Polyenyl Radicals Research Articles

Covalent Excited States of Polyenes C2nH2n+2 (n = 2-8) and Polyenyl Radicals C2n-1H2n+1 (n = 2-8): An Ab Initio Valence Bond Study.

The ab initio valence bond (VB) methods, VBSCF and VBCI, are applied to the ground states and the covalent excited states of polyenes C2nH2n+2 (n = 2-8) and polyenyl radicals C2n-1H2n+1 (n = 2-8). The excitation energy gap was computed at the ab initio VB level, which is in good agreement with the semiempirical VB method, VBDFT(s), and the experimental values as well as with the molecular orbital theory based methods, CASPT3 and MRCI. The ab initio VB wave functions of systems are also in very good agreement with those of the VBDFT(s) method, even though the former is based on the ab initio VB scheme while the latter is a semiempirical Hückel type method, in which no orbital optimization procedure is performed. The computational results show that the ab initio VB method is capable now of providing numerical accuracy not only for bond forming and breaking processes, as shown in the past, but also for excitation energies, as shown here. In addition, the computational results validate the efficiency of the VBDFT(s) method, which is a simple VB model with less computational effort but which provides intuitive insights into the excited states of conjugated molecules.

Radiation-Induced Modification of P(VDF/TrFE) Copolymers Studied by ESR and Vibrational Spectroscopy

Radical species produced in ferroelectric copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE) copolymers with (50/50) and (75/25) composition by fast-electron irradiation were studied by electron spin resonance (ESR). The irradiation was performed in both the para- and ferroelectric phases. The ESR spectra were found to be rather complex and changed with time. Three types of radicals were identified, e.g., the peroxy radical, and alkyl in-chain –CF2–C•H–CF2– and end-chain –C•H2 radicals. The decays of the radicals have a similar tendency and show a faster decaying process within a short period followed by a slower decaying process. The fast decay is related to the recombination reaction of alkyl radicals, whereas the slower one is due to the peroxy radicals recombination. In poly(VDF/TrFE) (75/25) copolymer irradiated in the paraelectric phase at T* = 420 K the decay of radicals, which were identified as the long-lived polyenyl radicals –CH2–(CF=CH)n–C•F–CH2–, is completed within one week and then the concentration of these radicals remains constant. The delocalization of unpaired electrons over up to five C=C bonds was estimated from the infrared studies.

Characterization of irradiated PEs/PA6 blends

In this work, changes caused by γ-radiation on binary blends of different PEs (high, low and linear low-density polyethylenes) with PA6 were studied. The structure and property changes in these samples induced by γ-irradiation were characterized by the ESR, DSC and FTIR techniques. These techniques provide relevant information concerning the reaction mechanisms of the structural modifications that take place during the irradiation of polymers. The experimental results suggest that allyl, alkyl and polyenyl radicals are generated in the materials by the irradiation process. The dynamics dominating the process kinetics seem to be crosslinking, chain scission and a coexistence of both, depending on the sample composition and the absorbed dose. The degradation of the materials produced a disruption in the ethylene sequences in polyethylenes thus decreasing their melting point. Bands attributed to carbonyl groups (C=O) were observed in FTIR spectra after the materials were irradiated.

EPR power saturation techniques and spectral differentiation are used to isolate and simulate radical species in UHMWPE

In this study, we found that the linear growth with the square of the power of each radical species in X-irradiated ultra high molecular weight polyethylene (UHMWPE) has a different slope than the other radicals present. We used this fact to isolate radicals, which are known to be in the crystalline regions of polyethylene (alkyl) from the signals due to radicals in the amorphous regions (allyl, polyenyl, peroxyl). A process to eliminate oxidation of irradiated polyethylene was also studied and is discussed at the end. Recording of the radical’s spectra was conducted at power settings as low as 10μW and as high as 100mW. Once the species were seen to be differentiated by using power saturation techniques (PST), various processing methods and spectral differentiation were used to verify the parameters used in performing simulations of selected spectra. The simulation parameters can be used to give a reasonable picture of the percentage of allyl, alkyl, peroxyl and polyenyl radicals present in UHMWPE EPR spectra. For example, simulation of a composite spectrum, obtained following the decay of the primary alkyl radicals, suggests the presence of allyl (n=1), dienyl (n=2), trienyl (n=3) and polyenyl (n>3) radicals (–C*–[CHCH–]n).

The ground and excited states of polyenyl radicals C2n-1H2n + 1 (n = 2-13): a valence bond study.

The semiempirical valence bond (VB) method, VBDFT(s), is applied to the ground states and the covalent excited states of polyenyl radicals C2n - 1H2n + 1 (n = 2-13). The method uses a single scalable parameter with a value that carries over from the study of the covalent excited states of polyenes (W. Wu, D. Danovich, A. Shurki, S. Shaik, J. Phys. Chem. A, 2000, 104, 8744). Whenever comparison is possible, the VB excitation energies are found to be in good accord with sophisticated molecular orbital (MO)-based methods like CASPT2. The symmetry-adapted Rumer structures are used to discuss the state-symmetry and VB constitution of the ground and excited states, and the expansion to VB determinants is used to gain insight on spin density patterns. The theory helps to understand in a coherent and lucid manner the properties of polyenyl radicals, such as the makeup of the various states, their geometries and energies, and the distribution of the unpaired electrons (the neutral solitons).

Preparation of Proton-Conducting Membranes by Direct Sulfonation. 1. Effect of Radicals and Radical Decay on the Sulfonation of Poly(vinyl fluoride) Films

The effect of irradiation treatment on the structure and sulfonation reactivity of polyvinyl fluoride (PVF) films was studied mainly with EPR and IR spectroscopy. The main radical species produced by electron irradiation are aliphatic radicals, whereas proton irradiation creates a significant amount of polyenyl radicals. The half-life time of the aliphatic radicals was about 6 h. FTIR study shows that sulfonation reaction of PVF is a single-stage process and thus the formed sulfonic acid structures do not react further and produce new structures. In addition, it is independent of the irradiation treatment. Despite the careful synthesis and removal of O2 from the sulfonation solution, the most important side reaction produced by the sulfonation is the formation of CO functionalities. Irradiation treatment increases the CO content. Both the sulfonation time and sulfonation reagent concentration demonstrate similar linear correlation with the extent of oxidation. Therefore, the oxidation cannot be decreased ...

Effect of carbonyls on the reactivity of polyenes in autoxidation

AbstractAutoxidation of the biologically active polyenes β‐carotene, canthaxanthin, retinyl acetate, methyl retinoate, retinal and 3,7,11,11‐tetramethyl‐10,15‐dioxo‐2,4,6,8‐hexadecatetraenal (diketoretinal) in solid amorphous films was investigated. The course of the process was followed by electronic and IR spectroscopy. The overall activation energies were obtained. It was shown that insertion in the polyene molecule of a carbonyl group conjugated with polyene chain results in a drastic decrease in the reactivity towards molecular oxygen. The results are discussed and explained on the basis of radical stabilization energy, ES(R·), as a driving force of the autoxidation process. Semi‐empirical AM1 calculations of ΔHf of some retinyl polyenes and polyenyl radicals were carried out and the C—H bond strengths and ES(R·) values of corresponding polyenyl radicals were evaluated. It was shown that the incorporation of a carbonyl group in the polyene results in a decrease in the overall autoxidation rate due to the decrease in both the initiation and propagation rates. Copyright © 2003 John Wiley & Sons, Ltd.

Beyond butadiene II: thermal isomerization of the [2 + 2] photodimer of an all-trans tetraene, (R)-4,4abeta,5,6,10a-hexahydro-10abeta-methyl-2(3H)-methyleneanthracene, to a 16-membered [8 + 8] cycle.

Enthalpies of stabilization of polyenyl radicals of increasing order previously obtained by thermal geometrical isomerization are applied to the ethylene-cyclobutane paradigm. Progressively lower enthalpies of activation for thermal cyclodimerization and its reverse, cycloreversion, are predicted and realized. Photochemical dimerization at -75 degrees C of the optically pure tetraene of the title (1) at the semicyclic double bond produces in the main only one (4-axx) of the three allowed cyclobutanes (4), to which the tentative configuration anti-exo,exo is assigned. Equilibration among the three cyclobutanes (4), a slower rearrangement to a thermodynamically considerably more stable, [8 + 8] cyclohexadecahexaene (16), and a surprisingly slow fragmentation to 1 are studied kinetically between -42.3 and -8.2 degrees C. Cycloreversion of the dimer 16 to monomer 1 occurs in the range 60.4-86.6 degrees C (DeltaH = 31.7 kcal mol(-1), DeltaS = +10.8 cal mol(-1) K(-1)). The ratio of the rates of stereomutation and cycloreversion is significantly larger in these 1,2-dihexatrienylcyclobutanes than in two less strongly stabilized, previously published examples. The possible extension of Doubleday's calculational finding of entropic control of products from cyclobutane is considered.

A study of long-lived free radicals in gamma-irradiated medical grade polyethylene

Ultra-high molecular weight polyethylene (UHMWPE), used in orthopaedic implants, was irradiated with gamma rays at room temperature (23°C) in air or vacuum, and it was then stored in the same environment for 2 yr at 23°C, 37°C, or 75°C. ESR measurements showed decay as well as changes from alkyl to peroxy and polyenyl radicals in air due to reaction with oxygen. In vacuum, on the other hand, radicals (primarily allyl) decayed without any observable transformation from one type to another. The residual concentrations obtained after 2 yr of post-sterilization storage at 23°C, 37°C, and 75°C were found to be 15–20%, 10–15%, and 2–10%, respectively.

Attempts to model neutral solitons in polyacetylene by ab initio and density functional methods. The nature of the spin distribution in polyenyl radicals

Different quantum chemical methods have been examined, in order to test their abilities to model long polyenyl radicals and, in the limit, a neutral soliton in polyacetylene. For polyenyl radicals up to C9H11, it was possible to use a range of methods up to CCSD(T). A number of interesting features of the geometries and spin distributions were found in these radicals. Coupled cluster methods reproduced rather well the experimental ratios of the spins at each of the carbons in allyl, pentadienyl, and heptatrienyl radicals. In contrast, both restricted and unrestricted Hartree–Fock methods gave very poor results. CASSCF gave good geometries and π spin distributions, but only if all the π MOs were included in the active space. Both pure (BLYP) and hybrid (B3LYP) DFT methods were found to over-estimate the widths of solitons in long polyenyl chains.

Reactivity of polyenes in solid‐state autoxidation

The reactivity of polyenes (β-carotene, canthaxanthin, lycopene and some retinyl polyenes as related compounds) in the solid state towards molecular oxygen at various temperatures and oxygen pressures was studied. The specimens were thin (ca 0.1 µm) amorphous films prepared by evaporation of one or two drops of appropriate solution on quickly rotating supports. The autoxidation was monitored by recording the changes in the electronic and infrared spectra of oxidizing films and was demonstrated to be a chain free-radical process with extremely high chain-initiation rates (10−5–10−6 mol l−1 s−1). For various polyenes the kinetic parameters of the initiation reaction were measured. In the films of several polyenes (retinyl acetate, retinal, methyl retinoate, β-carotene), the formation of free radicals proceeds in the absence of oxygen and the mechanism of the process was suggested. The observed ratios of the propagation and termination rate constants depend on the oxygen pressure, indicating the involvement of polyenyl radicals in the chain termination process. The observed trends are explained using the concepts of the stabilization energy of radicals, the reversibility of oxygen addition to polyenyls and morphological features of polyene films. Copyright © 1999 John Wiley & Sons, Ltd.

Radiation-sterilization and subsequent oxidation of medical grade polyethylene: an ESR study

Oxidation processes of irradiated ultra-high molecular weight polyethylene (medical grade, GUR 415) was examined by making free radical measurements before and after introduction of oxygen into the sample chamber. The free radicals (predominantly, alkyl and allyl), produced by X-irradiation at 45 K, remained frozen in the temperature regime 45–150 K. They, however, decayed rapidly (6 × 10 −3/K) in the temperature regime 150–190 K (γ-dispersion region) and slowly (2 × 10 −3/K) in the β-dispersion region, 190–300 K. The oxidation of the remaining radicals (about 50% of the initial) occurred in presence of oxygen at 300 K. While the presence of oxygen-centered radicals (PO 2 or PO ) could not be confirmed, evidences of vinyl type and polyenyl radicals of various conjugations were found.

The stabilization energies of polyenyl radicals

The resonance stabilization energies, Es, of polyenyl radicals can be estimated by the equation Es(N)=−13.2+[3.95–15.8(2) −2/n] kcal mol−1, where N is the number of C, C-π bonds in the polyenyl radicals. This correlation has been extended for predicting the weakest HC, CC and COH bond dissociation energies in vitamin A and similar compounds.

Spin-Coupled Study of the Electronic Structure of Polyenyl Radicals C3H5-C9H11

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSpin-Coupled Study of the Electronic Structure of Polyenyl Radicals C3H5-C9H11Peter B. Karadakov, Joseph Gerratt, Guido Raos, David L. Cooper, and Mario RaimondiCite this: J. Am. Chem. Soc. 1994, 116, 5, 2075–2084Publication Date (Print):March 1, 1994Publication History Published online1 May 2002Published inissue 1 March 1994https://doi.org/10.1021/ja00084a053RIGHTS & PERMISSIONSArticle Views44Altmetric-Citations15LEARN 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 (1 MB) Get e-Alerts Get e-Alerts

ChemInform Abstract: Stabilization Energy of Polyenyl Radicals: all‐trans‐Nonatetraenyl Radical by Thermal Rearrangement of a Semirigid (4‐1‐2) Heptaene. Model for Thermal Lability of β‐Carotene.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Stabilization energy of polyenyl radicals: all-trans-nonatetraenyl radical by thermal rearrangement of a semirigid {4-1-2} heptaene. Model for thermal lability of .beta.-carotene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStabilization energy of polyenyl radicals: all-trans-nonatetraenyl radical by thermal rearrangement of a semirigid {4-1-2} heptaene. Model for thermal lability of .beta.-caroteneW. v. E. Doering and Keshab SarmaCite this: J. Am. Chem. Soc. 1992, 114, 15, 6037–6043Publication Date (Print):July 1, 1992Publication History Published online1 May 2002Published inissue 1 July 1992https://doi.org/10.1021/ja00041a022RIGHTS & PERMISSIONSArticle Views373Altmetric-Citations32LEARN 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 (1 MB) Get e-Alerts Get e-Alerts

A quantum chemical study of the degradation and the maximum polyene length in PVC

The semi-empirical MINDO/3, AM1 and MNDO-PM3 methods were employed to study the stability of polyenes, polyenyl cations and polyenyl radicals. Both cations and radicals were subjected to computation since the elimination of HCl from PVC might proceed according to an ionic or radical mechanism. We subsequently investigated the effect of additional alkane chain ends, as well as PVC-like chain ends to both ends of the polyene sequence, in order to mimic degraded PVC better. The results are discussed in relation to the experimentally observed polyene length distribution in degraded PVC, as revealed from UV-Vis and Resonance Raman Spectroscopic data. The following conclusions may be drawn from the data presented. The MINDO/3 method has to be rejected. From the calculations, which basically refer to the molecules being in the gas phase, it was concluded that the formation of radical intermediates during polyene formation is much preferred (energetically) as compared to the corresponding ionic species. However, several researchers have suggested that the ionic process is preferred in solid PVC (‘solvation effect’); for solid PVC hard experimental evidence is still lacking with respect to either of the two mechanisms. The introduction of saturated chain ends markedly influences the energetics and the stability of the polyenes. HCl abstraction from an intact PVC chain was calculated to be about as probable as HCl abstraction adjacent to a polyene sequence. Calculations on polyenes with PVC-like chain ends do not give any indication that there is a maximum polyene length after which further elimination of HCl becomes less probable from an energetic point of view. The calculated data presented show no evidence for a finite polyene length, and thus do not support the unzipping mechanism. We propose a new one-step unzipping mechanism.

Thermal cis-trans rearrangement of semirigid polyenes as a model for the anticarcinogen .beta.-carotene: an all-trans-pentaene and an all-trans-heptaene

To assess the possible role of a 15,15' 90° -twisted, singlet diradical in the anticarcinogenicity of β-carotene, activation parameters for the thermal, cis-trans interconversion of all-trans-β-carotene and 15,15'-cis-β-carotene, polyenes of order 11, are needed. Indirect achievement of this goal is initiated here by determining activation parameters for cis-trans rearrangement in a set of semirigid, all-trans-polyenes of order 3, 5, and 7 (9 to follow). These data also provide a test of theoretical calculations of stabilization energies of polyenyl radicals

Benzophenone-sensitized photo-degradation of polyolefins: Influence of benzophenone on model compounds

The influence of benzophenone (BP) on the photo-degradation of model compounds of polyethylene and polypropylene was examined by means of ESR spectra of the compounds irradiated at − 196°C with light of λ>300 nm. BP-sensitized n-hexane, 2,4-dimethylpentane and 1-octene yielded 8-line, 4-line and 6-line spectra with splitting constants of 22 gauss, 24 gauss and 20 gauss, respectively. The spectral intensities increased with the amount of BP, but decreased gradually above a certain concentration of BP. The spectra recorded for the three samples disappeared when large amounts of BP were present, being replaced by a singlet spectrum with a line width of 14 gauss. By studying UV spectra, the formation of polyene with a triene structure was observed when 1-octene was irradiated in the presence of BP. BP is believed to influence the photo-degradation of model compounds such that it participates in the formation of alkyl radicals, but promotes the formation of polyenyl radicals in the system containing a large amount of BP.

Benzophenone-sensitized photo-degradation of polypropylene

The effects of benzophenone (BP) on the photo-degradation of polypropylene (PP) were investigated by analyzing the ESR and infrared spectra of the irradiated samples. In the system irradiated with light of λ>300 nm, samples irradiated at both −196°C and room temperature yielded a broad 4-line spectrum with a splitting constant of 24 gauss (alkyl radical), the intensities of which increased with the amount of BP. In the system irradiated with light of λ>220 nm, the sample irradiated at −196°C exhibited the broad 4-line spectrum, on which a sharp 4-line component with a splitting constant of 23 gauss (methyl radical) was superimposed, while the sample irradiated at room temperature mainly showed a singlet spectrum (polyenyl radical). Infrared spectra of the irradiated samples showed that the formation of double bonds on PP is extremely difficult as compared with polyethylene. Thus, it is believed that BP might abstract tertiary hydrogen atoms from the PP substrate to promote the formation of alkyl radicals and the contribution of BP to the formation of polyenyl radicals, which was observed for polyethylene, is small.