Effective dipolar couplings determined by dipolar dephasing of double-quantum coherences

Günne, Jörn Schmedt auf der

doi:10.1016/j.jmr.2006.02.009

Cited by 23 publications

(3 citation statements)

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“…The net P–P interatomic contacts in a glass may be assessed by 31 P 2QF experiments utilizing dipolar recoupling under MAS conditions ,,− ,− Here, 2QC are excited in a series of experiments with incremented excitation intervals (τ exc ), as shown in Figure a. The 2QC buildup rate from a pair j – k of 31 P– 31 P nuclei in the structure increases concomitantly with the size of its through-space-mediated homonuclear dipolar coupling constant ( b jk ; in units of rad s –1 ), which exhibits an inverse cubic relationship to the 31 P– 31 P internuclear distance r jk , where γ denotes the magnetogyric ratio of 31 P.…”

Section: Resultsmentioning

confidence: 99%

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

2014

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Melt-derived bioactive phosphosilicate glasses are widely utilized as bone-grafting materials for various surgical applications. However, the insight into their structural features over a medium-range scale up to ∼ 1 nm remains limited. We present a comprehensive assessment of the spatial distribution of phosphate groups across the structures of 11 Na2O-CaO-SiO2-P2O5 glasses that encompass both bioactive and nonbioactive compositions, with the P contents and silicate network connectivities varied independently. Both parameters are known to strongly influence the bioactivity of the glass in vitro. The phosphate distribution was investigated by double-quantum (31)P nuclear magnetic resonance (NMR) experiments under magic-angle spinning (MAS) conditions and by molecular dynamics (MD) simulations. The details of the phosphate-ion dispersion were probed by evaluating the MD-derived glass models against various scenarios of randomly distributed, as well as clustered, phosphate groups. From comparisons of the P-P interatomic-distance spreads and the statistics of small phosphate clusters assessed for variable cutoff radii, we conclude that the spatial arrangement of the P atoms in phosphosilicate glasses is well-approximated by a statistical distribution, particularly across a short-range scale of ≤ 450 pm. The primary distinction is reflected in slightly closer P-P interatomic contacts in the MD-derived structures over the distance span of 450-600 pm relative to that of randomly distributed phosphate groups. The nature of the phosphate-ion dispersion remains independent of the silicate network polymerization and nearly independent of the P content of the glass throughout our explored parameter space of 1-6 mol % P2O5 and silicate network connectivities up to 2.9.

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Section: Resultsmentioning

confidence: 99%

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

2014

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“…2D correlation spectra used rotor synchronized sampling for the indirect dimension. Double-quantum dephasing experiments (DoDe) were conducted at 8 kHz sample-spinning frequency. An INADEQUATE sequence prepared double-quantum coherence in 2.8 ms, dephasing was achieved with POSTC7, and reconversion was realized again with an INADEQUATE sequence preparing double-quantum coherence in 2.8 ms.…”

Section: Methodsmentioning

confidence: 99%

Diphosphines with Strongly Polarized P−P Bonds: Hybrids between Covalent Molecules and Donor−Acceptor Adducts with Flexible Molecular Structures

et al. 2009

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A series of P-phospholyl-substituted N-heterocyclic phosphines was prepared and characterized by single-crystal X-ray diffraction and solution and solid-state (31)P NMR spectroscopy. The molecular structures are distinguished by the presence of P-P bonds of exceptionally variable lengths (2.35-2.70 A) that are all well beyond the standard distance of 2.21 A. The unique flexibility is best illustrated by a specimen 4f where minor conformational changes of remote substituents induce a deviation in P-P bond lengths of some 5 pm between crystallographically independent molecules in the same unit cell. Computational studies suggest to rationalize the bond elasticity as the consequence of a very flat potential energy basin that allows even weak forces to have large impact on bond lengths. Solid-state (31)P NMR studies show that the bond distance variation coincides with substantial changes in the magnitude and sign of (1)J(PP), which is explained in the context of a dominant Fermi contact contribution. A relation between increasing internuclear distance and decreasing magnitude of (1)J(PP) was experimentally proven by determination of effective dipolar coupling constants by the double-quantum dephasing experiment (DoDe) for the crystallographically independent conformers of 4f and further supported by comparison with calculated coupling tensors with inclusion of the anisotropic J-coupling. NMR studies revealed large discrepancies in the values of (1)J(PP) measured in solution and the solid state and a substantial temperature dependence of the former. Interpretation of this behavior was feasible by taking into account that the value of (1)J(PP) in solution is affected by both temperature-dependent equilibria between trans and gauche conformers and additional bond length relaxation that accompanies the dissolution process. Consideration of experimental observations and population analysis of computed electron densities suggested to classify the P-P bonds in the molecules under study as "dative" rather than "normal" covalent bonds and to address the compounds 4 as hybrids between covalent diphosphines and phosphenium-phospholide contact ion pairs.

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“…Examples of such direct dipolar recoupling experiments are based either on the stimulation of the zero-quantum coherences such as the radio frequency-driven recoupling (RFDR) technique, , or on the excitation of double quantum coherences either via the back-to-back (BaBa) , technique or by symmetry-based windowless pulse sequences such as the R14 2 6 (ref ) or the POSTC7 methods. In such DQ-based methods, measurements of either DQ-buildup rates or DQ-dephasing rates have been rather powerful approaches for quantifying direct dipolar interactions in two- or multispin systems.…”

Section: Introductionmentioning

confidence: 99%

Indirect “No-Bond” ³¹P···³¹P Spin–Spin Couplings in P,P-[3]Ferrocenophanes: Insights from Solid-State NMR Spectroscopy and DFT Calculations

et al. 2014

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No-bond (31)P-(31)P indirect dipolar couplings, which arise from the transmission of nuclear spin polarization through interaction of proximal nonbonded electron pairs have been investigated in the solid state for a series of closely related substituted P,P-[3]ferrocenophanes and model systems. Through variation and combination of ligands (phenyl, cyclohexyl, isopropyl) at the two phosphorus sites, the P···P distances in these compounds can be varied from 3.49 to 4.06 Å. Thus, the distance dependence of the indirect no-bond coupling constant J(nb) can be studied in a series of closely related compounds. One- and two-dimensional solid-state NMR experiments serve to establish the character of these couplings and to measure the isotropic coupling constants J(iso), which were found to range between 12 and 250 Hz. To develop an understanding of the magnitude of J(nb) in terms of molecular structure, their dependences on intramolecular internuclear distances and relative orbital orientations is discussed by DFT-calculations on suitable models. In agreement with the literature the dependence of J(nb) on the P···P distance is found to be exponential; however, the steepness of this curve is highly dependent on the internuclear equilibrium distance. For a quantitative description, the off-diagonal elements of the expectation value of the Kohn-Sham-Fock operator in the LMO basis for the LMOs of the two phosphorus lone-pairs is proposed. This parameter correlates linearly with the calculated J(nb) values and possesses the same distance-dependence. In addition, the simulations indicate a distinct dependence of J(nb) on the dihedral angle defined by the two C-P bonds providing ligation to the molecular backbone. For disordered materials or those featuring multiple sites, conformers, and/or polymorphism, a new double-quantum NMR method termed DQ-DRENAR can be used to conveniently measure internuclear (31)P-(31)P distances. If conducted on compounds with known P···P distances, such measurements can also serve to estimate the magnitude of the anisotropy ΔJ of these no-bond indirect spin-spin couplings. The DFT results suggest that in the present series of compounds the magnitude of ΔJ is strongly correlated with that of the isotropic component, as both parameters have analogous distance dependences. While our studies indicate a sizable J-anisotropy for the model compound 1,8-bis(diphenylphosphino)napthalene (ΔJ ~ -70 Hz), the corresponding values for the P,P-[3]ferrocenophanes are significantly smaller, affecting their DQ-DRENAR curves only in a minor way. Based on the above insights, the structural aspects of conformational disorder and polymorphism observed in some of the P,P-[3]ferrocenophanes are discussed.

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Effective dipolar couplings determined by dipolar dephasing of double-quantum coherences

Cited by 23 publications

References 55 publications

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

Diphosphines with Strongly Polarized P−P Bonds: Hybrids between Covalent Molecules and Donor−Acceptor Adducts with Flexible Molecular Structures

Indirect “No-Bond” ³¹P···³¹P Spin–Spin Couplings in P,P-[3]Ferrocenophanes: Insights from Solid-State NMR Spectroscopy and DFT Calculations

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Effective dipolar couplings determined by dipolar dephasing of double-quantum coherences

Cited by 23 publications

References 55 publications

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by 31P Solid-State NMR and Molecular Dynamics Simulations

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by 31P Solid-State NMR and Molecular Dynamics Simulations

Diphosphines with Strongly Polarized P−P Bonds: Hybrids between Covalent Molecules and Donor−Acceptor Adducts with Flexible Molecular Structures

Indirect “No-Bond” 31P···31P Spin–Spin Couplings in P,P-[3]Ferrocenophanes: Insights from Solid-State NMR Spectroscopy and DFT Calculations

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Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

Assessing the Phosphate Distribution in Bioactive Phosphosilicate Glasses by ³¹P Solid-State NMR and Molecular Dynamics Simulations

Indirect “No-Bond” ³¹P···³¹P Spin–Spin Couplings in P,P-[3]Ferrocenophanes: Insights from Solid-State NMR Spectroscopy and DFT Calculations