Angle Spinning 13C NMR Spectra Research Articles

Unusual species of methane hydrate detected in nanoporous media using solid state 13C NMR.

Methane is considered to be a cubic structure I (CS-I) clathrate hydrate former, although in a number of instances, small amounts of structure II (CS-II) clathrate hydrate have been transiently observed as well. In this work, solid-state magic angle spinning 13C NMR spectra of methane hydrate formed at low temperatures inside silica-based nanoporous materials with pores in the range of 3.8-20.0nm (CPG-20, Vycor, and MCM-41) show methane in several different environments. In addition to methane encapsulated in the dodecahedral 512 (D) and tetrakaidecahedral 51262 (T) cages typical of the CS-I clathrate hydrate phase, methane guests in pentakaidecahedral 51263 (P) and hexakaidecahedral 51264 (H) cages are also identified, and these appear to be stabilized for extended periods of time. The ratio of methane guests among the D and T cages determined from the line intensities is significantly different from that of bulk CS-I samples and indicates that both CS-I and CS-II are present as the dominant species. This is the first observation of methane in P cages, and the possible structures in which they could be present are discussed. Broad and relatively strong methane peaks, which are also observed in the spectra, can be related to methane dissolved in an amorphous component of water adjacent to the pore walls. Nanoconfinement and interaction with the pore walls clearly have a strong influence on the hydrate formed and may reflect species present in the early stages of hydrate growth.

Aging‐induced structural relaxation in cornstarch plasticized with urea and glycerol

AbstractIn present work structural properties and aging of thermoplastic starches prepared by plasticization of cornstarch with urea, glycerol and their mixtures are studied using solid‐state1H NMR and13C NMR spectroscopy and WAXS measurements. Broad line1H NMR spectra reveal phase separation of plasticizers during aging of the samples with the same or higher relative amount of glycerol than the amount of urea. Glycerol in the TPS samples induces motion of starch chain segments, the mobility of which depends on the relative amount of glycerol. At the scale of nm, formation of B‐type crystallites in the samples containing glycerol and also of single‐helical crystallinity in all samples is observed during one‐year aging through cross‐polarization magic angle spinning13C NMR spectra. Urea, when used as the sole plasticizer, prevents the ordering of starch chains in B‐type crystallites. WAXS diffractograms show that regular crystals do not form in any of the samples.

Schiff base polymers derived from 2,5-diformylfuran

Polymerization of 2,5-diformylfuran with two primary amines was carried out in acetonitrile and ethanol at room temperature. The reaction was characterized using a combination of mass spectroscopy and NMR spectroscopy, which revealed the clean formation of the imine –CHN− functional group. Although some cyclic products were detected from mass spectroscopy, the ring size was limited to products that have the −CHN− group only in anti-geometry. The furan Schiff bases exhibit good thermal stability. While mass spectra evidenced oligomers of different lengths, cross-polarization magic angle spinning 13C NMR spectra of the insoluble polymer revealed the linear structure as proposed. © 2013 Society of Chemical Industry

Stereocopolyamides Derived from 2,3-Di-O-Methyl-d- and -l-Tartaric Acids and Hexamethylenediamine. 2. Influence of the Configurational Composition on the Crystal Structure of Optically Compensated Systems

The crystal structure of both the racemic copolyamide obtained from the racemic mixture of 2,3-di-O-methyl-d- and -l-tartaric acids and hexamethylenediamine and the equimolar mixture of the two configurationally homogeneous d- and l-polyamides was investigated with reference to the structure previously described for the optically pure poly(hexamethylene-2,3-di-O-methyl-l-tartaramide). DSC measurements showed that the two optically compensated systems have a crystallinity comparable to that displayed by the pure enantiomorphs whereas solid state cross-polarization−magic angle spinning 13C NMR spectra revealed structural differences between optically active and inactive forms. Structural data provided by X-ray diffraction and electron microscopy of powders, fibers, and single crystals were used for establishing the crystal structures of the inactive forms. In these systems the polymer chain assumed the same contracted conformation adopted by the optically pure polymer. The CERIUS2 software package was used ...

Functional group content of soil and sedimentary humic acids determined by CP/MAS 13C NMR related to conditional Zn 2+ and Cd 2+ formation constants

Cross polarization/magic angle spinning 13C NMR spectra, with varying dipolar dephasing times, were obtained for seven humic acids from a variety of terrestrial soil and aquatic sedimentary sources. Interpretation of the spectra revealed differences in functional group content, such as an enrichment in amino acid groups in the lignin-poor sedimentary samples relative to lignin-rich sedimentary and soil samples. Plots of the natural log of signal intensity above 100 ppm versus dipolar dephasing time, were used to measure the degree of aromatic substitution in each humic acid. The percentage of non-protonated aromatic carbon (relative to total aromatic carbon) ranged from 49% to 62% for the lignin-poor humic acids, while the values for lignin-rich samples were consistently higher and ranged from 64% to 86%, indicating a greater degree of aromatic substitution of the lignin-rich humic acids relative to the lignin-poor humic acids. In addition, conditional formation constants for each humic acid with Zn 2+ and Cd 2+ were evaluated by the use of Scatchard plots and the modified gel filtration technique. The sedimentary humic acids constants and this may be due to the higher amino acid content of these samples. Within a humic acid category (lignin-rich or lignin-poor), the samples with greater degrees of aromatic substitution were characterized by higher conditional formation constants, which may be due to a chelation effect.

Effects of unsaturation on 2H-NMR quadrupole splittings and 13C-NMR relaxation in phospholipid bilayers

Motional order and motional rates in unsonicated phospholipid bilayers were assessed as a function of unsaturation of the phospholipid. A measurement sensitive to motional order was obtained using 2H-NMR of 18:1, 18:1-phosphatidylcholine labelled at positions 9 and 10 with deuterium and included as a probe in phospholipid bilayers of interest at 10 mole percent. Spin lattice relaxation times from magic angle spinning 13C-NMR spectra of phospholipid dispersions of interest were used as a measure of motional rates. Measurements were made of phospholipid bilayers containing from 0 to 8 double bonds per molecule. No large effect of an increase in unsaturation was noted for the 2H-NMR quadrupole splittings or for the 13C-NMR spin lattice relaxation rate.

The solid state structure of arene-mercury(II) complexes from magic-angle spinning carbon-13 NMR and the solution carbon-13 NMR of some complexes of mercury(II) with arenes having bulky aliphatic substituents

The cross-polarization magic angle spinning 13C NMR spectra of Hg(SbF 6) 2 - 2 Arene (Arene = C 6HMe 5, 1,2,4,5-C 6H 2Me 4, 1,2,3,4-C 6H 2Me 4, or C 6H 6) have been measured. The spectra of the complexes of C 6HMe 5 and 1,2,4,5-C 6H 2Me 4 are consistent with static η 1-bonding of the mercury to the arene at an unsubstituted carbon atom, while the spectra of the 1,2,3,4-C 6H 2Me 4 and C 6H 6 complexes show the arene to have time-averaged C s or C 2, and C 6 symmetry respectively, at the temperature of measurement (300 K). The reduced temperature 13C NMR spectra of Hg(Arene) n 2+ ( n = 1 or 2; Arene = 1,3,5-C 6H 3R 3 (R = Me, i-Pr, or t-Bu)) in SO 2 solution are also reported and affirm that in these intramolecularly mobile species the mercury bonds in an η 1-manner, with unsubstituted aryl carbon atoms being the strongly preferred point of mercury attachment. This site preference is further demonstrated by the solution 13C NMR spectra of Hg(Arene) n 2+ (Arene = 1,2,3,4-C 6H 2-Me 4, n = 1 or 2; Arene = 1,4-C 6H 4R 2, R = Me or t-Bu, n = 1). The spectra of the 1,4-C 6H 4R 2 complexes and Hg( p-C 6H 4-t-BuMe) 2+ provide clear evidence for steric influence of the binding site. Like Hg(C 6Me 6) 2 2+, but unlike most of the complexes of substituted benzenes which have been studied, Hg(1,3,5-C 6H 3-i-Pr 3) 2 2+ exchanges only slowly with excess free ligand.

Magic angle spinning carbon-13 NMR of tobacco mosaic virus. An application of the high-resolution solid-state NMR spectroscopy to very large biological systems

Magic angle spinning 13C NMR was used to study tobacco mosaic virus (TMV) in solution. Well-resolved 13C NMR spectra were obtained, in which several carbon resonances of amino acids of the TMV coat protein subunits that are not observable by conventional high-resolution NMR spectroscopy can be designed. RNA resonance were absent, however, in the magic angle spinning 13C NMR spectra. Since three different binding sites are available for each nucleotide of the RNA, this is probably due to a line broadening caused by distributions of isotropic chemical shift values. In 13C-enriched TM 13C-13C dipolar interactions also gave rise to line broadening. By suitable pulse techniques that discriminate carbon resonances on the basis of their T1 and T1 rho values, it was possible to select particular groups of carbon nuclei with characteristic motional properties. Magic angle spinning 13C NMR spectra obtained with these pulse techniques are extremely well resolved.