Can a Cl−H···F Hydrogen Bond Replace a Cl···F Halogen Bond? H2XP:ClY:ZH versus H2XP:ClY:HZ for Y, Z = F, Cl

Abstract

Ab initio MP2/aug'-cc-pVTZ (where MP2 is Møller-Plesset perturbation theory) calculations have been carried out on four series of complexes, H

XP:ClF:HCl, H

XP:ClF:HF, H

XP:ClCl:HF, and H

XP: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 H

ClP:ClF:HF, where an ion-pair complex exists. Ion-pair complexes also result from the optimization of H

XP: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 H

XP:ClCl:FH for X = NC and CN, open ternary complexes stabilized by a pnicogen bond and a hydrogen bond are found. Optimization of H

P: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 H

P:ClCl:HCl leads to a reaction in which H

ClP 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 H

XP: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 H

XP:ClF:ZH for Z = F and Cl to give H

XP:ClF:HZ, thereby replacing a halogen bond by a hydrogen bond. Complexes H

XP:ClCl:HZ for X = NC and CN are stable complexes, but the corresponding halogen-bonded complexes H

XP:ClCl:ZH are not.