Proton transfer in the complex H3N...HCl catalyzed by encapsulation into a C60 cage

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.