Note that in earlier studies, it was not possible to observe appreciable amounts of pyridinium in Ga@SiO2 by FT-IR at room temperature, suggesting that N-py NMR is a privileged probe for Brønsted acid sites in these materials. Another common class of probe molecules used in NMR are phosphine oxides, which provide insight into the Lewis acidity of both molecules and materials. Thus, we chose to use triphenyl phosphine oxide (TPPO) as a probe molecule to explore the Lewis acid sites of Ga@SiO2. Adsorption of TPPO on Ga@SiO2 results in a phosphorus-31 ( 31 P) spectrum containing two dominant signals centered at 46 and 32 ppm (See SI S7). The signal at 46 ppm is assigned to the interaction of TPPO with GaLA (TPPO/GaLA) -confirming the presence of Lewis acidic Ga sites; the other signal at 32 ppm is ascribed to TPPO interacting with surface OH groups via H-bonding, e.g. Si-OH•••Ga (TPPO/GaBA) or Si-OH. In contrast to earlier reports, a CSA is observed for all species, suggesting a specific interaction with surface sites rather than highly mobile species reported previously (See SI S7 for fitting). Having identified the presence of two distinct families of Ga surface sites in Ga@SiO2 using probe molecules, we then turned to 71 Ga NMR, first investigating Ga@SiO2 at 14.1 and 21.15 T to gain further insights into the structure and distribution of the Ga sites present. Using WURST-QCPMG detection, in combination with a frequency-stepped approach, the NMR spectrum of Ga@SiO2 was obtained. The broad and continuous nature of the 71 Ga NMR signal obtained (Figure ) illustrates that there are, most likely, no large domains of Ga-oxides, which would result in signals with smaller CQ values and intense maxima, in addition to evidence for [6]-Ga species (δiso = 0-75 ppm). The shape of the spectrum is consistent with a Czjzek distribution, with an average η of 0.61. 44 N-pyridine, for (i) Lewis acid sites (GaLA, N-py/GaLA) and (ii) Brønsted acid sites (Ga-O(H)-Si), 15 N-py/GaBA). (c) Czjzek analysis of 71 Ga NMR spectra of (i) of Ga@SiO2 and (ii) Npy/Ga@SiO2 (14.1 and 21.15 T). The Czjzek spectra for a range of possible Czjzek parameters 𝜎 𝐶 and resulting second moments of the quadrupolar product √〈𝐶 𝑄 2 〉 are plotted as colored lines on top of the experimental data. For Ga@SiO2 best agreement of Czjzek and experimental lineshapes is obtained using √〈𝐶 𝑄 2 〉 ≈ 52 MHz the significantly narrower spectra of 15 N-py/Ga@SiO2 are best reproduced using √〈𝐶 𝑄 2 〉 ≈ 12 MHz.