Prediction of the p

• Published in: The Journal of Physical Chemistry A
• Main Authors: Loudermilk, Amanda, Dixon, David A
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Chemical Society 2024
• Subjects: Carbonic acid
• Online Access: https://doi.org/10.1021/acs.jpca.4c02879; https://pubmed.ncbi.nlm.nih.gov/38950028

The gas- and aqueous-phase acidities of hydrated metal dication carbonates, bicarbonates, and hydroxide complexes M(CO3)(H2O)n for n = 1 to 3, M(HCO3)2, M(HCO3)2(H2O)2, M(HCO3)(OH), and M(HCO3)(H2O)2(OH) for M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn were calculated at the CCSD(T)/aug-cc-pwCVDZ/cc-pwCVDZ level in the gas phase and at the B3LYP/aug-cc-pVTZ/cc-pVTZ(-PP) level with the COSMO self-consistent reaction field (SCRF) method in the aqueous phase. The composite correlated molecular orbital theory G3(MP2) and G3(MP2)B3 methods were used to predict the pKa's of the Mg structures and cis-cis carbonic acid to provide additional benchmarks. Using values scaled to experiment for H2CO3, the pKa's of bicarbonate ligands in group 2 and transition-metal complexes were compared to carbonic acid to gauge the effect of the metal complex on the bicarbonate. The group 2 metal complexes M(HCO3)2 and M(HCO3)(OH) decreased the acidity of the bicarbonate ligands, whereas their dihydrates were even less acidic. The transition-metal di-bicarbonate and bicarbonate hydroxide complexes generally made the bicarbonate more acidic especially when reduction of the metal occurs consistent with electron donation from the ligands; this is accompanied by spin transfer which typically increases in the order Mn < Fe < Co < Ni < Cu. The transition-metal dihydrates were less acidic than carbonic acid. Using values scaled to experiment for hydrated metal dications, the pKa's of water coordinated to group 2 and transition-metal complexes were generally more acidic than the hydrated metal dications, with the exception of Ca bicarbonate dihydrate, Co carbonate, Ni di-bicarbonate dihydrate, and Cu bicarbonate hydroxide di-bicarbonate.