Interaction of CHX(3) (X = F, Cl, Br) with HNO induces remarkable blue shifts of both C-H and N-H bonds.

The hydrogen-bonded complexes formed from interaction of trihalomethanes CHX(3) (X = F, Cl, Br) with nitrosyl hydride HNO were studied using ab initio MO calculations at the second-order perturbation theory (MP2/6-311++G(d,p)). Each interaction contains at least five separate equilibrium structures. Calculated binding energies range from 4 to 8 kJ mol(-1) with both ZPE and BSSE corrections. While CHBr(3) leads to the most stable complexes with HNO, CHF(3) forms the least stable counterparts. The strength of complexes thus tends to increase from F to Cl to Br, which is consistent with a decrease of deprotonation enthalpy of the corresponding C-H bonds. It is remarkable that all the C-H and N-H bonds are shortened upon complexation, giving rise to an increase of their stretching frequencies. A blue shift is thus observed for the N-H bonds of the type N-HX (X = F, Cl, Br); such a contraction of the covalent N-H bond is extremely rare.