Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X^- •••H₃ C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl^- •••H₃ N and Cl^- •••H₃ C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H₃ C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X^- •••C interaction of the HOMO of A^- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H₃ C-Y fragment. This X^- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H₃ C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.