NMR Cross-Polarization when TIS>T1ρ; Examples from Silica Gel and Calcium Silicate Hydrates

Probing structural features in potato starch granules at moderate hydration through the modelling of 1H->13C polarization transfer kinetics

The structural arrangement of starch polymers in presence of water is known to impact the functional properties of starchy products. In this study, the hydration of potato starch granules was investigated at the molecular level through various 1H->13C polarization transfer solid-state Nuclear Magnetic Resonance (ss-NMR) experiments. The impact of increasing the water content from 12.3 % to 45.9 % was assessed using 13C Cross Polarization Magic Angle Spinning (CPMAS), Variable Contact Time (VCT-CPMAS), Variable Spin Lock (VSL-CPMAS), and T One Rho QUEnching (TORQUE) NMR sequences. Of these, VCT-CPMAS proved to be the most promising. When applied with an optimal number of contact times, it enabled the application of several mathematical models that provided detailed insights into the structuring of protons in the hydrated potato starch granules. At low hydration (12.3 %), the models enabled various structural domains to be distinguished, which we suggest are associated with helical and amorphous structures. At moderate hydration (45.9 %), we tested two fitting models. Two pools of protons were revealed, corresponding to loosely ordered structures on the scale of tens of nanometers. These findings suggest varying water distribution during starch hydration and are likely to indicate variable hydration levels in the multilamellar amorphous structures of starch granules.

Effects of Using Aluminum Sulfate as an Accelerator and Acrylic Acid, Aluminum Fluoride, or Alkanolamine as a Regulator in Early Cement Setting

Aluminum sulfate was employed as the main accelerator in order to explore new non-chloride and alkali-free cement accelerators. Acrylic acid, aluminum fluoride, or alkanolamine were used as regulators to further accelerate cement setting. The setting time, compressive, and flexural strengths in cement early strength progress were detected, and both the cement (raw material) and hydrated mortar were fully characterized. The cement setting experiments revealed that only loading acrylic acid as the regulator would decrease the setting time of cement and increase the compressive and flexural strengths of mortar, but further introduction of aluminum fluoride or alkanolamine improved this process drastically. In the meantime, structural characterizations indicated that the raw material (cement) used in this work was composed of C₃S (alite), while hydrated mortar consisted of quartz and C₃A (tricalcium aluminate). During this transformation, the coordination polyhedron of Al³⁺ was changed from a tetrahedron to octahedron. This work puts forward a significant strategy for promoting the activity of aluminum sulfate in cement setting and would contribute to the future design of new non-chloride and alkali-free cement accelerators.

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 T_1ρ relaxation. By contrast, the MC-CP technique which alleviates most of the magnetization loss by ¹H T_1ρ 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 T_1ρ 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.

Sensitivity enhancement via multiple contacts in the {1H–29Si}–1H cross polarization experiment: a case study of modified silica nanoparticle surfaces

{¹H–²⁹Si}–¹H double cross polarization inverse detection (DCPi) solid-state NMR, has recently been shown to be a powerful tool for studying molecules adsorbed on the silica surface. In this contribution, we develop an improved version (MCPi) which incorporates a block of multiple contact pulses, and quantitatively compare the sensitivities of MCPi and DCPi over a typical range of experimental parameters. The MCPi pulse sequence aims at higher sensitivity and robustness for studying samples with various relaxation characteristics. In the case of dimethyl sulfoxide (DMSO) molecules adsorbed on the silica surface, MCPi performs equally well or up to 2.5 times better than DCPi over a wide range of parameters. The applicability to and performance of MCPi on composite materials was demonstrated using a sample of polymer–silica composite, where significantly higher sensitivity could be achieved at very long total mixing times. The results also showed that both techniques are surface specific in the sense that only the groups close to the surface can be detected.

In this paper we demonstrate {¹H–²⁹Si}–¹H multiple cross polarization inverse detection (MCPi) solid state NMR as a robust technique for studying modified silica nanoparticle surfaces.

Probing structural and motional features of organic and inorganic solids through extended family of cross-polarization experiments

Combined use of cross-polarization and magic-angle spinning in the middle of the seventies has opened a new era of high-resolution solid-state NMR spectroscopy. Cross-polarization procedure is commonly used to obtain a shorter measuring time and to investigate or exploit one nucleous by means of the other nucleous involved in the polarization transfer. An extended family of cross-polarization experiments including constant time cross-polarization approach, cross-polarization inversion and indirect observation of proton spin system is reviewed and illustrated with applications to a large range of solids.

Quantitative structural characterization of POSS and octavinyl-POSS nanocomposites by solid state NMR

The ratio between two different (29)Si atoms in chloromethylphenyl isobutyl Polyhedral Oligomeric Silsesquioxane (POSS) was determined based on the quantitative cross polarization (QCP) (Shu et al., Chem. Phys. Lett. 462 (2008) 125) in solid-state NMR. For a (29)Si/(1)H spin system, cross polarization and depolarization together with the reciprocity relation were performed with optimized experimental conditions. It saves considerable experimental time compared to the (29)Si direct polarization experiment. The same method was further applied to octavinyl-POSS nanocomposites containing perfluoropolyether (PFPE) for deriving directly and accurately the average numbers of reacted vinyl groups, which may not be obtained by combining FTIR and solution (1)H NMR. In principle, the aforementioned method proves to be valuable in quantitative characterization of silicon related structures in bulk materials.

Silica and Hybrid Silica Gels Revisited: New Insight by Solid State Nuclear Magnetic Resonance

The “old” 1H→29Si CP MAS (Cross Polarization Magic Angle Spinning) experiment is revisited in the frame of silica hybrid gels and silsesquioxanes. It is proved that the analysis of the CP curves can lead to erroneous interpretation in terms of quantification. We show that this results from false assumptions concerning the dynamical CP parameters THSi (cross relaxation time constant) and T1ρH (1H relaxation time in the rotating frame). In other words, at least one parameter must be measured independently, in order to constrain the fits of the CP curves. Moreover, we demonstrate that the well-known (and universally used…) “spin bath” assumption is not always valid in the frame of 1H→29Si CP MAS NMR. This point is clearly demonstrated on model silsesquioxanes exhibiting short Si-H bonds. In this case, the transfer of magnetization (called coherent transfer) presents clearly oscillations, which can lead to the precise measurement of Si-H distances by solid state NMR! Curiously, the coherent transfer of magnetization is also demonstrated for weakly coupled spin systems, encountered in the silsesquioxane (SiO1.5CH3)8 or T units gels. In this case, a numerical simulation of the CP curves gives a deep insight in the chemical environment of the 29Si sites in terms of Si-H distances and local molecular reorientations. For weakly coupled systems, 1H-1H spin diffusion must be suppressed, in order to reveal the coherent character of the transfer: the quenching of spin diffusion is demonstrated by using a modified version of the CP MAS experiment. We introduce here the Lee-Goldburg CP MAS experiment (CPLG MAS) for that purpose. The “off resonance” 1H irradiation (at the magic angle) ensures the strong suppression of the 1H-1H homonuclear dipolar interaction and therefore the efficient quenching of spin diffusion during the CP transfer.

Solid-state nuclear magnetic resonance studies of vinyl polymer/silica colloidal nanocomposite particles

Solid-state nuclear magnetic resonance (NMR) has been used to characterize the interface between the organic and inorganic components of "core-shell" colloidal nanocomposite particles synthesized by in situ aqueous (co)polymerization of styrene and/or n-butyl acrylate in the presence of a glycerol-functionalized silica sol. Polymer protons are in close proximity (<5 A) to surface silanol sites in all the nanocomposites studied, indicating that either styrene or n-butyl side groups extend between the glycerol-functional silane molecules toward the surface of the silica particles. For the poly(styrene-co-n-butyl acrylate)/silica nanocomposite n-butyl acrylate residues are located closer to the surface of the silica particle than styrene residues, suggesting a specific interaction between the former and the glycerol-functionalized silica surface. The most likely explanation is a hydrogen bond between the ester carbonyl and the glycerol groups, although this cannot be observed directly. For the Bindzil CC40 glycerol-functionalized silica sol the relative intensities of (29)Si NMR lines corresponding to T and Q(3) environments imply that there are approximately twice as many unreacted silanol groups on the silica surface as attached silane molecules.

Spectral deconvolution of NMR cross polarization data sets

The COmponent-REsolved (CORE) strategy has been employed, for the first time to solid state NMR spectroscopy. CORE was used to extract two time-dependent spectral components in 24 (29)Si{(1)H} NMR spectra, recorded on a meso-structured silica material under conditions of cross polarization evolution. No prior assumptions were made about the component bandshapes, which were both found to be skewed to higher chemical shifts. For the silica fragments close to protons this skewness could be rationalized by a distribution of the degree of condensation in the silica network; however, for the other component the non-Gaussian shape was unexpected. We expect that the same strategy could be applied to a range of experiments in solid-state NMR spectroscopy, where spectral distributions or kinetic parameters need to be accurately extracted.

Advanced solid state NMR techniques for the characterization of sol-gel-derived materials

A large array of advanced solid state NMR (nuclear magnetic resonance) techniques is presented in the frame of the structural characterization of sol-gel-derived materials. These techniques include the pertinent detection of (17)O chemical shifts, MAS (magic angle spinning) J spectroscopy in the solid state, high-resolution (1)H spectroscopy, heteronuclear and homonuclear D (dipolar)-derived multidimensional correlation experiments, and first-principles calculations of NMR parameters. This spectroscopic approach is suitable for the in-depth description of multicomponent sol-gel derivatives, crystalline and amorphous biocompatible silicophosphates, Al-O-P clusters, and templated porous materials. It offers unique perspectives for the description of the hybrid interfaces in terms of chemical and spatial connectivities.

Using liquid and solid state NMR and photoluminescence to study the synthesis and solubility properties of amine capped silicon nanoparticles

Water soluble silicon nanoparticles were prepared by the reaction of bromine terminated silicon nanoparticles with 3-(dimethylamino)propyl lithium and characterized with liquid and solid state nuclear magnetic resonance (NMR) and photoluminescence (PL) spectroscopies. The surface site dependent 29Si chemical shifts and the nuclear spin relaxation rates from an assortment of 1H-29Si heteronuclear solid state NMR experiments for the amine coated reaction product are consistent with both the 1H and 13C liquid state NMR results and routine transmission electron microscopy, ultra-violet/visible, and Fourier transform infrared measurements. PL was used to demonstrate the pH dependent solubility properties of the amine passivated silicon nanoparticles.

Optimization, Standardization, and Testing of a New NMR Method for the Determination of Zeolite Host−Organic Guest Crystal Structures

An optimized and automated protocol for determining the location of guest sorbate molecules in highly siliceous zeolites from (29)Si INADEQUATE and (1)H/(29)Si cross polarization (CP) magic-angle spinning (MAS) NMR experiments is described. With the peaks in the (29)Si MAS NMR spectrum assigned to the unique Si sites in the zeolite framework by a 2D (29)Si INADEQUATE experiment, the location of the sorbate molecule is found by systematically searching for sorbate locations for which the measured rates of (1)H/(29)Si cross polarization of the different Si sites correlate linearly with (1)H/(29)Si second moments calculated from H-Si distances. Due to the (1)H/(29)Si cross polarization being in the "slow CP regime" for many zeolite-sorbate complexes, it is proposed that the CP rate constants are best measured by (1)H/(29)Si cross polarization drain experiments, if possible, to avoid complications that may arise from fast (1)H and (29)Si T(1)rho relaxations. An algorithm for determining the sorbate molecule location is described in detail. A number of ways to effectively summarize and display the large number of solutions which typically result from a prediction of the structure from the CP MAS NMR data are presented, including estimates of the errors involved in the structure determinations. As a working example throughout this paper, the structure of the low loaded p-dichlorobenzene/ZSM-5 complex is determined under different conditions from solid-state (1)H/(29)Si CP MAS NMR data, and the solutions are shown to be in excellent agreement with the known single-crystal X-ray diffraction structure. This structure determination approach is shown to be quite insensitive to the use of relative rate constants rather than absolute values, to the detailed structure of the zeolite framework, and relatively insensitive to temperature and motions.

(1)H/(29)Si cross-polarization NMR experiments of silica-reinforced polydimethylsiloxane elastomers: probing the polymer-filler interface

Polydimethylsiloxane (PDMS) elastomers reinforced with fumed silica exhibit unusual strength characteristics that are necessary for their designed applications. The microscopic details of the surface interaction between the polymer and silica are not well characterized. (1)H/(29)Si cross-polarization (cp) experiments are used to characterize cured and uncured samples of Dow Corning silastic 745. Changes to the cp dynamics upon curing are evident by the variation in peak intensities in the variable contact-time spectra of the two samples. Estimates of the cp relaxation parameters are reported for the cured sample. Additional information can be obtained by expanding the (1)H/(29)Si cp to a two-dimensional heteronuclear correlation experiment. Dramatic differences between the cured and uncured (1)H/(29)Si HetCor spectra are observed that are not visible in the 1D spectra. These changes can be rationalized as a dehydration of the silica surface and an increased hardening of the polymer after the curing process. Furthermore, isolation of the NMR signal corresponding to nuclei at or near the polymer-filler interface may be achieved in the 2D (1)H/(29)Si HetCor spectrum.

Determination of the location of naphthalene in the zeolite ZSM-5 host framework by solid-state 1H/29Si CP MAS NMR spectroscopy

The location of naphthalene in the zeolite ZSM-5 has been determined from solid-state 1H/29Si cross-polarization (CP) magic-angle-spinning (MAS) NMR data alone. With the peaks in the 29Si spectrum assigned to the inequivalent Si sites in the zeolite from a two-dimensional INADEQUATE spectrum, the rates of cross polarization between the 1H nuclei of the guest sorbate molecules and the 29Si nuclei of the zeolite framework were used to determine the location of the naphthalene molecules by exploiting the proportional relationship between cross-polarization rate constants and 1H/29Si dipolar coupling second moments. The NMR structure determination was carried out on three different selectively deuterium-labeled naphthalene molecules (naphthalene-d0, α-naphthalene-d4, and β-naphthalene-d4). The average of the molecule locations in agreement with all three sets of NMR data was found to be in excellent agreement with an existing single crystal XRD structure of the naphthalene–ZSM-5 complex.Key words: solid-state NMR, cross polarization, zeolites, host–guest complexes, structure determination.

Measurement of NMR Cross-Polarization (CP) Rate Constants in the Slow CP Regime: Relevance to Structure Determinations of Zeolite−Sorbate and Other Complexes by CP Magic-Angle Spinning NMR

When analyzing I --> S variable contact time cross-polarization (CP) curves, the spin dynamics are usually assumed to be describable in the "fast CP regime" in which the growth of the S spin magnetization is governed by the rate of cross polarization while its decay is governed by the rate of I spin T1rho relaxation. However, in the investigation of the structures of zeolite-sorbate and other complexes by polarization transfer this will not necessarily be the case. We discuss the measurement of I --> S CP rate constants under the "slow CP regime" in which the rate of T1rho relaxation is fast compared to the rate of cross polarization, leading to a reversal of the usual assumptions such that the rate or growth is governed by the rate of I spin T1rho relaxation while the decay is governed by the rate of cross polarization (and the S spin T1rho relaxation). It is very important to recognize when a system is in the slow CP regime, as an analysis assuming the normal fast CP will lead to erroneous data. However, even when the slow CP regime is recognized, it is difficult to obtain absolute values for the CP rate constants from fits to standard CP curves, since the CP rate constant is correlated to the scaling factor, the contribution from 29Si T1rho relaxation is ignored, and it is difficult to obtain reliable data at very long contact times. The use of a 29Si{1H} CP "drain" or "depolarization" experiment, which measures absolute values of the CP rate constants, is therefore proposed as being most appropriate for theses situations. To illustrate the importance of these observations, measurements of the 1H-29Si CP rate constants in the p-dichlorobenzene/ZSM-5 sorbate-zeolite complex by 29Si{1H} CP and CP drain magic-angle spinning (MAS) NMR experiments are presented and compared and used to determine the location of the guest sorbate molecules in the cavities of the host zeolite framework.

Determining the geometry of strongly hydrogen-bonded silanols in a layered hydrous silicate by solid-state nuclear magnetic resonance

High-resolution solid-state NMR spectroscopy is exploited to obtain structural constraints involving strongly hydrogen-bonded silanols in octosilicate, a prominent member of the layered hydrous sodium silicates. Proton-silicon cross-polarization dynamics reveals that octosilicate contains two types of Q(3) silicons present in hydrogen-bonded -Si-O-Hcdots, three dots, centeredO-Si- and -Si-O(-)-type sites which can only be distinguished by their different abilities to cross polarize and the different mobilities of neighboring hydrous species. The theoretical analysis of the oscillating components of the polarization transfer buildup curves suggests that the model of heteronuclear pairs is an adequate description of the quantum spin system within hydrogen-bonded -Si-O-Hcdots, three dots, centeredO-Si- fragments. We also show that dipolar modulated, slow speed magic-angle (29)Si NMR spectrum provides unique geometric information on strongly hydrogen-bonded silanols. The dipolar modulated spinning sidebands contain all the information necessary to determine the internuclear Sicdots, three dots, centeredH distances as well as the magnitude and orientation of the principal elements of the (29)Si chemical shielding tensor in the molecular frame. The data provide definite proof of the intralayer character of strongly hydrogen-bonded silanol groups in a bridging, albeit not symmetric, position between neighboring tetrahedra. The approach developed in this work may be useful to obtain structural information on related layered alkali metal silicates, silica gels as well as on other classes of microporous materials.

Impact of remote protonation on 13C CPMAS NMR quantitation of charred and uncharred wood

The impact of inefficient cross polarization (long TCH values), caused by long 13C-1H internuclear distances, on 13C CPMAS NMR spectra of charred and uncharred woods is determined by simultaneously fitting data from complementary variable spin lock and variable contact time experiments. As expected, the impact is minimal for uncharred woods, but is very significant for the charred woods. Quantification of the decrease in CPMAS signal intensity caused by both inefficient cross polarization and rapid T1rhoH relaxation is achieved using an advanced spin counting methodology, for which the term "spin accounting" is proposed. 13C CPMAS NMR observabilities determined using the spin accounting methodology were close to 100% for the uncharred samples, and 69-82% for the charred samples. This represents a large improvement on the 30-40% observabilities determined using other spin counting techniques. Furthermore, it is shown that remote protonation and rapid T1rhoH relaxation are roughly equally responsible for the low signal intensity of standard (I ms contact time) 13C CPMAS spectra of charcoal.

Determination of T1rhoH relaxation rates in charred and uncharred wood and consequences for NMR quantitation

The performance of three different techniques for determining proton rotating frame relaxation rates (T1rhoH) in charred and uncharred woods is compared. The variable contact time (VCT) experiment is shown to over-estimate T1rhoH. particularly for the charred samples, due to the presence of slowly cross-polarizing 13C nuclei. The variable spin (VSL) or delayed contact experiment is shown to overcome these problems; however, care is needed in the analysis to ensure rapidly relaxing components are not overlooked. T1rhoH is shown to be non-uniform for both charred and uncharred wood samples; a rapidly relaxing component (T1rhoH = 0.46-1.07 ms) and a slowly relaxing component (T1rhoH = 3.58-7.49) is detected in each sample. T1rhoH for each component generally decreases with heating temperature (degree of charring) and the proportion of rapidly relaxing component increases. Direct T1rhoH determination (via 1H detection) shows that all samples contain an even faster relaxing component (0.09-0.24 ms) that is virtually undetectable by the indirect (VCT and VSL) techniques. A new method for correcting for T1rhoH signal losses in spin counting experiments is developed to deal with the rapidly relaxing component detected in the VSL experiment. Implementation of this correction increased the proportion of potential 13C CPMAS NMR signal that can be accounted for by up to 50% for the charred samples. An even greater proportion of potential signal can be accounted for if the very rapidly relaxing component detected in the direct T1rhoH determination is included; however, it must be kept in mind that this experiment also detects 1H pools which may not be involved in 1H-13C cross-polarization.

Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases

Self-assembled lamellar silica-surfactant mesophase composites have been prepared with crystal-like ordering in the silica frameworks using a variety of cationic surfactant species under hydrothermal conditions. These materials represent the first mesoscopically ordered composites that have been directly synthesized with structure-directing surfactants yielding highly ordered inorganic frameworks. One-dimensional solid-state 29Si NMR spectra, X-ray diffraction patterns, and infrared spectra show the progression of molecular organization in the self-assembled mesophases from structures with initially amorphous silica networks into sheets with very high degrees of molecular order. The silicate sheets appear to be two-dimensional crystals, whose structures and rates of formation depend strongly on the charge density of the cationic surfactant headgroups. Two-dimensional solid-state heteronuclear and homonuclear NMR measurements show the molecular proximities of the silica framework sites to the structure-directing surfactant molecules and establish local Si-O-Si bonding connectivities in these materials.