A recent article in Accounts of Chemical research discusses this very topic as well as some other interesting facts revolving around protons, their structure and their generation. What does H+ signify? Well it means different things to different disciplines. If you are a physicist it refers to one of the fundamental elementary particles, if you are a chemist it refers to a hydrogen ion. So what is it exactly? Well it is a very strong acid about 1056 times stronger than 100% sulphuric acid! Its place in chemistry is well documented even although you can only add H+ to a molecule in the gas phase whereas only a solvated proton can be added in less gaseous media. This has great implications for biology especially where proton pumping is an important function, thus the structure H(H 2 O)+ n turns out to be a very important structure. This is especially so because the degree of solvation affects the rate of protonation particularly in proton/electron transfer.

According to Professor Reed, the author of this article a self – ionising acid, HA is very unlikely to form a H 2 A+ cation, the structure is better represented by the following equation:

In the last few years it has emerged that the H+ is a 2-coordinate species, however, even this may extend to multi-coordinate when hydrogen bonding is involved. So what is the structure of H+ in water? This turns out to be a very difficult question to answer, and several instrumental methods have been applied to solve this problem, including IR and x-ray crystallography. These two methods and lots of hours have turned up H 13 O 6 + ions as the structure. The experimental evidence backing up this claim can be found in this paper, which is open access!

Talking about protons leads to the strongest acids known H(CHB 11 Cl 11 ) or H(CHB 11 F 11 ), however the latter species is very difficult to obtain due to the ease with which it gives up its proton. These acids can easily protonate benzene or better alkanes! They are very useful in the study of protonation due to the complete inertness of the anions that do not undergo the usual corrosive reactions associated with other types of super acids. The acid H(CHB 11 F 11 ) protonates butane to give the t-butyl cation at room temperature. Ethyl chloride is also protonated by H(CHB 11 Cl 11 ) with concomitant loss of HCl to give the diethyl chloronium salts which can be isolated.

Oxatriquinanes are tricyclic analogues of the H 3 O+ ion. These very strong acids can H-bond to the last lone pair of oxatriquinane, which is a tetravalent oxygen species with a 2+ charge and is an analogue of the H 4 O 2 + ion.

The Oxatriquinane oxonium cation.

So if you are searching for a proton source not accompanied by the usual destructive counter ions try one of these carborane acids.