School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK.
Science. 2011 Mar 18;331(6023):1423-6. doi: 10.1126/science.1197796. Epub 2011 Feb 3.
Solvent collisions can often mask initial disposition of energy to the products of solution-phase chemical reactions. Here, we show with transient infrared absorption spectra obtained with picosecond time resolution that the nascent HCN products of reaction of CN radicals with cyclohexane in chlorinated organic solvents exhibit preferential excitation of one quantum of the C-H stretching mode and up to two quanta of the bending mode. On time scales of approximately 100 to 300 picoseconds, the HCN products undergo relaxation to the vibrational ground state by coupling to the solvent bath. Comparison with reactions of CN radicals with alkanes in the gas phase, known to produce HCN with greater C-H stretch and bending mode excitation (up to two and approximately six quanta, respectively), indicates partial damping of the nascent product vibrational motion by the solvent. The transient infrared spectra therefore probe solvent-induced modifications to the reaction free energy surface and chemical dynamics.
溶剂碰撞常常会掩盖能量最初分配到溶液相化学反应产物的情况。在这里，我们通过皮秒时间分辨率获得的瞬态红外吸收光谱表明，CN 自由基与环己烷在氯化有机溶剂中反应生成的初生 HCN 产物优先激发 C-H 伸缩模式的一个量子和弯曲模式的两个量子。在大约 100 到 300 皮秒的时间尺度上，HCN 产物通过与溶剂浴的耦合松弛到振动基态。与已知在气相中与烷烃反应生成具有更大 C-H 伸缩和弯曲模式激发（分别高达两个和大约六个量子）的 HCN 的 CN 自由基反应的比较表明，溶剂部分阻尼了初生产物的振动运动。因此，瞬态红外光谱探测了溶剂诱导的反应自由能面和化学动力学的变化。
