Density-difference maps in quantum chemistry

Electron density-difference maps, used to study the changes that occur when a molecule changes its state or when the nuclei of a molecule change their relative positions, are generally useful only if the atomic densities cancel when one molecular density distribution is subtracted from the other. When, as in the case of the nonrigid internal rotation in ethane, such a cancellation of atomic densities is not possible the method of simple subtraction is no longer appropriate. It is shown that useful density-difference maps can nevertheless be obtained, when the changes in geometric parameters are small, by the calculation of two generalized density-difference functions: a point difference function which allows a comparison of the densities at corresponding points in the two systems, and a volume difference function to compare the amounts of charge in corresponding regions. The method is illustrated by consideration of a change in bond length of the nitrogen molecule and by the nonrigid internal rotation in ethane.Presented at the XIVth Quantum Theory Conference in Canterbury, England, September 1981.     

聚合物的玻璃化转变和孤立链的内旋转异构化能

玻璃化转变时的自由体积,对不同结构的聚合物不是常数,已由实验得到了说明。其原因是玻璃化转变时的体积膨胀系数部分地来自分子链构象变化的贡献。本文在文献[2]和[5]的基础上将Dimarzio定义的链倔硬能(?)与Lipotov关于玻璃化自由体积表达式联系起来,得到了T_g时的自由体积和孤立链的内旋转异构化能的关系式 上式的计算值和文献值较为一致,因此可以作为估算孤立链柔顺性参数的方法。     

Effect of solvent molecular structure on vibration—rotation spectra of DCl and CO solutes in cycloalkanes

Infrared vibration—rotation spectra have been obtained for DCl and CO in a variety of cyclic and bicyclic alkanes and in alkyl-substituted cyclopentanes and cyclohexanes. The band shapes for CO depend on solvent free volume. For DCl they are sensitive to solvent structure, the rotation being perturbed by surface features (“roughness”) of the surrounding molecules. Molecular roughness is assessed qualitatively either through inspection of molecular models or through _V*, the solvent volume at close-packing, obtained from equation of state data.     

The effect of solvent on the internal rotation of formamide: an ab initio study using a polarizable continuum model

The effect of solvent on the barrier to internal rotation of formamide has been studied using an ab initio 4-3IG method coupled with a polarizable continuum model. The trend of increasing rotation barrier with increasing solvent dielectric constant is reproduced. Conformational free energies were calculated by adding dispersion and cavitation terms to the electrostatic energy.     

Effective numbers of modes applied to analysis of internal dynamics of weakly bound clusters

The dependence of the volume of the chaotic component in the internal dynamics of triatomic van der Waals clusters on the angular momentum is calculated using the Monte Carlo and molecular dynamics methods. It has been found that this dependence is nonmonotonic and that its functional form varies for different values of the total energy. The effective number of rotational modes was used to clarify why a change in the volume of chaotic component of the phase space happens for certain values of the angular momentum. We conclude that a large fraction of regular trajectories in relation to all trajectories appears only when there is a possibility for the regular motion to perform a rotation different from that for a chaotic motion. When such difference is small, the regular motion disappears. The effective number of rotational modes can be used to estimate the difference in the type of rotation and is a convenient parameter which controls changes in the dynamics of the system.     

Optical Rotatory Feature of (R or S)-1,1′-Binaphthalene-2,2′-diol (BINOL) in Various Solvents

IntroductionElieletal.1,2 haveeverreportedthatsolventtypeandconcentrationwouldaffecttheequilibriumofcis 3 hydrox ythiane S oxideconformationwhichwascausedbyintermolec ularandintramolecularhydrogenbondformedwiththeinter actionbetweenthesolute solventandthesolute soluteinthesolutionandhaveimportantinfluenceonitsspecificrotation .Uptonowitisnotveryclear,however,whetherhydrogenbondingaffectsspecificrotation (inmagnitudeandoccasion allyinsign)directlyorwhetheritdoessoindirectly (byaf fectingconform…     

The photophysical properties of a julolidene-based molecular rotor

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.     

中变质煤小分子相对煤的吸附及渗流特性影响

为研究煤中小分子对煤的瓦斯吸附及流动特性的影响,采用四氢呋喃溶剂萃取煤中可溶有机小分子,得到萃取后煤样(残煤)。对原煤和残煤分别进行甲烷等温吸附、解吸实验和径向渗流实验;同时,采用氮气吸附法测试了它们的孔隙结构参数。结果表明,残煤的饱和吸附量a值低于原煤,吸附常数b值增大,萃取小分子降低了煤对甲烷的吸附能力,提高了低压段(<4 MPa)煤对甲烷的解吸速率;原煤的孔体积和平均孔径高于残煤,比表面积和微孔孔体积却减少;萃取后,煤粒表面传质阻力和扩散阻力均减小,残煤甲烷解吸速率和解吸量均高于原煤;同一渗流条件下,残煤的渗透率明显高于原煤。分析认为,煤中部分小分子被溶解后,煤孔隙结构的改变,降低了煤对瓦斯的吸附能力,减小了甲烷在煤粒中的内外扩散阻力,扩大了瓦斯在煤中的流动通道,改变了煤层储运特性,为煤储层的化学增透提供依据。     

Optical Rotatory Feature of （R or S）—1,1’—Binaphthalene—2,2’—diol（BINOL）in various Solvents

The changes of the specific rotation and sign of optically active BINOL have been studied in polar/non-polar solvents and at the different pH values of solvent.It is considered that these changes are determined by the equilibriurn studies between cisoid and transoid conformations of BINOL with the same configuration(R or S) which related to the change of the dihedral angle between two naphthalene ring planes of BINOL.     

Change of the molecular surface and volume during internal rotation and its effect on conformational equilibrium in solution

Conformational dependence of the molecular surface S and molecular volume V for hexane and 1,1,2-trichloroethane during rotation around a central bond in the molecules have been calculated. A model of overlapping spheres is used, the size of the spheres being determined by Van der Waals radii of individual atoms. Plots of S and V against torsional angle φ are compared with the potential of internal rotation of both molecules E(φ). The calculated molecular surfaces and volumes of the two molecules for the most stable conformers mutually differ by several percent as experimental results also indicate. We also show that the differences in S and V between individual conformers always affect conformational equilibrium in solution even if solvent-solute interaction energies are not explicitly considered. As a consequence of the mentioned volume changes during internal rotation in a molecule, the conformational energies for hydrocarbons in the condensed and gaseous phases can differ by as much as several kJ mol^?1.     

Compactization of rigid-chain amphiphilic macromolecules with local helical structure

Conformational characteristics of amphiphilic macromolecules with secondary local helical structuring are studied by the method of molecular dynamics for different properties of a helix (bending angles between neighboring vectors of the bond and internal rotation angle) and different rigidities of its fixation. Extended helices with high distances between helical turns and dense helices in which neighboring turns directly adjoin each other are studied. As the quality of a solvent deteriorates, extended helices experience a well-pronounced coil-globule transition, whose amplitude increases with an increase in chain rigidity, while the dimensions of dense helices gradually change. In a poor solvent, extended helices formed “collagen-like” structures, flexible chains of dense helices produce hairpin structures, and rigid macromolecules of dense helices form rodlike globules with an almost ideal local helical order. Independently of helix parameters, a deterioration in solvent quality leads to stabilization of the local secondary structure.     

Use of environmentally-responsive smart glasses in controlled release: Diffusion of molecules under applied stimuli

The use of organosilica sol-gels as environmentally-responsive materials for stimuli-controlled release of encapsulated molecules is reported. These sol-gels exhibit bulk volume changes with respect to applied stimuli such as pH, temperature, salt, and solvents. These volume changes result in expulsion/intake of solvent molecules which can be utilized as a means for externally-regulated release and delivery of dopant molecular entities. For these materials, which undergo a volume transition in response to environmental changes, the magnitude and response time of the material is related to diffusion of solvent molecules in and out of the material. The release of encapsulated molecules and diffusion of solvent accompanies the volume transitions, which can be initiated by changes in temperature or ionic concentration. The kinetics and mechanism of volume changes of organosilica network consisting of hydrophilic and hydrophobic segments are investigated which provide critical insights into the nature of underlying factors that influence release of encapsulated entities. The diffusion coefficient of water molecules in these gels is determined to be considerably faster as compared to organic polymer gels and silica gels indicating rapid volume responses which can be utilized as efficient trigger mechanisms for release of molecules. The results validate preliminary feasibility of stimuli-controlled release of molecules with these gels initiated by the diffusional flow of solvent established by the volume changes.     

CORRELATION BETWEEN GLASS TRANSITION OF POLYMERS AND ENERGY OF ROTATIONAL ISOMERIZATION OF MOLECULAR CHAINS

It has been shown that the free volume fraction at T_g is not a universal parameter for linear polymers of different molecular structure. The reason is that the volume expansion at T_g is partially contributed from the change of the numbers of conformations of isolated molecular chains due to internal rotation. In this paper, glassy transformation was connected with internal rotation of isolated molecular chains, and the relationship between free volume fraction of polymers at T_g and energy e of rotational isomerization of isolated molecular chains was formulated, e=-k·T_g·In (△α·T_g/1-△α. T_g). The values of calculated from the above formula are in good agreement with those published in the literatures. Thus, the method described in this paper can be used to estimate a parameter for the flexibility of isolated molecular chains.7     

Internal dynamics features in the free jet rotational spectrum of the acetaldehyde-Kr molecular complex

The structure and the dynamics of internal motions in the complex formed between acetaldehyde and Kr are studied by free jet absorption microwave spectroscopy performed in the range 60-78 GHz. The fourfold structure of each rotational line is evidence of the vibration-rotation coupling between the overall rotation of the complex, a tunneling motion of the Kr atom between two equivalent positions and the internal rotation of the methyl group in the acetaldehyde moiety. The four sets of transitions could be fitted with a coupled Hamiltonian which allows for the Coriolis interaction obtaining the energy separation between the vibrational energy levels related to the tunneling motion, while the observed splittings due to the methyl group internal rotation were analyzed independently with an appropriate model. The potential energy barriers for the tunneling motion and the internal rotation of the methyl group have been calculated and the interaction of the rare gas atom with the acetaldehyde moiety is reflected in the change of the V(3) barrier to internal rotation in going from the molecule to the weakly bound complex.     

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δε is analyzed on the basis of the two-site model. A linear relation between Δε/χξ and cos²θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.     

Fluorescence studies on volume phase transition in poly(acrylamide) and poly(N-isopropylacrylamide) gels

The changes in microenvironments during the volume phase transition of poly(acrylamide) and poly(N-isopropylacrylamide) gels induced by pH change or the change in solvent composition were studied by using dansyl or pyrenyl fluorescent probes.     

Entropy of cellulose dissolution in water and in the ionic liquid 1-butyl-3-methylimidazolim chloride

The entropic driving forces of cellulose dissolution in water and in the ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) are investigated via molecular dynamics simulations and the two-phase thermodynamic model. An atomistic model of cellulose was simulated at a dissociated state and a microfibril state to represent dissolution. The calculated values of entropy and internal energy changes between the two states inform the interplay of energetic and entropic driving forces in cellulose dissolution. In both water and BmimCl, we found that the entropy associated with the solvent degrees of freedom (DOF) decreases upon cellulose dissolution. However, solvent entropy reduction in BmimCl is much smaller than that in water and counteracts the entropy gain from the solute DOF to a much lesser extent. Solvent entropy reduction in water also plays a major role in making the free energy change of cellulose dissolution unfavorable at room temperature. In BmimCl, the interaction energies between solvent molecules and glucan chains and the total entropy change both contribute favorably to the dissolution free energy of cellulose. Calculations at different temperatures in the two solvents indicate that changes in total internal energy are a good indicator of the sign of the free energy change of cellulose dissolution.     

Culation of partial molar volume and its components for molecular dynamics models of dilute solutions

This paper is a review of our recent computational studies of volumetric characteristics using computer models of dilute solutions. Partial molar volume (PMV) and its components are calculated for simple and complex molecules in water (methane, noble gases, surfactants, polypeptides). Advantages and disadvantages of various computational methods are discussed. It is proposed to use the Voronoi-Delaunay technique to determine the reasonable boundary between a solute molecule and solvent molecules and to identify the PMV components related to the molecule, the boundary layer, and the solvent. It is noted that the observed increase in PMV with temperature for large molecules is due to an increase in the volume of voids in the boundary layer, i.e., due to the “thermal volume.” In this case, the solvent gives a negative contribution to the PMV. In contrast, for simple molecules (methane), the contribution from the solvent is positive and is the main factor in the increase in the PMV, which is associated with a specific change in water structure around a spherical hydrophobic particle outside the boundary layer. For surfactant molecules, the contribution from the solvent changes sign (from negative to positive) with increasing temperature.     

Influence of molecular weight on nanoscale modification of poly(methyl methacrylate) due to simultaneous mechanical and chemical stimulation

We report observations of poly(methyl methacrylate) films modified by the synergistic effect of solvent exposure and mechanical stress applied by the tip of an atomic force microscope (AFM). We show that these modifications are sensitive to polymer molecular weight as well as solvent strength and the force applied by the tip. Small-area scanning often produces localized patches of raised material as well as depressed areas. The volume change associated with the depressed areas generally increases with increasing solvent strength, increasing applied normal force, and decreasing polymer molecular weight. In contrast, the volume change associated with the raised patches is greatest for 25-145K Mw films in 60 and 100% ethanol solutions. In each case, the normal force applied by the AFM tip must exceed a threshold to significantly modify the surface; this threshold is associated with an increase in lateral force applied by the AFM tip during small-area scanning. We attribute the raised patches to mechanically enhanced swelling due to diffusion of solvent into near-surface material. Permanent net volume loss, when observed, is attributed to localized polymer dissolution.     

Raman scattering studies on n-heptane under high pressure

High-pressure Raman scattering studies at ambient temperature are performed on n-heptane. We observe a liquid-solid transition around 1.5 GPa from the changes in the Raman spectra. This has been reported in earlier works. With increasing pressure, we observe large changes in the Raman modes and the spectra show a distinct change around 7.5 GPa. This marks the solid-solid transition at 7.5 GPa observed in n-heptane for the first time. As predicted in theoretical work, we observe dampening of methyl rotation in n-heptane below 7.5 GPa. With increase in pressure above 7.5 GPa we observe a definitive conversion of gauche to trans conformation in the solid phase. Upon release of pressure we do not observe any hysteresis, which suggests that the solid-solid transition takes place with no volume change or is a second-order transition. In this paper we propose this transition to be an orientational order-disorder transition driven by the dampening of the rotation of the methyl group.