Gas Phase Chemistry of NHxCly⁺ Ions. II. Structure, Stability and Reactivity of Protonated Dichloramine

The structure and the reactivity of gaseous (NHCl 2 )H + ions formed upon CH 4 chemical ionization of monochloramine and dichloramine in water were investigated by FT−ICR (Fourier transform ion cyclotron resonance) mass spectrometry and theoretical methods. Bracketing experiments performed in order to evaluate the basicity of NHCl 2 suggested the existence of two (NHCl 2 )H + isomers. The basicity computed by MO SCF calculations allowed us to characterize the most stable (NHCl 2 )H + isomer as having the H 2 NCl 2 + structure, corresponding to dichloramine protonated on the nitrogen consistent with the higher basicity of this atom than of the chlorine atom of NHCl 2 and the less stable isomer as having the H 2 NCl−Cl + structure. The gas-phase basicity, GB, of dichloramine was estimated to be 721.4 ± 10 kJ mol -1 from bracketing experiments, according to the method based on the relationship between the efficiency of proton transfer and the standard free energy change of the process. This value is consistent with those from calculations at the B3LYP and CCSD(T)/6-311++G(3df,3pd) levels of theory. The proton affinity, PA, of NHCl 2 was evaluated to be 757 ± 10 kJ mol -1 . The (NHCl 2 )H + ion behaves as a protonating and chlorinating agent. An excellent correlation exists between the absolute Cl + affinities of ammonia, monochloramine, and dichloramine, estimated 546, 523, and 490 kJ mol -1 respectively and their proton affinities. The results show that as substitution of H by Cl atoms in NH 3 increases, both the PA and Cl + binding energy, BE, of chloramines decrease, owing to the electron-withdrawing effect of the Cl atoms on the nitrogen.