How Much Water Is Needed To Ionize Formic Acid?

Abstract

Structure, molecular properties, energy parameters, and vibrational IR spectra of hydrated clusters of formic acid (FOH), FOH·nH2O (n = 1-8), are presented following first principle based electronic structure theory. Several geometrical arrangements are considered as initial guess structures to look for the minimum energy equilibrium structures applying ωB97X-D density functional and aug-cc-pVDZ set of atomic basis functions. Results on FOH-water clusters suggest that the most stable structure of formic acid exists as a charge-separated ion pair, FO(-δ)---H(+δ)---O(-δ)H2, in hydrated clusters, FOH·nH2O, for cluster size n ≥ 7. The calculated interaction energy between FOH and the solvent water cluster increases significantly by adding the seventh solvent water molecule to the FOH·6H2O cluster whereas the solvent stabilization energy of FOH increases continuously on successive addition of solvent water molecules (n = 1-8). Energy partitioning of the solvent stabilization energy of FOH·nH2O clusters suggests an electrostatic component of energy to play the major role in solvent stability and does depict sudden variation for n = 7. Formation of a charge-separated ion pair for cluster size n ≥ 7 is manifested in simulated IR spectra of FOH·nH2O clusters. Static polarizabilities of hydrated formic acid clusters are calculated and observed to vary linearly with the size of the clusters, suggesting reliability in prediction of polarizability for larger size hydrated clusters of formic acid.