Competition and Interplay between σ-Hole and π-Hole Interactions: A Computational Study of 1:1 and 1:2 Complexes of Nitryl Halides (O2NX) with Ammonia

Quantum calculations at the MP2/cc-pVTZ, MP2/aug-cc-pVTZ, and CCSD(T)/c-cpVTZ levels have been used to examine 1:1 and 1:2 complexes between O2NX (X = Cl, Br, and I) with NH3. The interaction of the lone pair of the ammonia with the sigma-hole and pi-hole of O2NX molecules have been considered. The 1:1 complexes can easily be differentiated using the stretching frequency of the N-X bond. Thus, those complexes with sigma-hole interaction show a blue shift of the N-X bond stretching whereas a red shift is observed in the complexes along the pi-hole. The SAPT-DFT methodology has been used to gain insight on the source of the interaction energy. In the 1:2 complexes, the cooperative and diminutive energetic effects have been analyzed using the many-body interaction energies. The nature of the interactions has been characterized with the atoms in molecules (AIM) and natural bond orbital (NBO) methodologies. Stabilization energies of 1:1 and 1:2 complexes including the variation of the zero point vibrational energy (Delta ZPVE) are in the ranges 7-26 and 14-46 kJ mol(-1), respectively.