Advances in Molecular Relaxation and Interaction Processes最新文献

In our model the main attention is paid to the interrelations of the structure of a three-dimensional array and the behaviour of the molecules forming it. The method chosen consists of the analysis of a distribution of a molecular translation along kinetic units - that is sections of the array made of various number of molecules with a certain structure. The analysis shows that elementary acts of self-diffusion (translation) are of a collective character in the main. Such a distribution is one of the most significant characteristics of the dynamics of molecules. Thermodynamic properties are also related to it.

A preliminary study of aluminium isopropoxyde [(CD₃)₂CHO]₃Al in carbon tetrachloride solutions led to the characterization of the γ(CH), δ(CH) and ν(CH) vibrations in the isopropoxyde groups either free or coordinated in autoassociation complexes. Different species, dissociated or coordinated, formed when isopropanol(D₆) is adsorbed on to site pairs (O,Al) have been shown by the ν(CH) and {δ(CH),δ(OH)} studies of coadsorption of isopropanol(D₆) with pyridine(D₅) (a Lewis base) or aluminium pentachloride (a Lewis acid) or perfluorotertiobutanol, which is dissociatively adsorbed. The isopropanol(D₆) dissociation needs a very basic O site. The coordination takes over both the Lewis base and the H bond donor functions of the hydroxyl groups ; two coordinated species are formed on site pairs (O strong base, Al weak acid) and (O weak base, Al strong acid). The interaction energy is tentatively evaluated to − 40 kcal mol⁻¹. Very acidic hydroxyl groups are created when perfluorotertiobutanol is adsorbed : the energy of the hydrogen bridge which they form with physisorbed isopropanol(D₆) may be of − 12 kcal mol⁻¹.

The real component of shear impedance, R_L, was measured in concentrated aqueous solutions of sodium deoxycholate, by means of different experimental techniques, in the frequency range 30KHz – 830MHz, at temperatures from 20°to 40°C. The results obtained give direct confirmation on the existence of viscoelastic effects in such a system; they allow, also, to evaluate an upper limit for the dynamic shear viscosity values. From these data it is evident that shear viscosity relaxes well below the usual frequency range of longitudinal ultrasonic experiments. As a consequence, the previously observed large longitudinal ultrasonic absorption appear to be due mainly to volume viscosity contributions.

The A ← 0(H)CH(CD₃)₂ and A ← (CD₃)₂CHOH]₂ interactions were shown by an infrared study of complexes in carbon tetrachloride solutions, where A is a Lewis acid : SbCl₅, AlCl₃, AlBr₃, SnCl₄, BF₃, Eu(fod)₃, ICl, I₂ and S0₂. In the coordination, the ν(OH) and ν(CO) frequencies decrease and the intensities increase, it is the opposite trend for the ν(CH) vibration ; the ν(CD₃) band appears as a doublet ; there is no significant effect on the γ(CH) and {δ(CH), δ(OH)} frequencies. The complexation enthalpies, for Brönsted or Lewis acids, and the Δ$\text{ν}$(CH) shifts are linearly correlated.

Some aspects of the difficulties encountered in describing irreversible phenomena by means of classical mechanics are considered.

The molecular dynamics of CH₂Cl₂ in the liquid state have been simulated with two model representations of the intermolecular potential. These consist of a 3 × 3 and 5 × 5 atom-atom simulation with or without fractional charges at the atomic sites. A variety of thermodynamical and spectroscopic results show that the 5 × 5 potential is the more accurate. The effect of adding charges is significant but not pronounced. For example the simulated far infra-red spectrum looks 15% more like the real thing after an attempt has been made to include charge-charge electrodynamics. It is clear, however, that a better representation of the intermolecular pair potential of CH₂Cl₂ is needed to match the far infra-red results, and this can probably be obtained by measurements of the second $\text{dielectric}$ virial of CH₂Cl₂ over a sufficient temperature range. A coordinated experimental effort is needed to test the wide-range of simulated spectra now available for the asymmetric top CH₂Cl₂.

The usefulness of computer simulations of dense polyatomic fluids is asserted and illustrated by means of several examples (especially rotation-translation coupling and conformational transitions in alcanes). Technical points and present trends in simulations are discussed as well.

A molecular dynamics simulation of CHCl₃ is reported using a 5 × 5 Lennard-Jones atom-atom potential with partial charges at each atomic site. Thermodynamic and spectral properties have been computed for direct comparison with a range of experimental measurements. In general the agreement is good, given the semi-empirical nature of the pair potential used. Having checked the efficiency of the simulation in this way it is possible to use the algorithm to investigate molecular properties of liquid CHCl₃ which are not easily detectible with experimental or purely (non-numerical) theoretical methods. A range of mixed autocorrelation functions of the type 〈A(o)B(t)〉 has been used in this way to investigate: a) non Gaussian effects in the liquid state of CHCl₃; b) non-linear effects; c) rotation/translation effects too subtle for detection with present-day spectroscopic methods. It is clear that the classical theory of the Brownian motion [12] is in need of development because the simulation shows that it is not possible to factorise conditional probability density functions of rotation and translation into purely constituent parts. The pair-potential could be improved if measurements on the second dielectric virial coefficient of CHCl₃ vapour were to become available over a sufficient range of density.

We introduce (following Ciccotti et al.) the subject of cybernetic spectroscopy with reference to the computer simulation of rototranslational mixed a.c.f.'s in the molecule-fixed frame of CH₂Cl₂ in the liquid state (E.M.L.G. pilot project). The detailed acquisition of such “signatures” is possible only via numerical methods using fast computers and provides a corroborative challenge both in the fields of experimental spectroscopy and phenomenological theory. Up to nine elements each of the mixed a.c.f. matrices are observable depending on molecular symmetry. The properties of these elements are basically important in the interpretation of molecular diffusion and broad-band spectroscopy of liquids and mesophases.

Infrared and Raman spectra of 4-bromobutyric acid are reported. Raman spectra of the ν(C-Br) region for a range of concentrations below the critical micelle concentration have been interpreted in terms of $\text{gauche}$ C₄ - C₃ linkages becoming more dominant upon pre-micellar association. The results demonstrate the viability of using bromine substituted species in Raman studies of micelle formation.