Nutation NMR Spectroscopy Research Articles

Separation of Anti-Phase Signals Due to Parahydrogen Induced Polarization via 2D Nutation NMR Spectroscopy.

The present work introduces a novel method for the selective detection of 1 H NMR anti-phase signals caused by the pairwise incorporation of parahydrogen into olefins on noble-metal-containing catalysts. Via a two-dimensional (2D) nutation NMR experiment, the anti-phase signals of hyperpolarized 1 H nuclei are separated due to their double nutation frequency compared to that of thermally polarized 1 H nuclei. For demonstrating this approach, parahydrogen induced polarization (PHIP) was achieved via the hydrogenation of propene with parahydrogen on platinum-containing silica and investigated by in situ 1 H MAS NMR spectroscopy under continuous-flow conditions, that is, the hydrogenation reaction was performed inside the magnet of the NMR spectrometer. The 2D nutation NMR experiment described in the present work is useful for the separation of overlapping anti-phase and in-phase signals due to hyperpolarized and thermally polarized 1 H nuclei, respectively, which is important for research in the field of heterogeneous catalysis.

Off-resonance nutation nuclear magnetic resonance study of framework aluminosilicate glasses with Li, Na, K, Rb or Cs as charge-balancing cation

Framework aluminosilicate glasses with varying charge-balancing cation (Li, Na, K, Rb and Cs) have been studied with 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and 27Al on-resonance and off-resonance nutation NMR spectroscopy. This first application of off-resonance nutation NMR to disordered samples proves that it is a promising technique for the determination of mean quadrupole interactions in amorphous systems. Linewidths for Al decrease systematically with increasing size of the cation, due to a decrease in the quadrupole interaction from 5.0 MHz for the Li glass to 2.8 MHz for the Cs glass. A simple point-charge model effectively predicts the decrease in the quadrupole interaction. This indicates that the alkali ion is located close to aluminum. Looking at the residual linewidth after subtraction of the quadrupole broadening, the Al chemical shift distribution does not change significantly with the type of alkali ion. The same is true for the observed Si linewidth.

Off-Resonance Nutation NMR Spectroscopy of Half-Integer Quadrupolar Nuclei

The possibilities of off-resonance irradiation in nutation experiments of half-integer quadrupolar nuclei are demonstrated experimentally and theoretically. Off-resonance irradiation extends the applicability of nutation NMR spectroscopy at a given radiofrequency field strength to a larger range of quadrupole coupling constants. Moreover, it is possible to obtain several spectra with different resonance offsets on one spectrometer, thus facilitating the determination of the quadrupole parameters. A loss of signal to the uninformative zero-frequency signal which accompanies off-resonance irradiation can be avoided by using a frequency-stepped adiabatic half-passage or a soft selective 90° pulse as preparation for the experiment. Simulated off-resonance nutation spectra are presented as a function of quadrupole coupling constant and resonance offsets. The technique is demonstrated experimentally for a number of sodium compounds.

23Na and 13C solid state NMR spectroscopy of N,N,N′,N′-tetramethylethylenediamine-sodiumcyclopentadienide

The static 23Na NMR spectrum of N, N, N′, N′-tetramethylethylenediamine-sodiumcyclopentadienide revealed a lineshape that could not be unambiguously discussed. Nutation NMR spectroscopy combined with MAS allowed a definite assignment of the splitted signal to a second order quadrupolar interaction with vanishing asymmetry parameter. The quadrupole coupling constant is in the range of 2 MHz. The 13C CP-MAS spectrum yielded the expected signals.

The interaction of vanadia with sepiolite. Structural studies by 51V solid-state NMR and Raman spectroscopy

The interaction of vanadium with sepiolite, a layered magnesium silicate used to stabilize cracking catalysts against metal contaminants in crude oils, is investigated. To this end, field-dependent 51IV wideline, magic-angle-spinning (MAS), and nutation NMR studies, together with X-ray diffraction (XRD) and laser Raman spectroscopy (LRS) results, are reported on sepiolite samples impregnated with a solution of vanadyl napthenate in benzene. These results are discussed in connection with benchmark NMR data of crystallographically well-defined model compounds in the M9O-V 2O 5 system. When heated near 760°C in the presence of steam, sepiolite decomposes into enstatite (MgSiO 3) and silica. If the hydrothermal treatment is repeated in the presence of vanadium, a disordered microcrystalline phase of α-Mg 2V 2O 7 forms. Steam treatment at higher temperatures results in the formation of β-Mg 2V 2O 7. In addition, all samples examined contain an amorphous surface phase. Based on 51V nutation NMR spectroscopy, this amorphous constituent is inferred to have a structural environment similar to β-Mg 2V 2O 7. An MgV 2O 6-like, distorted octahedral environment is present only in samples with very high surface coverages and can be suppressed by choosing multiple-step over single-step impregnation.

Multiple-quantum MAS nutation NMR spectroscopy of quadrupolar nuclei

Theory, experiments, and simulations of two-dimensional nutation NMR spectroscopy of half-integer quadrupolar nuclei combined with magic-angle spinning (MAS) are presented. MAS enhances the overall sensitivity and improves resolution in the ω 2 2π dimension of the 2D nutation spectra. Compared to static nutation spectroscopy it is demonstrated that MAS nutation NMR puts less stringent requirements on the magnitude of the radiofrequency field strength, which is beneficial for nuclei with large quadrupole couplings. The finite nutation pulse excites all 2I(I + 1 2 ) multiple-quantum transitions for a spin- I nucleus. Multiple-quantum versions of the 2D nutation experiment are explored. Simulated 2D MAS nutation spectra are compared with experimental 23Na and 27Al spectra of NaNO 3 and Ca 3A1 2O 6·6H 2O (concrete), respectively, and with simulations of corresponding 2D nutation experiments for the static case.

Vanadium(V) environments in bismuth vanadates: A structural investigation using Raman spectroscopy and solid state 51V NMR

The Bi 2O 3V 2O 5 system was examined using Raman spectroscopy and solid state 51V wideline, magic-angle spinning (MAS), and nutation NMR spectroscopy. The methods are shown to be complementary in the identification of the various phases and in the characterization of their vanadium site symmetries. Most of the compositions examined (1:1 ≤ Bi:V ≤ 60:1) are multiphasic. Depending on the Bi:V ratio, the following phases have been identified: BiVO 4, Bi 4V 2O 11, a triclinic type-II phase, a cubic type-I phase, γ-Bi 2O 3 doped with V(V) (sillenite), and β-Bi 2O 3. Detailed spectroscopic characterization reveals that vanadium is tetrahedrally coordinated in all these compounds, and that the degree of symmetry increases with increasing Bi:V ratio. At the highest Bi:V ratios, the combined interpretation of the Raman and NMR data provides strong evidence for the presence of Bi 5+O 4 tetrahedra.

The determination of bond lengths in solids using the Carr-Purcell sequence

Nutation NMR spectroscopy has been used to measure bond lengths in orientationally disordered solids by suppression of the chemical-shift anisotropy broadening (I ). In this technique, removal of this unwanted broadening is accompanied by a reduction of the splitting in the Pake doublet (2) by a factor of two. In many cases of special interest, however, the isolated pairs of spins involved in the Pake doublet may belong to more than one class. For example, two different bond lengths (3) may be present leading to a superposition of two Pake doublet patterns. A similar situation may occur even if all pairs are characterized by a single bond length but are associated, for example, with two classes of molecules in quite different motional states. If one of these situations prevails, it is highly desirable to take advantage of the best possible resolution to permit correct assignments in increasingly complex spectra. The Carr-Purcell sequence (4) is another method for suppressing the chemical-shift anisotropy broadening; in principle, it should not scale down the dipolar interaction. Although a reduction of the homonuclear dipolar splitting does take place in practice, as shown below, it is not as severe as in the nutation experiment. For the determination of internuclear distances using the Carr-Purcell sequence it is essential to know the scaling behavior of the Pake doublet splitting as the pulse separation and pulse width are varied. We show that under ideal conditions this scaling should be a linear function of the duty factor. Our measurements in two different systems employing a modified Carr-Purcell sequence confirm this prediction over a wide range of duty factors. 13C pulsed NMR experiments were performed at a magnetic field strength of 1.4 1 T ( 15 MHz). Although the suppression of dipolar broadening by protons is partly

The geometry of reactive intermediates by nutation NMR spectroscopy: the tert-butyl cation

The geometry of reactive intermediates by nutation NMR spectroscopy: the tert-butyl cation

ChemInform Abstract: Determination of the Central Bond Length in Hexaarylethanes by Nutation NMR Spectroscopy

ChemInform Abstract: Determination of the Central Bond Length in Hexaarylethanes by Nutation NMR Spectroscopy

Determination of the central bond length in hexaarylethanes by nutation NMR spectroscopy

Determination of the central bond length in hexaarylethanes by nutation NMR spectroscopy

Mechanism of Ziegler-Natta polymerization of acetylene: a nutation NMR study

Abstract : Using nutation NMR spectroscopy the distance between adjacent 13C labels was measured in samples of polyacetylene prepared by polymerizing a dilute solution of double 13C-labelled acetylene in 12C-acetylene using titanium tetra-n-butoxide plus triethylaluminum as a catalyst. The experiments accord with expectation if the polymerization proceeds by a four-center insertion mechanism rather than by a metallacycle mechanism involving metal-carbenes. (Author)

Molecular Geometry ofcis- andtrans-Polyacetylene by Nutation NMR Spectroscopy

The carbon-carbon bond lengths in polyacetylene have been directly measured with use of nutation NMR spectroscopy. 1.36 and 1.44 \AA{} were found for the double and single bonds, respectively, in trans-${(\mathrm{CH})}_{x}$, and 1.37 \AA{} was found for the double bond in the cis isomer. Over the temperature range 4.2-300 K, the dynamical single-double bond interchange predicted for chains in trans-${(\mathrm{CH})}_{x}$ containing a mobile soliton defect was not observed.

NUTATION NMR STUDIES OF POLYACETYLENE

Abstract : Using nutation NMR spectroscopy we have been able to determine directly the carbon-carbon bond lengths in polyacetylene. For trans-polyacetylene we find double and single bond lengths of 1.36 and 1.44 angstrons, respectively; the double bond in cis-polyacetylene is found to be 1.38 angstrons. Interestingly, at 77 degrees K we do not observe a rapid interchange of the single and double bonds in trans-polyacetylene in the manner predicted for chains containing a soliton defect, suggesting that the majority of the carbons in the trans isomer are on conjugated segments which do not contain such mobile species. (Author)

Bond length measurements in amorphous solids by nutation NMR spectroscopy: The role of rf field homogeneity

Bond length measurements in amorphous solids by nutation NMR spectroscopy: The role of rf field homogeneity