Discussion of the 1 H{ Cl} RESPDOR Curve Fitting Procedure. As mentioned in the main-text, 20 Si-NS models were constructed that consist of two layers. Two Si-NS layers were required to accurately simulate the experimental RESPDOR dephasing curve because inter-sheet H-Cl internuclear distances are short when the Cl atom is near the H atom of interest (closest H-Cl inter-sheet distances are ca. 2.3, 2.8 or 3.4 Å when the Si-Si interlayer spacing is 5, 6 or 7 Å, respectively). Each Si-NS model contains 80 Si atoms, corresponding to sheet dimensions of ca. 13.5 Å × 13.5 Å. The position of Cl atoms that terminate Si atoms were randomized via a Monte Carlo procedure based on the population of SiSi3Cl determined expeirmentally (20 %) and the Cl natural isotopic abundance of 75 %. Practically, this was done by building a fully hydrogenated Si-NS model and assigning each H atom a random number that was between 0 and 1. If the random number was ≤ 0.15 (i.e., 20 % × 75 %), then the H atoms was replaced by a Cl atom, and if the random number was > 0.15, it remained an H atom. For all H atoms replaced by a Cl atom, the Si-H bond length was increased from 1.5 Å to 2.1 Å to accurately represent a Si-Cl bond. The population of SiSi3Cl varied between ca. 8 -28 % across the 20 models, with an average population of 19.5 % (Table ). For each chlorinated Si-NS models, the inter-sheet layer spacing was set to either 5, 6 or 7 Å by changing the Z-coordinate for each atom in a single layer. The XYZ coordinates of all 20 models with a 6 Å Si-Si interlay spacing is available in the Supporting Information.