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.