The molecular structure of allenes and ketenes. 17. Excited states of the allene chromophore: photoelectron, circular dichroism and absorption spectroscopy of alkyl- and halogenoallenes

Abstract

Photoelectron (PE) as well as near-and vacuum-ultraviolet (300-1 50 nm) absorption and circular dichroism (CD) spectra of the tert-butylallenes (CH,),CCH=C=CHX (X = C(CH3),, C1, Br, I) are reported. The emphasis of the work is on the excited states and electronic structures of the allenes. In the PE spectra interactions between the substituents across the allenic C%=C system are observed. These can be rationalized in terms of Coulombic field effects and "hyperconjugation". Effects of vibronic coupling observed in the PE spectra are discussed qualitatively. By the combined use of absorption and CD spectroscopy, a large number of excited states resulting from valence-shell and Rydberg transitions are observed. The assignments of Rydberg states rely essentially on the absorption spectra using ionization energies from the PE spectra. Referring to the hydrocarbon as a representative of the allene chromophore, the sequence of excited states resulting from intravalence e -e (HOMO -LUMO) transitions is found to be lA2 < 'BI < 'B2 < 'A,. The intravalence excited states of the halogenoallenes originating with the two highest energy orbitals can be correlated with those of the allene chromophore. The two lowest energy excited states of chloro-and iodoallene are shown to have bent C=C=C geometries. In the CD spectra of the halogenoallenes low-energy excited states are detected with the corresponding electronic transitions terminating at the antibonding C-X u orbitals ( u *~-~) and u*c-x comprising halogen d atomic orbitals.

Assignment of Absolute Configuration of Bromoallenes by Vacuum-Ultraviolet Circular Dichroism (VUVCD)

“…This approach of VUVCD measurement and interpretation has yielded the following three important arguments for this method. First, bromoallenes were expected to show intense Cotton effects at the wavelength corresponding to distinct absorption peaks at 180–190 nm, which were clearly observable by VUVCD and agreed well with the previous report [ 33 ]. This is also encouraging since a couple of reports demonstrated that VUVCD spectra were applicable to solutions of not only water but also organic solvents [ 35 , 36 ].…”

“…Positive Cotton effects were observed at the wavelength region of interest for all S bromoallenes and negative ones for all R bromoallenes, despite the presence of various functional groups such as hydroxy groups, acetals, esters, or silyl ethers. The relationship between the signs of the CD spectra of all of these compounds and their absolute configurations is consistent well with that obtained for other chiral allenes [ 33 ], implying that this π–π* transition of interest may be applicable to other types of allene derivatives. While these transitions were previously either non-observable or visualized only by gas-phase measurements, our results demonstrate the potential of VUVCD for the robust assignment of absolute configurations of bromoallenes.…”

“…The path length in the cell was adjusted using a Teflon spacer to 50 μm. The sample solutions were freshly prepared by dissolving in acetonitrile at an appropriate concentration, which was determined based on the reported molar absorption coefficient [ 33 ]. All of the spectra were recorded using a 1.0-mm slit, 4-s time constant, 20-nm/min scan speed, and four accumulations.…”

Helical orbitals and circular currents in linear carbon wires

“…The HOMO of (+)S(+) is an M-helix and the HOMOÀ1 is a P-helix; the order is reversed for the (À)S(À) conformation. possibility of experimental verication of the existence of helical orbitals spanning the length of linear molecules; a verication which ideally should be found by revisiting spectroscopic experiments on optical activity 11,[57][58][59] or with novel orbital imaging techniques. [60][61][62][63][64][65][66] Here, we will explore how the helical orbitals inuence the coherent electron transport properties of even [n]cumulenes with the aim of making explicit predictions for single-molecule conductance experiments.…”

Helical electronic transitions of spiroconjugated molecules

“…The effect of substituents and solvents on the optical properties of allene has been studied in great detail, and we refer to the extensive literature for analysis of their spectra. 15,[25][26][27] The transitions of dimethylallene are almost equally-weighted combinations of excitations from occupied pP/M to unoccupied pP/M*. Oscillator strengths and the specific MO weights will be method dependent, see computational details in ESI part A.…”