A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)₃(η⁶-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η⁶-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state.

Two molecules containing two phenylphosphaalkene moieties linked by an anthracene (1) or by a naphthalene (2) ring have been synthesized and their crystal structures have been determined. While electrochemical measurements show that these two systems are easily reduced, EPR spectra indicate that, at room temperature, the electronic structures of the two reduction compounds 1˙^− and 2˙^− are quite different. In 1˙^−, in good accordance with DFT predictions, the unpaired electron is delocalized on the full molecule while in 2˙^− it is confined on a single phosphaalkene moiety. This difference is attributed to the short distance between the two phenylphosphaalkene groups in 2˙^− which hinders their reorientation after addition of an electron. The role of this motion is consistent with the fact that two additional paramagnetic species are detected at 145 K: the dianion 2^2− characterized by a rather small exchange coupling constant and the radical monoanion 2*˙^− resulting from the formation of a one electron P–P bond. These two species are probably reaction intermediates which can lead to the formation of biphosphane.

A series of bis(TTF) donors containing aromatic linkers between the two TTF units has been synthesized in order to investigate on the electronic structure of the oxidized species from an experimental and theoretical point of view. A mono(TTF)-pyridine compound has been also prepared and characterized by single-crystal X-ray diffraction analysis. Oxidation of a solution of 2,6-bis(TTF)-pyridine (TTF-Pyr-TTF) or of 1,3-bis(TTF)-benzene (TTF-Bz-TTF) in CH₂Cl₂ with less than 0.1 equivalent of [Cp₂Fe][PF₆] gives rise to a seven-line EPR spectrum consistent with the hyperfine structure calculated by DFT for the corresponding radical monocation. Increasing the proportion of oxidant leads to a four-line hyperfine structure, similar to the quartet pattern observed after oxidation of mono(TTF)-pyridine (Pyr-TTF) or mono(TTF)-benzene (Bz-TTF). In good accordance with the very weak value of J calculated by DFT for the dicationic biradicals these four-line spectra are attributed to [2,6-bis(TTF)-pyridine]²⁺ and [1,3-bis(TTF)-benzene]²⁺. Similar experimental results are obtained for 1,4-bis(TTF)-benzene. In this case, however, electrochemical oxidation leads to the monoradical at low potential and to the diradical at higher potential, while only the diradical could be observed by electrochemical oxidation of 2,6-bis(TTF)-pyridine or of 1,3-bis(TTF)-benzene. 

A brief historical overview of physical chemistry at the University of Geneva as well as a description of the present research activities at the department of physical chemistry are presented.

A radical species characterized by a large g-anisotropy and a clearly resolved hyperfine structure with ^95/97Mo and ³¹P nuclei is formed, at 77 K, by radiolysis of a single crystal of Mo(CO)₅PPh₃. The corresponding EPR signals disappear irreversibly with increasing temperature and the angular dependence of the various coupling constants imply a spin delocalization of not, vert, ∼60% and not, vert, ∼4% on the molybdenum and the phosphorus atoms, respectively and are, a priori, consistent with the trapping of a one-electron deficient centre. The ability of DFT to predict the EPR tensors for a 17-electron Mo^(I) species is verified by calculating the g-tensor and the various ¹⁴N and ¹³C coupling tensors previously reported by Hayes for [Mo(CN)₅NO]^3-. Calculations at the B3LYP/ZORA/SOMF level of theory show that, in contrast to Mo(CO)₅PH₃, one-electron oxidation of Mo(CO)₅PPh₃ causes an appreciable change in the geometry of the complex. The g-tensor and the ^95/97Mo and ³¹P isotropic and anisotropic coupling constants calculated for [Mo(CO)₅PPh₃]^+· confirm the trapping of this species in the irradiated crystal of Mo(CO)₅PPh₃; they also show that the conformational modifications induced by the electron release are probably hindered by the nearby complexes.

[M(CO)₄PPh₃]^{•−} (M = Mo, W) were trapped at 77 K in X-irradiated single crystals of M(CO)₅PPh₃ and studied by EPR. Structures of [M(CO)₄PPh₃]^{•−} (M = Cr, Mo, W) were optimized by DFT; predicted g and ³¹P-hyperfine tensors agree with experiments for M = Mo, W. The anions adopt a slightly distorted pyramidal structure with PPh₃ in basal position and the spin mostly delocalized in a metal-d_z² orbital and carbon-p_z orbitals of carbonyls. The EPR tensors are slightly modified by annealing, they suggest that new constraints in the matrix distort the structure of [M(CO)₄PPh₃]^{•−} (M = Cr, Mo, W).

Paramagnetic complexes M(CO)₅P(C₆H₅)₂, with M = Cr, Mo, W, have been trapped in irradiated crystals of M(CO)₅P(C₆H₅)₃ (M = Cr, Mo, W) and M(CO)₅PH(C₆H₅)₂ (M = Cr, W) and studied by EPR. The radiolytic scission of a P−C or a P−H bond, responsible for the formation of M(CO)₅P(C₆H₅)₂, is consistent with both the number of EPR sites and the crystal structures. The g and ³¹P hyperfine tensors measured for M(CO)₅P(C₆H₅)₂ present some of the characteristics expected for the diphenylphosphinyl radical. However, compared to Ph₂P^•, the ³¹P isotropic coupling is larger, the dipolar coupling is smaller, and for Mo and W compounds, the g-anisotropy is more pronounced. These properties are well predicted by DFT calculations. In the optimized structures of M(CO)₅P(C₆H₅)₂ (M = Cr, Mo, W), the unpaired electron is mainly confined in a phosphorus p-orbital, which conjugates with the metal d_xz orbital. The trapped species can be described as a transition metal-coordinated phosphinyl radical.

We report the synthesis, crystal structure and electrochemical behaviour of a complex in which the Ph group of the phosphaalkene PhC(H)=PMes* (Mes*: 2,4,6-tri-tert-butylphenyl) is coordinated to a chromium tricarbonyl group. The EPR spectra resulting from electrochemical and chemical reductions are described and the experimental g and hyperfine tensors (³¹P)T, as determined from the EPR data, are compared with those predicted by DFT calculations for the radical anion (Cr(CO)₃, PhC(H)=PMes)^·−. The structural changes caused by the addition of an electron to the neutral complex are described, together with an estimation of the contribution of Cr(CO)₃ to the stabilization of the radical anion.

In many instances, the deduction of spectroscopic parameters from electron paramagnetic resonance spectra depends on spectrum simulation and parameter optimization. We have developed two software packages based on the approximate formulae of Iwasaki for the calculation of line positions and on the Levenberg-Marquardt algorithm for nonlinear least-squares optimization. Our software applies to systems having an anisotropicg-tensor and an arbitrary number of hyperfine interactions with nuclei. They are written in the FORTRAN 77 programming language. At present, neither the nuclear quadrupolar interaction nor the nuclear Zeeman interaction terms are handled. The programs CRISAJU and EPRPOWDERFIT apply to the cases of single crystals and powders, respectively. For use in the latter, thanks to the software ODYSSEE which implements automatic differentiation of algorithms, an ancillary subroutine, which contributes to the performance of the optimization, was created automatically.

Oxidation of the square planar Rh(I) complex [Rh(SPS^Me)(PPh₃)] (SPS^Me = 1-methyl-1-P-2,6-bis(diphenylphosphinosulfide)-3,5-(bisphenyl)-phosphinine) (1) based on mixed SPS-pincer ligand with hexachloroethane yielded the Rh(III) dichloride complex [Rh(SPS^Me)(PPh₃)Cl₂] (2), which was structurally characterized. The homoleptic Rh(III) complex [Rh(SPS^Me)₂][Cl] (4) was obtained via the stoichiometric reaction of SPS^Me anion (3) with [Rh(tht)₃Cl₃] (tht = tetrahydrothiophene). Complex 4, which was characterized by X-ray diffraction, was also studied by cyclic voltammetry. Complex 4 can be reversibly reduced at E = −1.16 V (vs SCE) to give the neutral 19-electron Rh(II) complex [Rh(SPS^Me)₂] (5). Accordingly, complex 5 could be synthesized via chemical reduction of 4 with zinc dust. EPR spectra of complex 5 were obtained after electrochemical or chemical reduction of 4 in THF or CH₂Cl₂. Hyperfine interaction with two equivalent ³¹P nuclei was observed in liquid solution, while an additional coupling with a spin ¹/₂ nucleus, probably ¹⁰³Rh, was detected in frozen solution. The ³¹P couplings are consistent with DFT calculations that predict a drastic increase in the axial P−S bond lengths when reducing (SPS^Me)₂Rh(III). In the reduced complex, the unpaired electron is mainly localized in a rhodium d_z² orbital, consistent with the g-anisotropy measured at 100 K.

The radical cation of the redox active ligand 3,4-dimethyl-3',4'-bis-(diphenylphosphino)-tetrathiafulvalene ( P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S₂C=CS₂ part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2^•+, one-electron oxidation of [Fe( P2)(CO)₃] gives rise to the metal-centered oxidation species [Fe^(I)( P2)(CO)₃], characterized by a coupling with two ³¹P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P–Fe bond is detected. Moreover, in presence of free ligand, [Fe^(I)( P2)(CO)₃] reacts to give the complex [Fe^(I)( P2)₂(CO)] containing two TTF fragments. The two-electron oxidation of [Fe( P2)(CO)₃] leads to decomplexation and to the P2^•+ spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe( P2)(CO)₃] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments.

EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P — P bond (3.479 Å), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P — P distance in neutral 1 is large (5.8 Å). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P — P single bond (P — P 2.389 Å). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C — Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P — P distance. The ability of the phosphinine radical anion to reversibly form weak P — P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P — P bond.

The reduction products of two diphosphaalkenes (1 and 2) and a bis(diphosphene) (3) containing sterically encumbered ligands and corresponding to the general formulas Ar−X==Y−Ar‘−Y==X−Ar, have been investigated by EPR spectroscopy. Due to steric constraints in these molecules, at least one of the dihedral angles between the CXYC plane and either the Ar plane or the Ar‘ plane is largely nonzero and, hence, discourages conformations that are optimal for maximal conjugation of P==X (or P==Y) and aromatic π systems. Comparison of the experimental hyperfine couplings with those calculated by DFT on model systems containing no cumbersome substituents bound to the aromatic rings shows that addition of an electron to the nonplanar neutral systems causes the X==Y−Ar‘−Y==X moiety to become planar. In contrast to 1 and 2, 3 can be reduced to relatively stable dianion. Surprisingly the two-electron reduction product of 3 is paramagnetic. Interpretation of its EPR spectra, in the light of DFT calculations on model dianions, shows that in [3]^2- the plane of the Ar‘ ring is perpendicular to the CXYC planes. Due to interplay between steric and electronic preferences, the Ar−X==Y−Ar‘−Y==X−Ar array for 3 is therefore dependent upon its redox state and acts as a “molecular switch�.

Cyclic voltammetry of Mes*P==C(NMe₂)₂ (1) and Mes*P==C(CH₃)NMe₂ (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. Electrochemical oxidation or reaction of 1 (or 2) with [Cp₂Fe]PF₆ leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental ³¹P and ¹³C isotropic and anisotropic coupling constants agree with density functional theory (DFT) calculations showing that the unpaired electron is strongly localized on the phosphorus atom, in accord with the description Mes*P^•−(C(NMe₂)₂)⁺. Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe₂)₂ moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)P−^•C(NMe₂)₂. No paramagnetic species is obtained by reduction of 2. The presence of the amino groups, responsible for the inverted electron distribution at the P−C double bond (P^-−C⁺), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal Ï€ electron distribution (P⁺−C^-):  no detection of the radical anion but easy formation of a rather persistent radical cation. For 1, this radical cation could even be isolated as a powder, 1^•+PF₆^-. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphorus−carbon bond caused by the presence of the amino groups.

Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) 4 in THF on a potassium mirror leads to EPR/ENDOR spectra characterized by a large coupling (~20 MHz) with two protons, similar to the spectra obtained after reduction of the m-disilylbenzene derivative 5, consistent with a localization of the extra electron on a single ring of 4. The spectra recorded after reduction of 4 at low temperature in the presence of an equimolar amount of 18-crown-6 exhibit couplings of ~10 MHz with four protons and indicate that embedding the counterion in crown-ether provokes the delocalization of the unpaired electron on the two phenyl rings of 4. The measured hyperfine interactions agree with those calculated by DFT for the optimized structure of 4^•-. Direct information on the structure of this anion is obtained from the X-ray diffraction of crystals grown at -18 °C in reduced solutions containing 4, potassium, and crown ether in a THF/hexane mixture. Both DFT and crystal structures clearly indicate the geometry changes caused by the addition of an electron to 4: the interphenyl distance drastically decreases, leading to a partial overlap of the two rings. The structure of 4^•- is a model for an electron transfer (ET) transition state between the two aromatic rings. The principal reason for the adoption of this structure lies in the bonding interaction between the LUMO (π* orbitals) of these two fragments; moreover, the constraints of the macrocycle probably contribute to the stabilization of this structure.

X-irradiation of single crystals of Tp–GeH₃ (Tp: triptycene) led to the trapping of the radical Tp–^√GeH₂. The angular variations of the resulting EPR spectra were recorded at 300 and 77 K. The drastic temperature dependence of the spectra was caused by both a strong anisotropy of the g-tensor and a rotation of the ^√GeH₂ moiety around the C–Ge bond. The determination of the EPR tensors as well as the analysis of this motion required to take the presence of disorder in the crystal into account. In accordance with DFT calculations, Tp–^√GeH₂ is shown to be pyramidal and to adopt, in its lowest energy structure, a staggered conformation. Rotation around the C–GeH₂ bond is blocked at 90 K and is almost free above 110 K. The experimental barrier, obtained after simulation of the EPR spectra as a function of the rotational correlation time, is equal to 1.3 kcal mol^−1, which is slightly inferior to the barrier calculated by DFT (3.6 kcal mol^−1). Calculations performed on Tp–CH₃, Tp–GeH₃ and Tp–^√GeH₂ show that the rotation barrier ΔE_rot around the C–Ge bond drastically decreases by passing from the germane precursor to the germanyl radical and that ΔE_rot increases by passing from the germane to its carbon analogous. Structural parameters involved in these barrier differences are examined.

Electrochemical and chemical reductions of Rh^(I) complexes of L_P4 (a macrocycle containing four phosphinine rings) and of L_P2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with ³¹P nuclei. An additional coupling (27 MHz) with ¹⁰³Rh is detected, in the liquid state, for the spectrum obtained with [L_P2S2Rh⁽⁰⁾]; moreover, resolved ³¹P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh^(I) complexes of model macrocycles L‘_P4 and L‘_P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for [L‘_P2S2Rh⁽⁰⁾], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (Ï�_Rh =0.35 for [L‘_P2S2Rh⁽⁰⁾]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle.

The paper shows the possibilities of the complementary use of the density matrix formalism for the simulation of the anisotropic EPR spectra and the DFT potential energy surface calculations to obtain a detailed picture of the motions of radical molecules. The combined approach is illustrated by a comparative EPR study of three phosphorus derivatives of barrelene. Three compounds were chosen as the model molecules for the observation of different temperature dependent dynamics of radical fragment. Each molecule based on the same barrelene skeleton has a different set of substituents which by influencing the local chemical environment are likely to modify the internal dynamics. The temperature dependent EPR spectra are simulated by means of the density matrix formalism and the geometry of radicals are calculated with DFT. The motion is described in terms of rotational barriers, DFT calculated energy profiles and hypothetical intramolecular distortions. These two approaches lead to a similar microscopic picture of the intramolecular radical motion.

Chemical and electrochemical reductions of the macrocycle 1 lead to the formation of a radical monoanion anion [1]^•- whose structure has been studied by EPR in liquid and frozen solutions. In accord with experimental ³¹P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding σ P−P orbital. Clearly, a one-electron bond (2.763 Ã…) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Ã… apart (crystal structure). A subsequent reduction of this radical anion gives rise to the dianion [1]^2- which could be crystallized by using, in the presence of cryptand, Na naphthalenide as a reductant agent. As shown by the crystal structure, in [1]^2-, the two phosphinine moieties adopt a phosphacyclohexadienyl structure and are linked by a P−P bond whose length (2.305(2) Ã…) is only slightly longer than a usual P−P bond. When the phosphinine moieties are not incorporated in a macrocycle, no formation of any one-electron P−P bond is observed: thus, one-electron reduction of 3 with Na naphthalenide leads to the EPR spectrum of the ion pair [3]^•- Na⁺; however, at high concentration, these ion pairs dimerize, and, as shown by the crystal structure of [(3)₂]^2-[{Na(THF)₂}₂]²⁺ a P−P bond is formed (2.286(2) Ã…) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon−sodium−phosphorus bridges.

The radical anion (tmbp)^•-, where tmbp = 4,4‘,5,5‘-tetramethyl-2,2‘-biphosphinine, was generated by reduction of tmbp on a potassium mirror. EPR/ENDOR spectra and DFT calculations show that, in contrast to the neutral species, this anion is planar and that the unpaired electron is mainly delocalized on the PCCP fragment with a large participation of the phosphorus p_π orbitals. This planar structure was confirmed by the first crystal structure of an anionic biphosphinine:  [tmbp][Li(2.2.1)]. Reduction of [Ni(tmbp)₂] led to the 19-electron complex whose g and ³¹P hyperfine tensors were obtained from EPR in liquid and frozen solutions. These results, together with DFT calculations on [Ni(bp)₂] and [Ni(bp)₂]^•-, indicate that, by accepting an extra electron, the neutral nickel complex distorts toward a more planar geometry and that the dihedral angle between the two phosphinine rings of each ligand slightly increases. In the reduced Ni complex, the unpaired electron is mainly delocalized on the ligands, in a molecular orbital which retains the characteristics of the SOMO found for the reduced isolated ligand. A charge decomposition analysis (CDA) shows that, in [Ni(bp)₂], metal−ligand back-donation strongly contributes to the metal−ligand bonding.

The g, ³¹P and ¹H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier ~= 0.5 kcal mol^−1) which correspond to the conformation of the PH₂ moiety in the only rotamer present in the dibenzobarrelene phosphine crystal.

As shown from the crystal structure, the oxygen atom of Ph₃P=CH---C(O)CH₃ forms both intra and intermolecular hydrogen bonds. X-irradiation of this compounds produces a room-temperature-stable radical which was studied by single crystal EPR/ENDOR spectroscopy. Comparison of the experimental hyperfine couplings with those obtained from ab initio calculations shows that the radical cation Ph₃P⁺---CH=C(OH)CH₂ is formed under radiolysis. The principal directions of the hyperfine tensors indicate that, in this process, some of the hydrogen bonds are broken and that the radical undergoes a drastic reorientation around the Ph₃P---C bond.

A "CO-like matrix", showing coordination analogous to that of carbonyl groups, is provided by silacalix[4]phosphinine macrocycles. Reaction with Au^I leads to the first gold(I) complexes of macrocycles, which can be reduced with sodium or potassium to the paramagnetic gold(0) complexes (an example is shown), as evidenced by cyclic voltammetry and EPR spectroscopy.

A new phosphine, the diphenyldibenzobarrelenephosphine 2, was designed to study the barrier to rotation of the P−H group around the C−^•P bond. After homolytic scission of a P−H bond by radiolysis, the EPR spectrum of the resulting phosphinyl radical, trapped in a single crystal of 2, was studied at 77 K and at room temperature. The directions of the ³¹P hyperfine eigenvectors were compared with the bond orientations of the undamaged compound as determined from its crystal structure. The temperature dependence of the EPR spectrum was analyzed by using the density matrix formalism; this showed that interaction between the phosphinyl hydrogen and the phenyl ring bound to the ethylenic bond is determinant for explaining the potential energy profile. DFT investigations are consistent with these experimental results.

Bis(N-methylimidazolidinethi-2-one)copper(I) chloride has been synthesized and its crystal structure determined. X-Irradiation of a single crystal of this compound leads to the formation of a CuII complex which was studied by EPR: it was shown that this species results from the addition of a radiogenic Cl atom on the CuI precursor. The structural changes induced by this reaction are revealed by the g-tensor and by the hyperfine tensors of one copper and two chlorine nuclei. The structure of this S2Cl2Cu type complex was compared with other sulfur- or chlorine-containing CuII complexes.

Two radiogenic radicals trapped in a single crystal of 1-[2,4,6-tri-tert-butylphenyl]-2-phenylphosphaethene have been studied by EPR and have been identified, from their ³¹P   hyperfine tensors, as being phosphoniumyl radical cations. The spectra modifications caused by ¹³C or ²D enrichment of the phosphaalkene moiety show that these species result from an intramolecular cyclization which can lead to two possible conformations of the radical. The experimental ³¹P, ¹³C, and ¹H hyperfine tensors are compared with those predicted by ab initio calculations on model phosphoniumyl radical cations. These calculations show that these interactions are very sensitive to the geometry of the radical and that their measurement can yield precise structural information.