Meyer Kelly S, Westerfield John H, Johansen Sommer L, Keane Jasmine, Wannenmacher Anna C, Crabtree Kyle N
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States.
J Phys Chem A. 2022 May 26;126(20):3185-3197. doi: 10.1021/acs.jpca.2c01761. Epub 2022 May 13.
Pyridyl is a prototypical nitrogen-containing aromatic radical that may be a key intermediate in the formation of nitrogen-containing aromatic molecules under astrophysical conditions. On meteorites, a variety of complex molecules with nitrogen-containing rings have been detected with nonterrestrial isotopic abundances, and larger nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have been proposed to be responsible for certain unidentified infrared emission bands in the interstellar medium. In this work, the three isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated with coupled cluster methods. For each species, structures were optimized at the CCSD(T)/cc-pwCVTZ level of theory and force fields were calculated at the CCSD(T)/ANO0 level of theory. Second-order vibrational perturbation theory (VPT2) was used to derive anharmonic vibrational frequencies and vibrationally corrected rotational constants, and resonances among vibrational states below 3500 cm were treated variationally with the VPT2+K method. The results yield a complete set of spectroscopic parameters needed to simulate the pure rotational spectrum of each isomer, including electron-spin, spin-spin, and nuclear hyperfine interactions, and the calculated hyperfine parameters agree well with the limited available data from electron paramagnetic resonance spectroscopy. For the handful of experimentally measured vibrational frequencies determined from photoelectron spectroscopy and matrix isolation spectroscopy, the typical agreement is comparable to experimental uncertainty. The predicted parameters for rotational spectroscopy reported here can guide new experimental investigations into the yet-unobserved rotational spectra of these radicals.
吡啶基是一种典型的含氮芳香自由基，可能是天体物理条件下含氮芳香分子形成过程中的关键中间体。在陨石上，已检测到多种具有含氮环的复杂分子，其同位素丰度与地球不同，并且有人提出较大的含氮多环芳烃（PANHs）是星际介质中某些未识别红外发射带的成因。在这项工作中，我们用耦合簇方法研究了吡啶基的三种异构体（2-、3-和4-吡啶基）。对于每个物种，在CCSD(T)/cc-pwCVTZ理论水平上优化结构，并在CCSD(T)/ANO0理论水平上计算力场。使用二阶振动微扰理论（VPT2）推导非谐振动频率和经振动校正的转动常数，并且用VPT2+K方法变分处理低于3500 cm的振动态之间的共振。结果得出了模拟每个异构体纯转动光谱所需的一整套光谱参数，包括电子自旋、自旋-自旋和核超精细相互作用，并且计算出的超精细参数与电子顺磁共振光谱的有限可用数据吻合良好。对于从光电子能谱和基质隔离光谱确定的少数几个实验测量的振动频率，典型的吻合程度与实验不确定性相当。本文报道的转动光谱预测参数可以指导对这些自由基尚未观测到的转动光谱进行新的实验研究。
