Step-by-step from amorphous phosphate to nano-structured calcium hydroxyapatite: monitoring by solid-state 1H and 31P NMR and spin dynamics.

The solid-state 1H, 31P NMR spectra and cross-polarization (CP MAS) kinetics in the series of samples containing amorphous phosphate phase (AMP), composite of AMP + nano-structured calcium hydroxyapatite (nano-CaHA) and high-crystalline nano-CaHA were studied under moderate spinning rates (5-30 kHz). The combined analysis of the solid-state 1H and 31P NMR spectra provides the possibility to determine the hydration numbers of the components and the phase composition index. A broad set of spin dynamics models (isotropic/anisotropic, relaxing/non-relaxing, secular/semi-non-secular) was applied and fitted to the experimental CP MAS data. The anisotropic model with the angular averaging of dipolar coupling was applied for AMP and nano-CaHA for the first time. It was deduced that the spin diffusion in AMP is close to isotropic, whereas it is highly anisotropic in nano-CaHA being close to the Ising-type. This can be caused by the different number of internuclear interactions that must be explicitly considered in the spin system for AMP (I-S spin pair) and nano-CaHA (IN-S spin system with N ≥ 2). The P-H distance in nano-CaHA was found to be significantly shorter than its crystallographic value. An underestimation can be caused by several factors, among those - proton conductivity via a large-amplitude motion of protons (O-H tumbling and the short-range diffusion) that occurs along OH- chains. The P-H distance deduced for AMP, i.e. the compound with HPO42- as the dominant structure, is fairly well matched to the crystallographic data. This means that the CP MAS kinetics is a capable technique to obtain complementary information on the proton localization in H-bonds and the proton transfer in the cases where traditional structure determination methods fail.

Kinetics of 1H-13C multiple-contact cross-polarization as a powerful tool to determine the structure and dynamics of complex materials: application to graphene oxide.

Hartmann-Hahn cross-polarization (HHCP) is the most widely used solid-state NMR technique to enhance the magnetization of dilute spins from abundant spins. Furthermore, as the kinetics of CP depends on dipolar interactions, it contains valuable information on molecular structure and dynamics. In this work, analytical solutions are derived for the kinetics of HHCP and multiple-contact CP (MC-CP) using both classical and non-classical spin-coupling models including the effects of molecular dynamics and several 1H, 13C relaxation and 1H-13C CP experiments are performed in graphene oxide (GO). HHCP is found to be inefficient in our GO sample due to very fast 1H T1ρ relaxation. By contrast, the MC-CP technique which alleviates most of the magnetization loss by 1H T1ρ relaxation leads to a much larger polarization transfer efficiency reducing the measuring time by an order of magnitude. A detailed analysis of the HHCP and MC-CP kinetics indicates the existence of at least two different kinds of hydroxyl (C-OH) functional groups in GO, the major fraction (∼90%) of these groups being in the unusual "slow CP regime" in which the rate of 1H T1ρ relaxation is fast compared to the rate of cross-polarization. This 13C signal component is attributed to mobile C-OH groups interacting preferentially with fast-relaxing water molecules while the remaining carbons (∼10%) in the usual "fast CP regime" are assigned to C-OH groups involved in hydrogen bonding with neighboring hydroxyl and/or epoxy groups.

Chemical shift powder spectra enhanced by multiple-contact cross-polarization under slow magic-angle spinning.

A simple multiple-contact cross-polarization (CP) scheme is applied to a powder sample of ferrocene and ß-calcium formate under static and magic-angle spinning (MAS) conditions. The method is described analytically through the density matrix formalism. We show that multiple equilibrations-re-equilibrations with the proton spin bath improves the polarization transfer efficiency at short contact times and provides higher signal enhancements than state-of-the art techniques such as adiabatic passage through the Hartmann-Hahn condition CP (APHH-CP) when MAS is applied. The resulting chemical shift powder spectra then are identical to the ones obtained by using ROtor-Directed Exchange of Orientations CP (APHH-RODEO-CP) with intensity gains of a factor 1.1-1.3.

Interaction Mechanisms between guaiacols and lignin: the conjugated double bond makes the difference.

Lignin is considered to be responsible for a selective sorption of phenolic compounds on wood. In order to investigate the mechanisms involved, two similar guaiacol compounds--only differing by the nature of the para side chain--were adsorbed on oak wood extracted lignin. Vapor sorption-desorption isotherms indicated that about 3.5 wt % of 4-vinylguaiacol is adsorbed near saturation whereas it is only 0.8% for 4-ethylguaiacol. For both compounds, the isotherms displayed a hysteresis though significantly greater for 4-vinylguaiacol. Analyses of the hydroxyl stretching region of FTIR spectra of the lignin/4-ethylguaiacol and lignin/4-vinylguaiacol complexes indicated that physisorption via hydrogen bonds occurs for both sorbates with lignin phenolic hydroxyl groups but which would be more condensed for the former than for the latter. According to NMR spectra, these phenolic hydroxyl groups correspond to non-etherified guaiacyl subunits. In contrast with other para substituents, the conjugated vinyl bond favors not only physisorption but also chemisorption as witnessed by the fact that upon desorption in the vapor phase, even after pumping under dynamic vacuum for several days, about 1 wt % of 4-vinylguaiacol remains adsorbed onto lignin.

Insertion of indigo molecules in the sepiolite structure as evidenced by 1H-29Si heteronuclear correlation spectroscopy.

Despite the numerous studies of the famous indigo-based pigment Maya Blue, there are still many questions regarding the elucidation of its structure. Here, two-dimensional (2D) (1)H-(29)Si heteronuclear correlation (HETCOR) spectroscopy with frequency-switched Lee-Goldburg (FSLG) homonuclear decoupling is applied to sepiolite and sepiolite-indigo complexes. Owing to the high resolution in the (1)H dimension of the 2D (1)H-(29)Si HETCOR spectrum obtained by FSLG homonuclear decoupling, the assignment of the (29)Si cross-polarization magic-angle spinning (CPMAS) spectrum of sepiolite is clearly confirmed. Moreover, 2D (1)H-(29)Si FSLG-HETCOR spectroscopy gives the first direct evidence that some indigo molecules are inserted in the sepiolite structure whereas no interaction between indigo and the external side surface (silanol groups) is observed in the (29)Si CPMAS spectra. These results are consistent with the fact that indigo molecules interact with water coordinated to magnesium and suggest that Maya Blue made from sepiolite is not a surface complex.

The incorporation of indigo molecules in sepiolite tunnels.

Evidence for access of molecules the size of acetone or pyridine to the intracrystalline tunnels of nanofibre clay (sepiolite) has indicated formation of a new type of organic-inorganic nanocomposites. However, the introduction of larger molecules has been a recurring problem. It is now agreed that for indigo, the molecules are located on the external surface and at the ends of the fibres, thus blocking access to internal tunnels. We claim, however, that it is possible for indigo molecules to access the internal channels of sepiolite. FTIR and XRD analyses have provided evidence for folding of the sepiolite structure preheated at high temperature (above 350 degrees C). By comparison, we have shown that for indigo/sepiolite mixtures treated in the same conditions, no change in the crystalline structure of the sepiolite is observed, and that blue samples, related to Maya blue, with indigo molecules incorporated deeply enough into sepiolite to prevent folding of the tunnels, can be obtained. NMR, FTIR and thermal analysis confirm the interaction of indigo with the water coordinated to magnesium(II) and located inside the internal and external channels of sepiolite. Two other hypotheses are excluded; we show both that zeolitic water is not blocked in the tunnels by indigo, and that if thermal decomposition products of indigo can be formed, they are in a minority.

Analytical solutions to several magic-angle spinning NMR experiments.

Using the Anderson-Weiss (AW) formalism, analytical expressions of the NMR signal are obtained for the following magic-angle spinning (MAS) experiments: total suppression of sidebands (TOSS); phase adjusted spinning sidebands (PASS); rotational-echo double-resonance (REDOR); rotor-encoded REDOR (REREDOR); cross-polarization magic-angle spinning (CPMAS); exchange induced sidebands (EIS); one-dimensional exchange spectroscopy by sideband alternation (ODESSA); time-reverse ODESSA (trODESSA); centerband-only detection of exchange (CODEX). In order to test the validity of the AW approach, the Gaussian powder approximation is compared with exact powder calculations. A quantitative study of the effect of molecular dynamics on the efficiency of the TOSS and REDOR pulse sequences is then presented.

13C/15N distance determination by CPMAS NMR in uniformly 13C labeled molecules.

The REDOR and CPMAS techniques are applied for measuring 13C-15N dipolar coupling constants in glycine. It is shown that the selective CP or SPECIFIC CP technique removes the coherent evolution of the spin system under homonuclear 13C-13C J couplings. While the large coupling constant (approximately 900 Hz) is readily determined because of the presence of large oscillations in the CPMAS dynamics, their absence precludes the measurement of the small coupling constant (approximately 200 Hz). The experimental results and numerical simulations demonstrate that the determination of 13C-15N coupling constants of medium size (<1 kHz) by the CPMAS technique is mainly limited by the strength of the 1H decoupling field and the size of the 13C and 15N chemical shift anisotropies.

1H/31P distance determination by solid state NMR in multiple-spin systems.

The results of two techniques of dipolar recoupling, REDOR and CPMAS, are compared in the case of a coupled multiple-spin system. A fundamentally different behavior is observed for these two techniques. In REDOR, the terms associated with each interaction S-I(k) commute with each other and no truncation takes place so that each addition of spin I(k) causes a splitting with its dipolar frequency. In CPMAS, the flip-flop terms of the dipolar Hamiltonian do not commute with the dominant term from the strongly coupled spin pair so that the weak coupling terms from the neighboring spin I(k) are effectively truncated by the dominant pair interaction. Spin dynamics calculations are in agreement with the experimental data in a cubane shaped cluster.

NMR polarization echoes in a nematic liquid crystal.

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the 1H-1H dipolar dynamics. In this way, the 13C becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P(00)(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P(00)(t) was well fitted to an exponential law with a characteristic time tau(C) approximately 310 micros. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P(00)(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible 1H-1H dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P(00)(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time tau(phi) approximately 527 micros. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that tau(phi) is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.

51V magic angle spinning NMR in VOPO4 phases.

51V magic angle spinning NMR was applied to the alpha(II), beta and gamma phases of VOPO4 at three magnetic field strengths (4.7, 7.1, and 11.7 T). The 51V quadrupole and chemical shift tensors were determined by iterative fitting of the NMR lineshapes at the three magnetic field strengths. The applicability of the method is illustrated by comparison with literature data. Although determined chemical shift tensors are completely axially symmetric and of the same magnitude, all studied phases can clearly be distinguished by their quadrupole coupling tensor. Relationships between the 51V NMR data and structural characteristics such as crystal symmetries are discussed.

59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO2 and HCoO2.

59Co and 23Na NMR has been applied to the layered cobalt oxides NaCoO(2) and HCoO(2) at three different magnetic field strengths (4.7, 7.1 and 11.7T). The 59Co and 23Na quadrupole and anisotropic shift tensors have been determined by iterative fitting of the NMR line shapes at the three magnetic field strengths. Due to the large 59Co quadrupole interaction in NaCoO(2), a frequency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the 59Co and 23Na shift and quadrupole coupling tensors in NaCoO(2) are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the 59Co NMR spectra clearly show the presence of structural disorder in HCoO(2). The 23Na chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO(2) is due to Van Vleck paramagnetism for Co(3+). Electric field gradient calculations using either the empirical point charge model or the ab initio full potential-linearized augmented plane wave method are compared with the experimental NMR data.

Synthesis and characterization of a novel cyclic aluminophosphinate: structure and solid-state NMR study.

We present the structure and a multinuclear solid-state NMR study of a new cyclic aluminophosphinate. The crystallographic structure of [Al(2)(HC(6)H(5)PO(2))(2)(C(4)H(9)OH)(8)]Cl(4) (compound 1) was obtained at low temperature (a = 11.830(7) A, b = 14.216(6) A, c = 17.790(6) A, beta = 91.25(4) degrees, monoclinic, P21/c, Z = 2). (13)C IRCP (inversion recovery cross polarization) and NQS (non quaternary suppression) NMR experiments allowed the complete assignment of the quaternary carbon atom of the phenyl ring and the precise determination of the isotropic /(1)J(P-C)/ coupling constant. (31)P CP MAS dynamics was carefully studied by varying the contact time. Dipolar oscillations even at slow MAS were observed. Up to 11 kHz, these oscillations were more pronounced, and the P-H distance was easily extracted. (27)Al NMR quadrupolar parameters for 1 were obtained with very good accuracy, and unusual satellite transition splitting was observed. Furthermore, the isotropic lines of the inner and outer transitions were clearly observable, leading to the unambiguous determination of the quadrupolar parameters.