The pK_BHX Hydrogen-Bond Basicity Scale: From Molecules to Anions

Hydrogen-bond basicity scales have been used to quantify the ability of molecules to act as acceptors in hydrogen bonding. These scales are well established for neutral species but remain limited for anionic hydrogen-bond acceptors. Prior research has shown that neutral HBAs can be ranked using pK_BHX values derived from complexation with 4-fluorophenol. However, no comprehensive scale exists for anions, which are important in many chemical processes. This gap motivated the development of a new scale that includes anionic HBAs. The current literature lacks a unified approach to measure and predict anion hydrogen-bond basicity. Existing methods focus on neutral systems or use solvents that do not reflect nonpolar environments. No prior work had resolved how substituent effects influence anion basicity in such conditions. This study addresses these limitations by extending the pK_BHX scale to anions using a combination of experimental and computational methods.

The study aims to expand the pK_BHX hydrogen-bond basicity scale to include anionic hydrogen-bond acceptors. This expansion is necessary to better understand how anions interact in nonpolar environments. The specific problem addressed is the lack of a standardized scale for anions, which limits the ability to predict hydrogen-bonding behavior in diverse chemical systems. The motivation comes from the need to unify basicity measurements across neutral and ionic species. By extending the scale to anions, the researchers hope to provide a more complete framework for hydrogen-bonding analysis. The study also seeks to determine whether substituent effects observed in neutral systems apply to anions. This work builds on prior research that established the pK_BHX scale for neutral HBAs. The ultimate goal is to enable more accurate predictions of hydrogen-bonding interactions in complex chemical environments.

The researchers extended the pK_BHX scale to anions using three complementary approaches. First, they measured complexation constants (K) via infrared spectroscopy of NBu₄⁺X⁻ ion pairs in CCl₄. Second, they estimated K values using linear free energy relationships between measured and literature-reported K values for various phenols in polar solvents. Third, they computed K values using density functional theory in CCl₄. These methods allowed the team to construct a pK_BHX scale for 26 anions. The study focused on anions with unipolar substituents such as O⁻ and S⁻. The researchers compared their results to Hammett-Taft σ constants to assess substituent effects. By combining experimental and computational data, the team ensured the scale's accuracy and applicability. This multi-method approach enabled a robust extension of the pK_BHX scale to anionic systems.

The pK_BHX scale was successfully extended to 26 anionic hydrogen-bond acceptors. The scale spans 9.4 pK_BHX units, from fluoride to tetraphenylborate. The researchers measured K values using infrared spectroscopy in CCl₄ and confirmed these with computational methods. Linear free energy relationships between measured and literature K values showed strong correlations. Anions with unipolar substituents followed the same pK_BHX versus σ relationships as dipolar ones. This suggests that field-inductive and resonance effects govern substituent influence in anions. Substituents at carbon and nitrogen showed both effects, while those at sulfur and phosphorus showed only field-inductive effects. These findings align with known substituent behavior in neutral systems.

The pK_BHX scale has been extended to anionic hydrogen-bond acceptors using a combination of experimental and computational methods. The scale spans 9.4 pK_BHX units, covering a wide range of anions. The researchers found that unipolar substituents in anions follow the same pK_BHX versus σ relationships as dipolar ones. This suggests that field-inductive and resonance effects are key in determining anion basicity. Substituents at carbon and nitrogen show both effects, while those at sulfur and phosphorus show only field-inductive effects. These findings align with prior observations in neutral systems. The study confirms that the pK_BHX scale can be applied to anions with confidence. The results support the use of Hammett-Taft σ constants to predict anion basicity in nonpolar environments.