Is trisulfuric acid hypothetical?

Question

In the preparation of sulfuric acid through contact process,

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated $\ce{H2SO4}$ in the absorption tower to form oleum:

$$\ce{H2SO4(l) + SO3(g) → H2S2O7(l)}$$

Oleum is reacted with water to form concentrated$\ce{H2SO4}$.

$$\ce{H2S2O7(l) + H2O(l) → 2 H2SO4(l)}$$

The sulfuric acid obtained is $\ce{96-98}$% pure. That means $\ce{2-4}$% is impure and contains byproducts and other oxoacids of sulfur including trisulfuric acid. If the sulfur trioxide to be adsorbed on $\ce{H2SO4}$ is in excess, it can form trisulfuric acid.

$$\ce{H2SO4(l) + 2SO3(g) → H2S3O10(l)}$$

But the amount of trisulfuric acid is negligible and can be omitted. But is it an actual compound? Can it be isolated? Can it exist in room temperature?

Pubchem identify it as a compound and also has a unique ID. It also has an unique CAS number.

It has $\ce{S=O}$ bonds and $\ce{S-OH}$ bonds (characteristics of oxoacid of sulfur) but still it is not mentioned in that list (link above). Why?

Answer 1

It seems that this compound has not yet been characterized, although traces of it could be found in mixtures of $\ce{H2SO4}$ and $\ce{SO3}$. The most recent paper I could find is this one from 2012, showing that salts of the respective anion exist and theoretical calculations imply that the acid is stable with respect to decomposition into $\ce{H2S2O7}$ and $\ce{SO3}$. The conditions under which it may be stable are however unknown.

Answer 2

$\ce{H2S3O_{10}}$ has not been isolated as an acid, but Logemann et al. [1] have isolated the lead salt $\ce{PbS3O_{10}}$ by heating a chloroplumbate salt in oleum.

The ion is found both experimentally and theoretically to have unequal sulfur-oxygen distances along the bridges between the sulfate units: each bridging oxygen is closer to the central sulfur atom (155 pm) than to the nearer terminal sulfur atom (169 pm). The bridging bond length in the disulfate ion is intermediare, 164.5 pm [2]. This suggests that the central sulfate unit is more weakly bound to the two outer $\ce{SO3}$ monomers than it would be to a single $\ce{SO3}$ monomer in the disulfate ion.

This bond-length feature, and the relatively weak bonding to the outer $\ce{SO3}$ monomers, is theoretically calculated to hold for the parent acid $\ce{H2S3O_{10}}$ as well. That would imply that although Ref. [1] renders $\ce{H2S3O_{10}}$ stable with respect to decomposition, it would react with monomeric $\ce{H2SO4}$ to form two molecules of the more strongly bridged $\ce{H2S2O7}$.

Reference

1. Logemann, Christian & Klüner, Thorsten & Wickleder, Mathias. (2012). "Synthesis and Characterization of the Trisulfate Pb[S3O10] and Theoretical Analysis of H2S3O10". Zeitschrift Fur Anorganische Und Allgemeine Chemie. 638. 758-762. https://doi.org/10.1002/zaac.201100533.

2. H. Lynton and Mary R. Truter (1960). "An accurate determination of the crystal structure of potassium pyrosulphate". J. Chem. Soc. 35. 5112-5118. https://doi.org/10.1039/JR9600005112.

Answer 3

The trisulfuric acid $\ce{H2S3O10}$ hasn't been isolated yet, but it exists and has been characterized as a chain of three $[\ce{SO4}]$ tetrahedra sharing vertices and one oxygen atom from each of the terminal $[\ce{SO4}]$ tetrahedra being protonated (the structure is oversimplified and doesn't account for accurate bond order and charge distribution — for more details see e.g. Why is hydrogen sulfate put together as it is?):

The formation of polysulfuric acids $\ce{H2S_nO_{2n+1}}$ with $n\in [1;4]$ in solutions of $\ce{SO3}$ in $\ce{H2SO4}$ has been proven by means of Raman [1] and infrared [2] spectroscopy.

According to Walrafen [1, p. 2340], $\ce{H2S3O10}$ possess at least five characteristic frequencies in Raman spectrum:

Five frequencies listed in Table III, viz., 230-250, ~325, ~350, ~685, and ~1455 cm⁻¹, are those of $\ce{H2S3O10}.$ The bands at 230-250, ~325, and ~350 cm⁻¹ are associated with deformation vibrations of the $\ce{S''-O-S'-O-S''}$ grouping of $\ce{H2S3O10}.$ The band at ~685 cm⁻¹ is probably produced by $\nu_\mathrm s\text{-}\ce{S''-O-S'-O-S''},$ and the band at ~1455 is related to asymmetric valence vibrations of the central or terminal $\ce{O=S=O}$ groups.

and is mostly present in oleums with the molar fraction $x(\ce{SO3}) \approx 0.75:$

Fig. 9. Comparison of $c(\ce{SO3})$ with the sum $[\ce{H2SO4}] + 2[\ce{H2S2O7}] +3[\ce{H2S3O10}] + [\ce{SO3}] + 3[\ce{S3O9}]$ obtained from Raman data.

Stopperka [2, p. 280] assigns characteristic antisymmetric valence vibrations of central $\ce{SO2}$ group (1480 cm⁻¹) as well as terminal $\ce{—O—SO2—OH}$ groups (1457 cm⁻¹). The intensities of the latter vibrations quickly decrease with an increase in molar concentration of $\ce{SO3}$:

Kleine Absorptionsmaxima bei 1457 und 1487 cm⁻¹, die in 90proz. Oleum den antisymmetrischen $\ce{SO2}$-Valenzfrequenzen der $\ce{H2S3O10}$ zugeordnet wurden (siehe hierzu Mitt. II) gehen in ihrer Intensität ebenfalls zurück. Bemerkenswert ist dabei, daß die schwache Bande bei 1480 cm⁻¹ in ihrer Intensität nicht so rasch abfallt, als die bei 1467 cm⁻¹ gelegene. In der vorangegangenen Mitteilung (1) hatten wir die Bande bei 1480 cm⁻¹ einer $\nu_\mathrm{as}$ $\ce{SO2}$ zugeordnet, die von einer mittelständigen $\ce{SO2}$-Gruppe der $\ce{H2S3010}$ herrührt. Dementsprechend wurde das schwache Absorptionsmaximum bei 1457 cm⁻¹ auf eine $\nu_\mathrm{as}$ $\ce{SO2}$ der $\ce{—O—SO2—OH}$-Endgruppe zurückgeführt. Der mit zunehmender $\ce{SO3}$-Konzentration beobachtbare unterschiedliche Intensitätsabfall führt zu dem Schluß, daß die Konzentration an $\ce{—O—SO2—OH}$-Endgruppen mit zunehmendem $\ce{SO3}$-Gehalt schnell absinkt, wogegen $\ce{—O—SO2}$-Gruppierungen noch deutlich bis zu relativ hohen $\ce{SO3}$-Konzentrationen nachgewiesen werden können.

A series of hydrogentrisulfate salts of alkali metals $\ce{A[HS3O10]}$ $(\ce{A} = \ce{Na}, \ce{K}, \ce{Rb}),$ (which is arguably the closest derivative of trisulfuric acid) have been isolated and characterized with single-crystal x-ray diffraction [3], also supporting the structural motif of found using spectroscopy:

Figure 2. Anions linked to dimers in the crystal structure of $\ce{Na[HS3O10]}.$

Given $\ce{H2S3O10}$ is predicted to be Brønsted-Lewis superacid [4], its isolation is far from trivial and existence under ambient conditions is highly questionable.

References

1. Walrafen, G. E. Raman Spectral Studies of Oleums. J. Chem. Phys. 1964, 40 (8), 2326–2341. DOI: 10.1063/1.1725511.

2. Stopperka, K. Infrarotspektroskopische Untersuchungen an den flüssigen Systemen $\ce{SO3-H2O}$ und $\ce{SO3-D2O}$. III. Die Schwingungsspektren von flüssigem Schwefeltrioxid. Z. Anorg. Allg. Chem. 1966, 345 (5–6), 277–289. DOI: 10.1002/zaac.19663450506.

3. Schindler, L. V.; Klüner, T.; Wickleder, M. S. Towards Polysulfuric Acids: The Hydrogentrisulfate Anion $\ce{[HS3O10]}$ − in $\ce{A[HS3O10]}$ $(\ce{A} = \ce{Na}, \ce{K}, \ce{Rb}).$ Chem. Eur. J. 2016, 22 (39), 13865–13870. DOI: 10.1002/chem.201602176.

4. Koppel, I. A.; Burk, P.; Koppel, I.; Leito, I.; Sonoda, T.; Mishima, M. Gas-Phase Acidities of Some Neutral Brønsted Superacids: A DFT and Ab Initio Study. J. Am. Chem. Soc. 2000, 122 (21), 5114–5124. DOI: 10.1021/ja0000753.