Cl dipolar couplings, respectively. Therefore, Si-Si interlayer spacings within the ensemble was set to 5 Å, 6 Å, or 7 Å. Comparison of the numerically simulated 1 H{ Cl} DE-RESPDOR dephasing curves with the experimental dephasing curve reveals an interlayer spacing of ca. 6 Å gives best agreement with the experiments. This result is in excellent agreement with the spacing we determined in our previous synchrotron X-ray diffraction study. Lastly, we used plane-wave DFT calculations to construct geometry optimized models of Si-NS containing 0, 20, 50 or 100 % Cl termination (Figure ). For simplicity, hydroxyl groups were not considered in these calculations. The calculations predict that the Si-Si interlayer distance increases with increasing chlorination to accommodate the longer Si-Cl bond length. The average interlayer spacing of 6.1 Å for the 20 % Cl terminated model is in excellent agreement with that determined from the 1 H{ 35 Cl} DE-RESPDOR dephasing curve. Interestingly, the 20 and 50 % Cl models exhibit slightly distorted sheets as compared to the 0 or 100 % Cl terminated models (Figure ). GIPAW calculated chemical shifts predict a large distribution in the Si chemical shifts for models with intermedia chlorine content, in good agreement with the experimental Si NMR spectrum (Figure ). We also calculated the electronic band structures and densities of states (DOS) for all Si-NS models (Figure , see Supporting Information for additional discussions). Fully hydrogenated Si-NS (0 % Cl) exhibits a direct transition of 2.1 eV at the zone-center G and a slightly lower band gap of 2.05 eV between A-G (Figure and). 20 % chlorination slightly narrows the band gap (2.04 eV, Figure and). Interestingly, 50 % chlorination is predicted to significantly decrease the direct transition by ca. 0.32 eV (band gap = 1.73 eV; Figure and). Fully chlorinated Si-NS exhibited the smallest band gap of 1.17 eV at A Figure and S8). The presence of a tunable direct transition via the chlorination of Si-NS should provide new pathways for the synthesis of 2D silicon semiconductors for many material science applications. The controlled chlorination of Si-NS is likely feasible via the reaction of hydrogenated Si-NS with PCl5 or Cl2 as these reagents have been utilized for the chlorination of H-terminated Si(111) surfaces. In conclusion, Cl} RESPDOR dephasing curve could be modelled and revealed that the Si-Si interlayer spacing is ca. 6 Å. Lastly, plane-wave DFT predicted electronic band structures suggest that increasing chlorination leads to the emergence of a direct transition that decreases with increasing chlorination. The presence of a tunable direct transition should enable the tailoring of 2D silicon semiconductors for many material science applications. Our results also show that 1 H{X} RESPDOR is a general method to rapidly map out CT quadrupolar powder patterns for I = 3/2 nuclei and that X{ Cl} RESPDOR should be applicable to detect the presence of Cl substituents in many materials.