Mass spectrometric and quantum chemical determination of proton water clustering equilibria

Solvation processes in steam: Ab initio calculations of ion–solvent structures and clustering equilibria

“…The situation for the hexamer H 3 O + Á(H 2 S) 5 (a5) is quite different to that of the H 3 O + Á(H 2 O) 5 (b5) cluster: The low energy structure H 3 O + Á(H 2 S) 5 maintains a core Eigen-type moiety with two additionally loosely bound H 2 S molecules (a5). The G3 and CBS-Q values we obtain for the enthalpy and entropy changes at the 4,5 hydration step in the H 3 O + Á(H 2 O) m system are À45.9 kJ mol À1 and À95.0 J K À1 mol À1 , and À47.4 kJ mol À1 and À92.1 J K À1 mol À1 , respectively, which are in fair agreement with reported DH o 4;5 and DS o 4;5 values of À53.2 ± 1.7 kJ mol À1 and À123.1± 5.0 J K À1 mol À1 , respectively (Likholyot et al, 2007). Unfortunately, experimental clustering equilibria for the H 3 O + Á(H 2 S) n series do not exceed n = 2 (Hiraoka and Kebarle, 1977) and uncertainties exist with respect to identification of H 3 O + Á(H 2 S) given the rapid and exoergic nature (À13.8 kJ mol À1 ) of the proton transfer reaction H 3 O + Á(H 2 S) = H 3 S + Á(H 2 O) (Williams et al, 1998).…”

“…However, we are not aware of any experimental or theoretical studies that have examined hydronium H 2 S clustering or mixed H 2 S and H 2 O clustering equilibria and the potential role of Eigenor Zundel motifs during cluster growth. S) n systems are À75.9 kJ mol À1 and À52.0 kJ mol À1 , respectively, of which the former is in good agreement with HPMS value of À76.6 ± 1.7 kJ mol À1 of Likholyot et al (2007). The principal low energy structures determined at the G3 and CBS-Q theory level for the H 3 O + Á(H 2 O) 4 and H 3 O + Á(H 2 S) 4 clusters differ substantially (Fig.…”

“…Prompted by these findings and our recent HPMS and quantum chemical study on hydronium water clusters (Likholyot et al, 2007), we have, in the present study, computed the clustering equilibria of the neat and mixed-proton clusters H 3 O + Á(H 2 S) m Á(H 2 O) n , NH 4 þ ÁðH 2 SÞ m ÁðH 2 OÞ n and H 3 S + Á(H 2 S) m Á(H 2 O) n in the gas phase. The principle goal of this study is to compute stepwise hydration enthalpies and entropies of the above clusters, to determine energetic trends for solvent exchange and to compute the temperature dependence of the clustering equilibria under conditions pertinent to natural hydrothermal/fumarolic environments.…”

“…Our current molecular-scale understanding of the interaction of ions, such as, H 3 O + , with solvent molecules in steam and/or low-density supercritical water and in magmatic/ volcanic gases is rudimentary. However, quantum chemical studies can provide fundamental insight into the structures and corresponding energies of such clusters, for instance, H 3 O + Á(H 2 O) m , and play an important role in our understanding of solute-solvent interactions at high-temperatures (Jiang et al, 2000;Boero et al, 2001;Walrafen et al, 2002;Shin et al, 2004;Boero et al, 2005;Headrick et al, 2005;Slanina et al, 2006;Likholyot et al, 2007). From these and other theoretical studies, we now know that, (1) such proton-water clusters structurally rearrange upon hydration, i.e.…”

“…We also consider the competitive exchange of H 2 O and H 2 S A þ ðH 2 OÞ m ðH 2 SÞ n þ ðH 2 OÞ $ A þ ðH 2 OÞ mþ1 ðH 2 SÞ nÀ1 þ H 2 S ð2Þ around the central ion as well as the effect of both temperature and water vapor pressure on cluster abundance. Interestingly, cluster geometries and energetic trends for high-temperature proton water clustering reactions are well known from both experimental (Magnera et al, 1991;Dalleska et al, 1993;Likholyot et al, 2007) and ab initio (Kochanski et al, 1997;Chang et al, 2005) studies. However, limited information is available regarding the energetics of competitive solvent exchange, ligand switching as well as to the level of stability of proton-water clusters in high-temperature low-density steam.…”