Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India.
Appl Spectrosc. 2011 Oct;65(10):1087-115. doi: 10.1366/11-06406.
The study of reaction mechanisms involves systematic investigations of the correlation between structure, reactivity, and time. The challenge is to be able to observe the chemical changes undergone by reactants as they change into products via one or several intermediates such as electronic excited states (singlet and triplet), radicals, radical ions, carbocations, carbanions, carbenes, nitrenes, nitrinium ions, etc. The vast array of intermediates and timescales means there is no single "do-it-all" technique. The simultaneous advances in contemporary time-resolved Raman spectroscopic techniques and computational methods have done much towards visualizing molecular fingerprint snapshots of the reactive intermediates in the microsecond to femtosecond time domain. Raman spectroscopy and its sensitive counterpart resonance Raman spectroscopy have been well proven as means for determining molecular structure, chemical bonding, reactivity, and dynamics of short-lived intermediates in solution phase and are advantageous in comparison to commonly used time-resolved absorption and emission spectroscopy. Today time-resolved Raman spectroscopy is a mature technique; its development owes much to the advent of pulsed tunable lasers, highly efficient spectrometers, and high speed, highly sensitive multichannel detectors able to collect a complete spectrum. This review article will provide a brief chronological development of the experimental setup and demonstrate how experimentalists have conquered numerous challenges to obtain background-free (removing fluorescence), intense, and highly spectrally resolved Raman spectra in the nanosecond to microsecond (ns-μs) and picosecond (ps) time domains and, perhaps surprisingly, laid the foundations for new techniques such as spatially offset Raman spectroscopy.
反应机制的研究涉及系统地研究结构、反应性和时间之间的相关性。挑战在于能够观察反应物通过一个或多个中间体（如电子激发态（单重态和三重态）、自由基、自由基离子、碳阳离子、碳阴离子、卡宾、氮宾、硝酮离子等）转化为产物所经历的化学变化。中间体和时间尺度的多样性意味着没有一种“万能”技术。当代时间分辨拉曼光谱技术和计算方法的同时进步，在亚微秒到飞秒时间域内可视化反应中间体的分子指纹快照方面取得了很大进展。拉曼光谱及其灵敏的共振拉曼光谱已被证明是确定溶液相中短寿命中间体的分子结构、化学键、反应性和动力学的有效手段，与常用的时间分辨吸收和发射光谱相比具有优势。如今，时间分辨拉曼光谱是一种成熟的技术；它的发展在很大程度上归功于脉冲可调谐激光器、高效光谱仪以及高速、高灵敏度多通道探测器的出现，这些探测器能够收集完整的光谱。本文将简要介绍实验装置的时间顺序发展，并展示实验人员如何克服众多挑战，在纳秒到微秒（ns-μs）和皮秒（ps）时间域内获得无背景（消除荧光）、强且高光谱分辨率的拉曼光谱，也许令人惊讶的是，为空间偏移拉曼光谱等新技术奠定了基础。
