Solvent dynamic effect barrier height

Many questions in the analysis of solvent dynamics effects for isomer-izations in solution have arisen, such as (1) when is a frequency-dependent friction needed (2) when does a change of solvent, of pressure, or of temperature change the barrier height (i.e., the threshold energy), and (3) when is the vibrational assistance model needed, instead of one based on Eq. (1.1) or its extensions ... 

First of all, liquid-phase studies generally do not obtain data which allows static and dynamic solvent effects to be separated [96,97], Static solvent effects produce changes in activation barriers. Dynamic solvent effects induce barrier recrossing and can lead to modification of rate constants without changing the barrier height. Dynamic solvent effects are temperature and viscosity dependent. In some cases they can cause a breakdown in transition state theory [96]. 

Some properties of excited merocyanine dyes are compiled in Table 36.6. The most frequently studied is merocyanine 540 (Ml). Ml is readily soluble in alcohols and Hpid membranes, making it attractive for many technical applications and biological activity studies.The solvent polarity has a significant influence on Tf and the related quantum yields. - An intrinsic barrier height of Ej, = 3 kj mol" for isomerization in Sj was determined from dynamical solvation effects on the rate of Ml in polar solvents. Environmental effects on radiative and nonradiative transitions have been studied for mero-cyanines in homogeneous and microheterogeneous systems. ... 

Summary Employing temperature-dependent dynamic NMR spectroscopy we have been able to measure both the activation enthalpy and entropy of inversion processes. Moreover we have performed a systematic study of the influence of the substituent groups, the nature of the cation, and solvent effects on the height of the inversion barrier. 

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