The existence and gas phase acidity of the HAlnF3n+1 superacids (n=1–4)

The saturation of the gas phase acidity of nHF/AlF3 and nHF/GeF4 (n = 1–6) superacids caused by increasing the number of surrounding HF molecules

“…The simplest case of the AlF 3 Lewis acid interacting with one HF molecule, HF/AlF 3 (HAlF 4 ), has already been characterized as a superacid composed of the hydrogen uoride donating its uorine's lone pair to the empty Al's 3p atomic orbital of AlF 3 quasi-planar fragment and additionally stabilized by the FH/F 3 Al hydrogen bond. 21,22,28 The DE of 279-280 kcal mol À1 and DG acid of 267-269 kcal mol À1 (depending on the theory level employed) 21,22,28 were predicted for this superacid, however, in this contribution we assume the DE ¼ 279 kcal mol À1 and DG acid ¼ 267 kcal mol À1 values for consistency with the results presented for the remaining nHF/ AlF 3 (n ¼ 2-6) systems. The 2HF/AlF 3 species might be viewed as formed by the attachment of the second HF molecule to the HF/ AlF 3 system, hence it resembles the deformed neutral AlF 3 molecule with two HF moieties attached, see structure 2HF/AlF 3 (1) in Fig.…”

“…It is worth noting that the strongest superacids proposed thus far were found to possess their Gibbs free energies of deprotonation in the 249-270 kcal mol À1 range. 11 Most recently, Srivastava and Misra also reported small DG acid values (indicating strong acidity) for HBeCl 3 (272 kcal mol À1 ), HPF 6 (281 kcal mol À1 ), and HLiCl 2 (284 kcal mol À1 ), 13 whereas our group demonstrated that even smaller Gibbs free deprotonation energies are estimated for HGaCl 4 (265 kcal mol À1 ), 28 HSn 3 F 13 (244 kcal mol À1 ), 25 HAl 4 F 13 (249 kcal mol À1 ), 21 and HSb 3 F 16 (230 kcal mol À1 ). 25 In fact, the last presented DG acid value of 230 kcal mol À1 predicted for the HF/ Sb 3 F 15 represents 25 the smallest gas phase Gibbs free energy of deprotonation reported in the literature thus far (including the corresponding values characterizing F(SO 3 ) 4 H and HSbF 6 superacids).…”

“…1,2 Since then, superacids remain the subject of continuing theoretical [10][11][12][13] and experimental [14][15][16][17][18][19] investigations concerning their structure, stability and acidity. Our group contributed to these studies by addressing the issue of the HAlCl 4 instability, 20 predicting the acidic strength of the aluminum-based HF/AlF 3 (HAlF 4 ), HF/Al 2 F 6 (HAl 2 F 7 ), HF/Al 3 F 9 (HAl 3 F 10 ), and HF/Al 4 F 12 (HAl 4 F 13 ) systems, 21 investigating the dissociative excess electron attachment to the HAlF 4 superacid 22 (whose properties were earlier determined by the Radom group 23,24 ), examining the strength of the Brønsted/Lewis superacids containing In, Sn, and Sb (i.e., HIn n F 3n+1 , HSn n F 4n+1 , and HSb n F 5n+1 (n ¼ 1-3)), 25 and, most recently, by demonstrating that the protonation of superhalogen anions 26,27 might be considered as the route to superacids' formation in selected cases only, 28 despite the fact that various superhalogens containing heavy metals as central atoms (e.g., InF 4 , SbF 6 , Sb 2 F 11 , SnF 5 , Sn 2 F 9 ) were utilized in the past to create atypical salts and complexes [29][30][31][32][33][34][35] even with noble gases (Kr and Xe). [36][37][38][39] The Lewis-Brønsted superacids consist of strong Lewis acid molecules (such as AlF 3 ) interacting with strong Brønsted acid molecules (e.g., HF) and thus their deprotonation process might be described by the following reaction scheme (that assumes the excess of a representative Brønsted acid): nHF/AlF 3 / ((n À 1)HF/AlF 4 ) À + H + .…”

Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages

“…[8,9] Due to their unusual properties, superacids are the subject of ongoing theoretical [10][11][12][13][14] and experimental [15][16][17][18][19][20][21][22][23][24] investigations focused mainly on their structure, acidity, and stability. Our contribution to these studies include predicting the acidity of the aluminumbased Al n F 3n /HF (n = 1-4) systems [25] and the compounds containing In, Sn and Sb, [26] Au, [27] Ti and Ge, [28] investigating the saturation of the acidity of nHF/AlF 3 and nHF/GeF 4 (n = 1-6) superacids caused by increasing the number of surrounding HF molecules, [29] describing the fragmentation process of the HAlF 4 and HGaF 4 superacids induced by an excess electron attachment, [30,31] and demonstrating the catalytic usefulness of various superacids. [32-36]] Theoretical approach commonly utilized to estimate the acid strength of superacids is based on calculating the Gibbs free energies of deprotonation reactions (DG 298 acid ).…”

“…It should also be noted that such a search for new systems possessing significant acid strength might be carried out by combining selected superhalogen anions (originally proposed by Gutsev and Boldyrev [38] and identified experimentally almost two decades later [39] ) with the additional proton. [25] even though several very strong acids have been recently proposed by following this route [25][26][27][28][40][41][42][43] most of them have not been experimentally obtained thus far. Therefore, it seems that the strongest superacids presently known (i. e., experimentally confirmed) are carborane acids whose enormous acid strength is likely related to the large stability of the icosahedral CB 11 'carborane core'.…”