首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 93 毫秒
1.
Variable-temperature 13C NMR spin-lattice relaxation times (T1) and Nuclear Overhauser Enhancements (NOE) at two magnetic fields have been used to study the dynamics of the amorphous part of a semicrystalline sample (33% of crystallinity) of poly(β-hydroxyoctanoate) (PHO). The interpretation of the relaxation data of the backbone carbons was made by employing a number of motional models. Among these, the DLM model offered the best interpretation of the relaxation data in terms of conformational transitions and librational motions of the backbone C? H vectors, and proved to be superior to unimodal distribution functions. The interpretation of temperature- and frequency-dependent T1 and NOE data of the carbon nuclei in the n-pentyl side chain was made by employing a newly developed composite spectral density function for multiple internal C? C bond rotations of restricted amplitude and chain segmental motion. The temperature dependence of the linewidths of the various protonated carbon resonances of PHO has been discussed in terms of the semicrystalline character of this polymer. © 1995 John Wiley & Sons, Inc.  相似文献   

2.
Using picoseconds time domain reflectometry, dielectric relaxation studies have been carried out for formamide (FMD)–propylene glycol (PLG) mixtures over the frequency range from 10?MHz to 20?GHz at various temperatures. The dielectric parameters, i.e. static dielectric constant (ε 0) and relaxation time (τ), have been obtained by Fourier transform and least squares fit methods. The excess dielectric properties and Kirkwood correlation factor of the mixtures have also been determined. The Kirkwood angular correlation factor is greater than one (g eff?>?1) in FMD-rich region and less than one (g eff?<?1) in PLG-rich region, which indicates that in the mixture the dipole pairs have been formed in such a way that their orientation is parallel in FMD-rich region and antiparallel in the PLG-rich region.  相似文献   

3.
Spin-lattice relaxation times (T1), spin-spin relaxation times (T2), and nuclear Overhauser enhancements (NOE), at 75.5 MHz are reported for α-13C nuclei of poly (γ-benzyl-L -glutamate) in deuterated dimethylformamide at 60°C and of poly(γ-hexyl-L -glutamate) in cyclohexanone at 48 and 79°C. It is shown that for molecular weights above 105, the polypeptides cannot be considered as essentially rigid helices with internal librational motions; additional backbone flexing motions contribute to the relaxation behavior.  相似文献   

4.
Proton-decoupled, ~(13)C FT-NMR (operating at 50.3 MHz) is used to determine spin-latticerelaxation time (T_1), nuclear Overhauser enhancement (NOE), linewidth and chemical shift of 1,2-polybutadiene as a function of temperature in CDCl_3 solution and the temperature dependence ofmolecular motion of 1,2-polybutadiene has been investigated with these NMR relaxation parameters.It is found that jumps of NOE and linewidth vs. temperature appear between-1℃and -30℃. Theminimum of nT_1 vs. temperature for all carbons occur at about -45℃.  相似文献   

5.
The syn and anti rotamers of o-tolyl-di-tert-butylcarbinol, 2a and 2b, respectively, have been studied by 1H NMR at 200 MHz and by natural abundance 13C NMR at 50 MHz. 1H-{1H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time, T1, of the 2-Me protons in 2b is unexpectedly higher than that for 2a. The 13C relaxation times of the 2-Me and the quaternary carbon of the tert-butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies for t-Bu rotation, measured by 1H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2-Me and t-Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG? values from this work and from the literature with MM2-calculated ΔH? values indicates that this force field systematically underestimates rotation barriers in open-chain systems by a factor of approximately 0.64.  相似文献   

6.
The reaction chemistry of C2N2? Ar and C2N2? NO? Ar mixtures has been investigated behind incident shock waves. Progress of the reaction was monitored by observing the cyano radical (CN) in absorption at 388.3 nm. A quantitative spectroscopic model was used to determine concentration histories of CN. From initial slopes of CN concentration during cyanogen pyrolysis, the rate constant for C2N2 + M → 2CN + M (1) was determined to be k1 = (4.11 ± 1.8) × 1016 exp(?47,070 ± 1400/T) cm3/mol · s. A reaction sequence for the C2N2? NO system was developed, and CN profiles were computed. By comparison with experimental CN profiles the rate constant for the reaction CN + NO → NCO + N (3) was determined to be k3 = 10(14.0 ± 0.3) exp(?21,190 ± 1500/T) cm3/mol · s. In addition, the rate of the four-centered reaction CN + NO → N2 + CO (2) was estimated to be approximately three orders of magnitude below collision frequency.  相似文献   

7.
The small negative magnetogyric ratio (γ) of the 15N nucleus decreases the efficiency of 15N? 1H dipole-dipole relaxation to about 25% of that for an analogous 13C nucleus. This may lead to greater competition from other relaxation mechanisms in 15N n.m.r. and consequent partial or total quenching of the negative nuclear Overhauser effect (NOE). In unfavorable circumstances nulling of the 15N resonance can occur. Previous 15N relaxation studies have examined isotopically enriched, low molecular weight compounds. The present study examines several small to intermediate size organic compounds containing nitrogen at natural isotopic abundance. In contrast to some of the earlier studies, 15N? 1H dipolar relaxation was found to be dominant for protonated nitrogen atoms, even for two tertiary nitrogens (the tertiary amine nitrogen in 1,2,3,4,6,7,12,12b-octahydroindolo[2,3-a] quinolizine and the oxime nitrogen in 3-methyl-2-pentanone ketoxime). The magnitude of the NOE and the moderate value of T1 indicate effective dipolar relaxation from neighboring but not directly bonded protons in these cases. Nitro groups were found, as expected, to have predominant contributions from non-dipolar mechanisms, and in one case (2-methyl-2-nitro-1, 3-propanediol) signal nulling (NOE of η = ?1) was observed. The effect of paramagnetic impurities was demonstrated for ethanolamine, which contains a basic nitrogen. In this case T1DD(15N? 1H) = 4·3 s; added Ni(acac)2 at 1 × 10?4 Molar reduced the 15N T1 to 0·065 s and consequently the NOE to η = 0.  相似文献   

8.
Carbon-13 spin-lattice relaxation times (T1) and 13C-{19F} nuclear Overhauser enhancement (NOE) factors have been measured for some simple fluorocarbons by pulse-Fourier transform methods—‘progressive saturation’ and ‘dynamic Overhauser enhancement’. The NOE factors are shown to be large for 13C with directly bonded fluorines, and the values for T1 and the NOE factors are similar to those obtained elsewhere on the corresponding hydrocarbons.  相似文献   

9.
Gamma radiation-induced free radical chain reactions in liquid mixtures of BrCH2CN, eyelohexane (RH), and haloalkanes (XCCl3) were studied. The kinetics of hydrogen and chlorine atom abstraction from CHCl3, CH3CCl3, CH2ClCCl3, CHCl2CCl3, CF3CCl3, C2Cl6, CCl3CN, and CCl4 by CH2CN radicals were investigated by a competitive method. The reactions investigated were Rate constant ratios k3/k2, k5/k6, k7/k2, and k3/k7 were determined at 180°C. In the CCl4? RH? BrCH2CN system k3/k2 was determined in the temperature range of 100–180°C, yielding log k2 k3 = ?0.11 ± 0.2 ?(3.34 ± 0.39/θ): where θ = 2.3RT in kcal/mol. The value E2? E3 was combined with existing data on E3 to yield E2(CCl4) = 17.57 kcal/mol. The reactivity trend of CH2CN is compared with that of R radicals. It is shown that in spite of a difference of about four orders of magnitude in kCl values, the reactive cyclohexyl radical is somewhat more selective than CH2CN. It is proposed that the relative reactivities log[k2(XCCl3)/k2(CH3CCl3)] can be correlated in terms of a dual-parameter Taft equation which takes into account both resonance and inductive substituent effects.  相似文献   

10.
The addition by redox catalysis (CuCl or FeCl3 with benzoin) of CCl4 and CCl3? CF2? CCl3 on pentafluorostyrene and on fluorinated allyl ether, C6F13? C2H4? O? CH2? CH?CH2 was performed. The monoaddition compounds were mainly obtained with satisfactory yields and characterized by NMR (13C, 1H, and 19F). These compounds were then used as telogens in telomerization by redox catalysis of acrylamide, methyl methacrylate, and acrylic acid and in radical-initiated telomerization of N-vinyl-pyrrolidone. The chain lengths of the hydrocarbonated sequences were determined from fluorine elemental analysis. A critical investigation of correlations between DP n and concentrations of products involved was made.  相似文献   

11.
Nuclear magnetic resonance spectroscopy is governed by longitudinal (T1) relaxation. For protein and nucleic acid experiments in solutions, it is well established that apparent T1 values can be enhanced by selective excitation of targeted resonances. The present study explores such longitudinal relaxation enhancement (LRE) effects for molecules residing in biological tissues. The longitudinal relaxation recovery of tissue resonances positioned both down‐ and upfield of the water peak were measured by spectrally selective excitation/refocusing pulses, and compared with conventional water‐suppressed, broadband‐excited counterparts at 9.4 T. Marked LRE effects with up to threefold reductions in apparent T1 values were observed as expected for resonances in the 6–9 ppm region; remarkably, statistically significant LRE effects were also found for several non‐exchanging metabolite resonances in the 1–4 ppm region, encompassing 30–50 % decreases in apparent T1 values. These LRE effects suggest a novel means of increasing the sensitivity of tissue‐oriented experiments, and open new vistas to investigate the nature of interactions among metabolites, water and macromolecules at a molecular level.  相似文献   

12.
The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH3? and CCl3? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ~1–3 × 109 M?1 sec?1. Rate constants for hydrogen atom abstraction by CH3? and CCl3? radicals are both ~1?2 × 105 M?1 sec?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu3Ge–H) is estimated to be approximately 84 kcal/mole.  相似文献   

13.
On Trichlorophosphazo Compounds from Nitriles. III. The Reaction between Acrylonitrile and PCl3. The reaction of PCl3 with acrylonitrile at higher temperatures gives CH2Cl? CCl2? CCl2? N? PCl3 ( II ). On pyrolysis of (II), CH2Cl? CCl2? CN (IV) is form- ed. Treatment of (II) with SO, results in CHzCL? CCl2? CCl?N-P(0)Cl2 ( III ). At lower temperatures and/or in the presence of PCl3, acrylonitrile reacts with PCl3 to give the cis/ trans isomers VIa and VIb .  相似文献   

14.
Proton decoupled, partially relaxed, Fourier-transform 50.3 MHz carbon-13 NMR in naturalabundance was used to determine spin-lattice times (T_1) and nuclear Overhauser enhancement fac-tors (NOE) of individual carbon of a serics of 1,2-polybutadienes with different structures in solutionin CDCl_2 The structure dependence of molecular metion and the internal motion of vinyl group in1,2-polybutadiene have been studied by nT_1 and NOE values. The nT_1 values of the carbons in cis-1,4-units are the highest and those of the carbons in 1.2-units are the lowest in three types of units in1,2-polybutadiene. The nT_1 values of carbons in the same unit become greater when the adjacent1,2-units are replaced by 1,4-units, and nT_1 values of the carbons in all units decrease sharply withthe increase of content of 1,2-units in the polymers. The fact that nT_1 values of --CH=are larger than those of=CH_2 in vinyl group impliesthat there are complex internal motions of vinyl group. It is shown by calculation that the dominantfactor causing the difference in nT_1 of--CH=and=CH_2 in vinyl group is a swing of vinyl group ina plane peopndicular to the chain backbone.  相似文献   

15.
The distribution of chlorine atoms in the chain of solution-chlorinated PVC (CPVC) composed of ? CH2? CHCl? (1), ? CHCl? CHCl? (2), and ? CCl2? CHCl? (3) monomeric units is described by first-order Markoffian statistics based on the assumption that the chlorination mechanism is independent of tacticity. In this case, all the statistical parameters can be obtained from experimentally determined concentrations of CH2, CHCl and CCl2 units. Populations of the sequences (11) and (111) calculated from the statistical parameters agree with experimental values determined from infrared and 220 MHz NMR spectra. From infrared spectra, suspension-chlorinated CPVC contains blocks of intact PVC.  相似文献   

16.
The effect of low-molecular-weight miscible additives on the sub-Tg (β) relaxation process in bisphenol-A polycarbonate (BPAPC) was studied using high-resolution carbon-13 solid-state NMR. The trend of the spin-lattice relaxation times T1 at 50 MHz suggests that strong intermolecular interactions occur upon mixing when BPAPC is physically stiffened by the antiplasticizing diluent, diphenylphthalate. The values of 13C T1 at 15 MHz in d-chloroform solutions for similar BPAPC-diluent mixtures suggest that diluent effects on the megahertz mobility of the polymer occur exclusively in the solid state. These results are explained using equilibrium thermodynamics, in the Ehrenfest sense, at the second-order glass transition temperature Tg. Theory predicts that the temperature dependence of the Flory–Huggins interaction parameter ?χ/?T changes abruptly as the polymer-diluent blends are cooled below Tg from the molten state. The difference between ?χ/?T in the liquid and glassy states is the major factor which determines the diluent concentration dependence of Tg. A method is developed to estimate the relative magnitudes of χ for polymerdiluent blends in the glassy state.  相似文献   

17.
A recently developed polarization model for representing polar bond effects in conformational energy calculations is applied to poly(vinylidene chloride) (PVDC). The geometries and conformational energies of a number of conformers of 2,2,4,4,6,6-hexachloroheptane were calculated. The geometries were found to be similar to the hydrocarbon analog polyisobutylene (PIB) in that steric crowding results in the usual T, G, G′ states being split into + or ? distortions of the torsional angles away from the traditional values. Only distortions of the same sign occur in the same pair of bonds interior to CCl2 groups. Distortions of G states towards eclipsed were found to be much more stable than those away. The interior skeletal valence angle is also distorted to an unusually large value, ca. 121°. The calculated dipole moments were used to infer a group moment for CCl2 of 1.56 D. The calculated conformational energies were fitted by linear combinations of interaction parameters representing the stabilities of G+, G? bonds (relative to T+, T?) and the interactions between bonds across intervening CCl2 groups. These parameters were used in statistical mechanical calculations of the characteristic and dipole-moment ratios. In order to make comparison with experiment, the dipole-moment/repeat unit of a 90% (by weight) PVDC copolymer with PVC was measured and found to be 1.42 ± 0.05 D. From this, the dipole-moment ratio for PVDC homopolymer is inferred to be ca. 0.8. The characteristic and dipole-moment ratios calculated from the interaction parameter set were somewhat too high but adjustment of the gauche energies downward brings the calculated ratios into agreement with experiment. The same statistical model along with energy parameters previously calculated also gives agreement with experiment for the characteristic ratio of PIB. The calculated geometries are in agreement with the conformation in the crystal being ? (T+G+T?G?)? .  相似文献   

18.
The dielectric properties of low-molecular-weight propylene glycols HO? [CH(CH3)CH2O]n? H (n = 3, 4, 5) were investigated to clarify the effect of chain length on the dielectric properties. The measurement of dielectric constant and dielectric loss was carried out over the frequency range 30 Hz to 30 MHz at temperatures of ?20 to ?65°C. The static dielectric constants of these glycols are between 10 and 30, slightly smaller than values for the corresponding ethylene glycols of the same degree of polymerization. All of the Cole–Cole arcs, even that for pentapropylene glycol, can be represented by the empirical Davidson–Cole equation. The dielectric properties of homologous propylene glycols are compared with those for the ethylene glycols and are discussed in terms of the effects of chain length and intermolecular hydrogen bonds.  相似文献   

19.
Partly epoxidized cis- and trans-1,4-polyisoprenes and cis-and trans-1,4-polybutadienes were prepared, and their 13C NMR spectra examined. All the prominent resonances in the spectra of the epoxidized polymers were assigned by using lanthanide shift reagent and off-resonance decoupling experiments. A 13C NMR method of quantitative assessment of the epoxide content was developed following determination of relative spin-lattice relaxation time (T1) and nuclear Overhauser effect (NOE) parameters of the various carbons in the epoxidized polyisoprenes and polybutadienes.  相似文献   

20.
Outer‐sphere reduction of hexacyanoferrate(III) by some enolizable/nonenolizable aldehydes (viz., aliphatic, heterocyclic, and aromatic aldehydes) in alkaline medium has been studied spectrophotometrically at λmax = 420 nm. The reactions are first order each in [aldehyde] and [Fe(CN)63?]. The rate increases with an increase in [OH?] in the oxidation of aliphatic and heterocyclic aldehydes, whereas it is independent of [OH?] in the reaction with aromatic aldehydes. The intervention of free radicals in the reaction mixture was carried out using both acrylonitrile and acrylamide scavenger in two different experiments. The kinetic results indicate that the oxidation of benzaldehyde in aqueous medium proceeds at a slower rate than the aliphatic aldehydes (other than formaldehyde) and furfural. The values of third‐order rate constant (k3) at 308 K in the oxidations of some aliphatic aldehydes and furfural follow the order (CH3)2CH? > CH3CH2? > CH3? > C4H3O? > H? . The rate constants correlate with Taft's σ* value, the reaction constant being negative (–9.8). The pseudo–first‐order rate constants in the oxidations of benzaldehyde and substituted benzaldehydes follow the order ? NO2 > ? H > ? Cl > ? OCH3. The Hammett plot is also linear with a ρ value (0.6488) for meta‐ and para‐substituted benzaldehydes. The kinetic isotope effect for benzaldehyde (kH/kD = 1.93 at 303 K) was obtained. The rate‐determining step is the outer‐sphere formation of Fe(CN)64? and free radicals, which is followed by the rapid oxidation of free radicals by Fe(CN)63? to give products. The kinetic data and hence thermodynamic parameters have been used to distinguish enolizable and nonenolizable aldehydes. An attempt has also been made to correlate kinetic data with hydration equilibrium constants of some aliphatic aldehydes. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 494–505, 2012  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号