Computational studies of σ-type weak interactions between NCO/NCS radicals and XY(X = H, Cl; Y = F, Cl, and Br)

The triatomic radicals NCO and NCS are of interest in atmospheric chemistry, and both the ends of these radicals can potentially serve as electron donors during the formation of sigma-type hydrogen/halogen bonds with electron acceptors XY (X = H, Cl; Y = F, Cl, and Br). The geometries of the weakly bonded systems NCO/NCSa <-XY were determined at the MP2/aug-cc-pVDZ level of calculation. The results obtained indicate that the geometries in which the hydrogen/halogen atom is bonded at the N atom are more stable than those where it is bonded at the O/S atom, and that it is the molecular electrostatic potential (MEP)-not the electronegativity - that determines the stability of the hydrogen/halogen bond. For the same electron donor (N or O/S) in the triatomic radical and the same X atom in XY, the bond strength decreases in the order Y = F > Cl > Br. In the hydrogen/ halogen bond formation process for all of the complexes studied in this work, transfer of spin electron density from the electron donor to the electron acceptor is negligible, but spin density rearranges within the triatomic radicals, being transferred to the terminal atom not interacting with XY.