Raman and ROA spectra of (-)- and (+)-2-Br-hexahelicene: experimental and DFT studies of a π-conjugated chiral system.

Christian Johannessen, Ewan W Blanch, Claudio Villani, Sergio Abbate, Giovanna Longhi, Nisha R Agarwal, Matteo Tommasini, David A Lightner

Index: J. Phys. Chem. B 117(7) , 2221-30, (2013)

Full Text: HTML

Abstract

The Raman optical activity (ROA) spectra of both enantiomers of 2-Br-hexahelicene in chloroform solution have been measured in the range 1700-300 cm(-1). Density functional theory (DFT) calculations accurately reproduce the observed features. The most intense ROA features are also the most intense Raman features, in the region 1350-1400 cm(-1), and correspond to the so-called D-modes, which play a major role in coronene and other PAHs (polycyclic aromatic hydrocarbons). Together with a detailed analysis of the normal mode structure, the polarizability tensors for the intense Raman features are investigated and related to the principal characteristics of helicene systems, namely, chirality and π-conjugation. Through electron-phonon coupling analysis, we propose a mechanism that justifies the intense ROA signals.