Symmetric Radical Research Articles

High-frequency dynamic nuclear polarization in the nuclear rotating frame.

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(epr) = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T(1rho)), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T(1n)). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T(1rho) and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T(1n)). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time(1/2) of epsilon(t) = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems.

Dibenzyl structure on the macromolecular chain. XII. Polyureas: structure-property relatonships

A series of polyureas with dibenzylic structures obtained from 2,2′-, 2,4′- or 4,4′-dibenzyl diamines and diisocyanates has been synthesized. Branched polyureas derived from a tetrafunctional monomer were also obtained. Their properties are strongly influenced by the isomeric structure of dibenzylic monomers. Polyureas containing symmetrical aromatic radicals like 4,4′- or 2,2′-dibenzyl exhibit low solubility in aprotic polar solvents and, their crystallinity lies in the range 20–66%. Polymers with non-symmetrical aromatic radicals, especially those with 2,4′-dibenzyl exhibit, increased solubility in polar solvents; they are amorphous and have film-forming properties. All polymers were characterized by their solubility and inherent viscosity, as well as IR spectra, thermogravimetric analysis, differential scanning calorimetry and X-ray diffraction measurements. © 2000 Society of Chemical Industry

Structure−Property Correlation on Second Hyperpolarizabilities of Symmetric One-Center and Three-Center Radicals

In order to elucidate the features of the second hyperpolarizabilities (γ) for n-center radical systems (n, integer), we investigate the γ of symmetric one-center radical models, i.e., BH3-, CH3, and NH3+, and three-center radical models, i.e., BH(CH2)2-, CH(CH2)2, and NH(CH2)2+, by various electron-correlation and a density functional methods using extended basis sets. Since these models are isoelectronic systems involving 9 and 23 electrons, respectively, the γ values are expected to reflect the features of their charge density distributions. It is actually found that the γ values for the isoelectronic systems sensitively depend on their charged states. For instance, the one-center radicals exhibit positive γ values at high-order electron correlation level using sufficiently large basis sets, while, for the three-center radicals, only NH(CH2)2+ is found to possess negative γ. These features are analyzed using γ density plots, which can provide a local view of spatial contributions of electrons to the γ, and are discussed in relation to our structure-property correlation rule.

Effects of the mixing of charge transfer and molecular excitations on the resonance Raman properties of symmetric radical dimers

The mixing of charge transfer (CT) and molecular excitation (ME) states in a molecular radical dimer is investigated based on a model Hamiltonian for a dimer with two orbitals per site filled with two electrons. The CT/ME mixing brings about an intensity enhancement of intramolecular modes in the Raman spectra resonant with the CT absorption. The relevant parameter for this effect is the transfer integral t ab for the hopping of one electron from orbital a in one molecule to orbital b in the other. This theoretical prediction is confirmed by Raman excitation profiles measured for the cation radical salt (TTF) 2(W 6O 19).

The gas phase nitrogen disulfide radical (SNS)

Collisional activation and neutralization—reionization mass spectrometric experiments provide evidence for the gas phase existence of the symmetrical nitrogen disulfide radical (S-N-S, 2A 1) as well as the corresponding cation (S-N-S +, 1Σ g +). Thiadiazolothiadiazole has been employed as the precursor. Ab initio molecular orbital calculations at the QCISD(T)/6–311 + + G(3df, 2p)/ /MP2/6–311 + + G(d, p) + ZPE level lead to the following thermochemical data: heats of formation at 0 K (Δ H f,0 0 in kcal/mol): S-N-S: 66 ± 3, N-S-S: 85 ± 3, S-N-S +: 240 ± 3, N-S-S +: 311 ± 3 and HS-N-S: 59 ± 3; proton affinity PA(SNS) = 169 ± 3 kcal/mol and ionization energies IE a(SNS) = 7.5 ± 0.2 eV and IE a(HS-N-S) = 8.7 ± 0.2 eV.

Chlorine Trioxide: Spectroscopic Properties, Molecular Structure, and Photochemical Behavior

Though postulated for years, now detected for the first time! The symmetric ClO3 radical was synthesized by vacuum flash pyrolysis of chlorine perchlorate and trapped by matrix isolation for UV and IR spectroscopic studies. The OClO bond angle is 113.5±2° and the ClO bond length 150.0±1 pm. The photolysis of ClO3 yields the unsymmetric isomer OClOO.

Electron-loss centres from methylmercury hydride, dimethylcadmium and diethylcadmium: an EPR study

Exposure of dilute solutions of methylmercury hydride in freon to ionizing radiation at 77 K generated two different radical cations. One, written as CH3. HgH+, has a large methyl proton hyperfine coupling with high spin density on the methyl ligand and the other (CH3Hg.H+) has a large coupling (460 G) to one proton, indicating a high spin density on the hydride ligand. In contrast, similar treatment of dimethyl- and diethyl-cadmium gave methyl and ethyl radicals directly at 77 K. These results are compared with those for various symmetrical R2Hg compounds which give symmetrical radical cations with high spin density on mercury.

Determination of the stabilities of CO 2+• (CO 2) n and O 2+• (CO 2) n clusters with n=1–6

Thermodynamic values for clustering reactions of the radical cations CO 2 + and O 2 + with carbon dioxide were measured with a pulsed electron beam mass spectrometer. CO 2 +• (CO 2) n has the form of a “shell structure”, i.e., (CO 2) 2 +• (CO 2) n-1 . Generally, the bond energies of symmetric radical cation dimers B +•—B increase with increasing ionization energy of B.

An ab initio SCF + CI study of the SH 3 and SF 3 radicals

The symmetric sulfuranyl radicals SH 3 and SF 3 are studied by means of ab initio SCF + CI calculations. All geometries are optimized at the UHF level using analytical gradients. SH 3 is found to be a transition state corresponding to a hydrogen exchange reaction, whereas SF 3 is stable with respect to decomposition to SF 2 + F.

Strongly enhanced infrared vibrational transitions in electronically degenerate states

Various types of strongly enhanced infrared vibrational transitions, with transition moments comparable to those characteristic of allowed electronic transitions in visible or UV spectra, may ensue in spatially degenerate electronic states of nonlinear molecules. The intimate correlation between the intensity of the enhanced infrared vibrational transitions and the anomalous pure rotational microwave spectra and the Stark effect is elucidated. The rules of Child and Longuet-Higgins with regard to infrared activity and infrared intensity enhancements in electronically degenerate states require revision and extension when properly considering the ramifications of the Jahn–Teller effect and invoking the action of mixed quadratic nuclear potential terms. Significantly, the enhancements of IR transition moments should allow the observation of symmetrical free radicals and molecular ions whose laboratory concentrations are often below the level of detectability for ‘‘normal’’ IR transition strengths. In addition, these effects could lead to multi-enhancement effects in nonlinear infrared and Raman vibrational spectra as well as enhanced radiative and nonradiative vibrational relaxations.

Reactions of bromonium salts of the carboborane series with nucleophiles

1. The reaction of phenyl-(9-m-carboranyl)bromonium borofluoride with nucleophiles is less regioselective than the analogous reactions of phenyl-(9-m-carboranyl)iodonium salts, and is more often accompanied by disruption of the carborane ring. 2. With the symmetrical bis(9-m-carboranyl)bromonium salts, both nucleophilic substitution and radical reactions proceed with much more difficulty than with the phenyl-(B-carboranyl) halonium and diphenylhalonium compounds.

Stereodynamical processes in β-substituted symmetric isopropyl radicals

Addition of trimethylsilyl (Me3Si·) and methylthiyl (MeS·) radical to Me3SiCH2–CHCH2 and MeSCH2–CHCH2 yields, respectively, the radicals Me3SiCH2ĊHCH2SiMe3(I) and MeSCH2ĊHCH2SMe (II) which were observed by e.s.r. spectroscopy. Restricted rotation about the ·CH–CH2 bonds was detected below –100 °C and the free energy of activation determined [3.4 for (I) and 2.85 kcal mol–1 for (II), respectively]. The features of the line shapes also showed that the conformation of (I) is symmetric whereas that of (II) is asymmetric. The geometry of these conformations was also assessed and the angle ϕ between the 2pz orbital of ·CH and the C–X bond was found to be 4–5°§ when X = SiMe3(I) and 10–11° when X = MeS (II). The dependence of the β-splitting on the conformation and on the electronic properties of silicon and sulphur was studied by INDO calculations and the hypothesis that sulphur-containing radicals must be distorted to account for their low aHβ splitting is challenged.

Electron-nuclear double resonance of horseradish peroxidase compound I. Detection of the porphyrin pi-cation radical.

Electron-nuclear double resonance spectra of horseradish peroxidase Compound I exhibit resonances from 14N and beta-protons, and most probably alpha-protons as well. The measurements prove that the oxyferryl iron of this enzymic intermediate is spin-coupled to a porphyrin radical. Comparison of experimentally obtained average pi-electron spin densities with theoretical predictions for a 4-fold symmetric porphyrin pi-cation radical is supportive of a 2A2u assignment for the radical's electronic state.

Spin correlation effects on reactions in spurs of radicals, radical ions and electrons after radiolysis

Dissociation and ionisation in condensed phases produce pairs of radicals (and electrons) or radical ions. Recombination of such pairs into ground state molecules depends on their spin state: singlets can recombine, triplets separate; theoretical work on ions in hydrocarbons is extended here to other substances, especially aqueous solutions. Single pairs are usually singlet initially. Recombination in spurs containing two or more pairs gives a mixture of singlets (S) and triplets (T). The cross-reactions in spurs, which in water give H2 and H2O2, normally have S:T = 1:3. Generation of a pair in a large spur via a triplet intermediate and Heisenberg spin exchange both transfer singlet character from the back reactions to the cross reactions. In mobile liquids, two processes are fast enough to cause correlation decay before recombination in the spurs: coherent (oscillatory) decay due to the hyperfine interaction in free hydrogen atoms and spin relaxation in hydroxyl and other cylindrically symmetrical radicals. Calculations of the spin relaxation time of ·OH radicals are presented : the mechanism is shown to depend on viscosity. Correlation effects on recombination of solvated electrons in viscous and glasses are discussed.It is shown that studies of spur dynamics must include spin effects. A variety of effects of magnetic fields and of deuteration on reaction rates in pairs and on yields of radiolysis products are predicted. Quantitative predictions are not attempted for lack of information, but effects will be small in liquid water, larger in organic liquids.

The symmetric radical of an algebra with involution

The symmetric radical of an algebra with involution

On the structure and properties of 2-silver(dimethylaminomethyl)ferrocene

Results of an X-ray structural study of 2-silver(dimethylaminomethyl)ferrocene are discussed. Joint thermal decomposition of this compound and cymantrenylsilver is shown to produce 2-dimethylaminomethyl-1-cymantrenylferrocene along with coupling products of symmetrical radicals.

EPR determination of the Co(II)-free radical magnetic geometry of the “doubleT” species arising in a coenzyme B-12-enzyme reaction

The physical significance of the observed structure of the EPR signal, commonly known as the “doublet” spectrum, is that it contains information not only about the exchange coupling but also about the geometry of the magnetic dipole-dipole spin-spin coupling. We can show this because we have developed a general method of analysis applicable to this type of system and because we demand a quantitative fit of theory to experiment at two microwave frequencies. We have chosen the “doublet” free radical signal, which arises in the ribonucleotide reductase-5′-deoxyadenosylcobalamin system (from Lactobacillus leichmannii, see Hamilton et al., Biochemistry 11, 4696–4705 (1972)), for study, for the particular reason that the 35 GHz “doublet” spectrum has three components (in this case) rather than two, which provides an important test of the recently proposed model of isotropic exchange coupling by Schepler et al. ((1975) Biochim. Biophys. Acta 397, 510–518). We find that a quantitative fit to the EPR “doublet” lineshape can be obtained with a model of isotropic exchange, and a “point” magnetic dipole-dipole interaction acting over a distance of 9.9 Å with the radical located approx. 34° off the principal g zz axis and <1° off the principal g xx axis of the Co(II) in the corrin ring. Quantitative fits of the doublet portion of the observed lineshape at both 9 and 35 GHz were achieved with this model, assuming an axially symmetric free radical signal and a Gaussian spin-packet lineshape with isotropic linewidth.

Konformationsuntersuchungen durch Elektronenspinresonanz Two-Jump-prozesse in Benzyl-phosphonium Salzen / Conformation Studies by Electron Spin Resonance Two-Jump Process in Benzyl-phosphonium Salts

Abstract The ESR spectra of 4-oxyl-benzyl-phosphonium-bromide radicals show a temperature dependence of the methylene protons and the phosphorus nucleus coupling constants which is explained by a hindered rotation of the phenoxyl ring of the radicals. Furthermore compounds with different phosphor substituents exhibit the phenomenon of a two jump process. This effect is described by an isomerisation of two symmetric radical conformations. Activation energies for this process are found to be around 7 kcal/Mol. The conformation of the radicals can be discussed as staggered ethane analogues. A lower limit value for the hyperconjugation Bᴾ -parameter is proposed.

Reversal of low-field CIDNP due to deuterium substitution

A perturbation method is applied to the problem of low-field CIDNP. It is shown that in the intermediate magnetic field region the most important deviation from the high-field formulae is due to a perturbation of the S—T 0 mixing by interaction with the T ± levels of the radical pair. In the case of a symmetrical radical pair this contribution decreases to zero and higher order contributions become dominant. An experimental example is the reversal of low-field CIDNP of ethane formed during the thermal decomposition of acetyl peroxide compared with that of ethane- d 3 during the decomposition of acetylperoxide- d 3. A qualitative rule is given for net CIDNP effects in the intermediate field region.

Intervalence (charge-transfer) bands in symmetrical and unsymmetrical bispyridinium-pyridinyl radical complexes, (Py.cntdot.)32+

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIntervalence (charge-transfer) bands in symmetrical and unsymmetrical bispyridinium-pyridinyl radical complexes, (Py.cntdot.)32+Edward M. Kosower and Avraham TeuersteinCite this: J. Am. Chem. Soc. 1976, 98, 6, 1586–1587Publication Date (Print):March 1, 1976Publication History Published online1 May 2002Published inissue 1 March 1976https://doi.org/10.1021/ja00422a055RIGHTS & PERMISSIONSArticle Views105Altmetric-Citations11LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (280 KB) Get e-Alerts Get e-Alerts