AB-INITIO STUDY OF THE STRUCTURE OF RADICAL CATIONS DERIVED FROM H-BONDED COMPLEXES - A COMPARISON BETWEEN [H2CO-CENTER-DOT-H2O](-DOT) AND [H2CO-CENTER-DOT-HF](+CENTER-DOT)(CENTER)

An ab initio Hartree-Fock and second-order Moller-Plesset study of the radical cation obtained by ionization of the formaldehyde-HF hydrogen -bonded dimer is reported. The results are compared with those obtaine d previously for the radical cations obtained from the formaldehyde-wa ter complex. An exploration of the [H2CO.HF]+' potential energy surfac e was done using the 3-21G basis set. The whole of the structures foun d were then refined using the 6-31G(d,p) basis set and - for the most interesting structures - 6-31+G(d,p) and 6-311++G(2d,2p). In all cases the calculations were performed using the unrestricted formalism, che cking always that spin contamination remained below normal thresholds. Although most of the structures found correspond closely to those fou nd in the case of water, the HF acceptor is not able, in general, to a bstract the proton from formaldehyde. Thus, contrary to the case of wa ter, the most stable CH2O.HF radical-cation complex has both the charg e and the unpaired electron located on the formaldehyde fragment. Howe ver, HF behaves in the same fashion as H2O in the sense that it is abl e to stabilize the hydroxymethylene radical cation with respect to the formaldehyde radical cation.