Phonon spectrum of a naphthalene crystal at a high pressure: Influence of shortened distances on the lattice and intramolecular vibrations

The Raman spectra of a naphthalene crystal have been measured at room temperature in the pressure range up to 20 GPa. The pressure shift and Grüneisen parameters for intermolecular and intramolecular phonons have been determined. The maximum rate of the pressure shift for intermolecular phonons is 44 cm^?1/GPa, and the rate of the pressure shift for intramolecular phonons lies in the range from 1 to 11 cm^?1/GPa for different modes. The pressure dependence of the phonon frequencies for direct and inverse pressure variations has a hysteresis in the pressure range from 2.5 to 16.5 GPa. It has been shown that the linear dependence of the intermolecular phonon frequency on the crystal density has a peculiarity, which indicates a possible phase transition at a pressure of 3.5 GPa. The pressure dependence of intramolecular phonons related to the stretching vibrations of hydrogen atoms exhibits features that are characteristic of intermolecular phonons, which is associated with the influence of shortened distances between the hydrogen atoms of the neighboring molecules on the intermolecular interaction potential.     

Crystal field and molecular structure in solid C60

The crystal field in the orientationally disordered phase of C₉₀-fullerite is derived from an intermolecular potential model, which takes into account the geometric difference between double bonds and single bonds. The molecules are modelled as rigid bodies, atoms and single bonds are treated as single interaction centers, while double bonds are described by a distribution of interaction centers along the bond. The crystal field is expanded in terms of cubic rotator functions. The calculated expansion coefficients are compared with empirical values derived from diffraction data. The angular dependence of the crystal field, resulting from an anticlockwise rotation of the molecule around the [111] axis, exhibits an absolute and a secondary minimum at angles of 98° and 38° respectively. The self interaction of the molecule in a deformable lattice is investigated.     

The polarized charge-transfer spectrum of crystalline anthracene-TNB complex

The intermolecular charge-transfer spectrum of the crystalline anthracene-TNB complex has been measured, using polarized light. The crystal spectrum differs from that of the solution to a much greater degree than is the case for ordinary aromatic systems. The transition is polarized along the intermolecular axis and is blue-shifted with respect to the solution. There is pronounced vibrational structure, including a prominent 250 cm^-1 interval. A Davydov splitting of approximately 200 cm^-1 is observed, and this is shown to be in reasonable agreement with the known crystal structure and with the symmetry assignment of the transition, which is A′ ← A′ for the complex in solution and A ← A in the crystal.     

Influence of pressure effect on Fermi resonance in binary solution

The Fermi resonance behaviours of the two groups of binary solutions --- pyridine and methanol, benzene and carbon tetrachloride, under different pressures are investigated according to their Raman spectra. The effect of pressure on Fermi resonance in binary solution differs significantly from that in pure liquid. In a binary solution, with the intermolecular distance shortening, the intermolecular interaction potential increases, the shift rates of the Raman spectral lines increase, the spectral line splitting occurs ahead of that in pure liquid, and the wavenumber separation Δ₀ between the unperturbed harmonic levels shifts more quickly, too. The Fermi resonance parameters, the coupling coefficient W and the intensity ratio R of the two Raman bands, decrease rapidly with pressure increasing, and the pressure at which Fermi resonance phenomenon disappears is much lower than that in pure liquid, especially in the solution whose molecules are of the same polarity. This article is valuable in the identification and the assignment of spectral lines under high pressure, as well as the study of high pressure effect, intermolecular interaction, and solvent effects in different cases, etc.     

Pressure-induced changes in the magnetic properties of the single crystal of Mn3 single-molecule magnet

The magnetic measurements of the single crystal of Mn3 single-molecule magnet under high pressure at T=2 K have been performed.We find both the antiferromagnetic intermolecular coupling parameter J and the effective energy barrier to vary compared with the measurements at low pressure.The increase of|J|is estimated to be 12%at 0.7 GPa when compared with that of0 GPa,whereas the effective energy barrier becomes smaller with increasing pressure.Our results demonstrate that the intermolecular interaction of single-molecule magnet can be changed by pressure.Compared with the normal magnetic alloy,the effect of pressure on the magnetic properties of Mn3 is much more prominent,which implies that Mn3 may have great potential in magnetic multifunctional material.     

Structural stability of nano-sized crystals of HMX: A molecular dynamics simulation study

The interaction potential energy and heat of sublimation of nanoparticles of HMX crystal polymorphs are studied by using molecular dynamics methods with a previously developed force field [Bedrov, et al., J. Comput.-Aided Mol. Des. 8 (2001) 77]. Molecular dynamics simulations of nanoparticles with 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 molecules of HMX are carried out at 300 K. The intermolecular, intramolecular and total interaction energies per mole for the nanoparticles are calculated at 300 K. Then, we have calculated sublimation enthalpy of HMX crystal polymorphs with different sizes. For the all sizes, the β-HMX is found to be the most stable phase, due to having the least total interaction energy. Also, α-HMX is more stable than δ-HMX. An increase in the sublimation enthalpy with the size of the nanoparticle can be seen.     

Hydration-dependent dynamical transition in protein: protein interactions at approximately 240 K

Interprotein motions in low and fully hydrated carboxymyoglobin crystals are investigated using molecular dynamics simulation. Below approximately 240 K, the calculated dynamic structure factor exhibits a peak arising from interprotein vibration. Above approximately 240 K, the intermolecular fluctuations of the fully hydrated crystal increase drastically, whereas the low-hydration model exhibits no transition. Autocorrelation function analysis shows the transition to be dominated by the activation of diffusive intermolecular motion. The potential of mean force for the interaction remains quasiharmonic. The results indicate useful experimental avenues on protein:protein interactions to be explored using next-generation neutron sources.     

Molecular Ordering of a Nematogen at Phase Transition Temperature - A Theoretical Study

Molecular ordering in 4-cyano-4'- n -propylbiphenyl ( CB3 ), a nematic liquid crystal, has been studied with the help of intermolecular interaction energy calculations. The CNDO / 2 method has been employed to compute the net atomic charge and atomic dipole moment at each atomic centre. Modified Rayleigh-Schrodinger perturbation theory along with multicentered-multipole expansion method has been employed to evaluate long-range intermolecular interactions, while a ' 6-exp ' potential function has been assumed for short-range interactions. The interaction energy values obtained through these computations were used to calculate the probability of each configuration at the phase transition temperature using Maxwell-Boltzmann formula. All possible geometrical arrangements between molecular pair have been considered during stacking, in-plane and terminal interactions. An attempt has been made to identify the most probable configuration at the phase transition temperature during stacking, in-plane and terminal interactions. Results obtained have been discussed in the light of experimental as well as other theoretical observations.     

Study of liquid crystal phases with a model soft ellipsoid contact potential

We propose a soft ellipsoid contact potential model (SECP) for a pair of identical uniaxial ellipsoidal molecules, considering the configuration dependent energy anisotropy explicitly along with their geometrical aspects. The present model is an extension of the ellipsoid contact potential (ECP), main contribution of which was to determine the correct contact function of the ellipsoidal core. Expressions for intermolecular forces and torques derived analytically from this new model pair interaction potential are presented hereto to make it useful for molecular dynamics simulation study. We also report on some observations of molecular dynamics simulation study to demonstrate the ability of this realistic coarse-grained potential in generating some important liquid crystal phases.     

三维超分子化合物(C12N2H8)的合成、晶体结构与荧光性质

合成了一个三维超分子化合物(C₂₀O₂H₁₄)(C₁₂N₂H₈)(命名为BP1),通过元素分析、红外光谱、核磁共振氢谱和X射线单晶衍射对其结构进行了表征,结果表明分子之间通过氢键和π-π堆积弱的相互作用形成超分子化合物。对所有合成的超分子化合物进行了紫外光谱和荧光光谱的测试。在室温DMSO溶液中,当激发波长为342nm时,化合物在373nm处有一强发射峰,呈现紫色荧光,这可以归属于分子内的π^*→π跃迁。X射线单晶衍射分析结果表明,该超分子化合物属于三斜晶系,P1空间群,晶胞参数a=1.0878(2)nm,b=1.1252(2)nm,c=1.1680(2)nm,α=97.89(3)°,β=110.91(3)°,γ=109.62(3)°,V=1.2032(4)nm³,Z=2,R₁=0.0531,wR₂=0.1634,GOF值为1.034。     

低温相H_2S晶体振动光谱计算

本文介绍了计算分子晶体基频振动的力常数模型并应用于Ｈ２Ｓ晶体低温相的拉曼，红外光谱分析。使用１１个可调参数（包括３个分子内部力常数和８个分子间作用力参数）计算得到６９个光学模式的振动频率及其对应的本征矢量。计算结果与实验值相符甚好。对已观测到的谱线作了全面指认。并给出了未观测到谱线的位置。计算表明，晶体中氢键为最重要的分子间相互作用。它使Ｈ２Ｓ分子Ｓ—Ｈ键力在晶体中减弱７％。     

The principle of corresponding states and the dynamic properties of simple liquids

We present a corresponding states analysis of existing data on sound velocity, transport coefficients and intermolecular interaction times of a number of simple fluids. With appropriate choices of parameters for the intermolecular pair potential, such properties as the adiabatic sound velocity, the shear and bulk viscosities, the thermal conductivity and the intermolecular interaction time can be reduced along the liquid-vapor equilibrium curve. Deviations from the simple law of corresponding states are most pronounced for the transport properties, indicating a stronger sensitivity of the dynamical properties to the interaction potential and the molecular structure.     

Compton scattering study of orthorhombic sulphur

The Compton profiles of orthorhombic sulphur crystals have been measured the scattering vector perpendicular to the crystal planes (110) and (001) using 60 keV radiation from a ²⁴¹Am source. Significant anisotropy was observed in the electron momentum distribution. Fourier transformed Compton profiles were compared with theoretical autocorrelation functions for a S₈ ring molecule. It is shown that the observed anisotropy can be only partially interpreted in terms of intermolecular interactions. The strong correlation, observed in the direction perpendicular to the plane of the S₈ ring indicates a meaningful ring — ring interaction in the orthorhombic sulphur crystal.     

Molecular forces and elastic constants of polyethylene single crystals

Polyethylene has an orthorhombic lattice for which nine elastic constants exist; they are obtained in terms of the intra- and intermolecular forces. Constants involved in the 6-12 Lennard-Jones potential approximating the London dispersion type of van der Waals' forces are obtained by computing the crystal potential energy and comparing it with the cohesive energy. First and second nearest-neighbor interactions are considered to establish relationship between the elastic constants and the interaction constants. The latter are obtained in terms of the C—C bond, stretching, bending, and repulsive force constants and the L-J potential constants. A limited type of central force assumption is applied. Values of Young and shear moduli are obtained along the three axes. The value along the chain compares with the experimentally determined and calculated values for oriented polyethylene. Young's modulus along the lateral direction is of the order of Young's modulus of bulk polyethylene, showing that intermo-lecular forces are the ones that determine the Young modulus of bulk polyethylene.     

A critical evaluation of isotropic potential functions for chlorine

A survey of many crystal structures shows that short intermolecular chlorine-chlorine contacts are anisotropic. This observation challenges the currently used potential function models which allow only for isotropic exp-6 and coulomb interactions. In this study the polymorphic p-dichlorobenzene DCB system is used to illustrate the difficulties encountered by relying on the isotropic atom-atom approximation. It is found that exp-6 potential functions cannot be extrapolated beyond the range of contacts used in their derivation. Addition of coulomb terms provides needed additional flexibility to the potential function but the derived C-Cl bond polarization is not found to be a constant of the molecule, i.e. the optimum value is not the same for each phase of DCB. Furthermore, no single exp-6 potential function could satisfy the structural constraints provided by the three phases of crystalline DCB and by hexachlorobenzene. These findings are typical of curve-fitting methods which employ an incorrect mathematical form for the curve and indicate the necessity of including anisotropic terms in the interaction potential.     

A biaxial nematic liquid crystal composed of matchbox-symmetric molecules

By means of Monte Carlo simulations in the isothermal-isobaric ensemble, we investigate the structure and phase behaviour of a thermotropic liquid crystal composed of matchbox-symmetric (or board-like) molecules. Besides the isotropic phase the liquid crystal exhibits also uniaxial and biaxial nematic phases. The interaction potential is derived through an expansion in terms of Stone's rotational invariants [A. J. Stone, Mol. Phys. 78, 241–256 (1978).] that can be reexpressed in terms of Cartesian tensors. This latter formulation is particularly well suited for computer simulations. We analyse the orientation distribution function which allows us to distinguish between intrinsic and extrinsic biaxiality. In addition, we study the orientation-dependent correlation functions. In the limit of large intermolecular separations, the value of the orientation correlation function corresponds to the uniaxial and biaxial order parameters which are coupled in a complex fashion.     

Theoretical calculation of the conformation and crystal structure of poly-L-hydroxyproline

The intra-and intermolecular forces determining the conformation of poly-L-hydroxyproline have been examined. The vacuum conformation of the isolated molecule has the left-hand trans configuration of poly-L-proline II with Ψ = 295°. A second less favorable low-energy form is the right-hand superhelix, also previously described for polyproline. No cis conformation was found to be feasible. Introduction of intermolecular interaction functions shows that the conformation in the solid state is strongly modified to accommodate such interactions. The rotation angle corresponding to the minimum energy in the solid is found to be Ψ = 343°. The crystal structure predicted from these calculations is a hexagonal lattice with a₀ = 12.5 Å and c₀ = 9.1 Å with space group P3₂. Three helices are required in the fundamental structural unit. These results are in excellent agreement with X-ray evidence.     

On the validity and properties of the atom-atom potential as a function of intermolecular separation,configuration, and partial wave order

The intermolecular partial wave expansion of the atom-atom potential U is reviewed briefly and developed, by using results due to Sack, so that the radial components of the expansion can be evaluated to arbitrary accuracy for all relevant partial wave orders and values of the intermolecular distance r. These results are used to study the convergence of the partial wave expansion of U as a function of partial wave order, r, intermolecular orientation, and the anisotropy of the interacting molecules. In marked contrast to previous work it is found that many of the higher order partial wave components of U are important relative to the isotropic term even for the interaction of relatively spherical molecules and that the results obtained from a truncated partial wave expansion depend significantly upon the method of summation due to the generally poor convergence of the expansion. The validity of the atom-atom potential as a representation of the correct attractive intermolecular potential is also discussed in some detail. There are basic problems associated with the representations furnished by both the isotropic and the anisotropic parts of the atom-atom potential at intermediate and large r. The different convergence properties of the r ^-1 expansions of the partial wave expansions of U and of the correct potential for these values of r is illustrated by using model interactions. While it appears that it may be possible to obtain a qualitatively reasonable representation of the attractive part of an intermolecular potential over a useful range of r from atom-atom results, this apparently cannot be achieved for wider ranges of r or for the purely anisotropic part of the potential.     

Ne-HF体系的相互作用势及散射截面的密耦计算

利用非线性最小二乘法拟合在CCSD(T)/aug-cc-pVQZ理论水平下计算的相互作用能,得到了基态Ne-HF体系相互作用势的解析表达式.基于拟合的CCSD(T)势,通过密耦计算得到了入射能量分别为60,75,100和150meV 时,Ne-HF散射的微分截面和分波截面,详细讨论了散射截面随能量的变化趋势以及态-态激发截面对总非弹性散射截面的影响. 关键词： 相互作用势 散射截面 密耦计算 Ne-HF体系     

A reliable single parameter interatomic potential for argon

A simple semiempirical approximation previously proposed for the isotropic intermolecular forces between two closed shell systems is tested in detail for the argon-argon interaction. The potential is based on the knowledge of the first-order coulomb interaction energy, a suitably damped three term long range asymptotic expansion of the second order coulomb energy, and a semiempirical representation of the exchange interaction energy which contains one adjustable parameter. The single adjustable parameter can be reliably determined by fitting the second virial coefficient for argon in the 130–773 K temperature range with the long range interaction coefficients being constrained within the theoretical bounds specified by Tang, Norbeck and Certain. The reliability of the potential is compared with that of several literature potentials by comparing the theoretical predictions obtained from the potentials with experimental results for the second virial coefficient, viscosity, thermal conductivity and thermal diffusion ratios for dilute argon gas, and with spectroscopic data for the dimer, and with SCF calculations of the Ar-Ar potential at small interatomic separations. Our best potential predicts these properties with a precision as good as or better than other recent potentials which generally contain more adjustable parameters and/or involve more input data. The results confirm earlier work that suggested that the scheme tested is capable of yielding reliable isotropic potentials for the interaction of closed shell systems for 0·3 ? R/R_m ? ∞ where R_m is the intermolecular distance at the van der Waals minimum. The scheme appears to offer a method for obtaining reliable potentials while avoiding problems associated with optimizing many parameters with respect to fitting experimental constraints.