School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom.
J Chem Phys. 2010 Feb 14;132(6):064305. doi: 10.1063/1.3292644.
The photophysics of gas phase pyrazole (C(3)N(2)H(4)) and 2H-1,2,3-triazole (C(2)N(3)H(3)) molecules following excitation at wavelengths in the range 230 nm>or=lambda(phot)>or=193.3 nm has been investigated using the experimental technique of H (Rydberg) atom photofragment translational spectroscopy. The findings are compared with previous studies of pyrrole (C(4)N(1)H(5)) and imidazole (C(3)N(2)H(4)), providing a guide to H atom loss dynamics in simple N-containing heterocycles. CASPT2 theoretical methods have been employed to validate these findings. Photoexcitation of pyrazole at the longest wavelengths studied is deduced to involve pi( *)<--pi excitation, but photolysis at lambda(phot)</=214 nm is characterized by rapid N-H bond fission on a (1)pisigma( *) potential energy surface. The eventual pyrazolyl radical products are formed in a range of vibrational levels associated with both the ground ((2)A(2)) and first excited ((2)B(1)) electronic states as a result of nonadiabatic coupling at large N-H bond lengths. The excitation energy of the lowest (1)pisigma( *) state of pyrazole is found to be significantly higher in energy than that of pyrrole and imidazole. Similar studies of 2H-1,2,3-triazole reveal that the lowest (1)pisigma( *) state is yet higher in energy and not accessible following excitation at lambda(phot)>or=193.3 nm. The N-H bond strength of pyrazole is determined as 37 680+/-40 cm(-1), significantly greater than that of the N-H bonds in pyrrole and imidazole. The correlation between the photochemistry of azoles and the number and position of nitrogen atoms within the ring framework is discussed in terms of molecular symmetry and orbital electron density. A photodissociation channel yielding H atoms with low kinetic energies is also clearly evident in both pyrazole and 2H-1,2,3-triazole. Companion studies of pyrazole-d(1) suggest that these slow H atoms arise primarily from the N-H site, following pi( *)<--pi excitation, and subsequent internal conversion and/or unintended multiphoton absorption processes.
气相吡唑（C(3)N(2)H(4)）和 2H-1,2,3-三唑（C(2)N(3)H(3)）分子在 230nm>=lambda(phot)=193.3nm 范围内的光物理性质已经使用 H（里德堡）原子光碎片平移光谱学的实验技术进行了研究。研究结果与先前对吡咯（C(4)N(1)H(5)）和咪唑（C(3)N(2)H(4)）的研究进行了比较，为简单的含氮杂环中 H 原子损失动力学提供了指导。采用 CASPT2 理论方法验证了这些发现。推断在研究的最长波长处对吡唑的光激发涉及 pi( *)<--pi 激发，但在 lambda(phot)<=214nm 处的光解以快速的 N-H 键断裂为特征，在一个（1）pisigma( *)势能面上。吡唑基自由基产物最终在与基态((2)A(2))和第一激发态((2)B(1))电子态相关的一系列振动能级中形成，这是由于在大 N-H 键长处的非绝热耦合。吡唑的最低（1）pisigma( *)态的激发能被发现明显高于吡咯和咪唑的激发能。对 2H-1,2,3-三唑的类似研究表明，最低（1）pisigma( *)态的能量更高，并且在 lambda(phot)<=193.3nm 处的激发下无法获得。吡唑的 N-H 键强度确定为 37680+/-40cm(-1)，明显大于吡咯和咪唑中 N-H 键的强度。唑类的光化学与环骨架内氮原子的数量和位置之间的相关性是根据分子对称性和轨道电子密度来讨论的。在吡唑和 2H-1,2,3-三唑中也明显存在一种导致氢原子具有低动能的光解通道。对吡唑-d(1)的伴随研究表明，这些缓慢的 H 原子主要来自 N-H 位点，随后是 pi( *)<--pi 激发，以及随后的内部转换和/或意外的多光子吸收过程。
