Spinning Sideband Analysis Research Articles

Phosphate uptake by TiO2: Batch studies and NMR spectroscopic evidence for multisite adsorption

Systematic studies, combining batch experiments with NMR spectroscopic methods, are carried out for phosphate sorption on titanium dioxide (TiO2). It is found that phosphate sorption on TiO2 decreases with increasing pH, whereas the phosphate uptake by TiO2 increases with increasing ionic strength of the solution. In I⩽0.1M, the sorption sharply increases and reaches a near maximum and then followed by little changes showing Langmuir-type behavior, whereas in I=0.7M, non-Langmuirian uptake becomes evident as equilibrium phosphate concentrations increase in solution. The sorption of phosphate on TiO2 is rapid and mostly irreversible at pH 4.5 and 7.0. At pH 9.0, however, the phosphate sorption is initially reversible and followed by resorption of phosphate on TiO2 at the system re-equilibration. 31P{1H} cross-polarization and magic angle spinning (CP/MAS) NMR spectra contain at least four main peaks which appear similar in position and width under all adsorption conditions, but vary in intensity with surface loading. The spectral characteristics of these peaks, including cross-polarization dynamics and chemical shift anisotropy obtained from spinning sideband analysis, suggest that they arise from distinct inner-sphere adsorption complexes, most of which are protonated. These results indicate that uptake of phosphate by TiO2 occurs by formation of several types of surface complexes.

ChemInform Abstract: Solid-State 199Hg MAS NMR and Vibrational Spectroscopic Studies of Dimercury(I) Compounds

The solid-state 199Hg MAS NMR spectra of Hg2X2 (X = Cl, SCN, NCO, CH3CO2, CF3CO2) have been measured, and the infrared and Raman spectra of these compounds have been recorded and analyzed to further characterize them and to assist in the interpretation of the NMR data. Spinning-sideband analysis has been used to determine the 199Hg shielding anisotropy and asymmetry parameters Δσ and η from the solid-state 199Hg MAS NMR spectra. In contrast to the case of the corresponding mercury(II) compounds, the shielding anisotropy is found to be relatively insensitive to the nature of the X group. This is consistent with the view that the electronic environment of the Hg atom in the mercury(I) compounds is dominated by the Hg−Hg bond. The changes in the 199Hg shielding parameters from the mercury(II) to the corresponding mercury(I) compounds, as well as the changes in these parameters in the mercury(I) compounds with changes in X, can be interpreted in terms of variations in the local paramagnetic contribution to th...

Interaction of Antimicrobial Oligomers with Lipids Studied by Solid-State NMR

A family of synthetic mimic of antimicrobial peptides (SMAMP), amphiphilic meta-phenylene ethynylene (mPE) molecules show a wide range of antimicrobial activity and specificity. The interaction of a specifically active mPE molecule (AMO-2) with mixed DOPE/DOPG lipid was studied by solid-state NMR. The AMO-2 molecules do not preferentially interact more strongly with either lipid component, but rather are well dispersed in the lipid matrix. AMO-2 intimately interacts with all parts of lipid molecules, including head groups. Magic-angle spinning sideband analysis indicated that in samples with co-existing lamellar and inversed hexagonal phases (HII), neither lipid component aggregate in either phase. The presence of AMO-2 molecules causes dynamic disorder in lipid head groups, as evidenced by the broadening of both static and MAS 31P spectra. AMO-2 molecules do not massively transform the lamellar lipid into HII phase.

Slow Motion of Confined Molecules: NMR and Broadband Dielectric Spectroscopy Investigations

Nuclear magnetic resonance (NMR) and broadband dielectric spectroscopy are used to investigate the dynamics of small glass-forming molecules confined to restricted geometries. Ethylene glycol molecules are embedded in the supercages of NaX zeolites. The combined application of NMR and broadband dielectric spectroscopy advances the understanding of the slowing down of the motion near the glass transition temperature of these confined molecules. In combination with nuclear spin relaxation and nuclear magnetic resonance spectroscopy, dielectric relaxation studies on glass forming molecules allow conclusions on the character of the motion. High resolution 1H magic angle spinning (MAS) NMR measurements not only enable a characterisation of the state of the adsorbed molecules via a chemical shift analysis. By means of an analysis of MAS spinning sidebands we may also estimate a correlation time the meaning of which will be discussed in comparison to the results of longitudinal proton spin relaxation measurements. In addition to broadband dielectric spectroscopy slow molecular motions of partially deuterated ethylene glycol adsorbed in NaX are studied by means of 2H NMR line-shape analysis.

Solid state 31P and 207Pb MAS NMR studies on polycrystalline O, O′-dialkyldithiophosphate lead(II) complexes

A number of lead(II) O, O′-dialkyldithiophosphate complexes were studied by 13C, 31P, and 207Pb MAS NMR. Simulations of 31P chemical shift anisotropy using spinning sideband analysis reveal a linear relationship between the S P S bond angle and the principal values δ 22 and δ 33 of the 31P chemical shift tensor. The 31P CSA data were used to assign ligands with different structural functions. In the cases of diethyldithiophosphate and di- iso-butyldithiophosphate lead(II) complexes, 2 J ( 31P, 207Pb)-couplings were resolved and used to confirm the suggested assignment of the ligands. The SIMPSON computer program was used to calculate 31P and 207Pb spectral sideband patterns.

A combined first principles computational and solid-state NMR study of a molecular crystal: flurbiprofen

The 1H, 13C and 19F magic-angle spinning NMR spectra have been recorded for Form 1 of flurbiprofen. In the case of 19F, spinning sideband analysis has produced data for the components of the shielding tensor. The chemical shift of the hydrogen-bonded proton was found to be 14.0 ppm. Shielding parameters for all three nuclei have been calculated using Density Functional Theory (DFT) together with the Gauge Including Projector Augmented Wave (GIPAW) method which takes full allowance for the repetition inherent in crystalline structures. Such computations were made for the reported geometry, for a structure with all the atomic positions relaxed using DFT, and with only the hydrogen positions relaxed. The relationships of the computed shifts to those observed are discussed. In general, the correlations are good.

Solid-state NMR studies of some tin(II) compounds

High-resolution NMR spectra of spin- 1 2 nuclei, particularly 119Sn and 31P, in solid tin(II) phosphite, SnHPO 3, and tin(II) phosphate, SnHPO 4, are presented. The results are discussed in relation to the crystal structures. Spinning sideband analysis has been carried out for both nuclei, giving information on the shielding tensors. Satellite peaks allow the indirect Sn,Sn coupling constants to be determined. Surprisingly large values of 2600±200 Hz and 4150±200 Hz are reported for SnHPO 3 and SnHPO 4 respectively. The satellite peaks were investigated by using a single Hahn echo for each refocusing time, which showed that the observed splittings result from 119Sn, 117Sn coupling. For SnHPO 3, the calculated relative intensities of the satellites for six intra-layer coupling interactions are in agreement with the experiment values, but for SnHPO 4 the coupling appears to be inter-layer in nature. Tin-119 (and in one case phosphorus-31) shielding tensor data derived from MAS NMR are also reported for four other crystalline tin(II) compounds, namely tin diphosphate, tin oxalate, tin sulphate and calcium tin ethylenediamine tetraacetate.

Magic-angle spinning NMR spectra and re-examined crystal structure of trimethyltin cyanide

The structure of crystalline trimethyltin cyanide has been studied by magic-angle spinning NMR, by DFT computations and by single-crystal X-ray diffraction. An earlier diffraction study reported some disorder in the SnCNSn chains. However, the 119Sn spectrum shows unequivocally that this is not so, since only a single centreband isotropic signal is observed. This finding is confirmed by DFT calculations, the results of which are consistent with an ordered CNSnCN environment for tin. The computed shielding anisotropy is in good agreement with the experimental value obtained from spinning sideband analysis. Carbon-13 CPMAS data are also reported. A new synthetic procedure provided high-quality crystals, enabling X-ray diffraction to be applied with modern equipment. The results confirmed the conclusion made from the NMR experiments and the DFT computations. The chain structure is ordered, though positions of some hydrogen atoms may be disordered. Revised geometrical data are reported and discussed.

REDOR-Based Heteronuclear Dipolar Correlation Experiments in Multi-Spin Systems: Rotor-Encoding, Directing, and Multiple Distance and Angle Determination

We review a variety of recently developed 1H–X heteronuclear recoupling techniques, which rely only on the homonuclear decoupling efficiency of very-fast magic-angle spinning. All these techniques, which are based on the simple rotational-echo, double-resonance (REDOR) approach for heteronuclear recoupling, are presented in a common context. Advantages and possibilities with respect to the complementary application of conventionally X and 1H-inversely detected variants are discussed in relation to the separability and analysis of multiple couplings. We present an improved and more sensitive approach to the determination of 1H–X dipolar couplings by spinning-sideband analysis, termed REREDOR, which is applicable to XH n groups in rigid and mobile systems and bears some similarity to more elaborate separated local-field methods. The estimation of medium-range 1H–X distances by analyzing signal intensities in two-dimensional REDOR correlation spectra in a model-free way is also discussed. More specifically, we demonstrate the possibility of combined distance and angle determination in H–X–H or X–H–X three-spin systems by asymmetric recoupling schemes and spinning-sideband analysis. Finally, an 1H–X correlation experiment is introduced which accomplishes high sensitivity by inverse 1H) detection and is therefore applicable to samples with 15N in natural abundance.

The Application of MAS Recoupling Methods in the Intermediate Motional Regime

We present investigations concerning the effect of molecular motions on the experimental timescale upon the recoupling of anisotropic interactions under magic-angle spinning conditions. An approach for the efficient simulation of spin dynamics occurring during complex pulse sequences, based on a linearization of the general solution of the stochastic Liouville–von Neumann equation, was developed. Using 13C CSA recoupling of the methyl carbon in dimethylsulfon as a sample interaction, we observed a characteristic signal decay under recoupling upon entering the intermediate motional regime, which can be well described by an apparent transverse relaxation time, T 2 rcpl . This quantity does not depend on the spinning frequency to a first approximation. Specific recoupling experiments, namely the measurement of tensor parameters by spinning sideband analysis, and the determination of rate constants with the CODEX experiment, are discussed with respect to possibilities and limits of their application in the intermediate motional regime. Important conclusions are drawn with regards to the limited applicability of popular recoupling methods like REDOR to samples exhibiting intermediate mobility.

Silicate-phosphate interactions in silicate glasses and melts: I. A multinuclear (27Al,29Si,31P) MAS NMR and ab initio chemical shielding (31P) study of phosphorous speciation in silicate glasses

An investigation of the structure of sodium aluminum silicate glasses [composition (x)Na2O, (1-x)Al2O3, 0.9SiO2 perturbed through the addition of 2 mol.% P2O5] was performed using a combination of 27Al, 29Si, and 31P solid state NMR spectroscopy and ab initio chemical shielding calculations (GIAO method). Speciation within these glasses was estimated using a physically consistent method for the deconvolution of inhomogeneously broadened 31P MAS-NMR spectra by employing a combination of spinning sideband analysis with the results of ab initio shielding calculations. A minimum of 13 different phosphorus-bearing species were estimated to contribute to the spectral complexity in these glasses over the compositional range investigated. The majority of these species are oxygen-bridged to the silicate lattice. These are described as (Al,Si)Qpn species, where n corresponds to the number of bridging oxygens (n = 1–4) and where there is variation in the Al/Si ratio of the tetrahedral nearest neighbors. In addition to the species bridging to the silicate lattice, nonlattice bridged Na-P type species, including ortho-, pyro-, and tripolyphosphates, are observed or deduced based on spectral behavior. A sharp crossover in the predominance of Na-P species relative to (Al,Si) Qpn dominant species occurs at low Al2O3 content. Within the group of nonbridging (Na-P) species, increased bulk aluminum content shifts the predominance from orthophosphate toward tripolyphosphate. Within the group of bridging [(Al,Si) Qpn] species, increasing bulk aluminum content shifts the predominance toward a greater number of oxygen bridges to the silicate lattice. To highlight the perturbative effect of 2 mol.% P2O5 addition on glass structure, 29Si MAS-NMR of P-bearing and P-absent glasses were compared. The speciation estimated from the combined 31P MAS-NMR and ab initio shielding calculations is consistent with 29Si MAS-NMR spectroscopic data. Analysis of 27Al MAS-NMR spectra for P-bearing and P-absent glasses reveals extensive Al-O-P interactions evident by systematic shifts in peak maxima to lower frequencies (greater shielding). Increases in the central transition peak half widths in the P-bearing samples reflect enhanced second-order quadrupolar interactions, also consistent with an abundance of (Al,Si) Qpn species. Analysis of the 27Al inner satellite transition sidebands reveals only a minor amount of VIAl. It is concluded that the addition of even a small amount of P2O5 (2 mol.%) results in a large perturbation of the structure of silicate glasses and should manifest significant effects on the physical properties of melts derived from them.

Solid-state NMR spectroscopy of paramagnetic metallocenes.

The paramagnetic metallocenes and decamethylmetallocenes (C(5)H(5))(2)M and (C(5)Me(5))(2)M with M=V (S=3/2), Mn (S=5/2 or 1/2), Co (S=1/2), and Ni (S=1) were studied by (1)H and (13)C solid-state MAS NMR spectroscopy. Near room temperature spinning sideband manifolds cover ranges of up to 1100 and 3500 ppm, and isotropic signal shifts appear between -260 and 300 ppm and between -600 and 1640 ppm for (1)H and (13)C NMR spectra, respectively. The isotropic paramagnetic signal shifts, which are related to the spin densities in the s orbital of ligand atoms, were discussed. A Herzfeld--Berger spinning sideband analysis of the ring carbon signals yielded the principal values of the paramagnetic shift tensors, and for metallocenes with a small g-factor anisotropy the electron spin density in the ligand pi system was determined from the chemical shift anisotropy. The unusual features of the (1)H and (13)C solid-state NMR spectra of manganocene were related to its chain structure while temperature-dependent (1)H MAS NMR studies reflected antiferromagnetic interaction between the spin centers.

Recoupled polarization-transfer methods for solid-state (1)H--(13)C heteronuclear correlation in the limit of fast MAS.

An in-depth account of the effects of homonuclear couplings and multiple heteronuclear couplings is given for a recently published technique for (1)H--(13)C dipolar correlation in solids under very fast MAS, where the heteronuclear dipolar coupling is recoupled by means of REDOR pi-pulse trains. The method bears similarities to well-known solution-state NMR techniques, which form the framework of a heteronuclear multiple-quantum experiment. The so-called recoupled polarization-transfer (REPT) technique is versatile in that rotor-synchronized (1)H--(13)C shift correlation spectra can be recorded. In addition, weak heteronuclear dipolar coupling constants can be extracted by means of spinning sideband analysis in the indirect dimension of the experiment. These sidebands are generated by rotor encoding of the reconversion Hamiltonian. We present generalized variants of the initially described heteronuclear multiple-quantum correlation (HMQC) experiment, which are better suited for certain applications. Using these techniques, measurements on model compounds with (13)C in natural abundance, as well as simulations, confirm the very weak effect of (1)H--(1)H homonuclear couplings on the spectra recorded with spinning frequencies of 25--30 kHz. The effect of remote heteronuclear couplings on the spinning-sideband patterns of CH(n) groups is discussed, and (13)C spectral editing of rigid organic solids is shown to be practicable with these techniques.

Cohydrolysis of Organotin Chlorides with Trimethylchlorosilane. Okawara's Pioneering Work Revisited and Extended

The synthesis of the stannasiloxanes {[R2(Me3SiO)Sn]2O}2 (1, R = Me; 2, R = Et), [R2(Me3SiO)Sn]2O (8, R = Me3SiCH2; 9, R = t-Bu), R2Sn(OSiMe3)2 (3, R = i-Pr; 4, R = Me3SiCH2; 5, R = t-Bu; 6, R = Cp(CO)3W; 7, R = Cp(CO)2Fe), and [Cp(CO)2Fe]2Sn(OSiPh3)2 (7a), the monoorganotin trichloride Me3SiCH2SnCl3 (13), and the organotin oxocluster (Me3SiCH2Sn)12O14(OH)6Cl2 (14) is reported. Their identity was confirmed by both solution and solid state multinuclear NMR spectroscopy and in the case of 1, 2, 6, 7a, and 14 also by single-crystal X-ray analyses. A spinning sideband analysis of the 119Sn MAS spectra reveals the coordination geometries of the tin atoms in the stannasiloxanes 1 and 2 to be different from those of related diorganotin oxides (R2SnO)n (R = Me, Et). In solution, 1 and 2 exhibit an intramolecular exchange process as well as monomer−dimer equilibria. The reaction of 4 with cyclo-(t-Bu2SnO)3 and of cyclo-[(Me3SiCH2)2SnO]3 with cyclo-(t-Bu2SnO)3 provided evidence for the formation in situ of the mixe...

X-ray diffraction and phosphorus-31 NMR studies of the dynamically disordered 3:2 phenol–triphenylphosphine oxide complex

The 3:2 phenol–triphenylphosphine oxide (phenol–TPPO) adduct was studied by means of X-ray diffraction together with high-resolution solid-state 31P NMR spectroscopy. The X-ray results showed that the crystalline structure, which belongs to the triclinic P[1 with combining macron] space group, involves disorder. There are two molecules of TPPO and three phenol molecules per unit cell. One of the latter is disordered across an apparent inversion centre. Analysis of NMR data in conjunction with the crystal structure allowed the origin of such disorder to be established, showing that it is dynamic in nature. Variable-temperature NMR experiments were performed and a coalescence temperature was found at 247 K. Spectra recorded below this temperature showed two phosphorus signals. The kinetics for the phenol residue exchanging between two different TPPO moieties (two-site exchange with equal populations) were determined. Thermodynamic parameters for the motion were calculated from Eyring plots. For temperatures ranging from 262.9 to 221.5 K, the bandshape analysis technique was used to derive the required data. For lower temperatures, the selective polarisation inversion experiment (SPI) was performed, whilst high temperature values were derived from variable-temperature T1ρ studies. The activation enthalpy (ΔH‡), calculated using the results obtained by bandshape analysis, T1ρ and SPI, was determined as 38 kJ mol−1, while the activation entropy (ΔS‡) was found to be −23 J mol−1 K−1 (assuming the transmission coefficient is ½). Phosphorus-31 shielding tensor anisotropies have been derived for this system by spinning sideband analysis at both fast and slow-exchange limits and it has been shown that the tensor is axially symmetric. Single-crystal experiments show that the symmetry axis of the tensor is along the P[double bond, length half m-dash]O bond (within experimental error).

Solid-State (199)Hg MAS NMR and Vibrational Spectroscopic Studies of Dimercury(I) Compounds.

The solid-state (199)Hg MAS NMR spectra of Hg(2)X(2) (X = Cl, SCN, NCO, CH(3)CO(2), CF(3)CO(2)) have been measured, and the infrared and Raman spectra of these compounds have been recorded and analyzed to further characterize them and to assist in the interpretation of the NMR data. Spinning-sideband analysis has been used to determine the (199)Hg shielding anisotropy and asymmetry parameters Deltasigma and eta from the solid-state (199)Hg MAS NMR spectra. In contrast to the case of the corresponding mercury(II) compounds, the shielding anisotropy is found to be relatively insensitive to the nature of the X group. This is consistent with the view that the electronic environment of the Hg atom in the mercury(I) compounds is dominated by the Hg-Hg bond. The changes in the (199)Hg shielding parameters from the mercury(II) to the corresponding mercury(I) compounds, as well as the changes in these parameters in the mercury(I) compounds with changes in X, can be interpreted in terms of variations in the local paramagnetic contribution to the shielding tensor.

Errors in chemical shift tensor components and orientation in the molecular frame as obtained from MAS NMR spinning sideband analysis

It is shown how to calculate random errors in chemical shift tensor components and in the Euler angles which fix the orientation of the σ tensor in the molecular frame, as obtained from spinning sideband analysis of MAS NMR spectra of powdered solids, when heteronuclear dipolar coupling interactions occur in a two spin system. The procedure was applied to experimental data corresponding to the chemical shift tensor of a carbon-13 bonded to a phosphorus-31 nucleus. Clues are given concerning the experimental variables to be set in order to obtain the desired accuracy in the orientation angles.

Solid-State199Hg MAS NMR Studies of Mercury(II) Thiocyanate Complexes and Related Compounds. Crystal Structure of Hg(SeCN)2

The solid-state 199Hg MAS NMR spectra of [Hg(SCN)2], [Hg(SeCN)2], M[Hg(SCN)3], M2[Hg(SCN)4] (M = K, Cs), and K2[Hg3(NCO)8] have been measured, and the (monoclinic) crystal structure of [Hg(SeCN)2] has been determined to assist in the interpretation of the NMR data. The asymmetric unit of Hg(SeCN)2 contains one molecule at an inversion center which shows linear Se−Hg−Se bonding (Hg−Se = 2.4738(10) A, Se−Hg−Se = 180°, Hg−Se−C = 97.5(3)°). Secondary Hg---Se and Hg---N contacts are 3.4246(9) and 2.835(12) A, respectively; this arrangement differs from that in Hg(SCN)2 where there are no secondary Hg---S contacts. A redetermination of the monoclinic crystal structure of K[Hg(SCN)3] is also reported, revealing substantial differences from the earlier work. Spinning sideband analysis has been used to determine the 199Hg shielding anisotropy and asymmetry parameters Δσ and η from the solid-state 199Hg MAS NMR spectra. The effects of changes in the mercury coordination number and of distortion of the coordination ...

Vibrational spectroscopy and solid-state MAS NMR studies of mercury(II) acetate complexes [Hg(X)OAc]: Crystal structure of [Hg(CN)OAc

The syntheses of [Hg(X)OAc] (OAc=acetate; X=CN, Cl, Br, I, SCN) are reported, and the crystal structure of the cyano complex has been determined. The asymmetric unit contains two [Hg(CN)OAc] molecules which show almost linear C–Hg–O bonding (Hg–C=2.019(13), 2.016(11) Å; Hg–O=2.067(9), 2.058(8) Å; C–Hg–O=176.0(4), 172.3(5)°), with only one of the two acetate oxygen atoms bound directly to the mercury atom. Secondary Hg⋯O and Hg⋯N contacts in the range 2.6–2.8 Å are about 0.2 Å shorter than the secondary Hg⋯O contacts in the corresponding X=Ph complex. The ν(HgX) and ν(HgO) modes have been assigned in the IR and Raman spectra of [Hg(X)OAc] (X=CN, Cl, Br, I, SCN); these spectra show that the complexes have structures with essentially linear O–Hg–X bonding, similar to that of the cyanide. Solid-state 199Hg MAS NMR spectra have been recorded for HgX 2 (X=CN, Cl) and [Hg(X)OAc] (X=Me, Ph, CN, Cl, SCN), and spinning sideband analysis has been used to determine the 199Hg shielding anisotropy and asymmetry parameters Δ σ and η. A semi-empirical method for the calculation of the local paramagnetic contribution to the shielding is given, and a linear relationship between Δ σ and the isotropic shielding σ iso which is predicted by this model for linear HgXY species is found to be obeyed reasonably well by the experimental data for HgX 2 and [Hg(X)OAc]. The same method is used to analyse the effects of secondary bonding on the 199Hg shielding parameters. The 13C MAS NMR spectrum of [Hg(SCN)OAc] shows 2J( 199 Hg 13 C) and 3J( 199 Hg 13 C) coupling to the acetate carbon atoms, with magnitudes similar to those found previously for Hg(OAc) 2. The CN carbon signals in Hg(CN) 2 and [Hg(CN)OAc] are split into 2:1 doublets due to residual dipolar coupling to the quadrupolar 14N nucleus.

Dipolar couplings and internuclear distances by double-quantum nuclear magnetic resonance spectroscopy of solids

The analysis of high-resolution double-quantum nuclear magnetic resonance spinning sidebands for measuring dipolar couplings and internuclear distances in dipolar solids is described. For this purpose, the response of a dipolar-coupled spin system in a rigid solid is investigated with respect to high-resolution multiple-quantum experiments using rotor-frequency synchronized pulse sequences. For isolated, magnetically equivalent spin-1/2 pairs, exact expressions for the double-quantum spinning-sideband patterns are derived. These patterns show spinning sidebands only at odd numbers of the rotor frequency. For longer excitation/reconversion cycles, the double-quantum spinning sidebands are sensitive to changes in the internuclear distances. Using this technique, the dipolar couplings for C13 spin pairs in double-labeled polyethylene were measured in crystalline and amorphous domains, respectively. In the former the dipolar coupling reflects the carbon–carbon distance, in the latter it is reduced due to molecular dynamics. The possibility to use multiple-quantum pulse sequences as a dipolar filter for the rigid domains is also shown.