Ab Initio Rotational and Vibrational Spectroscopy of C3H5 Radicals at the Coupled Cluster Level

Combustion and pyrolysis processes of allene and propyne are known to involve radicals with the structural formula C3H5, the most stable of which is the classic resonance-stabilized allyl radical. In addition to allyl, four other isomers of C3H5 are possible: the propene derivatives Z-1-propenyl, E-1-propenyl, and 2-propenyl, as well as the cyclopropane derivative cyclopropyl. Among these 5 species, the allyl radical has been extensively studied both theoretically and spectroscopically; however, little is known about the spectroscopy of the cyclopropyl radical, and virtually no experimental spectroscopic data are available for the remaining three. Here, we carry out an ab initio investigation of the C3H5 radicals at the coupled cluster level of theory with a focus on computing the spectroscopic parameters relevant for pure rotational and rotationally resolved vibrational spectroscopy. The rotational constants, dipole moments, spin–spin tensors, and Fermi contact parameters are evaluated at the CCSD(T)/cc-pwCVQZ level of theory, while vibrational properties such as vibration–rotation interaction parameters and centrifugal distortion constants are calculated at the frozen-core CCSD(T)/ANO1 level. Vibrational analysis was carried out using second-order vibrational perturbation theory with an explicit treatment of resonances. Finally, the inversion tunneling potential in cyclopropyl as well as the methyl internal rotation potentials in the 1- and 2-propenyl isomers are computed and relevant spectroscopic parameters for modeling their resultant tunneling splittings are derived. Results from the calculations compare favorably with the extensive body of spectroscopic literature on the allyl radical, giving confidence that the computed parameters for the remaining isomers will be useful for guiding the interpretation of future high-resolution experimental spectroscopy of these radicals.

众所周知，丙二烯与丙炔的燃烧及热解过程中会涉及分子式为C₃H₅的自由基，其中最稳定的是经典的共振稳定烯丙基自由基（allyl radical）。除烯丙基自由基外，C₃H₅还存在另外4种可能的同分异构体：丙烯衍生物顺式-1-丙烯基、反式-1-丙烯基、2-丙烯基，以及环丙烷衍生物环丙基自由基（cyclopropyl radical）。在这5种物种中，烯丙基自由基已在理论与光谱学层面得到广泛研究；然而人们对环丙基自由基的光谱学性质知之甚少，其余三种同分异构体几乎没有可用的实验光谱数据。本研究针对C₃H₅自由基开展了耦合簇（coupled cluster）理论水平的从头算（ab initio）研究，重点计算与纯转动光谱及转动分辨振动光谱相关的光谱学参数。我们在CCSD(T)/cc-pwCVQZ理论水平下计算了转动常数、偶极矩、自旋自旋张量以及费米接触（Fermi contact）参数；同时在冻结核近似CCSD(T)/ANO1理论水平下，计算了振动-转动相互作用参数、离心畸变常数等振动性质。研究采用二阶振动微扰理论并对共振进行显式处理，完成了振动分析。最后，我们计算了环丙基自由基的反转隧穿势，以及1-丙烯基与2-丙烯基同分异构体的甲基内转动势，并推导了用于模拟其隧穿分裂的相关光谱学参数。本计算结果与烯丙基自由基已发表的大量光谱学文献数据吻合良好，这让我们确信，其余同分异构体的计算参数将有助于指导未来对这些自由基的高分辨实验光谱研究的解析工作。