No abstract

The rates of thermal Z-E isomerization of 4-(dimethylamino)-4'-nitroazobenzene were measured in a variety of solvents, and the Kirkwood plot revealed that the rate constant increase is much larger in polar solvents than expected from the continuum theory. The result can be reasonably explained by the intervention of rotational isomerization which proceeds via a highly polar transition state. The large negative activation volumes support it. It is also shown that a strong electron-donating dialky lamino group makes the rotational transition state stable enough to compete with the inversional one without assistance from the electron-attracting substituent. On the contrary, an electron-attracting group like nitro increases the stability of the inversion transition state more than that of the transition state for the rotational isomerization.

No abstract

Pressure effects for the thermal Z-E isomerization of several azobenzenes were measured in various solvents. The activation volumes for 4-(dimethylamino)-4'-nitroazobenzene indicate that the reaction mechanism changes from inversion in n-hexane to rotation in benzene and other relatively polar solvents. The same change in mechanism takes place in the case of 4-anilino-4'-nitroazobenzene when the solvent is changed from benzene to acetone; for 4-methoxy-4'-nitroazobenzene and unsubstituted azobenzene, the reaction mechanism does not change greatly with solvent polarity. The activation enthalpy for azobenzene was redetermined; the result suggests that the original dipole moment is reduced by about 40% in the activation step.

Supplementary Material Available: Description of the X-ray diffraction determination of 3, including final atomic positional parameters (Tables 3 and 4), thermal parameters (Table 5), and selected bond distances and bond angles (Table 6), (6 pages); observed and calculated structure factors (Table 7) (12 pages). Ordering information is given on any current masthead page.

SynopsisLow-and high-density polyethylenes were irradiated by electron beams with dose of 2-50 Mrad and then immersed in aqueous solution of acrylic acid (monomer concentration from 30 to 100 wt %) for 10 min-5 h at a temperature of 25-4OoC. The degree of grafting increases with time and levels off. High density polyethylene shows lower grafting rate and higher final % grafting in compared with low-density polyethylene. Both grafting rate and final % grafting increase with total dose of preirradiation, but show some saturation a t high doses. The highest grafting rate was observed at 60 wt % of monomer concentration where the grafted polyethylene swells to the largest extent in the monomer mixture. Apparent activation energies for the grafting are 19.6 and 27.3 kcal/mol for low-and high-density polyethylenes, respectively, reflecting the process of monomer diffusion in the film. Grafting rate decreases with increasing film thickness. Graft polymerization starts on the surface of the film and proceeds to the inner part with monomer diffusion through the grafted layer.

No abstract

due to the differences in electronic configuration and thus differences in polarizabilities of these molecules. In this work, the polarizability differences are taken into account in the calculation of the adsorption potential as the mean polarizability and the polarizability ratio of the adsorbate molecule (ail/ais) assigned to each C force center of the molecule, derived from the corresponding molecular experimental data. The predicted s value for phenanthrene/anthracene adsorption on graphite is 1.44 when polarizability data are taken from Lefevre et al.39 and 1.55 from Schuyer et al.40 instead of 1.23 for the experimental value as measured by gas-adsorption chromatography. These calculations demonstrate how sensitive the model is to the value accepted for the mean polarizability of the carbon force center of the adsorbate molecule. ConclusionThese results show that the anisotropic adsorption potential model of Meyer and Dietz,14Js when applied to the molecular theory of adsorption, predicts with good agreement, at least as well as the empirical adsorption potential laws, the thermodynamic functions of adsorption of alkanes and aromatic hydrocarbons on graphite.The adsorption potential which takes into account the high anisotropic polarizability of graphite can be expressed with a good approximation by a Lennard-Jones (12-6) potential. The Kirkwood-Muller constant used for isotropic adsorption potential calculations on graphite is 4/3 larger than the attractive constant which assumes that the polarizability in the direction normal to the basal plane is zero. Therefore, adsorption potential laws which use the Kirkwood-Muller attractive constants to predict BAS and qst values for the adsorption on graphite give results which are too large.The relative retention of geometrical isomers are equal when calculated with the anisotropic or isotropic potential models if the same polarizability and diamagnetic susceptibility increments are assigned to the adsorbate force centers. Similarly the conclusions of Battezzati et al." which cencern the most stable equilibrium adsorbate conformations on graphite still hold with the anisotropic adsorption potential model.Finally, this work shows that correct knowledge of the polarizability of the adsorbate molecule and the structural properties of the adsorbed layers is of great importance in order to derive the constants of the adsorption potential laws.Two simple three-parameter equations are proposed as functions to describe kinetic and thermodynamic effects of pressure. The functions are found to reproduce experimental results more accurately than the most frequently used quadratic equation. The estimated activation volumes at zero pressure are almost independent of the experimental pressure range for most of the reactions examined and their standard deviations are reasonably small. The activation volumes at infinite pressure are, in many cases, in fairly good agreement with the intrinsic activation volumes calculated by an independent procedure. IntroductionActivation volume ...