The microwave and millimeter spectrum of ZnCCH (X̃ ²Σ ⁺): A new zinc-containing free radical

The pure rotational spectrum of the ZnCCH (X̃ ²Σ ⁺) radical has been measured using Fourier transform microwave (FTMW) and millimeter direct-absorption methods in the frequency range of 7-260 GHz. This work is the first study of ZnCCH by any type of spectroscopic technique. In the FTMW system, the radical was synthesized in a mixture of zinc vapor and 0.05 acetylene in argon, using a discharge assisted laser ablation source. In the millimeter-wave spectrometer, the molecule was created from the reaction of zinc vapor, produced in a Broida-type oven, with pure acetylene in a dc discharge. Thirteen rotational transitions were recorded for the main species, ⁶⁴ZnCCH, and between 4 and 10 for the ⁶⁶ZnCCH, ⁶⁸ZnCCH, ⁶⁴ZnCCD, and ⁶⁴Zn ¹³C ¹³CH isotopologues. The fine structure doublets were observed in all the data, and in the FTMW spectra, hydrogen, deuterium, and carbon-13 hyperfine splittings were resolved. The data have been analyzed with a ²Σ Hamiltonian, and rotational, spin-rotation, and H, D, and ¹³C hyperfine parameters have been established for this radical. From the rotational constants, an r _m ⁽¹⁾ structure was determined with r _Zn-C = 1.9083 Å, r _C-C = 1.2313 Å, and r _C-H = 1.0508 Å. The geometry suggests that ZnCCH is primarily a covalent species with the zinc atom singly bonded to the C≡C-H moiety. This result is consistent with the hyperfine parameters, which suggest that the unpaired electron is localized on the zinc nucleus. The spin-rotation constant indicates that an excited ²Π state may exist ∼19 000 cm ^-1 in energy above the ground state.