Persistence of Dual Free Internal Rotation in NH₄⁺(H₂O)·He_{<i>n</i>=0–3} Ion–Molecule Complexes: Expanding the Case for Quantum Delocalization in He Tagging

Abstract

To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH₄⁺(H₂O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH₄⁺(H₂O)·He_1–3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the −OH₂ and −NH₃ groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire −NH₃ rotor, effectively restoring <i>C</i>₃ symmetry for the embedded −NH₃ group