Vibrational Modes and Terahertz Phenomena of the Large-Cage Zeolitic Imidazolate Framework-71

Abstract

The zeolitic imidazole framework ZIF-71 has the potential to outperform other well-studied metal–organic frameworks due to its intrinsic hydrophobicity and relatively large pore size. However, a detailed description of its complex physical phenomena and structural dynamics has been lacking thus far. Herein, we report the complete assignment of the vibrational modes of ZIF-71 using high-resolution inelastic neutron scattering measurements and synchrotron radiation infrared spectroscopy, corroborated by density functional theory (DFT) calculations. With its 816 atoms per unit cell, ZIF-71 is the largest system yet for which frequency calculations have been accomplished employing the CRYSTAL17 DFT code. We discover low-energy terahertz dynamics such as gate-opening and shearing modes that are central to the functions and stability of the ZIF-71 framework structure. Nanoscale analytical methods based on atomic force microscopy (near-field infrared spectroscopy and AFM nanoindentation) further unravel the local chemical and mechanical properties of ZIF-71 single crystals.

Effects of Ligand Substituents on the Character of Zn-Coordination in Zeolitic Imidazolate Frameworks

“…Despite the general consensus regarding the ionic nature of the Zn-N bond in ZIFs, [27][28][29][30] the actual strength of this bond and the influence exerted on it by the chemical environment are still a matter of debate. 15,31,32 In particular, recent studies have unraveled non-trivial characteristics in the electronic structure of ZIF-8, [33][34][35] which point to the existence of some degree of covalence in the bond between Zn and N. This hypothesis is supported by ab initio calculations on the transport properties 33 and by the vibrational activity of ZIFs. 34,35 Experimental evidence of the (partial) covalent character of Zn-N coordination is, to date, only indirect.…”

“…From a fundamental perspective, these characteristics are ruled by the electronic charge distribution in the material and the nature of the chemical bonds therein. Despite the general consensus regarding the ionic nature of the Zn–N bond in ZIFs, − the actual strength of this bond and the influence exerted on it by the chemical environment are still a matter of debate. ,, In particular, recent studies have unraveled nontrivial characteristics in the electronic structure of ZIF-8, − which point to the existence of some degree of covalence in the bond between Zn and N. This hypothesis is supported by ab initio calculations on the transport properties and by the vibrational activity of ZIFs. , Experimental evidence of the (partial) covalent character of Zn–N coordination is, to date, only indirect. Observations of chemical bond stability of ZIF-8 and related compounds especially in acidic environments point to their susceptibility to hydrolytic cleavage, which is only known for covalent bonds .…”

“…15,31,32 In particular, recent studies have unraveled nontrivial characteristics in the electronic structure of ZIF-8, 33−35 which point to the existence of some degree of covalence in the bond between Zn and N. This hypothesis is supported by ab initio calculations on the transport proper-ties 33 and by the vibrational activity of ZIFs. 34,35 Experimental evidence of the (partial) covalent character of Zn−N coordination is, to date, only indirect. Observations of chemical bond stability of ZIF-8 and related compounds especially in acidic environments point to their susceptibility to hydrolytic cleavage, which is only known for covalent bonds.…”

Nanotrap Grafted Anionic MOF for Superior Uranium Extraction from Seawater

“…The nano-FTIR spectroscopy was employed to probe the hostguest interactions at the single-crystal level with a spatial resolution that beats the diffraction limit of light (Figure 5b,c). [38,39] This technique combines scattering-type scanning near-field optical microscopy (s-SNOM) with infrared nanospectroscopy to measure vibrational spectra reflecting local complex characteristics of individual sub-micron crystals where, notably even a singlecrystal X-ray diffraction is inaccessible. Rendering to local complex character of the framework, its symmetry is disrupted at a probing depth of 20 nm along the grain boundary of the iMOF-1A 2+ @iMOF-1A; b) nano-FTIR spectra of iMOF-1A, UO 2 2+ @iMOF-1A and UO 2 2+ salt, inset: nanoscopic spots corresponding to a probe size of ≈20 nm; c) local probing spots of UO 2 2+ @iMOF-1A; d) TFM for iMOF-1A; e) TFM quantification plots for UO 2 2+ @iMOF-1A, TFM reconstructions corresponding to different pressure points showing local stiffnesses of a single crystal.…”

“…The nano‐FTIR spectroscopy was employed to probe the host‐guest interactions at the single‐crystal level with a spatial resolution that beats the diffraction limit of light (Figure 5b,c). [ 38,39 ] This technique combines scattering‐type scanning near‐field optical microscopy (s‐SNOM) with infrared nanospectroscopy to measure vibrational spectra reflecting local complex characteristics of individual sub‐micron crystals where, notably even a single‐crystal X‐ray diffraction is inaccessible. Rendering to local complex character of the framework, its symmetry is disrupted at a probing depth of 20 nm along the grain boundary of the iMOF‐1A crystals that resulted in slightly broadened and additional peaks in nano‐FTIR spectra as compared to the far‐field ATR‐FTIR spectra.…”