Protonation Dynamics and Hydrogen Bonding in Aqueous Sulfuric Acid

Abstract

Hydration of sulfuric acid plays a key role in new-particle formation in the atmosphere. It has been recently proposed that proton dynamics is crucial in the stabilization of these clusters. One key question is how water molecules mediate proton transfer from sulfuric acid, and hence how the deprotonation state of the acid molecule behaves as a function concentration. We address the proton transfer in aqueous sulfuric acid with O K edge and S L edge core-excitation spectra recorded using inelastic X-ray scattering and with ab initio molecular dynamics simulations in the concentration range of 0-18.0 M. Throughout this range, we quantify the acid-water interaction with atomic resolution. Our simulations show that the number of donated hydrogen bonds per Owater increases from 1.9 to 2.5 when concentration increases from 0 to 18.0 M, in agreement with a rapid disappearance of the pre-edge feature in the O K edge spectrum. The simulations also suggest that for 1.5 M sulfuric acid SO4(2-) is most abundant and that its concentration falls monotonously with increasing concentration. Moreover, the fraction of HSO4(-) peaks at ∼12 M.

Sulphur Kβ emission spectra reveal protonation states of aqueous sulfuric acid

“…Figure 4(b) presents the coefficients of the different spectra with the values from AIMD simulations, and for the higher concentrations the agreement is found to be good. The most likely reason for the mismatch at lower concentration is the quality of the structural AIMD simulation, as we observe large discrepancies between different computational works 14 30 , both of which disagree with the experimental value 39 . We conclude that the lowest concentrations are difficult to simulate due to most likely long reaction times for the span of AIMD.…”

“…Secondly, as a more precise but also more challenging approach, we simulated ensemble-averaged S K β emission spectra by sampling the trajectories from previously published AIMD simulations 30 . The protonation distribution along the AIMD runs is presented in Table 1 .…”

“…According to the international panel for climate change report, IPCC 2013 3 , the net effect of aerosols is cooling, which compensates for the greenhouse effect caused by rising levels of antropogenic CO 2 in the atmosphere. The bulk and cluster systems of aqueous sulfuric acid have attracted considerable theoretical (for example, refs 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ) and experimental (for example, refs 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 ) interest. One frequently appearing question in these works considers the dynamics of the protons, which may be very relevant from the point of view of atmospheric new particle formation 40 41 .…”

“…Molecular liquids are an interesting class of samples for core-excitation and emission studies and, for example, X-ray photoelectron spectroscopic work on sulfuric acid was recently reported 39 in addition to works on sulphate compounds in the condensed phase other than H 2 SO 4 44 45 . We have also recently studied aqueous H 2 SO 4 using first-principles simulations and core-level excitations by inelastic X-ray scattering 30 .…”

α‐ and β‐Lapachone Isomerization in Acidic Media: Insights from Experimental and Implicit/Explicit Solvation Approaches

“…Previous studies showed that in highly concentrated solutions of sulfuric acid (between 75 and 100 %), high contents of undissociated H 2 SO 4 and HSO 4 À are observed (over 73 % of undissociated H 2 SO 4 ). [28] Furthermore, investigations of the proton transfer mechanism in sulfuric acid solutions using (H 2 SO 4 ) n ⋯(H 2 O) m clusters have shown that, at low water concentrations, the assistance of non-deprotonated H 2 SO 4 is mandatory for the stabilization of the conjugate base HSO 4 À and allows deprotonation to occur. [29] According to this hypothesis, the proton would be transferred via a H 2 SO 4 ⋯H 2 SO 4 ⋯B cluster, in which B is any base or solvent [27] molecule present in the mixture capable of accepting the proton.…”