Quantum rattling of molecular hydrogen in clathrate hydrate nanocavities

Abstract

We have performed high-resolution inelastic neutron scattering studies on three samples of hydrogenated tetrahydrofuran-water clathrates, containing either H2 at different para/ortho concentrtion, or HD. By a refined analysis of the data, we are able to assign the spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-H2. The H2 molecule rotates almost freely, while performing a translational motion (rattling) in the nanometric-size cage, resulting a paradigmatic example of quantum dynamics in a non-harmonic potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets, having different separation. The splitting is a consequence of a substantial anisotropy of the environment with respect to the orientation of the molecule in the cage, in the first case, or with respect to the center-of-mass position inside the cage, in the second case. The values of the transition frequencies and band intensities have been quantitatively related to the details of the interaction potential between H2 and the water molecules, with a very good agreement

Confirming a Predicted Selection Rule in Inelastic Neutron Scattering Spectroscopy: The Quantum Translator-RotatorH2Entrapped InsideC60

“…Inelastic neutron scattering (INS) spectroscopy is a powerful probe for investigating the highly quantum coupled translation-rotation (TR) dynamics of hydrogen molecules entrapped in the nanocavities of diverse host materials [1,2]. In particular, in recent years it has been used to probe in detail the TR dynamics of a hydrogen molecule encapsulated in the fullerene C 60 [3][4][5] and the azacyclic-thiacyclic open-cage fullerene (ATOCF) [6], as well as in the cages of clathrate hydrates [7,8]. INS was also used to investigate the quantum rotation of H 2 O inside C 60 [9].…”

Quantum Translator-Rotator: Inelastic Neutron Scattering of Dihydrogen Molecules Trapped inside Anisotropic Fullerene Cages

“…[4,5,6 ] The dynamics of several endohedral H 2 -fullerenes have been studied by NMR [7,8] and the data have been interpreted alongside models that incorporate quantised rotational and translational states of H 2 . However, the energy level separations are very much larger than the characteristic energy scale associated with the magnetic interactions so that NMR is only able to probe the translation-rotation manifold indirectly.…”

HD in C 60 : theoretical prediction of the inelastic neutron scattering spectrum and its temperature dependence

“…Besides H 2 @C 60 , the theoretical study of which will be reported shortly, this includes molecular hydrogen in clathrate hydrates [19,29,30], metal-organic frameworks [31][32][33][34][35] and zeolites [36,37], among others. Finally, methodological extensions are in progress in our group to the INS spectra of nanoconfined polyatomic molecules such as methane [38] and water [39].…”