Abstract
We report on our investigation of C-C bonding longer than 2.0 Å, which can be realized by perpendicularly facing two fluorenyl rings in the title compound. A small orbital overlap between the distantly positioned carbon atoms is observed as a small concentration of electrons on the X-ray electron density map. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the compound originate from the in-phase and out-of-phase interactions of the overlapping orbitals, respectively, with a gap of 2.39 eV. Solid-state 13C NMR spectroscopy shows a sharp peak at 82.9 ppm for the long-bonded carbons, and a CASSCF(6,6) calculation indicates small diradical character. The experimental and theoretical analyses reveal sufficient covalent-bonding interaction in the long-bonded carbon pair.
Highlights
The covalent bond is one of the most fundamental concepts in organic chemistry, wherein the strength of a covalent bond depends on the extent of the overlap integral between the orbitals of the atoms sharing the pair of electrons
The title compound 1 (Figure 2a) was prepared over four steps from commercially available 9-fluorenone, as outlined in Scheme 1. 9-(Dibromomethylene)-9H-fluorene was prepared according to the reported procedure.[16]
With the exception of the AIM model, all measurements and analyses carried out supported the presence of a sufficient covalent-bonding interaction between C2 and C3, even with an interatomic distance beyond 2.0 Å
Summary
The covalent bond is one of the most fundamental concepts in organic chemistry, wherein the strength of a covalent bond depends on the extent of the overlap integral between the orbitals of the atoms sharing the pair of electrons. The upfield position of the signals of C2 and C3, relative to those of other sp[2] carbon atoms, implies that these carbon atoms are not perfectly sp2-hybridized, but bear more p character, as determined by the fact that the 13C chemical shifts of C2 and C3 are comparable to those of sp[3] carbon atoms in other long C−C σ-bonds (Figure 5b).[5,7,13,33,34] It is notable that no signal broadening of the peak at 82.9 ppm was observed, thereby suggesting the small diradical character of 1. As the distance dC2−C3 changes, progressive changes were observed in the HOMO → LUMO transition (Figure 6d and Figure S11), 13C NMR chemical shift (Figure 6e and Figure S12), Wiberg bond index (Figure 6f), singlet−triplet energy gap (Figure 6g), and diradical character (Figure 6h) These results indicate a progressive weaking of the covalentbonding interaction between C2 and C3 upon elongation of the interatomic distance. It is notable that powdered 1 displayed no ESR signal in the temperature range of 90−400 K
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