The Acid-Base Through-the-Cage Interaction as an Example of an Inversion in a Cage Isomerism

• Main Authors: Jan C. Dobrowolski, Sławomir Ostrowski
• Format: Article
• Language: English
• Published: MDPI AG 2020-08-01
• Series: Symmetry
• Subjects: endohedral fullerenes; isomerism; acid-base interaction
• Online Access: https://www.mdpi.com/2073-8994/12/8/1291

We define a new inversion in a cage isomerism (<i>ic</i>): X@C∙∙∙Y <i>₪_ic</i> Y@C∙∙∙X, (<i>₪ </i>is the isomerism relation) as an isomerism in the three-component system of molecules X, Y, and a cage C, in which one of the molecules is located inside and the other outside the cage. The <i>ic</i> isomerism is similar to the endo-exo one, which occurs only if either the interior or exterior of C is empty. By contrast, <i>ic</i> occurs only if neither the interior nor the exterior of C is empty. We also discuss the other closely related types of isomerisms are also discussed. Calculations of the XH∙∙∙NH₃@C₆₀ and NH3∙∙∙HX@C₆₀<i>ic</i> isomers were performed at the ωB97XD/Def2TZVP level. The calculated energies demonstrated that the systems with the HX acid outside (X = F, Cl) and the NH₃ base inside the cage, XH∙∙∙NH₃@C₆₀, are more stable than their <i>ic</i> isomers, NH₃∙∙∙HX@C₆₀, by about 4–8 kcal/mol. This is because NH₃ is more stabilized inside the cage than HX (a matter of 6.5 kcal/mol). In the studied systems and subsystems, the HX molecules are Lewis acids and the NH₃ molecule is always a Lewis base. The C₆₀ molecule with HX inside or outside the cage is also an acid for the NH₃ base positioned outside or inside the cage. On the other hand, the C₆₀ cage is truly amphoteric because it is simultaneously an acid and a base.