Sodium Hexamethyldisilazide: Using <sup>15</sup>N–<sup>29</sup>Si Scalar Coupling to Determine Aggregation and Solvation States
Ryan A. Woltornist, David B. Collum
Abstract
29Si NMR spectroscopy, the method of continuous variations, and density functional theory computations show that sodium hexamethyldisilazide (NaHMDS) is a disolvated dimer in toluene, a mixture of disolvated dimer and tetrasolvated monomer in THF/toluene, and exclusively monomer in neat THF. The dioxane-solvated NaHMDS only partially deaggregates to monomer even in neat dioxane. 15N–29Si coupling constants and 29Si chemical shifts show a high and dependable correlation with the aggregation state. Monitoring either chemical shift or coupling constant versus THF concentration even in the high-temperature, rapid-exchange limit affords the solvation numbers consistent with DFT computations. The preparation of 15N-labeled NaHMDS has been improved.