Effect of Confinement on PH 3 and OH + 3 Inversion †

Abstract

Encapsulating molecules in nanocages such as C 60 provides a unique opportunity to probe how spatial confinement alters structure and dynamics. We examine umbrella inversion in hydronium (OH + 3 ) and phosphine (PH 3 ) in the gas phase and inside C 60 . Inversion potentials and eigenstates are computed with the dispersion-corrected DFT method (B97D/aug-cc-pVTZ). Barrier heights and tunnelling splittings for OH + 3 and PH 3 are benchmarked against CCSD(T)/aug-cc-pVQZ results. For free OH + 3 , the CCSD(T) barrier is computed to be ∼706 cm -1 while B97D yields a slightly lower value (613 cm -1 ). The predicted tunnelling doublets closely match the experimental findings. Encapsulation of hydronium in C 60 (denoted by OH + 3 @C 60 where we use the notation X@C 60 to indicate encapsulation of X within C 60 ) raises the barrier height from 613 to 871 cm -1 and markedly suppresses the splittings. In contrast, PH 3 exhibits an extremely high inversion barrier (∼11,000 cm -1 ), effectively quenching tunnelling. Upon confinement, the barrier is lowered marginally, and the vibrational eigenstate energies are found to shift upward. The interaction energies obtained using the DLPNO-CCSD(T)/def2-TZVP method confirm the stability of the encapsulated systems: -30.8 kcal mol -1 for OH + 3 @C 60 and -13.1 kcal mol -1 for PH 3 @C 60 . The energy decomposition analysis shows that OH + 3 @C 60 stabilization is predominantly electrostatic in nature, while the dispersion term for the PH 3 @C 60 is found to be much larger than that for the OH + 3 @C 60 .