Nitrogen as a pnicogen?: evidence for π-hole driven novel pnicogen bonding interactions in nitromethane–ammonia aggregates using matrix isolation infrared spectroscopy and ab initio computations

Abstract

The role of nitrogen, the first member of the pnicogen group, as an electron donor in hypervalent non-covalent interactions has been established long ago, while observation of its electron accepting capability is still elusive experimentally, and remains quite intriguing, conceptually. In the light of minimal computational exploration of this novel class of pnicogen bonding so far, the present work provides experimental proof with unprecedented clarity, for the existence of N(acceptor)⋯N(donor) interaction using the model nitromethane (NM) molecule with ammonia (AM) as a Lewis base in NM–AM aggregates. The NM–AM dimer, in which the nitrogen atom of NM (as a unique pnicogen) accepts electrons from AM (the traditional electron donor), was synthesized at low temperatures under isolated conditions within inert gas matrixes and was characterized using infrared spectroscopy. The experimental generation of the NM–AM dimer stabilized via N⋯N interaction has strong corroboration from ab initio calculations. Furthermore, confirmation regarding the directional prevalence of this N⋯N interaction over C–H⋯N and N–H⋯O hydrogen bonding is elucidated quantitatively by quantum theory of atoms in molecules (QTAIM), electrostatic potential mapping (ESP), natural bond orbital (NBO), non-covalent interaction (NCI) and energy decomposition (ED) analyses. The present study also allows the extension of σ-hole/π-hole driven interactions to the atoms of the second period, in spite of their low polarizability.