bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase diagram of water in the vicinity of the triple point
Authors: Tkachev Sergey N. , Nasimov Rashit M. , Kalinin Vladimir A.
Published: July 2002

The melting curves of ice VI and ice VII were measured using a diamond-anvil pressure cell up to P=2.8 GPa and T=400 K and their metastable extensions. We found the existence of the metastable extension of the phase boundary between ice VI and liquid water within the stable region of ice VII and confirmed the presence of the metastable extension of the phase boundary between ice VII and liquid water within the stable field of ice VI. We found that the curvature of the ice VI–ice VII phase boundary is more pronounced than previously reported. The complete interpretation of our results might be made on the basis of thermodynamics of liquids in small confining systems.
Hydrogen bond spectroscopy in the near infrared: Out-of-plane torsion and antigeared bend combination bands in (HF)2
Authors: Anderson David T. , Davis Scott , Nesbitt David J.
Published: July 2002

High-resolution near infrared spectra of the two ‘‘high’’ frequency intermolecular modes of (HF)2 have been characterized in HF-stretch excited states using a slit jet spectrometer. In the spectral region between 4280 and 4480 cm−1, four vibration–rotation–tunneling (VRT) bands are observed and assigned to tunneling pairs of the out-of-plane torsion (ν6) and antigeared bend (ν3) intermolecular modes, in combination with the hydrogen bond donor (ν2) and acceptor (ν1) high-frequency intramolecular HF stretches, respectively. Analysis of the jet-cooled, rotationally resolved spectra provide intermolecular frequencies, rotational constants, tunneling splittings, and predissociation rates for the ν3/ν6 intermolecular excited states. The relatively small changes in the hydrogen bond interconversion tunneling splitting with either ν3 or ν6 excitation indicate that neither intermolecular mode is strongly coupled to the tunneling coordinate. The high-resolution VRT linewidths reveal mode specific predissociation broadening sensitive predominantly to intramolecular excitation, but with significant additional effects due to low-frequency intermolecular excitation as well. The intermolecular vibrational frequencies in the combination states display a systematic dependence on intramolecular redshift that allows all four intermolecular fundamental frequencies to be extrapolated from the near-ir data. Agreement between full 6-D quantum calculations and experiment for the out-of-plane torsion (ν6) vibration is remarkably good (0.5%). However, significant discrepancies (≳10%) between theory and experiment are obtained for the antigeared bend (ν3), indicating the need for further refinement of the HF dimer potential surface. Finally, the observation of all four intermolecular modes allows zero-point contributions to the binding energy to be reliably estimated. The revised value for the binding energy, De=1580(35) cm−1, is slightly higher than semiempirical estimates but now in excellent agreement with recent high level ab initio calculations.
Random sequential adsorption of spheroidal particles: Kinetics and jamming limit
Authors: Adamczyk Zbigniew , Weroński Pawel/
Published: July 2002

Localized adsorption of hard (noninteracting) spheroidal particles on homogeneous interfaces was analyzed theoretically. In contrast to previous studies concentrated on flat (side on) adsorption in the present approach an unoriented (quasi-three-dimensional) adsorption of prolate and oblate spheroids was considered. By applying the random sequential adsorption (RSA) approach asymptotic analytic expressions were derived for the available surface function (surface blocking parameter) and adsorption kinetics in the limit of low and moderate surface concentrations. The range of validity of the approximate analytical results was determined by numerical simulations of adsorption kinetics performed using the Monte Carlo RSA technique. It was revealed by this comparison that the analytical approximation can be used with a good accuracy for the dimensionless adsorption time τ smaller than two. The numerical calculations also enabled us to determine the maximum (jamming) surface concentrations for unoriented adsorption of spheroids as a function of the elongation or flattening parameter A. It was demonstrated that these jamming concentrations θ∞ are approached for long adsorption times as τ−1/4, therefore deviating considerably from the Langmuir model used often in the literature.
Far-wing excitation study of the reactions in the Hg–H2 collisional quasimolecules. II. Rovibrational distributions of HgH, HgD (X 2Σ+) formed from Hg–H2, D2, HD
Authors: Ohmori K. , Takahashi T. , Chiba H. , Saito K. , Nakamura T. , Okunishi M. , Ueda K. , Sato Y.
Published: July 2002

We have applied the laser-pump/probe and double-beam absorption/dispersion approaches to the far wings of the Hg 3P1–1S0 resonance line broadened by collisions with H2, D2, and HD. Absolute reduced absorption coefficients of the Hg–D2 quasimolecules have been determined as a function of the wave-number shift Δ from the resonance-line center both in the red and blue wings. The nascent rotational distributions have been determined for the v=0 and 1 levels of HgH (X 2Σ+) and the v=0 level of HgD (X 2Σ+) formed from the Hg*(3P1)–H2, D2, and HD collisional–quasimolecular states Ã and B̃ attained by the red- and blue-wing excitation, respectively. Both of the intermediate states Ã and B̃ give quite similar rotational distributions peaking around N≂18 for HgH and N≂25 for HgD insensitive to the excitation-wave-number shift Δ. However, a small difference is found: the red-wing excitation gives larger populations in the low-N levels than the blue-wing one. The departing atom isotope effect is observed in these low-N populations of HgD from Hg–D2 and Hg–HD. The absolute ratio of the nascent yields of v=1 to 0 has been measured to be 0.3, being nearly constant against Δ in both the red and blue wings. These observations indicate that HgH is formed predominantly from a bent H–Hg–H configuration on both the pathways via the Ã and B̃ states. The different type of transition state, however, may be encountered on the pathways producing the minor components in the low-N levels.
The theta condition for linear polymer chains in continuous space and three dimensions
Authors: Yong C. W. , Clarke Julian H. R. , Freire Juan J. , Bishop Marvin
Published: July 2002

Configurational properties of isolated flexible linear chains of lengths in the range 72⩽N⩽576 have been investigated close to the theta regime by continuous space simulations using a configurational bias Monte Carlo algorithm. The polymer model consisted of beads interacting through a nontruncated Lennard-Jones potential and connected by a Gaussian distribution of link vectors. We use two criteria in order to characterize departures from ideal behavior at finite N, first the ratio of the mean squared end-to-end distance and the mean squared radius of gyration, and second the end-to-end vector distribution. Both the criteria lead, within the statistical errors, to the same prediction for the theta temperature as N→∞; the ratio criterion gives kBTθrat/ε=4.167±0.035 and the distribution criterion gives kBTθdis/ε=4.184±0.035, in close agreement with previous estimates for the same model. Deviations from ideal behavior were found to be independent of whether the finite number of beads constitute a whole chain or merely an inner part of a much larger chain. The N-dependencies of several configurational properties including the moments of the end-to-end distribution and the asphericity have been examined at the N→∞ theta temperature. Nonideal effects are manifest in different ways dependent on the property being considered. For instance the radius of gyration shows a slight contraction effect which diminishes with increasing chain length and which shows remarkable quantitative agreement with the prediction of tricritical renormalization group theory, while the end-to-end distribution shows slight expansion effects. It is suggested that each property emphasizes nonideal effects in slightly different ways.
A finitely extensible network strand model with nonlinear backbone forces and entanglement kinetics
Authors: Geurts Kevin R. , Wedgewood Lewis E.
Published: July 2002

In an earlier paper, a nonaffine network model of polymer melts was presented in which the rotation caused by shearing as well as the extension of the test strand are hindered by interactions with the network itself. In that work, it was shown that such a strand motion leads to qualitatively correct steady shear and elongational material properties, even though the strand disentanglement rate was constant and the strand force law was linear. These simplifications were accepted in order to emphasize the effects of the strand motion on material properties. In this paper, however, we show that these idealizations cause the model to fail in the start-up of shearing flow because no overshoot is seen in the shear stress growth function. To address this failure, the finitely extensible nonlinear elastic (FENE) network model is introduced in which the FENE force law replaces the Hookean force law used in the earlier finitely extensible network strand (FENS) model. Also, a nonlinear expression for the kinetics of strand disentanglement replaces the assumption of a constant rate of disentanglement. Material properties for the FENE network model are generated by stochastic simulation. The simulation results show that these modifications produce overshoot in the shear stress growth function and result in a more consistent description of finite network strand extensibility.
Satellite hole investigation of energy transfer between two different dyes
Authors: Kuo Ching-Tung , Chang Ta-Chau
Published: July 2002

Site-dependent hole filling of satellite holes has been studied by the excitation of 9-amino-6-chloro-2-methoxyacridine to fill the primary zero-phonon hole and satellite holes of 4,4-difluoro-5-(2-thienyl)-4-bora-3a,4a-diaza-s-indacene-3-propionic acid, succinimidyl ester. The results show that intermolecular energy transfer can trigger the filling of the primary holes. More important, two different filling rates are observed for four different satellite holes. The filling rate is more rapid on the red side than on the blue side of the absorption band.
Epitaxy and chainlength dependent strain in self-assembled monolayers
Authors: Fenter P. , Eberhardt A. , Liang K. S. , Eisenberger P.
Published: July 2002

We use grazing incidence x-ray diffraction to systematically study the structure of an archetypal self-assembled monolayer as a function of the hydrocarbon chain length, n. The monolayers consists of n-alkyl thiol molecules, CH3(CH2)n−1SH (Cn, 10⩽n⩽30), self-assembled on single crystal Au(111) surfaces. At room temperature, the 2D structure is described by a C(4×2) unit mesh for all chain lengths. However, we demonstrate that there is a systematic dependence of the tilt structure (i.e., the tilt angle and tilt direction) of the hydrocarbon chains as a function of the chain length. Furthermore, we show that the monolayer structures are characterized by distinct “long’’ (n⩾16) and “short’’ (n⩽14) chain length regimes, as well as a smooth variation of the structural parameters within each regime. We associate these systematic structural changes with the conflicting requirements of epitaxy and molecular packing, and argue that the driving force is the changing intra-layer interaction strength (which is proportional to hydrocarbon chain length). We believe that these phenomena should be characteristic of the behavior of self-assembled monolayers, as well as the more general class of “soft/hard’’ interfaces.
The 3A2, 1A2, 3B2, and 1B2 electronic states of CH2: Small bond angle states
Authors: Yamaguchi Yukio , Schaefer III Henry F.
Published: July 2002

Molecular structures with very small bond angles are a curiosity in chemistry. The two triplet (3A2 and 3B2) and two singlet (1A2 and 1B2) excited states of CH2 have been investigated systematically using ab initio electronic structure theory. For these four states total energies and physical properties including geometries, dipole moments, harmonic vibrational frequencies, and associated infrared intensities were determined with the single and double excitation configuration interaction (CISD) method using four different basis sets. It is confirmed in this study that the four states of CH2 all have bent structures with longer CH bond lengths and smaller bond angles than the four lower-lying (X̃, ã, b̃, and c̃) states of CH2. At the CISD optimized geometries single point energies were determined with complete active space self-consistent-field (CASSCF) and CASSCF second-order configuration interaction (SOCI) levels of theory. For the triplet excited states single point energies were also determined employing coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations methods. At the CISD level with the largest basis set, the triple zeta plus triple polarizations with two sets of higher angular momentum and two sets of diffuse functions basis set [TZ3P(2 f,2d)+2diff], the bond angles were predicted to be 40.6° (3A2), 46.1° (1A2), 76.3° (3B2), and 81.3° (1B2), while the dipole moments were determined to be 2.35 (3A2), 2.26 (1A2), 1.69 (3B2), and 1.60 debye (1B2), respectively. With the most accurate method in this study, the CASSCF-SOCI level with the TZ3P(2 f,2d)+2diff basis set, the energy separations (Te value) between the ground state (X̃ 3B1) and the four excited states were predicted to be 73.7 kcal/mol (3.20 eV, 25 800 cm−1) for the 3A2 state, 96.8 kcal/mol (4.20 eV, 33 800 cm−1) for the 1A2 state, 151.0 kcal/mol (6.55 eV, 52 800 cm−1) for the 3B2 state, and 182.5 kcal/mol (7.91 eV, 63 800 cm−1) for the 1B2 state, respectively.
Tricritical behavior of an off-lattice flexible polymer model: Monte Carlo calculation of second virial coefficients
Authors: Rubio Ana M. , Freire Juan J.
Published: July 2002

Simulation data of second virial coefficients have been obtained for an off-lattice model of linear polymer chains by means of Monte Carlo method with different values of the variable that describes intramolecular attractions between units. The data have allowed us to study the theta, or tricritical, region where binary interactions cancel out. These Monte Carlo results are quantitatively compared with numerical predictions from the tricritical renormalization group theory. With this purpose, we have employed the same values for the three-body and the cutoff theoretical parameters that were previously used to reproduce the mean size data.
Solvation and solid state effects on the structure and energetics of the tautomers of creatinine
Authors: Craw J. Simon , Greatbanks Stephen P. , Hillier Ian H. , Harrison Martin J. , Burton Neil A.
Published: July 2002

The tautomeric equilibrium involving the amine and imine forms of creatinine are studied theoretically in both the gas phase, the aqueous environment, and the solid state. High level ab initio calculations predict the imine form to be preferred in the gas phase by 2.0 kcal mol−1. In the aqueous environment, both continuum and explicit solvent models predict the amine form to be the most stable, with solute polarization being an important contributing factor. An embedding procedure is used to model solid state effects when again the amine is preferred, in agreement with experimental crystallographic studies. The predicted solid state molecular geometry is compared to the experimental structure.
Accurate numerical determination of Kohn-Sham potentials from electronic densities: I. Two-electron systems
Authors: Mura Michael E. , Knowles Peter J. , Reynolds Christopher A.
Published: July 2002

The extraction of the true Kohn-Sham exchange-correlation potential from near-exact electronic densities is demonstrated, with particular reference to the two-electron systems He and H2. It is shown that if the reference density is obtained from a standard configuration interaction calculation, very large orbital basis sets are required to obtain convergence in the resulting exchange-correlation potential. The correlation contribution to molecular bonding is analysed in terms of contributions from the potential in different regions of real space.
Coupled cluster prediction of vibrational band intensities for SiF2 and PF2+
Authors: Pak Youngshang , Woods R. Claude , Peterson Kirk A.
Published: July 2002

The electric dipole moment functions for SiF2 and PF2+ have been calculated at the CCSD(T)/aug-cc-pVTZ level. Using our previously obtained CCSD(T)/cc-pVQZ potential energy functions for SiF2 and PF2+, full three-dimensional variational calculations were carried out to obtain vibrational wave functions and band origins for SiF2 and PF2+. The CCSD(T)/aug-cc-pVTZ electric dipole moment functions and vibrational wave functions obtained from the variational calculations were then used to calculate the dipole moment matrix elements of various bands for SiF2 and PF2+.
Shear behavior of squalane and tetracosane under extreme confinement. I. Model, simulation method, and interfacial slip
Authors: Gupta S. A. , Cochran H. D. , Cummings P. T.
Published: July 2002

In this three part study, nonequilibrium molecular dynamics simulation of the rheology of confined films is used to explore the microscopic properties and response of model lubricants under shear. The rheological behavior of two alkanes that differ in molecular structural complexity is examined: tetracosane (C24H50), which is a linear alkane, and squalane (C30H62), which has six symmetrically placed methyl branches along a 24 carbon backbone. The model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. Shear flow is generated by moving the walls at constant velocity, and various properties are calculated after attainment of steady state. Heat generated by viscous dissipation is removed by thermostatting the first two atoms of the tethered molecules at 300 K, which allows a temperature profile to develop across the width of the lubricant layer. This paper details the molecular model and simulation method, and examines interfacial slip at the interface between the tethered chains and the fluid alkane. The effects of various parameters on the slip behavior are presented. Two subsequent papers respectively address the structural features of these liquid alkanes under shear flow and compare the viscosities from independent calculations of the bulk and confined fluids.
Dipole moments and hyperfine interactions in scandium monosulfide, ScS
Authors: Steimle T. C. , Marr A. J. , Goodridge D. M.
Published: July 2002

A supersonic molecular beam of scandium monosulfide (ScS), produced using a laser ablation/reaction source, was interrogated by laser induced fluorescence. The fine and magnetic hyperfine interactions in the (0,0) B 2Σ+–X 2Σ+ band system (origin=12 940.402 cm−1) were analyzed. The magnitude of the Stark effect was measured for the (RR24+RQ24)(0) and (PP13+PQ23)(1) branch features, giving dipole moments of μ(B 2Σ+)=5.60±0.04 D and μ(X 2Σ+)=5.64±0.04 D. The spectroscopic parameters and dipole moments are used to develop a molecular orbital bonding model and are compared with the isovalent molecules ScO and ScNH.
Temperature dependence of the rate constant for the Cl−+CH3Br reaction down to 23 K
Authors: Le Garrec Jean-Luc , Rowe B. R. , Queffelec J. L. , Mitchell J. B. A. , Clary David C.
Published: July 2002

Rate constants measured for the Cl−+CH3Br→CH3Cl+Br− reaction increase by over two orders of magnitude when the temperature is reduced from 300 to 23 K. Calculations done with the rotating bond approximation provide a simple explanation of this unusually strong negative temperature dependence of the rate constant.
Pulsed discharge nozzle Fourier transform microwave spectroscopy of the propargyl radical (H2CCCH)
Authors: Tanaka Keiichi , Sumiyoshi Yoshihiro , Ohshima Yasuhiro , Endo Yasuki , Kawaguchi Kentarou
Published: July 2002

The rotational spectrum of the propargyl radical (H2CCCH) was recorded with Fourier-trans-form microwave (FTMW) spectrometer coupled with a pulsed discharge nozzle (PDN). Fine and hyperfine components belonging to the 101−000, 202−101, 212−111, and 211−101 rota-tional transitions in the 2B1 ground vibronic state were observed. The analysis of the FTMW spectra resulted in the following ground-state molecular constants, with 3σ uncertainties given in parentheses. The rotational and centrifugal distortion constants are B0=9523.6775(60) MHz, C0=9206.8805(60) MHz, ΔN=3.44(63) kHz, and ΔNK=0.3753(28) MHz. The spin–rotation coupling constants are εaa=−529.386(60), εbb=−11.524(30), and εcc=−0.520(32) MHz. The hyperfine coupling constants are aF=−36.323(24), Taa=17.400(24), and Tbb=−17.220(37) MHz for the acetylenic (CH) proton, and aF=−54.21(11) and Taa=−14.121(19) MHz for the methylenic (CH2) proton. The derived hyperfine coupling constants indicate compar-able contributions of the allenyl (H2C=C=CH) and propargyl (H2C–C≡CH) forms to the resonance structure [H2Ċ–C≡CH↔H2C=C=ĊH].
Phase stability estimated for adhesive charged sphere fluids with an analytical solution
Authors: Kaneko Tetsuo
Published: July 2002

Phase separation, which can occur in a fluid mixture system (FMS) containing adhesive charged hard spheres (ACHSs) by the addition of a salt of pointlike ionic species, can be enhanced by increasing the absolute value of the charge z0e on the ACHS. Phase stability in the FMS can be revealed using an analytical solution based on the mean spherical approximation for the Ornstein–Zernike equation and can behave nonlinearly. For such a phenomenon, the effect given by the potential of mean force plays an important role. When the adhesive interaction between ACHSs is weak in an FMS containing ACHSs of low charge and low density, an increase in |z0e| can stabilize the dispersed state of the FMS. When each of the ACHSs reaches a highly charged state, the increase in |z0e| can enhance phase separation. When the adhesive interaction between ACHSs is strong in an FMS containing ACHSs of low charge, the stability of the dispersed state depends nonlinearly on the volume fraction φ of ACHSs. The clusters of the ACHSs resulting from phase separation can completely dissolve when φ reaches a certain value as φ increases.
On the second electronic state near the S1 state in dimethyldiazirine
Authors: Kim Taek-Soo , Kyu Kim Sang , Choi Young S. , Kwak Ilhwan
Published: July 2002

The fluorescence excitation (FE) spectra of the S1−S0 (1B2-1A1) transition of dimethyldiazirine have been obtained both in supersonic jet expansions and in a static cell at room temperature. Comparison of the jet and room-temperature FE spectra near the origin band demonstrates that the band, which is located 126 cm−1 red from the S1 origin and was previously assigned to the origin of another excited state, is really a hot band of the S1−S0 (1B2-1A1) transition. This observation implies that the existence of the so-called “second” excited electronic state, which allegedly has been observed within a few hundred wave numbers from the S1 origins of various diazirines, is questionable.
Energetics of adsorption and coadsorption of CN and O on Ni(100)
Authors: Yang Hong , Whitten Jerry L.
Published: July 2002

The adsorption of cyanide (CN) and the coadsorption of CN+O on Ni(100) are treated using an ab initio embedding theory. The Ni(100) surface is modeled as a three-layer, 30-atom cluster with the Ni atoms fixed at bulk lattice sites. The present calculations show that CN is able to bind to the surface either via the carbon, or nitrogen, or in a side-on geometry with very small differences in total energy, ⩽2 kcal/mol. Adsorption energies at fourfold, bridge and atop sites are comparable. For N-bonded CN, the adsorption energy is 114 kcal/mol at the most favorable (fourfold) site. For C-bonded CN, the adsorption energy is 115 kcal/mol at the most favorable (atop) site. For the side-on bonded CN, the calculated adsorption energy is 113 kcal/mol. Although CN is strongly bound to the surface, the molecule is nearly free to rotate to other geometries over very small energy barriers, ⩽2 kcal/mol. Dipole moment calculations show that the bonding of CN to the Ni surface is largely ionic. On coadsorption of CN+O on Ni(100), the adsorbed atomic oxygen tends to block CN adsorption at the nearby surface sites. With O coadsorbed at a fourfold site, the adsorption of CN at the next nearest bridge or on-top sites are energetically more favorable than adsorption at the adjacent fourfold site, and the C–N bond is perpendicular to the surface. It is predicted that the reaction of O+CN→OCN is exothermic and the formation of surface OCN is due to the formation of C–O bond by the interaction between the adsorbed O and the C atom of adsorbed CN.
A reanalysis of the k3Π state of CO
Authors: Berden Giel , Jongma Rienk T. , van der Zande Dorien , Meijer Gerard
Published: July 2002

The k3Π state of the CO molecule is investigated in the region between 91 000 and 97 000 cm−1 via 1+1 resonance enhanced multiphoton ionization spectroscopy on CO molecules prepared in a single quantum level of the a3Π(v=1) state. A new vibronic band is found which is at lower energy than the vibrational ground state reported in the literature, leading to a reassignment of the vibrational numbering of the k3Π state. The rotationally resolved spectra of the k3Π (v=0–6)←a3Π(v=1, J=1, Ω=1) of CO12 and CO13 have been observed and analyzed, confirming the new vibrational labeling and providing a full set of molecular constants of the k3 Π valence state.
Grand canonical Brownian dynamics simulation of colloidal adsorption
Authors: Oberholzer Matthew R. , Wagner Norman J. , Lenhoff Abraham M.
Published: July 2002

A dynamic simulation of colloidal adsorption has been developed to probe the effects of colloidal interactions on the kinetics and extent of adsorption. The simulation accounts for diffusion by Brownian dynamics to a homogeneous planar adsorption surface from a region of constant chemical potential. A grand canonical Monte Carlo routine is used periodically to re-equilibrate this region. Particle motion in the plane of the surface is subject to either unrestricted diffusion or zero diffusion. Deryaguin-Landau-Verwey-Overbeek pair potentials are used to characterize both particle–particle and particle–surface interactions. The pair potential parameters were chosen to mimic (separately) polystyrene latex microspheres and small globular proteins, two classes of charged colloidal particles for which experimental adsorption data exist. The simulation qualitatively captures the variation in adsorptive capacity with ionic strength distinct to each system: fractional coverage increases for polystyrene latex adsorption but decreases for protein adsorption with increasing salt concentration. In the former, strong lateral repulsion between adsorbed particles appears to govern the extent of adsorption, whereas in the latter, the extent of adsorption is more strongly affected by the screening of the weak attraction between the particle and the surface. Excellent quantitative predictions for polystyrene latex adsorption with and without surface diffusion are obtained without adjustable parameters.
Diagrammatic complete active space perturbation theory
Authors: Finley James P.
Published: July 2002

A second-order perturbation theory is formulated, called diagrammatic complete active space perturbation theory (diagrammatic CASPT2), that is based on multireference perturbation theory (MRPT) with one- and two-body energy denominator shifts. The energy denominator shifts are chosen so that they avoid the influence of intruder states by enlarging potentially problematic energy denominators and define the wave operator to be similar to the one used in the state specific formalisms, like CASPT2. The wave operator for this method is more efficient to compute than the state specific formalisms, but is expected to yield similar results. An approximation is suggested that eliminates the need to diagonalize the effective Hamiltonian.
High-resolution infrared spectrum and energy levels of the weakly bound complex, CO–paraH2
Authors: McKellar A. R. W.
Published: July 2002

The infrared spectrum of the weakly bound complex CO–paraH2, which accompanies the fundamental band of CO in the 4.7 μm region, has been recorded at high resolution (0.005 cm−1) using a long-path (200 m), low-temperature (48 K), absorption cell. A total of 135 transitions, constituting almost all of the observed lines, has been rotationally assigned in terms of 44 discrete quantum states of the complex, and the positions of these energy levels have been determined to high accuracy (<0.001 cm−1) for both the vCO=0 and 1 vibrational states. The binding energy of the complex, relative to the zero-point level, was determined to be about 22 cm−1. Predicted microwave and millimeter wave frequencies are given for the pure rotational spectrum of CO–paraH2. The energy level pattern derived here provides the information needed to make precise determinations of the CO–H2 intermolecular potential energy surface in the attractive region. However, determination of the dependence of this surface on the orientation of H2 within the complex will have to await the analysis of the more complicated spectrum of CO–orthoH2.
Experimental and theoretical studies of the gas-phase protonation of orthophosphoric acid
Authors: Gevrey S. , Luna A. , Haldys V. , Tortajada J. , Morizur J-P.
Published: July 2002

The gas-phase proton affinity (PA) and the gas-phase basicity (GB) of orthophosphoric acid (H3PO4) have been determined by the kinetic method to be 833 and 800 kJ/mol, respectively. High level ab initio calculations at the G2(MP2) level of theory give a PA of 827 kJ/mol. A good agreement is observed between the G2(MP2) and density functional theory (using B3LYP) results. B3LYP/6-311+G(2df,p) calculations and chemical ionization (CI) experiments have been performed in order to clarify the loss of water from protonated H3PO4. The oxo–oxygen atom is confirmed as the most basic site, the hydroxy–oxygen protonated isomer O=P(OH)2(OH2)+ being predicted to be 126 kJ/mol less stable than P(OH)4+. The isomerization barrier connecting both isomers is calculated as 200 kJ/mol and the dissociation products, water and protonated metaphosphoric acid O=P(OH)2+, are found to be 284 kJ/mol with regard to P(OH)4+. The proton affinity of metaphosphoric acid (HPO3) is also evaluated to be 712 kJ/mol at the G2(MP2) level of theory. In addition, a proton-bound complex [HO(O)PO⋯H⋯OH2]+ has been located as an intermediate for the elimination of HPO3 or H2O. The elimination of HPO3, which is not observed in CI experiments, was found to be more endothermic than the loss of water by 23 kJ/mol.
Instability of a nematic phase described by the Maier–Saupe model
Authors: Riccardi Carmen C. , Borrajo Julio , Williams Roberto J. J.
Published: July 2002

The stability of a nematic phase in binary mixtures of liquid crystals (LC) with non-nematogenic solvents, was analyzed using the combined Flory–Huggins and Maier–Saupe models. Stability was described using general criteria developed by Dorgan [Liq. Crys. 10, 347 (1991)], applied to the g(φ1,s) function describing the free energy density of the system (φ1=volume fraction of LC, s=orientational order parameter). Equivalent criteria were obtained by replacing s by the particular s(φ1) function that minimizes g and making d2g[φ1,s(φ1)]/dφ12>0, to define stability. Recent results reported by Shen and Kyu [J. Chem. Phys. 102, 556 (1995)] were reanalyzed using general stability criteria. It was found that the spinodal curve for the nematic phase extended beyond the limit fixed by these authors. In particular, the spinodal enters the region where the isotropic phase is the stable one, in agreement with other calculations reported by Dorgan for a nematic phase described by a modified Flory–Ronca model. A pure LC may exist in a metastable state in a small temperature range located above TNI (equilibrium temperature of the nematic to isotropic transition of the pure LC).
Monte Carlo simulations of the interlamellar spacing in model n-alkane crystals
Authors: McGann Mark R. , Lacks Daniel J.
Published: July 2002

Monte Carlo simulations are carried out on a simple one-dimensional model of n-alkane crystals to investigate the chain length dependence of the interlamellar spacing, which has implications with regard to the Raman spectra of n-alkane crystals. The results of these simulations show no significant chain length dependence for the interlamellar spacing. A quasiharmonic analysis reveals that the absence of a chain-length dependence is due to a cancellation of the effects of the intrachain vibrations, which act to increase the interlamellar spacing for longer chain lengths, by the effects of interchain vibrations, which act to decrease the interlamellar spacing for longer chain lengths.
The adsorption of small hydrocarbons on Cu(111): A combined He-atom scattering and x-ray absorption study for ethane, ethylene, and acetylene
Authors: Fuhrmann D. , Wacker D. , Weiss K. , Hermann K. , Witko M. , Wöll Ch.
Published: July 2002

Ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) adsorbed on Cu (111) are investigated using high-resolution helium atom scattering and x-ray absorption spectroscopy (NEXAFS). For C2H6/Cu(111) and C2H4/Cu(111) the excitation energies of the frustrated molecular translation normal to the surface (FTz) amount to 6.7 meV, suggesting the presence of a physisorbed species which is consistent with the NEXAFS data for ethylene. In contrast, for C2H2/Cu(111) the NEXAFS data indicate strong intramolecular distortions of the acetylene adsorbate compatible with a tilt of both CH ends away from the molecular axis. While the latter finding is in agreement with recent theoretical studies the theoretically predicted chemisorbed ethylene species could not be observed by the experiment. However, more detailed theoretical studies of the ethylene-Cu(111) interaction potential reveal two minima separated by an activation barrier. The minimum closer to the surface refers to strongly distorted chemisorbed C2H4 whereas the outer minimum is characterized by a free molecule-like physisorbed species. Thus the results from the present measurements are explained by the theoretically confirmed physisorbed species while chemisorbed C2H4 has to be excluded. Complementary results for ethylene and acetylene adsorbed on Pb(111) reveal a FTz-mode energy of 6.5 and 6.7 meV, respectively, thus revealing a much weaker acetylene–substrate binding than seen for Cu(111). Also in case of Pb(111) the FTz-mode showed an Einstein-like behavior with a flat dispersion curve, as for corresponding modes on the Cu(111)-substrate, see above.
The thermodynamics of nitrogen adsorption on nickel clusters: Ni19–Ni71
Authors: Parks E. K. , Nieman G. C. , Kerns K. P. , Riley S. J.
Published: July 2002

Equilibrium constants for the chemisorption reactions of molecular nitrogen with nickel clusters Nin have been determined as a function of temperature for n=19 to 71. Van’t Hoff analysis of the data yields standard-state changes in reaction enthalpy and entropy. These changes are related to what is known about nickel cluster structure and the nature of the cluster–N2 interaction. In general, the adsorption energy is highest for the smallest clusters studied, reaching values twice those for N2 adsorption on bulk nickel surfaces. In many cases, there is a correlation between enthalpy and entropy: high adsorption energy is accompanied by a large change in entropy, and vice versa. These effects are discussed in terms of the configurational entropy of reaction and the frequencies of the frustrated translational and rotational motions of the adsorbed N2 molecules.
On the time-dependent Lagrangian approach in quantum chemistry
Authors: Pedersen Thomas Bondo , Koch Henrik
Published: July 2002

We formulate the time-dependent variational principle in the form of the Euler–Lagrange equations, and demonstrate that standard variational as well as nonvariational wave functions may be obtained from these. We also demonstrate how inherently real expectation values of Hermitian operators can be constructed for nonvariational wave functions by using the time-dependent Hellmann–Feynman theorem which, in turn, is a simple consequence of the Euler–Lagrange equations. The procedure is illustrated by derivation of time-dependent Hartree–Fock and of time-dependent coupled cluster theory. Finally we give the fundamental equations for molecular dynamics within semiclassical electron nuclear dynamics (END) with a classical description of the nuclei and coupled cluster description of the electrons.
Dynamics of glass-forming liquids. IV. True activated behavior above 2 GHz in the dielectric α-relaxation of organic liquids
Authors: Hansen C. , Stickel F. , Richert R. , Fischer E. W.
Published: July 2002

We have measured the dielectric relaxation of butylbenzene and of the glass-former propylbenzene in the frequency range 10−2 Hz to 2×1010 Hz in order to characterize the variation of relaxation times with temperature for these low loss liquids. Additionally, salol has been remeasured above 1 GHz with improved resolution. Using the sensitive data representation [−dlog10(fmaxHz)/d(1/T)]−1/2 vs 1/T we find demarcation temperatures TA, at which the temperature dependence changes from a Vogel–Fulcher type law within the limits TB⩽T⩽TA to Arrhenius behavior for T>TA, corresponding to a position of the loss peak fmax>2 GHz. The activation energies derived from dielectric relaxation data for T>TA are associated with the energy of vaporization, Eη∝ΔEvap. A comparison of dielectric relaxation times τD to viscosity data in this wide range of temperatures suggests the relation τD∝η/T rather than τD∝η.
An energy function for dynamics simulations of polypeptides in torsion angle space
Authors: Sartori F. , Melchers B. , Böttcher H. , Knapp E. W.
Published: July 2002

Conventional simulation techniques to model the dynamics of proteins in atomic detail are restricted to short time scales. A simplified molecular description, in which high frequency motions with small amplitudes are ignored, can overcome this problem. In this protein model only the backbone dihedrals φ and ψ and the χi of the side chains serve as degrees of freedom. Bond angles and lengths are fixed at ideal geometry values provided by the standard molecular dynamics (MD) energy function CHARMM. In this work a Monte Carlo (MC) algorithm is used, whose elementary moves employ cooperative rotations in a small window of consecutive amide planes, leaving the polypeptide conformation outside of this window invariant. A single of these window MC moves generates local conformational changes only. But, the application of many such moves at different parts of the polypeptide backbone leads to global conformational changes. To account for the lack of flexibility in the protein model employed, the energy function used to evaluate conformational energies is split into sequentially neighbored and sequentially distant contributions. The sequentially neighbored part is represented by an effective (φ,ψ)-torsion potential. It is derived from MD simulations of a flexible model dipeptide using a conventional MD energy function. To avoid exaggeration of hydrogen bonding strengths, the electrostatic interactions involving hydrogen atoms are scaled down at short distances. With these adjustments of the energy function, the rigid polypeptide model exhibits the same equilibrium distributions as obtained by conventional MD simulation with a fully flexible molecular model. Also, the same temperature dependence of the stability and build-up of α helices of 18-alanine as found in MD simulations is observed using the adapted energy function for MC simulations. Analyses of transition frequencies demonstrate that also dynamical aspects of MD trajectories are faithfully reproduced. Finally, it is demonstrated that even for high temperature unfolded polypeptides the MC simulation is more efficient by a factor of 10 than conventional MD simulations.
Deriving the isothermal–isobaric ensemble: The requirement of a “shell” molecule and applicability to small systems
Authors: Corti David S. , Soto-Campos Gerardo
Published: July 2002

The isothermal–isobaric ensemble is derived from the canonical ensemble instead of the customary microcanonical ensemble. By explicitly accounting for interactions between the system and the external bath, an isothermal–isobaric partition function is obtained that is more general than a previous derivation and is better suited to study small systems. The ensemble requires a “wall” or “shell” molecule to identify and count nonredundantly all the volumes and associated configurations of the system, verifying the volume scale obtained by Koper and Reiss [J. Phys. Chem. 100, 422 (1996)] (under the condition of minimal interaction between the bath and system for which it is valid). In addition, the failure of information theory to arrive at the proper volume scale (or “shell” molecule) for the ensemble (again when interaction between the bath and system is ignored) is demonstrated.
c←X
laser excitation spectrum of the Hg–Ar vdW complex
Authors: Amano K. , Ohmori K. , Kurosawa T. , Chiba H. , Okunishi M. , Ueda K. , Sato Y. , Devdariani A. Z. , Nikitin E. E.
Published: July 2002

We have measured the c(v′=0−8)–X(v″=0) laser excitation spectrum of the Hg–Ar van der Waals complex. This corresponds to the first observation of the bound Hg–rare-gas complex in the dark c state. In this measurement, we employ the sequence of two laser pulses; the complex is excited by the first one to the dark bound level in the c state and then successively excited by the second one to another bright state for the optical detection. The Birge–Sponer (BS) plot of the v′ progression is well fitted by a straight line, suggesting that the c-state potential curve of Hg–Ar is well approximated by the Morse function. The linear fitting of this BS plot gives the potential parameters for the c state.
Singularity-free interpretation of the thermodynamics of supercooled water. II. Thermal and volumetric behavior
Authors: Rebelo Luis Paulo N. , Debenedetti Pablo G. , Sastry Srikanth
Published: July 2002

According to the singularity-free interpretation of the thermodynamics of supercooled water, the isothermal compressibility, isobaric heat capacity, and the magnitude of the thermal expansion coefficient increase sharply upon supercooling, but remain finite. No phase transition or critical point occurs at low temperatures. Instead, there is a pronounced but continuous increase in volume and a corresponding decrease in entropy at low temperatures, the sharpness of which becomes more pronounced the lower the temperature and the higher the pressure. We investigate the behavior of the response functions, equation of state, and entropy of a schematic waterlike model that exhibits singularity-free behavior, and thereby illustrate the simplest thermodynamically consistent interpretation that is in accord with existing experimental evidence on water’s low-temperature anomalies. In spite of its simplicity, the model captures many nontrivial aspects of water’s thermodynamics semiquantitatively.
A new algorithm for Reverse Monte Carlo simulations
Authors: da Silva Fernando Luı́s B. , Svensson Bo , Åkesson Torbjörn , Jönsson Bo
Published: July 2002

We present a new algorithm for Reverse Monte Carlo (RMC) simulations of liquids. During the simulations, we calculate energy, excess chemical potentials, bond-angle distributions and three-body correlations. This allows us to test the quality and physical meaning of RMC-generated results and its limitations. It also indicates the possibility to explore orientational correlations from simple scattering experiments. The new technique has been applied to bulk hard-sphere and Lennard-Jones systems and compared to standard Metropolis Monte Carlo results.
Cooperative effects in the structuring of fluoride water clusters: Ab initio hybrid quantum mechanical/molecular mechanical model incorporating polarizable fluctuating charge solvent
Authors: Bryce Richard A. , Vincent Mark A. , Malcolm Nathaniel O. J. , Hillier Ian H. , Burton Neil A.
Published: July 2002

A new hybrid quantum mechanical/molecular mechanical model of solvation is developed and used to describe the structure and dynamics of small fluoride/water clusters, using an ab initio wave function to model the ion and a fluctuating charge potential to model the waters. Appropriate parameters for the water–water and fluoride–water interactions are derived, with the fluoride anion being described by density functional theory and a large Gaussian basis. The role of solvent polarization in determining the structure and energetics of F(H2O)4− clusters is investigated, predicting a slightly greater stability of the interior compared to the surface structure, in agreement with ab initio studies. An extended Lagrangian treatment of the polarizable water, in which the water atomic charges fluctuate dynamically, is used to study the dynamics of F(H2O)4− cluster. A simulation using a fixed solvent charge distribution indicates principally interior, solvated states for the cluster. However, a preponderance of trisolvated configurations is observed using the polarizable model at 300 K, which involves only three direct fluoride–water hydrogen bonds. Ab initio calculations confirm this trisolvated species as a thermally accessible state at room temperature, in addition to the tetrasolvated interior and surface structures. Extension of this polarizable water model to fluoride clusters with five and six waters gave less satisfactory agreement with experimental energies and with ab initio geometries. However, our results do suggest that a quantitative model of solvent polarization is fundamental for an accurate understanding of the properties of anionic water clusters.
The internal axis system of molecules with one large amplitude internal motion
Authors: Szalay Viktor , Ortigoso Juan
Published: July 2002

An old puzzle of theoretical molecular spectroscopy, the derivation of the internal axis system (IAS) of molecules with one large amplitude internal motion and asymmetric top and asymmetric frame has been solved by a simple and transparent method. The basic idea has been to consider the large-amplitude-coordinate-dependent IAS as the result of quantum mechanical evolution in ρ of an initial system of axes, where ρ denotes the coordinate describing the large amplitude motion. A special choice of the operator governing ρ evolution guarantees that the ρ evolved system is the IAS. Floquet theory provides both a simple analytical expression for the evolution matrix and a means to efficiently calculate this matrix. Numerical examples have revealed fast convergence of the method to the IAS.
Mass-resolved two-photon and photoelectron spectra of Xe2 in the Xe(4f) region above the first molecular ionization limit
Authors: Hu X. K. , Mao D. M. , Shi Y. J. , Dimov S. S. , Lipson R. H.
Published: July 2002

Two-photon resonance enhanced multiphoton ionization (REMPI), time-of-flight (TOF) mass spectrometry, and photoelectron spectroscopy have been used to investigate the gerade Xe2 Rydberg states located just above the first molecular ionization limit. Effective vibrational analyses are presented for five dimer states that dissociate to Xe+Xe*(4f ). The electronic symmetries of the band systems in this region were deduced from separate REMPI spectra recorded with linearly and circularly polarized laser light while excited state bond lengths were estimated from Franck–Condon intensity simulations. Two photoelectron signals were measured for each molecular resonance examined in this work. The strongest one, having a near zero kinetic energy, is attributed to dimer electrons released through autoionization. The weaker photoelectron signals corresponding to faster electrons originate from excited state Xe atoms formed by predissociation.