3,4,5,6-Tetrafluorophenylnitren-2-yl: A Ground-State Quartet Triradical

Recent advances in chemistry of high-spin nitrenes

Experimental and theoretical studies on aromatic nitrenes bearing from three to six unpaired electrons and having quartet, quintet, sextet or septet ground spin states, published in the last 15 years are analyzed. A comparative analysis of the magnetic properties of high-spin nitrenes and all other known high-spin organic molecules is performed. Promising areas of practical application of high-spin nitrenes as molecular magnets and as qubits and qudits for quantum computations are discussed.

The bibliography includes 214 references.

Influence of molecular design on radical spin multiplicity: characterisation of BODIPY dyad and triad radical anions

A strategy to create organic molecules with high degrees of radical spin multiplicity is reported in which molecular design is correlated with the behaviour of radical anions in a series of BODIPY dyads. Upon reduction of each BODIPY moiety radical anions are formed which are shown to have different spin multiplicities by electron paramagnetic resonance (EPR) spectroscopy and distinct profiles in their cyclic voltammograms and UV-visible spectra. The relationship between structure and multiplicity is demonstrated showing that the balance between singlet, biradical or triplet states in the dyads depends on relative orientation and connectivity of the BODIPY groups. The strategy is applied to the synthesis of a BODIPY triad which adopts an unusual quartet state upon reduction to its radical trianion.

Photodissociation at various wavelengths: fragmentation studies of oxazine 170 using nanosecond laser pulses

The fragmentation of oxazine 170, a rhodamine-type dye, has been investigated by means of collisions and photodissociation with visible and ultraviolet radiation in a Fourier transform ion cyclotron resonance mass spectrometer. Because of an improved experimental setup, the photodissociation processes of stored ions are measured with high intensity with respect to the absorbed photons. By isotope labelling and quantum chemical calculations, the various fragmentation mechanisms are investigated. It is shown that the most important intermediate ion structure leading to the various ionic products is an even-electron azarine cation. Several new fragmentation mechanisms have been unveiled for the first time.

Triradicals

Consistent with the definition of diradicals, triradicals arespecies in which three electrons occupy three (nearly) degenerate orbitals, resulting in close-lying quartet and doublet states. The same concepts and rules that can be used to predict and rationalize the ground state multiplicity of diradicals also apply to triradicals, but the greater number of states in triradicals generally leads to more complex electronic structures. Most experimentally accessible triradicals are based onorganic π-systems; therefore triradicals are classified according to the σ/π symmetry of the nominally nonbonding molecular orbitals (NBMOs). The tridehydrobenzenes are prototypal σσσ triradicals with doublet ground states and significant doublet-quartet energy splittings. In all three isomers, the ordering of electronic states depends critically on the distance between the m-radical centers, which makes computational studies of these systems demanding. The experimental IR spectrum of matrix-isolated 1,2,3-tridehydrobenzene led to a revision of the previous ground state assignment based on computations. This work demonstrates the close interplay between experiment and theory in this realm of reactive intermediate chemistry. 1,3,5-Tridehydrobenzene can be isolated as its trifluoro derivative. The stabilization of dehydrophenyl nitrenes, typical members of the σσπ family of triradicals, also requires ortho-fluorination. Because of their quartet ground states, derivatives of 2-dehydrophenyl nitrene and 4-dehydrophenyl nitrene could be studied using IR or EPR spectroscopy. The zero-field splitting parameters of these systems provide direct evidence for the contribution of carbenoid resonance structures to the resonance hybrid of the high-spin systems. According to computations, the through-bond coupling of the in-plane electrons thermodynamically stabilizes the doublet ground states of m-dehydrophenyl nitrenes. But for the same reasons, these systems are prone to ring-opening reactions, which make them difficult to isolate. Remarkably, the m-phenylene unit leads to strongly antiferromagnetic coupling in σσπ triradicals, while o- or p-coupling results in high-spin systems. The more common all-π systems show the opposite pattern because the latter connectivity naturally results in closed-shell arrangements. Within the family of σππ triradicals, we could characterize 2-dehydro-m-xylylene and 4-dehydro-m-xylylene by EPR spectroscopy, whereas the 5-isomer features a doublet ground state. This observation is readily rationalized considering the nodal characteristics of the NBMOs involved and by simple spin polarization models. 1,3,5-Trimethylenebenzene strongly prefers ferromagnetic coupling and features a robust quartet ground state. We have synthesized this πππ triradical in cryogenic matrices and characterized it by IR and EPR spectroscopy. Interestingly, the triradical is photochemically much more stable than m-xylylene, a diradical that shows fascinating rearrangements upon irradiation in cryogenic matrices.

An ab initio MO study of heavy atom effects on the zero-field splitting tensors of high-spin nitrenes: how the spin–orbit contributions are affected

The CASSCF and hybrid CASSCF–MRMP2 methods reproduce the ZFS tensors of spin-septet 2,4,6-trinitrenopyridines, focusing on the heavy atom effects on the spin–orbit terms of the tensors.

Photochemistry of furyl- and thienyldiazomethanes: spectroscopic characterization of triplet 3-thienylcarbene

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (1), matrix isolated in Ar or N(2) at 10 K, yields triplet 3-thienylcarbene (13) and α-thial-methylenecyclopropene (9). Carbene 13 was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (s-E and s-Z) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|D/hc| = 0.508 cm(-1), |E/hc| = 0.0554 cm(-1); |D/hc| = 0.579 cm(-1), |E/hc| = 0.0315 cm(-1)). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in D values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the s-Z rotamer of 13 and the antibonding σ orbital between sulfur and the neighboring carbon-an interaction that is not observed in the s-E rotamer of 13. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (13) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ(max) = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (2), 3-furyldiazomethane (3), or 2-furyldiazomethane (4) provided further insight into the photochemical interconversions among C(5)H(4)S or C(5)H(4)O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (11), 3-furylcarbene (23), or 2-furylcarbene (22), respectively).

Is the spin-orbit coupling important in the prediction of the 51V hyperfine coupling constants of V(IV) O2+ species? ORCA versus Gaussian performance and biological applications

Density functional theory calculations of the (51)V hyperfine coupling (HFC) tensor A, have been completed for eighteen V(IV)O(2+) complexes with different donor set, electric charge and coordination geometry. A tensor was calculated with ORCA software with several functionals and basis sets taking into account the spin-orbit coupling contribution. The results were compared with those obtained with Gaussian 03 software using the half-and-half functional BHandHLYP and 6-311g(d,p) basis set. The order of accuracy of the functionals in the prediction of A(iso), A(z) and dipolar term A(z,anis) is BHandHLYP > PBE0 >> B3PW > TPSSh >> B3LYP >> BP86 > VWN5 (for A(iso)), BHandHLYP > PBE0 >> B3PW > TPSSh > B3LYP >> BP86 > VWN5 (for A(z)), B3LYP > PBE0 ∼ B3PW ∼ BHandHLYP >> TPSSh > BP86 ∼ VWN5 (for A(z,anis)). The good agreement in the prediction of A(z) with BHandHLYP is due to a compensation between the overestimation of A(iso) and underestimation of A(z,anis) (A(z) = A(iso) + A(z,anis)), whereas among the hybrid functionals PBE0 performs better than the other ones. BHandHLYP functional and Gaussian software are recommended when the V(IV)O(2+) species contains only V-O and/or V-N bonds, whereas PBE0 functional and ORCA software for V(IV)O(2+) complexes with one or more V-S bonds. Finally, the application of these methods to the coordination environment of V(IV)O(2+) ion in V-proteins, like vanadyl-substituted insulin, carbonic anhydrase, collagen and S-adenosylmethionine synthetase, was discussed.

Electron Paramagnetic Resonance Spectroscopic Characterization of α,2- and α,4-Didehydrotoluene

The elusive diradicals α,2- and α,4-didehydrotoluene 1 and 3, have been generated by photolysis of matrix-isolated 2-iodobenzyl iodide 7 and 4-iodobenzyl iodide 8, respectively. Diradical 3 could also be synthesized by flash vacuum thermolysis of 8, with subsequent trapping of the products in argon at 5 K. The diradicals in their triplet ground states were characterized by electron paramagnetic resonance spectroscopy. The triplet ground states were in accordance with previous theoretical calculations for these species.