Sakota Kenji, Inoue Naomi, Komoto Yusuke, Sekiya Hiroshi
Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan.
J Phys Chem A. 2007 May 31;111(21):4596-603. doi: 10.1021/jp070359a. Epub 2007 May 8.
The 7-azaindole-methanol 1:2 cluster [7AI(CH(3)OH)2] undergoes excited-state triple-proton/hydrogen atom transfer (ESTPT/HT) along the hydrogen-bonded network in the gas phase. The measurements of the resonance-enhanced multiphoton ionization (REMPI) spectra of 7AI(CH(3)OH)2-d(n) (n = 0-3), where subscript n indicates the number of deuterium, and the fluorescence excitation spectrum of 7AI(CH(3)OH)2-d(0) allowed us to investigate the ESTPT/HT dynamics. By comparing the intensity ratios of the vibronic bands between 7AI(CH(3)OH)2-d(0) and 7AI(CH(3)OH)2-d(3) in REMPI spectra, we obtained the lower limit of an acceleration factor (f(a)(low)) of 7AI(CH(3)OH)2-d(0), which is the ratio of the reaction rate for the excitation of a vibronic state to that of the zero-point state in S(1). The f(a)(low) values are 2.7 +/- 0.83 and 4.0 +/- 1.2 for an in-phase intermolecular stretching vibration (sigma(1)) and its overtone (2sigma(1)) observed at 181 cm(-1) and 359 cm(-1) in the excitation spectrum, respectively, while that of the vibration (nu(2)/sigma(1) or nu(3)/sigma(1)) at 228 cm(-1) is 1.1 +/- 0.83. Thus, vibrational-mode-specific ESTPT/HT occurs in the low-energy region (600 cm(-1)). The excitation of an intramolecular ring mode (nu(intra)) of 7AI at 744 cm(-1) substantially enhances the reaction rate (f(a)(low) = 4.4 +/- 0.98), but the increase of f(a)(low) is not prominent for the excitation of v(intra) + sigma(1) at 926 cm(-1) (f(a)(low) = 5.0 +/- 1.6), although the sigma(1) mode is excited. These results suggest that the ESTPT/HT reaction in 7AI(CH(3)OH)2-d(0) directly proceeds from the photoexcited states with the internal energy less than approximately 600 cm(-1), but it occurs from the isoenergetically vibrational-energy redistributed states when the internal energy is large. This shows a remarkable feature of ESTPT/HT in 7AI(CH(3)OH)2; the nature of the reaction mechanism changes from vibrational-mode specific to statistical fashion with increasing the internal energy. The hydrogen-bonded network in 7AI(CH(3)OH)2-d(0) is represented by a directed graph. This shows that ESTPT/HT is one of the simplest examples of cooperative phenomena.
7-氮杂吲哚 - 甲醇1:2团簇[7AI(CH(3)OH)2]在气相中沿着氢键网络经历激发态三质子/氢原子转移（ESTPT/HT）。对7AI(CH(3)OH)2-d(n)（n = 0 - 3，下标n表示氘的数量）的共振增强多光子电离（REMPI）光谱以及7AI(CH(3)OH)2-d(0)的荧光激发光谱进行测量，使我们能够研究ESTPT/HT动力学。通过比较REMPI光谱中7AI(CH(3)OH)2-d(0)和7AI(CH(3)OH)2-d(3)之间振动带的强度比，我们得到了7AI(CH(3)OH)2-d(0)的加速因子下限（f(a)(low)），它是S(1)中振动激发态与零点态反应速率的比值。对于在激发光谱中181 cm(-1)和359 cm(-1)处观察到的同相分子间伸缩振动（sigma(1)）及其泛频（2sigma(1)），f(a)(low)值分别为2.7 +/- 0.83和4.0 +/- 1.2，而在228 cm(-1)处的振动（nu(2)/sigma(1)或nu(3)/sigma(1)）的f(a)(low)为1.1 +/- 0.83。因此，振动模式特异性的ESTPT/HT发生在低能量区域（600 cm(-1)）。7AI在744 cm(-1)处的分子内环模式（nu(intra)）的激发显著提高了反应速率（f(a)(low) = 4.4 +/- 0.98），但对于926 cm(-1)处v(intra) + sigma(1)的激发，f(a)(low)的增加并不显著（f(a)(low) = 5.0 +/- 1.6），尽管sigma(1)模式被激发。这些结果表明，7AI(CH(3)OH)2-d(0)中的ESTPT/HT反应直接从内部能量小于约600 cm(-1)的光激发态进行，但当内部能量较大时，它从等能量振动能量重新分布的状态发生。这显示了7AI(CH(3)OH)2中ESTPT/HT的一个显著特征；随着内部能量的增加，反应机制的性质从振动模式特异性变为统计方式。7AI(CH(3)OH)2-d(0)中的氢键网络由有向图表示。这表明ESTPT/HT是协同现象最简单的例子之一。
