The quest for the carbene bent-pentadiynylidene isomer of C5H2

Highlights: • Relative energies of C₅H₂ isomers are computed at the CCSDT(Q)/CBS level. • Linear-pentadiynylidene $\left(1\text{,}{\stackrel{\sim }{X}}^3{\Sigma }_g^{-}\right)$ is found to the most stable geometry. • Bent-pentadiynylidene $\left(4\text{,}{\stackrel{\sim }{X}}^1{A}_1\right)$ is found to be 14.12 kcal mol⁻¹ above 1. • Isomers 2–6 are associated with a non-zero dipole moment $(\mu \ne 0)$. • However, except FTMW Spectroscopy identified all the other four isomers. The equilibrium geometry of the singlet ground electronic state of the bent isomer of C₅H₂, bent-pentadiynylidene (4; ${\stackrel{\sim }{X}}^1{A}_1\text{;}{C}_{2v}$), has been theoretically investigated by means of the highly accurate W3-F12 thermochemical protocol. Five isomers of C₅H₂, namely linear-pentadiynylidene (1; ${\stackrel{\sim }{X}}^3{\Sigma }_g^{-}\text{;}{D}_{\infty h}$), ethynylcyclopropenylidene (2; ${\stackrel{\sim }{X}}^1{A}^{\prime }\text{;}{C}_s$), pentatetraenylidene (3; ${\stackrel{\sim }{X}}^1{A}_1\text{;}{C}_{2v}$), ethynylpropadienylidene (5; ${\stackrel{\sim }{X}}^1{A}^{\prime }\text{;}{C}_s$), and 3-(didehydrovinylidene)cyclopropene (6; ${\stackrel{\sim }{X}}^1{A}_1\text{;}{C}_{2v}$) had already been identified in the laboratory. With respect to 1, the relative energy difference calculated at the CCSDT(Q)/CBS level of theory including zero-point vibrational energy corrections are: 0.66 (2), 13.53 (3), 14.12 (4), 15.40 (5), and 20.01 (6) kcal mol⁻¹, respectively. Isomers 2–6 are associated with a non-zero dipole moment ($\mu \ne 0$), however, except 4, all the other four isomers were identified by Fourier Transform Microwave spectroscopy, including 5 and 6 which lie higher in energy. Isomer 4 remains elusive to date. We believe that the theoretical data such as, optimal geometry, dipole moment, rotational and centrifugal distortion constants, harmonic vibrational frequencies, infra-red intensities, and isotopic shifts (¹²C–¹³C) in harmonic vibrational frequencies presented here would assist experimentalists in the identification of this elusive molecule (4).