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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Proton-coupled hole transfer in X-irradiated doped crystalline cytosine center dot H2O
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Proton-coupled hole transfer in X-irradiated doped crystalline cytosine center dot H2O

机译：X射线掺杂的晶体胞嘧啶中心点H2O中的质子耦合空穴转移

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摘要

Following exposure to X-irradiation at low temperatures, the main reactions taking place in single crystals of cytosine monohydrate doped with minute amounts of 2-thiocytosine are hole transfer (HT) from the electron-loss centers to the dopant and recombination of oxidation and reduction products, assurnedly by electron transfer. A huge deuterium kinetic isotope effect (KIE; > 10(2)-10(3)) at 100 K, together with the kinetic curves obtained and density functional theory (DFT) calculations of equilibrium energy changes, indicates that these reactions proceed through a concerted proton-coupled electron/hole transfer where the proton transfer occurs between hydrogen-bonded cytosine molecules. The temperature dependence of these reaction rates between 10 and 150 K in normal and partially deuterated samples was investigated by monitoring the growth and decay of the various radical species over time using electron paramagnetic resonance (EPR) spectroscopy. By assuming a random distribution of the hole donors and acceptors in the crystals, the data are consistent with an exponential distance-dependent rate, giving a distance decay constant (beta) around 1 angstrom(-1) for the HT, which indicates that a long-range single-step superexchange mechanism mediates the charge transfer. The reactions undergo a transition from a slow, weakly temperature-dependent rate to an Arrhenius-type rate at 40-50 K, presumably being activated by excitation of low-frequency intermolecular vibrations that couple to the process. Below this transition temperature, the transfer probability might be dominated by temperature-independent nuclear tunneling. A similar beta value in both temperature regions suggests that hopping is not activated.
机译：暴露于低温下的X射线辐射后，掺有少量2-硫代胞嘧啶的胞嘧啶一水合物单晶中发生的主要反应是从电子损耗中心到掺杂剂的空穴转移（HT）以及氧化和还原的重组产品，肯定是通过电子转移。在100 K时有巨大的氘动力学同位素效应（KIE;> 10（2）-10（3）），以及获得的动力学曲线和平衡能变化的密度泛函理论（DFT）计算，表明这些反应通过协调的质子耦合电子/空穴转移，质子转移发生在氢键合胞嘧啶分子之间。通过使用电子顺磁共振（EPR）光谱监测各种自由基随时间的增长和衰变，研究了正常和部分氘化样品中10至150 K之间这些反应速率的温度依赖性。通过假设晶体中空穴供体和受体的随机分布，数据与依赖于指数的速率相一致，从而给出了HT的距离衰减常数（β）约为1埃（-1），这表明远程单步超交换机制介导电荷转移。在40-50 K时，反应经历了从缓慢的，与温度无关的缓慢转变为Arrhenius型反应的转变，大概是通过激发与该过程耦合的低频分子间振动来激活的。在此转变温度以下，传输概率可能由与温度无关的核隧穿控制。在两个温度区域中相似的beta值表明未激活跳跃。

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