*2.2. 1H MAS NMR Study*

Figure <sup>3</sup> shows the 1H MAS NMR spectra for the studied forms of HCa2Nb3O10·*y*H2O acquired at 259 K. As one can see, depending on the hydration level of HCa2Nb3O10·*y*H2O, the spectra differ from each other by the number of spectral lines, their position, and the linewidths. This shows the presence of different proton-containing species in the α-, β-, and *γ*-forms and their different mobilities.

**Figure 3.** 1H MAS NMR spectra in the <sup>α</sup>- (**a**), <sup>β</sup>- (**b**), and *<sup>γ</sup>*-forms (**c**) of HCa2Nb3O10·*y*H2O at 297 K.

At room temperature (297 K), the 1H spectrum of α-HCa2Nb3O10 (Figure 3a) consists of two narrow intense Lorentzian lines at 3.1 and 6.8 ppm, L1 and L2, respectively, and two lines of lower intensities: Lorentzian line L3 at about 4.1 ppm and Gaussian line G4 at 6.0 ppm. For β-HCa2Nb3O10 (Figure 3b), it consists of only one rather broad Lorentzian line at 3.6 ppm, whereas for *γ*-HCa2Nb3O10 (Figure 3c), the main signal is observed at 8.2 ppm (L2), with a shoulder at 5.9 ppm (L1) (a signal at about −2 ppm can be associated with surface defects and is not discussed further).

To assign the spectral lines to the H-containing species, the evolution of the proton spectra with the temperature decreasing was studied; see Figure 4. As temperature decreases, the spectral lines broaden, and a redistribution of line intensities occurs. Let us first discuss the temperature evolution of the 1H MAS NMR spectrum of *<sup>γ</sup>*-form, HCa2Nb3O10·0.1H2O, which is characterized by the lowest water content. At room temperature, the contribution of the L2 line dominates, the relative intensity of L1 is of about 10%, and with sample cooling the line broadens and then disappears. Below 259 K, only the L2 line presents, and with the temperature further decreasing it splits into two lines: Lorentzian type at 8.9 ppm and Gaussian type at 7.8 ppm; see Figure 5. The temperature evolution of the spectral line parameters, namely the isotropic chemical shift (δiso), the full width at half maximum (Δν1/2), and the relative integral intensities are shown in Figure 6a–c, respectively.

**Figure 4.** 1H MAS NMR spectra in the <sup>α</sup>- (**a**), <sup>β</sup>- (**b**), and *<sup>γ</sup>*-forms (**c**) of HCa2Nb3O10·*y*H2O with the temperature decreasing.

**Figure 5.** 1H MAS NMR spectra at 151 K in *<sup>γ</sup>*-HCa2Nb3O10·*y*H2O and its decomposition.

**Figure 6.** Temperature dependencies of the 1H isotropic chemical shift (the upper row), the line width at half maximum (the middle row), and the integral intensities (the bottom row) for the α- (**a**), β- (**b**), and *γ*- (**c**) forms of HCa2Nb3O10·*y*H2O.

Based on the TG analysis, one can attribute L1 line to the bulk water. Normally its signal is expected at 5.5 ppm [22], but in a charged nanoconfinement it can be shifted towards a higher frequency. Its contribution is low with the temperature decreasing because of the slowing down of the molecular motion; thus, the line becomes too broad to be resolved. The most intensive line, L2, at about 8 ppm can be associated with the lattice protons in regular sites; e.g., in Ruddlesden–Popper phase H2La2Ti3O10·0.13H2O, the signal of isolated H<sup>+</sup> was reported at 11–13 ppm [13,37]. The splitting of the line at low temperatures may point to two inequivalent cation positions. It is worth noting that down to 151 K, the linewidth of the spectral lines is almost unchanged. This indicates that within the studied temperature range, the proton mobility (translational diffusion) does not change significantly.

The 1H spectra of α-HCa2Nb3O10 exhibits the most dramatic changes with temperature: the high field part of the spectrum rapidly disappears with cooling; see Figure 4a. The temperature changes of the spectral parameters are plotted in Figure 6a. As one can see, with the temperature decreasing, the intensities of the spectral lines L1 and G1 rapidly drop, and after cooling down below 245 K, only L2 and L3 remain. Below 200 K, only the L2 peak is visible. Such a complex temperature behavior of the 1H MAS NMR spectrum of the α-form, as well as its structure, reflect (i) the variety of types of interlayer proton-containing species due to the high content of intercalated water in comparison with the other studied forms, and (ii) the non-obvious mechanisms of interaction between them in a charged environment. Interestingly, in α-form there is no signal associated with isolated protons. Moreover, despite a rather high water content, the only signal that can be associated with the bulk water, the line G4 at 6.1 ppm, has a very low intensity and, similar to the *γ*-form, rapidly disappears with cooling. Altogether, this suggests the presence of charged water complexed like H<sup>+</sup> ... *x*H2O.

According to Ref. [43], the 1H chemical shift of H3O+ (*x* = 1), calculated for water solutions of mineral acids, is expected at 13.3 ppm. With *x* increasing, the 1H chemical shift decreases, e.g., for H<sup>+</sup> ... *2*H2O it was predicted at 8.0 ppm. Our calculations carried out for isolated complexes give 7.3 and 4.6 (17.5) ppm for the isotropic chemical shift for free H3O+ and H5O2 <sup>+</sup> clusters, respectively (the number in parenthesis corresponds to the central proton). These calculations are supported by several experimental studies of hydrated layered oxides, in which the signal at 8–11 ppm was assigned to the H3O+ [32,36,44–46]. Hence, following both theoretical and experimental studies of other complex layered oxides, and accounting that for α-form of HCa2Nb3O10·*y*H2O there are 1.6 H2O molecules and one interlayer proton per one formula, it can be suggested that one water molecule participates in the formation of H3O+, the signal L2 at about 7 ppm, whereas other signals correspond to water molecules that are localized in different sites of the charged interlayer space or are part of the more extended charged complexes, like H+ ... *2*H2O.

The temperature behavior of the L2 linewidth, Figure 6a, is typical for solids [13,47,48] and reflects the slowing down of the molecular motion. Using the onset temperature of motional narrowing, *T*MN = 150 K, one can estimate the activation energy of the line narrowing process within the semi-empirical Waugh-Fedin expression [49]:

$$E\_{\rm A}(\rm eV) \approx 1.61 \times 10^{-3} \cdot T\_{\rm MN}(\rm K). \tag{3}$$

This results in *E*<sup>a</sup> ≈ 0.24(2) eV.

The 1H MAS NMR spectrum of the β-form consists of one line centered at about 3.6 ppm, which almost does not shift within the studied temperature range; see Figure 5. Taking into account that, according to the TG analysis, the β-form contains 0.8 H2O molecules per formula unit, and hence per interlayer cation H+, and no signal from H3O+ or H<sup>+</sup> is observed, one can suppose that this line is the result of an exchange between the lattice protons (an expected signal at about 8 ppm as in the γ-form) and the non-hydrogenbounded water (an expected signal at about 0.8 ppm).
