Pure rotational spectra, structures, and hyperfine constants of OC-AuX (X = F, Cl, Br)

Rotational spectra of OC-AuX (X = F, Cl, Br) have been measured in the frequency range 5-21 GHz, using a pulsed jet cavity Fourier transform microwave spectrometer. The molecules were prepared by ablating Au metal with a Nd:YAG laser and allowing the vapor to react with CO plus a halide precursor contained in the Ar backing gas of the jets. For OC-AuCl and OC-AuBr these are the first high-resolution spectroscopic measurements; for OC-AuF these are the first observations of any kind. All three molecules are linear. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants have been precisely evaluated. The geometries determined for all three molecules show CO bond lengths close to that of free CO, plus relatively long Au-C bonds. These results are corroborated by ab initio calculations, which have also produced Mulliken populations showing significant sigma-donation from CO to Au, plus some pi-back-donation from Au to CO. There are major changes in the Au, Cl, and Br nuclear quadrupole coupling constants when CO bonds to AuX, consistent with the formation of strong Au-C bonds. The structural properties of OC-AuF are somewhat different from those of OC-AuCl and OC-AuBr.