The Helicenes: Potential Carriers of Diffuse Interstellar Bands

Abstract

In this work, the helicenes are postulated as potential carriers of the diffuse interstellar bands, DIBs. The helicenes are nonplanar cata-condensed molecules structurally related to the polyacenes, and polyacenes have been already proposed as conceivable carriers of the DIBs. In here, the possible identity of the carrier of the DIB at 4502 Å is studied and potentially identified as a cata-condensed helicene using spectroscopic, statistical, and stability methods. First, statistic and spectroscopic information allow the fitting of the regression equation that governs the variation of the location of the published absorbance p-bands as a function of the number of fused aromatic rings (nFAR) for planar and helical cata-condensed polycyclic aromatic hydrocarbons with 1−14 FAR, and the application of this equation to establish the identity of the possible carrier of the 4502 Å DIB, that is, [26]helicene (C 106 H 56 ). The difference between the calculated location of the p-band from the fitted equation and the DIB location under study is equal to 0.21%. Subsequently, an aromaticity argument demonstrates the presence of π-delocalizing benzene effect in helicenes with the general formula C 2+4N H 2N+4 , for example, [26]helicene, that would theoretically provide supplementary stability to the molecule to justify its occurrence. Finally, a feasible mechanism of formation of helicenes in the interstellar medium is proposed that involves successive additions of n-butane units to a phenanthrene moiety. To the best of our knowledge, this is the first report about the possible presence of cata-condensed helical molecules in the DIBs and the first study that theorizes that [26]helicene is the possible carrier of the DIB at 4502 Å.

Application of the Y-Rule and Theoretical Study to Understand the Topological and Electronic Structures of Polycyclic Aromatic Hydrocarbons from Atomic Force Microscopy Images of Soot, Coal Asphaltenes, and Petroleum Asphaltenes

“…In the case of the study of the single-triplet transitions and triplet–triplet transitions of asphaltenes, high-level quantum density functional theory (DFT) is necessary . as well as for the calculation of aromaticity using the magnetic-based aromaticity index nucleus-independent chemical shift (NICS). ,,,− The FAR-database which comprises the calculations of NICS(0), NICS(1), NICS(0) ZZ , and NICS(1) ZZ , of 660 neutral benzenoid PAHs and cyclopenta-fused PAHs provides NICS data of aromaticity of PAHs to be used in data science and machine learning. DFT has also been used to calculate properties such as NMR, solubility, and radical reaction energies, of steam-cracked tar asphaltene molecules and petroleum asphaltene obtained from the nc-AFM images. , …”

Polarizability in Astrochemical Studies of Complex Carbon-Based Compounds

“…Helicenes are a class of chiral molecules, and further chemical transformations of them may lead to chiral substrates relevant to biogenic space molecules. Curiously, a very recent computational study proposes the relevance of diffuse interstellar band 4502 Å to the high-molecular helicenes …”

“…Curiously, a very recent computational study proposes the relevance of diffuse interstellar band 4502 Å to the high-molecular helicenes. 67 Polycyclic aromatic hydrocarbons may have irregular structures that combine linear and zigzag, flat (two-dimensional (2D)) and nonplanar (three-dimensional (3D)) moieties. 52,68 In general, we can talk about the correlation: planar PAH isomers have low energy, whereas their nonplanar counterparts have low polarizability.…”

Gas-phase synthesis of racemic helicenes and their potential role in the enantiomeric enrichment of sugars and amino acids in meteorites

“…Although the gas-phase processes unraveled here form racemates of [5] and [6]helicenes, in deep space, ultraviolet circularly polarized light may preferentially photolyze the M or P enantiomer; this process could lead to an enrichment of one enantiomer over the other (enantiomeric excess), which can be propagated by further growth. Since PAHs, to which helicene belongs, represent fundamental molecular building blocks of nanometer-sized interstellar grains, 55–60 a bottom-up formation of carbonaceous nanoparticles via PAHs leads to chiral carbonaceous nanostructures 26,27,56,57,61 and may translate the enantiomeric excess of the helicenes from the gas-phase to the nanoparticles eventually resulting into chiral islands or chiral surfaces. Although there is no detailed description to date of how carbonaceous nanoparticles might form in deep space, PAHs are likely key contributors.…”