Can a Cl-H···F Hydrogen Bond Replace a Cl···F Halogen Bond? HXP:ClY:ZH versus HXP:ClY:HZ for Y, Z = F, Cl.

Ab initio MP2/aug'-cc-pVTZ (where MP2 is Møller-Plesset perturbation theory) calculations have been carried out on four series of complexes, HXP:ClF:HCl, HXP:ClF:HF, HXP:ClCl:HF, and HXP:ClCl:HCl, to answer the question raised in the title of this paper. When X is F or Cl, binary complexes containing a P(V) molecule hydrogen bonded to an acid are found on all potential surfaces except HClP:ClF:HF, where an ion-pair complex exists. Ion-pair complexes also result from the optimization of HXP:ClF:HF for X = NC, CN, and H. Changing the central molecule from ClF to ClCl has a dramatic effect on the nature of the optimized complexes when the substituents are NC, CN, and H. On the potential surfaces HXP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of HP:ClCl:FH leads to an ion pair. For H(NC)P:ClCl:HCl and H(CN)P:ClCl:HCl, cyclic ternary complexes stabilized by pnicogen, halogen, and hydrogen bonds result from optimization. Optimization of HP:ClCl:HCl leads to a reaction in which HClP and a second HCl molecule are formed, and the resulting cyclic ternary complex is stabilized by two hydrogen bonds and a pnicogen bond. Thus, the type of complex resulting from the optimization of the starting ternary complex HXP:ClY:HZ depends on the nature of the central molecule ClF or ClCl, the terminal molecule HCl or HF, and the substituent X. It is not possible to simply turn around the terminal HZ molecule in complexes HXP:ClF:ZH for Z = F and Cl to give HXP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes HXP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes HXP:ClCl:ZH are not.