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

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+center dot center dot center dot Ph4P+ interactions is confirmed in five other Ph4P+ salts. The strength of their repulsion depends more on the P center dot center dot center dot P distance than on the number of short-distance phenyl/phenyl interactions. Calculations on cation(2)anion(2) aggregates (extracted from the Ph4P+Cl- salt and presenting the same phenyl embrace found in this crystal) indicate that these cation(2)anion(2) aggregates are energetically stable due to the combined effect of the cation center dot center dot center dot anion interactions, which overweight the anion center dot center dot center dot anion and Ph4P+center dot center dot center dot Ph4P+ repulsions.