π-SCF and INDO calculations and the absorption, polarized excitation and circular dichroism spectra of [4]-helicenes

Abstract

The absorption and circular dichroism spectra of (-)D-l,12-dimethylbenzo[c]phenanthrene-5-acetic acid are reported, together with the polarized excitation and fluorescence spectra of the parent hydrocarbon, [4]-helicene. The transition energies of 1,12-dimethyl-[4]-helicene calculated in the ~r-SCF approximation, and the corresponding dipole and rotational strengths obtained by the dipole-velocity method, show that the optical isomer studied has the M-configuration with the stereochemical form of a segment of a left-handed helix. Analogous calculations in the INDO approximation using both the dipole-velocity and the dipole-length procedure with separate consideration of the one-centre and the two-centre transition moment integrals, indicate that the more satisfactory dipole and rotational strengths are obtained in the two-centre dipole--velocity formulation. The appearance of a~zt* transitions at higher energies limits the application of the 1r-SCF procedure, but the INDO method is found to place these transitions at too low an energy.

Theoretical and Experimental Studies on Circular Dichroism of Carbo[n]helicenes

“…Thus, (P)-and (M)-4,5-dimethylphenanethrene (DM-CH [3]), resolved by chiral HPLC below −70°C, were subjected to the CD spectral measurement in a mixture of ethanol and n-hexane at −60°C to exhibit mirror-imaged, coupled intense CD signals, from which the absolute configurations were successfully determined. 63 The optical resolution and CD spectral studies of 1,12-dimethylbenzo[c]phenanthrene-5-acetic acid (DM-CH[4]-CO 2 H) 64 [18][19][20]67 Only recently, the TD-DFT studies on CH [5] and CH[6], 68 as well as CH[7] 21 have been published. All of the theoretical studies successfully predicted the unique feature of the strong bisignate positive/negative CEs at the 1 B b / 1 B a bands, which are derived from the inherent chirality of helicenes.…”

“…Thus, ( P )- and ( M )-4,5-dimethylphenanethrene (DM- CH[3] ), resolved by chiral HPLC below −70 °C, were subjected to the CD spectral measurement in a mixture of ethanol and n -hexane at −60 °C to exhibit mirror-imaged, coupled intense CD signals, from which the absolute configurations were successfully determined . The optical resolution and CD spectral studies of 1,12-dimethylbenzo[ c ]phenanthrene-5-acetic acid (DM- CH[4] -CO 2 H) and of 1-fluoro-12-methylbenzo[ c ]phenanthrene (FM- CH[4] ) were also reported in the literature. The CD spectra, in particular the CE at the 1 B b band (Δε 299 nm = +46 M –1 cm –1 for DM- CH[4] -CO 2 H and Δε 290 nm = +68 M –1 cm –1 for FM- CH[4] ), resembled the theoretical one calculated for parent CH[4] (Δε 269 nm = +43 M –1 cm –1 ) with a slight shift in excitation energy.…”

Racemization Barriers of Helicenes: A Computational Study1

“…Methyl-Substituted Helicenes. Methyl substituents at the terminal rings of the helicene skeleton have a major influence on the structure of the ground state ,9b,, as well as on the racemization barriers. ,− …”

Absolute configuration of chiral fullerenes and covalent derivatives from their calculated circular dichroism spectra

“…Disagreement between experimental and theoretical transition energies had already been observed by Mason and co-workers in the case of other non-planar π-systems, the hexahelicenes, when they compared the π-SCF method, 5 a similar calculation to ours, with INDO calculations. 9 Comparatively, the INDO 10 method introduces also σ-π* transitions. However, our calculations using the SCF-CI-DV MO method reproduce well the shapes of the CD spectra and, as will be discussed below for C 76 , give the same results as INDO calculations.…”