New Horizons in Chemical and Photochemical Dynamics

化学变化，无论是由活性原子、自由基和分子之间的碰撞引起的，还是由光的吸收(光化学)引起的，在化学的所有分支中都是至关重要的。例如，合成复杂的有机分子，如那些自然存在于动植物中的分子，或构建现代功能材料所需的那些分子，需要深入了解反应机理以设计合成路径。基于热力学和反应速率理论的物理化学的想法支撑着我们预测化学变化的方向和这种变化发生的速度的能力。化学反应和光解离领域试图将这些理论建立在对化学键断裂和形成的量子力学的深刻理解的量化基础上。势能面(PES)(基于轻电子的快速运动与较重原子核的较慢运动的Born-Oppenheimer分离)是一个基本的概念，因为它们提供了发生化学变化的能量版图(S)。PESS上的极小值和势垒分别对应于原子的稳定构象和短暂的过渡态。光解涉及PESS上的动力学，其能量高于最低的基态，达到这些激发态所需的额外能量由光的吸收提供。在理解光化学和反应过程动力学方面取得进展的一个强大的驱动力是实验和理论研究之间的密切互动--可以说，该领域已经在很大程度上刺激了理论方法的发展，以计算PE景观并描述这些表面上的分子动力学。这种方法(经过简化近似)现在被广泛用于分子建模，例如药物设计、酶催化和许多其他领域。实验和理论方法的历史发展依赖于对只有少量原子的体系(如双原子和三原子分子的光解离；原子与双原子分子的反应)的补充研究，以便能够计算精确的PESS和进行精确的量子力学(QM)散射计算。这样的实验大多是在气相中进行的，在分子束的低温和稀薄环境中进行，因此可以忽略溶剂化或分子间相互作用的复杂因素。例如，这类系统的巨大成功揭示了奇特的QM效应在化学中的重要性，如穿透反应势垒、散射共振、PES之间的非绝热耦合以及不同路径之间对相同产品的干扰。对于具有3个原子的光化学或反应体系，只需要3个坐标来描述原子的所有可能的排列，因此可以计算跨越整个PE景观的代表性构型的相关PE。我们现在寻求一种多管齐下的方法来将这种研究扩展到更复杂的系统，目的是了解更大分子的PE景观(对于N原子，需要3N-6坐标来描述相关的PE超表面)，PE表面之间的跳跃的影响，并研究在溶剂存在下能量景观和化学动力学是如何改变的。通过这样做，我们将使反应和光解离动力学领域更接近有机、无机和生物化学家在合成中使用的化学反应类型。我们的战略包括发展新的实验和理论方法。这些重大挑战需要一种以联盟为基础的方法，在这种方法中，将两所大学的互补专业知识结合在一起，以解决选定的问题，我们可以从这些问题中学到很多关于化学变化的知识。

Chemical change, whether caused by collisions between reactive atoms, radicals and molecules, or by absorption of light (photochemistry), is of fundamental importance in all branches of Chemistry. For example, synthesis of complicated organic molecules, such as those naturally occurring in plant and animal life, or needed to construct functional modern materials, requires an in-depth understanding of reaction mechanisms to design synthetic pathways. Ideas from physical chemistry based on thermodynamics and reaction rate theory underpin our ability to predict directions of chemical change and how quickly such change will occur. The fields of chemical reaction and photodissociation have sought to place such theories on a quantitative foundation built on deep understanding of the quantum mechanics of breakage and formation of chemical bonds. Potential energy surfaces (PESs) (based on the Born-Oppenheimer separation of the fast motion of light electrons from the slower motion of heavier atomic nuclei) are an essential concept because they provide a map of the energy landscape(s) over which chemical change occurs. Minima and barriers on the PESs correspond, respectively, to stable conformations of the atoms and short-lived transition states. Photodissociation involves dynamics on PESs lying higher in energy than the lowest, ground state, with the extra energy needed to reach these excited states provided by absorption of light. A powerful driver for advances in understanding of the dynamics of photochemical and reactive processes has been a close interaction between experimental and theoretical studies - arguably, the field has done much to stimulate the development of theoretical methods to calculate PE landscapes and describe the molecular dynamics on these surfaces. Such methods (subject to simplifying approximations) are now finding widespread use in molecular modelling of, for example, drug design, enzyme catalysis, and many other fields. The historical development of experimental and theoretical methods has relied on complementary studies of systems with only a small number of atoms (e.g. photodissociation of diatomic and triatomic molecules; reaction of atoms with diatomic molecules) so that accurate PESs can be computed and precise, quantum-mechanical (QM) scattering calculations carried out. Such experiments were mostly conducted in the gas phase, in the low-temperature and rarefied environment of a molecular beam, so that complicating factors of solvation, or interaction between molecules can be ignored. Considerable success with such systems has, for example, revealed the importance of exotic QM effects in chemistry such as tunnelling through reaction barriers, scattering resonances, non-adiabatic coupling between PESs, and interference between different pathways to the same products. For a photochemical or reactive system with 3 atoms, only 3 coordinates are required to describe all the possible arrangements of the atoms and the associated PEs can thus be computed for representative configurations spanning the entire PE landscape. We now seek a multi-pronged approach to extend such studies to more complicated systems, with the intention of learning about PE landscapes for larger molecules (for N atoms, 3N-6 coordinates are needed to describe the associated PE hypersurface), the effects of jumps between PE surfaces, and to examine how the energy landscapes and chemical dynamics are changed in the presence of solvent. In so doing, we will bring the fields of reaction and photodissociation dynamics closer to the types of chemical reactions used in synthesis by organic, inorganic and biological chemists. Our strategy involves development of new experiments and theoretical methods. The substantial challenges necessitate a consortium-based approach, in which complementary expertise in two Universities is brought together to address selected problems from which we can learn much about chemical change.