High-Resolution\nSpectroscopy and Structure of Heavy\nCarbon Subchalcogenides: Tricarbon Selenide, C<sub>3</sub>Se

Linear\ntricarbon selenide, C<sub>3</sub>Se, has been\nstudied spectroscopically\nfor the first time using a combination of high-resolution infrared\nand microwave techniques. Probing laser ablation products from carbon–selenium\ntargets in a free jet expansion with He, initial spectroscopic detection\nwas accomplished in the infrared at a wavelength of 5 μm in\nsearch of the ν<sub>1</sub> vibrational fundamental. Along with\nthe band of the most abundant isotopic species C<sub>3</sub><sup>80</sup>Se found centered at about 2039 cm<sup>–1</sup>, the corresponding\nbands of the C<sub>3</sub><sup>82</sup>Se, C<sub>3</sub><sup>78</sup>Se, C<sub>3</sub><sup>76</sup>Se, and C<sub>3</sub><sup>77</sup>Se\nisotopologues were also detected. Pure rotational spectra of the five\nC<sub>3</sub>Se isotopologues in the 8–18 GHz frequency range\nwere observed in a supersonic jet expansion using chirped-pulse microwave\nspectroscopy of the discharge products of selenophene, c-C<sub>4</sub>H<sub>4</sub>Se. Spectroscopic analyses were guided by results from\nhigh-level quantum-chemical calculations carried out at the coupled-cluster\nlevel of theory using large correlation-consistent basis sets. Using\nthe experimentally derived ground-state rotational constants of five\nisotopologues and calculated zero-point vibrational corrections, an\naccurate semiexperimental equilibrium carbon–selenium bond\nlength is derived.