Isotropic Intermolecular Potential Research Articles

Spectral lineshapes of collision-induced absorption (CIA) using isotropic intermolecular potential for N2-CH4

Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures are computed for gaseous mixtures of molecular nitrogen and methane using theoretical values for the induced dipole moments and intermolecular potential as input. Comparison with theoretical absorption spectra shows satisfactory agreement. An empirical model of the dipole moment which reproduces the experimental spectra and the first three spectral moments more closely than the fundamental theory, is also presented. Good agreement between computed and experimental absorption lineshapes is obtained when a potential model which is constructed from the thermophysical and transport properties is used.

Visualizing the Geometry of Hydrogen Dimers.

The simplest molecular dimer, H2-H2, poses a challenge to both experiment and theory as a system with a multidimensional energy surface that supports only a single weakly bound quantum state. Here, we provide a direct experimental image of the structure of hydrogen dimers [(H2)2, H2-D2, and (D2)2] obtained via femtosecond laser-induced Coulomb explosion imaging. Our results indicate that hydrogen dimers are not restricted to a particular geometry but rather occur as a mixture of all possible configurations. The measured intermolecular distance distributions were used to deduce the isotropic intermolecular potential as well as the binding energies of the dimers.

Multi-property isotropic intermolecular potentials and predicted spectral lineshapes of collision-induced absorption (CIA), collision-induced light scattering (CILS) and collision-induced hyper-Rayleigh scattering (CIHR) for H2[sbnd]Ne, −Kr and −Xe

Quantum mechanical lineshapes of collision-induced absorption (CIA), collision-induced light scattering (CILS) and collision-induced hyper-Rayleigh scattering (CIHR) at room temperature (295 K) are computed for gaseous mixtures of molecular hydrogen with neon, krypton and xenon. The induced spectra are detected using theoretical values for induced dipole moment, pair-polarizability trace and anisotropy, hyper-polarizability and updated intermolecular potentials. Good agreement is observed for all spectra when the literature and the present potentials which are constructed from the transport and thermo-physical properties are used.

An Isotropic Empirical Intermolecular Potential for Solid H2 and D2: A Classical Molecular Calculation**Supported by the National Natural Science Foundation of China under Grant No U1430109.

We develop an isotropic empirical potential for molecular hydrogen (H2) and deuterium (D2) by fitting to solidstate data, which is appropriate for classical molecular dynamics (CMD) approach. Based on the prior isotropic intermolecular potential used in self-consistent phonon approximation, a zero-point energy term and an embedded energy term are introduced to describe the H2–H2 and D2–D2 interactions in CMD simulations. The structure, cohesive energy and elastic properties of solid H2 (D2) are used as the fitting database. The present method is tested by calculating the melting point of solid H2, and the pressure and bulk elastic modulus as a function of volume. The developed potentials well reproduce many properties of solid H2 and D2.

Spectral lineshapes of collision-induced absorption (CIA) using isotropic intermolecular potential for mixtures of molecular hydrogen with helium

Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures for the collisional complex of a hydrogen molecule and a helium atom are computed using the numerical results of the induced dipole moment and isotropic intermolecular potential as input. Comparison with measured spectra and first three spectral moments shows good agreement over the rototranslational band in the far infrared. The quality of the present potentials have been checked by comparing between calculated and experimental thermo-physical and transport properties, which are found to be in good agreement.

Spectral line shapes of collision-induced light scattering (CILS) and collision-induced absorption (CIA) using isotropic intermolecular potential for H2–Ar

ABSTRACTQuantum mechanical line shapes of collision-induced light scattering at room temperature (295 K) and collision-induced absorption at T = 195 K are computed for gaseous mixtures of molecular hydrogen and argon using theoretical values for pair-polarisability trace and anisotropy and induced dipole moments as input. Comparison with other theoretical spectra of isotropic and anisotropic light scattering and measured spectra of absorption shows satisfactory agreement, for which the uncertainty in measurement of its spectral moments is seen to be large. Ab initio models of the trace and anisotropy polarisability which reproduce the recent spectra of scattering are given. Empirical model of the dipole moment which reproduce the experimental spectra and the first three spectral moments more closely than the fundamental theory are also given. Good agreement between computed and/or experimental line shapes of both absorption and scattering is obtained when the potential model which is constructed from the transport and thermo-physical properties is used.

Quantum spectral rototranslational collision-induced absorption (CIA) in CO2 and CO2–Rg pairs (Rg = He, Ar and Xe): An insightful analysis based on new empirical multi-property isotropic intermolecular potentials

The rototranslational collision-induced absorption (CIA) of carbon dioxide CO2 and of carbon dioxide with inert gas mixtures CO2-He, CO2-Ar and CO2-Xe at different temperature are analyzed in terms of new isotropic intermolecular potentials and multipole-induced dipole function models, using quantum spectral lineshape computations. The irreducible spherical form for the induced operator of light absorption mechanisms was determined. The quality of the present potentials have been checked by comparing between calculated and experimental thermo-physical and transport properties over a wide temperature range. Quite a good agreement is observed for all carbon dioxide noble gas mixtures.

Thermophysical properties of CF4/O2 and SF6/O2 gas mixtures

Fitting formulae are presented for the calculation of the second interaction virial coefficients, mixture viscosities and binary diffusion coefficients for CF4/O2 and SF6/O2 gas mixtures in the temperature range between 200 K and 1000 K. The data recommended are obtained from the isotropic (n-6) Lennard-Jones intermolecular interaction potentials of the pure substances by using the Hohm-Zarkova-Damyanova mixing rules. In general, a good agreement is observed between our results and the experimental and theoretical data found in the literature.

Dipole–octupole polarisability of uranium hexafluoride and theoretical prediction of anisotropic light-scattering spectrum using new intermolecular potential

ABSTRACTThe higher order dipole–octupole polarisability of uranium hexafluoride has been determined from anisotropic collision-induced light scattering (CILS) of gaseous UF6. A new isotropic intermolecular potential was proposed. The CILS spectra were analysed using present and literature intermolecular potentials. Our estimates for the dipole–octupole polarisability is |E| = 507.3 a.u., which is in a good agreement with our new ab initio value and literature theoretical estimates. Our best theoretical values of (hyper)polarisabilities calculated at the CCSD(T) level of theory are as follows: α = 54.63 a.u., E = −555.3 a.u., C = 595.1 a.u., and γ = 2103 a.u.

Spectral lineshapes of collision-induced absorption (CIA) and collision-induced light scattering (CILS) for molecular nitrogen using isotropic intermolecular potential. New insights and perspectives

The rototranslational collision-induced absorption (CIA) at different temperatures and collision-induced light scattering (CILS) at room temperature of nitrogen gas are analyzed in terms of new isotropic intermolecular potential, multipole-induced dipole functions and interaction-induced pair polarizability models, using quantum spectral lineshape computations. The irreducible spherical form for the induced operator of light scattering mechanisms was determined. The high frequency wings are discussed in terms of the collision-induced rotational Rayleigh effect and estimates for the dipole–octopole polarizability E4, is obtained and checked with the ab initio theoretical value. The quality of the present potential has been checked by comparing between calculated and experimental thermo-physical and transport properties over a wide temperature range, which are found to be in good agreement.

Measurements and calculations of He-broadening and -shifting parameters of the water vapor transitions of the ν 1 + ν 2 + ν 3 band

The water vapor line broadening and shift in the ν 1 + ν 2 + ν 3 band induced by helium pressure were measured using a Bruker IFS 125HR FTIR spectrometer. The measurements were performed at room temperature at a spectral resolution of 0.01 cm−1 and in a wide He pressure range. The shifting coefficients δ are mainly positive, in contrast to the line center shift induced by other mono-atomic gases. Calculations of the coefficients γ and δ were performed in the framework of the semi-classical method. The influence of the rotational dependence of the isotropic intermolecular potential and accidental resonances in the H2O molecule on the shifting coefficients is discussed. Calculated values of line profile parameters are in a good agreement with observed parameters.

Rototranslational collision-induced absorption and collision-induced light scattering spectra of molecular hydrogen using isotropic intermolecular potentials

Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures and of collision-induced light scattering (CILS) at room temperature are computed for gaseous molecular hydrogen using theoretical values for induced dipole moments and pair-polarizability trace and anisotropy as input. Comparison with measured spectra of absorption, isotropic and anisotropic light scattering shows satisfactory agreement, for which the uncertainty in measurement of its spectral moments is seen to be large. Empirical models of the dipole moment and pair-polarizability trace and anisotropy which reproduce the experimental spectra and the first three spectral moments more closely than the fundamental theory are also given. Good agreement between computed and experimental lineshapes of both absorption and scattering is obtained when potential models which are constructed from the thermophysical, transport, total scattering cross-section and spectroscopic properties are used. Also, the use of the new potential in lattice dynamic calculations yields good results for several properties of solid hydrogen.

Calculation and analytical representation of self-pressure and air pressure broadening coefficients of ozone spectral lines

The coefficients γ of broadening by self-pressure, and pressure of nitrogen, oxygen, and air are calculated for absorption lines of the rotational band and for the ν2 band of the ozone molecule for temperatures 296, 252, and 212 K. The calculations are performed by the semiclassical method using rectilinear and exact trajectories for interacting molecules. It is shown that the experimental data obtained for the two bands at T = 296 K can be reconstructed better using different isotropic intermolecular interaction potentials. The experimental and calculated broadening coefficients of ozone absorption lines for the rotational band and for the ν2 and ν1 + ν3 vibrational bands were used to determine the parameters of an analytical model, which permits one to calculate γ in a wide range of rotational quantum numbers, 0 ≤ J ≤ 45, 0 ≤ Ka ≤ 20, and temperatures of 200–296 K.

Helium-induced halfwidths and line shifts of water vapor transitions of the ν 1 + ν 2 and ν 2 + ν 3 bands

The collisional broadening (γ) and shift (δ) coefficients of more than 100 absorption rovibrational lines of the ν 1 + ν 2 and ν 2 + ν 3 bands of the water vapor molecule in a H2O–He mixture were measured with the help of a Fourier-transform spectrometer over a large range of helium pressures. The shift coefficients δ are mainly positive and have an unusual rotational dependence, in contrast to the line center shift induced by other mono-atomic gases. The calculations of coefficients γ and δ are performed in the framework of the semi-classical method. The rotational dependence of these shift coefficients is explained by the rotational dependence of the isotropic intermolecular potential. The influence of accidental resonances in the H2O molecule is also discussed.

Infrared-active vibron bands associated with rare gas atom dopants isolated in solid parahydrogen

We report high-resolution infrared absorption spectroscopic studies of the dopant-induced Q1(0) vibron band in solid parahydrogen crystals doped with low concentrations of rare gas atoms. The frequency, line shape, and integrated absorption coefficient for the rare gas atom-induced Q1(0) vibron band are measured for Ne, Ar, Kr, and Xe. The observed line shapes and peak maxima frequencies are sensitive to the H2 vibrational dependence of the dopant–H2 isotropic intermolecular potential. Trends observed for Ar, Kr, and Xe indicate that the vibrational dependence is strong enough for Xe to trap the infrared-active vibron in its first solvation shell, while for Ar the vibron remains delocalized. The Ne-induced feature displays a qualitatively different line shape which is attributed to the weak intramolecular vibrational dependence of the Ne–H2 intermolecular potential relative to the H2–H2 interaction. The line shapes of the Ar, Kr, and Xe dopant-induced Q1(0) pure vibrational features agree well with recent first-principles calculations.

Multiple extrema in the intermolecular potential and the phase diagram of protein solutions

Recent experiments have revealed several surprising features of the phase equilibria in protein solutions: liquid-liquid phase separation which is, in some cases, metastable with respect to the liquid-solid equilibrium, and in others-unobservable; widely varying crystallization enthalpies, including completely athermal crystallization; the co-existence of several crystalline polymorphs; and others. Other studies have shown that the solvent molecules at the hydrophobic and polar patches on the protein molecular surfaces are structured, introducing repulsive forces at surface separations equal to several water molecule sizes. In search of a causal link between the latter and former findings, we apply Monte Carlo simulation techniques in the investigation of phase diagrams associated with globular biological molecules in solution. We account for the solvent structuring via short-range isotropic two-body intermolecular potentials exhibiting multiple extrema. We show that the introduction of a repulsive maximum or a secondary attractive minimum at separations longer than the primary attractive minimum has dramatic effects on the phase diagram: liquid-liquid separation curves are driven to lower or higher temperatures, the sensitivity of the solubility curve (liquidus) to temperature, i.e., the enthalpy of crystallization, is significantly reduced or enhanced, metastable liquid-liquid separation may become stable and vice versa, and both low- and high-density crystalline phases are observed. The similarity of these features of the simulated phase behavior to those observed experimentally suggests that at least some of the mysteries of the protein phase equilibria may be due to the structuring of the solvent around the protein molecular surfaces. Another conclusion is that at least some of the dense liquids seen in protein solutions may be stable and not metastable with respect to a solid phase.

Theoretical calculations for line-broadening and pressure-shifting in the fundamental and first two overtone bands of CO–H 2

The line broadening and shift coefficients for CO–H 2 in the fundamental and first two overtone bands are modeled using semi-classical calculations based on the Robert–Bonamy formalism and two isotropic and anisotropic intermolecular potentials. These potentials involve electrostatic contributions and (a) the atom–atom Lennard-Jones model, or (b) the Tipping–Herman model with adjustable parameters in order to fit the broadening coefficients. For the line shifts, which appear to arise only from vibrational dephasing effects, different values for the relative dependence of the attractive and repulsive parts of the isotropic potential are used. The calculated broadening and shift parameters are then compared with experimental results available in the literature. This comparison shows a reasonable agreement of the broadening parameters and a less satisfactory agreement of the line shifts for which the experimental data are on the whole rather inaccurate.

Pressure Lineshift and Broadening Coefficients in the 2ν3 Band of 16O12C32S

In this study we report the first measurements of the pressure-induced lineshift coefficients due to Ar, He, O2, and N2 for 22 rovibrational lines from P(53) to R(53), belonging to the 2ν3 band of 16O12C32S at 4100 cm−1. The lineshift results were obtained from the simultaneous record of the pressure-broadened and pure low-pressure OCS lines, using a tunable difference-frequency laser spectrometer. For four lines of the 2ν3 band we also report Ar-, He-, O2-, and N2-broadening coefficients by fitting Voigt and Rautian profiles to the measured shapes of these lines. The broadening and shift coefficients are compared to the results of theoretical calculations based on the semiclassical Robert–Bonamy formalism and two different isotropic and anisotropic intermolecular potentials. For OCS–Ar we also consider the Smith–Giraud–Cooper model including all orders of the interaction within the peaking approximation. In all cases, the calculated broadening coefficients are in reasonable agreement with the experimental data. By considering adjustable parameters for the vibrational dependence of the isotropic potential, the general trends of the lineshifts with J can be roughly predicted, except at low J values where an asymmetry behavior for P and R branches is generally observed.

Simultaneously predicted transport and equilibrium properties of boron trifluoride gas

An isotropic temperature-dependent intermolecular potential is proposed for simultaneous prediction of the second virial coefficient B and the viscosity of over a wide temperature range. Potential parameters at 0 K (repulsive power n = 24.28, equilibrium distance , potential well depth K) and the enlargement of the first excited state are determined by solving an inverse problem of minimization of the sum of squared deviations between calculated and measured data normalized to the relative experimental error. The temperature dependence of the average excited state parameters and is implied in the temperature dependence of the effective excited state enlargement calculated via the vibrational partition function. Tables with recommended potential parameters, second virial coefficient, viscosity and self-diffusion in a temperature range of 175-900 K are proposed.

Pressure-Induced Lineshifts in the 1 ← 0 Band of CO Perturbed by He, Ne, Ar, Xe, O2, and N2

The pressure-induced shifting coefficients for He-, Ne-, Ar-, Xe-, N2-, and O2-broadened lines of CO in the fundamental band have been measured by using a Fourier transform spectrometer. The lineshifts have been determined from the wavenumber differences between the lines of pure CO at low pressure and the pressure-broadened lines of CO in mixture with one of the perturbers above mentioned. Comparisons were made with results of theoretical calculations based on a semiclassical model and different isotropic and anisotropic intermolecular potentials. The general trends of the shifts with rotational quantum number are reproduced by the calculations, but not the finer details and the observed asymmetry behavior of the shifts forPandRbranches.