published: July 2002

The nonlinear susceptibility of acetonitrile has been measured in both the gas and liquid phases by dc electric field-induced second harmonic generation (dc-SHG or EFISH). The EFISH signal for this molecule is dominated by the first hyperpolarizability β. It is shown that local field factors do not adequately describe the effect of the solvent environment. Thus it is not possible to extract ‘‘gas phase’’ values of hyperpolarizabilities from measurements made in solution. First and second hyperpolarizabilities, β and γ, have been calculated for acetonitrile using ab initio techniques. These calculations are compared to the gas phase experimental measurements. Excellent agreement is obtained when electron correlation effects are included.
Isomerization reactions on the lowest potential energy hypersurface of triplet vinylidene and triplet acetylene
Authors: Vacek George , Thomas J. Russell , DeLeeuw Bradley J. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Triplet vinylidene, first predicted to have a sizeable barrier to unimolecular rearrangement in 1978 by theory, has now been observed under three different sets of experimental conditions. In order to quantitatively characterize the potential energy hypersurface of triplet vinylidene and triplet acetylene, high-level ab initio quantum mechanical methods have been employed. Basis sets as large as triple zeta plus two sets of polarization functions augmented with higher angular momentum functions [TZ(2df,2pd)] have been utilized in conjunction with correlated methods as sophisticated as the coupled cluster approach including all single, double, and perturbative triple excitations [CCSD(T)]. Of particular interest are predictions of the zero-point vibrational energy corrected barriers for rearrangement of ã 3B2 vinylidene to b̃ 3Bu trans-bent acetylene and of cis-bent ã 3B2 acetylene to trans-bent b̃ 3Bu acetylene. At the highest level of theory used here, TZ(2df,2pd) CCSD(T), these are predicted to be 47.9 and 13.0 kcal/mol, respectively. The physical properties of ã 3B2 vinylidene, including the dipole moment, harmonic vibrational frequencies, and the infrared intensities, have also been reported.