Infrared spectra of matrix-isolated [OC center dot center dot center dot X-2] and [CO center dot center dot center dot X-2] complexes (X = Cl, Br) and ab initio calculations

Co-deposition of argon or nitrogen dilute samples of natural and isotopic c arbon monoxide and halogen molecules (Cl-2, Br-2, BrCl) produced new narrow absorptions:lines assigned to X-2... CO complexes. With BrCl, in an argon matrix two 1:1 complexes were observed, one stronger than the Br2 CO comple x, the other one weaker than the Cl-2... CO complex. In the nitrogen matrix , one complex was only observed, namely, the ClBr .. CO complex. For all co mplexes, observed shifts were larger in nitrogen than in argon matrices and similar to available gas-phase results for Cl-2... CO and Br-2... CO compl exes. Irradiation of the samples with a filtered xenon lamp (lambda > 360 n m) induced in an argon matrix (not in a nitrogen matrix) the isomerization of the complexes into a metastable form CO ... X-2 identified for the first time. Back isomerization was observed by weak temperature increase. The st ructure and energetics of the four complexes formed between CO and BrCl wer e investigated by DFT ab initio calculations and compared to complexes form ed between CO and Cl-2 and Br-2.