Kalal Bhavika, Baweja Simran, Maity Surajit
Department of Chemistry, IIT Hyderabad, Sangareddy 502284, Telangana, India.
J Phys Chem A. 2024 Oct 24;128(42):9114-9121. doi: 10.1021/acs.jpca.4c04249. Epub 2024 Oct 10.
The article presents a comprehensive laser spectroscopic characterization of a nitrogen-rich indole derivative, namely, 2,6-diazaindole (26DAI), in the gas phase. A supersonic jet-cooled molecular beam of 26DAI was characterized using two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence spectroscopy (LIF) to investigate the electronic excitation. The S ← S origin transition was obtained at 33915 cm, which was red-shifted from that of one (indole) and two (7-azaindole) nitrogen-containing indole derivatives by 1317 and 713 cm, respectively. The molecular orbital and energy analysis for the S ← S transition shows the significant stabilization of LUMO on subsequent N-insertion, resulting in the lowering of the S ← S (ππ*) transition energy. The single vibronic level fluorescence spectrum from the vibrationless S state of the molecule was recorded. The spectrum displayed an extensive Franck-Condon activity until 2500 cm for the vibrational modes of the S state of the 26DAI molecule. The experimental ground state vibrational frequencies were compared to the calculated ones obtained at three different levels of theories. More accurate results were found at DFT B3LYP-D4 than those at the wave function-based MP2 and CCSD levels of theories. Further, the N-H stretching frequency of 26DAI in the S state was measured at 3524 cm using fluorescence-dip infrared (FDIR) spectroscopy. The stability of 26DAI against ionization radiation was probed by measuring the two-color photoionization energy (IE) of 26DAI at 71866 cm. The IE value is significantly higher than those of N-poor counterparts (indole and 7-azaindole). The NBO charges and spin density (SD) values of the 26DAI molecule have shown that electronegative N(6) makes the cationic ground state less stable due to the position of the positive centers on the N atom. The results provided insights into the stability of N-rich biomolecules against photodamage. The current investigation can shed light on nature's way of stabilizing biomolecules with a possible N-insertion mechanism.
本文介绍了一种富氮吲哚衍生物——2,6-二氮杂吲哚（26DAI）在气相中的综合激光光谱表征。使用双色共振双光子电离（2C-R2PI）和激光诱导荧光光谱（LIF）对超声喷射冷却的26DAI分子束进行表征，以研究其电子激发。S←S原点跃迁在33915 cm处获得，与含一个氮（吲哚）和两个氮（7-氮杂吲哚）的吲哚衍生物相比，分别红移了1317 cm和713 cm。对S←S跃迁的分子轨道和能量分析表明，在后续的N插入过程中，最低未占分子轨道（LUMO）有显著稳定化，导致S←S（ππ*）跃迁能量降低。记录了分子无振动S态的单振动能级荧光光谱。对于26DAI分子S态的振动模式，该光谱在2500 cm之前显示出广泛的弗兰克-康登活性。将实验基态振动频率与在三种不同理论水平下计算得到的频率进行了比较。在密度泛函理论（DFT）B3LYP-D4水平下得到的结果比基于波函数的MP2和耦合簇单双激发（CCSD）理论水平下的结果更准确。此外，使用荧光 dip 红外（FDIR）光谱在3524 cm处测量了26DAI在S态下的N-H伸缩频率。通过测量26DAI在71866 cm处的双色光电离能（IE），探究了26DAI对电离辐射的稳定性。该IE值显著高于贫氮类似物（吲哚和7-氮杂吲哚）。26DAI分子的自然键轨道（NBO）电荷和自旋密度（SD）值表明，由于正电荷中心在N原子上的位置，电负性的N(6)使阳离子基态不太稳定。这些结果为富氮生物分子对光损伤的稳定性提供了见解。当前的研究可以揭示自然界通过可能的N插入机制稳定生物分子的方式。
