Si-NS were synthesized via our previously reported procedure. In summary, CaSi2 was prepared by melting Si and Ca (1:2 molar ratio of Ca to Si) in a sealed Ta tube inside an Ar-filled glovebox for ca. 1 minute. While the contents were still molten, the Ta tube was carefully inverted for a few minutes until the material started to solidify. This procedure of heating and cooling was repeated four times. On the final step, the Ta tube was cooled with Al chips and washed with first HCl and then DI water. The material was then dried and transferred back into the glovebox. The synthesized CaSi2 was then ground in an N2-filled glovebox. Approximately 3 g of the ground CaSi2 was placed in a vial and cooled to - ( 1 H- Cl experiments) 14.1 T Bruker wide-bore magnet equipped with a Bruker AVANCE NEO console and a Phoenix 1.3 mm HXY MAS NMR probe. All experiments utilized N2 gas for spinning. H chemical shifts were referenced to 1 % tetramethylsilane (TMS) in CDCl3 with adamantane (diso( 1 H) = 1.71 ppm) as a secondary chemical shift reference. Si chemical shifts were indirectly referenced to neat TMS using the IUPAC recommended relative NMR frequency. NMR spectra were processed in Bruker Topspin (AVANCE III HD data) 3.6.1. or (AVANCE NEO data) 4.0.7. All experimental NMR parameters are given in Table .