Ah, the humble proton. Simple, stable, and able to drastically affect the chemistry of other molecules -- and nowhere more so than in the Interstellar Medium (ISM). H₂ molecules, for instance are readily protonated in dense interstellar clouds, forming H₃⁺, and playing a key role in the formation of hydrides like ammonia and methane. CO forms HCO⁺, N₂ forms HN₂⁺ and so on. So what about those polycyclic aromatic hydrocarbon (PAH) molecules I keep talking about...?

PAHs, it seems, may also be readily protonated in the ISM. After all, as any school kid knows, protons like electrons. Out in space, protons will happily stick to any region of electron density -- which means virtually any accessible chemical bond. A PAH molecule, with it's huge delocalised electron clouds, is a veritable smorgasbord for protonation!

The interesting thing is, because of the way aromatic molecules are, adding an extra proton into the fray disrupts the electron orbitals across the whole molecule (or at least its aromatic region). This has some interesting effects, spectroscopically. Regular PAHs are potent ultraviolet absorbers, with some intense spectral lines in the UV, near-UV and blue regions. Protonate them though, and you break their orbital degeneracies, causing them to absorb at lower wavelengths. Optical wavelengths.

The authors of this paper, use TDDFT calculations to study perturbations in the electron clouds of two molecules: coronene and ovalene. The results are really quite marked.

The paper presents a set of calculated spectra for both neutral and protonated PAH molecules. On looking at them, the differences are quite obvious! From a single strong absorption in the near UV for the neutral molecule, a whole slew of electronic transitions spring forth in the protonated molecule -- from near UV all the way to around 700nm (which is decidedly red).

So could this really be a hint towards solving the diffuse band problem? Frankly, that's a very good question. It is true, however, that this looks rather promising. From an astrochemical perspective, all that's going on is taking molecules that should exist and subjecting them to a process that should occur. Which seems perfectly natural, especially given the overwhelming abundance of hydrogen in the Universe.

The big problem with the diffuse bands is that they don't match up to anything that's been seen in the lab. Mind you, gas phase protonated PAHs aren't the sort of thing you can readily create in the lab to study. This would effectively seem to give a whole host of new candidate molecules to play with. In fact as far as the electrons are concerned, if you protonate one of the less symmetric molecules, each different protonation site yields an entirely different molecule with an entirely different electronic spectrum.

What remains to be seen is which molecules are more prevalent and why. It's a puzzle.
And I do love a good puzzle!


A. Pathak, P. J. Sarre (2008). Protonated PAHs as carriers of diffuse interstellar bands Monthly Notices of the Royal Astronomical Society: Letters DOI: 10.1111/j.1745-3933.2008.00544.x