The Peculiar Trend of Cyclic Perfluoroalkane Electron Affinities with Increasing Ring Size

Enhanced J-Couplings through Specially Solvated Electron in Perfluoro-[n]Prismanes and [n]Asteranes

Perfluoro-[n]prismanes ((C2F2)n, n = 3-8) and [n]asteranes ((C3F4)n, n = 3-5) exhibit a strong perfluoro cage effect that can stably encapsulate an additional electron inside the cage. The 2s-like distribution of solvated electron (esol-) not only changes the molecular structure but also affects the nuclear spin properties. In this work, we reveal how the esol- enhances and regulates indirect nuclear spin-spin coupling between two coupled F nuclei (JFF-coupling). Results show that esol- is mainly distributed in the central cavity, and a part of it penetrates into the C-shell and C-F bond regions due to the unique polyhedral C-shell structure. Such a 2s-like esol- creates a novel esol- based coupling mechanism, including the newly generated through-esol- (TSE) and esol--enhanced traditional through-bonds and through-space (esol--enhanced TB+TS) pathways, enhancing and regulating N(e)JFF-coupling, which crosses N bonds in the shortest TB pathway and is affected by esol-. The contribution of the TSE (JTSE) is positive and increases with the increase of the central angle between two coupled F nuclei (∠F⊗F), and the contribution of the esol--enhanced TB+TS (JTB+TS) is negative and |JTB+TS| decreases with the increase of N and straight linear distance between two coupled F nuclei (dFF). Interestingly, N(e)JFF exhibits a special dependence on N/dFF and ∠F⊗F due to the cooperation and competition between JTSE and JTB+TS. When ∠F⊗F < 70°, the esol--enhanced TB+TS can play a role; JTB+TS determines sign and magnitude of N(e)JFF. When ∠F⊗F > 70°, the TSE dominates, and JTSE determines sign and magnitude of N(e)JFF. This work not only further enriches information on the states, distributions, and properties of esol- but also provides insights into the nuclei spin properties in perfluorinated polyhedrons triggered by esol-.

About Perfluoropolyhedranes, Their Electron-Accepting Ability and Questionable Supramolecular Hosting Capacity

Polyhedral molecules are appealing for their eye-catching architecture and distinctive chemistry. Perfluorination of such, often greatly strained, compounds is a momentous challenge. It drastically changes the electron distribution, structure and properties. Notably, small high-symmetry perfluoropolyhedranes feature a centrally located, star-shaped low-energy unoccupied molecular orbital that can host an extra electron within the polyhedral frame, thus producing a radical anion, without loss of symmetry. This predicted electron-hosting capacity was definitively established for perfluorocubane, the first perfluorinated Platonic polyhedrane to be isolated pure. Hosting atoms, molecules, or ions in such "cage" structures is, however, all but forthright, if not illusionary, offering no easy access to supramolecular constructs. While adamantane and cubane have fostered numerous applications in materials science, medicine, and biology, specific uses for their perfluorinated counterparts remain to be established. Some aspects of highly fluorinated carbon allotropes, such as fullerenes and graphite, are briefly mentioned for context.

Understanding the Role of Aromaticity and Conformational Changes in Bond Dissociation Processes of Photo-Protecting Groups

Photoremovable protecting groups (PPGs) provide spatial and temporal control over the release of various chemicals. Using surface hopping studies with multireference electronic structure methods we have unravelled the nuclear and the electronic events at play. Furthermore, the electronic changes along the reaction path were probed using excited state aromaticity quantifiers and orbital analysis. We find that upon irradiation with light of appropriate wavelength on the substituted coumarin system a π-π* electronic excitation occurs which is followed by an electron loss from the aromatic ring on gaining proper alignment between the π* and the C-LG (LG = leaving group) σ*. This alignment is brought about by a critical dihedral angle change in the molecule, which subsequently triggers C-LG bond cleavage. The sequence of events is indicative of an intramolecular electron catalyzed process which is established through investigations of changes in aromaticity of the phenyl ring which acts as an electron reservoir.

Enthalpic and entropic contributions to the basicity of cycloalkylamines

Large-ring cycloalkylamines are slightly less basic than other cycloalkylamines such as cyclohexylamine, even though all have tetrahedral carbons and are strain-free. To understand why, enthalpy and entropy for protonation of a series of cycloalkylamines were accurately determined by isothermal titration calorimetry in 3 : 1 methanol–water. The study required resolving a discrepancy between these measurements and those in pure water. The data show that the lower basicity of large-ring cycloalkylamines is not due to enthalpy but to a more negative entropy of protonation. Computations show that this can be attributed in part to an entropy of conformational mixing, but the dominant contribution is steric hindrance to solvation, also corroborated by computation.

Large-ring cycloalkylamines are slightly less basic than other cycloalkylamines such as cyclohexylamine, even though all have tetrahedral carbons and are strain-free.

Unusual Indirect Nuclear Spin-Spin Exchange Coupling through Solvated Electron

Solvated electrons have been found to exist in various media which also exhibit more intriguing properties such as superconductivity, nonlinear optical response, and so on. However, how they affect the nuclear spin properties has not been proven. In this work, we present the first detailed study on solvated-electron-triggered indirect nuclear spin-spin J-coupling using density functional theory calculations. Taking ¹⁹F as a probe, we verify the presence of unusual J couplings between two distant F atoms in HF-containing anionic clusters. These couplings occur "through solvated electron", rather than through conventional covalent bonds or space. Solvated electron can serve as an additional channel to efficiently realize long-range J-coupling between far separated nuclei because of its dispersivity and Rydberg character. The coupling magnitude strongly depends on the unique distribution of solvated electron and its second-order interaction with solvating HF units. This work provides novel insights into the mediating roles of electrons, possibly opening up potential applications based on weakly bound electrons.

Synthesis of low-valent uranium fluorides by C–F bond activation

The uranium(iii) alkyl, Tp*₂UCH₂Ph (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), activates C–F bonds on a variety of fluorinated substrates, generating Tp*₂UF or Tp*₂UF₂.

Aromatic C20F20 cage and its endohedral complexes X@C20F20 (X = H−, F−, Cl−, Br−, H, He)

The structure and stability of endohedral X@C20F20 complexes (X = H-, F-, Cl-, Br-, H, He) have been computed at the B3LYP level of theory. All complexes in I (h) symmetry were found to be energy minimum structures. H-@C20F20 and F-@C20F20 complexes have negative inclusion energies, while other complexes have positive inclusion energies. Similarity between C20F20 and C20H20 has been found for X = H and He. On the basis of the computed nucleus independent chemical shift values at the cage center, both C20F20 and C20F20 are aromatic.

Structures of the Hexafluorocyclopropane, Octafluorocyclobutane, and Decafluorocyclopentane Radical Anions Probed by Experimental and Computational Studies of Anisotropic Electron Spin Resonance (ESR) Spectra

Anisotropic electron spin resonance (ESR) spectra are reported for the radical anions of hexafluorocyclopropane (c-C(3)F(6)(-)), octafluorocyclobutane (c-C(4)F(8)(-)), and decafluorocyclopentane (c-C(5)F(10)(-)) generated via gamma-irradiation in plastically crystalline tetramethylsilane (TMS) and rigid 2-methyltetrahydrofuran (MTHF) matrices. By combining the analysis of these experimental ESR spectra involving anisotropic hyperfine (hf) couplings with a series of quantum chemical computations, the geometrical and electronic structure of these unusual perfluorocycloalkane radical anions have been characterized more fully than in previous studies that considered only the isotropic couplings. Unrestricted Hartree-Fock (UHF) computations with the 6-311+G(d,p) basis set predict planar ring structures for all three radical anions, the ground electronic states being (2)A(2)(") for c-C(3)F(6)(-) (D(3h) symmetry), (2)A(2u) for c-C(4)F(8)(-) (D(4h)), and (2)A(2)(") for c-C(5)F(10)(-) (D(5h)), in which the respective six, eight, and ten 19F-atoms are equivalent by symmetry. A successful test of the theoretical computation is indicated by the fact that the isotropic 19F hf couplings computed by the B3LYP method with the 6-311+G(2df,p) basis set for the optimized geometries are in almost perfect agreement with the experimental values: viz., 19.8 mT (exp) vs 19.78 mT (calc) for c-C(3)F(6)(-); 14.85 mT (exp) vs 14.84 mT (calc) for c-C(4)F(8)(-); 11.6 mT (exp) vs 11.65 mT (calc) for c-C(5)F(10)(-). Consequently, the same computation method has been applied to calculate the almost axially symmetric anisotropic 19F hf couplings for the magnetically equivalent 19F atoms: (-4.90 mT, -4.84 mT, 9.75 mT) for c-C(3)F(6), (-3.54 mT, -3.48 mT, 7.02 mT) for c-C(4)F(8)(-), and (-2.62 mT, -2.56 mT, 5.18 mT) for c-C(5)F(10)(-). ESR spectral simulations performed using the computed principal values of the hf couplings and the spatial orientations of the 19F nuclei as input parameters reveal an excellent fit to the experimental anisotropic ESR spectra of c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-), thereby providing a convincing proof of the highly symmetric D(nh) structures that are predicted for these negative ions. Furthermore, using the computed 19F principal values and their orientations, the effective 19F anisotropic hf couplings along the molecular symmetry axes were evaluated for c-C(3)F(6)(-) and c-C(4)F(8)(-) and successfully correlated with the positions of the characteristic outermost features in both the experimental and calculated anisotropic spectra. In addition, the electronic excitation energies and oscillator strengths for the c-C(3)F(6)(-) , c-C(4)F(8)(-), and c-C(5)F(10)(-) radical anions were computed for the first time using time-dependent density functional theory (TD-DFT) methods.

On the Electronic Origin of Strain Energy: QTAIM Study of Perfluorocycloalkanes

The strain energies (SE) of the five smallest perfluorocycloalkanes (c-CnF2n; n=3, 4, 5, 6, and 7) were calculated by means of several homodesmotic processes using B3LYP/6-31++G(d,p) optimized molecular energies. These values were compared with the energy difference between the linear and cyclic CF2 groups calculated by means of the quantum theory of atoms in molecules (QTAIM) applied on charge densities obtained at the same computational level. The differences between the values computed with both methods vary from 255 (n=3) to 629 (n=7) kJ mol-1. These differences arise because QTAIM-computed SE contain the energy involved in opening the ring to give rise to nearly transferable central CF2 fragments of linear perfluoroalkanes, whereas homodesmotic energies contain energy terms corresponding to transformation of nontransferable linear CF2 fragments and a ring-opening energy, which depending on the process, transforms CF2 cyclic compounds into nearly transferable or nontransferable linear CF2 groups.

Structures of Tetrafluorocyclopropene, Hexafluorocyclobutene, Octafluorocyclopentene and Related Perfluoroalkene Radical Anions Revealed by Electron Spin Resonance Spectroscopic and Computational Studies

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C(4)F(6)(-)), octafluorocyclopentene (c-C(5)F(8)(-)) and perfluoro-2-butene (CF(3)CF=CFCF(3)(-)) in gamma-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d(12) matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C(4)F(6)(-) and c-C(5)F(8)(-) are characterized by three different sets of pairs of (19)F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C(4)F(6)(-) and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C(5)F(8)(-). By comparison with the results of ab initio quantum chemical computations, the large triplet (19)F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the C=C bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C(3)F(4)(-): C(2) symmetry ((2)A state) <-- C(2)(v) ((1)A(1)), c-C(4)F(6)(-): C(1) ((2)A) <-- C(2)(v) ((1)A(1)) and c-C(5)F(8)(-): C(1) ((2)A) <-- C(s) ((1)A'). The structural distortion arises from a mixing of the pi* and higher-lying sigma* orbitals at the C=C carbons similar to that previously found for CF(2)=CF(2)(-) with a C(2)(h) distortion. The isotropic (19)F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C(4)F(6)(-), c-C(5)F(8)(-) and CF(2)=CF(2)(-) were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of (19)F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF(2)=CF(2)(-), c-C(3)F(4)(-), c-C(4)F(6)(-) and c-C(5)F(8)(-) radical anions were computed for the first time by TD-DFT methods.

Reliable Electron Affinities of Perfluorocyclopropane and Perfluorocyclobutane from Convergent ab Initio Computations

To resolve discrepancies concerning the magnitude of the electron affinities of perfluorocyclopropane and perfluorocyclobutane, quantum chemical calculations have been carried out with the MP2 and CCSD(T) methods in conjunction with augmented correlation consistent basis sets (aug-cc-pVX Z, X = D, T, Q). Though no experimental values have been found for perfluorocyclopropane, we estimate its electron affinity to be 0.17 eV (0.00 eV without zero-point vibrational energy corrections). In addition, determination of the electron affinity of perfluorocyclobutane (0.61 and 0.44 eV with and without zero-point vibrational energy corrections, respectively) is in good agreement with experimental values reported by Miller and co-workers (0.63 +/- 0.05 eV). This study also demonstrates that the widely prescribed B3LYP/DZP++ model chemistry for computing electron affinities does not correctly describe these systems.