Microwave Study of Triflic Acid Hydrates: Evidence for the Transition from Hydrogen-Bonded Clusters to a Microsolvated Ion Pair

Abstract

Rotational spectra of the mono-, di-, and trihydrates of triflic acid, CF 3 SO 3 H•••(H 2 O) n=1−3 , have been recorded by pulsed nozzle Fourier transform microwave spectroscopy and spectroscopic constants obtained have been compared with values calculated at several levels of theory. The experimental results are consistent with the theoretical predictions presented here and elsewhere, indicating that with only one or two water molecules, triflic acid remains un-ionized in a cold molecular complex. The experiments further concur with theoretical predictions that the addition of a third water molecule transforms the system into what is best regarded as a hydrated hydronium triflate ion pair. Thus, only three water molecules are needed to induce ionization of triflic acid in a cold molecular cluster. This number is somewhat low compared with that for other simple protic acids and likely reflects the superacidity of triflic acid. Simple energetic arguments can be used to rationalize this result.

Stepwise hydrations of anhydride tuned by hydrogen bonds: rotational study on maleic anhydride-(H2O)1–3

“…14 Besides, charge distributions and structural changes can be observed to be prominent dependent on stepwise complexation with H 2 O. 15,16 Indeed, it has been shown that the bond lengths r(Na-Cl) increase with the growth of the number of water molecules. The structure of NaCl-(H 2 O) 3 is approaching that of the contact ion-pair, which is considered to be an intermediate species in the incipient solvation process.…”

Microwave and Computational Study of Methanesulfonic Acid and Its Complex with Water

“…These differences likely reflect the relative acidities of triflic acid and MSA. In this regard, it is satisfying to note that, at the M06-2X/6-311++G(3df,3pd) level of theory, the primary hydrogen bond (O9···H10) in the triflic acid monohydrate is 1.58 Å, which is somewhat shorter than the 1.66 Å distance in MSA–water obtained from the calculations reported here. The two lowest-energy conformers for MSA–water and triflic acid–water are very similar, structurally.…”

“…Microwave spectra of triflic acid and its −OD isotopologue have been reported and reveal a tunneling motion arising from the wagging of the −OH­(D) moiety between two equivalent minima on either side of a C s symmetric transition state. The hydrates of triflic acid, CF 3 SO 3 H···(H 2 O) n =1–3 , as well as its complex with trimethylamine, have also been investigated by microwave techniques. These studies have shown that triflic acid undergoes proton transfer when complexed with only one trimethylamine molecule but requires three water molecules to realize proton transfer in a cold molecular complex.…”

Partial Proton Transfer in the Gas Phase: A Spectroscopic and Computational Analysis of the Trifluoroacetic Acid – Trimethylamine Complex

“…Intuitively, increasing the strength of the acid should further increase the degree of proton transfer to any given base. Indeed, with the superacid triflic acid (CF 3 SO 3 H), complete or near-complete proton transfer been demonstrated in a 1:1 complex with N­(CH 3 ) 3 9 and in its complex with three water molecules …”

“…Indeed, with the superacid triflic acid (CF 3 SO 3 H), complete or near-complete proton transfer been demonstrated in a 1:1 complex with N(CH 3 ) 3 and in its complex with three water molecules. 10 In this paper, we report a microwave and computational study of the complex formed from trifluoroacetic acid (F 3 CCOOH, TFA) and trimethylamine (N(CH 3 ) 3 , TMA). Trimethylamine is a strong amine base and TFA is a stronger acid than most carboxylic acids (pK a = 0.47 at 298 K from an average of several literature values).…”