Water’s surface tension and cavity thermodynamics

The relationship between cavity thermodynamics in water and air-water surface tension is investigated in the present study. The effective hard sphere diameter of water molecules over a large temperature range is estimated from the experimental air-water surface tension, and cavity thermodynamics is calculated by means of classic scaled particle theory. The work of cavity creation proves to be a decreasing function of temperature and the cavity entropy change is a positive, practically constant, quantity, regardless of the cavity diameter, in marked contrast with well established theoretical and computer simulation results. This finding suggests that the relationship between cavity thermodynamics and surface tension is not a simple matter in the case of water.     

The shape of the Lorentz cavity and the internal field in anisotropic fluids

The predictions of the ellipsoidal Lorentz cavity model for different cavity axial ratios are compared with the predictions of a microscopic molecular crystal model for different molecular axial ratios. The models agree when the cavity depolarisation factor equals the crystal Lorentz factor. Numerical calculations show that the axial ratio of the cavity equivalent to the molecule is roughly equal to the molecular axial ratio up to ≈ 5. but increases rapidly thereafter. For molecular axial ratios of 9 and above there is no equivalent cavity. Implications of the results for the internal field in liquid crystals are considered.     

基于DFB型半导体激光器的腔增强吸收光谱研究

The Cavity enhanced absorption spectroscopy based on a tunable DFB diode laser (TDL-CEAS) was described. A brief introduction of cavity enhanced absorption spectroscopy development and experimental scheme was given, the effective absorption path of the medium in the optical cavity was interpreted from the way of Fabry Perot cavity. It is pointed out that the main reason why CEAS has high detection sensitivity is that the medium in the cavity can get a long absorption path. A tunable DFB diode laser which center wavelength is 1.573 μm was used as the light source, and an optical cavity which consists of two high reflectivity mirrors (near 1.573 μm, R about 0.994) separated at a distance of 34 cm was used as the absorption cell. Laser radiation was coupled into the optical cavity via accidental coincidences of laser frequency with the cavity mode when scanning the cavity and the laser. An absorption spectrum of carbon dioxide near 1.573 μm was obtained and a detection sensitivity of about 1.66×10-5 cm-1 was achieved. It is experimentally demonstrated that the CEAS is a highly sensitive and high resolution spectrum technology, and it has the advantage of simple experimental setup and easy operation.     

Hydrogen bonds seal single-molecule capsules

In the presence of methanol the tetrakis(benzoxazines) complex tetramethylammonium cation within the cavity, and the cavity is completely sealed by two intramolecular hydrogen bonds between amide groups. The Cl(-) anion is found external to the cavity. In CHCl(3), Me(4)N(+) is complexed within the cavity, but the Cl(-) anion acts as a stopper in the upper rim of the cavity, hydrogen-bonded to the amide groups. The solution results are supported by single-crystal X-ray structural studies of both the single-molecule molecular capsules, and those stoppered by Cl(-).     

Sensitivity limit of rapidly swept continuous wave cavity ring-down spectroscopy

The averaged transmitted intensity of a cavity excited by a linearly frequency swept laser with finite line width is derived and presented as a sum over passes, analytical integrals (where the sum of passes is converted to a continuous time variable), and an approximate but computationally more stable stationary phase approximation expression. The transmitted waveform is used to derive the bias in extraction of the cavity decay rate from such a cavity transient for three different fitting models. Numerical simulation of cavity excitation gives statistical fluctuations in the transmitted intensity that leads to noise in the cavity decay rate. For a range of parameters spanning those likely to be encountered in real experiments, numerical results are presented. These demonstrate that the theoretical signal-to-noise ratio and thus sensitivity of swept cavity (or equivalently, frequency) CRDS is substantially below that for CRDS where one attenuates the laser either with current modulation or with an external modulator.     

Pressure tensor in the wedge-shaped cavity between solids with dispersion forces

The pressure tensor in the wedge-shaped cavity between solids with dispersion forces is calculated. All four (three diagonal and one off-diagonal) pressure tensor are calculated as functions of the position inside the cavity. It is checked that, if the cavity walls belong to different solids, pressure tensor depends, not only on the opening angle of wedge-shaped cavity, but also on the position of interface between contacting solids. In this case, the absence of cavity symmetry leads to different values of normal pressure on opposite walls, thus complicating the determination of disjoining pressure. The region of configurations of bodies in which this determination can be made is found.     

Electrostatic contribution to the ion solvation energy: cavity effects

Ion solvation process has been analysed for the spherically symmetrical system where an ion is located inside a cavity surrounded by an isotropic nonlocal dielectric medium. It has been proven that for any dielectric properties of the medium, the electric field outside the cavity as well as the ion solvation energy depend only on the total ion charge but not of the particular distribution of the ion charge density inside the cavity. These characteristics remain unchanged if the charge is displaced from the external boundary of the cavity into it. Analytical formulas for them have been derived for a particular model of the nonlocal dielectric function. Comparison of results for the solvation energy on the basis of this new theory and of the conventional approach (disregarding the existence of the cavity) shows a significant difference between their predictions if the ion charge is displaced inside the ion cavity.     

The dependence of catalytic activities of secondary functional beta-cyclodextrins on cavity structures.

Secondary imidazole-appended beta-cyclodextrin 5 with a nondistorted cavity synthesized from a novel intermediate 3-amino-3-deoxy-beta-cyclodextrin exhibits much greater catalytic activity in the ester hydrolysis than its isomer 6 with a distorted cavity, indicating that the catalytic activities of secondary functional cyclodextrins are dependent on cavity structures.     

Local fields in cavity sites of rough dielectric surfaces revisited

The solution for the field near the tip of a conical cavity, given previously by Liver, Nitzan and Gersten in this journal, is reexamined. Their assumption of fields with rotational invariance about the cavity axis excludes some mathematically justified singular solutions. The field near the edge of a cylindrical cavity with triangular cross section is shown to be singular for Ag in the visible region under the assumption of locality. This may explain why cavity sites act as strong enhancement centers for surface-enhanced Raman scattering.     

Quantifying the protein core flexibility through analysis of cavity formation

We present an extensive analysis of cavity statistics in the interior of three different proteins, in liquid n-hexane, and in water performed using molecular-dynamics simulations. The heterogeneity of packing density over atomic length scales in different parts of proteins is evident in the wide range of values observed for the average cavity size, the probability of cavity formation, and the corresponding free energy of hard-sphere insertion. More interestingly, however, the distribution of cavity sizes observed at various points in the protein interior is surprisingly homogeneous in width. That width is significantly smaller than that measured for similar distributions in liquid n-hexane or water, indicating that protein interior is much less flexible than liquid hexane. The width of the cavity size distribution correlates well with the experimental isothermal compressibility data for liquids and proteins. An analysis of cavity statistics thus provides an efficient method to quantify local properties, such as packing, stiffness, or compressibility in heterogeneous condensed media.     

Noble gas clusters in model zeolite cavities

Noble gas atoms trapped in the intracrystalline cavities of zeolites may form clusters. A classical-mechanical isoenergetic molecular dynamics simulation is performed to simulate the dynamical behavior of noble gas clusters in zeolite cavities. To implement the simulation, a model is adopted of a homogeneous spherical cavity with Morse interaction between the noble gas atoms and cavity walls. The results for Ar₆ clusters indicate that the noble gas clusters in the cavity undergo the same solid/liquid phase changes as in free space, and, at high enough energies, a rapid exchange between atoms adsorbed on the inner surface and thosein the interior of the cavity. Mathematical quenching is used to investigated the multidimensional potential surface of Ar clusters in the cavity.     

Guest encapsulation and self-assembly of a cavitand-based coordination capsule

The synthesis and spectroscopic characterization of a cavitand-based coordination capsule 14 BF4 of nanometer dimensions is described. Encapsulation studies of large aromatic guests as well as aliphatic guests were performed by using 1H NMR spectroscopy in [D1]chloroform. In addition to the computational analysis of the shape and geometry of the capsule, an experimental approach to estimate the interior size of the cavity is discussed. The cavity provides a highly rigid binding space in which molecules with lengths of approximately 14 A can be selectively accommodated. The rigid cavity distinguished slight structural differences in the flexible alkyl-chain guests as well as the rigid aromatic guests. The detailed thermodynamic studies revealed that not only CH-pi interactions between the methyl groups on the guest termini and the aromatic cavity walls, but also desolvation of the inner cavity play a key role in the guest encapsulation. The cavity preferentially selected the hydrogen-bonded heterodimers of a mixture of two or three carboxylic acids 18-20. The chiral capsule encapsulated a chiral guest to show diastereoselection.     

A refined cavity construction algorithm for the conductor‐like screening model

A cavity construction algorithm based on the triangulation of an iso‐surface is introduced as a new standard for dielectric continuum solvation calculations with the Conductor‐like Screening Model COSMO. It overcomes deficiencies which have become apparent for the original COSMO standard cavity, especially in concave regions of the molecular shaped cavity. The new standard, called FINE Cavity, is described in this article with several application examples. The earlier COSMO cavity construction algorithms are described for comparison. © 2018 Wiley Periodicals, Inc.     

Temperature dependence of free volume of polyacrylamide gels studied by positron lifetime measurements

Changes of positronium (Ps) cavity radii in polyacrylamide and poly(N-isopropylacrylamide) gels were studied from 120 to 300 K by positron lifetime technique and it has been shown that the Ps cavity radius in the hydrogels changes by three or four stages. Temperature dependence of the Ps cavity radius exhibits variations similar to common polymers around the glass transition temperature. Hydrophilicity of the polymer chains significantly affects the Ps cavity radius just below 273 K. These results suggest an important role of free volume on the state of water in hydrogels.     

Spectral properties and supramolecular inclusion complexes of β-cyclodextrin with flexible amphiphilic and rigid compounds

Spectral properties and inclusion complexes of β-cyclodextrin (β-CD) with nonionic amphiphiles and rigid 1-bromonaphthalene (BrN) was investigated in detail. Fluorescence and ¹H NMR measurements give new insights into inclusion of the hydrophobic moiety of amphiphiles into the cavity of β-CD. Their apparent stability constants were well correlated with the structure of the hydrophobic moiety of amphiphiles. The long and flexible hydrophobic moiety may occupy the cavity in the compressed manner. The phosphorescence quenching and the binding strength of BrN in ternary complexes indicate that the inclusion depth and the rigidity of BrN in the cavity of β-CD are predominant factors in determining its phosphorescence. Further inclusion of rigid BrN into the cavity drives the built-in phenyl group of amphiphiles to expose to bulk water phase to a greater extent. Comparative analyses of molecular sizes and models reveal that the flexible hydrocarbon chain of an amphiphile in supramolecular inclusion complexes was located inside the crowded cavity of β-CD due to the filling of rigid BrN into the cavity.     

Autotuning of Vibrational Strong Coupling for Site-Selective Reactions

Site-selective chemistry opens new paths for the synthesis of technologically important molecules. When a reactant is placed inside a Fabry–Perot (FP) cavity, energy exchange between molecular vibrations and resonant cavity photons results in vibrational strong coupling (VSC). VSC has recently been implicated in modified chemical reactivity at specific reactive sites. However, as a reaction proceeds inside an FP cavity, the refractive index of the reaction solution changes, detuning the cavity mode away from the vibrational mode and weakening the VSC effect. Here we overcome this issue, developing actuatable FP cavities to allow automated tuning of cavity mode energy to maintain maximized VSC during a reaction. As an example, the site-selective reaction of the aldehyde over the ketone in 4-acetylbenzaldehyde is achieved by automated cavity tuning to maintain optimal VSC of the ketone carbonyl stretch during the reaction. A nearly 50 % improvement in site-selective reactivity is observed compared to an FP cavity with static mirrors, demonstrating the utility of actuatable FP cavities as microreactors for organic chemistry.     

Sedimentation of a charged colloidal sphere in a charged cavity

An analytical study is presented for the quasisteady sedimentation of a charged spherical particle located at the center of a charged spherical cavity. The overlap of the electric double layers is allowed, and the polarization (relaxation) effect in the double layers is considered. The electrokinetic equations that govern the ionic concentration distributions, electric potential profile, and fluid flow field in the electrolyte solution are linearized assuming that the system is only slightly distorted from equilibrium. Using a perturbation method, these linearized equations are solved for a symmetric electrolyte with the surface charge densities of the particle and cavity as the small perturbation parameters. An analytical expression for the settling velocity of the charged sphere is obtained from a balance among the gravitational, electrostatic, and hydrodynamic forces acting on it. Our results indicate that the presence of the particle charge reduces the magnitude of the sedimentation velocity of the particle in an uncharged cavity and the presence of the fixed charge at the cavity surface increases the magnitude of the sedimentation velocity of an uncharged particle in a charged cavity. For the case of a charged sphere settling in a charged cavity with equivalent surface charge densities, the net effect of the fixed charges will increase the sedimentation velocity of the particle. For the case of a charged sphere settling in a charged cavity with their surface charge densities in opposite signs, the net effect of the fixed charges in general reduces/increases the sedimentation velocity of the particle if the surface charge density of the particle has a greater/smaller magnitude than that of the cavity. The effect of the surface charge at the cavity wall on the sedimentation of a colloidal particle is found to increase with a decrease in the particle-to-cavity size ratio and can be significant in appropriate situations.     

Optimization of an Asymmetric Reaction in the Cavity of Chiral Aromatic Oligoamide Foldamers

An asymmetric reaction can be implemented and refined in the helical cavity of aromatic oligoamide sequences. These sequences, which bear a chiral inducer, can fold into helical structures with absolute control of the helical sense, whereby a ketone substrate covalently linked in the cavity can be asymmetrically reduced to diastereomers. The diastereoselectivity of the reduction is highly dependent on the shielding efficiency of the helical cavity. Iterative modifications of the sequence, such as addition and replacement of monomers, can fine-tune the cavity to realize the asymmetric reaction, thereby progressively increasing the diastereomeric excess up to 90 %.     

Structural Characterization of l-proline and Morpholine Appended Chiral Calix[4]resorcinarene Molecules

Calix[4]resorcinarenes serve as host molecules for small guest molecules. Recently calixarenes have been appended to chiral molecules in an attempt to promote chiral recognition. To take advantage of both cavity host and chiral substituent properties the position of the chiral moiety is important. We report the synthesis and structural characterization of two calix[4]resorcinarene based molecules that have helical chirality in the solid state. The calix[4]resorcinarene 1 has chiral l-proline ethyl ester substituents positioned perpendicular to the cavity whereas the calix[4]resorcinarene 2 has morpholines positioned parallel to the cavity which extend the depth of the cavity. Compound 1 is one of the first compounds to show the position of chiral centers with respect to the calixarene cavity. 1H and 13C NMR spectroscopy indicate that the helical chirality of 2 is retained at low temperature in nonpolar solvents.     

Mobility matrix of a spherical particle translating and rotating in a viscous fluid confined in a spherical cell, and the rate of escape from the cell

The mobility matrix of a spherical particle moving in a spherical cavity, filled with a viscous incompressible fluid, and with no-slip boundary condition at the wall of the cavity, is evaluated from the Oseen tensor for the cavity by the method used by Lorentz for a particle near a planar wall. For the case that the particle is a rigid sphere with no-slip boundary condition the comparison with exact calculations shows that the approximation is quite accurate, provided the radius of the particle is small relative to that of the cavity, and provided the particle is not too close to the wall. The translational mobility is used to derive the diffusion tensor of a Brownian particle via an Einstein relation. The approximate result for the diffusion tensor is employed to estimate the rate of escape of a Brownian particle from a cavity with semipermeable wall.