Dynamics of macromolecular chains. VI. Carbon 13 and proton nuclear magnetic relaxation of polystyrene in solution

Spin-lattice ¹H and ¹³C nuclear magnetic relaxation (NMR) times T₁ have been measured for solutions of polystyrene in hexachlorobutadiene at two different frequencies. Some nuclear Overhauser enhancements and linewidths have also been determined. At 15 and 25 MHz the relaxation times T₁ of the ortho and meta carbons show two different dependences on temperature. These measurements indicate internal motion of phenyl groups around the C_α—C_para axis. A single isotropic correlation time is inadequate to explain the relaxation data for the para carbon. Use of a diamond-lattice motional model reveals that segmental reorientation of the chain backbone of polystyrene can be described in terms of two correlation times, ρ characterizing the three-bond motion process, and θ reflecting either isotropic motions of subchains or departure from an ideal lattice. Data on low-molecular-weight polystyrene indicate the participation of overall rotatory diffusion in the relaxation process. This motion is no longer efficient in high-molecular-weight polymers, where relaxation is due to segmental reorientation.     

1H and 13C nmr studies of salicylalanilines

Several substituted salicylanilines (I) are studied by ¹H (chemical shifts) and ¹³C (chemical shifts and T₁ relaxation times) NMR in order to obtain information on molecular geometry changes and the transmission of the electronic effects due to substituents, as well as on the relative rates of the overall molecular tumbling and of the flipping of the phenyl rings. In particular, a good linear correlation of the OH proton shifts (affected by intramolecular hydrogen bonding) with Hammett's σ constants for p-substitution in the aniline moiety is found. Changes in rigidity of the rings expected as a result of the OH...N interaction and of p-substitution are reflected on the phenyl carbon relaxation times.     

Rotational correlation times of adenosine 5′-monophosphate in solution as deduced from 1H and 13C spin-lattice relaxation times

Rotational correlation times (τ_T) of the 5′-AMP molecule deduced from spin-lattice relaxation times (T₁) of different protons in the molecule agree fairly well with each other in the temperature range of 3.5–74°C. The same is true with τ_T values deduced from ¹³CT₁ values. These results indicate that the internal motions are slow as compared to the overall rotation of the 5′-AMP molecule.     

Carbon-13 NMR relaxation time measurements of molybdenum carbonyl physisorbed on alumina

In order to obtain information about the motion of Mo(CO)₆ physisorbed on high surface area alumina, the temperature dependences of ¹³C NMR relaxation times, T₁ and T₂, have been measured at 7.05 T. Eight samples prepared under various conditions were studied. Two types of alumina (γ and η) were used and the alumina in two samples was pretreated with iron nitrate. A T₁ minimum of 0.4 s at 220 K observed for most samples corresponds to a correlation time of 2 × 10⁻⁹ s. The dominant relaxation mechanism is not chemical shift anisotropy since T₁ measurements at 3.5 T indicated no field dependence. An increase in the iron content of the alumina decreased T₁ above 273 K but caused only minor changes in T₁ and T₂ at lower temperatures. T₁ was 0.73 s and T₂ was 11 ms for Mo(CO)₆ on the η-alumina sample at 300 K.     

Molecular dynamics in polyethylene and ethylene-1-butene copolymer investigated by NMR methods

1H and 13C NMR spectra and 1H spin-lattice relaxation times T₁ and T_1ρ have been employed to study the structure and molecular dynamics in polyethylene and ethylene-1-butene copolymer in the temperature range from 100 to 370 K. Results are interpreted in terms of α, β and γ -relaxation, as well as methyl group rotation. The activation energies for all motions were established. The incorporation of 1-butene into ethylene chain leads to an increase of mobility in amorphous and crystalline phases as well as appearance the 13C resonance characteristic to the monoclinic structure in addition to the orthorhombic observed in both polymers. The crystallinity degree derived from T_1ρ in studied polymers is close to that determined using DSC method.     

The structure and dynamics of the copper(II)—l-proline 1:2 complex in solution

The ¹³C relaxation times (T₁ and T₂) and isotropic contact shifts (Δω) of a one molar aqueous solution of l-proline at pH = 11 (or pD = 11.4) containing ca 10^?4 M copper(II) perchlorate are measured at 62.86 MHz over a temperature range of 26–70°C. The purely dipolar longitudinal relaxation of carbon-13 nuclei contrasting with purely scalar transverse relaxation allowed us to extract carbon-to-metal distances (through T₁ measurements) and hyperfine coupling constants and dynamic parameters (from T₂ and Δω measurements). The structure of the complex in solution is found closely similar to that in the solid state. Curve-fitting procedures allowed us to derive the hyperfine electron—carbon coupling constants A_c = ?1.95, + 0.42, + 1.90 and ?1.70 MHz for carbons α, β, γ, δ, of the pyrrolidinic ring, the reorientation correlation time of the complex, τ_R (25°C) = 1.15 × 10^?10 sec, the l-proline exchange rate, k_M (25°C) = 4.0 × 10⁵ sec^?1 (and the corresponding activation parameters ΔH^≠ = 9.0 kcal mol^?1 and ΔS^≠ = ?0.7 e.u.), and the electronic relaxation time, T_1e = 1.13 × 10^?8 sec (at 25°C). The latter value was found in agreement with the one computed from ESR data and the above τ_R value, showing the predominant contributions of spin—rotation interaction and, to a lesser extent, of the effect of g-tensor anisotropy to the electronic relaxation rate.     

Solid-state NMR study of biodegradable starch/polycaprolactone blends

Abundant literature exists on starch or modified starch blended with biodegradable polyesters to achieve good performance with cheap compost plastics. The level of miscibility in these blends is one of the most relevant parameters. In the present study, solid-state ¹H and ¹³C NMR spectra, as well as carbon spin-lattice relaxation times T₁(C) and proton spin-lattice relaxation times T₁(H) and proton spin-lattice relaxation times in the rotating frame T_1ρ(H) of biodegradable starch (or starch formate)/polycaprolactone (PCL) (or polyester (PE) oligomers) blends and samples of the neat components were measured. From the T_1ρ(H) and T₁(H) relaxation times it follows that blends starch/PCL, starch/PE-oligomers and starch formate/PE-oligomers are phase separated even on the scale of 20-110 nm. On the contrary starch formate/PCL blend is phase separated on the scale 2.5-12 nm but homogeneously mixed on the scale 20-90 nm. Moreover, shorter T₁(C) and especially T_1ρ(H) values found for the starch or starch formate component in all these blends in comparison with neat samples show that molecular mobility of starch and starch formate segments is affected by blending. This indicates some miscibility also in phase separated blends which can happen in amorphous channels of starch.     

High-Resolution Solid-State NMR Characterization of Morphology in Annealed Polylactic Acid

Single-pulse ¹³C NMR spectra and spin-lattice relaxation times T₁(¹H), detected indirectly via ¹³C carbons, and T₁(¹³C) were measured at 31°C for virgin pelletized and annealed polylactic acid (PLA) samples using the magic-angle spinning technique. The structural relaxation resulting in more regular crystals with narrower conformation distribution and increase in the lamellae thickness and crystallinity brought about by annealing at 100°C was deduced from the narrowing of the ¹³C NMR lines and proton spin-lattice relaxation times T₁(¹H). The spin-lattice relaxation times T₁(¹³C) related to the respective carbons of the α-polymorph of PLA are also discussed in the study.     

C-13 magnetic relaxation rates and H-1 and C-13 paramagnetic shifts of Ni(II) complex of dopamine

The ¹H and ¹³C isotropic contact shifts and the ¹³C relaxation times of dopamine in aqueous solution have been measured in the presence of the Ni(II) ion. The pD dependence of the ¹H and ¹³C paramagnetic shifts was also investigated. From the analysis of the shifts at pD = 6.5 and from the INDO MO calculations on selected models of dopamine radicals, a dominant σ delocalization mechanism of the spin density is proposed. From the spin distribution on the ligand carbon atoms, the metal centered as well as the ligand centered dipolar contributions of the modified Solomon—Bloembergen equation were calculated and an estimate of the correlation time τ_c was given.     

Wide-line <Superscript>1</Superscript>H NMR study of mesomorphic states of 4′-(octyloxy)-4-biphenylcarbonitrile

A sample of 4′-(octyloxy)-4-cyanobiphenyl (8OCB) was studied in the temperature range −60–80°C by wide-line ¹H NMR. The line shapes, half-widths, and second moments were determined. For the smectic phase of 8OCB, the relaxation times T ₁ and T ₂, the correlation time τ_c, and the degree of order were estimated. The ¹H NMR spectral patterns and characteristics were found to be quite different for the crystalline, smectic, and nematic phases of 8OCB, which makes it possible to reliably identify the corresponding transitions and assess the molecular dynamics and molecular order of a structure. The temperature ranges, stability conditions, and other characteristics of the liquid crystalline phases that form on heating 8OCB were determined.     

Investigation of molecular motions of crosslinked polyepichlorohydrin by nuclear magnetic resonance spectroscopy

The molecular motion of crosslinked polyepichlorohydrin (PECH) is studied qualitatively by NMR techniques. The results of temperature dependence of ¹H T₂ and T₁ indicate that the crosslinking (crosslink density < 3%) restricts molecular motions of the polymer even far above its T_g. The ¹H T₁ minimum, corresponding to the large-scale chain-motion of crosslinked PECH, shifts to higher temperatures with increasing crosslink density. ¹H T₂ data also show that the crosslinking hinders free chain motions of the polymer above its T_g. The ¹³C T₁ relaxation time is sensitive to such motional changes as well. ¹³C linewidths of crosslinked PECHs vary with the crosslink density in both the swollen state and the solid state. The mechanism of ¹³C linewidth broadening of crosslinked polymers is discussed in detail. In the case of PECH, the linewidth broadening is caused by changing molecular environment due to crosslinking (such as presence of various chemical shift structures and freezing effects in conformational environment as chain mobility decreases), rather than increasing correlation times, which shorten the relaxation time (T₂) of polymer chains. © 1994 John Wiley & Sons, Inc.     

Study on crystallographically inequivalent protons in RbH2AsO4 using static NMR and MAS NMR

Two inequivalent protons from ¹H NMR spectra of RbH₂AsO₄ in the paraelectric phase were distinguished using static NMR and MAS NMR. From the ¹H spin–lattice relaxation times in the laboratory frame, T₁, and rotating frame, T_1ρ, of the two crystallographically inequivalent hydrogen sites, i.e., H(1) and H(2), the temperature dependences of T₁ and T_1ρ for H(1) were related to the reorientational motion. The shorter H(1) bonds give rise to stronger H-bonds, and protons involved in stronger H-bonds have long relaxation times. Consequently, the RbH₂AsO₄ structure has two crystallographically inequivalent sites with two different hydrogen-bond lengths.     

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.     

Temperature dependence of rotational correlation times in tribromobenzene

The ¹³C relaxation times and nuclear Overhauser enhancements of the protonated carbons in 1,3,5-tribromobenzene were measured as a function of temperature in the solvent benzene-d₆. Rotational correlation times, τ_C(CH), calculated by the Microviscosity/Free Rotor and Hu-Zwanzig “slip” models are substantially below the measured values. In contrast, correlation times predicted by the Hynes—Kapral—Weinberg model are in near quantitative agreement with experiment at all temperatures studied.     

CP/MAS Carbon-13 NMR Study of Spin Relaxation Phenomena of Cellulose Containing Crystalline and Noncrystalline Components

Abstract

Cross-polarization, ¹³C rotating frame spin-lattice relaxation and C laboratory frame spin-lattice relaxation processes have been studied for different cellulose samples by CP/MAS ¹³C NMR spectroscopy. It was found that the CP process can be described by a simple thermodynamic model and relative intensities of the respective resonance lines are consistent with the atomic ratios for the spectra obtained at a contact time of about 1 ms. The observed rotating frame spin-lattice relaxation times T^C ₁₀ were dominantly dependent on the time constant T^D _CH by which ¹³C nuclei were coupled to the ¹H dipolar spin system. It was, therefore, impossible to obtain information about molecular     

Dynamics of superionic Rb3H(SeO4)2 single crystals studied by H and Rb spin-lattice relaxation

The ¹H and ⁸⁷Rb spin-lattice relaxation and spin-spin relaxation times in superionic Rb₃H(SeO₄)₂ single crystals grown by the slow evaporation method were measured over the temperature range 160-450 K. The temperature dependencies of the ¹H T₁, T_1ρ, and T₂ are measured. In the ferroelastic phase, T₁ differs from T_1ρ, which is in turn different from T₂, although these three relaxation times converge to similar values near 410 K. This transition seems to occur at temperature which is about 40 K lower than the superionic transition temperature. The observation of liquid-like values of the ¹H T₁, T_1ρ, and T₂ in the high temperature is compatible with the phase being superionic, indicating that the destruction and reconstruction of hydrogen bonds does indeed occur at high temperature. In addition, the ⁸⁷Rb T₁ and T₂ values at high temperature were similar (on the order of milliseconds), a trend that was also observed for ¹H T₁ and T₂. This behavior is expected for most hopping-type ionic conductors, and could be attributed to interactions between the mobile ions and the neighboring group ions within the crystal. The motion giving rise to this liquid-like behavior is related to the superionic motion.     

Utilisation de l'effet overhauser,des mesures de relaxation en RMN 1H et des deplacements chimiques en RMN 13C dans l'etude de la conformation des amides—II: Application a l'etude conformationnelle d'amides alicycliques: acyl-prolines

¹H and ¹³C chemical shifts in the two decoalesced rotamers of the deuteromethyl ester of N-acetyl-l-proline are established by use of NOE and relaxation measurements. This requires the previous determination of the rotational barrier ΔG_c^≠ and the measurement of the relaxation times T₁ of the α-proton in the two conformers. The observed results reinforce the previous structural conclusions inferred from ¹³C studies on several acyl l-prolines and hydroxy l-prolines. The introduction of an acyl group in the prolines does not affect the different carbon atoms of the cycle in the two rotamers in the same way. These results can be interpreted in terms of electric field effects by the examination of X-Pro dipeptides with the N-terminal amino group shifted from α (COCH₂NH₂) to δ position (CO(CH₂)₄NH₂).     

Raman spectroscopy in liquids at high pressure

The pressure dependence of the reorientational correlation function for chloroform has been measured by analysis of the Raman 3019 cm^{? 1} A₁ CH stretching lineshape at 1, 1000, and 2000 bar and 23°C. These reorientational correlation functions were obtained using the method of spectral Fourier deconvolution introduced by Bratos. The results are compared to the correlation times obtained from the NMR deuteron T₁ relaxation times for CDCl₃ and those calculated from high pressure viscosity measurements.     

NMR study of molecular reorientations in NH4H2AsO4

Zeeman (T_1Z) and dipolar (T_1D) spin-lattice relaxation times of protons in NH₄H₂AsO₄ were measured as a function of temperature. The existence of a slow motion (τ ≈ 10^?3 see) is established, which is most probably a low frequency hindered reorientation of H₂AsO₄ groups. This motion is slowed down below the Curie point T_c. A sharp increase of the dipolar relaxation rate above T = 314°K indicates the possibility of a new high temperature phase transition in this compound.     

1H and 13C NMR study of cyclopentadienyl metal carbonyls in the solid state

In this paper we deal with some structural and dynamic properties of Cp₂W₂(CO)₆ (I) and Cp₂Ru₂(CO)₄ (II) as shown by solid state ¹³C and ¹H NMR experiments. The IR and ¹³C CPMAS spectra of a polycrystalline sample of I show that this compound possesses the anti rotameric structure found in a previously reported X-ray diffraction study. The analysis of the spinning side-band manifold in the ¹³C CPMAS spectrum of I allows us to assess a different semi-bridging character between two CO-groups not seen from the X-ray results. The spectral features of compound II are fully consistent with the X-ray and solution structures previously reported. In both compounds the cyclopentadienyl ligands are involved in fast reorientation motions which modulate the magnetic interactions responsible for the relaxation of ¹³C resonances. The activation energies (E_a) associated with this reorientation process of the Cp ring along their C₅ coordination axis have been determined to be 15.5 and 10.2 kJ mol⁻¹ for I and II respectively on the basis of ¹H T₁ measurements at different temperatures. Furthermore, we show that an empirical relationship relates E_a values and T_min (the temperature at which proton relaxation is more efficient) in a related series of cyclopentadienyl compounds.