A comparative quantum chemical study of ethylcarbonium ion and hydroxymethylcarbonium ion

Nonempirical molecular orbital calculations of the energies of CH₃CH (ethylcarbonium ion) and HOCH (hydroxymethylcarbonium ion) as a function of rotation about the C? C or C? O bonds and deviation from coplanarity at the carbonium ion center are reported. As expected, and in agreement with previous work, both carbonium centers are planar and there is no barrier to rotation in the planar ethylcarbonium ion. However, for the planar configuration at carbon, the conjugative interaction between oxygen and carbon produces a barrier to rotation about the C? O bond of HOCH of 19.6 Kcal/mole. When a pyramidal geometry is imposed upon the carbonium ion center of CH₃CH, a typical three-fold barrier results. As the deviation from coplanarity increases there is a regular increase in the barrier height (1.72 Kcal/mole at the tetrahedral geometry), but the energy minimum remains at the same position in each case (60°). For HOCH, imposition of a pyramidal geometry on the carbonium ion center causes a change in both rotational barriers. One decreases slightly (from 19.6 to 15.4 Kcal/mole) and the other increases to 30.5 Kcal/mole. There is an accompanying change in the position of the minimum of the rotational potential, from 90° towards the gauche structure.     

Efficiency of radiation-induced main-chain scission of poly (methyl methacrylate) depends on the irradiation temperature because of coexisting monomer

Effect of irradiation temperature on the main-chain scission of poly (methyl methacrylate) (PMMA) caused by γ-irradiation was studied by means of gel permeation chromatography and ESR spectroscopy. Although no temperature dependency was observed on the scission efficiency for purified PMMA, the efficiency for crude or monomer-doped purified PMMA was decreased by decreasing the temperature below ca. 200 K. Above 200 K the efficiency was constant and did not depend on the purity of PMMA. ESR study of the irradiated PMMA revealed that the suppression of the scission below 200 K is induced by the addition of methyl methacrylate monomer to primary radical species, which otherwise cause the main-chain scission by warming the polymer above 200 K. The primary radical generated above 200 K immediately converts to the scission-type ? CH₂ ? ?(CH₃) COOCH₃ radical through the β-scission of the polymer main chain, so that the efficiency of the scission does not depend on both the impurity and the irradiation temperature. © 1994 John Wiley & Sons, Inc.     

Studies of ESR Nitroxide probe mobility in polymer blends

We report studies of the temperature-dependence of the ESR spectrum of the nitroxide spin radical 4-(2-bromoacetamide)-2,2,6,6 tetramethyl-1-oxyl piperidine (BRAMO) dispersed in poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), and PVDF/PMMA and PVAC/PMMA blends of varying composition. In PVDF/PMMA blends which show a single composition-dependent T_g, the mobility of BRAMO is identical to that in pure PMMA. On the other hand, in PVAC/PMMA blends, the mobility of BRAMO corresponds to that in pure PVAC. The results suggest that (1) BRAMO selectively binds to polymers based on hydrogen bonding affinity, (2) the spin probe is sensitive to segmental motions on a length scale shorter than those which give rise to the glass transition, and (3) compatible polymer blends are heterogeneous on the length scale of the BRAMO probe (ca. 8.3 Å).     

Molecular mechanics assessment of the configurational statistics of polyoxyethylene

Previous investigators have shown that statistical mechanical averages for configuration-dependent physical properties of long unperturbed polyoxyethylene chains are sensitive to the gauche–trans energy difference for rotation about C? C bonds. Agreement between theory and experiment could be obtained only by significant adjustment of this energy away from values predicted by semiempirical conformational energy computations. The present work examines the success of MM 2 in evaluating conformational properties of long unperturbed polyoxyethylene chains. Calculations are performed which identify the rotational isomers, and their energies, for the indicated bonds in CH₃OCH₂CH₂O? CH₂? CH₂? OCH₂CH₂OCH₃. These energies are used to assign statistical weights utilized in the configuration partition function for a rotational isomeric state chain with symmetric threefold interdependent rotations. The customary generator matrix scheme is employed to evaluate the mean-square unperturbed end-to-end distance, mean-square unperturbed dipole moment, and their temperature coefficients. Contrary to computational schemes employed previously, MM 2 is found to provide an estimate of the gauche–trans energy difference for rotation about C? C which is in harmony with the known dimensions and dipole moments of the unperturbed polymer. MM 2 also provides good estimates for most of the other parameters required in the rotational isomeric state treatment. A notable exception is provided by the gauche–trans energy difference for rotation about the C? O bond. This energy difference is overestimated by MM 2.     

Molecular motions of tactic poly(2-hydroxyethyl methacrylate) in solutions studied by 13C-NMR relaxation measurement

The spin-lattice relaxation time and the nuclear Overhauser enhancement were measured using Bruker AM 300 spectrometer operating at 75.5 MHz for ¹³C to investigate the molecular motional characteristics and its tacticity effect for tactic poly(2-hydroxyethyl methacrylate) (PHEMA) as a function of temperature in dimethyl sulfoxide and methanol solvents. The observed relaxation data have been analyzed for both backbone motion and methyl internal rotation according to the log-χ² distribution model and the diamond-lattice model. The correlation times thus obtained for the molecular motions show that isotactic PHEMA is more flexible than syndiotactic counterpart. The syndiotactic PHEMA seems to have intramolecular hydrogen bonding which restricts the motion of C-2 carbon at temperatures below 35°C, whereas the isotactic one indicated no hydrogen bonding at all temperatures examined in this study. The methyl group of isotactic PHEMA shows a greater degree of freedom for the internal rotation than that of syndiotactic one. From the temperature dependence of correlation times, the activation energies for both backbone segmental motion and methyl internal rotation are obtained. The activation energies, 20 kJ/mol for backbone motion and 19 kJ/mol for methyl internal rotation, for isotactic PHEMA are substantially lower than the corresponding activation energies of 30 and 32 kJ/mol obtained for syndiotactic one. An examination of these energies indicates that methyl side group and backbone motions in tactic PHEMA are linked together.     

Very low-pressure pyrolysis (VLPP) of pentynes. II. 4-methylpent-2-yne and 4,4-dimethylpent-2-yne. Heats of formation and resonance stabilization energies of methyl-substituted propargyl radicals

Studies of the unimolecular decomposition of 4-methylpent-2-yne (M2P) and 4,4-dimethylpent-2-yne (DM2P) have been carried out over the temperature range of 903–1246 K using the technique of very-low pressure pyrolysis (VLPP). The primary reaction for both compounds is fission of the C? C bond adjacent to the acetylenic group producing the resonance-stabilized methyl-substituted propargyl radicals, CH₃C??H(CH₃) from M2P and CH₃C?C?(CH₃)₂ from DM2P. RRKM calculations were performed in conjunction with both vibrational and hindered rotational models for the transition state. Employing the usual assumption of unit efficiency for gas-wall collisions, the results show that only the rotational model with a temperature-dependent hindrance parameter gives a proper fit to the VLPP data over the entire experimental temperature range. The high-pressure Arrhenius parameters at 1100 K are given by the rate expressions log k₂ (sec^?1) = (16.2 ± 0.3) ? (74.4 ± 1.5)/θ for M2P and log k₃ (sec^?1) = (16.4 ± 0.3) ? (71.4 ± 1.5)/θ for DM2P where θ = 2.303RT kcal/mol. The A factors were assigned from the results of recent shock-tube studies of related alkynes. Inclusion of a decrease in gas-wall collision efficiency with temperature would lower both activation energies by ～1 kcal/mol. The critical energies together with the assumption of zero activation energy for recombination of the product radicals at 0 K lead to DH⁰[CH₃CCCH(CH₃)? CH₃] = 76.7 ± 1.5, ΔH_f⁰[CH₃CCCH(CH₃)] = 65.2 ± 2.3, DH⁰[CH₃CCCH(CH₃)? H] = 87.3 ± 2.7, DH⁰[CH₃CCC(CH₃)₂? CH₃] = 72.5 ± 1.5, ΔH[CH₃CC?(CH₃)₂] = 53.0 ± 2.3, and DH⁰[CH₃CCC(CH₃)₂? H] = 82.3 ± 2.7, where all quantities are in kcal/mol at 300 K. The resonance stabilization energies of the 1,3-dimethylpropargyl and 1,1,3-trimethylpropargyl radicals are 7.7 ± 2.9 and 9.7 ± 2.9 kcal/mol at 300 K. Comparison with results obtained previously for other propargylic radicals indicates that methyl substituents on both the radical center and the terminal carbon atom have little effect on the propargyl resonance energy.     

Study of spin–lattice relaxation and local motion in dissolved poly[2,2-propanediylbis(3,5-dimethyl-4, hydroxyphenyl)carbonate]

Proton and carbon-13 spin–lattice relaxation times are reported for 10-wt % solutions of tetramethyl bisphenol-A polycarbonate. The relaxation times for both nuclei were measured at two Larmor frequencies and as a function of temperature. These relaxation times are interpreted in terms of three motions: segmental motion, restricted rotational diffusion, and backbone methyl-group rotation. The Hall–Helfand correlation function is used to describe the segmental motion. Internal rotation is described by the usual Woessner approach and restricted anisotropic rotational diffusion by the Gronski approach. As demonstrated by its higher activation energy, correlated segmental motion appears to be slower than the unsubstituted polycarbonate of BPA. In addition, the single-transition processes seem to be still less important than correlated backbone transitions. Phenylene-group rotation is described in terms of restricted rotational diffusion instead of complete anisotropic rotation. The time scale for backbone methyl-group rotation is comparable to that in BPA, a fact indicative of weaker cooperativity between this motion and the other motions. Rotation of the methyl group attached to the phenylene ring is too fast to significantly contribute to relaxation except by partially averaging the dipole–dipole interactions. The higher activation energies for segmental motion observed in solution for this methyl-substituted polycarbonate relative to the unsubstituted polycarbonate parallel a significant increase in the glass transition temperature observed for the substituted material. The restricted pheylene-group rotation in solution is also parallelled by a large upward shift of the low-temperature loss peak in the glassy polymer.     

Side-chain crystallinity,heat of melting,and thermal transitions in poly[N-(10-n-alkyloxycarbonyl-n-decyl)maleimides] (PEMI)

The heat of melting, the melting temperature T_m, and the sub-T_g transition temperature have been studied from –120°C to above T_m in a series of 11 poly[N-(10-n-alkyloxycarbonyl-n-decyl)]-maleimides (PEMI). Side-chains from ethyl to n-docosyl with n even have been included. The contribution to the heat of melting per methylene group shows that the hexagonal paraffin crystal modification is present in these poly(N-maleimides), in agreement with x-ray data for the same compounds. The enthalpy data show that only a part of the outer methylene groups are present in the crystalline aggregates. Furthermore, DSC traces exhibit a typical distribution of crystallite sizes, which become narrower as the side-chains become longer. The critical chain length needed to form a stable nucleus includes nine methylene groups in the outer part of the n-alkyl side-chain. The influence of the side-chain length and crystallinity on the γ-transition temperature of these polymers was also investigated. In the range where these polymers are essentially amorphous the sub-T_g transition temperature decreases continuously as the number of methylene groups in the side-chain increases. This transition is attributed to internal motion within the external side-group without any interaction with the main chain. This is presumably made possibly by the partial rotation of the oxycarbonyl group. We suggest that this transition is similar to the well known γ transition which has been attributed to various segmental motions in all ethylene copolymers and in all homopolymers containing a determined number of? CH₂? units in the main-chain or in the side-chain. Estimates based on the chemical structure, yield a value for the γ transition of ? CH₂? similar to that measured by other methods in polyethylene and related materials.     

Further calculations on the internal rotation spectrum of phenol

The influence of a small deformation of C?O angle in phenol (tilt), into the rotational far-infrared (FIR ) spectrum is analyzed using several approaches. In all of them, the CNDO /2 method is used to determine the potential energy functions. In a first step, the C? O bond and the rotation axis are both supposed to coincide with the C₂ symmetry axis of the phenyl group. With this assumption the torsional frequencies are calculated in both the symmetric and asymmetric rotor approximations. In a second step, the tilt of the C? OH bond is determined theoretically and found to be ?3°, measured from the C₂ symmetry axis, the C? OH bond crossing this axis, Using this second geometry, and taking as the rotation axis the C₂ axis, the torsional frequencies are again determined in both approximations. An improvement of the calculated transition energies is encountered at each stage of the calculation, when compared with experimental data. Finally the importance of the introduction of a tilt into the FIR torsional frequency calculations is discussed.     

Mobility of solid tert-butyl alcohol studied by deuterium NMR

The molecular mobility of solid deuterated tert-butyl alcohol (TBA) has been studied over a broad temperature range (103–283 K) by means of solid-state 2H NMR spectroscopy, including both line shape and anisotropy of spin–lattice relaxation analyses. It has been found that, while the hydroxyl group of the TBA molecule is immobile on the 2H NMR time scale (τC > 10(–5) s), its butyl group is highly mobile. The mobility is represented by the rotation of the methyl [CD3] groups about their 3-fold axes (C3 rotational axis) and the rotation of the entire butyl [(CD3)3-C] fragment about its 3-fold axis (C3′ rotational axis). Numerical simulations of spectra line shapes reveal that the methyl groups and the butyl fragment exhibit three-site jump rotations about their symmetry axes C3 and C3′ in the temperature range of 103–133 K, with the activation energies and preexponential factors E1 = 21 ± 2 kJ/mol, k(01) = (2.6 ± 0.5) × 10(12) s(–1) and E2 = 16 ± 2 kJ/mol, k(02) = (1 ± 0.2) × 10(12) s(–1), respectively. Analysis of the anisotropy of spin–lattice relaxation has demonstrated that the reorientation mechanism of the butyl fragment changes to a free diffusion rotational mechanism above 173 K, while the rotational mechanism of the methyl groups remains the same. The values of the activation barriers for both rotations at T > 173 K have the values, which are similar to those at 103–133 K. This indicates that the interaction potential defining these motions remains unchanged. The obtained data demonstrate that the detailed analysis of both line shape and anisotropy of spin–lattice relaxation represents a powerful tool to follow the evolution of the molecular reorientation mechanisms in organic solids.     

Application of the model‐free approach to low molecular weight systems with hindered internal rotation: cinnamoylmesitylene

We investigated the molecular dynamics of the molecule of cinnamoylmesitylene, a substituted chalcone. Known rotation barriers for the O?C‐4—C‐3?C‐2 bond of substituted chalcones are in the range of values accessible to modern NMR techniques. The internal rotation about the C‐3—C‐4 bond is found to be fast relative to the complete lineshape analysis (CLSA) time‐scale. To determine the activation parameters of overall and internal motions of the molecule, the Lipari–Szabo model‐free analysis of the relaxation times and heteronuclear NOE data was used instead. Simultaneous analysis of both heteronuclear spin–lattice relaxation times and NOE data for the two carbon atoms C‐2 and C‐7 in the O?C‐4—C‐3?C‐2 and mesitylene fragments at different temperatures was performed. The correlation times and activation energies of overall and internal motions and the generalized order parameter, which are measures of the molecular mobility, were thus determined. The standard enthalpies of activation, ΔH^≠, calculated from the experimental data for C‐2 and C‐7, are 5.6 and 6.6 kcal mol^?1, respectively. Theoretical estimates of the barriers to internal rotations by ab initio MO calculations were made to verify the experimental results. The agreement between the NMR and theoretical results was good. Copyright © 2003 John Wiley & Sons, Ltd.     

Effect of pressure on the rotational mobility of spin probes in polymers

The effects of pressure on the rotational mobility of three nitroxyl radicals (spin probes) in natural rubber, polyethylene, and butadiene-nitrile rubber SRN-26 have been studied. The activation volumes, activation energies, and pre-exponential factors of spin probe rotation at constant pressure and volume have been determined. The activation volumes of probe rotation (20–70 cm³/mol) increase with increasing size of radicals and differ insignificantly from the activation volumes of the β-relaxation process. In the polar polymer SRN-26, the activation volumes of rotation of radicals are appreciably more than in the nonpolar polymers, natural rubber and polyethylene. These features are apparently due to different volumes of the kinetic chain segment controlling probe rotation. The activation volumes of radical rotation around different molecular axes differ significantly. The activation energy of probe rotation at constant volume is appreciably less than at constant pressure. It has also been shown that the energy necessary for the formation of a fluctuation hole is the main factor that determines temperature dependence of the rotational mobility of low-molecular particles in the polymer.     

Mode-specific pressure effects on the relaxation of an excited nitromethane molecule in an argon bath

The vibrational and rotational mode-specific relaxations of CH₃NO₂ with 50 kcal/mol of initial internal energy in an argon bath is computed at 300 K at pressures of 10-400 atm. This work uses archived information from our previously published [J. Chem. Phys. 142, 014303 (2015)] molecular dynamics simulations and employs our previous published [J. Chem. Phys. 151, 034303 (2019)] method for projecting time-dependent Cartesian velocities onto normal mode eigenvectors. The computed relaxations cover three types of energies: vibrational, rotational, and Coriolis. In general, rotational and Coriolis relaxations in all modes are initially fast followed by an orders of magnitude slower relaxation. For all modes, that slower relaxation rate is approximately comparable to the vibrational relaxation rate. For all three types of energies, there are small-scale mode-to-mode variations. Of particular prominence is the exceptionally fast relaxation shared in common by the external rotation about the C N axis, the internal hindered rotation of the CH₃ group relative to the NO₂ group, and the symmetric stretch of the CH₃ group.     

Spin trapping studies of poly(methyl methacrylate) degradation in solution

Free radicals produced either by γ or ultrasonic irradiation of poly(methyl methacrylate) (PMMA) in benzene solution were stabilized by spin trapping; they were identified by analysis of ESR spectra of the trapped radicals (the spin adducts). The radical species identified after γ-irradiation were methyl, ester (COOCH₃), a pair of the chain scission radicals, ～CH₂C(CH₃)(COOCH₃) and CH₂C(CH₃)(COOCH₃)～, and phenyl radical originating from the solvent. The chain scission radicals were also detected by spin trapping after ultrasonic irradiation of the benzene solution. Taking account of the difference in the trapping rate for two spin trapping agents, 2,4,6-tri-t-butylnitrosobenzene (BNB) and penta-methyl-nitrosobenzene (PMNB) the radical species trapped by PMNB are assumed to be precursors of those trapped by BNB. Based on the radical species found by the spin trapping method, plausible degradation processes for PMMA in benzene solution are proposed.     

NMR observations of drawn polymers. VII. Nylon 66 fibers

Wide-line NMR spectra have been obtained on an oriented sample of drawn nylon 66 fibers at temperatures between ?196°C and 200°C and at alignment angles between the fiber axis and the magnetic field of 0°, 45°, and 90°. At ?196°C, 20°C, and 180°C, the complete angle dependence of the NMR spectrum has been measured. The second moments of these spectra have been compared to theoretical second moments calculated for various models of chain segmental motion in an attempt to elucidate the mechanisms involved in the low-temperature segmental motion (γ process) and the high-temperature segmental motion (α_c process). In agreement with earlier suggestions, the present results indicate that the γ process consists of segmental motion in noncrystalline regions. The overall decrease in second moment caused by the γ process is consistent with a model in which all noncrystalline segments rotate around axes nearly fixed in space. Furthermore, this decrease shows a pronounced dependence on the alignment angle. It is believed that this is due to tie molecules which become highly oriented along the fiber axis during drawing; their axes of rotation will therefore be nearly parallel to the fiber axis. The segments in noncrystalline entities such as chain folds and chain ends are less well oriented along the fiber axis and make an essentially isotropic contribution to the second moment decrease. The second moment at 180°C indicates the presence of considerable motion in the crystalline regions, and this motion is denoted the α_c process. The second moment S_c of the crystalline regions is strongly dependent on the alignment angle, the predominant feature being a relatively high value of the second moment when the fiber axis is directed parallel to the magnetic field. This is in qualitative, but not quantitative, agreement with the motional model recently advanced by McMahon, which assumes full rotation of the chains around their axes. Excellent quantitative agreement with experiment has been obtained by superimposition of rotational oscillation around the chain axis of amplitude roughtly 50°, and torsion of the chains with neighboring CH₂ groups oscillating around the C? C bond with a relative amplitude of about 40°. A model in which the chains perform rotational jumps of 60° between two equilibrium sites has also been considered (60° flip-flop motion). A distinction between this model and rotational oscillation has not been possible.     

Bindungsenergien von Phosphor–Stickstoff-Bindungen

The enthalpies of formation of P? N compounds with three-, four-and five-coordinated phosphorus have been determined by combustion calorimetry. From these data, using the energy values for atomisation, the P? N bond energies have been estimated to be 69 Kcal/Mole for P₄(NCH₃)₆ with three-coordinated P, and 79 Kcal/Mole for [PhNHP(O)NPh]₂ with tetrahedral phosphorus. For the compound C₂H₆Cl₆N₂P₂, in which the phosphorus is in a trigonal-bipyramidal hybrid function, the axial P? N bond energy is estimated to be 69 Kcal/Mole, and the equatorial one 77.5 Kcal/Mole.     

2H NMR characterization of phenylene ring flip motion in poly(p-phenylene vinylene) films

Solid-state ²H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of ²H spin lattice relaxation times have been obtained for films of poly(p-phenylene vinylene) deuterated in phenylene ring positions (PPV-d₄). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the ²H line shapes show that the jump motion is thermally activated, with a median activation energy, E_a = 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T₂ relaxation associated with ²H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. ²H NMR provides a method for determining the phenylene-vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.     

Effect of molecular motion and solvent interactions on nitrogen-15 relaxation in anilines

Dipolar relaxation of ¹⁵N in anilines and anilinium ions is influenced by overall motion of the molecule, by rotation about the aryl–-nitrogen bond, by inversion of the aniline nitrogen and by interactions of the NH₂ or NH₃⁺ group with the solvent. These factors are assessed by comparison of the ¹³C and ¹⁵N dipolar relaxation times as a function of para-substitution on the aryl ring. In the anilines (solvent CDCl₃), electron withdrawal brings about faster relative motion of the amine side-chain, contrary to expectation from consideration of C? N rotation but in agreement with the effects from nitrogen inversion. The ¹⁵N dipolar relaxation time correlates with the Hammett σ_p. For the anilinium ions (solvent Me₂SO-d₆), there is no correlation with σ_p and no qualitative relationship with either C? N rotation or N inversion. Nitrogen-15 relaxation, corrected for overall motion as judged by ring ¹³C relaxation, correlates with the inductive parameter σ_I. Electron withdrawal through induction reduces hydrogen bonding and increases side-chain mobility. For most of the anilines and for all of the anilinium ions, solvent interactions cause the nitrogen side-chain to be less mobile than the aryl ring. Under these circumstances, the Woessner approach cannot be used to calculate barriers. The hydrogen bond donor properties of the anilines are reduced in the absence of electron-donating substituents, and the first barriers to NH₂ rotation/inversion were calculated by this procedure: aniline in CDCl₃ 3.5 kcal/mol, p-chloroaniline in CDCl₃ 3.4 kcal/mol and p-nitroaniline in acetone 3.8 kcal/mol.     

Electron Spin Resonance of α-(Alkoxycarbonyl)alkyl Radicals in Solution

High-resolution ESR. spectra of the radicals CH₂COOR, CH₃CHCOOR and (CH₃)₂CCOOR with R?CH₃, CH₂CH₃, CH(CH₃)₂ and C(CH₃)₃ in liquid solution confirm planar energy-minimum structures with substantial barriers to internal rotation about the ?, CO-bonds (?40 kJ/mol) and partial π-electron delocalization. The assignments of coupling constants to protons in isomeric positions and the conclusions on radical structures are supported by INDO-calculations.     

Thermoluminescence of irradiated polystyrene

Thermoluminescence of irradiated polystyrene has been studied in the temperature range 100 to 440°K. Three glow peaks with maximum at 160, 221, and 378°K have been observed. These peaks are analyzed by different methods and the activation energies which were obtained are compared. The activation energies are found to be 0.22, 0.48, and 1.45 eV for the peaks with maxima at 160, 221, and 378°K, respectively. Second-order kinetics is appropriate to all these cases. The glow peaks are attributed to the decay of the free radicals formed on irradiation and subsequent thermal stimulation. The peak with the maximum at 160°K is attributed to electron trapping by the carbonyl groups or peroxy radicals formed on irradiation. The curve with the peak at 221°K is attributed to the cyclohexadienyl radical, and the curve with the peak at 378°K is attributed to the chain radical ? CH₂? C (C₆H₅)? CH₂? . The centers responsible for the observed thermoluminescence are identified by correlation with electron spin resonance (ESR) data obtained on the same samples.