The quest for the carbene bent-pentadiynylidene isomer of C5H2

The equilibrium geometry of the singlet ground electronic state of the bent isomer of C₅H₂, bent-pentadiynylidene (4; X̃¹ A¹; C_2v ), has been theoretically investigated by means of the highly accurate W3-F12 thermochemical protocol. Five isomers of C₅H₂, namely linear-pentadiynylidene (1; X̃³Σḡ; D_∞h), ethynylcyclopropenylidene (2; X̃¹A'; C_s ), pentatetraenylidene (3; X̃¹A₁; C_2v ), ethynylpropadienylidene (5; X̃¹A'; C_s ), and 3-(didehydrovinylidene)cyclopropene (6; X̃¹A₁; 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 (µ ≠ 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).