Concentration-Dependent Solvation Structure and Dynamics of Aqueous Sulfuric Acid Using Multinuclear NMR and DFT

Abstract

Sulfuric acid is a ubiquitous compound for industrial processes, and aqueous sulfate solutions also play a critical role as electrolytes for many prominent battery chemistries. While the thermodynamic literature on it is quite well-developed, comprehensive studies of the solvation structure, particularly molecular-scale dynamical and transport properties, are less available. This study applies a multinuclear nuclear magnetic resonance (NMR) approach to the elucidation of the solvation structure and dynamics over wide temperature (−10 to 50 °C) and concentration (0–18 M) ranges, combining the 17O shift, line width, and T 1 relaxation measurements, 33S shift and line width measurements, and 1H pulsed-field gradient NMR measurements of proton self-diffusivity. In conjunction, these results indicate a crossover between two regimes of solvation structure and dynamics, occurring above the concentration associated with the deep eutectic point (∼4.5 M), with the high-concentration regime dominated by a strong water–sulfate correlation. This description was borne out in detail by the activation energy trends with increasing concentration derived from the relaxation of both the H2O/H3O+ and H2SO4/HSO4 –/SO4 2– 17O resonances and the 1H self-diffusivity. However, the 17O chemical shift difference between the H2O/H3O+ and H2SO4/HSO4 –/SO4 2– resonances across the entire temperature range is nevertheless strikingly linear. A computational approach coupling molecular dynamics simulations and density functional theory NMR shift calculations to reproduce this trend is presented, which will be the subject of further development. This combination of multinuclear, dynamical NMR, and computational methods, and the results furnished by this study, will provide a platform for future studies on battery electrolytes where aqueous sulfate chemistry plays a central role in the solution structure.

17O NMR and DFT study of hydrogen bonding: Proton sharing and incipient transfer

“…The steeper slope can be explained to be due to the strong substituent effect that acts beside the H-bonding and PT. The overlap of both effects was deduced for 4-substituded PyOs and differentiated in the 13 C NMR spectra [19]. However, the physical origin of significant discrepancies between the calculated and measured δ (N-O) values for the PANO group is not clear.…”

“…1. The sequence of subjects was approximately ordered increasing the available values of acid dissociation constants (pKa) as well as the FTIR frequencies of PyO ring modes and the 13 C chemical shifts of ring carbons [24][25][26]. It is clearly seen that the series splits into two parts -the neutral B…H-A bonds with r(O-H)/R(O…O) < 0.5 and the ionic pairs BH + …Awith r(O-H)/R(O…O) > 0.5 that result from the proton transfer (PT).…”

“…This approach was proven to be adequate in various cases earlier. For example, a fairly good agreement between the calculated and experimentally measured 1 H and 13 C NMR chemical shifts was obtained for the molecular systems involved in a strong H-bonding, but also for rather 'inert' species, e.g. -CH 3 protons [24-26, 36, 37].…”

“…The sensitivity of 17 O chemical shifts and nuclear quadrupole coupling constants to H-bonding has attracted attention for several decades [4][5][6][7][8]. Due to difficulties arising from the low natural abundance of 17 O the investigations based on the 17 O properties have been gaining a wider publicity only recently [9][10][11][12][13]. However, investigations of examples with very short H-bonds are still rather scant [3,4,9,14,15].…”

“…Azine N-oxide complexes with carboxylic acids have been fairly extensively investigated in the past [18][19][20]. These works were aimed mostly at demonstrating the relationships between the differences of pK's of the components and the spectroscopic evidence of the strength of the resulting H-bonds, such as the centre of gravity of the broad OH absorption in the FTIR spectra as well as 1 H and 13 C chemical shifts. The results were reviewed in Ref.…”