Variable-Temperature ¹⁹F NMR and Theoretical Study of 1,9- and 1,7-C₆₀F(CF₃) and C_s- and C₁-C₆₀F₁₇(CF₃):  Hindered CF₃ Rotation and Through-Space J_FF Coupling

Abstract

Milligram amounts of the new compounds 1,9- and 1,7-C60F(CF3) (ca. 85:15 mixture of isomers) and C60F3(CF3) were isolated from a high-temperature C60/K2PtF6 reaction mixture and purified to 98 mol % compositional purity by two-dimensional high-performance liquid chromatography using Buckyprep and Buckyclutcher columns. The previously observed compounds C60F5(CF3) and C60F7(CF3) were also purified to 90+ mol % for the first time. Variable-temperature 19F NMR spectra of the mixture of C60F(CF3) isomers and the previously reported mixture of C(s)- and C1-C60F17(CF3) isomers demonstrate for the first time that fullerene(F)n(CF3)m derivatives with adjacent F and CF3 substituents exhibit slow-exchange limit hindered CF3 rotation spectra at -40 +/- 10 degrees C. The experimental and density functional theory (DFT) predicted deltaH++ values for CF3 rotation in 1,9-C60F(CF3) are 46.8(7) and 46 kJ mol(-1), respectively. The DFT-predicted deltaH++ values for 1,7-C60F(CF3), C(s)-C60F17(CF3), and C1-C60F17(CF3) are 20, 44, and 54 kJ mol(-1), respectively. The (> or = 4)J(FF) values from the slow-exchange-limit 19F spectra, which vary from ca. 0 to 48(1) Hz, show that the dominant nuclear spin-spin coupling mechanism is through-space coupling (i.e., direct overlap of fluorine atom lone-pair orbitals) rather than coupling through the sigma-bond framework. The 2J(FF) values within the CF3 groups vary from 107(1) to 126(1) Hz. Collectively, the NMR data provide an unambiguous set of (> or = 4)J(FF) values for three different compounds that can be correlated with DFT-predicted or X-ray diffraction derived distances and angles and an unambiguous set of 2J(FF) values that can serve as an internal standard for all future J(FF) calculations.

Synthesis and molecular structures of heptafluoroisopropylated fullerenes: C60(i-C3F7)8, C60(i-C3F7)6, and C60(CF3)2(i-C3F7)2

“…Difluoride, C 60 F 2 , was shown to be a 1,9 isomer with two F atoms in ortho position on a 6:6 junction [21,22]. For a mixed fluorotrifluoromethyl compound, C 60 F(CF 3 ), DFT calculations predict even higher stability of the 1,9 isomer over the 1,7 one (with addends in a para position), but the energy difference is as low as 9.8 kJ mol À1 [23]. In the case of C 60 (CF 3 ) 2 , 1,7 isomer becomes 34.7 kJ mol À1 more stable than the 1,9 isomer due to steric reasons.…”

Synthesis, Characterization, and Theoretical Study of Stable Isomers of C70(CF3)n (n = 2, 4, 6, 8, 10)

“…[29] Fullerene C 70 A C H T U N G T R E N N U N G (CF 3 ) 8 : Two isomers were isolated and studied by 1D and 2D 19 F NMR spectroscopy. Both exhibit NMR spectra with four CF 3 multiplets, only one of which is a simple quartet.…”

“…The J F,F coupling between CF 3 groups in meta-and para-C 6 A C H T U N G T R E N N U N G (CF 3 ) 2 hexagons on fullereneA C H T U N G T R E N N U N G (CF 3 ) n derivatives is mediated predominantly, if not almost exclusively, by through-space Fermi-contact coupling, as discussed elsewhere. [14,29] This coupling is believed to be due to the overlap of lone-pair electrons between pairs of proximate fluorine atoms. [30] In the case of fullereneA C H T U N G T R E N N U N G (CF 3 ) n compounds, a single fluorine atom from a rapidly rotating CF 3 group is positioned over the C 6 A C H T U N G T R E N N U N G (CF 3 ) 2 hexagon, putting it in close contact (ca.…”

1H–1H scalar coupling across two stacked aromatic rings: DFT calculations and experimental proof

“…[3] Moreover, throughspace scalar couplings associated with van der Waals dimers and CH/π interacting systems have been calculated by Bagno and coworkers. [4] In his article, Bagno states that the results obtained by ab initio and density functional theory (DFT) calculation could be confirmed experimentally studying inclusion compounds or conformationally restricted species.…”