The Fourier transform microwave spectrum of the arsenic dicarbide radical (CCAs: X̃²Π_1/2) and its ¹³C isotopologues

The pure rotational spectrum of the CCAs radical in its ground electronic and spin state, X̃²Π_1/2, has been measured using Fourier transform microwave techniques in the frequency range of 12-40 GHz. This species was created in a supersonic expansion from a reaction mixture of AsCl₃ and C₂H₂ or CH₄ diluted in high pressure argon, using a pulsed nozzle containing a dc discharge source. Three rotational transitions were measured for the main isotopologue, ¹²C¹²CAs, in the ω=1/2 ladder; both lambda-doubling and arsenic (I=3/2) hyperfine interactions were observed in these spectra. In addition, two to four rotational transitions were recorded for the ¹³C¹³CAs, ¹³C¹²CAs, and ¹²C¹³CAs species. In these three isotopologues, hyperfine splittings were also resolved arising from the ¹³C nuclei (I=1/2), creating complex spectral patterns. The CCAs spectra were analyzed with a case (a) Hamiltonian, and effective rotational, lambda-doubling, and arsenic and carbon-13 hyperfine constants were determined for the ω=1/2 ladder. From the effective rotational constants of the four isotopologues, an r _m⁽¹⁾ structure has been derived with r_C-C=1.287 Å and r_C-As=1.745 Å. These bond lengths indicate that the predominant structure for arsenic dicarbide is C=C=As·, with some contributing C≡C and C≡As triple bond characters. The hyperfine constants established in this work indicate that about 2/3 of the unpaired electron density lies on the arsenic atom, with the remaining percentage on the terminal carbon. The value of the arsenic quadrupole coupling constant (eqQ=-202 MHz) suggests that the As-C bond has a mixture of covalent and ionic characters, consistent with theoretical predictions that both π backbonding and electron transfer play a role in creating a linear, as opposed to a cyclic, structure for certain heteroatom dicarbides.