Experimental <sup>13</sup>C and <sup>1</sup>H Solid-State NMR Response in Weakly Tetrel-Bonded Methyl Groups
Scott A. Southern, Michael S. West, Maressa J. Z. Bradshaw, David L. Bryce
Abstract
Like halogens, pnictogens, and chalcogens, group 14 tetrel atoms have the ability to exhibit areas of elevated electrostatic potential on their surfaces (σ-holes) which can engage in noncovalent interactions with electron donors. Evidence for tetrel bonds involving spin-1/2 nuclei, such as 13C, may be found by careful observation of the isotropic chemical shift in solid-state NMR experiments. Here, we study the 13C and 1H solid-state NMR spectroscopic responses to weak tetrel bond formation by examining a series of eight caffeine and theophylline cocrystals along with similar compounds not featuring tetrel bonds. Overall, we observe a moderate increase in the methyl 13C chemical shift on the order of ppm in cocrystals featuring tetrel bonds as compared to their nonbonded counterparts. The value of δ(13C) shows a weak inverse correlation with the tetrel bond length, while no strong correlation with the tetrel bond angle is observed experimentally. Methyl proton chemical shifts are also influenced by the presence of a tetrel bond, but no strong correlations with structural features are noted based on the experimental data. With the aid of DFT calculations, we explore the relationship between the tetrel bond and the 13C chemical shift tensor, furthering our understanding of how tetrel bonds and potentially competing weak CH···O hydrogen bonds affect the NMR response, which is of importance in NMR crystallography applications. While computations generally show clear correlations between chemical shifts and structural features, this experimental work demonstrates that other interactions and crystal packing effects can weaken and even obscure the expected correlations.