Reorientation and isomerization of trans-stilbene in alkane solutions
Reorientation and isomerization of trans-stilbene in alkane solutions
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DOI：
10.1021/j100319a018
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发表时间：
1988-04
期刊：
影响因子：
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通讯作者：
S. K. Kim;G. Fleming
中科院分区：
文献类型：
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作者：
S. K. Kim;G. Fleming
非极性溶剂施加的摩擦对ZraŦi-stilbene光异构化的定性影响现在似乎很清楚了。正如障壁交叉理论所预测的那样，随着摩擦力的增加，流速首先上升，达到最大值，然后随着摩擦力的进一步增加而下降。在高压气相中速率最大，为1·2，在液体溶液中速率随溶剂粘度的减小而减小。对曲线上升3,6部分和下降4,5部分进行定量描述的尝试遇到了困难。在本文中，我们集中在高摩擦（溶液）部分的曲线。如果用粘度的函数表示二苯乙烯异构化速率，通过溶剂或温度的变化得到不同的粘度，如果溶剂摩擦等于宏观剪切粘度，则速率下降的速度比Kramers方程预测的要慢。对这种行为有几种可能的解释：(1)分子内电位表面可能依赖于溶剂。6,8(2)溶剂粘度可能不能充分衡量异构化分子所感受到的摩擦，要么是因为分子内屏障的存在强调了溶剂的短时间响应（频率依赖性摩擦*“12），要么仅仅是因为斯托克斯定律在异构化过程中涉及的分子尺度上是不充分的。(3)除反力坐标外的其他自由度可能使摩擦依赖关系复杂化。13“15也许Stokes定律的不足是最明显的解释，最近Leeet等人（16）和Courtney等人（17）都采纳了Velsko等人（18）的建议，并使用旋转重新定向时间作为微观摩擦的度量。两个小组都发现，斯托克斯-爱因斯坦关系不能很好地描述室温下一系列烷烃溶剂中的重定向时间。如果用逆取向时间作为摩擦测量，则得到了对Kramers方程（在相同溶剂下）的改进拟合。16,17因此，克雷默斯方程在描述一系列烷烃在室温下的异构化时的失败，在很大程度上可以追溯到斯托克斯-爱因斯坦关系的失败。
Introduction The qualitative influence of the friction exerted by nonpolar solvents on the photoisomerization of ZraŦi-stilbene now seems clear. 1" 6 As predicted by barrier crossingtheories, 7 the rate first rises as friction is increased, reaches a maximum, and then turns over and decreases as friction is increased further. The maximum occurs in high-pressure gas phase, 1· 2 andin liquid solutions the rate is a decreasing function of solvent viscosity. Attempts to provide a quantitative description of both rising3, 6 and falling4, 5 parts of the curve have met with difficulties. In this paper we concentrate on the high-friction (solution) portion of the curve. If the stilbene isomerization rate is plotted as a function of viscosity with different viscosities being obtained by change of solvent or temperature, the rate falls off more slowly than is predicted by Kramers’ equation if the solvent friction is equated with macroscopic shearviscosity. 4, 5 There are several possible explanations for this behavior:(1) The intramolecular potential surface could be solvent dependent. 6, 8 (2) The solvent viscosity may not be an adequate measure of the friction felt by the isomerization molecule, either because the presence of the intramolecular barrier places an emphasis on the short-time response of the solvent (frequency-dependent friction*" 12) or simply because Stokes’s law is inadequate on the molecular scale involved in the isomerization process.(3) Other degrees of freedom than the reaction coordinate may complicate the friction dependence. 13" 15Perhaps the possible inadequacy of Stokes’ law is the most obvious explanation, and recently both Leeet al. 16 and Courtney et al. 17 have taken up the suggestion of Velsko et al. 18 and used rotational reorientation times as a measure of microscopic friction. Both groups found that the Stokes-Einstein relation gave a poor de-scription of reorientation times in a series of alkane solvents at room temperature. Improved fits to Kramers’ equation (in the same solvents) were obtained if the inverse reorientation time was used as the friction measure. 16, 17 Thus, much of the failure of Kramers’ equation to describe stilbeneisomerization in a series of alkanes at room temperature can be traced to the failure of the Stokes-Einstein relation.