Using protonation to change a Cl⋯N halogen bond in N-Base:ClOH complexes to a Cl⋯O halogen bond.

Graphical abstract Highlights • Protonation changes the halogen bonding scheme in complexes N-Base:ClOH. • Traditional N⋯Cl halogen bonds become covalent N Cl bonds. • Covalent Cl O bonds become traditional Cl⋯O halogen bonds. • Proton affinity of ClOH increases dramatically in complexes N-Base:ClOH. • EOM-CCSD coupling constants show the evolution of halogen bonds upon protonation of N-Base:ClOH. Abstract Ab initio MP2/aug′-cc-pVTZ calculations demonstrate that protonation of N-Base:ClOH complexes at O leads to complexes (N-Base-Cl)+:OH 2 , as a traditional N⋯Cl halogen bond is replaced by a chlorine-transferred Cl⋯O bond. For a fixed base, the binding energies of the latter complexes are more than an order of magnitude greater than the former. As a function of the Cl N distance, EOM-CCSD Cl N coupling constants show the evolution of the traditional halogen bond in the N-Base:ClOH complexes, to a chlorine-shared halogen bond in N 2 :ClOH 2 +, to a covalent bond in (N-Base-Cl)+:OH 2. 1J(Cl N) values for these complexes resemble 1J(Cl N) for the ions (N-Base-Cl)+. [ABSTRACT FROM AUTHOR]