NbF5 and TaF5: Assignment of 19F NMR resonances and chemical bond analysis from GIPAW calculations
Graphical abstract
The complete assignment of the 19F NMR lines of NbF5 and TaF5 allow establishing relationships between the 19F δiso values, the nature of the fluorine atoms (bridging or terminal), the position of the terminal ones (opposite or perpendicular to the bridging ones), the fluorine charges, the ionicity and the length of the M–F bonds.
Introduction
Unambiguous assignment of complex 19F solid-state NMR spectra of crystalline fluoride compounds having multiple crystallographic sites or fluoride glasses often remains a challenging task. In several studies [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], assignments of 19F NMR resonances to environments were based on similarities to crystalline model compounds (similar 19F isotropic chemical shifts (δiso) values intending to indicate similar structural environments). Nowadays, high resolution two-dimensional heteronuclear and homonuclear correlation experiments can be used for helping in the assignment of fluorine sites. The efficiency of these experiments to probe interatomic spatial proximities and through bond connectivities has been described in various studies [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. An alternative approach consists in the calculation of 19F isotropic chemical shieldings (σiso). These calculations have been first achieved in various crystalline fluoride materials [29], [30], [31], [32], [33], [34], [35] using a semi-empirical model, called superposition model [36]. Ab initio approaches devoted to molecular systems have been used for studying extended systems. In these cases, clusters of atoms were build such that the central atom experiences an environment similar to that in true extended solid, i.e., to mimic the crystalline periodic structures [14], [30], [37], [38], [39], [40], [41], [42], [43], [44]. Recent advances in the theoretical calculations of NMR parameters for extended solids lead to the development of the Gauge Including Projector Augmented Wave (GIPAW) method [45], [46] which integrates explicitly the periodic boundary conditions. This major breakthrough enables consequently the calculations of NMR tensors in solids [47], [48] and was applied on inorganic fluorides for the calculation of 19F σiso values [19], [20], [21], [22], [23], [27], [49], [50], [51], [52], [53].
When interested in the prediction of δiso values, the calculated 19F σiso values have to be converted into the isotropic chemical shift scale. Assuming that the 19F σref can be obtained, the calculated 19F σiso values can be converted into “calculated” 19F δiso values applying the relation δiso≈σref–σiso [14], [19], [42], [49], [54]. Calculated 19F σiso values can also be converted into “calculated” 19F δiso values [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [43] using an experimental absolute scale for fluorine [55], [56]. To avoid these referencing problems and possible errors coming from the calculation of the 19F σref value, “calculated” 19F δiso values can be deduced from the linear regression established between calculated 19F σiso values and experimental 19F δiso values for the compounds under study [20], [21], [23], [50], [51], [53], implying that an assignment was already done. This third approach seems to become the standard practice [47], [48]. Alternatively a linear regression previously established on various compounds [50] can be used [22], [27], [52]. For many nuclei, deviations from the theoretically expected slope of minus one have been reported for GIPAW calculations [48] using GGA (generalized gradient approximation) for the exchange and correlation potential. 19F is not an exception with a large dispersion of reported slopes: −0.68 [20], −0.80 [50], 0.86 [49] and 0.83 [21], the last two positive values being obtained when “calculated” and experimental δiso are compared. Pedone et al. [53] were able to reach a nearly ideal slope value of −0.963 but only on few compounds.
The two binary pentafluorides NbF5 and TaF5 are isomorphs and consist of tetrameric structural units [M4F20] (M=Nb,Ta), build up of MF6 octahedra linked to each other by corners in a cis-configuration. Both the compounds involve six fluorine sites, 2 sites with multiplicity 4 and 4 sites with multiplicity 8 [57], [58]. These two compounds, known to be very hygroscopic, have not been yet studied by solid state NMR. In the present work, we report for the first time the one dimensional (1D) 19F magic angle spinning (MAS) NMR spectra of NbF5 and TaF5, allowing us to determine their experimental 19F δiso values. Due to the hygroscopic character and reactivity (chemical attack of the caps of the rotors by NbF5) of NbF5 and TaF5, we choose to avoid 19F–19F correlation experiments since they require long acquisition time. Anyway, in these cases, only fragmentary information could be obtained since some of the fluorine atoms have similar environments and since at least two 19F NMR resonances overlap. Consequently, the only way to complete the initial partial assignment, based on NMR line relative intensities and 19F δiso and chemical shift anisotropy (δcsa) values, was to perform calculations of the 19F chemical shielding tensors. These DFT (density functional theory) computations have been performed using the GIPAW [45], [46] method implemented in the NMR-CASTEP code [59], [60]. Since the agreement between the calculated and experimental NMR parameters is generally significantly improved after a DFT structural optimization [16], [21], [23], [31], [47], [48], [49], [51], [52], [61], GIPAW calculations have been achieved on both experimental and optimized structures. In NbF5 and TaF5, however, [M4F20] units are held together by van der Waals (vdW) interactions and those dispersion forces are not described by most of the exchange-correlation DFT functional used for the GIPAW calculations. A recently proposed method to work around this problem consists in adding a semi-empirical dispersion energy term to the conventional Kohn–Sham DFT energy [62]. For this purpose, structural optimizations were also done with the semi-empirical DFT-D2 approach of Grimme as implemented in the VASP package [63], [64]. The semi-empirical parameters needed for describing the three elements under study (F, Nb and Ta) were taken from the work of Grimme [62].
In the present work, we show that GIPAW calculations enable a complete and convincing assignment of the lines of the 19F NMR spectra of NbF5 and TaF5. Moreover, the effects of the geometry optimisations are carefully examined and the structural features deeply discussed. Relations are also clearly established between the fluorine chemical shifts and the nature of the fluorine atoms (bridging or terminal), the position of the terminal ones (opposite or perpendicular to the bridging ones), the fluorine Mulliken charges or the ionicity and length of the M–F bonds. Additionally, for three of the 19F NMR lines of NbF5, distorted multiplets, arising from 1J-coupling and residual dipolar coupling between the 19F and 93Nb nuclei, were simulated yielding to values of 93Nb–19F 1J-coupling for the corresponding fluorine sites.
Section snippets
Materials and methods
The samples of TaF5 and NbF5 were purchased from Alfa Aesar (lot numbers D20L30 and l11T005, respectively) and were kept in a dry glove box under nitrogen atmosphere. As both compounds are very hygroscopic, the rotors were filled inside the glove box to avoid any hydration of the samples.
Solid-state NMR experiments were performed on an Avance 300 Bruker spectrometer operating at 7.0 T (19F Larmor frequency of 282.2 MHz), using a 2.5 mm CPMAS probehead. 19F one dimensional (1D) MAS NMR spectra were
Calculation
The chemical shift tensor is described by three parameters, the isotropic chemical shift (δiso), the chemical shift anisotropy (δcsa) and the asymmetry parameter (ηcsa), determined experimentally, and defined as , , , with the principal components defined in the sequence .
The chemical shielding tensor is also described by three parameters, the isotropic chemical shielding , the chemical
Results and discussion
TaF5 and NbF5 are isomorphs and crystallize in a monoclinic cell (space group C2/m, a=9.62 Å, b=14.43 Å, c=5.12 Å and β=96.1° for NbF5 [57] (ICSD [71] file no. 26647) and a=9.5462 Å, b=14.3678 Å, c=5.0174 Å and β=97.086° for TaF5 [58] (ICSD [71] file no. 171155)). They consist of tetrameric structural units, [M4F20], build up of MF6 octahedra linked to each other by corners in a cis-configuration (Fig. 1). The structures of both compounds contain six F crystallographic sites, two sites of
Conclusions
The 19F δiso values in NbF5 and TaF5 are determined from the reconstruction of 1D 19F MAS NMR spectra. An initial partial assignment is achieved, based on NMR line relative intensities and δiso values previously determined in several compounds containing also bridging and terminal fluorine atoms bonded to Nb and Ta atoms.
The atomic position optimizations and full geometry optimizations were performed using the VASP package [63] with and without vdW corrections (i.e. with and without using the
Acknowledgments
The authors thank the Région Pays de la Loire for the financial support of the RMN3MPL project, especially M. Biswal (doctoral grant) and A. Sadoc (post-doctoral fellowship). The computational presented in this work have been carried out at the Centre Régional de Calcul Intensif des Pays de la Loire (CCIPL), financed by the French Research Ministry, the Région Pays de la Loire, and Nantes University. We thank CCIPL for CASTEP licenses financial support.
References (96)
Prog. Nucl. Magn. Reson. Spectrosc
(1996)- et al.
J. Solid State Chem.
(1998) - et al.
J. Non-Cryst. Solids
(1999) - et al.
J. Non-Cryst. Solids
(2000) - et al.
J. Non-Cryst. Solids
(2001) - et al.
J. Non-Cryst. Solids
(2002) - et al.
J. Non-Cryst. Solids
(2004) - et al.
Solid State Nucl. Magn. Reson.
(2005) - et al.
J. Solid State Chem.
(2013) - et al.
C.R. Chim.
(2004)
J. Solid State Chem.
Chem. Phys.
Chem. Phys. Lett.
Chem. Phys. Lett.
Solid State Nucl. Magn. Reson.
Chem. Phys. Lett.
J. Fluorine Chem.
J. Fluorine Chem.
J. Organomet. Chem.
Prog. Nucl. Magn. Reson. Spectrosc
Inorg. Chim. Acta
C.R. Chim.
J. Phys. Condens. Matter
Am. Mineral.
J. Am. Ceram. Soc.
J. Am. Chem. Soc.
Am. Mineral.
Chem. Mater.
Chem. Commun.
Dalton Trans.
Inorg. Chem.
Phys. Chem. Chem. Phys.
Phys. Chem. Chem. Phys.
J. Am. Chem. Soc.
Inorg. Chem.
Inorg. Chem.
J. Mater. Chem.
Phys. Chem. Chem. Phys.
Chem. Mater.
J. Phys. Chem. C
Dalton Trans.
Inorg. Chem.
Inorg. Chem.
Inorg. Chem.
Dalton Trans.
J. Phys. Chem. C
J. Chem. Soc., Faraday Trans.
J. Phys. Chem. A
Cited by (24)
Advances in the computation of nmr parameters for inorganic nuclides
2023, Comprehensive Inorganic Chemistry III, Third EditionProgress in Our Understanding of <sup>19</sup>F Chemical Shifts
2018, Annual Reports on NMR SpectroscopyCitation Excerpt :Exact assignment of complex 19F solid-state NMR spectra of either fluoride compounds with multiple crystallographic sites or fluoride glasses is a difficult task. Recently, researchers have begun using high-resolution 2-D heteronuclear or homonuclear NMR correlation experiments for this task [174]. In the past studies, it has been reported that double-quantum single-quantum (DQ–SQ) MAS NMR correlation experiments [175], with homonuclear dipolar couplings [176], were able to determine the fluorine–fluorine spatial proximities in other fluoride materials [177–180].
Identify OH groups in TiOF<inf>2</inf> and their impact on the lithium intercalation properties
2017, Journal of Solid State ChemistryCitation Excerpt :The lines located at higher chemical shift values, i.e. ~ 146 and 182 ppm (relative intensities equal to 18%), are assigned to fluorine atoms close to titanium vacancies Ti-F-□. These assignments are in agreement with the increase of the chemical shift of the anion when the number of surrounding cations decreases, as observed for 19F NMR in NbF5 and TaF5 [24] and in the anatase Ti0.78□0.22O1.12(OH)0.48F0.40 [7] and for 17O NMR in TiO2(B) [25]. The 19F δiso values of the fluorine atoms close to titanium vacancies Ti-F-□ are significantly larger than those measured for F-Ti□2 in the anatase Ti0.78□0.22O1.12(OH)0.48F0.40 (98 ppm [7]) and for terminal F in K2TiF6 (71.4 ppm [26]) and in hybrid hydroxyfluorotitanates (IV) (76–82 ppm [27]).
NMR parameters in column 13 metal fluoride compounds (AlF<inf>3</inf>, GaF<inf>3</inf>, InF<inf>3</inf> and TlF) from first principle calculations
2014, Solid State Nuclear Magnetic ResonanceCitation Excerpt :These findings therefore clearly confirm that the previously proposed relationship [9] could allow predicting 19F NMR spectra for a broad range of crystalline metal fluoride compounds with a relatively good accuracy. For completeness, it should be noted that Pedone et al. [51] obtain a slope of −0.963 on five compounds among which three contain Ca2+, by applying a larger shift of the local potential of the USPP of Ca2+ and that a slope close to −1 has been obtained for NbF5 [50]. As mentioned above, a nice agreement between experimental quadrupolar parameters and PAW [19,20] calculated values for the IS structures of α-AlF3 and InF3 is observed (Table 2), showing the accuracy of these crystalline structures.