Solid-state NMR characterization of tri-ethyleneglycol grafted polyisocyanopeptides.

In aqueous media, ethylene glycol substituted polyisocyanopeptides (PICPs) change their state (undergo a sol-to-gel transition) as a response to temperature. This makes them promising materials for various biomedical applications, for instance, for controlled drug release and non-damaging wound dressing. To utilize PICP in biomedical applications, understanding of the origin of the gelation process is needed, but this is experimentally difficult because of the notoriously low gelator concentration in combination with the slow polymer dynamics in the sample. This paper describes a detailed characterization of the dried state of PICPs by solid-state NMR measurements. Both the (13) C and the (1) H NMR resonances were assigned using a combination of 1D cross-polarization magic angle spinning, 2D (13) C-(1) H heteronuclear correlation spectra and (1) H-(1) H single quantum-double quantum experiments. In addition, the chemical groups involved in dipolar interaction with each other were used to discuss the dynamics and spatial conformation of the polymer. In contrast to other PICP polymers, two resonances for the backbone carbon are observed, which are present in equal amounts. The possible origin of these resonances is discussed in the last section of this work. The data obtained during the current studies will be further used in elucidating mechanisms of the bundling and gelation. A comprehensive picture will make it possible to tailor polymer properties to meet specific needs in different applications. Copyright © 2015 John Wiley & Sons, Ltd.

The influence of thermochemical treatments on the lignocellulosic structure of wheat straw as studied by natural abundance 13C NMR.

The effects of thermochemical treatments (aquathermolysis, pyrolysis, and combinations thereof) on the lignocellulosic structure and composition of wheat straw were studied with (13)C and (1)H solid state NMR spectroscopy and proton T1ρ relaxation measurements. Results show that aquathermolysis removes hemicellulose, acetyl groups, and ash minerals. As a result, the susceptibility of lignocellulose to pyrolysis is reduced most likely due to the removal of catalytically active salts, although recondensation of lignin during aquathermolysis treatment can also play a role. In contrast to pyrolysis of wheat straw, pyrolysis of aquathermolysed wheat straw leaves traces of cellulose in the char as well as more intense lignin methoxy peaks. Finally, it was found that both pyrolysis chars contain aliphatic chains, which were attributed to the presence of cutin or cutin-like materials, a macromolecule that covers the aerial surface of plants, not soluble in water and seemingly stable under the pyrolysis conditions applied.

The electronic structure and ionic diffusion of nanoscale LiTiO2 anatase.

Upon lithium insertion in the pristine TiO2 anatase phase the theoretical maximum of LiTiO2 can be reached in crystallite sizes less than approximately 10 nm, whereas bulk compositions appear limited to Li(x) approximately 0.6TiO2 at room temperature. Both X-ray absorption spectroscopy (XAS) and ab initio calculations have been applied to probe the electronic structure of the newly formed LiTiO2 phase. These results indicate that a large majority of the Li-2s electrons reside at the Ti-3d(t2g)/4s hybridized site. About 10% of these electrons are transferred to non-localized states which makes this compound a good electronic conductor. Ionic conductivity is probed by nuclear magnetic resonance (NMR) relaxation experiments indicating relatively small hopping rates between the Li-ion sites in LiTiO2. Formation of the poor ionic-conducting LiTiO2 at the surface of the particles explains why micro-anatase Li(x)TiO2 is not able to reach the theoretical maximum capacity at room temperature, and why this theoretical maximum capacity reached in nano-sized materials cannot be (dis)charged at high rates.

High-resolution liquid- and solid-state nuclear magnetic resonance of nanoliter sample volumes using microcoil detectors.

The predominant means to detect nuclear magnetic resonance (NMR) is to monitor the voltage induced in a radiofrequency coil by the precessing magnetization. To address the sensitivity of NMR for mass-limited samples it is worthwhile to miniaturize this detector coil. Although making smaller coils seems a trivial step, the challenges in the design of microcoil probeheads are to get the highest possible sensitivity while maintaining high resolution and keeping the versatility to apply all known NMR experiments. This means that the coils have to be optimized for a given sample geometry, circuit losses should be avoided, susceptibility broadening due to probe materials has to be minimized, and finally the B(1)-fields generated by the rf coils should be homogeneous over the sample volume. This contribution compares three designs that have been miniaturized for NMR detection: solenoid coils, flat helical coils, and the novel stripline and microslot designs. So far most emphasis in microcoil research was in liquid-state NMR. This contribution gives an overview of the state of the art of microcoil solid-state NMR by reviewing literature data and showing the latest results in the development of static and micro magic angle spinning (microMAS) solenoid-based probeheads. Besides their mass sensitivity, microcoils can also generate tremendously high rf fields which are very useful in various solid-state NMR experiments. The benefits of the stripline geometry for studying thin films are shown. This geometry also proves to be a superior solution for microfluidic NMR implementations in terms of sensitivity and resolution.

Homonuclear correlation experiments for quadrupolar nuclei, spinning away from the magic angle.

We present a set of homonuclear correlation experiments for half-integer quadrupolar spins in solids. In all these exchange-type experiments, the dipolar interaction is retained during the mixing time by spinning the sample at angles other than the "regular magic angle" (54.7 degrees celsius). The second-order quadrupolar interaction is averaged by different strategies for the different experiments. The multiple-quantum off magic angle spinning (MQOMAS) exchange experiment is essentially a regular MQMAS experiment where the quadrupolar interaction is averaged by combining magic angle spinning with a multiple- to single-quantum correlation scheme. The sample is spun at the magic angle at all times except during the mixing time which is added to establish homonuclear correlation. In the multiple-quantum P4 magic angle spinning (MQP4MAS) exchange experiment, the sample is spun at one of the angles at which the fourth-order Legendre polynomial vanishes (P4 magic angle), the remaining second-order quadrupolar interaction now governed by a second-rank tensor is refocussed by the multiple to single-quantum correlation scheme. In the dynamic angle spinning (DAS) exchange experiment, the second-order quadrupolar interaction is averaged by correlating the evolution from two complementary angles. These experiments are demonstrated and compared, in view of their specific advantages and disadvantages, for 23Na in the model compound Na2SO3.

Sonochemical polymerization of diphenylmethane.

Sonolysis of diphenylmethane (DPhM) has been studied under the effect of 20 kHz ultrasound (absorbed acoustic power 0.45 W/ml, surface area of sonotrode 1 cm(2), volume of sonicated solution 100 ml) under argon at 60 degrees C. The solid product of the sonolysis was characterized by elemental analysis, FTIR, 13C MAS NMR, TGA/DSC, XRD and TEM techniques. It was found that the sonolysis of DPhM causes formation of the polymer with the composition similar to crosslinked polystyrene. Assumed mechanism of DPhM sonolysis consists of DPhM molecules dissociation inside the cavitating bubble. Secondary radical scavenging and radical recombination processes yields the sonopolymer in the liquid phase. The breakdown of the aromatic ring during DPhM sonolysis confirms that a very high temperature established in the cavitating bubble.

Observation of hydroxyl groups by 17O solid-state multiple quantum MAS NMR in sol-gel-produced silica.

17O NMR parameters (CQ, eta, delta(iso) and T1) are reported for both Si-O-Si and Si-OH fragments within a silica gel. The Si-OH units have a wide spread of parameters but are typically characterised by a very short T1 (approximately 0.1 ms) and CQ < 200 kHz. These observations have extremely important implications for the quantification of such units in these gels and related glassy materials by 17O NMR. In light of these observations, the 17O NMR experiments have been optimised and a distinct resonance from the OH group is observed in 1D static and magic angle spinning (MAS) NMR measurements as well in the multiple quantum (MQ) experiment.

Spin density description of rotational-echo double-resonance, transferred-echo double-resonance and two-dimensional transferred-echo double-resonance solid state nuclear magnetic resonance.

The spin density matrix formalism has been applied to rotational-echo double-resonance (REDOR), transferred-echo double-resonance (TEDOR) and two-dimensional (2D) TEDOR experiments in order to obtain an expression for the signal intensities. TEDOR spectra of 15N-labeled glycine were measured with different dipolar evolution times. 2D-TEDOR spectra were measured of doubly labeled glycine-2-13C, 15N and of 15N-labeled glycine. Both the TEDOR and the 2D-TEDOR spectra were readily obtained although the 2D-TEDOR experiment on 15N-labeled glycine used a lot of machine time. Even though the 15N-1-13C dipolar coupling is relatively small (200 Hz), the 1-C resonance can still be observed.

The determination of the average 27Al-31P distance in aluminophosphate molecular sieves with SEDOR NMR.

The average 27Al-31P distances in an aluminophosphate, AlPO4-5, and in a silicoaluminophosphate, SAPO-11, were determined by using Spin Echo DOuble Resonance (SEDOR). A calculated SEDOR curve was fitted to the data in order to obtain the dipolar coupling constant. The tetrahedral surrounding of 27Al by four 31P atoms in the second coordination sphere was taken into account in the calculation of the theoretical SEDOR curve. The average 27Al-31P distances obtained by using this technique proved to be in good agreement with X-ray diffraction data.