Experimental ¹³C and ¹H Solid-State NMR Response in Weakly Tetrel-Bonded Methyl Groups

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.

Characterization of Type I and II Interactions between Halogen Atoms

“…Unlike most of the other properties, this coupling constant is considerably larger for T1 than for T2. It should be pointed out finally that these various spectral changes are not unique to the FX dimers examined here but are general features of halogen bonds in general and even occur in complexes containing closely related interactions such as chalcogen and pnicogen and even hydrogen bonds. ,− …”

Noncovalent interactions in proteins and nucleic acids: beyond hydrogen bonding and π-stacking

“…26,47,48 Besides, a change in chemical shift observed from NMR experiments and vibrational stretching frequency obtained from IR and Raman experiments are used as spectroscopic rulers for NCIs. [49][50][51][52][53][54][55][56] The variable temperature NMR/IR and concentration-dependent titrations are significantly helpful in the solution phase to extract thermodynamic parameters such as the enthalpy and entropy of dimerization and complexation. 49,57,58 In addition to the techniques mentioned above, isothermal calorimetric titration (ITC) is also employed to determine the equilibrium constant and other thermodynamic parameters such as Gibbs free energy, entropy and enthalpy.…”

“…They showed that the oxygen of THF interacts with the dicyano-substituted carbon atom of the cyclopropane ring via C-bond formation. Recently, Southern et al 52 performed solid-state 13 C-NMR experiments to establish the C-bond in cocrystals based on caffeine and theophylline. Fig.…”

Influence of Substituents in the Benzene Ring on the Halogen Bond of Iodobenzene with Ammonia

“…For example, the signal of the bridging proton moves downfield by several ppm upon formation of a H-bond, and the degree of this shift is closely related to the strength of the bond. There have been several studies in recent years [43][44][45][65][66][67][68][69][70][71] that suggest that these ideas can be extended to halogen and other related noncovalent bonds.…”