The sensitivity of 129Xe NMR to the size and shape of the cavities makes, both thermally-polarized and hyper-polarized techniques, invaluable for the characterization of porous materials, such as MOFs and molecular crystals [1-2]. In this presentation, the chemical shift anisotropy (CSA) pattern mirrors not only shape and symmetry of the explored rooms, but also the dynamics of xenon jumping from one site to the next. This study belongs to a program for generating ultra-fast molecular rotors and understanding dynamics in porous crystals [3-4]. Tetra-carboxylic molecules could be obtained in the permanently porous form for the stability imparted by 8 hydrogen bonds: the material was explored by xenon and the anisotropic 129Xe signal resonates in the 220-320 ppm range, indicating a tight fit of the gas atom in the adsorption sites [5]. The intriguing line-shape evolution (see figure) denotes a temperature dependent exchange dynamics (observed at 10 bar loading pressure) and was interpreted as the effect of thermally activated Xe jumps to adjacent vacant sites along the channels. Each site is marked by the relative orientations. Indeed, the explored cavities show elliptical cross-sections (e = 0.75), alternatively rotated by 90° about the channel axis, as CSA profile below 240 K depicts. At higher temperatures, xenon atoms dynamically explore the two orientations and, when the exchange rates exceed the frequency span of the tensor principal components, Xe perceives an averaged interaction. This interpretation allowed us to simulate the anisotropy and assign at each pattern a specific jump-rate and to measure, by an Arrhenius plot, the energy barrier for an individual jump of 1.8 kcal/mol. Ab initio calculations and molecular dynamics confirm the high mobility of xenon atoms inside the structure and evaluate a consistent energy barrier. The experimental single-jump frequency and energy were the basis to establish xenon diffusion rate in the narrow channels. Source of founding PRIN 2017-20 -NAZ-104, Cariplo Foundation 2017-19 BALANCE and INSTM-Lombardy 2017-18. References: [1] Porous Materials Explored by Hyperpolarized Xenon NMR; P. Sozzani, S. Bracco, A. Comotti in Hyperpolarized Xenon-129 Magnetic Resonance T. Meersman and E. Brunner (Eds.), 2015, 164. [2] A. Comotti, S. Bracco, P. Sozzani, S. Horike, R. Matsuda, S. Kitagawa JACS, 2008, 130,13664. [3] Review: Molecular rotors build in porous materials; S. Bracco, A. Comotti, P. Sozzani, Acc. Chem. Res., 2016, 49, 1701. [4] S. Bracco, F. Castiglioni, A. Comotti, S. Galli, M. Negroni, A. Maspero, P. Sozzani, Chem. Eur. J., 2017, 23, 11210. [5] I. Bassanetti, S. Bracco, A. Comotti, M. Negroni, C. Bezuidenhout, S. Canossa, P. P. Mazzeo, L. Marchio’, P. Sozzani J Mater. Chem A (in press).
Negroni, M., Bracco, S., Comotti, A., Bezuidenhout, C., Bassanetti, I., Marchiò, L., et al. (2018). Reorientation jumps and energy profile for Xe diffusing from site to site along the channels of porous molecular crystals. In Book of Abstracts.
Reorientation jumps and energy profile for Xe diffusing from site to site along the channels of porous molecular crystals
Negroni, M
Membro del Collaboration Group
;Bracco, SMembro del Collaboration Group
;Comotti, A;Bezuidenhout, CMembro del Collaboration Group
;Bassanetti, IMembro del Collaboration Group
;Sozzani, PMembro del Collaboration Group
2018
Abstract
The sensitivity of 129Xe NMR to the size and shape of the cavities makes, both thermally-polarized and hyper-polarized techniques, invaluable for the characterization of porous materials, such as MOFs and molecular crystals [1-2]. In this presentation, the chemical shift anisotropy (CSA) pattern mirrors not only shape and symmetry of the explored rooms, but also the dynamics of xenon jumping from one site to the next. This study belongs to a program for generating ultra-fast molecular rotors and understanding dynamics in porous crystals [3-4]. Tetra-carboxylic molecules could be obtained in the permanently porous form for the stability imparted by 8 hydrogen bonds: the material was explored by xenon and the anisotropic 129Xe signal resonates in the 220-320 ppm range, indicating a tight fit of the gas atom in the adsorption sites [5]. The intriguing line-shape evolution (see figure) denotes a temperature dependent exchange dynamics (observed at 10 bar loading pressure) and was interpreted as the effect of thermally activated Xe jumps to adjacent vacant sites along the channels. Each site is marked by the relative orientations. Indeed, the explored cavities show elliptical cross-sections (e = 0.75), alternatively rotated by 90° about the channel axis, as CSA profile below 240 K depicts. At higher temperatures, xenon atoms dynamically explore the two orientations and, when the exchange rates exceed the frequency span of the tensor principal components, Xe perceives an averaged interaction. This interpretation allowed us to simulate the anisotropy and assign at each pattern a specific jump-rate and to measure, by an Arrhenius plot, the energy barrier for an individual jump of 1.8 kcal/mol. Ab initio calculations and molecular dynamics confirm the high mobility of xenon atoms inside the structure and evaluate a consistent energy barrier. The experimental single-jump frequency and energy were the basis to establish xenon diffusion rate in the narrow channels. Source of founding PRIN 2017-20 -NAZ-104, Cariplo Foundation 2017-19 BALANCE and INSTM-Lombardy 2017-18. References: [1] Porous Materials Explored by Hyperpolarized Xenon NMR; P. Sozzani, S. Bracco, A. Comotti in Hyperpolarized Xenon-129 Magnetic Resonance T. Meersman and E. Brunner (Eds.), 2015, 164. [2] A. Comotti, S. Bracco, P. Sozzani, S. Horike, R. Matsuda, S. Kitagawa JACS, 2008, 130,13664. [3] Review: Molecular rotors build in porous materials; S. Bracco, A. Comotti, P. Sozzani, Acc. Chem. Res., 2016, 49, 1701. [4] S. Bracco, F. Castiglioni, A. Comotti, S. Galli, M. Negroni, A. Maspero, P. Sozzani, Chem. Eur. J., 2017, 23, 11210. [5] I. Bassanetti, S. Bracco, A. Comotti, M. Negroni, C. Bezuidenhout, S. Canossa, P. P. Mazzeo, L. Marchio’, P. Sozzani J Mater. Chem A (in press).
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