Proton spin-lattice relaxation rate studies. 1-Aryl-4,4-dimethyl-2-methylthio-2-imidazolin-5-ones

Proton spin-lattice relaxation rates (R₁ values) have been measured, at 270 MHz, for a series of 1-aryl-4,4-dimethyl-2-methylthio-2-imidazolin-5-ones, using both null-point and non-linear regression methods; the two procedures give the same rates. A normalization procedure has been used to enable comparisons to be made between the R₁ values of different compounds; this is necessary because the experimental values are affected by changes in motional correlation times associated with changes in molecular structure. A substantial (five-fold) dynamic range of R₁ values has been observed for the protons of individual compounds, and the values have been correlated with the molecular environments of the nuclei. Evidence for anisotropic molecular motion and inter-ring proton relaxation in these compounds has been considered.     

A general survey of the proton spin–lattice relaxation rates of steroids

The spin-lattice relaxation rates of the aromatic, alkene, hydroxyl, methine and methyl protons of 19 steroid derivatives have been measured using the null point method. A simple procedure is described whereby the R₁ values of molecules which have different motional tumbling rates can be directly inter-compared, and it is shown that such ‘normalized’ relaxation data can provide novel insight concerning both the geometry and the local molecular motion of these substances in solution.     

A proton spin–lattice relaxation rate and nuclear Overhauser effect investigation of the stereochemistry of some 1-aryl derivatives of 1,2-dihydro-s-triazines

Proton spin–lattice relaxation rates (R₁ values) have been measured, at 400 MHz, for a number of 2-methyl- and 2,2-dimethyl-substituted 4,6-diamino-1-aryl-1,2-dihydro-S-triazines. These compounds have high barriers to internal rotation about the aryl C? N bond, and exist in solution as mixtures of enantiomeric or diastereomeric rotational isomers. Diastereotopically related 2-methyl groups in enantiomeric rotamers, and 2-methyl and 2-methine protons in diastereomeric rotamers, typically have different relaxation rates. In favourable circumstances this information may be used to identify the individual rotamers. Unequivocal direct identification of rotamers may be obtained from nuclear Overhauser effect difference spectra.     

Dynamic viscosity of dilute aqueous solutions of poly(acrylic acid) at frequencies of 2–500 kHz

The dynamic viscosity of aqueous solutions of poly(acrylic acid) at a polymer concentration of ca. 0.15 g/100 ml has been measured at frequencies from 2 to 500 kHz as a function of degree of polymerization P, degree of neutralization α, and salt (NaCl) concentration C_s. Relaxation spectra have been obtained from the dynamic viscosity. The spectra in the short relaxation time region can be approximated by the Zimm theory for the conformational relaxation of nonionic polymers. The maximum relaxation time τ₁ of the Zimm spectra is proportional to P² and depends rather moderately on α and C_s. Increased deviation is found, however, in the long relaxation time region, in particular for high values of P and α and low values of C_s. The major part of the deviation is interpreted in terms of rotational relaxation of a molecule as a whole. The rotational relaxation time τ_R is proportional to P³ and increases with increasing α and decreasing C_s. The remaining part of the excess spectra located between τ₁ and τ_R is ascribed to the deviation of the conformational relaxation from the Zimm theory arising from ionization of the polymer.     

Effect on structural relaxation of the poly(methyl‐methacrylate) copolymers chain flexibility

The structural relaxation of poly(methyl‐methacrylate) (PMMA)‐based copolymers with different chain flexibility has been studied by DSC with the classical procedure of the isothermal and dynamical approach. Modified PMMA with different chain flexibility have been prepared by free radical polymerization in solution using a mixture of monomers containing 10 mol % of alkyl methacrylate (i.e., ethyl, buthyl, and hexyl methacrylate). The molecular characteristics of all the prepared copolymers have been performed by a multiangle laser light scattering (MALS) photometer on‐line to a size exclusion chromatography (SEC) system (SEC‐MALS) after and before the thermal treatments, NMR (¹H and ¹³C) and MALDI‐TOF mass spectrometry. A comparison of the apparent relaxation rate (R_H) was appraised from the enthalpy loss by annealing the different samples at the same level of undercooling (T_a = T_g ? 18 °C). It was found an increase of R_H increasing the chain flexibility in the copolymers. Dynamical tests, performed at different cooling rates, have been used to estimate the apparent activation energy of the relaxation process. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 596–607, 2009     

Porous sol gel silica films doped with crystalline NiO nanoparticles for gas sensing applications

A reliable sol gel route to synthesize NiO doped SiO₂ films with different NiO content is here described. The films showed detectable and reversible changes in both optical and electrical properties when exposed to some reducing/oxidizing gaseous species at temperatures in the 250°C–350°C range. A functional characterization protocol has been designed and some of the sensing properties of the materials have been investigated for detecting NO₂, CH₄, CO and H₂. An optical transmittance increase up to 2% has been detected for 1% CO in dry air atmospheres, while relative resistance response (R _R = R _gas/R _air) values up to 4.97 for 850 ppm H₂/air mixtures have been registered for conductometric gas sensing. Films at all NiO molar concentrations in the 10% NiO - 40% range showed an optical response to the target gas, while only 30% and 40% NiO films provided a detectable gas induced resistance change.     

Comparison Between Volume and Enthalpy Relaxations in Non-crystalline Solids Based on the Fictive Relaxation Rate

The volume and enthalpy relaxation rate of inorganic glasses and organic polymeric materials subjected to temperature jump T has been analyzed. It is shown that the relaxation behavior in isothermal conditions can be compared on the basis of the fictive relaxation rate defined as R_f=(dT_f/dlogt)_i. No significant difference between volume and enthalpy relaxation rate has been found for all materials examined. A simple equation relating the R_f and parameters of Tool-Naraynaswamy-Moynihan (TNM) phenomenological model has been derived. This equation predicts increasing R_f with the magnitude of temperature jump. It seems that correct determination of TNM parameters might be problematic for slowly relaxing polymers as the effect of these parameters becomes comparable with experimental uncertainty.This revised version was published online in November 2005 with corrections to the Cover Date.     

Acidic peroxo salts. A new class of initiators for vinyl polymerization. I. Kinetics of polymerization of acrylonitrile initiated by potassium monopersulfate catalyzed by Ag(I)

The kinetics of polymerization of acrylonitrile initiated by KHSO₅ and catalyzed by Ag(I) have been investigated in an aqueous medium over the temperature range of 35–50°C. The rates of polymerization R_p have been calculated and studied with respect to monomer and initiator. The catalytic activity of various metal ions on the initiator has been determined from a comparison of R_p values. The effects of monomer, catalyst, neutral salts, various amines, and inhibitor (hydroquinone) on the initial rate as well as maximum conversion have been studied. From the kinetics results a suitable reaction scheme has been proposed.     

Ligand–protein recognition studies as determined by nuclear relaxation analysis

The ligand–macromolecule interaction is analysed in terms of selective (R₁^SE) and non-selective (R₁^NS) spin–lattice relaxation rates, considering the contribution arising from all possible specific and non-specific receptor sites. A linear relation between ΔR₁^SE and macromolecule concentration is demonstrated to exist under any coordination conditions. The slope of the straight line obtained is defined as the ‘affinity index' which is proposed as a general criterion for assessing the total ability of a ligand to interact with a macromolecule.     

Treatment of Peak Intensity Uncertainties in NMR Relaxation Data Analysis Can Lead to Severe Artifacts

Summary. Accurate estimations of experimental uncertainties of relaxation rates are of vital importance for the interpretation of relaxation data, in particular for the Lipari-Szabo model free approach and for comparative relaxation studies. Here we report a systematic investigation of different methods for the estimation of experimental uncertainties on longitudinal R₁ and transversal R₂ rates using two different schemes for sampling the rates. We show that certain combinations of sampling strategies and methods of estimating experimental uncertainties result in wrong rates and rate errors. Practical recommendations for obtaining proper rate and rate uncertainties are deduced.     

Transverse Relaxation of Scalar‐Coupled Protons

In a preliminary communication (B. Baishya, T. F. Segawa, G. Bodenhausen, J. Am. Chem. Soc. 2009 , 131, 17538–17539), we recently demonstrated that it is possible to obtain clean echo decays of protons in biomolecules despite the presence of homonuclear scalar couplings. These unmodulated decays allow one to determine apparent transverse relaxation rates R₂^app of individual protons. Herein, we report the observation of R₂^app for three methyl protons, four amide H^N protons, and all 11 backbone H^α protons in cyclosporin A. If the proton resonances overlap, their R₂^app rates can be measured by transferring their magnetization to neighboring ¹³C nuclei, which are less prone to overlap. The R₂^app rates of protons attached to ¹³C are faster than those attached to ¹²C because of ¹³C–¹H dipolar interactions. The differences of these rates allow the determination of local correlation functions. Backbone H^N and H^α protons that have fast decay rates R₂^app also feature fast longitudinal relaxation rates R₁ and intense NOESY cross peaks that are typical of crowded environments. Variations of R₂^app rates of backbone H^α protons in similar amino acids reflect differences in local environments.     

Preparation, spectroscopic and high field NMR relaxometry studies of gadolinium(III) complexes with the asymmetric tetraamine 1,4,7,11-tetraazaundecane

The reaction of Gd(III) with asymmetric tetramine 1,4,7,11-tetraazaundecane (2,2,3-tet, L¹) ligand has been studied via NMR spectroscopy. The ligand proton longitudinal relaxation rates (R₁) have been used to estimate the distances of these protons from the Gd(III) center, in Gd(III)–L¹ reaction solutions, in H₂O/D₂O 5/1 mixtures.Two Gd(III) complexes [Gd(III)(L¹)(NH₃)(H₂O)₄](CH₃COO)₃·2H₂O (1) and [Gd(III)(L¹)(NH₃)(H₂O)₂]Cl₃·EtOH (2) have been isolated and characterized by elemental analyses, TGA, IR, NMR and relaxometry measurements. The NMR relaxation measurements of 2 in aqueous solutions have been performed, under various temperature or concentration conditions, and compared with those of the commercial contrast agents Gd(III)–DTPA and Gd(III)–DTPA-BMA. It has also been studied the influence of (i) the Gd(III) inner-sphere water molecule number (q) alteration and (ii) the steric constraint enhancement on the metal site, over the relaxation rate values of the parent aqueous solution of Gd(III)–2,2,3-tet, and of the aqueous solutions of 2.     

Monitoring dendrimer conformational transition using 19F and 1H2O NMR

The conformational transition of a fluorinated amphiphilic dendrimer is monitored by the ¹H signal from water, alongside the ¹⁹F signal from the dendrimer. High-field NMR data (chemical shift δ, self-diffusion coefficient D, longitudinal relaxation rate R₁, and transverse relaxation rate R₂) for both dendrimer (¹⁹F) and water (¹H) match each other in detecting the conformational transition. Among all parameters for both nuclei, the water proton transverse-relaxation rate R₂(¹H₂O) displays the highest relative scale of change upon conformational transition of the dendrimer. Hydrogen/deuterium-exchange mass spectrometry reveals that the compact form of the dendrimer has slower proton exchange with water than the extended form. This result suggests that the sensitivity of R₂(¹H₂O) toward dendrimer conformation originates, at least partially, from the difference in proton exchange efficiency between different dendrimer conformations. Finally, we also demonstrated that this conformational transition could be conveniently monitored using a low-field benchtop NMR spectrometer via R₂(¹H₂O). The ¹H₂O signal thus offers a simple way to monitor structural changes of macromolecules using benchtop time-domain NMR.     

Polymers from renewable resources. II. A study in the radio chemical grafting of poly(styrene‐alt‐maleic anhydride) onto cellulose extracted from pine needles

A binary mixture of styrene and maleic anhydride has been graft copolymerized onto cellulose extracted from Pinus roxburghii needles. The reaction was initiated with gamma rays in air by the simultaneous irradiation method. Graft copolymerization was studied under optimum conditions of total dose of radiation, amount of water, and molar concentration previously worked out for grafting styrene onto cellulose. Percentage of total conversion (Pg), grafting efficiency (%), percentage of grafting (Pg), and rates of polymerization (R_p), grafting (R_g), and homopolymerization (R_h) have been determined as a function of maleic anhydride concentration. The high degree of kinetic regularity and the linear dependence of the rate of polymerization on maleic anhydride concentration, along with the low and nearly constant rate of homopolymerization suggest that the monomers first form a complexomer which then polymerizes to form grafted chains with an alternating sequence. Grafting parameters and reaction rates achieve maximum values when the molar ratio of styrene to maleic anhydride is 1 : 1. Further evidence for the alternating monomer sequence is obtained from quantitatively evaluating the composition of the grafted chains from the FT‐IR spectra, in which the ratio of anhydride absorbance to aromatic (CC) absorbance for the stretching bands assigned to the grafted monomers remained constant and independent of the feed ratio of maleic anhydride to styrene. Thermal behaviour of the graft copolymers revealed that all graft copolymers exhibit single stage decomposition with characteristic transitions at 161–165°C and 290–300°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1763–1769, 1999     

Motional Anisotropy of 1-Phenyladamantane

¹³C-NMR longitudinal relaxation rates are analysed with the Woessner equations: in CDCl₃ solution at room temperature, 1-phenyladamantane has rotational diffusion coefficients R_‖ = 8.1 × 10¹⁰ rad · s^?1 and R_⊥ = 1.1 × 10¹⁰ rad · s^?1 corresponding to high motional anisotropy (σ = 7.4).     

Atomic level picture of stress relaxation in polymer melts

We have been developing a physical picture on the atomic level of stress relaxation in polymer melts by means of computer simulation of the process in model systems. In this article we treat a melt of freely jointed chains, each with N = 200 bonds and with excluded-volume interactions between all nonbonded atoms, that has been subjected to an initial constant-volume uniaxial extension. We consider both the stress relaxation history σ(t) based on atomic interactions, and the stress history σ_e(t; N_R) based on subdividing the chain into segments with N_R bonds each, with each segment regarded as an entropic spring. It is found that at early times σ(t) > σ_e(t; N_R) for all N_R, and that, for the remainder of the simulation, there is no value of N_R for which σ(t) = σ_e(t; N_R) for an extended period; by the end of the simulation σ(t) has fallen just below the value σ_e(t; 50). The decay of segment orientation, 〈P₂(t; N_R)〉, and of bond orientation 〈P₂(t; 1)〉, is computed during the simulation. It is found that the decay of the atom-based stress σ(t) is closely related to that of 〈P₂(t; 1)〉. This result may be understood through the concept of steric shielding. The change in local structure of the polymer melt during relaxation is also studied. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 143–154, 1998     

<Superscript>1</Superscript>H and <Superscript>31</Superscript>P nuclear spin-lattice relaxation in C-phosphorylated oximes

The ¹H spin-lattice relaxation times of the proton-bearing groups and the ³¹P spin-lattice relaxation times in C-phosphorylated oximes R¹C(=NOH)P(=O)R²R³ (R¹ = Ph, R² = R³ = OMe; R¹ = Ph, R² = OMe, R³ = OCH²CH²Br; R¹ = PhCH², R² = R³ = OCHMe²) and dioxime R²P(=O)C(=NOH)(CH₂)₄C(=NOH)P(=O)R² (R = OMe) in DMSO-d₆ were measured. The characteristic reorientation times of the whole molecules were estimated using the measured values of the ¹H relaxation times and the results of semiempirical PM3 quantum chemical calculations of the molecular geometries. The reorientation times were used to identify the contributions of different relaxation mechanisms to the rate of ³¹P spin-lattice relaxation. The anisotropy of the chemical shielding of ³¹P nuclei was evaluated from the difference between the ³¹P relaxation rates measured at 101.27 and 161.92 MHz.     

Synthesis and Chiroptical Properties of Chiral Carbazole-Based BODIPYs

A series of carbazole-based boron dipyrromethenes (BODIPYs) 2 a – g bearing binaphthyl units have been synthesized by the Et₂AlCl-mediated reaction of the corresponding BODIPY difluorides 1 a – g with 1,1′-binaphthalene-2,2′-diol. Substituents such as halogen, nitrile, and amino groups were tolerated under the reaction conditions, and the reaction of the phenylethynyl-substituted 1 h gave (R,R)- 3 h bearing two binaphthyl units. The chiroptical properties of these dyes with different substituents were investigated by UV/Vis, CD, fluorescence, and circularly polarized luminescence (CPL) spectroscopy. The CD spectra showed Cotton effects in the absorption region of the BODIPY moieties. In addition, they showed CPL both in solution and in the solid state. Interestingly, several dyes recorded higher g_lum values in the solid state, probably due to intermolecular interactions. Because (R,R)- 3 h recorded relatively low g_lum values, the diastereomer (R,S)- 3 h was prepared. The (R,S) diastereomer showed intense CPL, which suggests a synergetic effect of the two binaphthyl groups. Finally, chiral carbazole-based BODIPY dimers have been synthesized for the first time and their chiroptical properties were investigated. They showed redshifted fluorescence and CPL, which reached the near-IR (NIR) region in the solid state.     

The origin of DC electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) films

We present here the evidence for the origin of dc electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) (P3HT) films. P3HT has been synthesized and purified to obtain pristine P3HT polymer films. P3HT films are chemically doped to make conducting P3HT films with different conductivity level. Temperature (77–350 K) dependent dc conductivity (σ_dc) and dielectric constant (ε′(ω)) measurements on pristine and doped P3HT films have been conducted to evaluate dc and ac electrical conduction parameters. The relaxation frequency (f_R) and static dielectric constant (ε₀) have been estimated from dielectric constant measurements. A correlation between dc electrical conduction and dielectric relaxation data indicates that both dc and ac electrical conductions originate from the same hopping process in this system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1047–1053, 2010     

13C Fourier transform NMR and partially relaxed Fourier transform NMR relaxation study on the ethanol–manganese(II) system

The paramagnetic contribution to the ¹³C and ¹H nuclear relaxation rates in the ethanol-Mn(II) system has been calculated. Both T₁ and T₂ experiments have been performed by means of Fourier transform and partially relaxed Fourier transform NMR spectroscopy. The correlation times for the dipolar and scalar parts have been discussed and evaluated. Calculations of distances and of hyperfine coupling constants have been carried out.