Proton Transfer in the Complex H 3 N⋅⋅⋅HCl Catalyzed by Encapsulation into a C 60 Cage

Caged up : In contrast to acid–base behavior in solution, single molecules of NH 3 and HCl do not react to form the ion pair NH 4 + Cl − in isolation. Proton transfer occurs in the complex H 3 N⋅⋅⋅HCl inside the C 60 cage, to form the ion pair NH 4 + Cl − under the catalytic action of C 60 (see picture).We report proton transfer in the complex H 3 N⋅⋅⋅HCl to form the ion pair NH 4 + Cl − , which is favored inside the C 60 cage according to quantum chemical calculations. The results show that the NH 4 + Cl − @C 60 is stable with an interaction energy of −2.78 kcal mol −1 . Compared with the complex H 3 N⋅⋅⋅HCl without proton transfer, it is found that the C 60 cage plays the role of a catalyst for proton transfer. In NH 4 + Cl − @C 60 a negative charge area in the C 60 cage is near the cation NH 4 + whereas a positive charge area is near the anion Cl − . Also, a confinement effect of the C 60 cage is noticed, as the endohedral structure of NH 4 + Cl − is more compact than the structure of NH 4 + Cl − in the gas‐phase complex. These findings indicate that the catalysis by the C 60 cage comes from two effects: 1) electrostatic inducement between the C 60 cage and endohedral molecules and 2) the confinement effect that compresses endohedral molecular structures inside the C 60 cage. In the infrared spectrum, it is found that the confinement effect of the cage can cause large blue shifts of the N–H stretching vibrations in NH 4 + Cl − @C 60 compared with those in the NH 4 + Cl − ⋅⋅⋅H 2 O complex.