Competition and Interplay between sigma-Hole and pi-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) cc-pVTZ levels have been used to examine 1:1 and 1:2 complexes between O2NX (X = Cl, Br, and I) with NH3. The 1:1 complexes can easily be differentiated using the stretching frequency of the N–X bond. Thus, those complexes with σ-hole interaction show a blue shift of the N–X bond stretching whereas a red shift is observed in the complexes along the π-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 (ΔZPVE) are in the ranges 7–26 and 14–46 kJ mol–1, respectively.