Si-NS were synthesized via the topotactic deintercalation of CaSi2 in aqueous HCl at ca. -30 °C; CaSi2 was prepared via the melting of elemental Ca and Si. Si solid-state NMR spectra of Si-NS were recorded with either direct excitation or 1 H→ 29 Si cross-polarization (CP) and CPMG for detection (Figure ). The 1 H→ 29 Si CP-CPMG NMR spectrum recorded with a long CP contact time (tCP = 8 ms) reveals three broad Si NMR signals centered at ca. 10, -5 and -100 ppm. Only the Si NMR signal at -100 ppm was observed with the use of a short CP contact time (tCP = 0.5 ms), confirming the assignment of the -100 ppm NMR signal to H terminated Si atoms (SiSi3H). As discussed below, the Si NMR signals at 10 and -5 ppm likely correspond to Si atoms terminated with OH groups and Cl atoms, respectively. To experimentally confirm the presence of SiSi3OH, we recorded a 2D 1 H→ 29 Si Lee-Goldberg CP (LG-CP) heteronuclear correlation (HETCOR) NMR spectrum (Figure ). LG-CP suppresses 1 H-1 H spin-diffusion during the CP step and yields a spectrum primarily free of relayed correlations. The 2D 1 H→ 29 Si LG-CP HETCOR spectrum reveals two 1 H NMR signals at 1. Si NMR signals. However, the 10 ppm Si NMR signal exhibits increased relative signal intensity as compared to the CP-CPMG NMR spectrum, suggesting this Si NMR signal corresponds to Si terminated with an OH group. The 1 H shift of 1.4 ppm is also typical for non-hydrogen bonded silanols, further corroborating our assignment. Next, we performed 29 Si{ 35 Cl} phase-modulated RESPDOR (PM-RESPDOR) NMR experiments to probe Cl terminated Si atoms. In a Si{ 35 Cl} PM-RESPDOR experiment, the Si NMR signal will decrease in intensity due to the application of a Cl saturation pulse if the Si spin is in close spatial proximity (dipolar coupled) to Cl. The rate of signal dephasing is dependent on the inverse cube of the Si-Cl internuclear distance. The 29 Si{ 35 Cl} PM-RESPDOR dipolar dephasing curve shows that both Si NMR signals at ca. -5 and -100 ppm exhibit significant signal dephasing, directly confirming that Cl is incorporated within the sheets (Figure ). However, the rate of signal dephasing is much faster for the -5 ppm 29 Si NMR signal. Numerical simulation of the 29 Si{ 35 Cl} RESPDOR experiment suggests that the Si-Cl internuclear distance is 2.1 Å, confirming that the -5 ppm Si NMR signal corresponds to Cl terminated Si atoms (SiSi3Cl, Figure ). We note that To further confirm Cl termination of Si-NS and alleviate the challenges associated with the 29 Si{ 35 Cl} PM-RESPDOR experiment, we turn to fast magic angle spinning (MAS) 1 H{ Cl} NMR spectroscopy. The indirect detection of Cl through 1 H is beneficial because 1 H exhibits a significantly higher gyromagnetic ratio than Si and 1 H is ca. 100 % abundant. In addition, the use of small diameter RF coils (1.3 mm probe) and a double resonance probe configuration enables access to higher Cl RF fields of ca. 85 kHz that can provide significant signal dephasing in RESPDOR experiments.