# What is the structure of trifluoromethanesulfonic acid in the gas phase?

During an interesting discussion in our group’s seminar, the question arose what actually causes trifluoroacetic acid to be a much better nucleophile than trifluoromethanesulfonic acid. Part of the discussion was whether the oxygens of trifluoromethanesulfonic acid were equal or not.

Traditionally, $\ce{TfOH}$ is drawn as shown in the structure below on the left. The structure would assume the molecule having point group $C_\mathrm{s}$ due to the non-equality of the oxygen atoms only one of which bears a hydrogen. The triflate anion $\ce{TfO-}$ in solution, on the other hand, has three homotopic oxygens as can be seen in the structure on the right, leading to the more symmetric point group $C_\mathrm{3v}$.

One of the opinions voiced in the discussion was that even the gas-phase molecule $\ce{TfOH}$ adopted a $C_\mathrm{3v}$ point group, the hydrogen being in the same axis as carbon and sulfur and somewhat centred between the three oxygens. This makes perfect sense when thinking about it. Therefore, my questions are:

• Has the structure of trifluoromethanesulfonic acid or a structurally strongly related acid been studied in the gas phase? (Please provide a reference)

• Does the experiment suggest equivalent or non-equivalent oxygens?

• How well can that idea be extended? e.g. should one consider the structure of formic acid to be $C_\mathrm{2v}$ symmetric rather than $C_\mathrm{s}$?

• Do (solid-state) crystal structures or other experiments exist that confirm a different structure for solid $\ce{TfOH}$?

• If solid and gas phase structures differ substantially, what about the structure of the molecule in liquid phase/solution?

• I can't see any good reason why it should be symmetric. It would be symmetric bifurcated H-bond there. I wonder if they even exist. – Mithoron Jun 3 '16 at 14:55
• I agree with @Mithoron. In the more compact $\ce{-COOH}$ group the angle does not even get close to allowing a substantial attractive interaction $\ce{-OH\bond{~}O\bond{-}}$. See this question and my answer to it. It's an interesting thought provoking question nevertheless. – Martin - マーチン Jun 9 '16 at 8:19
• Also, dear compchem people, if you want to contribute with calculations, you are hereby warmly encouraged. That is, if no reference exists already. – Jan Sep 15 '16 at 19:43
• Solid state crystal structures do exist, but everything I've seen indicate hydrates, making the question of symmetry a bit complicated. – Geoff Hutchison Sep 15 '16 at 20:01
• – DavePhD Sep 20 '16 at 12:15

Has the structure of trifluoromethanesulfonic acid or a structurally strongly related acid been studied in the gas phase? (Please provide a reference)

Yes. See Gas Phase Structure of Methyl Trifluoromethanesulfonate, CH3OSO2CF3, and Conformational Properties of Covalent Sulfonates Inorg. Chem., 1999, 38 (13), pp 3051–3055 and references cited therein, especially Electron Diffraction Investigation of the Molecular Structure of Trifluoromethanesulfonic acid (triflic acid) Z. Naturforsch. Teil A 1981, 36, 917

Does the experiment suggest equivalent or non-equivalent oxygens?

Non-equivalent.

The 1981 paper finds

S=O 141.7 ( 1) pm
S-O 155.7 ( 2) pm

How well can that idea be extended? e.g. should one consider the structure of formic acid to be C2v symmetric rather than Cs?

From the following gas phase study, I don't think that gas phase formic acid monomer is C2v. http://actachemscand.dk/pdf/acta_vol_23_p2848-2864.pdf

Do (solid-state) crystal structures or other experiments exist that confirm a different structure for solid TfOH?

Quoting from Trifluoromethyl Chlorosulfonate, CF3OSO2Cl: Gas Phase and Crystal Structure, Conformation and Vibrational Analysis Studied by Experimental and Theoretical Methods Inorg. Chem. 2003, 42, 7297-7303

"The structure of triflic acid, HOSO2CF3 (trifluoromethansulfuric acid), was also studied by GED, and an anti orientation was assumed in the analysis of the diffraction pattern. [reference 6: the 1981 reference above] In contrast, an X-ray investigation of 2HOSO2CF3 [dot] H2O [ie. the hemihydrate] resulted in gauche orientation of the O-H bond,[reference 7] in accordance with theoretical calculations. [references 8 and 9]"