Distinguishing between COOH, COO−, and hydrogen disordered COOH sites in solids with 13C chemical shift anisotropy and T1 measurements

Abstract

Since 1993, it has been known that ¹³C chemical shift tensor (i.e., δ₁₁, δ₂₂, and δ₃₃) provides information sufficient to distinguish between COOH and COO⁻ sites. Herein, four previously unreported metrics are proposed for differentiating COOH/COO⁻ moieties. A new relationship is also introduced that correlates the asymmetry (i.e., δ₁₁–δ₂₂) of COOH sites to the proximity of hydrogen bond donating partners within 2.6 Å with high accuracy (±0.05 Å). Conversely, a limitation to all proposed metrics is that they fail to distinguish between COO⁻ and hydrogen disordered COOH sites. To reconcile this omission, a new approach is proposed based on T₁ measurements of both ¹H and ¹³C. The ¹³C T₁ values are particularly sensitive with the T₁ for hydrogen disordered COOH moieties found to be nearly six times smaller than T₁'s from COO⁻ sites.