Abstract
Bicyclo[3.3.1]nonane-2,6-dione (1) and bicyclo[3.3.1]nona-3,7-diene-2,6-dione (2) have been examined by vibrational circular dichroism (VCD), which, as for most C2-symmetric systems, exhibits strong VCD signals. In the case of 2, VCD signals are stronger and sharper with several bisignate doublets; for 1, signals are less intense and broader. The VCD and IR spectra are excellently predicted by DFT calculations: only one conformer is present for 2, while for 1, three main conformers, related through concerted skeleton torsional motions are present (two of them being interchanged by C2-rotation). The VCD spectrum shows specific features for the different conformers, such that correct population factors are crucial for reproducing experimental data. Also, the TD-DFT prediction of ECD (electronic circular dichroism) spectra is good. By comparing the spectroscopic signature of the two molecules (both VCD and ECD) and by careful analysis of the theoretical results, the role of the C=C double bond in compound (2) is evidenced. The double bond contributes toward enhancing the CD response both electronically and vibrationally.
Highlights
Bicyclo[3.3.1]nonane-2,6-dione (1) and bicyclo[3.3.1]nona-3,7-diene-2,6-dione (2) in Scheme 1 was synthesized earlier and studied [1,2,3,4,5,6] to learn whether and to what extent electron delocalization through homo-conjugation and transannular interactions, defined by Hoffman et al [7], manifest in the characteristics of a “coupled” chromophore [8,9,10]
Evidence was provided by 13 C-NMR chemical shift δ(C=O), photoelectron spectroscopic data [1], as well as electronic circular dichroism (ECD) data [2]
We examined whether vibrational circular dichroism (VCD) [11,12] can provide further independent information on (1) the interaction of symmetrically disposed ketone carbonyl moieties within the same molecule and (2) the influence of C=C double bonds on spectroscopic response
Summary
Bicyclo[3.3.1]nonane-2,6-dione (1) and bicyclo[3.3.1]nona-3,7-diene-2,6-dione (2) in Scheme 1 was synthesized earlier and studied [1,2,3,4,5,6] to learn whether and to what extent electron delocalization through homo-conjugation and transannular interactions, defined by Hoffman et al [7], manifest in the characteristics of a “coupled” chromophore [8,9,10]. Evidence was provided by 13 C-NMR chemical shift δ(C=O), photoelectron spectroscopic data [1], as well as electronic circular dichroism (ECD) data [2]. The interchromophoric interaction present in unsaturated ketones shows clear spectroscopic signatures and is amply studied in the literature [2]. We examined whether vibrational circular dichroism (VCD) [11,12] can provide further independent information on (1) the interaction of symmetrically disposed ketone carbonyl moieties within the same molecule and (2) the influence of C=C double bonds on spectroscopic response. The presence of the two moieties promises exquisite spectroscopic
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