Effect of confinement on PH₃ and OH₃⁺ inversion

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Nanotechnology

Background:

• Encapsulating molecules in nanocages like C60 offers insights into spatial confinement effects on molecular structure and dynamics.
• Understanding these effects is crucial for designing novel materials and controlling chemical reactions at the nanoscale.

Purpose of the Study:

• To investigate the impact of C60 nanocage confinement on the umbrella inversion dynamics of hydronium (OH3+) and phosphine (PH3).
• To computationally model and compare the inversion barrier heights and tunneling splittings of these molecules in both gas phase and confined environments.

Main Methods:

• High-level correlated electronic structure methods (CCSD(T)/aug-cc-pVTZ and aug-cc-pVQZ) were employed for gas-phase calculations.
• Dispersion-corrected Density Functional Theory (DFT) (B97-D/aug-cc-pVTZ) was used to model the computationally intensive confined systems (OH3+@C60 and PH3@C60).
• Benchmarking and interaction energy calculations (DLPNO-CCSD(T)/def2-TZVP) were performed to validate results and assess system stability.

Main Results:

• Gas-phase OH3+ exhibits a computed inversion barrier of ~706 cm-1, with predicted tunneling doublets matching experimental data.
• Confinement in C60 (OH3+@C60) significantly increases the inversion barrier to 871 cm-1 and suppresses tunneling splittings.
• PH3 has a very high gas-phase inversion barrier (~11,000 cm-1), precluding tunneling; confinement marginally lowers this barrier and shifts vibrational energies upwards.

Conclusions:

• Spatial confinement within C60 dramatically alters the inversion dynamics of OH3+ and PH3, primarily by modifying their respective energy barriers.
• The stabilization of OH3+@C60 is mainly electrostatic, while PH3@C60 shows a larger contribution from dispersion forces.
• These findings highlight the significant influence of nanocage environments on molecular behavior and quantum phenomena like tunneling.