Dynamic Nuclear Polarization NMR Spectroscopy Allows High-Throughput Characterization of Microporous Organic Polymers

Dynamic nuclear polarization (DNP) solid-state NMR was used to obtain natural abundance 13C and 15N CP MAS NMR spectra of microporous organic polymers with excellent signal-to-noise ratio, allowing for unprecedented details in the molecular structure to be determined for these complex polymer networks. Sensitivity enhancements larger than 10 were obtained with bis-nitroxide radical at 14.1 T and low temperature (∼105 K). This DNP MAS NMR approach allows efficient, high-throughput characterization of libraries of porous polymers prepared by combinatorial chemistry methods

Solid-State Dynamic Nuclear Polarization at 263 GHz: Spectrometer Design and Experimental Results

Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz 1H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water–glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period.National Institutes of Health (U.S.) (NIH grant EB-002804)National Institutes of Health (U.S.) (NIH grant EB-002026

Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13C and 29Si NMR Enhanced by Dynamic Nuclear Polarization

We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of13C and 29Si nuclei located in mesoporous organic/inorganic hybrid materials, at several hundreds of nanometers from stable radicals (TOTAPOL) trapped in the surrounding frozen disordered water. The approach is demonstrated using mesoporous silica nanoparticles (MSN), functionalized with 3-(N-phenylureido)propyl (PUP) groups, filled with the surfactant cetyltrimethylammonium bromide (CTAB). The DNP-enhanced proton magnetization is transported into the mesopores via 1H–1H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements εon/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to εon/off ≈ 30 for both nuclei. A quantitative analysis of the signal enhancements in MSN with and without surfactant is based on a one-dimensional proton spin diffusion model. The effect of solvent deuteration is also investigated

Probing the Nonrandom Aluminum Distribution in Zeolite Merlinoite with Ultra-High-Field (18.8 T) 27Al and 29Si MAS NMR

Two crystallographically distinct tetrahedral Al sites in zeolite merlinoite are directly identified by ultra-high-field (18.8 T) (27) Al MAS NMR spectroscopy. The experimental chemical shifts and peak areas indicate a preferential siting of Al in site T-II. This non-random aluminum distribution is independently confirmed by ultra-high-field (18.8 T) (29) Si MAS NMR spectroscopy. (C) 2002 Elsevier Science B.V. All rights reserved.X1113sciescopu

Observing Apparent Nonuniform Sensitivity Enhancements in Dynamic Nuclear Polarization Solid-State NMR Spectra of Polymers

International audienceHigh-field dynamic nuclear polarization (DNP)may enhance the sensitivity of solid-state NMR experimentson a wide range of systems, including synthetic polymers,owing to the transfer of electron spin polarization from radicalsto nuclei upon microwave irradiation (usually at cryogenictemperatures). Provided that the radicals are homogeneouslydispersed in the sample, a uniform DNP enhancement isexpected for all the signals in the 13C cross-polarization magicangle spinning (CPMAS) spectrum. Here, we show that, in thecase of methyl group containing polymers, a change in thecross-polarization (CP) dynamics induced by the moderate increase in sample temperature due to microwave irradiation maylead to the observation of apparent nonuniform enhancements in the DNP-enhanced 13C CPMAS spectra. This peculiarbehavior should be accounted for when measuring 13C CP DNP enhancements on polymer materials, especially forheterogeneous polymer samples (for which truly nonuniform DNP enhancements could potentially be detected), or whenquantitative results are sought

A case study concerning the use of vocabulary learning strategies among Malaysian sojourners in the United Kingdom

This thesis investigates the Vocabulary Learning Strategies (VLS) used by Malaysian sojourners in learning English in UK in relation to context and learners’ agencies. Strategies used in formal education and daily life are explored and their vocabulary language learning experiences in Malaysia are taken into account. The study investigates the case of ten Malay native speakers who were living temporarily in the UK for a period of three to four years. Four participants were students, three had full-time jobs, two had part-time jobs and the other one was stay-at-home spouse. The participants were asked about the strategies they used in the UK, and also to reflect on their language learning in Malaysia. This study also asked how their VLS progressed and changed as they moved from their home country to the UK. This study adopted a mixed-method approach and data was collected through semi-structured interviews, learning diaries, Vocabulary Levels Tests and VLS questionnaires in three phases; over a period of fourteen months. Interview data was coded into themes and was supplemented with data from learning diaries and questionnaires. In relation to research question one, it was found that Malaysian sojourners used a mix of vocabulary learning strategies throughout their vocabulary learning journey starting from school, HE, their workplace and Coventry. The participants’ VLS were more or less consistent across their learning English journey. Frequency of use for each strategy very much depended on who or what was available to them and their willingness to learn new words. The participants also started to use more technological tools as these tools became widely available. In respect to research question two, Malaysian sojourners encountered new words in all sorts of situations. These encounters could be a mix of intentional and incidental as one could be purposely read an academic article and found new words. However, a large number of the encounters came from technological based tools such as watching online movies, surfing the Internet and reading online. The frequency of encounters differed depending on the opportunities the participants had. Some opportunities were specific to participants’ status such as being a student, working, and not working. Other opportunities were common and were experienced by all, e.g. travelling, seeking health advice and using media. The study offers a model of Vocabulary Learning in Context (VLC) in which context and agency are considered. In particular, the model shows that learners have to make decisions whether to encounter new vocabulary and what actions they should take about it. If they choose to try and learn new vocabulary systematically, they would engage in the process of transferring the new vocabulary from short-term memory to long-term memory. The vocabulary learning process will start again when learners use the new words in context as the context will, repeatedly, provide new opportunities of encountering other new words. Recommendations for L2 learners, L2 teachers, researchers and those supporting language learning more informally are given