Cosmic coronene's phantom spectral bands

Anomalies in the spectra of the aromatic planar molecule coronene can be explained with powerful calculations on infrared and Raman spectroscopic data. The evidence suggests that minute out-of-plane molecular distortions from D(6h) to C(2h) symmetry are the underlying cause.

Petar Todorov and Leonardus Jenneskens of the Chemical Biology and Organic Chemistry department at Utrecht University, in The Netherlands working with colleague Joop van Lenthe in the Theoretical Chemistry Group have investigated one of the archetypal aromatic organic chemicals, coronene.

Coronene occurs naturally as the mineral carpathite, which forms flakes in sedimentary rock and is thought to have its origins in ancient hydrothermal vent activity the molecule is colloquially known, among chemists, as super-benzene. Where benzene is an aromatic ring of six carbon atoms and attendant hydrogens, the highly symmetrical coronene is a "flat" ring of six fused benzene molecules, each sharing an edge with a neighbour.

Its intriguing structure and vibrational activity are now attracting a lot of interest from astronomers interested in the diffuse infrared emission bands they observe between the stars due to a miscellany of organic molecules and as potential indicators of organic life elsewhere in the universe. Coronene is also of interest to materials scientists who see it as the smallest possible unit, benzene excepted, of graphene and graphitic materials, which are now being keenly researched as future molecular scale electronics components.

The coronene structure exists as twenty canonical forms and its most stable has a somewhat higher resonance energy per pi electron than benzene itself. Its system of conjugated double bonds endows this yellow substance an interesting asymmetry between its absorption and emissions - it fluoresces blue under ultraviolet.

However, superaromatic coronene remains enigmatic in some ways to the organic chemists who study it. Jenneskens and colleagues explain that the molecular geometry and the normal modes properties of coronene can be investigated using density functional theory DFT(B3LYP) and restricted/Hartree-Fock calculations.

"The search of structure-property relationships, infrared and Raman spectroscopy have come forward as indispensable experimental and theoretical tool," the team explains. As a model system coronene is frequently used in countless studies. " Clearly, a deep understanding of the molecular structure and vibrational motion of coronene is essential for the correct modelling of carbon-based materials on macroscale, as well as for the unambiguous molecular spectral identification in the observed interstellar radiation," the team says.

"The most remarkable feature of all solid-state infrared spectra of coronene is the unexpected medium-strong band at about 960 cm?1," the teams explains. Previous researchers have reported a doublet at 958 cm?1 medium and 962 cm?1 weak. Others saw a clearly visible medium-strong band at approximately 960 cm?1. But the calculated spectrum has no infrared active modes in this region.

The predictions then allow them to interpret and explain afresh the solid-state infrared and Raman spectra of coronene, with particular regard to so-called phantom bands that were not previously understood and are missing from the molecule's gas phase spectra (hence the name). Their study suggests that despite planarity being a key feature of aromaticity in organic molecules, these phantom bands arise because of tiny deviations from an absolutely flat molecule and a change in symmetry.

The team explains that they have now correctly assigned all the phantom infrared/Raman spectral features of crystalline coronene. The phantom bands occur because of the small symmetry reduction caused by the distortion of the molecule from its totally planar state, a fact corroborated by the available single-crystal X-ray crystallographic data.

What remains to be clarified is the exact nature of the deformation and its scale, important factors that will ultimately influence research on both the astronomical and nanoscopic scales. "Apparently, the molecular structure and vibrational motion of aromatic hydrocarbons still pose considerable theoretical challenges," the team concludes.