Search

Lipid oxidation is one of the major causes of oil deterioration causing off-flavors and consumer rejection. Fast, easy, and dependable assays for predicting lipid oxidation rates in foods are important for shelf-life prediction. In this study, an electron paramagnetic resonance (EPR) spin-trapping technique with N-tert-butyl-α-phenylnitrone (PBN) was tested to determine the lag phase of lipid oxidation in stripped soybean oil (SSO), SSO with added α-tocopherol, and commercial soybean, canola and corn oils. EPR intensity of spin-trapped products from SSO correlated well with lipid hydroperoxides formation for samples stored at 37 and 55 °C respectively. When the antioxidant α-tocopherol was added, the EPR signal intensity of oil samples increased—indicating sample deterioration—after 50–65% of α-tocopherol was consumed. When using the EPR method with commercial soybean, canola or corn oil stored at 55 °C, there was a poor relationship between EPR intensity and lipid hydroperoxides lag phases. However, a linear correlation was found between EPR signal intensity and hexanal formation. For example, EPR signal intensity lag phases were 5, 13 and 27 days for soybean, canola and corn oils, respectively which was similar to the hexanal lag phases of 5, 13 and 25 days for the same oils. The EPR spin-trapping assay method has several advantages over headspace hexanal measurements, especially with regard to easier sample handling and shorter analysis times.

The source of unoccupied Ti 3d states in the case of stoichiometric anatase structured (TiO2)n clusters has been investigated using ab initio methods. These unoccupied gap states appear for example in the case of a stoichiometric (TiO2)38 cluster. We show that the origin of these gap states is related to effective subcluster formation which gives rise to empty defect-like gap states, when these states are split off from conduction band. © 2015 Wiley Periodicals, Inc.

Towards highly luminescent aggregates: A highly twisted triphenylamine displays aggregation-induced enhanced emission. A solvent-specific critical molecular aggregate size, once reached, gives rapid onset of enhanced emission in polar solvent mixtures that favor quenching of solvated individual molecule excited states.

The synthesis and the structural and magnetic characterization of two new cobalt(II) compounds [Co4(TTA)4(OMe)4(EtOH)4] (1) and [Co4(TTA)4(OMe)4(MeOH)4] (2) (TTA = 4,4,4-trifluoro-1-thienoyl-2,4-butanedione) are reported. The structures consist of a tetranuclear Co4O4 core in a cubane framework. Both systems were modeled by a spin Hamiltonian with two isotropic exchange interactions and local orthogonal anisotropy axis configuration. The temperature-dependent susceptibility was successfully simulated yielding many low-lying mixed eingenstates that cannot be approximated by a giant spin model. A surprising fact is that despite the structural similarities there are striking differences between their magnetic properties. DC magnetic susceptibilities showed only ferromagnetic interactions between cobalt(II) ions for 2, whereas 1 has ferromagnetic and weaker antiferromagnetic interactions among their ions. In addition, compound 1 presented split zero-field-cooled and field-cooled susceptibility around 6 K and a frequency dependent ac susceptibility typical of slow magnetization not observed in 2. The temperature dependence of this relaxation is similar to what is observed for cluster-glass systems and may be related to a complex process involving many low-level mixed states.

Hydroxycarbamide (hydroxyurea) provides laboratory and clinical benefits for adults and children with sickle cell anaemia (SCA). Given its mechanism of action and prior reports of genotoxicity, concern exists regarding long-term toxicities and possible carcinogenicity. We performed cross-sectional analyses of chromosome stability using peripheral blood mononuclear cells (PBMC) from 51 children with SCA and 3-12 years of hydroxycarbamide exposure (mean age 13·2 ± 4·1 years), compared to 28 children before treatment (9·4 ± 4·7 years). Chromosome damage was less for children receiving hydroxycarbamide than untreated patients (0·8 ± 1·2 vs. 1·9 ± 1·5 breaks per 100 cells, P = 0·004). There were no differences in repairing chromosome breaks after in vitro radiation; PBMC from children taking hydroxycarbamide had equivalent 2 Gy-induced chromosome breaks compared to untreated patients (30·8 ± 16·1 vs. 31·7 ± 8·9 per 100 cells, P = not significant). Radiation plus hydroxycarbamide resulted in similar numbers of unrepaired breaks in cells from children on hydroxycarbamide compared to untreated patients (95·8 ± 44·2 vs. 76·1 ± 23·1 per 100 cells, P = 0·08), but no differences were noted with longer exposure (97·9 ± 42·8 breaks per 100 cells for 3-6 years of hydroxycarbamide exposure vs. 91·2 ± 48·4 for 9-12 years of exposure). These observations provide important safety data regarding long-term risks of hydroxycarbamide exposure for children with SCA, and suggest low in vivo mutagenicity and carcinogenicity.

Soluble para-phenylenevinylenes (PPVs) with amino side groups and different spacers have been synthesized. Photophysical properties showed that these compounds emitted light in blue region when dissolved in methanol. They have been designed in order to be platforms for attaching different functional groups to the amino units. Molecule 5 was a model compound which showed a quantum efficiency of 52 % in methanol. Different spacers were added to the model compound to yield molecules 6 and 7 . The preliminary experiment showed that molecule 5 can interact with various metal ions in solution phase. The interaction was observed by the changes of fluorescent emission intensity of the molecule. This molecule will be further functionalized to be more specific in binding certain metal ions.

Background Demethylating agents may alter the expression of genes involved in chemotherapy resistance. We conducted a phase I trial to determine the toxicity and molecular effects of the demethylating agent, decitabine, followed by doxorubicin and cyclophosphamide in children with refractory solid tumors. Procedure Stratum A included children with any solid tumor; Stratum B included neuroblastoma patients only. Patients received a 1-hr decitabine infusion for 7 days, followed by doxorubicin (45 mg/m2) and cyclophosphamide (1 g/m2) on day 7. Pharmacokinetic studies were performed after the first dose of decitabine. Biological studies included methylation and gene expression analyses of caspase-8, MAGE-1 and fetal hemoglobin (HbF), and expression profiling of pre- and post-treatment peripheral blood and bone marrow cells. Results The maximum-tolerated dose of decitabine was 5 mg/m2/day for 7 days. Dose-limiting toxicities at 10 mg/m2/day were neutropenia and thrombocytopenia. Decitabine exhibited rapid clearance from plasma. Three of 9 patients in Stratum A and 4/12 patients in Stratum B had stable disease for ≥4 months. Sustained MAGE-1 demethylation and increased HbF expression were observed in the majority of patients post-treatment (12/20 and 14/16, respectively). Caspase-8 promoter demethylation and gene expression were seen in 2/7 bone marrow samples. Differentially expressed genes were identified by microarray analysis. Conclusion Low-dose decitabine when combined with doxorubicin/cyclophosphamide has tolerable toxicity in children. However, doses of decitabine capable of producing clinically relevant biologic effects were not well tolerated with this combination. Alternative strategies of combining demethylating agents with non-cytotoxic, biologically targeted agents such as histone deactelyase inhibitors should be explored. Pediatr Blood Cancer. 2010;55:629–638. © 2010 Wiley-Liss, Inc.

Thymomas are thymic epithelial neoplasms, associated with a variety of autoimmune disorders (especially myasthenia gravis), that apparently result from aberrant intra-tumourous thymopoiesis and export of inefficiently tolerized T-cells to the periphery. The autoimmune regulator (AIRE) drives the expression of self-antigens in the thymic medulla and plays an essential role in ‘central’ tolerance in both humans and mice. However, while inactivating AIRE mutations result in the ‘autoimmune polyendocrinopathy syndrome type 1’ (APS-1), its major features are not well reproduced in AIRE-knock-out mice. Therefore, alternative human disease scenarios with concomitant AIRE deficiency may be valuable tools to test conclusions drawn from mouse models. Here we show, in a large series, that ∼95% of thymoma patients are ‘chimeric’; expression of AIRE and major AIRE-related autoantigens (eg insulin) were undetectable in their tumours but maintained in their remnant thymic tissue and lymph nodes. Notably, despite the AIRE-deficient thymopoiesis in thymomas, disorders and autoantibodies typical of APS-1 were distinctly uncommon in these patients. The one striking similarity was in the recently observed neutralizing anti-type I interferon (IFN) antibodies, which are found at diagnosis in 100% of patients with APS-1 and in ∼60% of patients with thymomas, as we show here. We conclude that APS-1 type autoantigens must be protected from autoimmunity by mechanisms that do not extend to the muscle autoantigens so frequently targeted in thymoma patients but so rarely recognized in APS-1. Thus our findings argue strongly for a tolerogenic function of AIRE beyond its role in negative T-cell selection in human thymopoiesis, and/or for specific autoimmunization against muscle in thymomas. Copyright © 2007 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The photoluminescence (PL) and electroluminescence (EL) of four 2,7-bis(phenylethenyl)fluorenes (OFPVs) and two 2,7-diphenylfluorenes (OFPhs) are compared to evaluate effects of nonconjugating peripheral substitution and conjugation length on their EL emissions. The OFPVs exhibit very similar PL spectra with 460–480 nm emission maxima but show large variation in the organic light-emitting diode (OLED) efficacy: from a material that does not give persistent emission in test OLEDs (9,9-diheptyl substitution on the fluorene ring) to materials with luminance efficiencies of 0.5 cd A–1 and greater (9,9-diethyl substitution on the fluorene ring, methoxy and methoxy/heptyloxy substituents on the phenylethenyl rings). The best OFPV in an ITO/PEDOT:PSS/(emitter)/Ca–Al (ITO: indium tin oxide; PEDOT: poly(ethylenedioxythiophene); PSS: poly(styrene sulfonate)) OLED configuration has 9,9-diethyl substitution and terminal heptyloxy substitution (maximum luminance of 1500 cd m–2 at 12 V). Unlike the OFPVs, the neat OFPhs show not only EL at the desired blue output of ca. 400–410 nm emission maxima but also an undesired green emission component at 500–550 nm. Blending the OFPhs with poly(methyl methacrylate) eliminates the long-wavelength component when the emitter load is 10–25 %, but the OFPh luminance efficiencies, turn-on voltages, and maximum luminance tend to be poorer than those of the OFPVs. The deficiencies of the OFPhs appear to be attributable to thermal degradation and oxidative reactivity, although solid-state annealing and a nonoptimal bandgap match to the OLED device configuration may also contribute.

The zero field splitting parameters of two 2,6-dinitrenopyridines were estimated to be |D/hc| ≅ 0.19–0.20 cm−1 and |E/hc| ≅ 0.064–0.066 cm−1 by lineshape simulation of frozen solution X-band ESR spectra. This large ratio of |E/D| is consistent with a dominating dipolar interaction between the two nitrene units having a relative interaction vector angle of about 114°–116°. UB3LYP/6-31G* computations of 2,6-dinitrenopyridine predict a very similar interaction vector angle. These results are quite different from analogous results observed for meta-dinitrenobenzenes. The experimental zero-field splitting (zfs) and computational results indicate that the ESR spectroscopy of 2,6-dinitrenopyridines are dominated by the one-center interactions between unpaired electrons on the nitrene units (the triplet mononitrene zfs), and the relative geometry of interaction between the mononitrenes caused by the molecular geometry of the dinitrenopyridine (the interaction vector angle). Copyright © 2006 John Wiley & Sons, Ltd.

Radio-tagged adult grayling Thymallus thymallus(n = 22), monitored from mid August to mid December 1999 in the River Kuusinkijoki, Finland, shifted by the end of September (water temperature 10·0–14·5° C) from riffle sites to deeper and slower pool sites 0·7–1·6 km up- or downstream. In early winter (c. 0° C water temperature), eight of 13 fish still under study made a further shift into new pool sites, possibly triggered by ice formation. The summer range of grayling in the riffles was smaller (mean ± s.d. length: 75 ± 146 m) than the autumn range (99 ± 46 m) in the pools, but gross daily movements were equally short in both the seasons (18 ± 34 m and 15 ± 7 m, respectively). In late summer, adult grayling preferred water depths 80–120 cm and mean velocities >40 cm s−1. In autumn, the preferred ranges were 100–240 cm and

AIM d-myo-inositol-1,2,6-trisphosphate (alpha-trinositol, PP56), is a synthetic isomer of the intracellular second messenger, d-myo-inositol-1,4,5-trisphospahate. The pharmacological actions of alpha-trinositol include potent anti-inflammatory properties and inhibition of the secretion induced by cholera toxin and obstructive ileus. In the present study, we investigated whether alpha-trinositol was able to influence the secretion induced by heat-stable ST-toxin from Escherichia coli in the rat jejunum. METHODS A midline abdominal incision was performed in anaesthetized male Sprague-Dawley rats and a 6-7 cm long jejunal segment was isolated with intact vascular supply and placed in a chamber suspended from a force displacement transducer connected to a Grass(R) polygraph. Intestinal net fluid transport was continuously monitored gravimetrically. Crystalline ST-toxin (120 mouse units) was introduced into the intestinal lumen and left there for the rest of the experiment. When a stable secretion was observed, alpha-trinositol (60 mg kg-1 h-1) or saline were infused during 2 h, followed by a 2-h control period. RESULTS alpha-Trinositol induced a significant (P < 0.001) inhibition of ST-toxin secretion within 30 min, lasting until 2 h after infusion had stopped. The agent also moderately increased (P < 0.05) net fluid absorption in normal jejunum. Mean arterial pressure (P < 0.001) and heart rate (P < 0.001) were reduced by alpha-trinositol. CONCLUSION The inhibition by alpha-trinositol of ST-toxin induced intestinal secretion is primarily secondary to inhibition of secretory mechanisms and only to lesser extent due to increased absorption. The detailed mechanisms of action have not been clarified but may involve suppression of inflammation possibly by means of cellular signal transduction.

Carbon—carbon bond-forming polymerization of 2-bromo-3-(3′,5′-di-t-butyl-4′-methoxyphenyl)-thiophene yields poly[3-(3′,5′-di-t-butyl-4′-methoxyphenyl)-2,5-thienylene] with regiospecific connectivity and a degree of polymerization of about six. Lewis-acid-moderated-cleavage of the methoxy groups on the pendant phenyl group yield the corresponding polyphenolic polymer, which is oxidized under solution conditions to yield the title polyradical. Poly[3-(3′,5′-di-t-butyl-4′-oxyphenyl)-2,5-thienylene] exhibits a strong, persistent electron spin resonance spectrum and a UV–visible spectrum consistent with formation of the pendant phenoxyl spin-bearing units. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 779–788, 1999

Changes in cortical activity during foot shock were assessed under conscious and propofol-anesthetized conditions using functional magnetic resonance imaging (fMRI). Increases in signal intensity were observed in the contralateral somatosensory cortex in response to electrical shock of the hindpaw under both conditions. These increases in cortical signal ranged from 6% to 26% while awake and from 1% to 6% under propofol anesthesia. In each of the six animals studied, the largest increase in blood oxygenation level-dependent (BOLD)-based signal intensity was observed during consciousness. In three of six animals, propofol anesthesia depressed signal intensity by as much as 10-fold, showing that the level of cortical activity during foot shock is dampened by anesthesia. These results indicate it would be advantageous to use fully conscious animals to maximize BOLD-based MRI signal in certain behavioral studies using MR spectrometers with modest field strengths (1.0-2.0 T).

A photochemical precursor to a pendant conjugated polyradical has been synthesized, poly[3,5-di-tert-butyl-4-[(2,4,6-tri-tert-butylphenyl)oxalato] phenylacetylene], 3. Irradiation of 3 at 77 K in the solid state at < 300 nm yielded poly(3-5-di-tert-butyl-2-oxyphenyl acetylene), 2, with 30-40% of the expected number of radical spins. Spin yields on the surface of solid samples appears to be considerably higher. Electron spin resonance experiments showed no evidence of cooperative exchange interaction between the pendant spins. Computational modeling indicated that a major reason for the failure of this and other polyphenylacetylenes to show ferromagnetic exchange between spins is the substantial twisting of the polyacetylene backbone required by steric interactions, leading to deconjugation and a loss of exchange interaction between pendant radicals along the chain.

This chapter gives an introduction to the basic concepts of magnetism in organic paramagnetic soft matter materials. Key concepts are emphasized using example case studies. Detailed analysis covers radicals functionalized with phenols and with benzimidazole functionalities, which induce various degrees of crystal self-assembly, depending on specific structures. A review with over 200 references and notes.

A series of m,n′-diazidodiphenyl ethers (m ≤ n, m = 3,4; n = 3,4) was photolyzed at 77 K in frozen, glassy 2-methyltetrahydrofuran matrices to generate the corresponding diphenyl ether m,n′-dinitrenes for study by electron spin resonance (ESR) spectroscopy. 3,4′-Diazidodiphenyl ether gave an ESR spectrum dominated by a mononitrene peak with ∣D/hc∣ = 0·972 cm−1, and also showed a weak dinitrene quintet spectrum with ∣D/hc∣ = 0·162 cm−1 having ESR spectral intensity vs temperature dependence (Curie law) consistent with either a high-spin ground state or a very small singlet–quintet gap. Di(3-azidophenyl) ether gave a strong mononitrene peak with ∣D/hc∣ = 0·996 cm−1 and a quintet dinitrene ESR spectrum (∣D/hc∣ = 0·162 cm−1) which exhibited non-linear Curie law intensity behavior consistent with the quintet being a thermally populated excited state 40 cal mol−1 above a singlet ground state. Di(4-azidophenyl) ether gave a strong mononitrene peak with ∣D/hc∣ = 0·961 cm−1, but no observable spectrum related to a high-spin open-shell dinitrene. The results are consistent with oxygen being a weak exchange coupling linker in pi-conjugated open-shell molecules. The observed ground-state spin multiplicities are in accord with qualitative superexchange and connectivity models, despite any perturbations due to resonance effects between the oxygen linker and p-nitrene sites.

Triplet zero-field splitting parameters were obtained in 2-methyltetrahydrofuran glass at 77 K for phenylene-1,4-dinitrene (1), biphenyl-4,4′-dinitrene (2), (E)-1,2-bis(4′-nitrenophenyl)ethene (3), 1,4-bis(4′-nitrenophenyl)buta-1,3-diene (4) and 1,8-bis(4′-nitrenophenyl)octa-1,3,5,7-tetraene (5). The results were (1) |D/hc| = 0·169 cm−1 |E/hc| = 0·004 cm−1, (2) |D/hc| = 0·189 cm−1, |E/hc| = 0·00 cm−1, (3) |D/hc| = 0·122 cm−1, |E/hc| = 0·00 cm−1, (4) |D/hc| = 0·0865 cm−1, |E/hc| = 0·00 cm−1 and (5) |D/hc| = 0·0442 cm−1, |E/hc| = 0·00 cm−1. All these biradicals are ground-state singlets. Based on the observed decrease in triplet signal intensities as temperature decreases. The substantial magnitudes of |D/hc| for 3–5, despite the large distance between localized nitrene electrons, is much more than can be explained by a simple dipolar interaction between localized electrons, and is attributed at least partly to spin polarization effects on the π-electron clouds of these systems.

The electronic states of open-shell stilbene diradicals were studied. Previously used semiempirical MO-CI methods were applied to stilbenes having dimethylene, dioxyl, and dinitroxide centers in o,o'-, o,m'-, o,p'-, m,m'-, m,p'-, and p,p'-substitution patterns, and the vinous singlet-triplet energy gaps were computed to predict qualitative ground state spin multiplicities. Singlet and triplet spin states were essentially degenerate in the m,m'-isomers of disjoint connectivity, while nondisjoint o,m'- and m,p'-isomers had triplet computed ground states. Nitroxide-based diradicals showed considerably smaller computational exchange coupling energies than did other diradical models studied. As a test of these computations, stilbenes with two N-tert-butyl nitroxide groups at the o,m'-, m,m'-, and m,p'-positions were prepared, and their magnetic properties were studied by ESR and magnetic susceptibility measurements. The m,m'- and m,p'-isomers were found experimentally to have singlet and triplet ground states, respectively, in accord with the qualitative computed predictions

Photolysis of 10,10-dimethylanthrone-2,7-diazide in a glassy matrix at 77 K gives the corresponding quintet dinitrene molecule with a singlet ground state (ΔE(S → Q) = 810 J/mol). This system shows disjoint exchange behavior in a case where conjugation is assured by an enforced planar geometry.

Chemically stable poly[2-(3,5-di-tert-butyl-4-oxyphenyl)-1,4(p)-phenylenevinylene], poly[4-(3,5-di-tert-butyl-4-oxyphenyl)-1,3(m)-phenylenevinylene], and poly[4-(3,5-di-tert-butyl-4-oxyphenyl)-1,2(o)-phenylenevinylene] were synthesized via the polymerization of 4-bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)styrene, 5-bromo-2-(3,5-di-tert-butyl-4-acetoxyphenyl)styrene, and 2-bromo-4-(3,5-di-tert-butyl-4-acetoxyphenyl)styrene, respectively, using a palladium catalyst. The o- and p-polyradicals, even with their spin concentration of 0.6 spin/unit, displayed S values of 4/2 to 5/2 (S = 5/2 for the o-polyradical with a degree of polymerization of 17 and a spin concentration of 0.68), while the m-polyradical has a singlet ground state. A long-range intramolecular through-bond ferromagnetic exchange interaction was realized between the pendant unpaired electrons for the o- and p-polyradicals, in accordance with theoretical predictions based on the molecular connectivity. The average value of the exchange coupling...

A three-pot synthesis of 4-azidobenzaldehyde from 4-nitrobenz-aldehyde in overall yield of 71% is described. The new synthesis is superior in its ease of reproducibility and reduction of waste side products.

Various examples are described of a multi-strategy approach to improvement of multispin exchange design. For example, H-bonded benzimidazole 4-t-butylnitroxide shows apparent simple planar 2D-type interaction, with a Weiss constant of Θ = −4.24 K and an intraplane exchange interaction of J = −1.60 K.

Time domain electromagnetic (TDEM) soundings were utilized in site assessment studies for the purpose of accomplishing two objectives: (1) measuring the migration of brines from oil and gas field evaporation pits, and (2) mapping the continuity of clay strata. Both case histories are representative of common objectives in site assessment. The data for both examples were analyzed by one-dimensional ridge regression inversions. Both case histories illustrate that TDEM is an effective method for determining the lateral and vertical resistivity distribution (geoelectric section) of the subsurface in the depth range from about 5 m to 100 m, and it is known from other investigations that depths of investigations up to 2500 m can be achieved with TDEM. Frequently, the lateral extent of contaminant plumes emanating from localized sources are of limited areal extent. The case history involving the use of TDEM to map a brine plume shows that a TDEM measurement has a relatively small zone of influence, so that meaningful information about the lateral and vertical extent of the plume can be obtained. Both case histories demonstrate the ability of TDEM to determine geoelectric sections below layers of low resistivity.

para-Xylylenes are generated by treatment of various 1,4-bis(dialkylsulfoniomethyl)arene dihalides with base in water, methanol, and aqueous acetonitrile, as shown by UV-Vis spectroscopy. This procedure allows the monitoring of the transient xylylene monomers that yield polyelectrolyte precursor polymers for poly(arylene vinylene)s, formed by variations of the chemistry developed originally by Wessling and co-workers. Alkoxy, alkyl, and halogen ring substituents on the sulfonium salt precursors do not greatly affect the ability to generate and observe the growth and decay of steady-state concentrations of the para-xylylene intermediates. Use of strong resonance-acceptor substituents—such as cyano or nitro—on the ring reduces production of a strong para-xylylene absorption, possibly due to reluctance of the ylides initially formed from the bis-sulfonium salts to eliminate to the xylylenes. By variation of UV-Vis conditions, it was found that use of 20% aqueous acetonitrile rather than water allowed formation of low to modest molecular weight polyelectrolytes in cyano-substituted cases (Mw = 8000–37,000). Use of UV-Vis test experiments should be useful for screening of bis sulfonium salt precursors that may be expected to give high molecular weight polyelectrolytes—in cases where para-xylylene formation is easy—as well as for finding reaction conditions that will optimize polymer formation. © 1992 John Wiley & Sons, Inc.

Derivatized poly(α-tetrahydrothiophenio para-xylylene) chloride were produced by aqueous or methanolic base-induced polymerization of 1,4-bis (tetrahydrothiopheniomethyl) benzene dichloride, both with and without a variety of added polymerization inhibiting agents. The results suggest that production of high molecular weight poly(α-tetrahydrothiophenio-p-xylene) chloride proceeds through a radical chain propagation sequence

Synthesis of poly(4′-vinyl-2,4,6-tri-tert-butyl-diphenyloxalate) and its 65 : 35 and 19 : 81 copolymers with styrene are described. Rigid phase broad-band photolysis of the homopolymer with a quartz filtered xenon arc at 77 K results in production of up to 25% of the theoretical number of spins/mol expected for quantitative production of pendant phenoxyl radicals, and shows no major loss of radical signal in the ESR at temperatures below 100 K. Curie law analysis of the temperature dependence of the ESR radical signal intensity for the neat photolyzed homopolymer 1 shows curvature consistent with antiferromagnetic pairing of radical spins at low temperatures. Since through-bond conjugation of radical spins is not possible in this system, the antiferromagnetic interaction is interpreted in terms of intrachain and/or interchain through-space exchange interactions.

Photolysis of biphenyl-3,4′ -diazide in a rigid glassy matrix at 77 K yields quintet state biphenyl-3,4′ -dinitrene with zero-field splitting parameters of |D/hc- = 0.153 cm−1 and |E/hc| = 0.019 cm−1 determined by electron spin resonance spectroscopic studies. Curie plot studies are consistent with assigning the quintet to be the ground state in this species. This finding confirms qualitative connectivity-based predictions for this general connectivity type of openshell system, and is in qualitative agreement with spectral INDO-CI computational predictions for both planar and twisted geometries of the dinitrene.

We have shown that the MNDO-PM3 semiempirical molecular orbital method is capable of yielding calculated structures of oxaphosphetanes and ylides that are comparable with the experimentally known structures. The MNDO-PM3 method also reproduces the geometric and thermodynamic parameters of the “mythyical Witting half-reaction”, previously calculated using a more elaborate and costly ab initio molecular orbital approach. In addition, using the SADDLE routine, we were able to find a transition state for this half-reaction. This transition state resembles geometrically the transition state found using the ab initio approach (4-31G* level). The energy of activation was calculated to be 9.2 kcal/mole, which is fairly comparable with the ab initio reslt of 5.2 kcal/mole. According to our calculations, and in general agreement with findings of Volatron and Eisenstein [11], the mythical Witting reaction reaction would be expected to proceed through a planar transition state for the cycloaddition of the ylide and the aldehyde in a process that superficially seems to be a synchronous one. However, an analysis of the evolution of the bond orders and the localized molecular orbitals throughout the reaction indicates that the PO bond has not been formed in the transition state, whereas the CC bond is about 40% formed. Therefore, the mythical Wittig half-reaction is best described as a very asynchronous cycloaddition (a bordeline two step mechanism) with a planar geometry in the transition state, composed of a multi-centered mPCCO bond orbital and with the phosphorus and the oxygen atoms sharing a strong attractive charge interaction. A similar description results from computations of the Wittig reactions of H3CHCH3 and (C6H5)3PCHCH3 with H3CCHO, which is inconsistent with the Vedejs geometry.

The literature reveals that the structures of four unusual oxaphosphetanes have been established by x-ray diffraction studies, and the structure of one mythical oxaphosphetane (6) has been deduced by theoretical calculations. We have used these known structures to calibrate the MMX87 force field, the only necessary modification being the stretching term for the P-O bond. The bond length, 1o, was set at 1.80A, which is an “average” of the P-O axial bond distances of the stable oxaphosphetanes. The AMPAC (Ver. 1.0) molecular orbital package utilizing the MNDO hamiltonian was also applied to the same problem, starting with the minimized geometries of the modified MMX87 force field. Fixed bond lengths and full minimizations were performed. The computed geometries of the four-member ring of each of the four oxaphosphetanes of known structures were found to be in very good agreement with the values obtained by x-ray diffraction. Furthermore, the method was applied to the mythical oxaphosphetane, and the results of MMX87 and MNDO calculations were found to be in very good agreement with the results of ab initio calculations. The MMX force field and the MNDO semiempirical method have been used to calculate the geometries and the steric energies (or heats of formation) of diastereomeric oxaphosphetanes formed in a theoretically real Wittig reaction.