bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytic energy derivatives for ionized states described by the equation-of-motion coupled cluster method
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

The theory for analytic energy derivatives of excited electronic states described by the equation-of-motion coupled cluster (EOM-CC) method has been generalized to treat cases in which reference and final states differ in the number of electrons. While this work specializes to the sector of Fock space that corresponds to ionization of the reference, the approach can be trivially modified for electron attached final states. Unlike traditional coupled cluster methods that are based on single determinant reference functions, several electronic configurations are treated in a balanced way by EOM-CC. Therefore, this quantum chemical approach is appropriate for problems that involve important nondynamic electron correlation effects. Furthermore, a fully spin adapted treatment of doublet electronic states is guaranteed when a spin restricted closed shell reference state is used—a desirable feature that is not easily achieved in standard coupled cluster approaches. The efficient implementation of analytic gradients reported here allows this variant of EOM-CC theory to be routinely applied to multidimensional potential energy surfaces for the first time. Use of the method is illustrated by an investigation of the formyloxyl radical (HCOO), which suffers from notorious symmetry breaking effects.
On the classical theory of the rate of isomerization of HCN
Authors: Tang Hui , Jang Soonmin , Zhao Meishan , Rice Stuart A.
Published: July 2002

We report the results of calculations, using classical mechanics, of the rate of the isomerization reaction HCN↔CNH. The three purposes of the calculations are (i) to test whether or not the Zhao–Rice approximate version of the Davis–Gray theory provides an accurate description of the rate of isomerization when there is a large scale atomic rearrangement; (ii) to determine if the quasi-two-dimensional reaction path representation of dynamical evolution on a multidimensional potential energy surface preserves the major features of the phase space mappings in two dimensions that are the key features of the Davis–Gray formulation of unimolecular reaction rate theory; and (iii) to determine if the reaction path representation is useful when the energy of the system is considerably greater than that along the minimum energy path. We find that both the Zhao–Rice (ZR) and the reaction path calculations of the isomerization rate constant are in reasonable agreement with the rate constant estimated from trajectory calculations; the ZR rate constant is about a factor of 2 smaller, and the reaction path rate constant is about a factor of 2 larger, than that obtained from trajectory calculations. For the few data points available, the energy dependence of the ZR rate constant agrees very well with that obtained from trajectory calculations. The energy dependence of the reaction path rate constant is uniformly accurate over the range we have studied, but is in moderately good agreement with the values obtained from the trajectory studies. We conclude that the reaction path representation remains useful even when the energy of the reacting system is considerably greater than that along the minimum energy path.
Study of polymer diffusion across the interface in latex films through direct energy transfer experiments
Authors: Liu Yuan Sheng , Feng Jianrong , Winnik Mitchell A.
Published: July 2002

Efforts have been made to improve the method of using direct nonradiative energy transfer to follow the interdiffusion of polymer molecules across an interface. Two data analysis techniques are developed. The first one presents the extent of interdiffusion in terms of an acceptor concentration distribution; the second one recovers the diffusion coefficient and the concentration profile based on a Fickian diffusion model. The first method is model independent while the second method can be readily extended for other diffusion models. Polydisperse systems are characterized by a distribution of molecular mobilities. The energy transfer experiment provides a mean diffusion coefficient averaged over the sample history. For such systems, the concept of ‘‘instantaneous’’ effective diffusion coefficient and a method to derive it are discussed. These techniques are examined using computer simulations and tested in the study of molecular diffusion process of a poly(butyl methacrylate) latex system.
Photoionization spectroscopy of the In–N2 van der Waals complex
Authors: Brock L. R. , Duncan M. A.
Published: July 2002

A vibrationally resolved electronic spectrum is observed for the metal atom van der Waals complex In–N2. Two electronic band systems are detected with mass resolved two-color photoionization spectroscopy. A lower energy system is observed slightly to the blue of the In ( 2D←P) atomic asymptote. It is characterized by a progression in the In–N2 stretching mode with a frequency of ω′e=76.7 cm−1. The higher energy system is slightly to the blue of the In (4P←2P) asymptote. It also exhibits a progression in the In–N2 stretch with a frequency of ω′e=87.7 cm−1. Extrapolation of the vibrational progressions leads to determination of the excited state dissociation energies. Energetic cycles based on the electronic transition energies, excited state dissociation energies, and atomic asymptotes lead to a determination of the ground state dissociation energy of D″0=1519 cm−1 (0.188 eV). A single-photon photoionization experiment determines the ionization potential to be 43 372 cm−1 (5.377 eV). This IP value, together with the atomic IP and the ground state neutral dissociation energy, yields a dissociation energy of D″0=4817 cm−1 (0.597 eV) for the In+–N2 ion–molecule complex.
Spectral diffusion of individual pentacene molecules in P-terphenyl crystal: Stochastic theoretical model and analysis of experimental data
Authors: Reilly Philip D. , Skinner J. L.
Published: July 2002

We propose a microscopic theoretical model to explain recent experiments involving the spectral diffusion of individual pentacene molecules embedded in p-terphenyl crystal. The experimental spectral diffusion trajectories are analyzed in terms of three stochastic characterizations: The time autocorrelation of transition frequency fluctuations, the time-dependent distribution of spectral jumps, and the equilibrium distribution of frequencies. The observed spectral diffusion is well described by our model, and we determine detailed quantitative information about the localized excitations that are responsible for the fluctuations in the pentacene transition frequency. We find that the spectral diffusion of the pentacene transition is caused by the reorientation of the central phenyl ring in the p-terphenyl molecule, and that this reorientation can only occur at a wall between domains of different central phenyl ring ordering. Furthermore, we find that only those pentacene molecules that reside within a few lattice spacings of these domain walls exhibit spectral diffusion.
Predissociation dynamics of the O2 B 3Σ−u state: Vibrational state dependence of the product fine-structure distribution
Authors: Leahy David J. , Osborn David L. , Cyr Douglas R. , Neumark Daniel M.
Published: July 2002

The predissociation of the O2B 3Σ−u state (υ=0–11) is investigated using fast beam photofragment translational spectroscopy. The energy resolution of the experiment, 7–10 meV, is sufficient to yield the correlated fine structure distribution P(j1,j2) for the two O(3Pj) fragments. These spin–orbit branching ratios depend markedly on the vibrational quantum number, providing detailed insight into a relatively unexplored facet of molecular dissociation dynamics. No less than four repulsive states are expected to mediate the predissociation of the B 3Σ−u state, primarily via spin–orbit coupling, and the couplings among these states at long range (R∼5–7 Å) determine the final spin–orbit distributions P(j1,j2). We have attempted to model these distributions in both the adiabatic and diabatic limits, with neither limit proving very successful. A more phenomenological approach to fitting our data suggests that products with j1=j2=2 result from single transitions between adiabatic potentials at long range, whereas the populations in the other product states are determined by multiple transitions among the repulsive states.
The Kelvin equation and self-consistent nucleation theory
Authors: Wilemski Gerald
Published: July 2002

Issues of self-consistency are reviewed for several unary equilibrium size distributions based on the capillarity approximation. Some apparent difficulties of interpretation are resolved. In terms of the kinetic approach to nucleation theory, the influence of self-consistency on the nucleation rate is shown to arise entirely from differences in the dimer evaporation rates for nearly all versions of classical theory. The nucleation rate behavior of the Kelvin model is explored. In this model, the Kelvin equation is used to prescribe all cluster evaporation rates. Nucleation rates predicted by the Kelvin model are quantitatively similar to those of the self-consistent classical (SCC) theory, but not to other simple versions of the classical theory. This behavior arises entirely from the relatively close coincidence of the SCC and Kelvin dimer evaporation rates. This means that, for the distribution-based versions of classical theory, the SCC model is the closest analogue of the Kelvin model. Because the Kelvin equation is fundamentally inadequate for very small clusters, the close relationship between the Kelvin and SCC formulations indicates that both are equally lacking in fundamental justification. The Kelvin model may, however, have some pragmatic utility, and a simple analytical rate expression is also derived for it to simplify the calculation of nucleation rates for this model.
A mean-field model for semiflexible chains
Authors: Ha B.-Y. , Thirumalai D.
Published: July 2002

We derive a mean-field model for a semiflexible chain using a functional integral approach. The resulting model satisfies the global constraint 〈u2(s)〉=1 rather than the hard constraint that u2(s)=1 for all s. The function u(s) is the tangent vector ∂r/∂s, where r(s) represents the conformation of the chain and s is the arc length.
Quantification of the selective activation of C–H bonds in short chain alkanes: The reactivity of ethane, propane, isobutane, n-butane, and neopentane on Ir(111)
Authors: Johnson Dale F. , Weinberg W. Henry
Published: July 2002

The initial probabilities of precursor-mediated, dissociative chemisorption of the saturated hydrocarbons 13C-labeled ethane, propane, isobutane, n-butane, and neopentane on the close-packed Ir(111) surface have been measured. The selective activation of primary (1°), secondary (2°), and tertiary (3°) C–H bonds has been quantified by examining the reactivities of the selectively deuterated isotopomers of propane, C3H8, CH3CD2CH3, and C3D8, and of isobutane, (CH3)3CH, (CH3)3CD, and (CD3)3CH. With respect to the bottom of the physically adsorbed well for each hydrocarbon, the apparent C–H bond activation energies have been found to be 10.4±0.3 kcal/mol (ethane), 11.4±0.3 kcal/mol (propane), 11.5±0.3 kcal/mol (n-butane), 11.3±0.3 kcal/mol (i-butane), and 11.3±0.3 kcal/mol (neopentane). For all the alkanes examined, the ratios of the preexponential factors of the rate coefficients of reaction and desorption are 1×10−2. The C–D bond activation energies are higher than the corresponding C–H bond activation energies by 480 cal/mol (ethane), 630 cal/mol (propane), and 660 cal/mol (i-butane). By analyzing the primary kinetic isotope effects for the selectively deuterated isotopomers of propane and isobutane, the 2° C–H bond activation energy is found to be 310±160 cal/mol less than the 1° C–H bond activation energy on this surface, and similarly, 3° C–H bond cleavage is less by 80±70 cal/mol. The quantification of the branching ratios within the C–H bond activation channel for propane and isobutane on this surface shows that the formation of 1°-alkyl intermediates is, in general, favored over the formation of either 2°- or 3°-alkyl intermediates. This result is a direct consequence of the disproportionate number of 1° C–H bonds relative to the number of 2° and 3° C–H bonds in these alkanes. These results are compared to those for the reaction of these alkanes on the reconstructed Pt(110)-(1×2) surface, and the influence of surface structure on the selective activation of 1°, 2°, and 3° C–H bonds is discussed.
Hindered internal rotation and torsion–vibrational coupling in ortho-chlorotoluene (S1) and ortho-chlorotoluene+ (D)
Authors: Richard Erik C. , Walker Robert A. , Weisshaar James C.
Published: July 2002

The techniques of resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure absorption spectra of ortho-chlorotoluene (S1, Ã 1A1) and of ortho-chlorotoluene+ (D0, X̃ 2A1; the cation ground state) for internally cold molecules in a pulsed nozzle expansion. The adiabatic ionization potential is 71 318±5 cm−1=8.8423±0.0006 eV. Most of the observed low lying torsion–vibrational structure in both S1 and D0 can be assigned using a one-dimensional torsional model plus low frequency vibrational modes whose identity is corroborated by the ab initio normal modes of D0. The intensities of certain weak, forbidden torsion–vibration combination bands in the S1–S0 spectrum are well predicted by a nuclear coordinate dependence of the electric dipole transition moment. The threefold methyl torsional barrier is V3=144.2±2.0 cm−1 in S1 and V3=456.5±2.0 cm−1 in D0. Ab initio calculations at the HF/6-31G* level find V3=481 cm−1 in S0 with the minimum energy conformation pseudo-trans, i.e., with one CH bond lying in the plane of the ring on the opposite side of the chlorine substituent. Spectral band intensities show that the minimum is pseudo-trans in S1 and D0 as well.

In both S1 and D0, excitation of either of the two lowest frequency out-of-plane bending modes, ν38 or ν37, leaves the methyl torsional potential essentially unchanged. In S0, S1, and D0, the barrier is substantially larger for ortho-chlorotoluene than for ortho-fluorotoluene, consistent with greater steric repulsion between the 3p chlorine lone pair and the CH bond pairs. The effects of π excitation and π ionization on the barrier are similar in ortho-chlorotoluene and ortho-fluorotoluene. Apparently both chlorine and fluorine are weak π donors that have similar effects on the crucial ring CC bond orders closest to methyl. Both the S1 and D0 spectra reveal several examples of torsion–vibrational coupling which perturbs torsional state energies and produces extra bands. Simple zeroth-order models of energy levels and coupling strengths fit experimental frequencies and band intensities well. Deperturbation yields coupling matrix elements between torsion and out-of-plane bending vibrations that fall in the narrow range 6–15 cm−1 in all cases. For three different molecules with widely varying methyl rotor barriers from 10 to 450 cm−1, the magnitude of such coupling matrix elements is similar, always in the range 3–15 cm−1. This provides guidance for theoretical models of intramolecular vibrational energy redistribution.
Basis set dependence of the locality of the kinetic energy operator
Authors: Hoch Douglas E. , Harriman John E.
Published: July 2002

We have analyzed the locality of matrices of the kinetic energy operator in even-tempered Gaussian and Slater-type (STO) bases, as well as in several three-dimensional, one-dimensional, and pseudo-one-dimensional basis sets of orthogonal polynomials and trigonometric functions. We find that the locality of the kinetic energy matrix in Gaussian bases is dependent upon the basis function parameters, while STO bases are found to always produce kinetic energy matrices which are very nearly equal to the matrices of local operators. In addition, it is observed that in the limit of completeness, the locality measure for all of the one-dimensional basis sets appears to converge to a common value. In the case of a basis set of particle on a ring eigenfunctions, an exact value for this limit has been determined analytically. Three-dimensional and l=0 pseudo-one-dimensional basis sets of orthogonal polynomials give kinetic energy matrices that do not behave similarly in the limit of completeness. It is found that all of these basis sets result in kinetic energy matrices which clearly exhibit nonlocal behavior, except for those involving exponentials. Such bases, like the STO bases, appear always to yield kinetic energy matrices that remain nearly local regardless of the basis function parameters and the number of basis functions used.
The molecular structure of ferrocene
Authors: Koch Henrik , Jo/rgensen Poul , Helgaker Trygve
Published: July 2002

The vertical iron–cyclopentadienyl distance in ferrocene has been determined using ab initio coupled-cluster theory. At the CCSD level, a TZV2P basis set yields a metal–ligand distance of 1.664 Å with all 96 electrons correlated and 1.672 Å with 66 electrons correlated. With 66 electrons correlated at the CCSD(T) level the equilibrium distance is 1.660 Å, in agreement with experiment (1.66 Å).
Structural transformation in (CO2)N clusters, N<100
Authors: Torchet G. , de Feraudy M.-F. , Boutin A. , Fuchs A. H.
Published: July 2002

In this paper, the structure of CO2 clusters made of less than a hundred of molecules is studied by means of electron diffraction experiments and molecular dynamics simulations. According to previous works, CO2 clusters are expected to undergo a transition in this size range, from an icosahedral structure to the bulk cubic structure. Electron diffraction patterns have been recorded from clusters produced in a free jet expansion of gaseous CO2. These patterns provide evidence of a structural change occurring in clusters made of a few tens of molecules. (CO2)N models, with 13<N<80, have been constructed in the cubic structure and annealed through a molecular dynamics calculation. Calculated diffraction functions show significant changes for a critical size of about 30 molecules per cluster. In the N=25 model, carbon atoms exhibit a polyicosahedral arrangement, similar to the one already identified in argon cluster models. In the N=32 model, most of the molecules build up a cubic lattice, meanwhile some of them adopt a pentagonal arrangement allowing a higher cluster stability. In addition to what has been observed previously, the present study confirms that the transformation from an icosahedral to a crystalline structure is a general process which occurs during the growth of van der Waals clusters made of small molecules. The relation between the critical size and the molecular species is briefly discussed.
Ultrafast infrared study of the ultraviolet photodissociation of Mn2(CO)10
Authors: Owrutsky J. C. , Baronavski A. P.
Published: July 2002

The 310 nm photodissociation of dimanganese decarbonyl, Mn2(CO)10, in cyclohexane has been studied using ultrafast infrared spectroscopy. Subpicosecond IR detection near 5 μm, in the region of carbonyl stretch vibrations, is carried out using a frequency-resolved broad (>100 cm−1) IR probe pulse. The evolution of infrared spectra provide information on Mn2(CO)10 and its photoproducts. In the terminal carbonyl region (1970–2050 cm−1), bleach signals are observed due to loss of the parent compound. A broad, featureless absorption appears promptly and narrows upon vibrational cooling (on the 50 ps time scale) to reveal IR spectral features consistent with those previously assigned to two photoproducts, Mn(CO)5 and Mn2(CO)9. The bleach signals partially recover, indicating ground state recovery of vibrationally excited Mn2(CO)10, which is consistent with previous reports of geminate recombination. Mn2(CO)9 signal appearance times are 54(4) ps for the terminal CO band at 2052 cm−1 and 31(5) ps for the bridge band at 1760 cm−1. The direct observation of the bridging band establishes 31 ps as an upper limit for the bridge bond formation. The time dependence of the signal is probably due primarily to vibrational cooling of low frequency modes. The times are somewhat longer than the shorter of two times measured in the visible region and attributed to low frequency mode thermalization by Zhang and Harris [J. Chem. Phys. 95, 4024 (1991)]. Vibrational coherence effects were observed at early times (close to the vibrational dephasing time).
A Monte Carlo study of CO oxidation with oscillations induced by site blocking
Authors: Jansen A. P. J. , Nieminen R. M.
Published: July 2002

We show that introduction of an additional adsorbate with appropriate adsorption and desorption rates leads to sustained oscillations in the CO oxidation on a catalytic surface. The adsorbate does not participate in the formation of CO2, but only blocks sites. We speculate on the possibility of experimental verification of our results by calculating the properties that the extra adsorbate should have. We have studied the reactions using a master equation that we have solved with the dynamical Monte Carlo method, using macroscopic rate equations, and using a combination of these two.
Novel procedure to determine coexistence lines by computer simulation. Application to hard-core Yukawa model for charge-stabilized colloids
Authors: Meijer Evert Jan , El Azhar Fouad
Published: July 2002

We describe a novel procedure to determine a phase coexistence line by computer simulation. The procedure combines elements of the Clausius–Clapeyron integration technique with free-energy difference calculations. As an application, the phase diagram of a simple hard-core Yukawa model of a charge-stabilized colloid, which exhibits liquid–solid and solid–solid phase coexistence lines, is computed.
Third order nonlinear response of liquid water
Authors: Saito Shinji , Ohmine Iwao
Published: July 2002

Third order nonlinear response of liquid water is investigated by using a classical molecular dynamics simulation. The calculated response function yields a marked agreement with the recent experimental results. The physical origins of the third order spectrum is analyzed in terms of the translational and the librational motions. Its temperature dependence and isotope effect are also analyzed.
Ab initio pseudopotential calculation of the photo-response of metal clusters
Authors: Pacheco J. M. , Martins José Luı́s
Published: July 2002

The photoabsorption cross section of small sodium and lithium clusters is computed in the time-dependent local density approximation to density functional theory, making use of two different types of ab initio nonlocal pseudopotentials. The equilibrium geometries of the clusters have been obtained via Langevin quantum molecular dynamics. It is found that the average bond length of the clusters and their static polarizabilities depend on the input pseudopotential. Nonetheless, it is found that the different pseudopotentials lead to the same equilibrium shape for the clusters, and to multipeaked line shapes for the photoabsorption cross sections which are nearly identical, apart from small overall energy shifts. For sodium, it is found that the local reduction of the pseudopotential obtained by keeping only its s-part provides, in all cases, an excellent approximation to the full pseudopotential, whereas for lithium the same procedure proves inaccurate.
An ab initio MO study on structures and energetics of C3H−, C3H, and C3H+
Authors: Ikuta Shigeru
Published: July 2002

The geometrical structures of cyclic and linear conformers in C3H−, C3H, and C3H+ are optimized using the full-valence CASSCF and CCSD(T) levels of theory. The atomic natural orbital ([5s3p2d1f/3s2p1d]) and correlation consistent basis (aug-cc-pVTZ) sets are used. The relative stability between the cyclic and linear conformers is investigated using the CCSD(T) and multireference CI levels of theory with the aug-cc-pVTZ basis set. The basis set dependency is checked with the 6-311+G(3d2 f,2pd) basis set. The most stable conformer in C3H− and C3H is C2v cyclic with a C3-ring, and that in C3H+ C∞v linear. The energy difference between the cyclic and linear in C3H radical is really small, being around 1.0 kcal/mol. The π-electron population on the C3-ring in cyclic C3H− is 2.00, which is a typical value predicted on the basis of the 4n+2 (n=0) aromatic-rule. The π-electron population on the C3-ring decreases in the order of C3H−, C3H, and C3H+, consistent with the order of the stability of the cyclic conformers. The adiabatic electron affinity and ionization potential in C3H are calculated to be 2.01 and 9.06 eV, respectively. The excitation energies from the most stable isomer are also calculated at the multireference CI level of theory with the aug-cc-pVTZ basis set.
A tensor approach to two-electron matrix elements
Authors: Adams Terry R. , Adamson Ross D. , Gill Peter M. W.
Published: July 2002

We present a new algorithm, the COLD PRISM, for computing matrix elements in molecular orbital calculations. Whereas traditional approaches form these from two-electron repulsion integrals (ERIs) which, in turn, are formed from shell-pair data, we introduce several alternative paths that do not involve ERIs as intermediates. Tensor multiplication can be used as the basic arithmetic operation on all of the new PRISM paths and the associativity of tensor multiplication plays a key role. We have implemented our approach in the Q-Chem program.
Natural steric analysis: Ab initio van der Waals radii of atoms and ions
Authors: Badenhoop J. K. , Weinhold F.
Published: July 2002

We employ natural steric analysis (introduced in a previous paper) to evaluate a set of effective ab initio van der Waals radii for free and covalently bonded atoms and ions of H–Ar (Z=1–18) determined using a helium atom probe. We critically examine the degree of anisotropy, dependence on charge state, and other intrinsic limitations of a simple atomic van der Waals hard sphere representation of the accurate steric surface. We also evaluate the ab initio steric force (gradient of steric energy at van der Waals contact) as a measure of “hardness” of the atomic van der Waals spheres. Comparison with empirical van der Waals radii shows reasonable agreement (within the acknowledged uncertainties of the latter values in the most important cases), but suggest a wider range of variability and anisotropy than could be adequately represented by any fixed constant radius. Simple expressions for incorporating the dependence on natural atomic charge or correcting for other types of intermolecular contact are given, extending the accuracy and usefulness of the atomic van der Waals sphere concept.
A new He–CO interaction energy surface with vibrational coordinate dependence. II. The vibrational deactivation of CO(v=1) by inelastic collisions with He3 and He4
Authors: Reid J. P. , Simpson C. J. S. M. , Quiney H. M.
Published: July 2002

Vibrational relaxation cross-sections and rate constants have been calculated for the deactivation of CO(v=1) by He3 and He4 on a new intermolecular potential with vibrational coordinate dependence [T. G. A. Heijmen, R. Moszynski, P. E. S. Wormer and Ad van der Avoird, J. Chem. Phys. 107, 9921 (1997)]. The new surface is found to resolve the qualitative discrepancy between theory and experiment which existed in earlier theoretical calculations. The low impact energy regime has also been investigated focussing in particular on impact energies of less than 15 cm−1 above the vibrational (v=1) threshold. Resonance structure has been found to occur and a comparison is made with an earlier investigation of the low temperature region.
Effect of patterned surface on diblock-copolymer melts and polymer blends near the critical point
Authors: Petera D. , Muthukumar M.
Published: July 2002

We investigate the density profile of diblock-copolymer melts and polymer blends close to a patterned surface. For diblocks we find a dependence of the orientation of the polymer chains and the lamellae on the ratio of the natural bulk period to the pattern period L. For a very large pattern period, the diblocks experience locally a homogeneous surface and orient perpendicular to it. For commensurable periods, their orientation is parallel and the lamellae are perpendicular to the surface. A sudden change in orientation is seen at a ratio Rg/L=2∛/2π for symmetric diblocks, where Rg is the radius of gyration of the diblock in the disordered melt. Polymer blends always follow the surface pattern close to the surface if the surface interaction is strong enough.
Tagged chain diffusion equation and effective friction tensor in dense polymer solutions
Authors: Szamel Grzegorz
Published: July 2002

We start from the many-chain Smoluchowski equation that describes dynamics of dense polymer solutions and derive an effective diffusion equation for a tagged (probe) chain distribution function. In the tagged chain diffusion equation the effects of the inter-chain interactions are incorporated through an effective friction tensor. We propose a simple phenomenological formula for the friction tensor in which the friction on a given bead depends only on the positions of its two neighbors along the chain. This formula is used in conjunction with an exact lower bound for the center of mass self-diffusion coefficient. We show that the necessary condition for reproducing N−2 scaling of the self-diffusion coefficient is not weaker-than-linear dependence of the anisotropy on the chain length N. To check whether this also a sufficient condition, we perform a set of single chain Brownian dynamics simulations. We show that the linear chain length dependence of the anisotropy leads to Ds∼N−1.6 whereas the N2 and N3 scaling of the anisotropy result in the chain length dependence of D that is consistent with the observed N−2 behavior.
Calculations of the direct electromagnetic enhancement in surface enhanced Raman scattering on random self-affine fractal metal surfaces
Authors: Sánchez-Gil J. A. , Garcı́a-Ramos J. V.
Published: July 2002

We study the classical electromagnetic enhancement at the excitation wavelength related to surface enhanced Raman scattering (SERS) experimental configurations in the vicinity of random rough metal surfaces possessing self-affine scaling behavior. The scattered electromagnetic intensity is obtained by means of numerical calculations based on the rigorous integral equations formulation of the electromagnetic wave scattering, free from the limitations of electrostatic and/or dipolar approximations. From the enhancement of the scattered field intensity in the immediate vicinity of the surface, originated in the excitation of transversal-magnetic surface plasmon polaritons, the SERS electromagnetic mechanism on substrates of Ag, Au, and Cu is explored as a function of the surface fractal dimension, rms height, and excitation wavelength. It is found that fractality favors the occurrence of large electromagnetic enhancements, which in turn appear to be maximum at an optimum wavelength as a result of the compromise between roughness-induced light coupling into surface plasmons and absorptive losses. This optimum wavelength is shorter for Ag than for Au and Cu. Maximum local enhancements on the order of 103 are encountered for the three metals being considered.
Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis
Authors: Marzocchi Mario P. , Mantini Anna R. , Casu Maurizio , Smulevich Giulietta
Published: July 2002

The scheme of energy levels previously proposed to describe dual excitation and emission associated to excited state intramolecular proton transfer (ESIPT) of some hydroxyanthraquinones (HAQ’s) has been made more quantitative in the present paper. The zero-point energy and the frequency of the νOH mode for the HAQ’s have been calculated on the basis of the Lippincott–Schroeder double-minimum potential for the O–H⋯O hydrogen bond. The second derivative absorption (D2) spectra show that the vibrational structures of the electronic excited state of HAQ’s giving rise to ESIPT are characterized by the progression of the νOH stretching mode. The νOH mode in the ground state is observed as a very strong band in the vibrational structure of the short wavelength emission for HAQ’s showing ESIPT. The combined resonance Raman band assignment of four hydroxyanthraquinones and transform analysis show that the visible transition involves the hydrogen bonded cycle and induces proton transfer in the excited state in most cases. On the basis of the isotopic effects, some vibrations of the hydrogen bonded cycle, namely the νC=O, δC=O, νCOH, and δOH modes, have been identified. The transform method, including the combined analysis of the absorption and D2 spectra in terms of sum-over-states, was checked by directly deriving the displacement parameters (Franck–Condon factors) of 1,4-DHAQ from the high resolution free-jet spectrum. The values of the displacement parameters of the νOH mode are quite large for the HAQ’s showing ESIPT, while are negligible for 1,4-DHAQ. High values of the displacement parameters for the other vibrations of the hydrogen bonded cycle were found for all HAQ’s.
Structures and vibrational frequencies in the full configuration interaction limit: Predictions for four electronic states of methylene using a triple-zeta plus double polarization (TZ2P) basis
Authors: Sherrill C. David , Leininger Matthew L. , Van Huis Timothy J. , Schaefer Henry F.
Published: July 2002

Benchmark energies, geometries, dipole moments, and harmonic vibrational frequencies are determined for four states of methylene (CH2) by solving the electronic Schrödinger equation exactly for a triple-ζ plus double polarization (TZ2P) basis with the restrictions that the core orbital remains doubly occupied and the highest-lying virtual orbital is deleted. Approximate models of electron correlation are evaluated based on their ability to match the exact, full configuration interaction results. Predictions from configuration interaction with all single, double, triple, and quadruple substitutions (CISDTQ) and coupled-cluster including singles, doubles, and triples (CCSDT) are virtually identical to the full CI results for all but the c̃ 1A1 state, which is poorly described by a single-configuration reference. In agreement with previous work, the c̃ state remains slightly bent at the TZ2P full CI level of theory, with a bond angle of 170.1° and a barrier to linearity of only 25 cm−1.
Mixed silicon–carbon dianions and their stability in the gas phase
Authors: Dreuw A. , Sommerfeld T. , Cederbaum L. S.
Published: July 2002

Free dianionic mixed silicon–carbon clusters SixCy2− (x=1,2 y=4–9) have been studied by standard ab initio methods. Branched, chainlike, and cyclic isomers have been investigated in detail. The smallest system that has been found to be clearly stable with respect to electron autodetachment and fragmentation is a triangular seven atomic SiC62− dianion, better referred to as Si(C2)32−. Chainlike isomers are less stable than the corresponding branched isomers. The onset of electronic stability within the chainlike structural class is predicted to occur at the eight atomic cluster dianion Si2C62−. Small stable cyclic isomers have not been found. The bonding mechanisms of the excess electrons in the different isomers are discussed and the systems are compared with other small gas-phase dianions. Furthermore, the essentially different principles of building up dianionic, monoanionic, and neutral mixed silicon–carbon clusters are explained and discussed.
A molecular dynamics study of the structure of water layers adsorbed on MgO(100)
Authors: Marmier A. , Hoang P. N. M. , Picaud S. , Girardet C. , Lynden-Bell R. M.
Published: July 2002

Molecular dynamics simulations are performed at various temperatures (150-300 K) and coverages (1-3 layers) on the adsorption of water on a clean MgO(100) surface using semiempirical potentials. At the monolayer coverage, a number of very stable (m×n) structures are obtained which differ only by the mutual orientations of the molecules. The p(3×2) phase observed above 180 K in low-energy electron diffraction (LEED) and helium atom scattering (HAS) experiments is shown to be the most stable at 200 K and above this temperature. It contains six inequivalently oriented molecules which lie flat above the cation sites with the hydrogens pointing approximately along the Mg rows. When the water coverage increases, a layer of icelike hexagonal structure within which the water molecules are hydrogen bonded is formed above the stable monolayer. This overlayer, which is stable at 150 K, is not hydrogen bonded to the stable monolayer. At 300 K it tends to break up and to aggregate into a 3D ice structure with strong hydrogen bonding. Examination of the calculated oxygen-oxygen distances dOO in the monolayer and in the icelike overlayer, and the comparison with the correlation diagram of the frequency shift and bandwidth of the water infrared spectrum versus dOO give a very consistent interpretation of the observed polarized infrared signals.
Density distributions of diatoms in carbon nanotubes: A grand canonical Monte Carlo study
Authors: Khan Ifadat Ali , Ayappa K. G.
Published: July 2002

Grand canonical Monte Carlo simulations have been carried out for dumbbell (D) and spherical (S) models of N2 and Br2 in single-walled carbon nanotubes, with nanotube radii ranging from 4.029 to 25.142 Å. The radial density distributions between models S and D in the nanotube were compared under similar bulk conditions. For gas and liquid bulk densities of N2, differences between the pore density distributions for the S and D models in the nanotube were small and the inclusion of quadrupole moments to model D had little effect. In contrast, the density distributions between the S and D models for Br2 showed greater differences for the bulk liquid case. Angular density distributions show that molecules adjacent to the nanotube wall orient parallel to the wall. At high densities the wall layer contains a mixture of wall-parallel and wall-perpendicular orientations, giving rise to a shoulder in the density distributions for Br2.
A theoretical study of Gemini surfactant phase behavior
Authors: Layn Kristine M. , Debenedetti Pablo G. , Prud’homme Robert K.
Published: July 2002

Gemini surfactants are a relatively new type of surface active material, characterized by multiple hydrophilic head groups. The phase behavior of a Gemini surfactant lattice model is studied by Monte Carlo simulations and quasichemical (QC) calculations. The predictions of these methods are in excellent agreement, justifying use of the analytical QC theory. The dependence on temperature, surfactant solubility, surfactant rigidity, and oil chain length of the global phase behavior of ternary mixtures of Gemini surfactant, oil, and water is investigated. Three-phase equilibrium exists only at low temperatures, with a transition to two-phase equilibrium as temperature is increased. Surfactants with moderate solubilities (i.e., those which do not have a strong preference for either solvent) exhibit three-phase equilibrium at low temperatures, while surfactants with extreme solubility preferences yield two-phase equilibrium only. When a flexible surfactant exhibits three-phase equilibrium, imposing rigidity promotes a transition from three- to two-phase equilibrium. Increasing molecular size of the hydrophobic solvent (oil) causes a transition from two- to three-phase equilibrium, and finally back to two-phase equilibrium, but with reversed relative surfactant solubility in the oil and water phases.
Electron impact ionization of the SF5 and SF3 free radicals
Authors: Tarnovsky V. , Deutsch H. , Martus K. E. , Becker K.
Published: July 2002

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the SF5 and SF3 free radicals from threshold to 200 eV using the fast-neutral-beam technique. The total single ionization cross sections at 70 eV were found to be 5.1×10−16 cm2 (SF5) and 3.2×10−16 cm2 (SF3). The cross sections reach their maximum at about 100 eV for both radicals, with values of 5.4×10−16 cm2 (SF5) and 3.5×10−16 cm2 (SF3). Dissociative ionization is important only for SF5, where the SF5+ parent ionization cross section and the SF4+ fragment ionization cross sections have roughly the same value. By contrast, the ionization of SF3 is dominated by the formation of SF3+ parent ions. A comparison of the measured total single ionization cross sections with calculated cross sections, using a modified additivity rule, shows excellent agreement for both radicals in terms of the absolute cross section values and the cross section shapes. Total single ionization cross sections were also calculated for SF4, SF2, and SF.
Hydrogen bonding in glassy liquid water from Raman spectroscopic studies
Authors: Tulk C. A. , Klug D. D. , Branderhorst R. , Sharpe P. , Ripmeester J. A.
Published: July 2002

In order to investigate subtle differences in the local structure and bonding of amorphous ices, glassy liquid water was produced by rapid cooling of micrometer sized water droplets. Samples were first studied with power x-ray diffraction (PXRD) and differential scanning calorimetry (DSC) to ensure that the production technique resulted in amorphous material with low crystalline content. Solutions containing 5% D2O in H2O and 5% H2O in D2O were vitrified, cooled to 12 K, and used to collect Raman spectra of the decoupled O–D (ΔνOD=2440 cm−1) and O–H (ΔνOH=3302 cm−1) vibrations, respectively. Samples were then annealed from 90 to 180 K in temperature intervals of 10 K. During each annealing step the samples were held isothermally for 15 min before rapidly cooling them back down to 12 K for data collection. The transition from amorphous to crystalline material was observed to occur between 150 and 160 K. Raman frequency shift data were then used to estimate the distribution in the hydrogen-bonded O–H––O distances using known correlations. The local bond strengths and O–H––O distances in vitrified liquid water were found to be very similar to those in vapor-deposited amorphous water.
What is the best alternative to diagonalization of the Hamiltonian in large scale semiempirical calculations?
Authors: Daniels Andrew D. , Scuseria Gustavo E.
Published: July 2002

Recently, several linear scaling approaches have been introduced which replace the time dominating diagonalization step in semiempirical methods, enabling practical calculations to be performed on very large molecules. This paper compares the accuracy and performance of pseudodiagonalization (PD), conjugate gradient density matrix search (CG-DMS), the Chebyshev polynomial expansion method (CEM), and purification of the density matrix (PDM) as linear scaling substitutions for diagonalization. The scaling, speed, and reliability of these methods are compared for AM1 single point energy calculations on polyglycine chains (up to 20 000 atoms), water clusters (up to 12 300 atoms), and nucleic acids (up to 6300 atoms).
One-electron pseudopotential study of NanFn−1 clusters (2⩽n⩽29). I. Electronic and structural properties of the ground state
Authors: Durand G. , Giraud-Girard J. , Maynau D. , Spiegelmann F. , Calvo F.
Published: July 2002

We introduce a one-electron pseudopotential model to study the structural and electronic properties of excess-electron alkali halide clusters. This model assumes total charge transfer between alkali and halide atoms. This ionic part of the system is described via repulsive and Coulomb potentials. The remaining electrons of the excess metal atoms are treated within an explicit quantal scheme via ion–electron pseudopotentials. Moreover, explicit core-polarization and core-electron correlation contributions are taken into account. This model is used to derive ground state structural, energetics, and electronic properties of one-excess electron NanFn−1 clusters in the range 2⩽n⩽29. We show that the structural characters are closely related with electron localization and we propose a classification into five types, two of them exhibiting rather strong localization namely F-centers and Na-tail structures, the others exhibiting a less bound electron localizing in a surface-state, an edge-state, or on an atom-depleted face of the cluster. Although we observe an energetical predominance of cubiclike structures, hexagonal isomers are seen to appear as stable ones and exhibit similar localization features. The various energy contributions to the stability are examined. All studied NanFn−1 clusters are found stable with respect to fragmentation. The ionization potentials, which are seen to reflect faithfully the localization character, are discussed in details and compared with consistent recent experimental data.
One-electron pseudopotential study of NanFn−1 clusters (2⩽n⩽29). II. Absorption spectra, spectral signature, and classification
Authors: Durand G. , Spiegelmann F. , Poncharal Ph. , Labastie P. , L’Hermite J.-M. , Sence M.
Published: July 2002

A one-electron pseudopotential model is used to calculate the excited states of NanFn−1 clusters in the range 2⩽n⩽29. The transition dipole moments are derived to determine the vertical absorption lines from the ground state. Monte Carlo simulations based on the Franck–Condon principle are also achieved to obtain finite temperature absorption spectra. The analysis of ground and excited orbitals in various situations illustrates the rather simple nature of the excitations which are interpreted through an analogy with s−p type transitions in atoms. The influence of electron localization in the initial state is pointed out. Comparison of the simulated spectra with experimental data provides additional keys to understand and discuss the relationship between structure, electron localization, and spectroscopic properties.
Thermodynamic aspects of the glass transition phenomenon. II. Molecular liquids with variable interactions
Authors: Alba-Simionesco C. , Fan J. , Angell C. A.
Published: July 2002

As a contribution to the understanding of the thermodynamics of the glass transition phenomenon a series of molecules having the same steric character, but differing in the strength and nature of intermolecular interactions, has been investigated. The series is based on systematic changes of substituents on disubstituted benzene ring compounds, the simplest example of which is meta-xylene. Meta-isomers are chosen in each instance because of their greater tendency to supercool. In particular, m-fluoroaniline cannot be crystallized at ambient pressure. The principal measurements performed were of heat capacity and enthalpy change, using the technique of differential scanning calorimetry, and these have been examined in the light of literature data on the liquid viscosities and some recent data for dielectric relaxation. As the strength of hydrogen-bonding interactions between the ring substituents on adjacent molecules increases, the glass transition temperature Tg increases by almost 100 degrees from the lowest value in the series, 122.5 K, for m-fluorotoluene. Empirical rules involving Tb/Tm and Tg/Tm are found wanting. The important thermodynamic characteristic of the glass transition, viz., the change in heat capacity at the glass transition, ΔCp, remains approximately constant until the −OH substituent is introduced, whereupon a new element appears. This is a specific component of ΔCp which appears at temperatures above an initially small jump at Tg. It is well accounted for by the addition of a two-state H-bond breaking component (with the usual H⋯−OH bond energy) to the total excess heat capacity. The liquid ground state (or Kauzmann) temperature TK assessed from thermodynamic data acquired in this study, falls 20%–30% below the glass transition temperature. From the limited transport data available, these liquids appear to be quite fragile in character implying that the phenyl group influence dominates the hydrogen bond factor which has often seemed responsible for decreased fragility. In the case of cresol the hydrogen bonding apparently produces dielectric/shear relaxation anomalies of a character previously only seen in certain aliphatic monoalcohols.