The endofullerenes H2@C60 and HF@C60 : Inelastic neutron scattering spectra from quantum simulations and experiments, and a selection rule that should not exist
Lundi 4 mai 2020 11:00
- Duree : 1 heure
Lieu : ILL4, seminar room, first floor - 71 avenue des Martyrs - Grenoble
Orateur : Zlatko BACIC (1-Department of Chemistry, New York University, New York, NY, 10003, USA / 2-NYU-ECNU Center for Computational Chemistry at NYU Shanghai, 3663 Zhongshan Road North, Shanghai, 200062, China)
Light-molecule endofullerenes (LMEFs) have molecules like H2, HF, H2O, and CH4, characterized by small masses and large rotational constants, encapsulated inside C60 and other fullerenes. A distinguishing feature of LMEFs, one that has motivated a large number of experimental and theoretical studies, is the dominance of quantum effects in the dynamics and spectroscopy of the guest molecules [1], for the low temperatures at which the spectroscopic measurements are carried out, typically between 1.5 K and 30 K. The quantum effects arise from several sources. One of them is the quantization of the translational center-of-mass degrees of freedom (DOFs) of the encapsulated
molecules due to their tight confinement inside the fullerene (particle-in-a-box effect). The confining potential of the fullerene interior couples the quantized translational DOFs to those of the rotational eigenstates of the guest molecule (M), giving rise to a sparse and surprisingly intricate translation-rotation (TR) energy level structure [1]. In this talk,
I will survey the three main directions of our research in the past decade involving these remarkable species.
(1) Fully coupled quantum calculations of the TR eigenstates of M@C60, M = H2, HF, H2 O. These calculations (treating the guest molecule and C60 as rigid) in 5D for M = H2 and HF, and in 6D for M = H2O, have provided the quantitative description of their TR level structure. They have also elucidated its salient features, as well as the quantum numbers appropriate for the assignments [1–3].
(2) Rigorous quantum simulations of the INS spectra of H2, HD, and HF in C60. We have developed the methodology for accurate quantum calculation of the INS spectra of (rigid) diatomic molecules, homo- and heteronuclear, confined inside a (rigid) nanocavity of an arbitrary shape [4–6]. The distinguishing feature of this approach is the incorporation
of the coupled quantum 5D TR eigenstates of the entrapped diatomic as the spatial components of the initial and final states of the INS transitions. Consequently, the simulated INS spectra for the first time reflect in full the complexity of the TR dynamics of the caged molecule. They have played a key role in the assignment of the measured INS spectra of H2@C60 [7] and HF@C60 [3] (measured at ILL).
(3) Discovering the first selection rule in the INS spectroscopy of molecular compounds. Our quantum calculations of the INS spectra of H2 [7] and HD [8] in C60, followed by a rigorous derivation [7], have established the first ever selection rule in the INS spectroscopy of discrete molecular compounds. It states that for para-H2 and HD inside a near-spherical nanocavity, e.g., that of C60, INS transitions out of their ground TR state to certain excited TR states are forbidden [7]. This selection rule contradicted the widely held view that the INS spectroscopy is not subject to any selection rules. Nevertheless, its predictions were soon confirmed by the combined experimental (ILL) and theoretical INS investigation of H2@C60 [9]. Shortly thereafter, we presented an analytical derivation of a generalized version of this INS selection rule, that applies to any diatomic molecule inside a nanocavity with near-spherical symmetry, and a wider range of initial TR states [10]. The INS spectrum recorded for HF@C60 [3] agrees very well with theory.
[1] Z. Baˇci´c, J. Chem. Phys. 149, 100901 (2018).
[2] Z. Baˇci´c, M. Xu, and P. M. Felker, Adv. Chem. Phys. 163, 195 (2018).
[3] M. Xu, P. M. Felker, S. Mamone, A. J. Horsewill, S. Rols, R. J. Whitby, and Z. Baˇci´c, J. Phys. Chem. Lett. 10, 5365
(2019).
[4] M. Xu, L. Ulivi, M. Celli, D. Colognesi, and Z. Baˇci´c, Phys. Rev. B 83, 241403(R) (2011).
[5] M. Xu and Z. Baˇci´c, Phys. Rev. B 84, 195445 (2011).
[6] M. Xu, L. Ulivi, M. Celli, D. Colognesi, and Z. Baˇci´c, Chem. Phys. Lett. 563, 1 (2013).
[7] M. Xu, S. Ye, A. Powers, R. Lawler, N. J. Turro, and Z. Baˇci´c, J. Chem. Phys. 139, 064309 (2013).
[8] M. Xu, S. Ye, R. Lawler, N. J. Turro, and Z. Baˇci´c, Phil. Trans. R. Soc. A 371, 20110630 (2013).
[9] M. Xu, M. Jim´enez-Ruiz, M. R. Johnson, S. Rols, S. Ye, M. Carravetta, M. S. Denning, X. Lei, Z. Baˇci´c, and A. J.
Horsewill, Phys. Rev. Lett. 113, 123001 (2014).
[10] M. Xu, S. Ye, and Z. Baˇci´c, J. Phys. Chem. Lett. 6, 3721 (2015).
Contact : tellier@ill.fr
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