Impact of configuration and fluorination on the solubility of octyl ester benzoate dimers in CO2

Data are reported on the phase behavior of hydrocarbon and semifluoroinated octyl ester benzoate dimers in CO2 to temperatures of 100 °C and pressures of 1 600 bar. The experimental data at 75 °C demonstrate that the non-fluorinated head-to-head (H-H) dimer dissolves in CO2 at ∼400 bar lower pressures than the non-fluorinated tail-to-tail (T-T) dimer. Even though partially fluorinating the octyl tails of the H-H and T-T dimers renders them soluble in CO2 at pressures near 200 bar, it still takes ∼40 bar more pressure to dissolve the fluorinated T-T dimer as compared to the H-H dimer. The difference in pressures needed to dissolve these dimers is attributed to steric constraints on the coplanarity of the benzene rings imposed by the H-H regiochemistry that do not exist with T-T dimers. Semi-empirical quantum mechanics calculations suggest that the H-H dimer has a twisted, non-coplanar conformation due to the steric effect of the octyl ester groups while the T-T dimer has a less twisted conformation. Steric hindrance in the H-H dimer reduces considerably resonance or conjugation between the π electrons of the aromatic groups which also reduces the dipole moments of the H-H dimers compared to those of the T-T dimers.