Largeurs des raies spectrales de CO autoperturbe et perturbe par N2, O2, H2, HCI,NO et CO2

Linewidths of CO self-broadening and broadened by N₂, O₂, H₂, HCI, NO, and CO₂ have been calculated using different contributions in the intermolecular dispersion potential.The quadrupole moment of some perturbers has been determined by comparison between calculated and observed linewidths. The values obtained for the quadrupole moments may depend on the dispersion potential, especially when it is low (as is the case for N₂, O₂ and H₂). For CO-CO and CO-NO, the electrostatic interactions including the octupole moment yield good results for the linewidths for high |m|-values.     

A classical molecular dynamics study of the anomalous ionic product in near-critical and supercritical water

This paper reports a molecular dynamics simulation study on the ionic dissociation of water. Experimentally the ionic product of water, K_W , is known to exhibit an anomalous variation with temperature. It rises to a maximum at near-critical temperatures before falling off sharply as the temperature becomes super-critical. The paper describes a molecular dynamics simulation study, using the SPC/E water model, to gain insights into this surprising behaviour. Results for K_W are in good agreement with experiment up to the critical point and the anomalous trend appears to be the result of a balance between the effects of a positive gas phase enthalpy of reaction, a decreasing electrostatic stabilisation of the ions with increasing temperature and more subtle entropic effects. The possibility of ion-pairing effects are considered and a simple estimate of the equilibrium dimerisation constants shows the dimer to be preferred at low temperatures, and the separated ions to be increasingly favoured at higher temperatures. The experimental value of pK_W was fitted to simulation results at 700 K to give an estimated water dipole of 2.06 Debye.     

Force-field dependence on the interfacial structure of oil–water interfaces

We investigate the performance of different force-fields for alkanes, united (TraPPE) and all atom (OPLS-AA) models, and water (SPC/E and TIP4P-2005), in the prediction of the interfacial structure of alkane (n-octane, and n-dodecane)–water interfaces. We report an extensive comparison of the interfacial thermodynamic properties as well as the interfacial structure (translational and orientational). We use the recently introduced intrinsic sampling method, which removes the averaging effect of the interfacial capillary waves and provides a clear view of the interface structure. The alkane interfacial structure is sensitive to the environment, i.e. alkane–vapour or alkane–water interfaces, showing a stronger structure when it is in contact with the water phase. We find that this structure is fairly independent of the level of detail, full or united atom, employed to describe the alkane phase. The water surface properties show a small dependence on the water model. The dipole moment of the SPC/E model shows asymmetric fluctuations, with a tendency to point both towards the alkane and water phases. On the other hand the dipole moment of the TIP4P-2005 model shows a tendency to point towards the water phase only. Analysis of the intrinsic electrostatic field indicates that the surface water potential is confined to an interfacial region of about 8 Å. Overall we find that the intrinsic structure of alkane–water interfaces is a robust interfacial property, which is independent of the details of the force-field employed. Hence, it should provide a good reference to interpret experimental data.     

The dipole moment distribution on water is improved by using large flexibility and large bending angle

A highly flexible model of water with fixed charges is used to study properties of water. The bending angle of an isolated molecule is 125^○ that was chosen to match the experimental dipole moment. The geometry of water in the liquid phase is made closer to that of the rigid SPC/E model by decreasing the bending angle spring constant, k _Θ. The new model, called SPCE-FHΘ, is a modified version of the recently proposed SPCE-FH [J. Alejandre, G.A. Chapela, F. Bresme and J.-P. Hansen, J. Chem. Phys. 130, 174505 (2009)] to simulate ionic solutions which includes short ranged interactions on the hydrogen atoms. By increasing angle flexibility it is possible to obtain, in the liquid phase at ambient conditions, bending angles ?Θ(HOH)? ～ 109^○, dipole moment ?μ? ～ 2.5 D and dielectric constant ?ε? ～ 80. The dipole moment distribution at room temperature goes from 1.5 to 3.5 D due to large fluctuations in bending angle and has the same trend found in ab initio simulations of liquid water. The dipole moment profile at the interface of water varies from 1.9 D in the vapour phase to 2.5 D in the liquid region at 400 K. The SPCE-FHΘ gives dipole moment, dielectric constant, coexisting densities and surface tension along the liquid–vapour coexistence line closer to the experimental values than those obtained for the SPC/E force field.     

超临界水中硫酸钠结晶动力学的分子动力学模拟

在超临界水反应器中，硫酸钠是易造成堵塞的一种常见无机盐，研究其结晶动力学对于防盐沉积反应器的设计具有重要意义.本文采用LAMMPS分子动力学模拟软件研究硫酸钠在超临界水中的微观结晶过程，其中水分子采用SPC/E模型，离子-离子、离子-水分子相互作用采用Coulumb和Lennard-Jones联合势能函数.结果表明：水对离子的静电屏蔽作用随温度升高而增强、随密度减小而减弱；增大超临界水的温度和密度有利于离子扩散，进而促进离子相互碰撞、成核；在模拟的超临界水参数范围内，其成核速率的数量级为10²⁹cm^-3·s^-1，生长速率为(19.8~25.8) m·s^-1.     

Role of translation-rotation coupling on phase diagrams of N2-solids and related systems

Molecular solids composed of N₂-molecules are studied by Monte Carlo simulations in the constant-stress ensemble utilizing Lennard-Jones and electrostatic interactions. A phase transition was found from a high-temperature orientationally-disordered cubic phase to a low-temperature phase with Pa3 structure. The transition temperature and the jump in volume are in qualitative agreement with experimental findings. An increase in the elastic constants C₁₁ and C₄₄ and a decrease in C₁₂ at the fcc-Pa3 transition are predicted. An additional study was done by neglecting the electrostatic interaction in order to study the role of the intermolecular potential. In this case a transition to a low-temperature phase with trigonal structure was obtained. If, however, translationrotation coupling is omitted, the Lennard-Jones model exhibits the Pa3 phase too. In this study, phase transitions to hexagonal phases are suppressed by the choice of periodic boundary conditions. Many similarities are found with theoretical predicition of the translation-rotation coupling induced phase instabilities in molecular C₆₀- and C₇₀-solids.     

Self-diffusion coefficients and orientational correlation times in aqueous NaCl solutions: Complementarity with structural investigations

The dynamical properties of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-6 m) at ambient conditions are characterized by molecular dynamics (MD) simulations. MD simulations are performed with a flexible SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. In this paper, attention has been focused on the self-diffusion coefficients (D_i) of ions and water molecules and on orientational correlation time of water molecules. It is found that the self-diffusion coefficients decrease with ion concentration. Moreover, the self diffusion coefficients of sodium and chloride at higher salt concentrations are very comparable which may be due to the formation of clusters of these ions. The deduced rotational dynamics speeds up as the salt concentration increases. Some complementarities between dynamical properties and structural ones, recently obtained, are carried out.     

2-三氟甲基吡啶的转动光谱及分子结构研究

本文测定了2-三氟甲基吡啶在2∽20 GHz频率范围内的高分辨转动光谱. 测定了转动常数、¹⁴N核四极耦合常数及离心畸变常数等一系列光谱参数. 同时还在自然丰度下测定了5个¹³C和1个¹⁴N单取代同位素异数体的光谱数据. 实验结果结合从头算准确地推导出2-三氟甲基吡啶的骨架结构. 实验测得同位素异数体的平面转动惯量P_cc数值均为44.46 u?²,表明此分子具有C_s对称性. 此外,本文计算了吡啶、2-氟吡啶、2-甲基吡啶和2-三氟甲基吡啶的分子表面静电势,以此分析了三氟甲基的取代对电子分布的影响.     

Electric quadrupole interaction of210Po(6+) in Bi single crystal

The electric quadrupole coupling constant of the²¹⁰Po(6⁺) isomer in a Bi single crystal was measured as a function of the temperature using the time-differential perturbed angular correlations technique. The electric field gradient (EFG) for Po in Bi is found to be temperature independent in the range 77–477 K. Using a calculated value of the nuclear quadrupole moment, the magnitude of the EFG is derived, and in turn serves to re-examine the systematics of quadrupole moments of λh ₉ ^2/2 configurations in the lead region.     

The microwave spectrum of cyanoformamide

The spectrum of cyanoformamide, NCCONH₂, has been measured between 18 and 40 GHz using a Hewlett-Packard spectrometer with Stark modulation. The molecule is somewhat unstable and could most conveniently be measured in a flow system. The quadrupole hyperfine structure due to the two nitrogen nuclei has been analyzed for the ground state, and quadrupole coupling constants, rotational constants, and centrifugal distortion constants have been determined for the ground state. A rough determination of the components of the electric dipole moment was possible from the Stark shifts of suitable transitions.     

Deuteron magnetic resonance of monodeuteroethene: Isotropic and anisotropic phase spectra

Deuteron magnetic resonance spectrum of monodeuteroethene in isotropic phase solution is shown to give the relative signs of the indirect spin-spin coupling constants, while the proton spectrum cannot provide this information. In agreement with previous experimental and theoretical evidence, J _gem is found to have the same sign as the two vicinal coupling constants. High resolution deuteron resonance spectra of monodeuteroethene in the nematic phase solution are analysed in terms of the order matrix and quadrupole splitting. The quadrupole coupling constant and the asymmetry parameter are determined by two partly independent methods.     

The microwave spectrum and structure of chloryl fluoride

The microwave spectra of three isotopic species of chloryl fluoride, FClO₂, in its ground vibrational state, have been measured in the frequency region 8–37 GHz. The spectra have yielded values for the rotational constants, centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants, as well as the molecular dipole moment, 1.722 ± 0.03 D. The molecule has been shown to have C_s symmetry, and a pyramidal configuration, with the chlorine atom at the apex of the pyramid. The following internuclear parameters were obtained:r(Cl?F)1.697±0.003 A r(Cl?F)=1.418±0.002AThe structural parameters, quadrupole coupling constants, dipole moment and force field are explained in terms of a bonding scheme in which a fluorine 2p atomic orbital overlaps with the highest occupied orbital of ClO₂; there is considerable evidence for withdrawal of electron density from this singly occupied antibonding orbital of ClO₂ toward the fluorine atom.     

Puzzle of the <Superscript>6</Superscript>Li quadrupole moment: Steps toward solving it

The problem of the origin of the quadrupole deformation in the ⁶Li ground state is investigated with allowance for the three-deuteron component of the ⁶Li wave function. Two long-standing puzzles related to the tensor interaction in the ⁶Li nucleus are known: that of an anomalous smallness of the ⁶Li quadrupole moment (being negative, it is smaller in magnitude than the ⁷Li quadrupole moment by a factor of 5) and that of an anomalous behavior of the tensor analyzing power T_2q in the scattering of polarized ⁶Li nuclei on various targets. It is shown that a large (in magnitude) negative exchange contribution to the ⁶Li quadrupole moment from the three-deuteron configuration cancels almost completely the “direct” positive contribution due to the αd folding potential. As a result, the total quadrupole moment proves to be close to zero and highly sensitive to fine details of the tensor nucleon-nucleon interaction in the ⁴He nucleus and of its wave function.     

System size and trajectory length dependence of the static structure factor and the diffusion coefficient as calculated from molecular dynamics simulations: The case of SPC/E water

The effect of the applied trajectory length on the convergence of the self-diffusion coefficient was examined for the SPC/E water model in the NVT ensemble with different system sizes at 293 K. Temperature dependence and isotope effects, via using D₂O instead of H₂O, were also investigated. A simulation for the polarizable SWM4-DP model was also carried out to compare the effect of different potential models. Radial distribution functions and the neutron weighted structure factor were also calculated; they were found to be insensitive to changing the system size in the range of 216 to 16,000 molecules. On the other hand, the diffusion coefficient is rather sensitive to the applied trajectory length, system size and the method of calculation. The diffusion coefficient is therefore not appropriate for assessing, and distinguishing between, potential models of water, whereas the structure factor could serve as a more stable measure.     

Influence of quadrupole pairing on backbending

The influence of the different quadrupole pairing forces ∝ Y_2m (m = 0, 1, 2) and the spin-dependent particle-hole force on backbending (BB) is studied. A cranked Hartree-Fock-Bogoliubov approach with particle number projection before the variation of the important degrees of freedom is used. To discuss the numerical results qualitatively perturbative formulas for the moment of inertia and the gap parameters are given. The results are the following: (i) The quadrupole pairing Y₂₁ is not affecting the backbending. (ii) The Y₂₀ pairing is reducing the moment of inertia at low angular momenta by about 20 %. This just cancels the increase of the moment of inertia by Y₂₁ pairing at low angular momenta, (iii) The Y₂₁ and Y₂₀ pairing together shift the backbending point to higher angular momenta and better agreement with the experimental data. (iv) A spin-dependent ph force does not affect the moment of inertia at low angular momentum. But above backbending it reduces the moment of inertia by about 13 % to the correct experimental value if a strength parameter adapted in ²⁰⁸Pb is used.     

Electronic structure of arsabenzene: Microwave spectrum,dipole moment,and nuclear quadrupole coupling constants

The microwave spectrum of arsabenzene was analyzed; a dipole transitions were observed. The following rotational constants were obtained; A = 4871.03 ± 0.18 MHz, B = 2295.87 ± 0.01 MHz, C = 1560.10 ± 0.01 MHz. The dipole moment was 1.10 ± 0.04 D. The nuclear quadrupole coupling constants due to the ⁷⁵As nucleus were χ_aa = ?186.4 ± 0.1 MHz, χ_bb = 43.5 ± 0.2 MHz, χ_cc = 142.9 ± 0.2 MHz, and the asymmetry parameter, η = 0.533 ± 0.002. Analysis of the quadrupole coupling constants indicated that the population of the 4p orbitals on arsenic decrease in the order n_a > n_b > n_c.     

Sintering mechanism of CaO during carbonation reaction in the presence of water vapor

In this paper, CaO sintering in the presence of water vapor for CO₂ capture were carried out by ReaxFF(Reactive Force Field) molecular dynamics. The CaO sintering model was simulated at different temperatures (873 K-1273 K) and atmospheres (CO₂, H₂O), respectively. The results showed that water vapor could significantly promote the sintering process of CO₂ capture by CaO. The Mean-square displacement (MSD) and Boltzmann–Arrhenius dependency were used to study the diffusion properties of CaO particles. The decreased diffusion activation Ea and increased pre-exponential factor D₀ indicate that CaO particles have a stronger initial diffusivity and a lower diffusion barrier in the presence of CO₂ and H₂O. The inner and outer regions of CaO atoms were analyzed and it was found that the activation energy is the main factor to enhance the diffusion in the presence of CO₂ for CaO sintering process, whereas the pre-exponential factor dominates with both CO₂ and H₂O. Water vapor enhanced the sintering pf CaO carbonation reaction is mainly achieved by promoting atoms in the inner layers of CaO particles. The types and numbers of sintering atoms during the sintering process were counted, and the distances between Ca and O atoms were calculated, which found that water vapor first dissociates into hydroxyl and H protons on the CaO surface, and the hydroxyl group will stay on the surface of CaO and combine with CO₂, while the H proton will combine with O inside CaO to promote the sintering of CaO further.     

Microwave spectra and structure of an isoxazole-water complex

The rotational spectrum of a hydrogen-bonded isoxazole-water complex has been measured between 6–18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. In addition to isoxazole-H₂O, the complexes with HDO and D₂O as well as isoxazole-¹⁵ N-H₂O have been investigated in order to determine the structure of the complex. Rotational constants, quartic centrifugal distortion constants and quadrupole coupling constants, where applicable, have been fitted to the measured transition frequencies of the isotopomers. Structural data, which have been deduced from the planar moments of inertia and the quadrupole coupling constants of the isotopomers, have established conclusively that water binds to nitrogen in the ring plane of isoxazole. Ab initio calculations have revealed that complexes with a hydrogen-bond to nitrogen or to oxygen are both stable. The complex with water attached to nitrogen has been found to be more strongly bound than that with water attached to oxygen. Small splittings of the rotational transitions of the two complexes with H₂O have been interpreted as being the result of an internal rotation of water with respect to isoxazole.     

Microwave spectra of chloro,bromo, and hydroxy derivatives of acetyl fluoride

The microwave spectra of CH₂ClCOF, CH₂BrCOF, and CH₂OHCOF have been analyzed. From very limited isotopic substitution data it was concluded that all molecules have C_s symmetry with the α-substituent cis with respect to the carbonyl group; in the hydroxy compound a hydrogen-bonded five-membered ring was found. The dipole moment components of the latter substance were determined. The nuclear quadrupole coupling constants for CH₂ClCOF and CH₂BrCOF are compatible with an electron distribution which is cylindrically symmetric about the carbon-halogen bond.