Tert-butyl Radical Research Articles

Oxidative stability of plant hydroalcoholic extracts assessed by EPR spin trapping under forced ageing conditions: A myrtle case study

The oxidative stability of myrtle hydroalcoholic extracts was measured, over storage time, with the EPR spin trapping method under forced ageing conditions. The extracts were prepared with 150 and 300 g l−1 of berries and extraction media with ethanol ranging from 60 to 90%. Two radicals were detected: the PBN–1-hydroxyethyl adduct and the tert-butyl aminoxyl radical. A dimensionless parameter (Ω) calculated on the basis of the lag time, the rate of formation and concentration of the radical species was used to estimate the extracts’ oxidative stability. Ω was strongly influenced by the extraction medium, being lower in extracts with ethanol 60%, and by the time of storage. An inverse correlation was calculated between Ω and ellagic acid concentration, thus suggesting the role of this phenolic acid in the antioxidant properties of the extracts. The radical scavenging activity of the extracts against the hydroxyl radical was also measured.

Dose-dependency of multiple free radical-scavenging activity of edaravone

BackgroundEdaravone is a powerful free radical scavenger that is in clinical use. However, data concerning its dose-response relationship against multiple free radicals remain sparse. The purpose of the present study was to demonstrate the dose-dependency of direct scavenging activity of edaravone against multiple free radical species. Materials and methodsFree radical-scavenging activities of edaravone against six free radical species were evaluated by electron spin resonance spectroscopy using spin-trapping method. ResultsEdaravone scavenged the following free radicals in dose-dependent manners with reaction rate constants (kedaravone) or 50% inhibitory concentration (IC50) as indicated: hydroxyl radical (kedaravone = 5.2 × 1010 M−1 s−1), superoxide anion (kedaravone/kG-CYPMPO = 0.63), tert-butyl peroxyl radical (kedaravone/kG-CYPMPO = 8.8), ascorbyl free radical (IC50 = 0.17 ± 0.06 mM), 2,2-diphenyl-1-picrylhydrazyl (DPPH, IC50 = 4.7 ± 0.3 μM), and nitric oxide (kedaravone = 7.0 × 103 M−1 s−1). ConclusionsThe dose-dependent scavenging activities of edaravone against multiple free radical species were clearly illustrated. It is speculated that edaravone acts as antioxidant by dose-dependently scavenging multiple free radical species along the chain reactions of oxidative stress in surgery.

Investigation of the two- and three-fragment photodissociation of the tert-butyl peroxy radical at 248 nm.

The photodissociation dynamics of the tert-butyl peroxy (t-BuOO) radical are studied by fast-radical-beam coincidence translational spectroscopy. The neutral t-BuOO radical is formed by photodetachment of the corresponding t-BuOO- anion at 700 nm (1.77 eV), followed by dissociation at 248 nm (5.00 eV). Photofragment mass and translational energy distributions are obtained. The major channel is found to be three-body fragmentation to form O, CH3, and acetone (83%), with minor two-body fragmentation channels leading to the formation of O2 + tert-butyl radical (10%) and HO2 + isobutene (7%). Experimental results show that the translational energy distribution for two-body dissociation peaks is close to zero translational energy, with an isotropic angular distribution of fragments. These results indicate that two-body fragmentation proceeds via internal conversion to the ground electronic state followed by statistical dissociation. For three-body dissociation, the translational energy distribution peaks closer to the maximal allowed translational energy and shows an anisotropic distribution of the plane of the dissociating fragments, implying rapid dissociation on an excited-state surface. A small shoulder in the three-body translational energy distribution suggests that some three-fragment dissociation events proceed by a different mechanism, involving internal conversion to the ground electronic state followed by sequential dissociation.

The fate of the tert-butyl radical in low-temperature autoignition reactions.

Alkyl combustion models depend on kinetic parameters derived from reliable experimental or theoretical energetics that are often unavailable for larger species. To this end, we have performed a comprehensive investigation of the tert-butyl radical (R• in this paper) autoignition pathways. CCSD(T)/ANO0 geometries and harmonic vibrational frequencies were obtained for key stationary points for the R• + O2 and QOOH + O2 mechanisms. Relative energies were computed to chemical accuracy (±1 kcal mol-1) via extrapolation of RCCSD(T) energies to the complete basis-set limit, or usage of RCCSD(T)-F12 methods. At 0 K, the minimum energy R• + O2 pathway involves direct elimination of HO2∙ (30.3 kcal mol-1 barrier) from the tert-butyl peroxy radical (ROO•) to give isobutene. This pathway lies well below the competing QOOH-forming intramolecular hydrogen abstraction pathway (36.2 kcal mol-1 barrier) and ROO• dissociation (35.9 kcal mol-1 barrier). The most favorable decomposition channel for QOOH radicals leads to isobutene oxide (12.0 kcal mol-1 barrier) over isobutene (18.6 kcal mol-1 barrier). For the QOOH + O2 pathways, we studied the transition states and initial products along three pathways: (1) α-hydrogen abstraction (42.0 kcal mol-1 barrier), (2) γ-hydrogen abstraction (27.0 kcal mol-1 barrier), and (3) hydrogen transfer to the peroxy moiety (24.4 kcal mol-1 barrier). The barrier is an extensive modification to the previous 18.7 kcal mol-1 value and warrants further study. However, it is still likely that the lowest energy QOOH + O2 pathway corresponds to pathway (3). We found significant spin contamination and/or multireference character in multiple stationary points, especially for transition states stemming from QOOH. Lastly, we provide evidence for an A∼-X∼ surface crossing at a Cs-symmetric, intramolecular hydrogen abstraction structure.

Slow photoelectron velocity-map imaging of cold tert-butyl peroxide.

Photoelectron spectra of cryogenically cooled X∼1A' tert-butyl peroxide anions are obtained using slow electron velocity-map imaging. The spectra show highly structured bands corresponding to detachment to the X∼2A″ and A∼2A' electronic states of the neutral radical and represent a notable improvement in resolution over previous photoelectron spectra. We report an electron affinity of 1.1962(20) eV and a term energy T0(A∼2A') of 0.9602(24) eV for the tert-butyl peroxy radical. New vibrational structure is resolved, providing several frequencies for both neutral states. Additionally, the threshold behavior of the photodetachment cross section is investigated within the context of Dyson orbital calculations.

An Experimental Study of the Kinetics of the Reactions of Isopropyl,sec-Butyl, andtert-Butyl Radicals with Molecular Chlorine at Low Pressures (0.5-7.0 Torr) in the Temperature Range 190-480 K

In this work, we have measured the rate coefficients of the reactions of isopropyl (propan-2-yl), sec-butyl (butan-2-yl), and tert-butyl (2-methylpropan-2-yl) radicals with molecular chlorine as a function of temperature (190–480 K). The experiments were done in a tubular laminar flow reactor coupled to a photoionization quadrupole mass spectrometer employing a gas-discharge lamp for ionization. The radicals were homogeneously produced in the reactor by photolyzing suitable precursor molecules with 193-nm pulsed exciplex laser radiation. The bimolecular rate coefficients were obtained by monitoring the radical decay signals in real time under pseudo–first-order conditions. The rate coefficients of all three reactions showed negative temperature dependence. The bath gas used in the experiments was helium, and the rate coefficients appeared to be independent of the helium concentrations employed ([2.4–14] × 1016 cm−3) for all three reactions. The rate coefficients of the reactions can be approximated in the studied temperature range by the following parameterizations: k(i‐C3H7•+ Cl 2)=(3.70±0.12)×10−11×(T/300K)−(1.98±0.10) cm 3s−1 k(s‐C4H9•+ Cl 2)=(4.90±0.20)×10−11×(T/300K)−(1.92±0.19) cm 3s−1 k(t‐C4H9•+ Cl 2)=(4.46±0.18)×10−11×(T/300K)−(2.97±0.14) cm 3s−1 We estimate that the overall uncertainties of the measured rate coefficients are ±20%. We were able to observe 2-chloropropane (i-C3H7Cl) product for the i-C3H7● + Cl2 reaction. No products were observed for the other two reactions, and the reasons for this are briefly discussed in the text.

Removal of AOX, Chroma and TOC in Chemical Dyestuff Wastewater with Iron Scraps-Fenton-Coagulation Combined Process

Iron scraps-Fenton-coagulation process was applied to chemical dyestuff wastewater. The removal performance of absorbable organic halogens(AOX), chroma and total organic carbon (TOC) was investigated at different molar ratios of Fe2+ to H2O2 (1:3-1:15), iron scraps reaction time (2-5 h) and Fenton reaction time (20-80 min). The results showed that the removal ratios of AOX, chroma and TOC firstly increased and then decreased with the decrease of the molar ratio of Fe2+ to H2O2, while continuously increased with the increase of iron scraps and Fenton reaction time. The optimal condition was determined as Fe2+:H2O2 ratio of 1:8, iron scraps reaction time of 4 h and Fenton reaction time of 60 min, under which 94.2% of AOX, 93.7% of chroma and 27.2% of TOC were removed. A comparison study revealed that the iron scraps-Fenton-coagulation combined process could achieve much better removal of AOX, chroma and TOC than any other single or combined processes of iron treatment, Fenton oxidation and coagulation. GC-MS analysis revealed that halogenated compounds and anilines were efficiently removed, as well as nitrobenzenes, phenols, benzaldehydes, ethers, nitriles and heterocyclic compounds.·OH was found to devote much in the Fenton reaction according to the tert-butyl alcohol trapping hydroxyl radicals test.

Benchmarking Compound Methods (CBS-QB3, CBS-APNO, G3, G4, W1BD) against the Active Thermochemical Tables: Formation Enthalpies of Radicals.

The 298.15 K formation enthalpies of 38 radicals with molecular formula CxHyOz have been computed via the atomization procedure using the five title methods. The computed formation enthalpies are then benchmarked against the values recommended in the Active Thermochemical Tables (ATcT). The accuracy of the methods have been interpreted in terms of descriptive statistics, including the mean-signed error, mean-unsigned error, maximum average deviation, 2σ uncertainties, and 2×root-mean-square-deviations (2RMSD). The results highlight the following rank order of accuracy for the methods studied G4 > G3 > W1BD > CBS-APNO > CBS-QB3. The findings of this work are also considered in light of a recent companion study, which took an identical approach to quantifying the accuracies of these methods for 48 closed-shell singlet CxHyOz compounds. A similar order of accuracies and precisions were observed therein: G3 > G4 > W1BD > CBS-APNO > CBS-QB3. Both studies highlight systematic biases/deviations from the ATcT for the methods investigated, which are discussed in some detail, with methods having clear tendencies to over- or underpredict the recommended formation enthalpies for radical and/or closed-shell CxHyOz compounds. We show that one can improve the accuracy of their computation, and simultaneously reduce the uncertainty, by taking unweighted average formation enthalpies from various combinations of methods used. The reader should note that the statistical analyses preceding these conclusions also highlight that these error cancellation effects are unique for closed-shell and radical species. By extension, these error-cancellation effects can be expected to be different for various homologous series and chemical functionalities and their closed- and open-shell subgroups. Hence, further benchmarking studies are advised for other homologous series, such that the scientists and engineers (e.g., combustion/atmospheric/astrochemical) who frequently use these methods can assign reasonable uncertainties to their computations, while simultaneously optimizing their computational costs. For CxHyOz compounds, a combination of the CBS-APNO/G3/G4 methods is shown to be quite powerful when the atomization method is employed and is capable of reproducing the ATcT to within "near-chemical-accuracy", with 2RMSD (≈95% confidence interval) values of 0.0 ± 4.34 kJ mol(-1) computed for CxHyOz radicals, 0.0 ± 4.22 kJ mol(-1) for closed-shell CxHyOz compounds, with a total uncertainty of 0.0 ± 4.27 kJ mol(-1) subsequently computed considering all 85 CxHyOz compounds. Given the performance of these methods for determination of formation enthalpies when the atomization procedure is employed, we expect isodesmic reactions involving these methods to be capable of achieving chemical accuracy, as illustrated for the case of the tert-butyl radical. We also highlight that there is still disagreement between experiment and theory for this radical, despite its significance in gas-phase chemistry. Kineticists, thermodynamicists, and chemical kinetic modellers alike are warned that the popular CBS-QB3 method is found to have particularly poor performance, with a computed 2RMSD of 0.0 ± 12.51 kJ mol(-1), indicating that one should not apply this method in isolation for formation enthalpy determination unless other error-cancellation strategies are employed.

Direct access to dithiobenzoate RAFT agent fragmentation rate coefficients by ESR spin-trapping.

The β-scission rate coefficient of tert-butyl radicals fragmenting off the intermediate resulting from their addition to tert-butyl dithiobenzoate-a reversible addition-fragmentation chain transfer (RAFT) agent-is estimated via the recently introduced electron spin resonance (ESR)-trapping methodology as a function of temperature. The newly introduced ESR-trapping methodology is critically evaluated and found to be reliable. At 20 °C, a fragmentation rate coefficient of close to 0.042 s(-1) is observed, whereas the activation parameters for the fragmentation reaction-determined for the first time-read EA = 82 ± 13.3 kJ mol(-1) and A = (1.4 ± 0.25) × 10(13) s(-1) . The ESR spin-trapping methodology thus efficiently probes the stability of the RAFT adduct radical under conditions relevant for the pre-equilibrium of the RAFT process. It particularly indicates that stable RAFT adduct radicals are indeed formed in early stages of the RAFT poly-merization, at least when dithiobenzoates are employed as controlling agents as stipulated by the so-called slow fragmentation theory. By design of the methodology, the obtained fragmentation rate coefficients represent an upper limit. The ESR spin-trapping methodology is thus seen as a suitable tool for evaluating the fragmentation rate coefficients of a wide range of RAFT adduct radicals.

UV/persulfate and UV/hydrogen peroxide processes for the treatment of salicylic acid: effect of operating parameters, kinetic, and energy consumption

Two advanced oxidation processes (AOPs), based on UV light irradiation with potassium persulfate (UV/KPS) and with hydrogen peroxide (UV/H2O2), were employed for the treatment of salicylic acid (SA) in aqueous media. The influence of operating parameters, initial pH and concentration of the oxidant reagents, was investigated. Under the optimum conditions of pH 6 and [KPS] = 1,000 mg/L for UV/KPS and pH 4 and [H2O2] = 140 mg/L for UV/H2O2, about 94 and 98% SA degradations were achieved after 60 min treatments. Total organic carbon removal was also obtained as 80% and 87%, respectively, after 150 min. Rate of SA degradation in these processes obeys pseudo-first-order kinetic. For UV/KPS process, the role of active radicals was studied using ethanol and tert-butyl alcohol radical scavengers, indicating the contribution of hydroxyl and sulfate radicals in the degradation of about 56% and 44%, respectively. The electrical energy consumption, for nearly complete degradation of SA, was evaluated and the preference was shown in comparison with previous works.

Thermochemistry of C7H16 to C10H22 alkane isomers: primary, secondary, and tertiary C-H bond dissociation energies and effects of branching.

Standard enthalpies of formation (ΔH°f298) of methyl, ethyl, primary and secondary propyl, and n-butyl radicals are evaluated and used in work reactions to determine internal consistency. They are then used to calculate the enthalpy of formation for the tert-butyl radical. Other thermochemical properties including standard entropies (S°(T)), heat capacities (Cp(T)), and carbon-hydrogen bond dissociation energies (C-H BDEs) are reported for n-pentane, n-heptane, 2-methylhexane, 2,3-dimethylpentane, and several branched higher carbon number alkanes and their radicals. ΔH°f298 and C-H BDEs are calculated using isodesmic work reactions at the B3LYP (6-31G(d,p) and 6-311G(2d,2p) basis sets), CBS-QB3, CBS-APNO, and G3MP2B3 levels of theory. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) level for contributions to entropy and heat capacities. Enthalpy calculations for these hydrocarbon radical species are shown to have consistency with the CBS-QB3 and CBS-APNO methods using all work reactions. Our recommended ideal gas phase ΔH°f298 values are from the average of all CBS-QB3, CBS-APNO, and for G3MP2B3, only where the reference and target radical are identical types, and are compared with literature values. Calculated values show agreement between the composite calculation methods and the different work reactions. Secondary and tertiary C-H bonds in the more highly branched alkanes are shown to have bond energies that are several kcal mol(-1) lower than the BDEs in corresponding smaller molecules often used as reference species. Entropies and heat capacities are calculated and compared to literature values (when available) when all internal rotors are considered.

Synthesis of 2-pyrolyl-2-hydroxy-2-cyanoacetamide through FeCl3–TBHP mediated hydroxylation of captodative stabilized radical intermediate

A series of 2-hydroxy-2-(4-oxo-1,2-diaryl-4,5,6,7-tetrahydro-1H-3-indolyl)-2-cyanoacetamides have been synthesized through selective CH oxidation of active methylene carbon of cyanoacetamide derivatives using tert-butyl hydroperoxide as oxidant and ferric chloride as radical initiator. In this reaction the tert-butyl oxo radical abstracts hydrogen from the active methylene group of 2-pyrolyl-2-cyanoacetamides to generate a stable captodative radical intermediate. Then recombination of this radical intermediate with tert-butyl oxo radical followed by acid catalysed thermal elimination of the tert-butyl group produces the quaternary hydroxyl cyanoacetamide derivatives in good yield.

Reduction and alkylation of (η6-arene)tricarbonylmanganese complexes using alkyl-substituted coenzyme NADH models

The direct tert-butyl group transfer was observed between t-BuBNAH and (η6-arene)Mn(CO)3+ complexes in reflux acetonitrile without a radical initiator, and the corresponding tert-butyl radical addition products were achieved. The substituents on the aromatic ring significantly influence the reactivity and chemoselectivity. The electron-withdrawing group helps to promote the reactions. The radical inhibition experiment indicated that the reaction is probably a radical chain process initiated by single electron transfer.

Radical Migration–Addition of N-tert-Butanesulfinyl Imines with Organozinc Reagents

A novel migration-addition sequence was discovered for the reaction of enantioenriched N-tert-butanesulfinyl iminoacetate 1a with functionalized benzylzinc bromide reagents, producing tert-leucine derivatives in excellent diastereoselectivity (dr 98:2). The absolute configurations of two new chiral centers were unambiguously assigned by chemical transformations and X-ray crystallography. In addition, the regio- and diastereoselectivities of this novel reaction were both explained through the key N-sulfinamine intermediate M6 generated by the tert-butyl radical attack on the imine. Computational analysis of this reaction process, which was performed at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31G*-LANL2DZ level, also supported our proposed two-stage mechanism.

Catalytic ozonation of 2,4-Dichlorophenoxyacetic acid using alumina in the presence of a radical scavenger

Using a laboratory-scale mixed reactor, the performance of alumina in degrading 2,4-Dichlorophenoxyacetic acid with ozone in the presence of tert-butyl alcohol radical scavenger was studied. The operating variables investigated were the dose of alumina catalyst and solution pH. Results showed that using ozone and alumina leads to a significant increase in 2,4-D removal in comparison to non-catalytic ozonation and adsorption processes. The observed reaction rate constants (kobs ) for 2,4-D during ozonation were found to increase linearly with increasing catalyst dose. At pH 5, the kobs value increased from 19.3 to 26 M−1 s−1 and 67 M−1 s−1 when varying the alumina dose from 1 to 2 and 4 g L−1, respectively. As pH was increased, higher reaction rates were observed for both non-catalytic ozonation and catalytic ozonation processes. Thus, at pH 3 and using a catalyst dose of 8 g L−1, the kobs values for non-catalytic ozonation and catalytic ozonation processes were 3.4 and 58.9 M−1 s−1, respectively, whereas at pH 5 reaction rate constants of 6.5 and 128.5 M−1 s−1 were observed, respectively. Analysis of total organic carbon suggested that catalytic ozonation with alumina achieved a considerable level of mineralization of 2,4-D. Adsorption of 2,4-D on alumina was found to play an important role in the catalytic ozonation process.

Intermolecular Hydrogen Transfer in Isobutane Hydrate

Electron spin resonance (ESR) spectra of butyl radicals induced with γ-ray irradiation in the simple isobutane (2-methylpropane) hydrate (prepared with deuterated water) were investigated. Isothermal annealing results of the γ-ray-irradiated isobutane hydrate reveal that the isobutyl radical in a large cage withdraws a hydrogen atom from the isobutane molecule through shared hexagonal-faces of adjacent large cages. During this “hydrogen picking” process, the isobutyl radical is apparently transformed into a tert-butyl radical, while the sum of isobutyl and tert-butyl radicals remains constant. The apparent transformation from isobutyl to tert-butyl radicals is an irreversible first-order reaction and the activation energy was estimated to be 35 ± 3 kJ/mol, which was in agreement with the activation energy (39 ± 5 kJ/mol) of hydrogen picking in the γ-ray-irradiated propane hydrate with deuterated water.

A First‐Principles Study on the Interaction between Alkyl Radicals and Graphene

The interaction between alkyl radicals and graphene was studied by means of dispersion-corrected density functional theory. The results indicate that isolated alkyl radicals are not likely to be attached onto perfect graphene. It was found that the covalent binding energies are low, and because of the large entropic contribution, ΔG(298)° is positive for methyl, ethyl, isopropyl, and tert-butyl radicals. Although the alkylation may proceed by moderate heating, the desorption barriers are low. For the removal of the methyl and tert-butyl radicals covalently bonded to graphene, 15.3 and 2.4 kcal mol(-1) are needed, respectively. When alkyl radicals are agglomerated, the binding energies are increased. For the addition in the ortho position and on opposite sides of the sheet, the graphene-CH(3) binding energy is increased by 20 kcal mol(-1), whereas for the para addition on the same side of the sheet, the increment is 9.4 kcal mol(-1). In both cases, the agglomeration turns the ΔG(298)°<0. For the ethyl radical, the ortho addition on opposite sides of the sheet has a negative ΔG(298)°, whereas for isopropyl and tert-butyl radicals the reactions are endergonic. The attachment of the four alkyl radicals under consideration onto the zigzag edges is exergonic. The noncovalent adsorption energies computed for ethyl, isopropyl, and tert-butyl radicals are significantly larger than the graphene-alkyl-radical covalent binding energies. Thus, physisorption is favored over chemisorption. As for the ΔG(298)° for the adsorption of isolated alkyl radicals, only the tert-butyl radical is likely to be exergonic. For the phenalenyl radical we were not able to locate a local minimum for the chemisorbed structure since it moves to the physisorbed structure. An important conclusion of this work is that the consideration of entropic effects is essential to investigate the interaction between graphene and free radicals.

Interaction between alkyl radicals and single wall carbon nanotubes

The addition of primary, secondary, and tertiary alkyl radicals to single wall carbon nanotubes (SWCNTs) was studied by means of dispersion corrected density functional theory. The PBE, B97-D, M06-L, and M06-2X functionals were used. Consideration of Van der Waals interactions is essential to obtain accurate addition energies. In effect, the enthalpy changes at 298 K, for the addition of methyl, ethyl, isopropyl, and tert-butyl radicals onto a (5,5) SWCNT are: -25.7, -25.1, -22.4, and -16.6 kcal/mol, at the M06-2X level, respectively, whereas at PBE/6-31G* level they are significantly lower: -25.0, -19.0, -16.7, and -5.0 kcal/mol respectively. Although the binding energies are small, the attached alkyl radicals are expected to be stable because of the large desorption barriers. The importance of nonbonded interactions was more noticeable as we moved from primary to tertiary alkyl radicals. Indeed, for the tert-butyl radical, physisorption onto the (11,0) SWCNT is preferred rather than chemisorption. The bond dissociation energies determined for alkyl radicals and SWCNT follow the trend suggested by the consideration of radical stabilization energies. However, they are in disagreement with some degrees of functionalization observed in recent experiments. This discrepancy would stem from the fact that for some HiPco nanotubes, nonbonded interactions with alkyl radicals are stronger than covalent bonds.

Mechanism of the intramolecular hydrogen transfer reaction at ground and excited state of tert-butyl radical: An ESR and DFT study

Hydrogen transfer reactions at the ground and excited states of tert-butyl radical to form iso-butyl radical have been investigated by means of ab initio calculation and electron spin resonance (ESR) spectroscopy. It was found that tert-butyl radical irradiated with 254nm ultraviolet light converts quite efficiently to iso-butyl radical. Also, it was suggested that this conversion occurs as an intramolecular hydrogen transfer from a methyl group of tert-butyl radical to a radical site (central carbon atom). The theoretical calculations showed that barrier height of hydrogen transfer at the excited state is significantly lower than that of ground state. The mechanism of internal hydrogen atom conversion from tert-butyl to iso-butyl radicals was discussed.

Intramolecular Hydrogen Bonding in Calix[4]arene-Based Nitroxide Monoradical and Biradical as Studied by CW-ESR and Pulse-ESR HYSCORE Spectroscopy

A calix[4]arene-based biradical with two tert-butyl nitroxide radicals and a monoradical derived from the biradical have been studied by continuous-wave electron spin resonance (CW-ESR) and pulse-ESR-based hyperfine sublevel correlation (HYSCORE) spectroscopy. The two nitroxide radical sites antiferromagnetically interact with each other, generating a thermally accessible triplet state located 4 cm−1 above the singlet ground state. The present fine-structure and hyperfine spectral simulation for the spin Hamiltonian parameters of the biradical is sensitive to the local molecular structure at the spin-bearing site, illustrating a salient electronic structure of the radical sites with the π-orbitals on the nitrogen (and oxygen) atoms cross-facing each other. The derived structure contrasts with the molecular structure determined by an X-ray crystal analysis for the hydroxylamine precursor of the biradical. The distance between the two midpoints of the nitrogen–oxygen bonds at the radical sites is by 0.234 nm longer than the one (=0.314 nm) of the two hydroxyl groups of the precursor determined by the X-ray analysis. The lack of intramolecular hydrogen bonds between the nitrogen and hydrogen of the hydroxyl groups, caused by the oxidation of the hydroxyamino precursor, gives rise to such a sizable increase in the distance between the radical sites. The HYSCORE experiments gave a direct evidence of the local molecular structure of the radical site of the partially oxidized monoradical. The experimentally derived molecular structures of both the bi-, monoradicals and the precursor are in good agreement with those obtained by density functional theory calculations.