Noble gas-metal chemical bonding? The microwave spectra, structures, and hyperfine constants of Ar-CuX (X = F, Cl, Br)

The rotational spectra of the complexes Ar-CuF, Ar-CuCl, and Ar-CuBr have b een observed in the frequency range 5-22 GHz using a pulsed-jet cavity Four ier transform microwave spectrometer. All the complexes are linear and rath er rigid in the ground vibrational state, with the Ar-Cu stretching frequen cy estimated as similar to 200 cm(-1). Isotopic data have been used to calc ulate an r(0) structure for Ar-CuF, while for Ar-CuCl and Ar-CuBr partial s ubstitution structures have also been obtained. To reduce zero-point vibrat ional effects a double substitution method (r(d)) has also been employed to calculate the structures of Ar-CuCl and Ar-CuBr. The Ar-Cu distance has be en found to be rather short and to range from 2.22 Angstrom in Ar-CuF to 2. 30 Angstrom in Ar-CuBr. Ab initio calculations at the MP2 level of theory m odel the geometries and stretching frequencies well and predict an Ar-Cu bo nd energy in Ar-CuF of similar to 47.3 kJ mol(-1). Large changes in the Cu nuclear quadrupole coupling constant on complex formation show that extensi ve charge rearrangement occurs upon formation of the complexes. This, in co njunction with the sizable dissociation energy, suggests that the Ar-Cu bon ds in these complexes are weakly covalent. The rotational spectrum of CuF h as also been reinvestigated to improve the hyperfine constants. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)00521-3].