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.