Unique O=N•••O Pnicogen Interactions in Nitromethane Dimers: Evidence Using Matrix Isolation Infrared Spectroscopy and Computational Methodology

Geometries of nitromethane homodimers have been revisited using ab initio and density functional methodologies, following their formation within cryogenic matrices, confirmed using infrared spectroscopy. In contrast to the claim that the intermolecular interactions are due to dispersion forces or very weak hydrogen bonds, in the present work, concrete evidence for the prevalence of O=N center dot center dot center dot O pnicogen bonding has been presented. The pnicogen bonds have been found to be primarily responsible for the characteristic geometry of a homodimer. The formation of a nitromethane dimer with pnicogen bonding stabilization is evidenced using matrix isolation infrared spectroscopy with Ne and Ar matrices and computations. The interactions within homodimers have been characterized using quantum theory of atoms in molecules, natural bond orbital, energy decomposition, electrostatic potential mapping, and noncovalent interaction analyses. The larger intermolecular separations in liquid nitromethane indicated by previous molecular dynamics-based studies alongside the supposed invariance of infrared spectra in the gas phase and liquid phase could have led to the assumption of a lack of intermolecular interactions. However, the prevalence of hydrogen and pnicogen bonds across these larger than usual separations is affirmed by the geometries presented here.

Automated summary

This study reexamines the geometries of nitromethane homodimers and presents concrete evidence for the prevalence of O=N center dot center dot center dot O pnicogen bonding, contrary to previous claims. The formation of a nitromethane dimer with pnicogen bonding stabilization is confirmed using matrix isolation infrared spectroscopy and computations. The study also characterizes the interactions within homodimers using various methods, highlighting the prevalence of hydrogen and pnicogen bonds.