Prototypical Cyclohexane Dimers: Spectroscopic Evidence for σ Stacking at Low Temperatures
Owing to the fact that σ stacking is as important as π stacking in determining the structural motifs of aliphatic saturated cyclic hydrocarbons, in this work, we have provided the first unambiguous spectroscopic evidence for the existence of σ stacking interaction in cyclohexane dimers at low temperatures. Molecular beam experiments performed using effusive nozzle and supersonic jet sources on cyclohexane in N2 matrix generated cyclohexane dimers stabilized through σ stacking and the dimers were characterized by infrared spectroscopy. The ab initio computations carried out on cyclohexane dimers identified eclipsed (face-to-face), parallel displaced and T-shaped structures, which are predominantly stabilized by σ stacking interactions. While Natural Bond Orbital analysis substantiated a significant amount of σ→σ* interactions involved in the stabilization, the Atoms in Molecules analysis indicated that the stacking is induced by a plausible ‘dihydrogen bonding’ interaction. Energy Decomposition analysis disclosed that a large measure of dispersion interactions effectively contributes for the overall stability of cyclohexane dimers.