Conflicting Observations Resolved by a Far IR and UV/Vis Study of the NO3 Radical

Codeposition of NO and O ₂ diluted in Ne at 6 K yield a ON???OO complex that exhibit strong UV absorption. This complex is converted into NO₃ radicals by UV irradiation, and is regenerated by radiation of visible light (see spectra).

     

EPR spectroscopy of multicomponent,multispin molecular system obtained by the photolysis of 2,4,6-triazido-3-cyano-5-fluoropyridine in solid argon

Complex multicomponent, multispin molecular system, consisting of a septet trinitrene, two quintet dinitrenes, and three triplet mononitrenes, was obtained by the photolysis of 2,4,6-triazido-3-cyano-5-fluoropyridine in solid argon. To identify these paramagnetic products, electron paramagnetic resonance spectroscopy in combination with line-shape spectral simulations and density functional theory calculations was used. The products of the photolysis was found to be triplet 2,4-diazido-3-cyano-5-fluoropyridyl-6-nitrene (D_T = 1.000 cm⁻¹, E_T = 0), triplet 2,4-diazido-3-cyano-5-fluoropyridyl-2-nitrene (D_T = 1.043 cm⁻¹, E_T = 0), triplet 2,6-diazido-3-cyano-5-fluoropyridyl-4-nitrene (D_T = 1.128 cm⁻¹, E_T = 0 cm⁻¹), quintet 4-azido-3-cyano-5-fluoropyridyl-2,6-dinitrene (D_Q = 0.211 cm⁻¹, E_Q = 0.0532 cm⁻¹), quintet 2-azido-3-cyano-5-fluoropyridyl-4,6-dinitrene (D_Q = 0.208 cm⁻¹, E_Q = 0.0386 cm⁻¹), and septet 3-cyano-5-fluoropyridyl-2,4,6-trinitrene (D_S = −0.1017 cm⁻¹, E_S = −0.0042 cm⁻¹) in a 38:4:7:22:14:4 ratio, respectively.     

Synthesis and EPR/UV/Vis‐NIR Spectroelectrochemical Investigation of a Persistent Phosphanyl Radical Dication

The reaction of the bis(imidazoliumyl)‐substituted P^I cation [(2‐Im^Dipp)P(4‐Im^Dipp)]⁺ ( 10 ⁺) (2‐Im=imidazolium‐2‐yl; 4‐Im=imidazolium‐4‐yl; Dipp=2,6‐di‐isopropylphenyl) with trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) yields the corresponding protonated [(2‐Im^Dipp)PH(4‐Im^Dipp)]²⁺ ( 11 ²⁺) and methylated [(2‐Im^Dipp)PMe(4‐Im^Dipp)]²⁺ ( 12 ²⁺) dications, respectively. EPR/UV/Vis‐NIR spectroelectrochemical investigation of the low‐coordinated P^I cation 10 ⁺ predicted a stable and “bottleable” P‐centered radical dication [(2‐Im^Dipp)P(4‐Im^Dipp)]^2+. ( 13 ^2+.). The reaction of 10 ⁺ with the nitrosyl salt NO[OTf] yields the persistent phosphanyl radical dication 13 ^2+. as triflate salt in crystalline form. Quantum chemical investigation revealed an exceptional high spin density at the P atom.     

Functionalization of heteropolyanions-osmium and rhenium nitrido derivatives of Keggin- and Dawson-type polyoxotungstates: synthesis, characterization and multinuclear ((183)W, (15)N) NMR, EPR, IR, and UV/Vis fingerprints

Reaction of K(10)[alpha(2)-P(2)W(17)O(61)] or K(10)[alpha(1)-P(2)W(17)O(61)] or [Bu(4)N][OsCl(4)N] in a water/methanol mixture, and subsequent precipitation with (Bu(4)N)Br provided [alpha(2)-P(2)W(17)O(61){Os(VI)N}](7-) and [alpha(1)-P(2)W(17)O(61){Os(VI)N}](7-) Dawson structures as tetrabutylammonium salts. Reactions of [(Bu(4)N)(4)][alpha-H(3)PW(11)O(39)] with either [ReCl(3)(N(2)Ph(2))(PPh(3))(2)] or [Bu(4)N][ReCl(4)N] are alternatives to the synthesis of [(Bu(4)N)(4)][alpha-PW(11)O(39){Re(VI)N}]. (183)W and (15)N NMR, EPR, IR, and UV-visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu(4)N)(4)][alpha-PW(11)O(39){Os(VI)N}] Keggin derivative.     

EPR spectroscopy of quintet 4-amino-3,5-dichloropyridine-2,6-diyldinitrene isolated in solid argon

An EPR spectrum of solid Ar isolated quintet 4-amino-3,5-dichloropyridin-2,6-diyldinitrene that formed by the photolysis of 4-amino-2,6-diazido-3,5-dichloropyridine at 15 K was recorded. Using computer simulation based on numerical diagonalization of the quintet spin Hamiltonian matrices, it was established that this EPR spectrum corresponds to a quintet spin state with the magnetic parameters g = 2.0023, |D _q| = 0.2100 cm⁻¹, and |E _q| = 0.0560 cm⁻¹. Owing to high resolution of the experimental spectrum, the zero-field splitting parameters of the quintet intermediate were determined to an accuracy of at least 5·10⁻⁴ cm⁻¹. Calculations of the fine-structure energy levels in external magnetic field and the dependences of the EPR signal positions and intensities of the quintet dinitrene on the direction of external magnetic field were performed for the first time. This allowed unambiguous assignment of all EPR lines of quintet molecules having both in-principal-axis and off-principal-axis orientations. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2284–2289, December, 2007.     

Molecular Magnets: The Synthesis and Characterization of High-Spin Nitrenes

Among all C-, N-, and O-centered polyradicals, high-spin nitrenes possess the largest magnetic anisotropy and are of considerable interest as multi-level molecular spin systems for exploration of organic molecular magnetism and quantum information processing. Although the first representatives of quintet and septet nitrenes were obtained almost 50 years ago, the experimental and theoretical studies of these highly reactive species became possible only recently, owing to new achievements in molecular spectroscopy and computational chemistry. Meanwhile, dozens of various quintet dinitrenes and septet trinitrenes were successfully characterized by IR, UV/Vis, and EPR spectroscopy, thus providing important information about the electronic structure, magnetic properties and reactivity of these compounds.     

Infrared, optical absorption, and EPR spectroscopic studies on natural gypsum

The natural gypsum has been investigated by infrared, Raman, X-ray diffraction, optical absorption and electron paramagnetic resonance spectroscopy. The fundamental stretching and bending vibrations observed in the infrared region for SO₄²⁻ and H₂O are compared with the near-infrared overtones and combinations of these vibrations. MIR and Raman spectral features are attributed to sulfate fundamentals and lattice vibrations of H₂O, SO₄²⁻. The charge transfer and ligand field transition bands were observed near 490, 630, and 800–900 nm and were compared to those of iron oxides. The optical absorption spectrum indicates the presence of ferric and ferrous ions in the mineral. The site symmetry of Fe(III) in the sample is tetragonally distorted. EPR results indicate the presence of the ferric ion in a tetragonally distorted state.     

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

The photochemistry of 2‐iodo‐3,4,5,6‐tetrafluorophenyl azide ( 7 d ) has been investigated in argon and neon matrices at 4 K, and the products characterized by IR and EPR spectroscopy. The primary photochemical step is loss of a nitrogen molecule and formation of phenyl nitrene 1 d . Further irradiation with UV or visible light results in mixtures of 1 d with azirine 5 d ′, ketenimine 6 d ′, nitreno radical 2 d , and azirinyl radical 9 . The relative amounts of these products strongly depend on the matrix and on the irradiation conditions. Nitreno radical 2 d with a quartet ground state was characterized by EPR spectroscopy. Electronic structure calculations in combination with the experimental results allow for a detailed understanding of the properties of this unusual new type of organic high‐spin molecules.     

Generation and EPR Spectroscopy of the First Silenyl Radicals,R2C=Si.−R: Experiment and Theory

The first two persistent silenyl radicals (R₂C=Si^.?R), with a half‐life (t_1/2) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(²⁹Si_α)=137.5–148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (?C=Si?R=134.7–140.7°). In contrast, the analogous vinyl radical, R₂C=C^.?R (t_1/2≈3 h), exhibits a small hfcc (a(¹³C_α)=26.6 G) and has a nearly linear geometry (?C=C?R=168.7°).     

IR spectroscopic study of thioacrolein and its photoisomerization to methylthioketene

The molecules of thioacrolein (1) and metylthioketene (3), which are labile under normal conditions, have been studied by the matrix Fourier transform infrared spectroscopy method. Compound1 was obtained by vacuum pyrolysis (1120 K, 10^–4 Torr) of diallyl sulfide. The analysis of its IR spectrum shows that1 exists as a mixture ofs-trans ands-cis conformers. UV irradiation ( > 248 nm) of matrix isolated1 results in a 1,3 hydrogen shift and the formation of3, which is characterized by a strong band of antisymmetric stretching vibrations of the thioketene fragmentvC=C=S at 1777.2 cm^–1. The experimental IR bands of molecules1 and3 were assigned to fundamental modes. The relatively low frequencies of the stretching vibrations of the double bonds (vC=C at 1598.0 cm^–1 andvC=S at 1071.8 cm^–1) and the heightened frequency of the C-C single bond stretching (vC-C 1173.7 cm^–1) in the spectrum of1 indicate appreciable delocalization of the electron density in the conjugated -orbital system.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 464–470, March, 1995.     

EPR studies of amine radical cations, part 1: thermal and photoinduced rearrangements of n-alkylamine radical cations to their distonic forms in low-temperature freon matrices

The thermal and photochemical transformations of primary amine radical cations (n-propyl 1.+, n-butyl 5.+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spectroscopy. Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterioamines. The identifications were supported by quantum chemical calculations on the geometry, electronic structure, hyperfine splitting constants and energy levels of the observed transient radical species. The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of 1.+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation .C(2CH2CH2NH3+. By comparison, the formation of the corresponding .CH2CH2CH2CH2NH3+ species from 5.+ is considerably more facile and already occurs within the short irradiation time. These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines, a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines. The greater ease of the thermal rearrangement of 5.+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hydrogen shifts, the lower barrier for the former being associated with minimisation of the ring strain in a six-membered transition state. For 1.+, the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to approximately 350 nm light, although there is also evidence for the 1,3-hydrogen shift requiring a higher energy. A more surprising result is the photochemical formation of the H2C=N. radical as a minor product under hard-matrix conditions in which diffusion is minimal. It is suggested that this occurs as a consequence of the beta-fragmentation of 1.+ to the ethyl radical and the CH2=NH2+ ion, followed by consecutive cage reactions of deprotonation and hydrogen transfer from the iminonium group. Additionally, secondary ion-molecule reactions were studied in CFCl2CF2Cl under matrix conditions that allow diffusion. The propane-1-iminyl radical CH3CH2CH=N. was detected at high concentrations of the n-propylamine substrate. Its formation is attributed to a modified reaction sequence in which 1.+ first undergoes a proton transfer within a cluster of amine molecules to yield the aminyl radical CH3CH2CH2N.H. A subsequent disproportionation of these radicals can then yield the propane-1-imine precursor CH3CH2CH=NH, which is known to easily undergo hydrogen abstraction from the nitrogen atom. The corresponding butane-1-iminyl radical was also observed.     

Properties of Triplet Tetrachloro-4-pyridylnitrene Isolated in Inert Matrices

Photolysis of 4-azidotetrachloropyridine isolated in inert matrices at low temperatures leads to formation of triplet tetrachloro-4-pyridylnitrene as the sole product of the photochemical reaction. In the EPR spectrum, this nitrene gives an intense signal and is characterized by zero-field splitting parameters. The UV spectrum of nitrene in solid argon contains a series of high-intensity and medium-intensity absorption bands. The IR spectrum in solid argon agrees well with nonempirical and semiempirical calculations. On exposure to light, nitrene easily reacts with molecular carbon(II) oxide, forming the corresponding isocyanate. Irradiation with light does not result in formation of bicyclic azirine, but initiates the reverse reaction of fixation by nitrene of the molecular nitrogen contained in the solid argon, leading to the starting azide.     

EPR study of the radicals formed upon UV irradiation of ceria-based photocatalysts

EPR measurements reveal remarkable differences on the type of radicals produced after UV illumination of TiO₂, CeO₂ and 0.8% CeO₂/TiO₂ photocatalysts. Photoactivation of the TiO₂ sample in vacuum results in the formation of Ti⁴⁺–O⁻ species and a small amount of Ti³⁺ centers. In the presence of adsorbed oxygen, irradiation of this material also generates Ti⁴⁺–O₃⁻ radicals. In the case of the CeO₂/TiO₂ catalyst, the ceria component is present in a highly dispersed state, as indicated by XRD and UV–Vis diffuse reflectance spectra (DRS) results. Accordingly, the only type of Ce⁴⁺–O₂⁻ adducts generated on the CeO₂/TiO₂ sample are indicative of the presence of two-dimensional patches of ceria on the anatase surface. On the other hand, photoactivation of the CeO₂/TiO₂ sample in the presence of oxygen also leads to the formation of some Ti⁴⁺–O⁻ and Ti³⁺ centers. In the case of the CeO₂ sample, superoxide radicals are observed upon irradiation in vacuum and subsequent oxygen adsorption. Further irradiation of this material in the presence of oxygen increases the amount of Ce⁴⁺–O₂⁻ radicals and simultaneously generates new species, which are tentatively assigned to Ce⁴⁺–O₂H radicals. Photocatalytic activity was tested for toluene oxidation, and the results obtained show that the photodegradation rate is slightly lower for CeO₂/TiO₂ than for the TiO₂ sample. However, the selectivity towards benzaldehyde (6–13%) is comparable for both materials. In the case of CeO₂, the photo-oxidation rate is an order of magnitude lower than for TiO₂, although mineralization of toluene is almost complete. Photoactivity results are discussed in connection with the characteristics of the radicals observed.     

Fourier Transform EPR Spectroscopy of Trityl Radicals for Multifunctional Assessment of Chemical Microenvironment

Pulse techniques in electron paramagnetic resonance (EPR) allow for a reduction in measurement times and increase in sensitivity but require the synthesis of paramagnetic probes with long relaxation times. Here it is shown that the recently synthesized phosphonated trityl radical possesses long relaxation times that are sensitive to probe the microenvironment, such as oxygenation and acidity of an aqueous solution. In principle, application of Fourier transform EPR (FT‐EPR) spectroscopy makes it possible to acquire the entire EPR spectrum of the trityl probe and assess these microenvironmental parameters within a few microseconds. The performed analysis of the FT‐EPR spectra takes into consideration oxygen‐, proton‐, buffer‐, and concentration‐induced contributions to the spectral shape, therefore enabling quantitative and discriminative assessment of pH, pO₂, and concentrations of the probe and inorganic phosphate.     

An EPR and ENDOR investigation of a series of diazabutadiene-group 13 complexes

Paramagnetic diazabutadienegallium(II or III) complexes, [(Ar-DAB)2Ga] and [{(Ar-DAB*)GaX}2] (X = Br or I; Ar-DAB = {N(Ar)C(H)}2, Ar = 2,6-diisopropylphenyl), have been prepared by reactions of an anionic gallium N-heterocyclic carbene analogue, [K(tmeda)][:Ga(Ar-DAB)], with either "GaI" or [MoBr2(CO)2(PPh3)2]. A related InIII complex, [(Ar-DAB*)InCl2(thf)], has also been prepared. These compounds were characterised by X-ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal(III) complexes incorporating the Ar-DAB ligand, [(Ar-DAB(.))MX(2)(thf)(n)] (M = Al, Ga or In; X = Cl or I; n = 0 or 1) and [(Ar-DAB)2Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al (a = 2.85 G), Ga (a = 17-25 G) or In (a = 26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert-butyl groups in the gallium complex, [(tBu-DAB*)GaI2] (tBu-DAB={N(tBu)C(H)}2), changes the unpaired electron spin distribution producing 1H and 14N couplings of 1.4 G and 8.62 G, while the aryl-substituted complex, [(Ar-DAB*)GaI2], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely aGa approximately 1.4 G for [(tBu-DAB*)GaI2] and aGa approximately 25 G for [(Ar-DAB*)GaI2]. The EPR spectra of the gallium(II) and indium(II) diradical complexes, [{(Ar-DAB*)GaBr}2], [{(Ar-DAB*)GaI}2], [{(tBu-DAB*)GaI}2] and [{(Ar-DAB*)InCl}2], revealed doublet ground states, indicating that the Ga-Ga and In-In bonds prevent dipole-dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, [(Ar-BIAN*)GaI2] (Ar-BIAN = bis(2,6-diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert-butyl protons were obtained by ENDOR spectroscopy. In [(Ar-DAB*)GaI2], gallium hyperfine and quadrupolar couplings were detected for the first time.     

A spectroscopic study of the reduction of geometrically restrained viologens

A small series of N,N'-dimethyl-4,4'-bipyridinium dication derivatives (commonly known as viologens) has been synthesized and fully characterized; a short dialkoxy tether attached at the 3,3'-positions is used to alter the central dihedral angle. These angles were determined by both single-crystal X-ray diffraction and by computational studies made for the dication, radical cation, and neutral species in a solvent reservoir. The dihedral angle derived for the dication controls the first reduction potential, whereas the geometry of the resultant pi-radical cation determines the magnitude of the second reduction potential. The optical absorption spectra recorded for the various species, and especially those of the radical cations, and the EPR spectral parameters of the pi-radical cations also depend on the molecular geometry. In particular, the central dihedral angle influences the spin density distribution around the aromatic nucleus and, by way of comparison to the parent viologen, it has been possible to resolve the angle dependence from the inherent inductive effect of the strap. These results are considered in terms of the degree of electronic communication between the two aromatic rings, as controlled by the length of the tether.     

A Stable and Crystalline Triarylgermyl Radical: Structure and EPR Spectra

A radical thing : After being obtained unexpectedly in low yields, the synthesis of the first stable triarylgermyl radical ^.Ge[3,5‐tBu₂‐2,6‐(EtO)₂C₆H]₃ ( 1 ; C gray, O blue) was considerably optimized, and the product was investigated by X‐ray analysis and EPR spectroscopy. The results were compared with DFT‐MO studies for the model compound ^.Ge[2,6‐(MeO)₂C₆H₃].