Are the phenyl embrace motifs between Ph4P+cations in crystals attractive? An accurate theoretical evaluation

Abstract

The energetic stability of the six, four and two phenyl embraces (6PE, 4PE, and 2PE) found in ionic crystals that present Ph₄P⁺ ions, is evaluated by means of ab initio theoretical studies on the Ph₄P⁺Cl⁻ salt, which is a good representative example of crystal structures exhibiting phenyl embraces. Despite the attractive energetic influence of the short-distance phenyl⋯phenyl interactions, all phenyl embraces found in the Ph₄P⁺Cl⁻ salt are found to be energetically destabilizing, with interaction energies within the 30 to 34 kcal mol⁻¹ range. The destabilizing energetic nature of Ph₄P⁺⋯Ph₄P⁺ interactions is confirmed in five other Ph₄P⁺ salts. The strength of their repulsion depends more on the P⋯P distance than on the number of short-distance phenyl⋯phenyl interactions. Calculations on cation₂anion₂ aggregates (extracted from the Ph₄P⁺Cl⁻ salt and presenting the same phenyl embrace found in this crystal) indicate that these cation₂anion₂ aggregates are energetically stable due to the combined effect of the cation⋯anion interactions, which overweight the anion⋯anion and Ph₄P⁺⋯Ph₄P⁺ repulsions.