Thetransndash;cisisomerization rates for stiffhyphen;diphenylbutadiene (Shyphen;DPB) innhyphen;alkane solvents were measured using single photon counting methods and the rotational reorientation times tgr;Rfor Shyphen;DPB andtransstilbene were obtained by picosecond polarization spectroscopy. In neither case did tgr;Rvs viscosity show Stokesndash;Einsteinndash;Debye (SED) behavior. The values of tgr;Rwere used to calculate the angular velocity correlation frequencies bgr; using the Hubbard relation. The variation of isomerization rate with bgr; was found to be predicted well by the Kramers equation when barrier frequencies of 154 cmminus;1for stilbene and 16 cmminus;1for Shyphen;DPB were used. This Kramershyphen;Hubbard fit finesses questions regarding the validity of the one dimensional Kramers model and focuses attention on the SED equation. The dynamical relationship between the torsional friction, important in isomerization, and rotational friction, which determines the overall angular motion of the molecules, is discussed.
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