The Role of Nonstatistical Dynamics in the Chemistry of Reactive Intermediates

直到最近，对有机反应机制感兴趣的实验化学家不得不依靠间接技术/通常基于反应动力学/来推断反应中间体的性质，因为这些瞬态物质通常不能直接观察到。随着皮秒和飞秒光谱技术的出现，光化学反应中瞬态中间体的直接检测有时成为可能。然而，对于热引发反应，中间体只有在寿命超过1微秒时才能被检测到，因此间接方法在大多数机理研究中仍然占主导地位。将获得的关于反应物和产物的数据转化为对未检测到的中间体的见解，必然依赖于某种动力学模型。显然，如果选择的模型是错误的，那么关于反应机制的结论也可能是错误的。现在有令人信服的证据，不仅来自PI的实验室，而且来自世界各地的许多其他实验室，至少对于某些反应，这确实发生了。问题是，大多数熟悉的动力学模型，如过渡态理论，依赖于所谓的统计近似的有效性。然而，实验和分子动力学（MD）模拟表明，许多热生成的反应中间体表现出非统计动力学行为。非统计动态可以总结如下。大多数反应性中间体是在其可用振动模式的一小部分的选择性激发下产生的。在热反应中，这种选择性可以有两个来源：对于中间体处于能量平台或非常浅的最小值的反应，选择性主要源于需要将反应物的多余能量定位在反应坐标中以形成中间体。对于中间体在势能面（PES）上占据相对较深的局部最小值的反应，最大的贡献者将是伴随从第一过渡态到中间体的过程的势能（PE）到动能（KE）的转换。这种转换在模式中储存能量，在很大程度上是由中间结构和过渡结构的固定点的几何差异决定的。对于光化学反应，选择性通常来自于PE到KE的转换，伴随着通过锥形交叉。选择性激发的提议本身与反应动力学的标准统计近似并不矛盾。区别在于人们认为这些选择性兴奋的物种发生了什么。统计模型，就其本质而言，假设分子内振动能量再分配（IVR）发生的速度比中间产物转化为任何产物的速度要快得多，因此选择性激发没有机制后果。我们和其他人一直在质疑的就是这个假设。在本建议中，我们力求实现两个主要目标。一个是使用最先进的超快光谱学，首次为非统计动力学的MD模拟产生的一些主要预测提供直接实验测试。另一个是扩大反应类型的范围，以进行非统计行为的测试。后者将使我们更接近以一般方式理解非统计动态效应何时发生以及其后果的长期目标。

Until quite recently, experimental chemists interested in the mechanisms of organic reactions have had to rely on indirect techniques / commonly based on reaction kinetics / to infer something about the properties of reactive intermediates, because these transient species have usually not been directly observable. With the advent of picosecond and femtosecond spectroscopies direct detection of transient intermediates is sometimes possible for photochemical reactions. However, for thermally initiated reactions intermediates can only be detected if they have lifetimes in excess of ~1 microsecond, and so the indirect methods still hold sway in most mechanistic studies. The transformation of data obtained on reactants and products into insights about undetected intermediates necessarily relies on some kind of kinetic model. Obviously, if the chosen model were wrong, the conclusions about the reaction mechanisms would probably be wrong as well. There is now persuasive evidence, not only from the PI's lab but also from many other labs around the world that, at least for some reactions, this is exactly what has happened. The problem is that most of familiar kinetic models, such as Transition State Theory, rely on the validity of the so-called statistical approximation. However, experiments and molecular dynamics (MD) simulations have revealed that many thermally generated reactive intermediates exhibit nonstatistical dynamical behaviour. Nonstatistical dynamics can be summarised as follows. Most reactive intermediates are created with selective excitation of a small subset of their available vibrational modes. In thermal reactions there can be two sources of this selectivity: for reactions in which the intermediate sits on an energetic plateau or in a very shallow minimum, the selectivity arises primarily from the need to localize the excess energy of the reactant(s) in the reaction coordinate for formation of the intermediate. For reactions in which the intermediate occupies a relatively deep local minimum on the potential energy surface (PES), the largest contributor will be the potential energy (PE) to kinetic energy (KE) conversion that accompanies the progress from the first transition state to the intermediate. This conversion deposits energy in modes that are, in large measure, determined by the geometry differences of the stationary points for an intermediate and the transition structure from which it was formed. For photochemical reactions, the selectivity commonly arises from the PE to KE conversion that accompanies passage through a conical intersection. The proposal of selective excitation is not, by itself, at odds with the standard statistical approximation to reaction kinetics. The distinction arises in what one thinks happens to these selectively excited species. Statistical models, by their very nature, assume that intramolecular vibrational energy redistribution (IVR) occurs much faster than conversion of the intermediate to any product, and hence that selective excitation has no mechanistic consequence. It is that assumption that we and others have been questioning. In the present proposal we seek to accomplish two principal goals. One is to use state-of-the-art ultrafast spectroscopies to provide, for the first time, direct experimental tests of some of the principal predictions arising from MD simulation of nonstatistical dynamics. The other is to expand the range of reaction types for which tests of nonstatistical behaviour can be conducted. The latter will move us a step closer to the long-term goal of understanding in a general way when nonstatistical dynamical effects will occur, and what their consequences will be.