On the stability of X2NO radicals (X = F, Cl, Br, I).

The stability of X(2)NO radicals has been studied by investigating the three possible dissociative channels, namely X+XNO, X(2)+NO, and NX(2)+O. While all the radicals have been found stable with respect to the latter, the second pathway shows that Br(2)NO and I(2)NO are unstable with respect to dissociation. The first dissociative channel has been thoroughly investigated with the aim of understanding whether F(2)NO and Cl(2)NO are stable and how much. This implied the molecular structures and energies of X(2)NO and XNO, with X = F, Cl, to be computed at high level of theory. The coupled cluster ansatz in conjunction with hierarchical series of basis sets has been employed, thus accounting for extrapolation to the complete basis-set limit. Core correlation as well as higher excitations in the electronic-correlation treatment have also been taken into account. It is particularly noteworthy that explicit inclusion of quadruple excitations allowed to obtain for the first time equilibrium geometries of FNO and ClNO in full quantitative agreement with their experimental counterparts.