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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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送鮮花一朵gmy1990(金幣+2): 2011-07-14 21:35:11
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不錯。這個說法以前我也聽過，但沒有認(rèn)真思考過。 我想，這也是為什么在很多體系計算中，反應(yīng)焓、活化焓都可以跟實(shí)驗(yàn)值對得很好，而反應(yīng)自由能變、活化自由能則總是相差很多的原因之一吧。通常的文獻(xiàn)描述中，會用Gaussian對熵的計算很不準(zhǔn)確來解釋（今天群里聊到這個問題，孤獨(dú)狼提到誤差在50%左右）。 結(jié)合這份描述，原因則歸責(zé)于尋找過渡態(tài)鞍點(diǎn)的時候，Gaussian尋找到的是Eelec的鞍點(diǎn)，而非自由能的鞍點(diǎn)——而這兩個點(diǎn)未必相同。造成尋找到的鞍點(diǎn)，未必是真實(shí)反應(yīng)過程的真正鞍點(diǎn)。然而顯然Gaussian不能做自由能面的掃描以尋找到真正的鞍點(diǎn)。這個在齊格勒的早期的文獻(xiàn)中發(fā)現(xiàn)他似乎用了自己編寫的程序跑過自由能面掃描鞍點(diǎn)，但沒有詳述過程，只是提出對熵值做了矯正。 那么，如果Gaussian程序不能掃描自由能的鞍點(diǎn)，就一直不會解決活化自由能與實(shí)驗(yàn)值無法對應(yīng)的問題了。 不知我理解的是否有誤，有專家給指點(diǎn)一下否？ [ Last edited by zhangmt on 2011-7-19 at 10:25 ] |
4樓2011-07-14 20:53:18
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