The quest for the carbene bent-pentadiynylidene isomer of C5H2

Abstract

The equilibrium geometry of the singlet ground electronic state of the bent isomer of C5H2, bent-pentadiynylidene (4; X∼1A1;C2v), has been theoretically investigated by means of the highly accurate W3-F12 thermochemical protocol. Five isomers of C5H2, namely linear-pentadiynylidene (1; X∼3Σg-;D∞h), ethynylcyclopropenylidene (2; X∼1A′;Cs), pentatetraenylidene (3; X∼1A1;C2v), ethynylpropadienylidene (5; X∼1A′;Cs), and 3-(didehydrovinylidene)cyclopropene (6; X∼1A1;C2v) 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).