Dipole Moment Propels π-Stacking of Hetero-Dimers of Fluorophenylacetylenes.

Electronic and vibrational spectroscopic investigations in combination with quantum chemical calculations were carried out to probe the formation of four sets of heterodimers of phenylacetylene with 2-fluorohenylacetylene, 3-fluorophenylacetylene, 4-fluorophenylacetylene, and 2,6-difluorophenylacetylene. The interaction of phenylacetylene with fluorophenylacetylenes leads to marginal (2-9 cm(-1)) red-shifts in the acetylenic C-H stretching frequencies of fluorophenylacetylenes, which suggests that constituent monomers are minimally perturbed in the heterodimer. On the other hand, the density-functional-theory-based calculations indicate that pi-stacked structures outweigh other structures incorporating C-H center dot center dot center dot pi and C-H center dot center dot center dot F interactions by about 8 kJ mol(-1) or more. The IR spectra in the acetylenic C-H stretching region were interpreted based on the perturbed dipole model, which suggests formation of predominantly antiparallel pi-stacked structures, propelled by dipole moment. However, the energy decomposition analysis suggests that among stabilizing components dispersion dominates, while electrostatics plays a pivotal role in the formation of the pi-stacked structures. Interestingly, the ability of 2-fluorophenylacetylene and 2,6-difluorophenylacetylene to pi-stack differs significantly, even though both of them have almost identical dipole moments and the dipole moment propels the formation of pi-stack structures. These results suggest pi-stacking transcends the classical electrostatic description in terms of dipole moment.