Syntheses, optical and intramolecular magnetic properties of mono- and di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to 2,6-bispyrazolylpyridine cores

This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm(-1)) is larger than the OVZ biradical (|D/hc| = 0.00601 cm(-1)). Additionally both biradicals displayed forbidden half-field transitions (ΔM(s) = ±2; g(av) ~ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM(s) = ±2 signal intensities (χ(EPR)) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J(NN)?J(OVZ).     

Darstellung und EPR-spektroskopie m-phenylen-verknuepfter biradikalischer bistetracyclon-metallketyle

The synthesis of several substituted bistetracyclones bridged by phenyl rings at meta positions is described. From the corresponding metal ketyls—obtained by alkali metal reduction—EPR triplet powder spectra have been recorded. The spectra can be interpreted in terms of electron-electron dipolar coupling between the unpaired spins of randomly oriented molecules. The influences of substitution, concentration of the paramagnetic species and of the nature of the alkali metal counter ion on the magnitude of the zero field splitting parameters are investigated. Assuming two different conformations of the bis-tetracyclone ketyl biradicals (cis cis and cis trans—referred to the position of the oxygen atoms) the EPR data can be explained satisfactorily.     

Modulation effects in the electron spin echo of trans bis-(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals

Electron spin echo modulation effects have been investigated in trans bis(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals involving ¹⁴N or ¹⁵N nuclei in different positions. The modulation frequencies are shown to be the nitrogen nuclear transition frequencies in radical fragments at m_S = 0 of a triplet electronic spin. Quadrupole splitting parameters for these nitrogen nuclei have been determined.     

Studies on electron spin resonance spectroscopy of biradical molecules containing N-O and N-O moieties

The biradicals with ¹⁴N-Oxide and ¹⁵N-Oxide at the both ends of a molecule are synthesized for the molecular ruler of protein structure, and a potential device for quantum computing. We also establish a general synthetic method for reliable biradical formation. ESR spectra are recorded for the biradicals containing ¹⁵N-Oxide and ¹⁴N-Oxide with various interdistance separations. We find that two types of biradicals yielded different ESR spectra depending upon the distance between the ¹⁵N-O and ¹⁴N-O moieties in a molecule. This is due to electron spin dipole–dipole interaction occurring between the radicals. We also find that there is an indication of isotopic nuclear effects in the dipole–dipole interactions. The present study implies feasibility of the distance measurement between two different N-Oxides containing ¹⁴N and ¹⁵N isotopes. We conclude that quantum entanglement effects are observed through the dipolar interactions, which enable application of quantum computing devices operating in the liquid state.     

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.     

Zero-field splitting of quintet conjugated dinitrenes: 2,6-biphenylenedinitrene

2,6-Diazidobiphenylene was synthesized and photolyzed in frozen matrix. The resulting electron spin resonance spectrum showed the formation of 2,6-biphenylenedinitrene as a quintet species with zero field splitting (zfs) parameters ∣D/hc∣=0.260±0.002 cm⁻¹, ∣E/hc∣≤0.0005 cm⁻¹. The zfs parameters are in excellent accord with dipolar models for a quintet state produced by interaction between triplet state nitrene sites.     

Analysis of the magnetic coupling in nitroxide organic biradicals

The magnetic coupling in organic biradicals has been analyzed by means of ab initio wave function-based methods. Attention is focused on the coupling between the spin moments localized on the NO-groups in meta and para phenylene-bridged nitroxides, and bis(nitronyl) nitroxide and bis(imino) nitroxide biradicals. The leading mechanisms governing the coupling have been isolated by means of class-partitioned CI calculations. It was found that the mechanisms of the coupling in the para and meta phenylene-bridged nitroxides are similar to that found in transition metal complexes, while for the other biradicals the dominance of other mechanisms (like the spin polarization) imposes restrictions on the computational strategy to be followed to best estimate the coupling.     

Kinetically distinct triplets in the photorearrangement of 2-phenylcyclohexanone to cis- and trans-6-phenyl-5-hexenals

A reinvestigation of the photochemistry of 2-phenylcyclohexanone reveals that the two aldehyde products, cis- and trans-6-phenyl-5-hexanal, come from triplets of different lifetimes. That the two distinct triplets are not simply the two conformers with phenyl axial and equatorial is demonstrated by the similar behavior of cis-4-t-butyl-2-phenylcyclohexanone. The trans isomer of this ketone is photostable. It is concluded that trans-enal arises by an almost concerted out-of-plane cleavage which forms a 1,6-biradical in the perfect geometry for disproportionation to trans-enal.The precursor to cis-enal may be a minor rotamer of 1 which is forced by nonbonded interactions into a cleavage mode which twists the biradical into a geometry suitable for at least partial formation of cis-enal. In both cases biradicals must be very short-lived and not rotationally equilibrated.     

Synthesis and properties of air-stable 1,3-diphosphacyclobutane-2,4-diyls and the related compounds

A kinetically stabilized phosphaalkyne bearing a bulky Mes^∗ (2,4,6-tri-t-butylphenyl) group is useful compound to prepare an enormous number of highly stable 1,3-diphosphacyclobutane-2,4-diyls through reactions with a lithium reagent and an electrophile. By utilizing this synthetic protocol, we prepared several non-symmetrical 1,3-diphosphacyclobutane-2,4-diyls in which the substituents on the phosphorus are different. Furthermore, we succeeded in preparation and characterization of novel air-tolerant symmetrical 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyls bearing the identical alkyl substituents on the phosphorus atoms. Structures and properties of the 1,3-diphosphacyclobutane-2,4-diyls indicate characters as singlet ground-state carbon centered biradicals. In addition to those biradicals, we succeeded in preparation and isolation of a novel P-heterocyclic air-stable neutral radical as well as a P-heterocyclic cation radical.     

Zero field splittings in the lowest excited triplet states of aza- and diazaphenanthrenes and their protonated species

ESR spectra of the lowest excited ππP^* triplet states of a series of azaphenanthrenes (AP) and diazaphenanthrenes (DAP) are measured in rigid glasses. The zfs parameters are evaluated for 4-AP, 1,7-DAP, 4,7-DAP, 1,10-DAP and for their mono- and diprotonated species, BH⁺ and BH²⁺₂. The zfs parametesr of the bases are a litte higher than those of phenanthrene. On single protonation they fall below the latter values. Second protonation causes again an increase in zfs, with the exception of the 1,10-DAP divalent cation which behaves peculiarly. Spin-orbi contributions to the zfs seem not to be responsible for the observed differences.     

Spirocyclic Nitroxide Biradicals: Synthesis and Evaluation as Dynamic Nuclear Polarizing Agents

A method for the synthesis of rigid nitroxide biradicals with various spatial orientations between the radical centers is reported. Diketones were employed as substrates for tin amine protocol (SnAP) reagents to provide the parent spirocyclic diamines. Oxidation by peroxyacids provided the corresponding nitroxide biradicals. A set of four different biradicals with various interelectron distances and torsion angles between the radical planes was synthesized using this method. The exact geometries were determined by X-ray crystallography and the biradicals were investigated by EPR spectroscopy and evaluated for their dynamic nuclear polarization (DNP) performance. ¹H-DNP enhancements in the range of 1.2–2.1 at 14.1 Tesla (600 MHz spectrometer) were achieved. This synthetic methodology opens a promising alternative to access nitroxide biradicals with various torsional angles and inter radical distances.     

Heterospin biradicals based on new piperidineoxyl-substituted 3,6-di-<Emphasis Type="Italic">tert</Emphasis>-butyl-<Emphasis Type="Italic">o</Emphasis>-benzoquinone

A nucleophilic addition reaction of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (OH-TEMPO) to 3,6-di-tert-butyl-o-benzoquinone was used to obtain a new sterically hindered o-benzoquinone (1) containing 2,2,6,6-tetramethylpiperidineoxyl functional group, which was characterized by IR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction. A one-electron reduction of 1 with potassium and thallium is an efficient method for the generation of earlier unknown heterospin biradicals 5a and 5b, respectively, containing nitroxide and o-semiquinone radical centers. Analysis of the hyperfine structure of the ESR spectra of biradicals 5a and 5b in solution showed that they belong to the group of heterospin biradicals with strong (J >> a) and fast exchange interaction between the radical centers.     

ST0 CIDNP from triplet biradicals. Determination of the rate constant for biradical scavenging from magnetic field dependent CIDNP

ST₀ type nuclear spin polarization has been observed in the reaction products of photochemically generated acylalkyl biradicals in the presence of a scavenger. From the magnetic field dependence of biradical CIDNP the rate constant k of proton abstraction from 2-methyl-2-propanethiol by the alkyl site of the biradicals has been determined (k = 5.0 × 10⁷ M^?1 s^?1).     

Hyperfine splitting in the ESR spectra of random oriented alkali earth biradical chelates

The ESR spectra of rigid glass solutions of biradicals of glyoxal diimine anions and alkali earth cations were recorded. The spectra could be computer simulated only by adding to the spin hamiltonian

= gβH·S + D[S_Z²?$\text{1}\text{2}$S(S+1)] a hyperfine term due to the anisotropic interaction of the nitrogen nuclei. Two different models were tested: the first one with the two radicals in the same plane and the second one with the two radicals perpendicular; agreement with the experiment was found for the latter.     

ESR of ortho-dimesitoylbenzene-alkali earth metal chelates

The ESR spectra of rigid glass solutions of biradicals of ortho-dimesitoylbenzene anion and alkali earth ions, were recorded. The zero field splitting parameter D is compared with theoretical values calculated for different interionic distances. The results show that the biradical species should be of the contact type with metal to oxygen distance very close to the sum of the van der Waals radii of oxygen and metal ions.     

Quantum-chemical study of degradation of molecular analogs of butadiene-styrene copolymer fragments during stretching by end atoms

A comparative study of changes in the electronic structure during stretching of molecular fragments by end atoms is performed by using semiempirical quantum chemical method PM3 with geometry optimization for cis- and trans-hexa-3-ene, 3-methyl-1-pentene and sec-butylbenzene, which model butadiene-styrene copolymer units. In contrast to molecules modeling butadiene-1,4 units, the degradation of molecules modeling butadiene-1,2 and styrene units is found to proceed in two stages. At the first stage, when a maximum in the corresponding stress-strain curves is attained, the molecules with side vinyl and phenyl groups do not break down but experience intramolecular structural transition that is accompanied by the isomerization and displacement of double bonds. At the second stage, the molecules degrade.     

Comparative studies of reactions of commercial polymers with molecular oxygen,singlet oxygen,atomic oxygen and ozone—I. Reactions with cis-1,4-polybutadiene

The oxidation of cis-1,4-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone has been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO₂ and its reaction with polymer) are discussed. Chain scission was observed during all types of oxidation. A new mechanism involving opening of double bonds and formation of biradicals has been considered in detail.     

Röntgenkristallstrukturanalyse und Organische Chemie

An introduction into X-ray crystal structure analysis as method to determine the structure of organic compounds is given. In this connection two new methods for phase determination of X-ray reflections of macromolecules are reported. A few specially selected samples of structures are given to demonstrate the efficiency of X-ray crystal structure analysis with regard to the elucidation of stereochemical problems in the field of organic chemistry. The examples of application are heterocyclic organic compounds (cycloaddition products of nitrile-ylides, a stable azomethinimine, a cyanine dyestuff) and biologically relevant molecules (aminoacids of the amatoxins, the main poison of the green deathcup toadstool). The mentioned molecular structures determined by X-ray crystal structure analysis are discussed under the following aspects.

1. Elucidation of constitution, configuration and conformation.
2. Identification and verification of chemical pathways for new classes of compounds.
3. Bond relationships, influence of ring strain and other intramolecular interactions upon bond lengths and angles.
4. The attempt to find general stereochemical principles among related molecules or molecular parts points to the fundamental aspects of these experiments. Generally, the 1,4-diaza-s-trans-1,3-butadiene system, the steric influence of a free electron pair at a double bonded nitrogen and the CF₃-group are studied.
5. Obtaining of initial parameters for quantum mechanical calculations and comparison with theoretical results.
6. With regard to the structure analysis of two amino acid fragments of the amatoxins the configuration of the hydroxylated isoleucines of the amatoxins and the configuration at the chiral SO center of the natural amatoxins and of the synthetic phalloidin sulphoxides are fixed. The configuration at the SO center is decisive for the poisonous effect.