Nutation Spectra Research Articles

Single- and multiple-quantum cross-polarization in NMR of quadrupolar nuclei in static samples

Cross-polarization from a spin I=1/2 nucleus (e.g., 1H) to a spin S = 3/2 nucleus (e.g., 23Na) or a spin S = 5/2 nucleus (e.g., 27A1 or nO) in static powder samples is investigated. The results of conventional (single-quantum), three-quantum, and five-quantum cross-polarization experiments are presented and discussed. Based on a generalization of an existing theory of cross-polarization to quadrupolar nuclei, computer simulations are used to model the intensity and lineshape variations observed in cross-polarized NMR spectra as a function of the radio-frequency field strengths of the two simultaneous spin-locking pulses. These intensity and lineshape variations can also be understood in terms of the spin S = 3/2 or 5/2 nutation rates determined from experimental quadrupolar nutation spectra. The results of this study are intended as a preliminary step towards understanding single- and multiple-quantum cross-polarization to quadrupolar nuclei under MAS conditions and the application of these techniques to the MQMAS NMR experiment.

Calculations of Multipulse Sequence in NQR of Spins [formula omitted

The general formalism of the interaction representation with respect to an operator which is its own inverse is developed and applied to pure NQR of spins I = 32. Under the assumption of no relaxation and no dipolar coupling, it is shown that the calculation of the response to pure NQR multipulse sequences can be performed with the same concepts used in high field NMR, such as coherence pathways. All the tools and mathematical expressions to predict the time evolution of the signal created by a pure NQR multipulse sequence are presented explicitly. It takes into account the off-resonance irradiation as well as the angular dependence of the excitation and detection for every value of the electric field gradient asymmetry parameter. Particular attention is devoted to the powder average, which is performed via a probability function derived analytically for the first time, leading to a drastic reduction of simulation times. The theory is illustrated by the study of the optimization and excitation bandwidths of one- to three-pulse sequences and compared to experimental results on Chloranil. We show that the three-pulse “stimulated echo” sequence gives a more uniform excitation profile than the traditional two-pulse echo sequence for powder samples. Thus, the “stimulated echo” sequence could be useful to cover a large spectrum when the experiment duration, or the signal to noise ratio, are not critical parameters. Analytical expressions for the nutation spectra obtained by one or two-pulse sequences are also derived for the first time.

First Observation of the Photoexcited Quintet State in Fullerene Linked with Two Nitroxide Radicals

The bisadduct of fullerene (1) having two nitroxide radicals at the trans-3 position was newly synthesized. The ground and photoexcited states of 1 were studied by continuous-wave and pulsed two-dimensional electron paramagnetic resonance spectroscopy. An exchange interaction between the radicals was evaluated to be much smaller than an isotropic hyperfine splitting, showing a doublet (S = 1/2) character in the ground state. It was found from a 2D nutation spectrum that a photoexcited state has a quintet character (S = 2) which is generated via interactions between photoexcited triplet fullerene (S = 1) and the two nitroxide radicals (S = 1/2).

The structure and electronic state of photoexcited fullerene linked with a nitroxide radical based on an analysis of a two-dimensional electron paramagnetic resonance nutation spectrum

An electron paramagnetic resonance (EPR) study of 3,4-fulleropyrrolidine-2-spiro-4′[2′,2′,6′,6′-tetramethyl]piperidine-1′-oxyl (1) was performed on the photoexcited quartet state in toluene glass. The spectrum of the |S,Ms〉=|3/2,±3/2〉⇔|3/2,±1/2〉 transitions was observed selectively by using a two-dimensional (2D) nutation method and analyzed with a spectral simulation in a randomly oriented system. A position of the nitroxide moiety was determined with respect to the zero-field splitting (zfs) axes of excited triplet fullerene (3C60) by taking into account of the dipolar–dipolar interaction between the radical and 3C60, the hyperfine coupling, the anisotropic g-value of the nitroxide radical, and the zfs of the 3C60 moiety. It was found that none of the zfs axes of the 3C60 moiety coincide with the local C2 axis of the molecule which is defined by the position of addition. A symmetry of the electronic structure in 3C60 is discussed on the basis of the result.

Lineshape Analysis of 2D NMR and NQR Nutation Spectra of Integer and Half-Integer Quadrupolar Nuclei

Abstract A lineshape analysis of the NMR and NQR powder nutation spectra of integer and half-integer quadrupolar nuclei is presented. Simulated NMR nutation spectra of spin I = 1 and 3 nuclei are reported. For I = 1 the formulas for the singularities of the NMR nutation powder patterns as a functions of η, e2Qq and 7B x are given. The NQR nutation powder patterns for spin I = 3/2, 5/2, 7/2, and 9/2 for different induced transitions are calculated, and some experimental aspects of the method are discussed. A universal empirical formula to facilitate the determination of the asymmetry parameter from the NQR nutation frequency singularities for any arbitrary spin or transition is found. The NQR nutation spectra for half-integer and integer spins are compared. For integer spins the two-frequency excitation of the nutation spectrum is analysed. Application of the 2D-nutation lineshape analysis for the determination of the quadrupole interaction parameters is emphasised.

Influence of the rf Field Inhomogeneity on Nutation NQR Spectra of Spin 3/2 Nuclei in Powders

Abstract The influence of the rf field inhomogeneity on the NQR nutation spectra of spin / = 3/2 nuclei in powder samples is investigated. To eliminate the rf field inhomogeneity effects, a method of reconstruction of the NQR nutation spectra, based on finding the deconvolution of the Fourier nutation spectrum with a function of the rf field distribution, is used. The method is successfully demonstrated for simulated and experimental NQR nutation spectra of 35C1 in TiCl4 . The lineshape analysis of reconstructed nutation spectra allowing the determination of the EFG asymmetry parameter is given. The real advantage of the proposed method is that the high-resolution nutation spectrum may be obtained for a sample filling up the entire coil.

新しいＥＳＲ法を用いた石炭ラジカルの研究

In this review, it is described that new information can be obtained by applying new ESR methods to the radicals contained in coals. High frequency ESR measurements enable us to observe the highly resolved ESR signals. Thus, the mechanism of linewidth broadening of the ESR signals for coal radicals can be discussed in terms of the spin exchange, the hyperfine interaction and the anisotropy of g-value. By pulsed ESR measurements, various kinds of spectra such as nutation spectra and HYSCORE (hyperfine sublevel correlation spectroscopy) spectra are obtained. The nutation method confirms the spin multiplicity of coal radicals. The 1H-and 13C-hyperfine signals are separately observed in HYSCORE spectra. In comparison with the model compounds, main coal radicals are attributed to the radicals with highly symmetrically condensed aromatic hydrocarbons.

Relaxation effects in two-dimensional zero-field nutation NQR spectroscopy

The relaxation and off-resonance effects on the NQR (nuclear quadrupole resonance) nutation frequency and on the damping constant of the nutation spectrum for spinsI = 3/2 are considered. Using the solution of dynamic equations for nuclear magnetisation in the rotating frame, we obtained a general expression for determination of the asymmetry parameter η, which includes the relaxation time and frequency offset. Specific requirements for the RF (radio frequency) field allowing the presence of nutation and the sufficient resolution of nutation spectra are determined. It is shown that at low RF field it is necessary to take into account theT 2 relaxation effect on the location of frequency singularities in the nutation spectrum. The values ofH 1 andT 2 must be exactly known in nutation NQR spectroscopy. The 2D NQR spin-echo separation technique was used for determination of transverse relaxation of individual spin packets in case of inhomogeneously broadened NQR lines, so that the asymmetry parameter could then be determined by the nutation experiment with consideration of relaxation effects and frequency offset. The theoretical results are confirmed experimentally for the narrow and inhomogeneously broadened 2D-nutation NQR spectra in several molecular compounds containing the35Cl and75As nuclei.

Determination of the asymmetry parameter of EFG tensor from moments of NQR nutation spectra in powders

A novel approach to determination of the asymmetry parameter of the EFG tensor from zero-field nutation NQR spectra of the spinI = 3/2 nuclei in powder samples is reported. The proposed theoretical treatment uses lineshape analysis of the nutation NQR spectra by the method of line moments. The analytical formulas for the lineshape of the powder nutation spectrum are given. It is shown that the asymmetry parameter can be determined from the second moment 〈ω2〉 and the frequency of only one singularity ω2 of the nutation spectrum. It is also shown that the asymmetry parameter can be determined from the second and fourth spectrum moments alone. The method is successfully demonstrated for the simulated nutation NQR spectra of the spinI = 3/2 nuclei in powder samples.

2D Off-Resonance Nutation NQR Spectroscopy of Spin 3/2 Nuclei

Abstract The effects of off-resonance irradiation in nutation NQR experiments are demonstrated both experimentally and theoretically. The theoretical description of the off-resonance effects in 2D nutation NQR spectroscopy is given, and general exact formulas for the asymmetry parameter are obtained. It is shown that the outcome of the off-resonance nutation experiments depends on the data acquisition procedure, leading to one- or three-line nutation spectra. To explain this fact and to describe the off-resonance nutation experiment properly, the transient response theory of a quadrupolar spin system to an RF pulse was modified using the wave-function approach. The 2D separation of interactions technique has been applied to separate a static and a randomly time-fluctuating dynamic part of the quadrupole interaction in the antiferroelectric phase of poly-crystalline ammonium dihydrogen arsenate. Two-dimensional off-resonance nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin-3/2 nuclei in several molecular crystals containing the 35Cl and 75As nuclei. The off-resonance phenomena in 2D NQR are very important from the practical point of view (determination of ƞ in multiple or broad-line spectrum) and also provide interesting information on the dynamic properties of a quadrupolar spin system.

High-Spin States of Hyperbranched Polycationic Organic Polymers as Studied by Ft Pulsed-Esr/Electron Spin Transient Nutation

As a novel electron magnetic resonance technique for the characterization of organic molecule-based magnetic materials, an electron spin transient nutation method based on Fourier-transformed pulsed ESR spectroscopy was applied to polycationic organic polymers of π-topology-mediated molecular design. The polymers are triphenylamine-based in the hyperbranched three-dimensional network with the unpaired electrons at the nitrogen cation sites. The FID-detected nutation spectrum from one of the polymers exhibited several nutation-frequency components corresponding to high-spin species containing S=l to S=3. The static magnetic susceptibility of the polymer supported the high-spin ground state in the system under study. The present results demonstrate the potentialities of the pluri-charged high-spin polymeric systems as well as the methodological advantage of nutation spectroscopy in identifying the spin multiplets of the molecule-based magnetic materials in the frequency domain.

Polycationic High-Spin States of One- And Two-Dimensional (Diarylamino)Benzenes as Studied By Pulsed Esr/Electron Spin Transient Nutation (Estn) Spectroscopy

Polycationic high-spin states of star-burst molecules based on (diarylamino)benzenes have been studied by cw and pulsed ESR spectroscopy. An electron spin transient nutation (ESTN) method based on pulsed ESR spectroscopy was applied to the dicationic and tricationic high-spin states of N,N,n',n'-tetra(4-anisyl)-2,4-dimethyl-1,5-phenylenediamine and N,N,n′,n′,N″,N″-hexa(4-anisyl)-1,3,5-triaminobenzene, respectively, in order to identify the spin multiplicity of the polycationic states. The observed 2D nutation spectra of the dicationic and tricationic states unequivocally showed that the dicationic and tricationic states are ground-state triplet and quartet, respectively.

A Two-Dimensional EPR Nutation Study on Excited Multiplet States of Fullerene Linked to a Nitroxide Radical

Photoexcited states of 3,4-fulleropyrrolidine-2-spiro-4‘-[2‘,2‘,6‘,6‘-tetramethyl]piperidine-1‘-oxyl (1) were studied by the two-dimensional (2D) EPR nutation method in toluene glass. 2D nutation spectra were observed by varying a pulse length of the first microwave pulse in the spin echo experiment. Three kinds of nutation frequencies were obtained from analyses of the 2D spectra and assigned to those of the excited quartet and doublet states on the basis of the fictitious spin-1/2 theory. The obtained EPR parameters such as g-values, hyperfine coupling constants, and zero-field splittings were discussed in terms of the spin Hamiltonian for doublet and quartet states.

The asymmetry parameter of the electric field gradient tensor of TiCl 4 and its complexes studied by NQR nutation spectroscopy

Nuclear Quadrupole Resonance (NQR) Nutation Spectroscopy studies and INDO calculations of the electronic structure of TiCl 4 and its complexes have been performed. An explanation of the changes of the NQR frequencies and asymmetry parameters upon complex formation have been proposed. The populations of the TiCl 4 orbitals have been estimated for different crystallographic structures and compared with the population of the TiCl 4·2CH 3CN orbitals. The values of the experimental 35 Cl NQR frequencies correlated with the calculated ones. Moreover, there is good agreement between the experimentally determined and calculated nutation spectra asymmetry parameter for TiCl 4. The results of the INDO calculations showed that the Ti–Cl bond order decreases upon complex formation and consequently the 35 Cl NQR frequencies decrease and the asymmetry parameter increases. On the basis of the results of the quantum chemical calculations the population of the 3p π chlorine orbital decreases when the molecular structure changes from TiCl 4 (T d) to TiCl 4 (D 4 h). The electron density transfer from the p π orbital of the chlorine atom to the d π orbital of the Ti atom decreases due to complex formation. It was found that the INDO method is suitable for determining the electron distribution in TiCl 4 and its complexes.

Determination of the spectral parameters of nuclear quadrupole resonance from nutation spectra of powdered samples

An analytical expression I(ω) is obtained for a normalized function of the shape of an idealized nuclear quandrupole resonance nutation line of a powdered sample for spins I=3/2 (η≠0). Calculations are made of the initial moments of the nutation spectrum of the powder in the form of functions of ω0=γB1 and the asymmetry parameter η of the electric field gradient tensor. A method is proposed for determining the spectral parameters η and eQqzz from the experimentally measured values of , , , and ω1/2 of the nuclear quadrupole resonance nutation spectrum of the powder.

FT pulsed-ESR/electron spin transient nutation of hyperbranched polycationic organic high-spin polymers

As a novel electron magnetic resonance technique for the characterization of organic molecule-based magnets, an electron spin transient nutation method based on Fourier-transformed pulsed ESR spectroscopy was applied to polycationic organic polymers. The polymers are triphenylamine-based in the hyperbranched three-dimensional network with the unpaired electrons at the nitrogen cation sites. The FIDdetected nutation spectrum from one of the polymers exhibited several nutation-frequency components corresponding to high-spin species containing S=3. The static magnetic susceptibility of the polymer supported the high-spin ground state in the system under study. The present results demonstrate the potentialities of the charged high-spin polymers as well as the methodological advantage of nutation spectroscopy in identifying the spin multiplets of the molecule-based magnets in the frequency domain.

Pulsed ESR/ENDOR and ESTN(Electron Spin Transient Nutation) study of polycationic high-spin states of one-and two-dimensional (diarylamino)benzenes

Polycationic high-spin states of 1,3-bis(diarylamino)benzenes, 1,3,5-tris(diarylamino)benzenes, and their homologues have been studied by cw and pulsed ESR/ENDOR spectroscopy. An electron spin transient nutation (ESTN) method based on pulsed ESR spectroscopy was applied to the dicationic and tricationic high-spin states of N,N,N′,N″-tetra(4-anisyl)-2,4-dimethyl-1,5phenylenediamine and N,N,N′,N′,N″,N″-hexa(4-anisyl)-1,3,5-triaminobenzene, respectively, in order to identify the spin multiplicity of the polycationic states. The observed 2D nutation spectra of the dicationic and tricationic states unequivocally showed that the dicationic and tricationic states are ground-state triplet and quartet, respectively. We have also carried out cw-and pulsed ENDOR measurements to the tricationic state of 1,3,5-tris(diarylamino)benzene, clarifying the electronic structure of the cationic quartet spin state. On the central phenyl ring there are carbon sites with large π-spin densities which are additively brought from each diarylamino moiety by spin polarization mechanism. In the triplet and quartet ground states, the topological spin polarization mechanism is dominant for forming the high-spin states.

Solid-state NMR studies of the aluminophosphate molecular sieve VPI-5

MAS 27Al, 31P, and 27Al nutation NMR spectra show that at 294 K fully hydrated VPI-5 contains three Al and P crystallographic sites in an 1:1:1 occupancy ratio and one-third of Al is 6-coordinate. The results are consistent with the P6 3 space group. Above 353 K two 31P MAS resonances in a 2:1 intensity ratio are found, and quadrupole nutation spectrum gives only one line from 4-coordinate Al, indicating that fully hydrated VPI-5 undergoes a structural transformation to a higher framework symmetry P6 3cm space group. The preparation of partially hydrated VPI-5 can be monitored by 2H NMR and by 27Al and 31P MAS NMR. Both partially and fully hydrated VPI-5 undergo a symmetry change from P6 3 space group at 294 K to P6 3cm above 353 K, which indicates that the breakdown of hydrogen-bonded water structure inside the pores is not a factor in the process.

Off-resonance effects on 2D NMR nutation spectra of [formula omitted] quadrupolar nuclei in static samples

The off-resonance effects on 2D NMR nutation of I = 3 2 quadrupolar nuclei are demonstrated with perturbation theory and numerical calculation in static samples. The off-resonant (Δω) rf field (ω 1) enlarges a nutation frequency and consequently increases the measurement range of nuclear quadrupolar interaction parameters. When ω e > ω Q max, and arctg( ω 1 Δω ) = ±54.7 ° (magic angle), the satellite lines (produced by coherence transfers) in a nutation spectrum are superimposed with the line of central transition, and hence the nutation spectrum is simplified and its sensitivity is enhanced. The nuclear quadrupolar interaction parameters of 23Na nuclei in NaΩ molecular sieve are obtained using 2D NMR nutation.

FT Pulsed and CW EPR Studies of Stable Molecular Spin Crystals

Abstract The potential application of time domain EPR spectroscopy to an organic molecular spin system was examined, particularly focusing upon quantum spin transient nutation phenomena. A nitronyl nitroxide-based biradical with an excited triplet state (S=1), p-phenylenebis(α-nitronylnitroxide), was adopted as a simple model system for studying the nutation spectroscopy which enables us to discriminate between S≥1 and S=½ states. In the low temperature region below 10 K, an S=1 spin of the themally excited biradical molecule and an S=½ spin of an impurity molecule were found to coexist in the crystalline solid state of p-phenylenebis(α-nitronylnitroxide). Field-swept echo-detected EPR spectra reproduced the cw spectra of the biradical powder. Echo-detected nutation spectra exhibited two distinct maxima of the echo intensity at the nutation frequencies expected for S=1 and S=½ spins, indicating that the biradical solid was identified to be a mixture of the S=1 and S=½ species as expected from the static s...