A femtosecond study of the infrared-driven cis-trans isomerization of nitrous acid (HONO) -: art. no. 044509
A femtosecond study of the infrared-driven cis-trans isomerization of nitrous acid (HONO) -: art. no. 044509
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DOI：
10.1063/1.1834567
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发表时间：
2005-01-22
影响因子：
4.4
通讯作者：
Hamm, P
中科院分区：
文献类型：
--
作者：
Schanz, R;Bolan, V;Hamm, P
用超快时间分辨红外光谱研究了低温氪基质中亚硝酸(HONO)的红外顺反异构化反应动力学和机理。在OH-伸缩模式的激发下，反式HONO态在8ps和260ps的时间尺度上呈双指数衰减。初始激发的顺HONO态在20ps的时间尺度上衰变。顺式HONO异构化为反式HONO，反应时间为20ps，量子产率为10%。我们观察到的量子产率明显低于之前报道的100%，这可能意味着存在额外的、慢得多的反应通道。我们进一步建立了该体系的四维模型，其中包括HONO分子内的三个质子自由度和晶笼中分子的一个分子间平移自由度。我们发现顺反异构化必然伴随着整个分子在晶笼中的平移。平移自由度可以调节HONO分子内的相互质子状态。当共振发生时，质子态可能耦合并转移布居。我们提出了一种可能的反应途径，其中顺OH伸展激发态首先耦合到高顺式扭转模式，然后该模式几乎可以瞬间转移到反式侧。该模型定性地解释了所有的实验观测。(C)2005年美国物理研究所。
We investigate the dynamics and mechanism of the IR-driven cis-trans isomerization of nitrous acid (HONO) in a low-temperature krypton matrix applying ultrafast time resolved IR spectroscopy. After excitation of the OH-stretching mode the trans HONO state decays biexponentially on a 8 and 260 ps time scale. The initially excited cis HONO state decays on a 20 ps time scale. Cis HONO isomerizes with 10% quantum yield on a 20 ps time scale to trans HONO. The quantum yield we observe is significantly smaller than the previously reported 100%, which could imply that additional, much slower reaction channels exist. We furthermore developed a four-dimensional model of the system, which includes the three proton intramolecular degrees of freedom of HONO fully quantum mechanically and one intermolecular translational degree of freedom of the molecule in the crystal cage. We find that cis-trans isomerization necessarily is accompanied by a translation of the molecule as a whole in the crystal cage. The translational degree of freedom tunes the intramolecular proton states of HONO with respect to each other. When resonances occur, the proton states might couple and transfer population. We suggest a possible reaction pathway, where the cis OH-stretch excited state first couples to a high cis torsional mode, which then may transfer almost instantaneously to the trans side. The model qualitatively explains all experimental observations. (C) 2005 American Institute of Physics.