Ability of IR and NMR Spectral Data to Distinguish between a Tetrel Bond and a Hydrogen Bond.

The placement of a nucleophile X along the R-CH3 axis of a methyl group can be described as either a trifurcated H bond or as a tetrel bond between C and X. Quantum calculations of the vibrational and NMR spectral features of a number of systems are used to provide a framework by which to distinguish between the presence of a tetrel or H bond. Both cationic and neutral methyl-containing Lewis acids (SMe3+, NMe4+, SMe2) are paired with both neutral and anionic Lewis bases (NH3, OH2, OCHNH2, OCH3-, OH-, HCOO-). As the base is moved away from the R-CH3 axis of the Lewis acid (tetrel bond) toward a position along a CH axis (CH···X H bond), the methyl stretching frequencies shift to the red, and the bending modes shift to the blue. This modification in the geometry also causes the methyl C atom NMR chemical shielding to increase, while the methyl H atom is progressively deshielded. These changes are of sufficiently large magnitude that they should provide a means to distinguish these two bonding situations from one another.