π-Electron absorption and circular dichroism spectra of [6]- and [7]-helicene

Abstract

East Anglia, NorwichThe absorption and c.d. spectra of [7]-helicene are reported, together with a re-examination of the electronic spectrum of [6]-helicene. The transition energies and dipole and rotational strengths of [6]-and [7]-helicene have been evaluated in the n-SCF approximation, and theoretical absorption and c.d. spectra are derived on the assumption that the absorption bands have Gaussian forms. It is found that the degree of correspondence between the experimental and theoretical spectra declines with increasing molecular size over the series [5]-, [6]-, and [7] -helicene, and the significance of u-n interaction and of n-electron exchange between the nearest-neighbour non-bonded carbon atoms in the terminal rings of the higher helicenes are discussed. It is concluded that the (+)-isomers of [4] -, [5] -, [6] -, and [7]helicene have a right-handed helical configuration. THE sixth and seventh members of the helicene series, now extending to [13]-helicene,l are of particular electronic and stereochemical interest in that [6]helicene completes one turn of the general helicene structure and [7]-helicene3 is the first member of the series in which the terminal benzene rings facially overlap. The enantiomers of these overcrowded aromatic hydrocarbons have large optical activities, [a]D26 3750 for [6]-helicene and [ U . ]~~O 6200 for [7]-helicene,6 and the rotatory power of the former, representing the classical example of an inherently dissymmetric chromophore, has attracted considerable attention.Fitts and Kirkwood? by using the polarisability theory of optical activity,' showed that six benzene rings disposed in the form of a right-handed helix with a pitch of 3-80A gives a positive rotation, [a4, +3010. The 0.r.d. curve of [6]-helicene was calculated in the Huckel n-approximation from the one-electron theory of optical activity by Moscowitz,Q who obtained a quantitative agreement between the theoretical and experimental curves over the first two absorption bands and to lower frequencies. A more approximate 0.r.d. curve of [6]helicene was calculated by Tinoco and Woody,l0 by the free-electron method and with a simpler structural model of the hydrocarbon, which consisted of a singlestrand helix with radius 2-50 and pitch 3.80 A. It was found lo that in the free-electron model, right-handed helical x-systems are generally dextrorotatory at wavelengths longer than those of the absorption bands of the system. The same general conclusion emerges from the bond-polarisability helix model of Brewster, who has calculated for [6]-helicene a molecular rotation of the order of magnitude observed.llThe polarised excitation and partial c.d. spectrum of [6]-helicene has been recorded by Weigang and cot Present address .

Chiral Polyaryl Cyclophanes

“…The ellipticities for 5 are comparable in magnitude to those observed for the resolved enantiomers of the closely related cyclophane 2 . The CD effect for 6 is considerably larger, but still small when compared to the values often observed for the carbohelicenes and related compounds (Δ ε ≈ 100–700 m –1 cm –1 ) , , …”

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.…”

Chiral Molecular Science: How were the absolute configurations of chiral molecules determined? “Experimental results and theories”

“…For such compounds, the π‐electron SCF‐CI‐DV MO method is useful, where SCF‐CI‐DV is the abbreviation of Self Consistent Field–Configuration Interaction–Dipole Velocity. In this method, the rotational strength R ba and the dipole strength D ba of the transition a → b are formulated as follows where Ψ a and Ψ b are the wave functions; ∇ is the del operator; r is the distance vector; μ B is the Bohr magneton; and σ ba is the wave number of the excitation a → b .…”

“… where ∑ indicates the summation for all π‐bonds r – s ; C ra is the coefficient of atomic orbital r in the molecular orbital Ψ a ; <∇> rs is the expected value of ∇ rs ; the vector ∇ rs is along the bond r–s and directs from r to s , i.e., r → s ; X r , Y r , and Z r are the x , y , and z coordinates of atom r ; R rs is the distance between atoms r and s . The x‐ and y‐axis components of electric and magnetic transition moments are similarly formulated …”