The pure rotational spectrum of the T-shaped AlC2 radical (2A1)

Abstract

The pure rotational spectrum of the AlC₂ radical (²A₁) has been measured using Fourier transform microwave/millimeter-wave (FTMmmW) techniques in the frequency range 21–65 GHz. This study is the first high-resolution spectroscopic investigation of this molecule. AlC₂ was created in a supersonic jet from the reaction of aluminum, generated by laser ablation, with a mixture of CH₄ or HCCH, diluted in argon, in the presence of a DC discharge. Three transitions (N_Ka,Kc = 1₀₁ → 0₀₀, 2₀₂ → 1₀₁, and 3₀₃ → 2₀₂) were measured, each consisting of multiple fine/hyperfine components, resulting from the unpaired electron in the species and the aluminum-27 nuclear spin (I = 5/2). The data were analyzed using an asymmetric top Hamiltonian and rotational, fine structure, and hyperfine constants determined. These parameters agree well with those derived from previous theoretical calculations and optical spectra. An r₀ structure of AlC₂ was determined with r(Al–C) = 1.924 Å, r(C–C) = 1.260 Å, and θ(C–Al–C) = 38.2°. The Al–C bond was found to be significantly shorter than in other small, Al-bearing species. The Fermi contact term established in this work indicates that the unpaired electron in the valence orbital has considerable 3p_za₁ character, suggesting polarization towards the C₂ moiety. A high degree of ionic character in the molecule is also evident from the quadrupole coupling constant. These results are consistent with a T-shaped geometry and an Al⁺C₂⁻ bonding scheme. AlC₂ is a possible interstellar molecule that may be present in the circumstellar envelopes of carbon-rich AGB stars.