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majun04

木虫 (著名写手)

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[交流] 【求助】前线轨道分析

好久前发过帖子,现在想起来,还没有太明白,请高手帮忙答疑一下.

就是在反应过程中,怎么才能看出来是那些化学键的断裂或者形成起到决定性的作用?这个从前线轨道分析可以看出来吗?怎么样分析?
在文献中看到这样一段分析，感觉其中有一句不明白（文中加粗部分）恳请高手指点。
问题：这个过渡态对应着多个键的反应O—H键的断裂C—N键的转化，C—O键和N—H键的形成，怎么能够从HOMO-2的轨道信息中得到，这个轨道就是对应着H的转移呢？怎么样从输出文件中看出分子轨道对应着那些原子的变化，恳请帮忙。

It is interesting to notice the molecular orbital interaction between the H2O molecule and amino acetonitrile (1). We plotted the frontier orbitals of 1, the complex (H2O + 1, precursor of reactants), and transition state 2 in Figure 3. Assuming that the molecular plane of 1 passes through the central carbon atom, the nitrile group, and the nitrogen atom, we then can distinguish the HOMO-1 as being symmetric to the molecular plane, from HOMO-2, on the molecular plane of 1. These two MOs feature the two e orientations of the CN group. When H2O starts to approach 1, the shapes of these MOs as well as the energy order do not change (as shown in the complex portion of MOs in Figure 3). However, we found that the energy of HOMO-2 of the complex starts to increase as the two approaching moieties get closer and closer, and it became HOMO at the transition state. We can trace these points along the reaction coordinate and plot the change in these MO energies, shown in Figure 4. At point 12 where the CaaaO distance is 2.53 ? and the HaaaOH distance is 1.01 ?, the MO energy of the original HOMO-2 jumps over and becomes HOMO-1. This MO energy keeps on rising, and at point 16 where the C...O distance is 1.89 ? and the HaaaOH distance is 1.02 ?, it becomes HOMO and remains HOMO to the transition state. This MO energy trend is similar to the potential energy profile of the transition state. On the contrary, the energies of the original HOMO and HOMO-1 of the precursor do not follow the trend and become HOMO-1 and HOMO-2, respectively, at the transition state. This result reminds us not to ignore the molecular interaction occurring in HOMO-2 of the precursor, which almost dominates the energy change of the reaction process. We found out that the formation of HOMO-2 and HOMO-3 in the precursor was mainly an MO combination from the HOMO of H2O and the HOMO-2 of amino acetonitrile, drawn in Figure 3. The positive (or same phase) combination forms HOMO-3, and the negative (or opposite phase) combination forms HOMO-2. Along the reaction coordinate, we inspected the changes in these MOs and assured that HOMO-2 represented the transfer of the hydrogen to the N atom of the nitrile group in amino acetonitrile, whereas that of HOMO-3 represented the formation of the C-O bond of the C atom of the nitrile group with the O atom of H2O. Because the MO energy of HOMO-3 does not change significantly (in fact, it decreases a little bit in Figure 4) during the whole reaction process, we are convinced that the crucial part of this nitrile hydrolysis process was the transfer of the hydrogen atom of H2O to the N atom of the nitrile group but not the formation of the C-O bond between H2O and the nitrile group.

从IRC路径上的各点，在过渡态前是水中氢原子断键远离和氧原子对腈基碳亲核过程。这个协同过程，应该有个“主次”，如果找到主要过程（也就是能垒高得过程），并通过引入催化剂的方法解决掉，才是催化的目的。
是否是原子轨道发生位相相符的重叠就能够形成稳定的化学键（《图解量子化学》福井谦一）。水分子的HOMO轨道主要由O原子的原子轨道贡献的。
而在上述figure3中，因为水分子的HOMO轨道和aminoacetoitrile的HOMO-2轨道以位相相符合的方式重叠，能够形成稳定的C－O键。所以这两个轨道转化成的precursor和过渡态的HOMO-3轨道代表着O原子对C原子的亲核进攻过程。
而水分子的HOMO轨道和aminoacetitrile的HOMO-2轨道以位相不相符合的情况重叠是不能够形成稳定的化学键的，对应着水分子中质子氢的转移。
不知道上述分析得对不对，希望大家发表意见，讨论讨论。

把上面的例题算了一下，
水分子的轨道信息如下：
Molecular Orbital Coefficients
                        1       2       3       4       5
                     (A1)--O (A1)--O (B2)--O (A1)--O (B1)--O
EIGENvalueS -- -20.56173  -1.33881  -0.69998  -0.56941  -0.49714
其中5为HOMO轨道
aminoacetoitrile的分子轨道信息如下：
                     11       12       13       14       15
                     (A"

--O (A')--O (A')--O (A"

--O (A')--O
EIGENvalueS -- -0.59079 -0.55724 -0.47552 -0.46248 -0.42548
其中15为HOMO轨道
那么可以看出，水分子的HOMO轨道能量和aminoacetoitrile的HOMO-2的轨道能量最接近，也就是能差最小，所以随着两个分子相互作用，这两个轨道将相互叠加作用。
在这个过程中，有O－H键的断裂，和C－O键的形成。那么就出现了上述的HOMO-3对应着O－C键的形成，HOMO-2对应着O－H键的断裂。

不知道这样分析对不对，恳请大家发表一下意见。

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