New Horizons in Chemical and Photochemical Dynamics

化学变化，无论是由反应性原子、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具有根本的重要性。例如，合成复杂的有机分子，如植物和动物生命中自然存在的分子，或需要构建功能性现代材料，需要深入了解反应机制来设计合成途径。基于热力学和反应速率理论的物理化学思想支撑着我们预测化学变化方向和这种变化发生速度的能力。化学反应和光解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定量基础上。势能面（PESs）（基于玻恩-奥本海默（Born-Oppenheimer）对轻电子的快速运动与较重原子核的慢速运动的分离）是一个重要的概念，因为它们提供了化学变化发生的能量景观图。ps上的最小值和势垒分别对应于原子的稳定构象和短暂的过渡态。光解作用涉及到能量高于最低基态的PESs的动力学，而达到这些激发态所需的额外能量是由光的吸收提供的。在理解光化学和反应过程的动力学方面，一个强大的推动力是实验和理论研究之间的密切相互作用——可以说，该领域已经做了很多工作来刺激理论方法的发展，以计算PE景观和描述这些表面上的分子动力学。这些方法（服从于简化近似）现在被广泛应用于分子建模，例如药物设计，酶催化和许多其他领域。实验方法和理论方法的历史发展依赖于对只有少量原子的系统的互补研究（例如双原子和三原子分子的光解作用；原子与双原子分子的反应），从而可以计算精确的PESs，并进行精确的量子力学（QM）散射计算。这类实验大多在气相、低温和稀薄的分子束环境中进行，因此可以忽略溶剂化或分子间相互作用的复杂因素。例如，这种系统的相当大的成功揭示了奇特的QM效应在化学中的重要性，如穿过反应势垒的隧道，散射共振，ps之间的非绝热耦合，以及不同途径产生相同产物之间的干扰。对于具有3个原子的光化学或反应系统，只需要3个坐标来描述原子的所有可能排列，因此可以计算出整个PE格局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变化的。这样，我们将使反应和光解动力学领域更接近于有机、无机和生物化学家在合成中使用的化学反应类型。我们的策略包括发展新的实验和理论方法。面对巨大的挑战，必须采取一种以联盟为基础的方法，将两所大学的互补专业知识汇集在一起，解决选定的问题，从中我们可以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.