Coexistence of Neutral and Ion-Pair Clusters of Hydrated Sulfuric Acid H₂SO₄(H₂O)<i>_n</i> (<i>n</i> = 1−5)A Molecular Orbital Study

Abstract

Various isomeric structures of the hydrated clusters of sulfuric acid, H2SO4(H2O)n (n = 1−5), are examined using a density functional molecular orbital method. Due to the small energy difference between trans and cis conformations about two OH groups of sulfuric acid, there are three types of isomeric forms of the hydrated clusters of sulfuric acid which involve the proton nontransferred trans conformer, the proton transferred trans conformer, and the proton nontransferred cis conformer of sulfuric acid. In the case of transoid H2SO4, the proton transferred ion-pair structures become more stable than the proton nontransferred structures as the number of water molecules increases. The hydrated clusters of the cis conformation remain neutral hydrogen-bonded structures even if the number of water molecules increases. All stable clusters tend to form multi-cyclic structures. While both protons of sulfuric acid participate in cyclic hydrogen bonding in the neutral structures, the OH group of HSO4- in the ion-pair structures remains dangling because the counterion H3O+ prefers to make strong hydrogen bonds with water molecules and/or the HSO4- moiety. The energy difference between the most stable structures of proton transferred and proton nontransferred isomers is found to be less than 1 kcal/mol in the case of n = 3 and 4 clusters. The ion-pair structure of HSO4-H3O+(H2O)4 becomes 2 kcal/mol more stable than the hydrogen-bonded neutral cluster H2SO4(H2O)5 in the case of n = 5. Analyzing the interaction energies, many-body interaction is shown to be essential to describe the stability between neutral and ionic clusters owing to the difference of charge flow on the neutral and ion-pair structures in multi-cyclic hydrogen bonding. The calculated IR spectra of stable isomers of H2SO4(H2O)n clusters clearly demonstrate the significant red-shift of OH stretching of sulfuric acid and hydrogen-bonded OH stretching of water molecules as the number of cluster size increases. The IR spectra of the OH stretching of hydrated sulfuric acid are predicted to appear in three regions, hydrogen-bonded OH stretching of H3O+ (2500∼2800 cm-1), hydrogen-bonded OH stretching of water molecules (3100∼3500 cm-1), and nonhydrogen-bonded OH stretching of water molecules (3800∼3900 cm-1).