High-resolution spectroscopy of CoS (X4Deltai): examining 3d transition-metal sulfide bonds

The pure rotational spectrum of CoS, the cobalt sulfide radical, has been measured using direct absorption techniques in the frequency range of 180-540 GHz. This study is the first spectroscopic investigation of any kind of this molecule. CoS was created by reacting cobalt vapor with H2S. Four spin components were identified in the spectra of this species, one of which exhibited lambda doubling, identifying the ground state as 4Deltai. Transitions arising from the lowest spin component of the less abundant Co 35S isotopomer have also been detected, as well as from v=1 and v=2 of the main species. The spectra were readily identified because each spin component exhibited an octet pattern arising from the 59Co spin of I=72. The data were fit using Hund's case (a) Hamiltonian, and rotational, fine-structure, hyperfine, and lambda-doubling constants were determined. The hyperfine parameters support a delta3pi2 electron configuration and are consistent with some orbital overlap between the metal and sulfur atoms. From the rotational constant, the bond length of CoS was calculated to be r0=1.977 985 06(10) A. This bond length is significantly shorter than that of MnS or FeS, in contrast to the bond distances found in the oxide analogs which are all similar in value. These results indicate that the 3d metal sulfides differ somewhat from their oxide counterparts, probably due to the availability of sulfur p orbitals for bonding.