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xiaozufeng

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[交流] 关于过渡态与过渡结构已有2人参与

The distinction between a transition state and a transition structure. Strictly speaking, a transition state is a thermodynamic concept, the species an ensemble of which are in a kind of equilibrium with the reactants in Eyring's1 transition- state theory. Since equilibrium constants are determined by free energy differences, the transition state, within the strict use of the term, is a free energy maximum along the reaction coordinate (in so far as a single species can be considered representative of the ensemble). This species is also often (but not always) also called an activated complex. A transition structure, in strict usage, is the saddle point on a theo- retically calculated PES. Normally such a surface is drawn through a set of points each of which represents the enthalpy of a molecular species at a certain geometry; recall that free energy differs from enthalpy by temperature times entropy. The
transition structure is thus a saddle point on an enthalpy surface. However, the energy of each of the calculated points does not normally include the vibrational energy, and even at OK a molecule has such energy (ZPE). The usual calculated PES is thus a hypothetical, physically unrealistic surface in that it neglects vibrational energy, but it should qualitatively, and even semiquantitatively, resemble the vibrationally-corrected one since in considering relative enthalpies ZPEs at least roughly cancel. In accurate work ZPEs are calculated for stationary points and added to the "frozen-nuclei" energy of the species at the bottom of the reaction coordinate curve
in an attempt to give improved relative energies which represent enthalpy differences at OK (and thus, at this temperature where entropy is zero, free energy differences also). It is also possible to calculate enthalpy and entropy differences, and thus free energy differences, at, say, room temperature. Many chemists do not routinely distinguish between two terms, and in this book the commoner term, transition state, is used. Unless indicated otherwise, it will mean a calculated geometry, the saddle point on a hypothetical vibrational-energy-free PES. ----from

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zhangmt

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gmy1990(金币+2): 2011-07-14 21:35:11

不错。这个说法以前我也听过，但没有认真思考过。
我想，这也是为什么在很多体系计算中，反应焓、活化焓都可以跟实验值对得很好，而反应自由能变、活化自由能则总是相差很多的原因之一吧。通常的文献描述中，会用Gaussian对熵的计算很不准确来解释（今天群里聊到这个问题，孤独狼提到误差在50%左右）。
结合这份描述，原因则归责于寻找过渡态鞍点的时候，Gaussian寻找到的是Eelec的鞍点，而非自由能的鞍点——而这两个点未必相同。造成寻找到的鞍点，未必是真实反应过程的真正鞍点。然而显然Gaussian不能做自由能面的扫描以寻找到真正的鞍点。这个在齐格勒的早期的文献中发现他似乎用了自己编写的程序跑过自由能面扫描鞍点，但没有详述过程，只是提出对熵值做了矫正。
那么，如果Gaussian程序不能扫描自由能的鞍点，就一直不会解决活化自由能与实验值无法对应的问题了。
不知我理解的是否有误，有专家给指点一下否？

[ Last edited by zhangmt on 2011-7-19 at 10:25 ]