title: Modeling the antisymmetric and symmetric stretching vibrational modes of aqueous carboxylate anions.
abstract: The infrared spectra of six aqueous carboxylate anions have been calculated at the M05-2X/cc-pVTZ level of theory with the SMD solvent model, and validated against experimental data from the literature over the region of 1700 cm(-1) to 1250 cm(-1); this region corresponds to the stretching modes of the carboxylate group, and is often interrogated when probing bonding of carboxylates to other species and surfaces. The anions studied here were formate, acetate, oxalate, succinate, glutarate and citrate. For the lowest energy conformer of each anion, the carboxylate moiety antisymmetric stretching peak was predicted with a mean signed error of only 4 cm(-1) using the SMD solvent model, while the symmetric peak was slightly overestimated. Performing calculations in vacuum and scaling was found to generally over-predict the antisymmetric vibrational frequencies and under predict the symmetric peak. Different conformers of the same anion were found to have only slightly different spectra in the studied region and the inclusion of explicit water molecules was not found to significantly change the calculated spectra when the implicit solvent model is used. Overall, the use of density functional theory in conjunction with an implicit solvent model was found to result in infra-red spectra that are the best reproduction of the features found experimentally for the aqueous carboxylate ions in the important 1700 cm(-1) to 1250 cm(-1) region. The development of validated model chemistries for simulating the stretching modes of aqueous carboxylate ions will be valuable for future studies that investigate how carboxylate anions complex with multivalent metal cations and related species in solution.
authors.affiliation: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia.
authors.forename: Catherine C R
authors.initials: CC
authors.lastname: Sutton
authors.affiliation: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia.
authors.forename: George V
authors.initials: GV
authors.lastname: Franks
authors.affiliation: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia. Electronic address: gdasilva@unimelb.edu.au.
authors.forename: Gabriel
authors.initials: G
authors.lastname: da Silva
mesh_terms: D000838:Anions / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D008958:Models, Molecular; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry
chemical_list: D000838:Anions; D002264:Carboxylic Acids; D014867:Water
keywords: Aqueous carboxylate anion; Continuum solvation model; DFT; IR spectra; Vibrational frequencies