bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutron diffraction study of lithium hydrogen phthalate monohydrate: A material with two very short intramolecular O⋅⋅⋅H⋅⋅⋅O hydrogen bonds
Authors: Küppers Horst , Takusagawa Fusao , Koetzle Thomas F.
Published: July 2002

The structure of lithium hydrogen phthalate monohydrate has been refined based upon neutron diffraction data obtained at three temperatures: 15, 100, and 298 K. All bond distances have been determined with precision better than 0.002 Å. The two crystallographically independent hydrogen phthalate anions in the unit cell both possess very short intramolecular O⋅⋅⋅H⋅⋅⋅O hydrogen bonds with O⋅⋅⋅O≊2.4 Å and ∠O⋅⋅⋅H⋅⋅⋅O≊170°. One of these linkages is very decidedly asymmetric, with O⋅⋅⋅H=1.122(1) Å and H⋅⋅⋅O=1.294(1) Å at 15 K, reflecting the fact that the two O atoms have quite different environments in the crystal. The second O⋅⋅⋅H⋅⋅⋅O bond is more nearly symmetric, with O⋅⋅⋅H=1.195(1) Å and H⋅⋅⋅O=1.205(1) Å at 15 K, but the difference between the two distances is still significant. The asymmetry of the short hydrogen bonds appears somewhat more pronounced at 298 K than at the lower temperatures. A corresponding shift with temperature of the H atom in the short intramolecular hydrogen bond was found previously in 2,3-pyridinedicarboxylic acid, which has a molecular geometry similar to that found here.
Bond forces in long-chain molecules
Authors: Weiner J. H. , Berman D. H.
Published: July 2002

Theoretical formulas are developed for the mean and mean-square deviation of the forces in the bonds of a freely jointed long-chain molecule with arbitrary fixed end-to-end displacement r. The mean bond force is the same for all bonds in the molecule and is nonzero even when r=0 and the mean axial force on the molecule vanishes. The mean-square deviation of the bond forces is large and computer simulation results are presented to illustrate the time variation of these forces.
Studies of energy transfer processes in triplet states of N2. I. Energy pooling by vibrationally selected N2 (A 3Σ+u,v) molecules
Authors: Nadler I. , Rosenwaks S.
Published: July 2002

Emissions from the second positive (C–B) and from the Herman infrared (HIR) systems of N2, generated by the N2 (A,v)+N2(A,v′) energy pooling (EP) reaction, were studied at room temperature in a flowing afterglow apparatus. Using ‘‘vibrational quenchers’’ of N2 (A,v), the emission resulting from the EP reaction of 2N2(A,v=0) was compared to that of 2N2 (A,v=0,1,2). The dependence of the vibrational distribution of both the N2 (C,v) and the upper N2(HIR,v) states on that of N2(A,v) was accurately measured and analyzed. It is shown that Franck–Condon factors and energy matches for the EP reactions can account for this dependence. The spectroscopic and kinetic data suggests that the EP reaction is governed by long range interaction which involves direct excitation transfer. It also suggests that the HIR system may originate from transitions from the unobserved 3∏u(II) state to G 3Δg.
Time resolved mode-to-mode vibrational energy flow in the isolated s-tetrazine argon complex
Authors: Heppener Marc , Kunst André G. M. , Bebelaar Dick , Rettschnick Rudolf P. H.
Published: July 2002

This paper reports the first direct observation of intramolecular vibrational redistribution (IVR) where two consecutive steps have been distinguished. The initially excited vibronic state ∼(6b2) of the van der Waals complex of s-tetrazine and argon changes into the intermediate state ∼(16a116b1) from which subsequently the final state ∼(16a2) originates. The temporal intensity profiles of selectively detected emission from each of these states of the complex have been measured on a picosecond time scale.
Assignment of a surface vibrational mode by chemical means: Modification of the lattice modes of Al2O3 by a surface reaction with H2O
Authors: Chen J. G. , Crowell J. E. , Yates J. T.
Published: July 2002

The adsorption of water on a well-characterized Al2O3 film produced by oxidizing an Al(111) surface has been studied using electron energy loss spectroscopy (EELS). Water reacts with this oxide layer to form surface hydroxyl species characterized by a sharp O–H (O–D) stretching vibration at 3720 (2740) cm−1. The assignment of the three-peak vibrational spectrum of Al2O3 to surface and bulk Al–O modes is confirmed by chemical means based on the observation that the 625 cm−1 loss feature assigned to a surface Al–O stretching vibration is preferentially affected upon formation of surface hydroxyl groups. The remaining two bulk modes are uninfluenced by surface hydroxylation. The surface hydroxyl species can be quantitatively removed by electron stimulated desorption (ESD), reversing the spectroscopic changes observed upon exposure of Al2O3 to water, further substantiating the vibrational assignments.
A modified coupled pair functional approach
Authors: Chong Delano P. , Langhoff Stephen R.
Published: July 2002

A modified coupled pair functional (MCPF) method is presented that dramatically improves properties for cases where the Hartree–Fock reference configuration is not a good zeroth-order description of the wave function. This new methodology is compared to singles-plus-doubles configuration interaction and the CPF method of Ahlrichs et al. for the ground states of NiH, CuH, and ZnH.
Viscoelastic relaxation of segment orientation in dilute polymer solutions. II. Stiffness dependence of fluorescence depolarization
Authors: Perico Angelo , Guenza Marina
Published: July 2002

The memory functions for the orientation of a segment in any position on a linear chain are calculated for a diffusion Gaussian model in the optimized Rouse–Zimm approximation for semiflexible polymer models. The crossover from flexible to rod chains is described and expectations for fluorescence anisotropy, fluorescence quenching, and the mean orientational relaxation time are outlined and discussed.
Hyperpolarizabilities of the hydrogen fluoride molecule: A discrepancy between theory and experiment?
Authors: Sekino Hideo , Bartlett Rodney J.
Published: July 2002

Due to the recent availability of an experimental value for the second- and third-order electric polarizabilities of the HF molecule, possible errors in the previous prediction of these quantities have been considered. These include basis sets, vibrational corrections, frequency dependence, infinite-order correlation corrections, and the effect of triple excitations. Despite the inclusion of all of these effects, the discrepancy between experiment and theory remains. Our best results predict χ(2)∥ to be −3.3 to −3.8×10−32 and χ(3)∥ to be 45 to 48×10−39 esu which are at best 81% and 69% of the experimental values, respectively. Possible additional corrections are considered with emphasis on basis set completeness. Considering the difficulty in predicting such sensitive quantities, this is rather good agreement. The paper also addresses questions of reliability of ab initio calculations for such high-order properties as hyperpolarizabilities, while identifying some places where the experimental results are susceptible to error.
Carbon clusters revisited: The ‘‘special’’ behavior of C6 and large carbon clusters
Authors: Cox D. M. , Reichmann K. C. , Kaldor A.
Published: July 2002

We have performed a series of experiments in which the conditions necessary for synthesis and detection of carbon clusters have been systematically investigated. The effects of ionizing laser intensity and frequency, vaporization and condensation conditions, and graphite substrate composition (pure and metal loaded) on the carbon cluster ion signals show that ‘‘special’’ conditions are needed to observe locally enhanced ion signals on particular clusters (e.g., a large C+60 signal). We present evidence for two sources of carbon clusters, one source which produces mostly smaller (n<32) and the other which leads to mostly larger (n>32) carbon clusters. In addition we find that several, not just one, metal atom can be attached to C60. Lastly we propose that large variations in ion signals may be explained in terms of the electronic properties of the carbon clusters.
A model for ultrafast vibrational cooling in molecular crystals
Authors: Hill Jeffrey R. , Dlott Dana D.
Published: July 2002

A model is presented to describe vibrational cooling (VC) in crystals of large molecules. Vibrational cooling is the process by which a vibrationally excited crystal returns to the ground state. This process may consist of many sequential and parallel vibrational relaxation (VR) steps. The model describes a highly excited, vibrationally dense molecular crystal at zero and finite temperatures. An initially excited vibration relaxes via anharmonic coupling by sequential emission of many lattice phonons until all vibrational energy is destroyed. The time evolution of vibrational excitation probability is described with a Master equation. Various models for the phonon density of states, which exerts primary control over the VR process, are considered. It is found that VC occurs on a much slower time scale than VR, and that the rate of VC is only weakly dependent on temperature, even in systems where VR is highly temperature dependent. An important conclusion of this work is that vibrational cooling is described by an ensemble averaged vibrational population distribution function which moves to lower energy states and broadens as time increases. The motion to lower energy is described by a ‘‘vibrational velocity’’ (emitted energy per unit time) which is independent of temperature, while the width of the distribution increases with increasing temperature. The model is then used to calculate experimental observables including time resolved absorbance, emission, and Raman scattering following excitation of a high frequency vibration.
Computer modeling of a liquid–liquid interface
Authors: Meyer M. , Mareschal M. , Hayoun M.
Published: July 2002

Molecular dynamics method and Lennard-Jones potential functions are employed to model liquid–liquid interfaces. The simulations are carried out in a range of temperature and pressure near the triple point. The investigated systems are symmetric and composed of two identical liquids L1 and L2. The interactions between the atoms of L1 and L2 are obtained from modified Lennard-Jones potential functions where extra parameters are introduced to reduce the miscibility between the two liquids. The interfacial tensions and the miscibilities are varied by using different parameters. The interfaces thus obtained are stable on the time scale of the simulation as shown by the density and pressure profiles. This is also confirmed by a geometrical analysis performed in order to characterize the fluctuations of the interfaces. The calculation of the diffusion coefficients shows clearly an anisotropy of the diffusion process in the interfacial region.
Doubly ionized states of ethylene: Auger spectrum, potential energy surfaces and nuclear dynamics
Authors: Ohrendorf E. , Köppel H. , Cederbaum L. S. , Tarantelli F. , Sgamellotti A.
Published: July 2002

Theoretical investigations of the outer valence doubly ionized states of ethylene are presented using ab initio Green’s function and configuration interaction methods. The vertical double ionization potentials computed by the Green’s function method using the ADC(2) scheme are discussed in connection with the Auger spectrum of ethylene and found to reproduce the experimental spectrum quite accurately. Another main purpose of the present work is the investigation of the potential energy surfaces of selected states via ADC(2) and the study of the nuclear dynamics on them. It is shown that only the ground state of the dication is nonplanar with a torsional angle of 90°. The investigation of the topology of the surfaces reveals a low energy conical intersection of the dicationic ground and first excited states. The associated vibronic coupling problem turns out to include, additionally, the second excited state and thus, we encounter here a three-state problem. In the approximation of linear vibrational and vibronic coupling the static and dynamic aspects of the vibronic interaction are investigated by a model Hamiltonian comprising three nonseparable nondegenerate vibrational modes. The input data for the model are extracted from the ADC(2) results. With the aid of this Hamiltonian we have computed a hypothetical ‘‘sudden’’ double ionization spectrum of ethylene whose complex structure is analyzed stepwise and by cuts through the pertinent adiabatic and diabatic surfaces. The analysis exhibits that the conical intersection of the two lower lying states dominates the vibronic dynamics and leads to strong vibronic mixing between them. This mixing is indirectly reinforced by the interaction between the intermediate and the upper states. As a consequence, strong nonadiabatic effects occur in the ethylene dication. Furthermore, two of the three surfaces involved in the conical intersection are also involved in a multidimensional avoided crossing. A more detailed analysis presented earlier has exhibited that the states belong to an interesting class where two potential energy surfaces are likely not to coincide although plenty of nuclear degrees of freedom exist, in principle, to allow for an intersection to occur.
Dynamics of radiation damping in nuclear magnetic resonance
Authors: Warren W. S. , Hammes S. L. , Bates J. L.
Published: July 2002

The time evolution of spins-1/2 subject to radiation damping, which is commonly encountered for solvent peaks at high field, is examined in detail. The well-known analytic results for rectangular pulses on undamped spin-1/2 systems are extended to the radiation damped case, and reveal surprisingly complex dynamics. Explanations in terms of Bloch vectors are also presented, and composite pulse sequences which would also be insensitive to radiation damping are proposed. In addition, gradient optimization programs were developed to find shaped π and π/2 pulses insensitive to radiation damping. The optimized pulses compensate for radiation damping effects even when the characteristic damping time is shorter than the pulse length.
State selective photodissociation dynamics of Ã state ammonia. II
Authors: Biesner J. , Schnieder L. , Ahlers G. , Xie Xiaoxiang , Welge K. H. , Ashfold M. N. R. , Dixon R. N.
Published: July 2002

The photodissociation dynamics of Ã state ammonia molecules (both NH3 and ND3) has been further investigated using the technique of H(D) atom photofragment translational spectroscopy. The resulting NH2(ND2) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v′2 level excited. Dissociation from the v2=0 level of the Ã state yields ground state NH2(ND2) fragments, primarily in their zero-point level, but with high levels of rotational excitation specifically concentrated about the a-inertial axis; the population distribution over the energetically accessible product rotational levels with N≂Ka appears near to statistical. In contrast, dissociation from the parent v′2=1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical trajectory calculations employing the best available ab initio potential energy surfaces for the Ã and X̃ states of the ammonia molecule. The energy disposal following excitation to the parent v2=2 and 3 levels is found to mimic that observed for, respectively, the v′2=0 and 1 levels. These results provide clear evidence for the importance of anharmonic coupling (whereby an even number of bending quanta are redistributed into stretching motions) in promoting the fragmentation of parent levels with v2≥2. The threshold energy for producing electronically excited NH2(Ã2A1) fragments is 6.02 eV [∼6.16 eV for ND2(Ã)]. The present studies of NH3 photolysis suggest that this fragmentation channel opens at threshold and clearly indicate that branching into this channel occurs with much higher quantum yield than hitherto believed.
Theoretical study of GeHn, AsHn, and SeHn: Ionization energies
Authors: Binning R. C. , Curtiss L. A.
Published: July 2002

A b initio molecular orbital theory (Mo/ller–Plesset theory to fourth order with a series of extended basis sets) has been used to calculate the total energies of GeH+n (n=1–4), AsH+n (n=1–3) and SeH+n (n=1–2). In combination with previously published results on the neutral hydride molecules, the data are used to derive ionization energies of the neutral species and appearance potentials of the ions with an expected accuracy of ±0.15 eV. Experimental data are reviewed in light of these results. Theoretical predictions of the structures of several cations are discussed including the Jahn–Teller distortion in GeH+4.
Spectroscopic properties of the X 1Σ+ and a 3Π electronic states of CF+, SiF+, and CCl+ by multireference configuration interaction
Authors: Peterson Kirk A. , Woods R. Claude , Rosmus Pavel , Werner Hans-Joachim
Published: July 2002

Potential energy and dipole moment functions have been calculated using multireference configuration interaction (MRCI) techniques for CF+, SiF+, and CCl+ in their ground 1Σ+ and first 3Π electronic states with large Gaussian basis sets. Where experimental data is available, the MRCI values of re and ωe are accurate to within 0.006 Å and 7 cm−1, respectively. The Te values for the a 3Π–X 1Σ+ transitions for CF+, SiF+, and CCl+are calculated to be 4.77±0.05 eV, 4.78±0.05 eV, and 3.19±0.05 eV (3.16 eV), respectively (experimental value in parentheses). Rotational and vibrational spectroscopic constants are also predicted for all three species in the 3Π state. Infrared transition probabilities have been calculated from the MRCI potential energy and dipole moment functions. Intense infrared transitions are predicted for all three species in both their ground and first excited state.
Rotationally inelastic rates over a wide temperature range based on an energy corrected sudden–exponential-power theoretical analysis of Raman line broadening coefficients and Q branch collapse
Authors: Millot G.
Published: July 2002

We study the ability of the energy corrected sudden (ECS) scaling law associated with a hybrid exponential-power (EP) fitting law for the basis rate constants to model the rotational dependence of isotropic Raman linewidths. We determine the temperature dependence of the hybrid law (EP) parameters and give several applications for various molecular collisional systems over a wide temperature range. In particular, we find that the ECS–EP law gives a very good description of both rotational and temperature dependences of the Raman line broadening coefficients of N2 perturbed by H2O contrary to the ECS–P law. For all the collisional systems studied the ECS–EP law is found very suitable to describe the line broadening coefficients. We give a complete set of ECS–EP parameters for these collisional systems which play an important role in coherent anti-stokes Raman spectroscopy thermometry. We also test the ability of the ECS–EP law to predict accurate collapsed Q branch at high density. An application performed on the Raman Q branch of pure N2 at high density demonstrates that the ECS–EP law properly models the rates of state-to-state rotational energy transfer. The overall collisional line shift obtained from the collapsed Q branch is in perfect agreement with low pressure measurements on isolated lines.
Raman spectroscopic study of dilute HOD in liquid H2O in the temperature range − 31.5 to 160 °C
Authors: Hare D. E. , Sorensen C. M.
Published: July 2002

We present Raman data for the OD stretch mode of 10 mol % HOD in H2O for the liquid phase from −31.5 to 160 °C. We find that an exact isosbestic does not hold, but rather the crossing of isotherms slowly but uniformly changes with temperature. We present an analysis based on Boltzmann statistics which gives evidence for a distribution of deuterium hydrogen bond strengths with minimum energy near the frequency (2440 cm−1) also found in the solid ice and amorphous solid phases. This analysis also gives evidence for a band of frequencies above 2630 cm−1 due to OD oscillators all at essentially the same high energy relative to the strongest hydrogen bonds, and we interpret this band as due to broken hydrogen bonds. This allows us to calculate hydrogen bond probabilities, and we find this probability increases with decreasing temperature and approaches a value equal to the four bonded percolation threshold near the singular temperature Ts ≂−45 °C for the anomalies of supercooled water. Peak frequency and full width at half-maximum of the OD stretch band are found to drop precipitously to the amorphous solid values as T→Ts implying the ultimate state of supercooled water is similar to the amorphous solid.
The ground state infrared spectra of four diatomic deuterides (GaD, InD, TlD, AgD) and the determination of mass-independent molecular parameters
Authors: Urban Rolf-Dieter , Birk Helmut , Polomsky Petra , Jones Harold
Published: July 2002

The infrared spectra of two isotopic forms of the monodeuterides of the metals gallium, indium, thallium, and silver have been measured in their ground electronic state (1Σ) in the gas phase using a diode laser spectrometer. The analysis of the experimental data was carried out in two ways: (a) a complete set of Dunham parameters was determined for each isotopic species independently and (b) all the available data over both deuterated and protonated species of a particular compound were simultaneously fitted to obtain a set of mass-independent parameters. Mass scaling coefficients, which take into account effects arising from a breakdown of the Born–Oppenheimer approximation, were determined for the hydrogen atom.
Wave functions with terms linear in the interelectronic coordinates to take care of the correlation cusp. I. General theory
Authors: Kutzelnigg Werner , Klopper Wim
Published: July 2002

The matrix elements needed in a CI-SD, CEPA, MP2, or MP3 calculation with linear r12-dependent terms for closed-shell states are derived, both exactly and in a consistent approximate way. The standard approximation B guarantees that in the atomic case the error due to truncation of the basis at some angular momentum quantum number L goes as ∼L−7, at variance with L−3 in conventional calculations (without r12 terms). Another standard approximation A has errors ∼L−5, but is simpler and—for moderate basis sets—somewhat better balanced. The explicit expressions for Mo/ller–Plesset perturbation theory of second and third order with linear r12 terms (MP2-R12 and MP3-R12, respectively) are explicitly given in the two standard approximations.
Analysis of the two-photon D 1Δ–X 1Σ+ transition in CO: Perturbations in the (10–0) band
Authors: Garetz Bruce A. , Kittrell Carter , Le Floch André C.
Published: July 2002

Spin-orbit perturbations among the D 1Δ(v=10), d 3Δ(v=14), a 3Π(v=18), and a′ 3Σ+(v=19) levels of carbon monoxide have been observed in the two-photon laser excitation spectrum of the D 1Δ–X 1Σ+ (10–0) band. Spectra have been analyzed using a weighted nonlinear least-squares-fitting method. Detailed assignments and wave numbers are presented for the D–X (10–0), a–X (18–0), and d–X (14–0) bands. Molecular constants and perturbation parameters have been determined, and mixing coefficients have been calculated. Electronic perturbation parameters associated with the D 1Δ∼a 3Π spin-orbit interaction and the d 3Δ∼D 1Δ isoconfigurational interaction are obtained for the first time. The latter parameter could be determined because of an unusual multiple crossing among the a, d, and D states. The v=18 level is the highest vibrational level of the a 3Π state ever observed.
Electronic and chemical changes in shocked liquid carbon disulfide inferred from time resolved reflection experiments and analysis
Authors: Gustavsen R. , Gupta Y. M.
Published: July 2002

Time resolved, spectroscopic reflection measurements (300–500 nm) have been used to examine the electronic and chemical changes in liquid carbon disulfide shocked to peak pressures as high as 110 kbar. Multiple-shock loading and unloading and double-shock experiments were performed to examine the influence of temperature, and to compare the present data with previous absorption and continuum measurements. The reflectance increases markedly with pressure in multiple-shock loading experiments. At 300 nm, the reflectance increases from less than 0.33% at ambient conditions to 10% at 105 kbar. However, the reflectance changes exhibit wavelength dependence, being smaller for longer wavelengths, and are reversible upon pressure unloading. A phenomenological model was developed to calculate the complex refractive index for carbon disulfide. This model in conjunction with Fresnel’s reflection equations can be used to analyze the multiple-shock reflection and absorption data in a consistent manner. The experimental results can be understood in terms of the growth of absorption bands due to increasing overlap of the π electron wave functions of neighboring molecules due to compression; this overlap is likely a precursor to associative chemical reactions.

The complete reversal in reflectance at pressures above 90 kbar is in contrast to the absorption data and arises because the present reflection measurements do not sample the bulk material, but, instead, are collected from a very thin, cooled, unreacted layer of carbon disulfide. The double-shock experiments show no evidence of a chemical reaction below 90 kbar and are in agreement with multiple-shock data. At higher pressures, the double-shock experiments show evidence of chemical reactions and display a complicated reflectance history that depends markedly on the wavelength. The wavelength dependence and associated complexities in the double-shock experiments are consequences of pressure dependent changes in absorption bands, cooling due to heat conduction to the sapphire optical windows, and the temperature dependence of the reaction threshold pressure. The present work has provided a link between the absorption measurements obtained under multiple-shock loading and continuum measurements obtained under double-shock loading.
A study of the electronic structures of Pd−2 and Pd2 by photoelectron spectroscopy
Authors: Ho Joe , Ervin Kent M. , Polak Mark L. , Gilles Mary K. , Lineberger W. C.
Published: July 2002

The ultraviolet negative ion photoelectron spectrum of Pd−2 is presented for electron binding energies up to 3.35 eV. The anion is prepared by sputtering in a flowing afterglow ion source. Multiple low-lying electronic states of Pd2, all unidentified previously, are observed with resolved vibrational structure. The spectrum shows two strong electronic bands, each with similar vibrational progressions. Franck–Condon analyses are carried out on the two transitions and molecular constants are extracted for the anion and the two neutral electronic states. With the help of simple molecular orbital arguments and ab initio calculations, these two electronic bands are assigned as the triplet ground state (3Σ+u) and a singlet excited state (1Σ+u). The adiabatic electron affinity is E.A.(Pd2)=1.685±0.008 eV and the singlet excitation energy T0(1Σ+u) is 0.497±0.008 eV (4008±65 cm−1 ). The bonding in the palladium dimers is discussed and we find that the anion bond strength is 1.123±0.013 eV stronger than that of the neutral. Related studies of Pd− yield an improved electron affinity of E.A.(Pd)=0.562±0.005 eV.
A Monte Carlo simulation of the CO oxidation on probabilistic fractals
Authors: Casties A. , Mai J. , von Niessen W.
Published: July 2002

We study a model of the CO oxidation on percolation clusters which represent the surface of our system in the Monte Carlo simulation. We observe two phase transitions where the surface is completely covered (poisoned) by one species (in our system CO or O). These phase transitions are described by their order and the values of the mole fraction yCO of CO in the gas phase at y1 (O poisoning) and at y2 (CO poisoning). The interval (y1,y2) represents the reactive regime. The influence of the occupation probability p for generating a spanning cluster on the underlying square lattice, of the diffusion of CO and of the lattice size on the value and the character of the kinetic phase transitions is studied. Increasing p leads to a shift of y2 to larger values of yCO for all ratios of adsorption to diffusion events but the value of y1 is increased to a larger value of yCO only if diffusion is not allowed. In the case of diffusion the value of y1 is maximally independent of p. A change in the character of the phase transition at y2 from first order on regular lattices to second order is observed on percolation clusters (p≥pC=0.592 75). Here pC is the percolation threshold on the square lattice. The character becomes again first order if diffusion is allowed or if p exceeds 0.97. The observed effects may be understood as a result of the different ramification of the percolation clusters which strongly depends on p. The cluster structures are characterized by pair correlation functions. The lattice size shows for p near unity no significant effect on the values of y1 and y2 but its influence increases with decreasing p. We will show that the fractal dimension of the surface is not a useful quantity for predicting the positions and the character of the phase transitions for the reaction system studied here.
Vertical and adiabatical ionization potentials of MH−K+1 anions. Ab initio study of the structure and stability of hypervalent MHk+1 molecules
Authors: Boldyrev Alexander I. , Simons Jack
Published: July 2002

Vertical and adiabatic ionization potentials of the free MHk+1− anions, where M is a nontransition atom and k is the maximal formal valence of the atom M, have been studied by ab initio methods. Thermodynamic stabilities of the corresponding neutral hypervalent MHk+1 species have been calculated as well. LiH2−, BeH3−, MgH3−, BH4−, AlH4−, and SiH5− are very stable to loss of the extra electron and are stable geometrically at high symmetry structure, except for SiH5− which is not stable thermodynamically. LiH2, NaH2, BeH3, MgH3, and BH4 have minima at C2v (2B2) structures; however, only BH4 is thermodynamically stable to all possible dissociation asymptotes. The NaH2− anion is not adiabatically electronically stable.
Temperature and time dependencies of surface patterns in constrained ionic N-isopropylacrylamide gels
Authors: Li Chunfang , Hu Zhibing , Li Yong
Published: July 2002

Surface patterns in ionic N-isopropylacrylamide (N-IPA) gels have been investigated under external constraint. Hexagonal, grain, and bubble patterns have been observed at temperatures below, near, and above the phase transition temperature Tc. It is found that the behavior of these patterns depends not only on temperature, time, and external constraint, but also on the thermal path. The time dependence of the gel thickness has revealed that there is a plateau period and that the end of the plateau period corresponds to the onset of the bubble pattern. This observation is in agreement with previous measurements of free spherical N-IPA beads. The experiments show that each hexagonal cell evolves into a bubble for N-IPA gels, while a bubble can be present in the middle of a hexagonal cell for acrylamide gels. The mechanism for the shrinking patterns will be also discussed.
Calculated structure, heat of formation and decomposition energetics of 1,3-dinitro-1,3-diazacyclobutane
Authors: Grice M. Edward , Habibollahzadeh Dariush , Politzer Peter
Published: July 2002

Using HF/6-31G* optimized geometries, the heat of formation of 1,3-dinitro-1,3-diazacyclobutane and the energetics of two possible composition reactions were computed by a nonlocal density functional procedure (deMon). ΔHf° and 25 °C is 351 cal/g; the N–NO2 dissociation energy is 36.6 kcal/mole, and the activation barrier for symmetrical ring fragmentation is 44.2 kcal/mole.
The origin of β relaxation in organic glasses
Authors: Murthy S. S. N. , Sobhanadri J. , Gangasharan  , Sobhanadri J.
Published: July 2002

Using dielectric spectroscopy, some organic liquids consisting of mainly alkylhalides and acetic acid esters, have been studied over a wide frequency (106 to 10−3 Hz) and temperatures (down to 77 K). Measurements have also been made in the true liquid state at four microwave frequencies of 33, 22.4, 10, and 5.4 GHz in the temperature range 303–363 K. The high-temperature data reveals a significant contribution from the intramolecular (end- or side-group) rotation. The data corresponding to the supercooled state reveals a secondary (β) process in addition to the primary (α) process. The magnitude of the β process is found to be relatively large in systems which have greater contribution from the intramolecular process to the overall polarization at room temperature. The activation energies together with the side/end-group dependence of the magnitude of the β process suggest that the intramolecular process, though very much hindered during the process of supercooling, may survive to some extent in the glassy state to be realized as the β process. Our study suggests that the origin of the β process in these glasses may not be intermolecular in nature.
Photodissociation study on Mg+(H2O)n, n=1–5: Electronic structure and photoinduced intracluster reaction
Authors: Misaizu F. , Sanekata M. , Fuke K. , Iwata S.
Published: July 2002

Photodissociation spectra of Mg+(H2O)n (n=1–5) cluster ions were examined in the wavelength region from 720 to 250 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as 17 000 cm−1 with respect to the 2P–2S resonance line of the free Mg+ ion and were explained by the shift of this transition as a result of hydration. The spectra also exhibit clear evolution of solvation shell with the first shell closing at n=3, being consistent with the theoretical prediction. The mass spectra of the fragment ions show the existence of two dissociation processes: the evaporation of water molecules and the photoinduced intracluster reaction to produce the hydrated MgOH+ ion, MgOH+(H2O)m. The branching fraction between the two processes depends strongly on the solvent number n and also on the photolysis wavelength. The energetics and the dynamics of the dissociation processes were discussed in conjunction with the results of ab initio calculations.
Acceleration of intramolecular vibrational redistribution by methyl internal rotation. II. A comparison of m-fluorotoluene and p-fluorotoluene
Authors: Timbers Peter J. , Parmenter Charles S. , Moss David B.
Published: July 2002

Comparisons of rate constants for intramolecular vibrational redistribution (IVR) within the S1 manifolds of aromatic molecules with and without methyl internal rotation have been extended to meta-fluorotoluene (mFT). The IVR rate constant, as determined by the method of O2 fluorescence quenching (chemical timing), is 8.2×1011 s−1 for the zero-order level 31 with εvib≊1250 cm−1. This rate is approximately 10 times greater than that for the comparable level in para-fluorotoluene (pFT). The IVR rate in pFT in turn exceeds that for a comparable level in para-difluorobenzene (pDFB) by a factor of 40. We consider these differences using the Fermi golden rule formulation of the IVR rate constant. The enhanced value for mFT vs pFT may be a consequence of increased coupling strength between the internal rotation and molecular vibration plus a combination of factors that increase the coupled state density. This result is in contrast to the comparison between pFT and pDFB, where IVR acceleration is attributed primarily to the increased density of coupled states provided by introducing the internal rotation–vibration interaction.
Fine-structure dependence of predissociation linewidth in the Schumann–Runge bands of molecular oxygen
Authors: Lewis B. R. , Gibson S. T. , Dooley P. M.
Published: July 2002

Fine-structure-specific predissociation linewidths have been calculated for the v=0–18, N=0–40 levels of the B 3Σ−u state of 16O2 using a predissociation model which takes into account spin–orbit interactions between the B 3Σ−u state and the 1 1Πu, 1 3Πu, 1 5Πu, and 2 3Σ+u states, and spin-electronic and L-uncoupling interactions between the B 3Σ−u and 1 3Πu states. The model parameters were optimized by comparing the calculated widths with existing measurements of fine-structure linewidth for v=0, 14–18, and new determinations for v=1, 2, 5, 7, 9–13 which have been obtained from existing cross sections for the (v,0) and (v,1) Schumann–Runge bands using a fitting procedure which assumes unequal linewidths for the triplet fine-structure components. The L-uncoupling interaction is found to be important for all vibrational levels in determining the width ratios for the fine-structure components, even at quite low rotational levels. The calculated linewidths vary from 0.03 (for v=16, N=30, F1) to 4.8 cm−1 (for v=4, N=40, F3), and good agreement is found between the measurements and the model calculations for most vibrational levels. The calculations are expected to be useful in the construction of state-of-the-art models describing the photochemistry of the middle atmosphere. Greatly improved predissociation model parameters, in good agreement with ab initio calculations, have been obtained for the 3Πu and 1Πu states, while the 3Σ+u parameters remain the most uncertain. The model parameters resulting in the best fit to the widths also give good agreement with observed perturbations in the Schumann–Runge band origins. It is found that the actual separations between triplet fine-structure levels consistently exceed values predicted from generally accepted spectroscopic constants for the B 3Σ−u state, suggesting that a reevaluation of those constants may be in order.
Dielectric relaxation and structural study of aniline–methanol mixture using picosecond time domain reflectometry
Authors: Fattepur R. H. , Hosamani M. T. , Deshpande D. K. , Mehrotra S. C.
Published: July 2002

Complex permittivity of aniline–methanol mixture for various temperatures have been measured as a function of frequency between 10 MHz and 10 GHz using time domain reflectometry technique (TDR). Dielectric parameters viz. static dielectric constant εs, relaxation time τ were obtained from complex permittivity spectra using nonlinear least-square fit method. With these parameters, excess dielectric properties, Kirkwood correlation factor and thermodynamic parameters were determined and discussed to yield the information on the molecular structure and dynamics of the mixture. At low methanol concentration, strong interaction between the solute–solvent molecules was observed through hydrogen bonding. The values of ΔH* and Kirkwood correlation factors support the same. The dielectric constant of the mixture fits well with the modified Bruggman equation.
Breaking bonds with the state-selective multireference coupled-cluster method employing the single-reference formalism
Authors: Piecuch Piotr , Adamowicz Ludwik
Published: July 2002

The state-selective (SS) multireference coupled-cluster (CC) method exploiting the single-reference (SR) formalism [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)] is applied to BH and H2O at equilibrium and displaced geometries. Different selections of active spaces are considered. Comparison with the SR CC approaches involving single and double (CCSD), single, double, and triple (CCSDT) and single, double, triple, and quadruple (CCSDTQ) excitations, and with the full configuration interaction method, indicates remarkable stability and accuracy of the SS CC results for difficult bond breaking cases. For the first time, the ab initio SS CC calculations include the most complete SS CCSD(TQ) approximation, which emerges through selection of the most essential clusters appearing in the full SR CCSDTQ theory.
Molecular dynamics simulation of iron(III) and its hydrolysis products in aqueous solution
Authors: Rustad James R. , Hay Benjamin P. , Halley J. W.
Published: July 2002

A simple potential model is described which allows molecular dynamics simulations to be performed for ferric iron ions in dissociating aqueous solutions. The model was parametrized by fitting the polarizable dissociating water model of Halley et al. [J. Chem. Phys. 98, 4110 (1993)] to a single water molecule–ferric iron ion potential energy surface taken from the work of Curtiss et al. [J. Chem. Phys. 86, 2319 (1987)]. The model gives very good results for the structure of the solvated hexaaqua iron(III) complex; the proper coordination number of 6 was obtained when the Fe–O interaction was fit directly to the ab initio calculations without further modification. The model produces adequate results for the first hydrolysis constant, but breaks down for the second hydrolysis constant, which is overestimated by 18 kcal/mol.
Symmetric diblock copolymers under large amplitude oscillatory shear flow: Entanglement effect
Authors: Zhang Yuanming , Wiesner Ulrich
Published: July 2002

The effect of large amplitude oscillatory shear flow on the morphology of two nearly symmetric polystyrene–polyisoprene diblock copolymers of intermediate molecular weights has been studied. Using two dimensional SAXS experiments and electron microscopy micrographs the orientation of the lamellae after shear flow has been determined. It has been shown that under the appropriate experimental conditions an orientation component can be observed with the unit normal of the lamellae parallel to the flow which on the timescale of hours is not observed in low molecular weight samples. It has been proposed that this orientation is the result of topological constraints, i.e., entanglement effects, on the diblock copolymer dynamics, as evidenced by dynamic mechanical experiments on the samples before flow. With the help of mechanical data collected during the flow, the orientation dynamics has been studied. Using concepts from the tube model of entangled polymer melts and characteristic features of the chain conformational statistics of diblock copolymers, a mechanism for the formation of this orientation component is suggested.
A comparison of two approaches to perturbation triple excitation corrections to the coupled-cluster singles and doubles method for high-spin open-shell systems
Authors: Crawford T. Daniel , Schaefer Henry F.
Published: July 2002

An analysis of molecular properties is presented for several first-row diatomic molecules as determined at the CCSD(T) level of theory. In particular, a comparison of spectroscopic constants predicted by the ROHF-CCSD(T) methods of Scuseria vs Gauss et al. is given. Accurate determination of the values of equilibrium bond lengths, dissociation energies, harmonic vibrational frequencies, anharmonic constants, vibration–rotation coupling constants, and centrifugal distortion constants show only small differences between the two methods. In particular, we show that the average absolute differences between the two methods for the equilibrium bond length, harmonic vibrational frequency, and the dissociation energy are 0.000 09 Å, 0.8 cm−1 and 0.1 kcal/mol, respectively.
Properties of phosphorus compounds by density functional theory: CH3P species as a test case
Authors: Nguyen Minh Tho , Creve Steven , Vanquickenborne Luc G.
Published: July 2002

A comparison of different density functional theory (DFT) and molecular orbital (MO) methods for calculating molecular and energetic properties of low-coordinated phosphorus compounds is reported. While DFT methods include both Becke–Lee–Yang–Parr (BLYP and B3LYP) nonlocal functionals, MO methods involve second-order perturbation theory (MP2), quadratic configuration interaction [QCISD(T)], and coupled-cluster theory [CCSD(T)], in conjunction with the 6-31G(d,p), 6-311++G(3df,2p), and 6-311++G(3df,3pd) basis sets. Properties examined include geometrical parameters of the different CH3P equilibrium structures (phosphaethene, phosphinocarbene, methylphosphinidene, and a phosphacarbyne) and relevant transition structures for isomerisations and rearrangements in both the lowest-lying singlet and triplet states, vibrational wave numbers, relative energies, barrier heights, and singlet–triplet energy gaps. In addition, the heat of formation, ionization energy, and proton affinity of phosphaethene are also evaluated. Overall, the B3LYP method, when employed with a large basis set, yields energetic results comparable to the CCSD(T) results. Nevertheless, both DFT methods fail to predict the behavior of the addition/elimination reactions of the hydrogen atom in the triplet state.
Cubic response functions in the multiconfiguration self-consistent field approximation
Authors: Jonsson Dan , Norman Patrick , Ågren Hans
Published: July 2002

Cubic response functions and various related residues are derived for multiconfigurational self-consistent field reference states. Compact computable expressions are given which render the possibility to simultaneously consider several types of electric and magnetic response properties referring to ground and excited states and to consider multiphoton spectra between ground and excited states or between different excited states. The perturbational expression for the excited-ground state difference in a frequency dependent property can be identified from a double residue of the cubic response function, and from one single reference wave function the property of interest can in principle be given for the whole manifold of excited states. With the present theory calculations for excited state properties can thus be conceived in two ways; by cubic response calculations with the ground state as the reference state and by linear response calculations using the particular excited state as reference state. The two approaches are identical at the full configuration interaction limit, while at the Hartree–Fock level only the former is possible. The applications include calculations of frequency dependent hyperpolarizabilities and excited state polarizabilities of lithium hydride and carbon monoxide and correlated Cotton–Mouton constants for hydrogen fluoride. The results for LiH and CO indicate that the cubic response function approach is quite rewarding already at the self-consistent field level, giving excited state polarizabilities comparable to those obtained from excited state multiconfiguration self-consistent field calculations. At this level the cubic response function calculation scales as ordinary self-consistent field calculations with applicability to large systems. The role of the presently proposed method for quantum chemistry of excited states is briefly discussed.
Calculations of structure and IR-spectrum for small UF6 clusters
Authors: Beu T. A. , Onoe J. , Takeuchi K.
Published: July 2002

A new site–site intermolecular potential model for UF6, featuring exchange, dispersion, electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine UF6 cluster structures up to the hexamer and, by means of a second order line shift formalism, to calculate the corresponding IR-spectra in the region of the ν3 vibrational mode (at 627.724 cm−1). The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is found to be the resonant dipole–dipole coupling. The theoretical spectra are compared and interpreted against recent Fourier transform IR-spectroscopy measurements.