High-resolution Solid-state 13C NMR Spectra Research Articles

Remarkably enhanced H2 evolution activity of oxidized graphitic carbon nitride by an extremely facile K2CO3-activation approach

The photocatalytic activity enhancement of graphitic carbon nitride (g-C3N4) in water splitting were commonly achieved by introducing various functional groups onto/into its planar structure. Herein, oxidized g-C3N4 (CNO) was prepared via an extremely facile K2CO3 activation approach. The high-resolution XPS spectra and solid-state 13C NMR spectra confirmed the successful introduction of O-containing groups and O atoms were inherited the original molecular framework of g-C3N4, but exhibited effectively enlarged bonded with C atoms rather than N atoms in the tri-s-triazine structure of g-C3N4. CNO inherited the original molecular framework of g-C3N4, but exhibited effectively enlarged surface area and enhanced visible light absorption, and more attrractively, the band structure of g-C3N4 could be well-tuned with the introduction of oxygen species. Importantly, the surface O-containing moieties could strongly interact with cocatalyst Pt and thus remarkably promote interfacial electron transfer and consequent photo-generated charge carrier separation. Consequently, the as-obtained CNO exhibited the remarkably enhanced photocatalytic performance with a H2 evolution rate of 199.7 μmol h−1, which is 9 times that of g-C3N4 (22.2 μmol h−1).

Dynamics and packing mode of long-chained n-alkane molecules in the nano-channel of MCM-41

High-resolution solid-state 13C NMR spectra of long-chained n-alkanes included in the nano-channel of MCM-41 have been observed. Each spectrum has two sets of peaks: one from very flexible molecules and the other from molecules with no significant flexibility. These molecules are characterized by carbon spin lattice relaxation times, T 1, of < 1 s and > 10 s, respectively. This phenomenon is ascribed to phase separation of the alkanes in the nano-channel, i.e., the amorphous and crystalline phases, respectively. The mechanism of this phase separation and the dynamics of these molecules in the nano-channel are discussed.

Structural characterization of drawn (ethylene–vinyl alcohol) copolymer by high-resolution solid-state 13C NMR

High-resolution solid-state 13C NMR spectra of drawn (ethylene–vinyl alcohol) copolymer (EVOH) and their 13C spin-lattice relaxation times ( T 1) were measured in order to elucidate the structure and dynamics of EVOH in the drawn state. It is found that the 13C chemical shift of the CH carbon for drawn EVOH moved upfield as compared with that of undrawn EVOH. Further, the mobility for the CH carbons of low molecular motion components are increased by drawing the EVOH sample. From these experimental results, the changes in the structure and dynamics of the drawn EVOH sample can be elucidated.

High resolution solid‐state 13C‐NMR study of as‐cured and irradiated epoxy resins

AbstractThis article presents the effects of strong ionizing radiations on the physico‐chemical modifications of aliphatic or aromatic amine‐cured epoxy resins based on diglycidyl ether of bisphenol A (DGEBA). Such epoxy resins have a considerable number of applications in the nuclear industrial field and are known to be very stable under moderate irradiation conditions.Using extensively high resolution solid‐state 13C‐NMR spectroscopy we show that the aliphatic amine‐cured resin (DGEBA‐TETA) appears much more sensitive to gamma rays than the aromatic amine‐cured one (DGEBA‐DDM). On the one hand, qualitative analyses of the high resolution solid‐state 13C‐NMR spectra of both epoxy resins, irradiated under similar conditions (8.5 MGy), reveal almost no change in the aromatic amine‐cured resin whereas new resonances are observed for the aliphatic amine‐cured resin. These new peaks were interpreted as the formation of new functional groups such as amides, acids and/or esters and to alkene groups probably formed in the aliphatic amine skeleton. On the other hand, molecular dynamics of these polymers are investigated by measuring the relaxation times, TCH, T1ρH and T1C , before and after irradiation. The study of relaxation data shows the formation, under irradiation, of a more rigid network, especially for the aliphatic amine‐cured system and confirms that aromatic amine‐cured resin [DGEBA‐4,4′‐diaminodiphenylmethane(DDM)] is much less affected by ionizing radiations than the aliphatic amine‐cured resin [DGEBA‐triethylenetetramine(TETA)]. Moreover, it has been shown that the molecular modifications generated by irradiation on the powder of the aliphatic‐amine‐cured resin appear to be homogeneously distributed inside the polymers as no phase separations can be deduced from the above analyses. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Metal-substituted ketenes: first 13C and 31P CP/MAS NMR determinations

Cross-polarization, combined with magic-angle spinning, has been employed to obtain high-resolution solid-state 13C and 31P NMR spectra of a series of solid, peculiar ketenes, also known as η 1-ketenyl derivatives, bearing as substituents a PPh 3 moiety and a coordinatively unsaturated Pt(II) fragment. Solid-state NMR data found for trans-[PtCl 2( η 2-C 2H 4){ η 1-C(PPh 3)CO}], trans-[PtCl 2{ η 1-C(PPh 3)CO} 2], Ph 3PCCO, trans-[PtCl 2( η 2-C 2H 4){ η 1-CH(PPh 3)COMe}], trans-[PtCl 2{ η 1-CH(PPh 3)COMe} 2] and Ph 3PCHCOMe, are compared with NMR values obtained in solution. The general utility of the CP/MAS NMR technique in the study of unstable or insoluble complexes is discussed.

A structural study of (ethylene–vinyl alcohol) copolymers by high-resolution solid-state 13C NMR

High-resolution solid-state 13C NMR spectra of (ethylene–vinyl alcohol) copolymers (EVOH) with various ethylene contents in the solid-state and their 13C spin-lattice relaxation times, T1, were measured, in order to elucidate the structure and dynamics of the copolymers. From these experimental results, the structural change of the EVOHs with changes of the ethylene content was successfully elucidated. Further, it is found that the 13C T1 for the CH carbon of the vinyl alcohol unit is mainly composed of two components. The fractions of short and long T1 components increase and decrease with an increase in the ethylene fraction, respectively. From these experimental results, it can be said that changes of the ethylene content in the copolymers lead to large changes in their structure and dynamics.

13C CP/MAS and 7Li NMR Study of Lithium Perchlorate/Poly(ethylene oxide)

To study compositional dependent conductivity of Li+-ions in lithium perchlorate/poly(ethylene oxide) (LiClO4/PEO) electrolytes, mobility of the Li+-ions and morphology of the electrolytes are investigated by examining the spin-lattice relaxation time (T1) of 7Li and by high-resolution solid-state 13C NMR methods. It is shown that the amount of the mobile Li+-ion estimated from the shorter 7Li-T1 component is not directly compared with the reported conductivity. The high-resolution solid-state 13C NMR spectra show that there exists five different CH2 carbons in the LiClO4/PEO electrolytes. The five peaks are assigned to (1) the crystalline phase of pure PEO, (2) the amorphous phase of PEO with the Li+-ions, and three complex phases of the following ratio of (3) PEO : Li+=3 : 1, (4) x : 1 (3<x<6), and (5) 6 : 1 . Furthermore, it is shown that the domain size of the PEO : Li+=6 : 1 phase in the LiClO4/PEO = 1/6 electrolyte is large and the mobility of the Li+-ions in the phase is so fast as to give a short 7Li-T1 comparable to that in the amorphous phase. The former ions are confined in the PEO : Li+=6 : 1 phase and do not contribute to conductivity while the latter does. Therefore, care should be taken in correlating the amount of the mobile Li+-ions obtained from the 7Li-T1 data to the Li+-ion conductivity.

Conformational analysis of the biphenyl moiety of dimer liquid crystals in the solid state by high-resolution solid-state13C NMR andab initio GIAO-CHF calculations

High-resolution 13C NMR spectra of α,ω-bis[(4,4′-cyanobiphenylyl)oxy]alkanes (CBA-n, n = 7, 9 and 10) were measured in chloroform solution and in the polycrystalline state. The spectra of the samples in solution are essentially the same; in the polycrystalline state, on the other hand, the spectrum of CBA-9 presents some differences in the aromatic carbon region in relation to those of CBA-7 and CBA-10′ which are similar. Therefore, ab initio GIAO-CHF calculations were carried out and an analysis of the conformation of the biphenyl moiety was attempted by comparing the results of the calculations with the measured high-resolution solid-state 13C NMR spectra. From these results it was concluded that the torsion angle for the biphenyl moiety of CBA-7 and CBA-10 stays at about 40°. For CBA-9 it is suggested that conformers with torsion angles of about 30° and 60° are also present. Copyright © 1999 John Wiley & Sons, Ltd.

A structural and dynamic study of poly(vinyl alcohol) in the gel state by solid-state 13C NMR and 1H pulse NMR

1H pulse NMR and high-resolution solid-state 13C NMR spectra of poly(vinyl alcohol) (PVA) gel were measured to clarify the structure and dynamics of the immobile component of the gel. From 1H pulse NMR experiments, it was found that the T 2 signal is mainly composed of three components. The long T 2 component was assigned to the mobile component which comes from the noncrosslinked region, the intermediate T 2 component corresponds to the intermediate immobile component which comes from the vicinity of the crosslinked region, and the short T 2 component corresponds to the immobile component which comes from the crosslinked region in the PVA gel. From high-resolution solid-state 13C NMR experiments, the mechanism of gel formation was satisfactorily elucidated. Further, dynamic viscoelastic modulus measurements were carried out. These experimental results were satisfactorily explained on the basis of the NMR results.

Some observations on the high-resolution solid-state CP-MAS 13C-NMR spectra of periodate-oxidised cellulose

Oxidation of cellulose by sodium metaperiodate leads to a decrease in the crystallinity. A wide-angle X-ray diffraction (WAXRD) study of oxidised celluloses [2,3-dialdehyde celluloses (DAC), 2,3-dicarboxycelluloses (DCC) and sodium 2,3-dicarboxylate celluloses (NaDCC)] showed that the crystalline peak at 2Θ = 22.7 ° decreased almost proportionately with the degree of oxidation of the starting cellulose. In the WAXRD spectra, no new crystalline peaks were observed for the oxidised celluloses, further indicating their amorphous nature. High-resolution, solid-state, cross-polarisation-magic angle spinning (CP-MAS) 13C nuclear magnetic resonance (NMR) spectra of these same oxidised cellulose samples were recorded in order to investigate the changes occurring with increasing oxidation levels. For each series of spectra of increasing oxidation levels of DAC, DCC and NaDCC an increasing trend towards the formation of a multiplicity of peaks was observed, owing to the numerous stereochemical configurations and rotameric conformations possible. The 98% oxidised cellulose samples represent almost pure compounds (new, linear, cellulose-derived polymers). The spectra and observed chemical shifts for this series are being reported for the first time in this paper. It is shown that solid-state NMR can be useful in investigations of oxidatively degraded celluloses.

High-resolution solid-state 13C NMR of the LH1 from Rhodospirillum rubrum

High-resolution solid-state 13C NMR spectra of the light-harvesting antenna complex (LH1) from Rhodospirillum rubrum were observed for the first time by cross-polarization (CP), magic angle spinning (MAS) methods with a total elimination of spinning side band technique (TOSS). Chemical shift analysis of the CP/MAS/TOSS 13C NMR spectra confirmed that the LH1 consists mainly of α-helices in the solid state. Time constants of cross polarization (TCH) and relaxation time T1 in a rotating frame (T1ρH) were determined from the experiments at various contact times. Smaller values of TCH were obtained for the carbons attached directly with protons in a rigid state. Relaxation times T1ρH revealed the dynamic structure of the complex and showed that bacteriochlorophyll a in the LH1 has high internal mobility even in the solid state. The proton spin-lattice relaxation time in a laboratory frame (T1H) determined by the 13C NMR signal amplitude changes suggested that protons in the LH1 proteins have such strong interaction among them that the spins of all protons in the protein can diffuse through spin-lattice-relaxation.

Spectroscopic studies of solid α-α trehalose

Infrared, Raman and high-resolution solid-state 13C NMR spectra are reported for the crystalline hydrate and anhydrous forms of solid α-α trehalose. A new form of anhydrous crystalline trehalose is also reported which has been identified from its vibrational and NMR characteristics. These show that it is distinct from the previously known anhydrous form. Detailed assignments of the spectra for all three solids are given. Whilst the vibrational spectra have been shown to be consistent with the crystal structures reported for the individual disacharide molecules, the NMR results sugest that solid-state NMR spectra are primarily determined by packing effects rather than by inequivalencies within the disaccharide molecule. The longer-range effects affect different carbon atoms to different extents, having a negligible effect on the C1,C1′ atoms of the hydrated form, which displays one singlet resonance. The results obtained indicate, therefore, that the presence of a singlet resonance in a solid-state 13C NMR spectrum cannot be taken as evidence of the existence of only one environment for the carbon atoms corresponding to that resonance, and may simply reflect a lower sensitivity of solid-state NMR to local environments.

Second-order quadrupolar effects for directly bonded and remote13C–79/81Br spin pairs in high-resolution13C NMR spectra of solids

High-resolution solid-state 13C NMR spectra of 1,4-dibromobenzene, 1,3,5-tribomobenzene, 1,2,4,5-tetrabromobenzene and hexabromobenzene (recorded at several different magnetic field strengths from 4.7–14.1 T) are reported. The bandshapes for 13C nuclei directly bonded to 79/81Br nuclei are characteristic multiplets arising from the effects of second-order quadrupolar interactions and both direct and indirect dipole–dipole interactions between 13C and 79/81Br. Numerical simulations have been used to obtain data on dipolar coupling constants and/or anisotropies in indirect coupling. In these simulations, effects arising from interaction with the directly bonded 79/81Br nucleus, intramolecular interactions with 79/81Br nuclei in the ortho, meta and para positions relative to the 13C nucleus, and intermolecular interactions with 79/81Br nuclei in neighbouring molecules were considered. Spectra are also presented for a range of other organic solids containing bromine.

Crystal and Molecular Structures of a Low-Melting Polymorph of Allocinnamic Acid

Abstract Crystals of a polymorph of allocinnamic acid having a melting point of 58 °C are monoclinic, space group P21/c with a = 6.2508(8), b = 15.524(2), c = 8.5067(5) Å, β = 109.295(7)°, and Z = 4. The structure was solved by direct methods and refined to a final R value of 0.042 for 1321 reflections [I &amp;gt; 3σ(I)]. The molecules form centrosymmetric dimers with two O–H···O bonds of carboxylic acid groups [O···O distance, 2.666(1) Å, O···H–O angle, 160.79°]. The plane formed by the olefin group is twisted with respect to the phenyl ring at an angle of 37.8(2)°. The high-resolution solid-state 13C NMR spectra were also measured and may be useful for distinguishing polymorphs of allocinnamic acid.

Crystal and Molecular Structures of Allocinnamic Acid

Abstract Crystals of allocinnamic acid having a melting point of 68 °C are monoclinic: space group P21/n with a = 10.157(4), b = 9.798(2), c = 15.770(4) Å, β = 91.35(3)°, and Z = 8. The structure was solved by direct method and refined to a final R value of 0.059 for 3832 reflections [I &amp;gt; 3σ(I)]. There are two independent molecules (A, B) in the asymmetric unit. The two molecules are hydrogen bonded through carboxylic acid groups [O1A···O2B 2.643(5) Å, O2A···O1B 2.629(5) Å]. Geometric differences between the two molecules are found in the torsion angle around the C(phenyl)–C(olefin) bond and the olefinic C=C bond length. In the high-resolution solid-state 13C NMR spectra resonances for the C2, C3, and C4 carbons are observed as doublet features in accordance with the conformational variations observed in X-ray crystallography.

Faraday communications. Carbon–halogen second-order quadrupolar and indirect spin–spin coupling effects in high-resolution solid-state13C NMR spectra of halobenzenes

High-resolution solid-state 13C NMR spectra of halobenzenes containing C—X bonds (X = CI, Br, I) are reported and the effects of the second-order quadrupolar interaction and indirect spin–spin coupling between 13C and the quadrupolar halogen nuclei 35/37Cl, 79/81Br and 127I are discussed.

A high-resolution solid-state 13C-NMR study on [1-13C]Ala and [3-13C]Ala and [1-13C]Leu and Val-labelled bacteriorhodopsin. Conformation and dynamics of transmembrane helices, loops and termini, and hydration-induced conformational change.

We have recorded 100.7-MHz high-resolution solid-state 13C-NMR spectra of [3-13C]Ala, [1-13C]Ala-labelled, Leu-labelled and Val-labelled bacteriorhodopsin (bR), to analyze the conformation and dynamics of transmembrane alpha helices and hydration-induced conformational changes. We assigned the 13C-NMR signals of these 13C-labelled amino acid residues to portions of the transmembrane alpha helices, loops and N-terminus and C-terminus, based on the conformation-dependent 13C chemical shift. The assignment of peaks to the transmembrane alpha helices is straightforward in view of the characteristic 13C chemical shifts of the C beta and carbonyl carbons, referred to the data of the model system. The signals of the transmembrane alpha helices were further divided into three or four peaks which are ascribed to either a dispersion of torsion angles of the alpha helices or variation of environments around the helices. In addition, we found that conformation of the N-terminus and C-terminus of bR is virtually the random-coil form which undergoes rapid reorientational motion in the hydrated system. Further, we show that the higher-order (secondary and/or tertiary) structure of bR is influenced by the hydration/dehydration process of the purple membrane, as viewed from the peak profile of the Ala C beta 13C signals. This change of structure occurs between relative humidities of 2% and 4%, consistent with the shift of the absorption maxima of retinal, which arise from the protonation/deprotonation processes of the Schiff base. In contrast, no such change of the carbonyl signals occurs for the Leu, Val and Ala residues. For the latter two residues, some spectral changes were noted at the stage of full hydration.

(13C,2H) residual dipolar and indirect spin–spin coupling effects in high-resolution13C nuclear magnetic resonance spectra of solids

We report high-resolution solid-state 13C NMR spectra of urea and thiourea inclusion compounds containing deuteriated guest molecules. The spectra (recorded under conditions of magic-angle sample spinning and high-power 1H decoupling at several magnetic fields from 4.7 to 11.7 T) for CD, CD2 and CD3 groups are triplets, pentets and heptets, respectively. These multiplets arise from the effects of residual dipolar interaction and indirect spin–spin coupling between 13C and 2H. A full analysis of these lineshapes is presented, and the effect of molecular motion is discussed. It is shown that the 13C NMR lineshape for deuteriated materials which are in a rapid motion regime (with respect to the 2H NMR timescale) can be rationalized by considering the motionally averaged 2H quadrupole coupling constant 〈χ〉.

Asp 85 is the only internal aspartic acid that gets protonated in the M intermediate and the purple-to-blue transition of bacteriorhodopsin A solid-state 13C CP-MAS NMR investigation

High-resolution solid-state 13C NMR spectra of the ground state and M intermediate of the bacteriorhodopsin mutant D 96N with the isotope label at [4- 13C]Asp and [11- 13C]Trp were recorded. The NMR spectra show that Asp 85 is protonated in the M intermediate. The environment of Asp 85 is quite hydrophobic. On the other hand, Asp 212 remains deprotonated and a slight shift to lower field indicates a more hydrophilic environment. Asp 85 also protonates in the purple-to-blue transition or bacteriorhodopsin in the deionized membrane, where it experiences a similar environment to M. The shift of Trp resonances in M reflect a conformational change of the protein in forming the M intermediate.

High-Resolution Solid-State Cross-Polarization Magic-Angle Spinning 13C NMR of Metal-Free Phthalocyanines

Abstract The high-resolution solid-state cross-polarization magic-angle spinning 13C NMR spectra of α, β, τ and X-form metal-free phthalocyanines (H2pc) were investigated. Chemical shifts differed according to crystal forms. As a result of the temperature dependence of chemical shifts, we could observe the change in the spectra because of the inner protons.