Fabian Gerson

Electron spin resonance spectroscopy of organic radicals

Preface.Abbreviations and Symbols.A. GENERAL PART.1. Physical Fundamentals of Electron Spin Resonance.2. Paramagnetic Organic Species and Their Generation.3. Electron-Nuclear Magnetic Interaction.4. Spin Density, Spin Population, Spin Polarization, and Spin Delocalization.5. Multiresonance.6. Taking and Analyzing ESR Spectra.B. SPEICAL PART.7. Organic Radicals Centered on One, Two, or Three Atoms.8. Conjugated Hydrocarbon Radicals.9. Conjugated Radicals with Heteroatoms.10. Saturated Hydrocarbon Radicals.11. Biradicals and Triplet-state Molecules.Appendices.References.Index.

Oxidation of phosphines containing two or three tetrathiafulvalene (TTF) or o-dimethyl-TTF moieties. Evidence for formation of radical polycations

Stepwise oxidation of tetrathiafulvalenyl(diphenyl)phosphine (1), 4′,5′-dimethyltetrathiafulvalen-4-yl(diphenyl)phosphine (1a), bis(tetrathiafulvalenyl)phenylphosphine (2), bis(4′,5′-dimethylt-tetrathiafulvalen-4-yl)phenylphosphine (2a), tris(tetrathiafulvalenyl)phosphine (3) and tris(4′, 5′-dimethyltetrathiafulvalen-4-yl)phosphine (3a) was followed by EPR and ENDOR spectroscopy, and cyclic voltammetry. Each of the tetrathiafulvalene (TTF) or 4,5-dimethyltetrathiafulvalene (o-DMTTF) moieties in 1–3a donates two electrons, so that exhaustive oxidation at potentials below 1 V (vs. SCE) leads to the dications 12+ or 1a2+, the tetracations 24+ or 2a4+ and the hexacations 36+ or 3a6+. In 2, 2a, 3 and 3a, the electrons are removed one-by-one from different TTF or o-DMTTF moieties. In the initially formed radical cations 2˙+, 2a˙+, 3˙+ and 3a˙+, the electron hole is delocalized over all TTF or o-DMTTF moieties, two in 2˙+ or 2a˙+ and three in 3˙+ or 3a˙+. On the other hand, in paramagnetic species produced by further oxidation and also giving rise to well defined EPR and ENDOR spectra, the unpaired electron appears on the hyperfine time-scale as localized in only one donor moiety. These species are supposed to be the radical trications 2˙3+ or 2a˙3+ and the radical pentacations 3˙5+ or 3a˙5+. The intermediately occurring dication 22+ or 2a2+ and 32+ or 3a2+, as well as the tetracations 34+ or 3a4+, should have a triplet ground-state, while the trications 33+ or 3a3+ are expected to be quartet species.

The ESR spectrum of the cis-stilbene radical anion

Abstract The ESR spectrum of the cis-stilbene radical anion has been observed under conditions where its conversion into the trans isomer is relatively slow. Assignment of the proton coupling constants has been based on studies on the 4,4′- and 7,7′- dideuterio derivatives, as well as on comparison with structurally related species. The values for the radical anions of cis- and trans-stilbene are very similar, except those (2.68 and 4.49 G, respectively) arising from protons in the 7,7′-positions. The difference is attributed to strong distortions of the π-system in the C1C7′C1′ fragment of the cis isomer.

The radical cation of naphthalene: First correct analysis of its ESR spectrum

Abstract The radical cations of naphthalene (1) and its 1,4-dideutero derivative (1-d2), generated from the corresponding neutral compounds by γ-rays in a CFCl3 matrix at 77 K, have been studied by ESR and ENDOR spectroscopy. At 140 K, the hyperfineanisotropy tensors of the protons in 1+• appear to be quasi-axial, presumably as a consequence of the fast rotation about the twofold symmetry axis perpendicular to the molecular plane. The isotropic coupling constants are −0.74±0.01 and −0.187 ± 0.014 mT for the two sets of four equivalent protons in the 1,4,5,8- and 2,3,6,7-positions, respectively. The relative increase in the coupling constant of the 1,4,5,8-protons on going from the radical anion 1−• (−0.495 mT) to the radical cation 1+• (−0.74 mT) is similar to that found for the 1,4-protons in the corresponding radical ions (2−• and 2+•) of s-trans-buta-1,3-diene (2).

Tricyclopropylamine and Its Radical Cation

A surprisingly high first vertical ionization energy (8.44 eV) and an unusually high oxidation potential are exibited by tricyclopropylamine (1)-in sharp contrast to triisopropylamine. These are attributed to the near-tetrahedral geometry of 1 at the N atom and the perpendicular orientation-with respect to the N-orbital axis-of the cyclopropyl groups. gamma-Irradiation of 1 led to the radical cation 1(.)(+), which, in accord with computations, has a planar C(3h)-symmetrical structure. The EPR-spectroscopic and computational results disclose a dramatic, previously not reported, conformational change on going from 1 to 1(.)(+).

Radical anion of 1,8-bis(dimethylamino)naphthalene (“proton sponge”)

Abstract The ring protons in the radical anion of 1,8-bis(dimethylamino)naphthalene (I) exhibit hyperfine coupling constants which are comparable to the corresponding values for the radical anion of unsubstituted naphthalene. On the other hand, the splittings from the 14 N nuclei and the N-methyl protons are surprisingly small. These findings can be rationalized in terms of the steric hindrance characteristic of the compound 1. An estimate of φ = 60–70° has been made for the dihedral angle between the orbital axes of a nitrogen lone pair and the 2pπ-AO of a substituted carbon atom (1 or 8). The effect of sp 3 -hybridization on the 14 N coupling constant is discussed.

The radical ions of hexacene

Abstract The radical anion and the radical cation of hexacene have been characterized by ESR spectroscopy. The electronic absorption spectrum of the former has been recorded and analyzed. Based on a comparison with photoelectron spectroscopic data, the lowest excited state of the hexacene (and pentacene) radical ions is identified as a non-Koopmans state.

An ESR. and ENDOR. Study of the Radical Anions of Cyano‐ and Nitro‐Monosubstituted Naphthalenes

1H- and 14N-coupling constants have been determined by ESR. and ENDOR. spectroscopy for the radical anions of 1- and 2-cyano-, and 1- and 2-nitronaphthalene.

Radical Anions and Radical Trianions of a Phosphabenzene and Some Diphospha Derivatives of Biphenyl,p-Terphenyl andp,p′-Quaterphenyl: an ESR and ENDOR Study

The radical anions and radical trianions of 2,4,6‐triphenylphosphabenzene (1), 3,3′,5,5′‐tetraphenyl‐4λ3,4′λ3‐diphosphabiphenyl‐1,1′ (2), 3,3″,5,5″‐tetraphenyl‐4λ3,4″λ3‐diphosphaterphenyl‐1,1′:4′,1″ (3) and 3,3‴,5,5‴‐tetraphenyl‐4λ3,4‴λ3‐diphosphaquaterphenyl‐1,1′:4′,4″:1″,1‴ (4) were studied by ESR and (in part) ENDOR spectroscopy. The singly occupied orbital in the radical anions is symmetric with respect to the vertical mirror plane passing through the phosphorus atom in 1·‐ or two such atoms in 2·‐–4·‐. Accordingly, the prominent feature of these species is the large and positive 31P coupling constant (1·‐,+3.31; 2·‐,+2.62; 3·‐,+2.02; and 4·‐,+1.67 mT), which gives rise to a pronounced hyperfine anisotropy. The anions 1·‐–4·‐ can thus be regarded as ‘P‐centred radicals’ (g=2.0045±0.0003). In contrast, the trianions 1·3‐–4·3‐ have to be classified as ‘hydrocarbon radicals’ (g=2.0027±0.0001). For 1·3‐, the hyperfine data could be precisely analysed with the use of the ENDOR technique. As the nodal plane of the singly occupied orbital in this trianion passes through the phosphorus atom, the 31P coupling constant is small and negative (‐0.267 mT); the largest value (‐0.473 mT) belongs to the two protons in the 3,5‐positions of the phosphabenzene ring. The hyperfine patterns of the radical trianions 2·3‐–4·3‐ resemble that of 1·3‐ with the π‐spin population being thus restricted to one phosphabenzene ring and its two phenyl substituents. Such a ‘localization’ is considered to be due to a degeneracy or near‐degeneracy of the singly occupied orbital and to a perturbation caused by the association with three alkali metal counterions.

Powerful Oxidizing Properties of a Planar [8]Annulene: Tetrakis(perfluorocyclobuta)cyclooctatetraene and Its Radical Anion

The central eight-membered ring of tetrakis(perfluorocyclobuta)cyclooctatetraene (1) is flat both in the crystal and in solution, but tetrakis(perfluorocyclopenta)cyclooctatetraene (2) assumes a tub shape similar to that of parent cyclooctatetraene. This sterically induced structural difference strongly affects the physical properties of compounds 1 and 2. The first and second reduction potentials of 1 in solution, +0.79 and +0.14 V vs. SCE, are considerably more positive than the corresponding values, +0.20 and −0.21 V, of 2. Accordingly, mere contact with Hg metal in N,N-dimethylformamide converts 1 to an unusually stable radical anion 1.− that is persistent in air. The temperature-independent coupling constant, aF=+0.320 mT, of the sixteen equivalent β-19F-nuclei in 1.− is much smaller than the expected value, and the g factor of 1.− is markedly lower than those generally found for structurally related radical anions. In contrast, the characteristic data for 2.−, generated from 2 with K metal in 1,2-dimethoxyethane, comply with the expected data. The coupling constants aF=+0.976 and +0.935 mT, each of eight β-19F-nuclei in 2.−, average +0.951 mT upon raising the temperature from 220 to 260 K. The gas-phase electron affinity of 1, A=3.4±0.2 eV, which was estimated from the reduction potential in solution and from charge-transfer absorption spectra obtained from a series of methylated benzenes and naphthalenes, is among the highest known for neutral organic molecules.