| Summary: | Fourteen highly reactive isomers of C<sub>5</sub>H and their ionic counterparts have been theoretically investigated using density functional theory (DFT) and coupled-cluster methods. The linear C<sub>5</sub>H (<i>l</i>-C<sub>5</sub>H) radical, pent-1,3-diyn-5-yliden-1-yl (<b>1</b>), along with its cationic form and the cyclic C<sub>5</sub>H (<i>c</i>-C<sub>5</sub>H), 1-ethynylcycloprop-1-en-2-yl-3-ylidene (<b>2</b>), have recently been detected in the Taurus Molecular Cloud-1. By using the UCCSD(T)/cc-pCVTZ level of theory, the calculated rotational constants and other spectroscopic parameters are found to be in good agreement with the available experimental data for isomers <b>1</b> and <b>2</b>. Therefore, the current theoretical study may assist synthetic chemists and molecular spectroscopists in detecting other isomers in the laboratory or in the interstellar medium (ISM). Thermodynamically favorable rearrangement schemes for forming low-lying isomers <b>1</b>, <b>2,</b> and <b>3</b> have also been studied theoretically, and (2λ<sup>3</sup>-cycloprop-2-en-1-ylidene)ethenylidene (<b>3</b>) with a large dipole moment (μ = 4.73 Debye) is proposed to be a plausible candidate for detection in the ISM.
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