The pure rotational spectra of AuCl and AuBr

The pure rotational spect ra of AuCl and AuBr have been measured in the v = 0 and 1 (also for v = 2 for (AuCl)-Cl-35) vibrational states in the 5-22 G Hz spectral region using a cavity pulsed-jet Fourier transform microwave sp ectrometer. The molecules were prepared by ablating Au metal in the presenc e of Cl-2 or Br-2 entrained in the Ar or Ne backing gas of the jet. The equ ilibrium internuclear distances r(e) have been determined along with estima tes of the harmonic vibration frequencies and the dissociation energies. Th e ionic characters of the AuCl and AuBr bonds have been estimated from the halogen nuclear quadrupole-coupling constants to be i(c) similar to 38%. Th e gold nuclear quadrupole coupling constants change appreciably from AuF to AuCl to AuBr (including a sign change), indicating substantial differences in their electronic structures at Au; these variations could not be accoun ted for with a simple Townes-Dailey calculation. (C) 2000 Academic Press.