Coexistence of neutral and ion-pair clusters of hydrated sulfuric acid H2SO4(H2O)n (n=1-5) - A molecular orbital study

Various isomeric structures of the hydrated clusters of sulfuric acid, H2SO 4(H2O)(n) (n = 1-5), are examined using a density functional molecular orbi tal method. Due to the small energy difference between trans and cis confor mations about two OH groups of sulfuric acid, there are three types of isom eric forms of the hydrated clusters of sulfuric acid which involve the prot on nontransferred trans conformer, the proton transferred trans conformer, and the proton nontransferred cis conformer of sulfuric acid. In the case o f transoid H2SO4, the proton transferred ion-pair structures become more st able than the proton nontransferred structures as the number of water molec ules increases. The hydrated clusters of the cis conformation remain neutra l hydrogen-bonded structures even if the number of water molecules increase s. All stable clusters tend to form multicyclic structures. While both prot ons of sulfuric acid participate in cyclic hydrogen bonding in the neutral structures, the OH group of HSO4- in the ion-pair structures remains dangli ng because the counterion H3O+ prefers to make strong hydrogen bonds with w ater molecules and/or the HSO4- moiety. The energy difference between the m ost stable structures of proton transferred and proton nontransferred isome rs 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 owi ng to the difference of charge flow on the neutral and ion-pair structures in multi-cyclic hydrogen bonding. The calculated IR spectra of stable isome rs of H2SO4(H2O)(n) clusters clearly demonstrate the significant red-shift of OH stretching of sulfuric acid and hydrogen-bonded OH stretching of wate r 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 re gions, hydrogen-bonded OH stretching of H3O+ (2500 similar to 2800 cm(-1)), hydrogen-bonded OH stretching of water molecules (3100 similar to 3500 cm( -1)), and nonhydrogen-bonded cm OH stretching of water molecules (3800 simi lar to 3900 cm(-1)).