A research paper titled Effect of confinement on PH3 and OH3+ inversion, coauthored by Dr Brijesh Kumar Mishra, Associate Professor, Chemistry, Professor S Sivakumar, Professor, Physics and Dean – Research, and the SIAS alumni Kaustav Mehta, Shreya Chidambaram, and Netra Krishna, has recently been published in the scientific journal Physical Chemistry Chemical Physics (PCCP).
Accessible Summary:
Molecules trapped in cages (bigger molecules that enclose a volume) exhibit features distinct from those when they are free. In this work, the authors study the “inversion” of caged pyramidal molecules. Inversion is the process by which the atom at the top of the pyramid tunnels through the bottom plane of atoms to reach the symmetrically located position on the other side of the plane. Essentially, a transition from an erect pyramid to an inverted pyramid, and hence the name inversion. The cage has two prominent effects on the pyramidal molecules: it shifts the energy levels corresponding to the bending motion and changes the tunnelling barrier between the two structures. The authors carried out accurate calculations of these changes and, in many cases, produced results that compare well with measured values, surpassing earlier estimates in the literature. Apart from their importance from a fundamental perspective, caged molecules are also potential candidates for quantum information processing and metrology.
Technical Abstract:
Encapsulating molecules in nanocages such as C60 provides a unique opportunity to probe how spatial confinement alters structure and dynamics. We examine umbrella inversion in hydronium (OH3+) and phosphine (PH3) in the gas phase and inside C60. Inversion profile computations for OH3+ and PH3 are based on high-level correlated methods [CCSD(T)/aug-cc-pVTZ and aug-cc-pVQZ]. Modelling confined systems requires dealing with the cage and the encapsulated molecules together, which is computationally complex. Therefore, results pertaining to encapsulated systems are based on dispersion-corrected DFT (B97-D/aug-cc-pVTZ). Barrier heights and tunnelling splittings for OH3+ and PH3 are benchmarked against CCSD(T)/aug-cc-pVQZ results. For free OH3+, the CCSD(T) barrier is computed to be ∼706 cm−1, while B97-D yields a slightly lower value (612 cm−1). The predicted tunnelling doublets closely match the experimental findings. Encapsulation of hydronium in C60 (denoted as OH3+@C60, where X@C60 indicates the encapsulation of X within C60) raises the barrier height from 612 to 871 cm−1 and markedly suppresses the splittings. In contrast, PH3 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 shifted 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 OH3+@C60 and −13.4 kcal mol−1 for PH3@C60. Energy decomposition analysis shows that OH3+@C60 stabilization is predominantly electrostatic in nature, whereas the dispersion term in PH3@C60 is considerably larger.