A refined potential energy surface and the rovibrational states for the electronic ground state of ozone

A potential energy surface for the electronic ground state of ozone has been optimized by using a variational procedure with the exact vibrational Hamiltonian in bond length-bond angle coordinates. In the optimization, the ab initio force field of Borowski, P., Andersson, K., Malmquist, P.-A., and Roos, B. O., 1992, J. chem. Phys., 97, 5568 is taken as the starting point, and the recent observed vibrational band origins up to 4900 cm^-1 reported by Floud, J.-M., Barbe, A., Camy-Peyret, C., and Plateaux, J. J., 1996, J. molec. Spectrosc., 177, 34 are involved. The root mean square error of this fit for the 39 observed vibrational energy levels is 0.83 cm^-1. In order to test the refined potential, the rovibrational energy levels up to J = 15 are calculated and compared with the observed values.     

A revised many-body potential energy function for the description of the H3O+(H2O)n clusters

A revised potential energy function that has been fitted to the latest set of Kebarle and coworkers [1982, J. Am. chem. Soc, 104, 1462] entropy and enthalpy measurements at T = 300 K is presented. The model assumes a rigid hydronium unit and accounts for all orders of many-body interactions explicitly. The difference with the older function that had been based on earlier measurements by Kebarle and co-workers [1972, J. Am. chem. Soc, 94, 7627; 1967, J. Am. chem. Soc, 89, 6393] is that more compact clusters are generated. We have studied the structural properties of water clusters in the size range 5–80 at T = 250 K within the framework of the (μPT) Grand Canonical ensemble. Clusters with sizes less than about 10 water molecules consist of a four-coordinated first shell, where the fourth water molecule is hydrogen bonded to the oxygen atom of the hydronium ion. The hydration number goes through a minimum value ～1.6, for a cluster size around 50, and it starts increasing again with further cluster growth, to ～2.5 for a cluster size of 250 water molecules, which is the largest cluster examined. On the other hand the water molecule coordination number shows a monotonic increase with cluster size. In small clusters, less than 10, water molecules prefer to be arranged in a chain-like fashion; at sizes around 50, tri-coordinated clathrate-like structures dominate whereas with further size increase the coordination number eventually levels off to the experimental bulk value, at 4.6.     

Constrained reaction coordinate dynamics for systems with constraints

In the context of molecular dynamics simulations of rare events, the application of constraints on a suitable reaction coordinate has often been found useful for sampling of the free energy barrier. The efficiency of these calculations is hampered by geometrical difficulties, related to the metric factor and inertial forces. Some years ago Mulders et al. [1996, J. chem. Phys., 104, 48691 suggested a way to simplify the approach. Their idea was demonstrated shortly afterwards by Sprik and Ciccotti [1998, J. chem. Phys., 109, 77371. The present paper extends these results to vector reaction coordinate and molecular systems modelled with holonomic constraints.     

Cooperative binding of drugs on human serum albumin

In order to explain the adsorption isotherms of the amphiphilic penicillins nafcillin and cloxacillin onto human serum albumin (HSA), a cooperative multilayer adsorption model is introduced, combining the Brunauer–Emmet–Teller (BET) adsorption isotherm with an amphiphilic ionic adsorbate, whose chemical potential is derived from Guggenheim's theory. The non-cooperative model has been previously proved to qualitatively predict the measured adsorption maxima of these drugs [Varela, L. M., García, M., Pérez-Rodríguez, M., Taboada, P., Ruso, J. M., and Mosquera, V., 2001, J. chem. Phys., 114, 7682]. The surface interactions among adsorbed drug molecules are modelled in a mean-field fashion, so the chemical potential of the adsorbate is assumed to include a term proportional to the surface coverage, the constant of proportionality being the lateral interaction energy between bound molecules. The interaction energies obtained from the empirical binding isotherms are of the order of tenths of the thermal energy, therefore suggesting the principal role of van der Waals forces in the binding process.     

Assignment of a perturbation in the FT-ICLAS spectrum of 12C2H2 around 12 709.5 cm−1

The vibrational state perturbing the J = 17 and 18 rotational states of the zero-order v ₁ + 3v ₃ state of ¹²C₂H₂ is assigned to the state with vibrational energy predicted at G _ν = 12 685.1 cm^?1 using the cluster model (El Idrissi, M. I., Liévin, J., Campargue, A. and Herman, M., 1999, J. chem. Phys., 110, 2074). The assignment is discussed also in terms of the very special pressure shift behaviour demonstrated previously for absorption lines reaching these levels (Herregodts, F., Hepp, M., Hurtmans, D., Vander Auwera, J. and Herman, M., 1999, J. chem. Phys., 111, 7961). The experimental information arising from a set-up newly running at ULB, called FT-ICLAS brings decisive information in the assignment process. This set-up is described briefly.     

Transport and relaxation properties of isotopomeric hydrogen–helium binary mixtures. II. HD,D2, T2–He mixtures

The comprehensive comparison between calculated bulk non-equilibrium properties of hydrogen–helium isotopomeric mixtures and experiment that has previously been carried out for H₂–helium mixtures [2004, Molec. Phys., submitted] has been extended to mixtures of HD, D₂ and T₂ with ³He and ⁴He. For HD–⁴He mixtures, comparison is also made, where possible, with previous calculations of Köhler and Schaefer [1983, Physica A, 120, 185]. The phenomena examined herein include low temperature interaction second virial coefficients, binary diffusion and thermal conductivity coefficients, rotational relaxation, transport property field effects and flow birefringence. Scattering calculations have been carried out for the HD–He PES of Schaefer and Köhler [1985, Physica A, 129, 469], and for both the Köhler–Schaefer and Tao [1994, J. chem. Phys., 100, 4947] potential surfaces for the D₂–He and T₂–He interactions. Comparisons between calculated and experimental results for HD, D₂, T₂–He mixtures confirm the conclusion, reached earlier from the H₂–He comparisons, that these potential surfaces are very close to the correct one for the hydrogen–helium interaction, and that the small differences between them cannot be distinguished readily by measurements of bulk gas phenomena unless the attendant experimental uncertainties are better than ±0.3%.     

Three-body interaction-induced light scattering in krypton gas: a computer simulation of the spectral line shapes

The two-, three- and four-body effective collision induced scattering spectral line shapes are calculated for dense gaseous krypton using the pairwise additivity (PA) approximation and different polarizability models. These spectra and several interaction induced spectra calculated at various densities are compared with the experimental measurements of Barocchi et al. [1988, Europhys. Lett., 5, 607]. The potential effect on the spectrum is found to be weak. The results obtained with the Meinander et al. [1986, J. chem. Phys., 84, 3005] empirical polarizability model and molecular dynamics fit well the experimental two- and three-body spectral shapes. The irreducible contribution to the spectral shape is evaluated using the dipole induced dipole irreducible polarizability [buckingham, A. D., and Hands, I. D., 1991, Chem. Phys. Lett., 185, 544]. This contribution is found to be relatively weak for the anisotropic spectra in the frequency and density range studied, explaining the good agreement between the pairwise approximation calculations and the experimental data. The spectra radiated by the quasi-molecules Kr₂, Kr₃, and Kr₄ (the total spectrum within the PA approximation) are also simulated.     

New accurate binary hard sphere mixture radial distribution functions at contact and a new equation of state

New and very accurate formulae for additive binary hard sphere (HS) mixture radial distribution functions (RDFs) at contact are proposed in a simple analytical form. Using the virial theorem, the formulae also provide a new HS mixture equation of state (EOS). The new RDF formulae are the most accurate currently available. The new EOS is of comparable accuracy with that of Malijevsky, A., and Veverka, J. (1999, Phys. Chem. chem. Phys., 1, 4267), which is the most accurate HS mixture EOS currently available. However, the new EOS proposed here is of much simpler analytical form.     

Simulation of hydrogen adsorption in carbon nanotubes

Computer simulations are reported of hydrogen adsorption in multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). The gas-solid interaction was modelled both as pure dispersion forces and also with a hypothetical model for chemisorption introduced in a previous paper (CRACKNELL, R., F., 2001, Phys. Chem. chem. Phys., 3, 2091). A two-centre model for hydrogen was employed and the grand canonical Monte Carlo methodology was used throughout. Uptake of hydrogen in the internal space of a carbon nanotube is predicted to be lower than in the optimal graphitic nanofibre with slitlike pores (provided the gas-solid potential is consistent). Part of the difference arises from the assumption of pore surface area used in converting the raw simulation data to gravimetric adsorption; however, the majority of the differences can be attributed to the curvature of the pore. This reduces the uptake of hydrogen (on a gravimetric basis) in spite of deepening the potential minimum inside the pore associated with dispersion forces. It is concluded that for the uptake of hydrogen in SWNTs of 5–10% reported by Heben (DILLON, A. C., JONES, K. M., BEKKEDAHL, T. A., KIANG, C. H., BETHUNE, D. S., AND HEBEN, M. J., 1997, Nature, 386, 377), gas-solid forces other than dispersion forces are required and most of the adsorption must occur in the interstices between SWNTs.     

An approximate formula for the intermolecular Pauli repulsion between closed shell molecules

The exchange repulsion formula proposed by Murrell and co-workers (Proc. Roy. Soc. (Lond.) 1965, A284, 566; J. chem. Phys., 1967, 47, 4916) is considered in detail. Potentially important terms missing in the formalism of Murrell and co-workers are identified and evaluated for the water dimer using several basis sets. Insights into the contributing terms are obtained by using localized molecular orbitals. The results point towards a relatively simple expression for intermolecular exchange repulsion, based on the isolated wavefunctions of the two overlapping species.     

Phase coexistence and interface structure of a two-component Lennard-Jones fluid in porous media: application of Born—Green—Yvon equation

The Born-Green-Yvon equation with smoothed density approximation is used to calculate the liquid-liquid density profiles of a symmetric Lennard-Jones fluid in a hard sphere disordered matrix. The phase diagrams are evaluated for model systems characterized by different matrix densities and compared with the results of theoretical predictions and the Monte Carlo simulations of Gordon, P. A., and Glandt, E. D., 1996, J. chem. Phys., 105, 4257. It was found that increasing the matrix packing fraction reduces the magnitude of the miscibility gap and smooths the density profiles between two coexisting phases.     

Rotational spectra and internal dynamics of Ne—H2S

Rotational spectra of 15 isotopomers of the Ne-H₂S van der Waals complex were measured in the frequency range 4–22 GHz using a pulsed molecular beam Fourier transform spectrometer. Two K = 0 progressions were observed for each of the symmetric isotopomers (with H₂S or D₂S). This doubling is attributed to an internal rotation motion of the H₂S subunit within the complex. These two states can be correlated with the 0₀₀ and 1₀₁ rotational states of free H₂S and D₂S. By contrast, symmetry constraints no longer apply to isotopomers with DHS. The excited internal rotor state is no longer metastable, and only one K = 0 progression could be observed. The rotational constants obtained were compared with those of Ar-H₂S and Ar—H₂O. The ground state rotational constant remained almost constant upon substitution of H with D, showing an unusual isotope effect, similarly to a previous observation in Ar-H₂S (GUTOWSKY, H. S., EMILSSON, T., and ARUNAN, E., 1997, J. chem. Phys., 106, 5309). This behaviour is in agreement with the ab initio study by OLIVEIRA, G. D., and DYKSTRA, C. E., 1999, J. chem. Phys., 110, 289. An approximate substitution analysis was carried out to deduce structural information from the ground state rotational constants. Nuclear quadrupole hyperfine structures were observed and resolved or partially resolved for isotopomers containing ³³S and D, respectively, and the corresponding nuclear quadrupole coupling constants were determined. These were used to derive information about the internal dynamics of the dimer. Different sensitivities of the quadrupole coupling constants of D and ³³S to the extent of out-of-plane motion were revealed.     

Evidence of maximum in the melting curve of hydrogen at megabar pressures

Hydrogen at high pressures of ∼400 GPa might be in a zero-temperature liquid ground state (N. Ashcroft, J. Phys.: Condens. Matter A 12, 129 (2000), E. G. Brovrnan et al., Sov. Phys. JETP 35, 783 (1972)). If metallic hydrogen is liquid, the melting T _melt(P) line should possess a maximum. Here we report on the experimental evaluation of the melting curve of hydrogen in the megabar pressure range. The melting curve of hydrogen has been shown to reach a maximum with T _melt = 1050 ± 60 K at P = 106 GPa and the melting temperature of hydrogen decreases at higher pressures so that T _melt = 880 ± 50 K at P = 146 GPa. The data were acquired with the aid of a laser heating technique where diamond anvils were not deteriorated by the hot hydrogen. Our experimental observations are in agreement with the theoretical prediction of unusual behavior of the melted hydrogen [S. Bonev et al., Nature 481, 669 (2004)]. The article is published in the original.     

An accurate,global, ab initio potential energy surface for the H+ 3 molecule

A new global, ground-state, Born-Oppenheimer surface is presented for the H⁺ ₃ system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45 000 cm^?1 above the H⁺ ₃ minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753).     

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne₂ ⁺, Ar₂ ⁺ and Kr₂ ⁺. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar₂ ⁺, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne₂ ⁺ suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne₂ ⁺ and Ar₂ ⁺ to possess a double-well potential and that of Kr₂ ⁺ to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr₂ ⁺ and Xe₂ ⁺ dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar₂ ⁺ does not.     

The equilibrium geometry of the SC3H radical: an ab initio study

The SC₃H radical is known by experiment to have a linear equilibrium structure, but even rather high-level ab initio computations give a bent equilibrium geometry. A theoretical study of the SCCCH radical has been carried out in order to analyse the influence of several factors in the computed equilibrium structure. Quadratic configuration interaction QCISD(T) and restricted coupled cluster RCCSD(T) computations have been performed in combination with large basis sets. Spin-orbit effects have been taken into account through the Breit-Pauli Hamiltonian using multi-configuration SCF and configuration interaction wavefunctions. Our final results indicate that the equilibrium structure must be linear, in agreement with the experimental studies [McCarthy, M. C., Vrtilek, J. M., Gottlieb, C. A., Wang, W., and Thaddeus, P., 1994, Astrophys. J., 431, L127; Hirahara, Y., Ohshima, Y, and Endo, Y, 1994, J. chem. Phys., 101, 7342]. Both spin-orbit and electron correlation effects appear to be of comparable importance, but an adequate computation of the correlation energy has been much more difficult and has ultimately required basis set extrapolations.     

A study of Bi–Pb–Sn–Cd–Sb penta-alloys rapidly quenched from melt

Optical microscopy, X-ray diffractometry, the double bridge method, the Vickers microhardness testing and dynamic resonance techniques have been used to investigate structure, electrical resistivity, hardness, internal friction and elastic modulus of quenched Bi–Pb–Sn–Cd–Sb penta-alloys. The properties of these penta-alloys are greatly affected by rapid quenching. The intermetallic compound χ(Pb–Bi) or Bi₃Pb₇ is obtained after rapid quenching using the melt-spinning technique, and this is in agreement with reports by other authors [Marshall, T. J., Mott, G. T. and Grieverson, M. H. (1975). Br. J. Radiol., 48, 924; Kamal, M., El-Bediwi, A. B. and Karman, M. B. (1998). Structure, mechanical properties and electrical resistivity of rapidly solidified Pb–Sn–Cd and Pb–Bi–Sn–Cd alloys. J. Mater. Sci.: Mater. Electron., 9, 425; Borromêe-Gautier, C., Giessen, B. C. and Grrant, N. J. (1968). J. Chem. Phys., 48, 1905; Moon, K.-W., Boettinger, W. J., Kattner, U. R., Handwerker, C. A. and Lee, D.-J. (2001). The effect of Pb contamination on the solidification behavior of Sn–Bi solders. J. Electron. Mater., 30, 45.]. The quenched Bi_43.5Pb_44.5Cd₅Sn₂Sb₅ alloy has important properties for safety devices in fire detection and extinguishing systems.     

A quasiharmonic model calculation for non-markovian far-infrared spectra of HCl in Kr and Xe liquids

The far-infrared spectra (0–200 cm^?1) of dilute solutions of HCl in liquid Kr and Xe have been calculated by applying of a non-markovian spectral theory called PTOC (partial time ordering cumulants) and by using a quasiharmonic model for the liquid structure recently reported by us (A. Calvo Hernández et al., 1987, J. chem. Phys., 86, 4597) and successfully applied to HCl?Ar solution (Ibid., 86, 4607). The bath correlation functions which appear in the spectral theory involve a reduced set of parameters regarding both the HCl?Kr and HCl?Xe interactions and the liquid structure. From comparison between theoretical and experimental spectra it is possible to deduce quantitative values for the above parameters. The pronounced rotational fine structure of the experimental spectra for HCl in Kr and Xe liquids is quite well explained by the actual model.