Nutation Spectra Research Articles

Through-Space Magnetic Interaction of cis-Azobenzene Biradical

Two chiral PROXYL groups bridged with azobenzene show the change of the magnetic properties with the trans-cis photoisomerization of the azobenzene moiety. The isomerization brought the radicals closer together and increased the exchange interactions. The spin state of the cis-isomer was determined from two-dimensional nutation spectra as a mixture of S = 1/2 and S = 1. This is an example of the optical control of through-space intramolecular magnetic interactions.

Measuring of short spin-spin relaxation times distributions using NQR nutation experiments

Experiments using nutation interferograms and nutation spectra in nuclear quadrupole resonance (NQR) are used for the first time to find the distribution of short spin-spin relaxation times in powder samples. Instead of the traditional method of the inversion of the Laplace transform, the inversion of the integral transformations of nutation interferograms and nutation spectra are used in this work. The use of the distributions of NQR relaxation times can provide new information on dynamic processes in complex molecular systems in the solid state. To illustrate the capabilities of the method, experimental results for 35Cl, 75As NQR and for 1H NMR are provided.

Oxidation of allenes bearing 1,8‐diphenoxy or diaryloxyacridene moieties

New allene compounds bearing 1,8‐diphenoxy or bis(3,5‐di‐tert‐butylphenyl) oxyacridene (diaryloxyacridene) moieties were synthesized. These compounds were more easily oxidized than Cl‐substituted compounds. The allene compounds were oxidized to generate thermally stable triplet carbenes. Electron spin resonance and electron spin transient nutation spectra of the oxidation reaction products suggested the generation of triplet species; however, the main isolated product from the 1,8‐diphenoxy allene was a cyclized compound that was characterized by X‐ray analysis. The product derived from diaryloxy allene was also characterized by X‐ray analysis as a singlet species, which included a vinyl cation stabilized by a neighboring O(3,5‐di‐tert‐butylphenyl) group. This singlet species was stable even at room temperature and did not show any electron spin resonance signals in solution, indicating that the triplet species was not in equilibrium with the singlet. Copyright © 2016 John Wiley & Sons, Ltd.

High radio-frequency field strength nutation NMR of quadrupolar nuclei

Owing to the introduction of microcoils, high RF field strength nutation NMR is a viable candidate for the study of quadrupolar nuclei with strong quadrupolar couplings, not accessible using contemporary NMR techniques. We show powder 23Na nutation spectra on sodium nitrite for RF field strengths of up to 1170kHz, that conform to theoretical predictions. For lanthanum fluoride powder, 139La nutation spectra taken at elevated RF field amplitudes show clear discrepancies when compared to the theory. These errors are shown to be mainly caused by pulse transients at the end of the pulse, which proved to be detrimental to the shape of the nutation spectra. Using a nutation pulse which ends in a sudden frequency jump, we show that these errors can be reduced, and nutation spectra that conform to theory can be readily acquired. This enables nutation NMR for the study of quadrupolar nuclei with a strong quadrupolar coupling, bridging the gap between NMR, which can only analyse nuclei with a weak to medium quadrupolar coupling, and NQR, were extensive searching for the right quadrupolar frequency is the limiting factor.

RF inhomogeneity and how it controls CPMAS

In this report we discuss the effect of radiofrequency field (RF) inhomogeneity on cross-polarization (CP) under magic-angle spinning (MAS) by reviewing the dependence of the CP-detected signal intensity as a function of the position in the sample space. We introduce a power-function model to quantify the position-dependent RF-amplitude profile. The applicability of this model is experimentally verified by nutation spectra obtained by direct signal detection, as well as by CPMAS signal detection, in two commercial MAS probes with different degrees of RF inhomogeneity. A conclusion is that substantial sections of a totally filled rotor, even in a probe with rather good homogeneity, do not contribute at all to the detected spectra. The consequence is that in CPMAS-based recoupling experiments, such as the CP-with-variable-contact-time (CPVC), spatial selectivity of the Hartmann–Hahn matching condition overcomes complications that could be caused by RF inhomogeneity permitting determination of accurate spectral parameters even in cases with high inhomogeneity.

Oxidation of an allene compound bearing 1,8‐dichloroacridene moieties and photolysis of the halogenated allene compound for the generation of triplet carbenes

New allene compounds, 5c and 5d, bearing 1,8‐dichloroacridene moieties were synthesized, and the oxidation of 5c and 5d was performed to generate thermally stable triplet carbenes. Since the oxidation resulted in the formation of decomposition products, the photolysis of brominated allene compound 18 was performed. UV–vis measurement at low temperatures indicated the generation of intermediates with decreasing starting material 18, although the efficiency of photolysis was quite low. ESR and electron spin transient nutation (ESTN) spectra suggested that the multiplicity of intermediates is triplet, probably the target triplet carbene 6d is generated. In terms of |D| values of the intermediates, ESR showed that unpaired electrons are more effectively delocalized in 6d by the introduction of nitrogen atoms in the skeleton (biradical character in Scheme 1). The observed triplet species 6d was not stable enough to isolate. Copyright © 2014 John Wiley & Sons, Ltd.

Complete description of the interactions of a quadrupolar nucleus with a radiofrequency field. Implications for data fitting

We present a theory, with experimental tests, that treats exactly the effect of radiofrequency (RF) fields on quadrupolar nuclei, yet retains the symbolic expressions as much as possible. This provides a mathematical model of these interactions that can be easily connected to state-of-the-art optimization methods, so that chemically-important parameters can be extracted from fits to experimental data. Nuclei with spins >1/2 typically experience a Zeeman interaction with the (possibly anisotropic) local static field, a quadrupole interaction and are manipulated with RF fields. Since RF fields are limited by hardware, they seldom dominate the other interactions of these nuclei and so the spectra show unusual dependence on the pulse width used. The theory is tested with 23Na NMR nutation spectra of a single crystal of sodium nitrate, in which the RF is comparable with the quadrupole coupling and is not necessarily on resonance with any of the transitions. Both the intensity and phase of all three transitions are followed as a function of flip angle. This provides a more rigorous trial than a powder sample where many of the details are averaged out. The formalism is based on a symbolic approach which encompasses all the published results, yet is easily implemented numerically, since no explicit spin operators or their commutators are needed. The classic perturbation results are also easily derived. There are no restrictions or assumptions on the spin of the nucleus or the relative sizes of the interactions, so the results are completely general, going beyond the standard first-order treatments in the literature.

Exact calculation of the response of a quadrupolar nucleus to radio frequency irradiation

The effect of a pulse in NMR is usually considered as a rotation of the frame of reference of the spin system. For spins-1/2, this concept is an important and very useful tool. The assumption behind this concept is that while the radio frequency irradiation is on, this term dominates all other interactions. Although this is usually true for spins-1/2, typical interactions for a quadrupolar nucleus can be very large and the assumption is no longer valid. The full solution is complex, but two extreme cases are already solved. If the quadrupole interaction is very small, then the assumption is valid and the pulse does act like a rotation of the frame of reference. At the other extreme, if the interaction is large and the spin, I, is half-integral, then the central transition remains relatively narrow and can be treated as a fictitious spin-1/2. The pulse then acts as a rotation, but with a scaling factor of I + 1/2. This paper treats the general case, where no approximations are made. The effects can be observed in a nutation experiment, in which the observed signal is plotted as a function of pulse width, in a simple one-pulse experiment. If the pulse acts as a rotation, then the nutation plot will be a sine wave, but otherwise it will be a sum of sinusoids. This is true even for a single-crystal sample with a single quadrupolar coupling. If the sample is a powder, then the nutation plot will be a sum of many sinusoids, since the quadrupole coupling will vary with the powder average. This paper sketches out the theory of these effects based on a full and exact description of a quadrupolar system and illustrates it with some nutation spectra of 23Na in a powdered sample of sodium nitrate.

Recording 2-D Nutation NQR Spectra by Random Sampling Method

The method of random sampling was introduced for the first time in the nutation nuclear quadrupole resonance (NQR) spectroscopy where the nutation spectra show characteristic singularities in the form of shoulders. The analytic formulae for complex two-dimensional (2-D) nutation NQR spectra (I = 3/2) were obtained and the condition for resolving the spectral singularities for small values of an asymmetry parameter η was determined. Our results show that the method of random sampling of a nutation interferogram allows significant reduction of time required to perform a 2-D nutation experiment and does not worsen the spectral resolution.

Application of the Elliptically Polarized Radio Frequency Fields in Spin-3/2 Nuclear Quadrupole Resonance Spectroscopy

A possibility to excite the spin-3/2 quadrupolar nuclei in sites with a non-zero asymmetry parameter of the electric field gradient (EFG) tensor by means of an elliptically polarized radio frequency (RF) magnetic fields is discussed. Closed analytical formulas for the intensities of nuclear quadrupole resonance (NQR) nutation spectra and nutation frequencies of powder samples were obtained. Characteristic singularities in the nutation spectra were determined which allow the measurement of the asymmetry parameter η. It was found that in the general case of η ≠ 0 the excitation of the nuclear spins in +m and −m states by using the circularly polarized RF fields is not fully selective.

Stochastic Liouville equation studies of FT-EPR spectra of singlet–triplet mixing photo-chemically generated radical pair system

A stochastic Liouville equation (SLE) was numerically solved to obtain pulsed Fourier-transform (FT) EPR spectra on a radical pair system created in a photo-induced chemical reaction. Numerical calculations were applied to the photo-chemical reaction of deuterated acetone and 2-propanol at low temperatures. In this reaction system, the antiphase structures of the EPR signals, so called spin-correlated radical pair (SCRP) signals of two identical isopropyl ketyl radicals and spin-polarized free isopropyl ketyl radicals were observed by FT-EPR and continuous wave time-resolved (CW TR) EPR techniques. In the present work, FT-EPR spectra of the antiphase structure signals of the radical pair themselves as well as spin-polarized free radical signals were simulated. Additionally, rising behavior of free radical signals polarized by the radical pair mechanism (RPM) was also clarified. Furthermore two-dimensional (2D) FT-EPR nutation spectra were simulated in the both cases with and without the radical pairs by the use of SLE. In these simulations, strong DC components in the nutation frequency dimension, were well reproduced as was obtained in experiments. It was shown that relaxation during the microwave pulse was essential for the appearance of the DC components.

Nutation Spectra of Nuclear Quadrupole Resonance in Off-Resonance Conditions

The modes of recording the multidimensional NQR nutation spectra have been analyzed using different off-resonance methods. A method of recording the nutation spectra in off-resonance conditions, based on optimal filtration, has been proposed. For the first time, an experimental spectrum of 3D-35Cl NQR nutation of chloral hydrate is presented.

Application of two-dimensional pulsed EPR nutation spectroscopy to a disordered system with largeg-anisotropy

Two-dimensional (2-D) pulsed electron paramagnetic resonance (EPR) nutation spectroscopy is applied to a disordered system with strongly anisotropicg-factors. The analysis and interpretation of a 2-D nutation spectrum is performed by theoretical calculations appropriate for ferric myoglobin cyanide in frozen solution. The observed and calculated 2-D spectra show broad signals which correspond to the principalg-values. The advantage of nutation spectroscopy is demonstrated by measuring theg x=0.93 signal, which cannot be observed in a conventional field-swept spectrum due to largeg strain broadening. It is shown that the 2-D nutation experiment can be considered as a sort of an angle-selected method but a nutation spectrum gives a powder pattern even for magnetic field settings at extreme field values of an EPR spectrum. This is in contrast to angle-selected electron nuclear double resonance, which gives a single-crystal type spectrum. The advantage of sine Fourier transform (FT) over complex FT is demonstrated for data processing in the nutation domain. Problems of experimental and data-processing procedures are also discussed.

Two-dimensional Exchange and Nutation Exchange Nuclear Quadrupole Resonance Spectroscopy

A theoretical treatment of the 2D exchange NQR pulse sequence is presented and applied to a quantitative study of exchange processes in molecular crystals. It takes into account the off-resonance irradiation, which critically influences the spin dynamics. The response to the threepulse sequence of a system of spins I = 3/2 in zero applied field, experiencing electric quadrupole couplings, is analysed. The mixing dynamics by exchange and the expected cross-peak intensities as a function of the frequency offset have been derived. The theory is illustrated by a study of the optimization procedure, which is of crucial importance for the detection of the cross- and diagonal-peaks in a 2D-exchange spectrum. The systems investigated are hexachloroethane and tetrachloroethylene. They show threefold and twofold reorientational jumps about the carbon-carbon axis, respectively. A new method of direct determination of rotational angles based on two-dimensional nutation exchange NQRspectroscopy is proposed. Themethod involves the detection of exchange processes through NQR nutation spectra recorded after the mixing interval. The response of a system of spins I = 3/2 to the three-pulse sequence with increasing pulse widths is analyzed. It is shown that the 2D-nutation exchange NQR spectrum exhibits characteristic ridges, which manifest the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read directly from elliptical ridges in the 2D spectrum, which are also sensitive to the asymmetry parameter of the electric field gradient tensor.

Two-dimensional nutation exchange NQR spectroscopy: direct determination of rotational angles

A new method for the direct determination of rotational angles based on 2-dimensional nutation exchange NQR spectroscopy is proposed. The method involves the detection of exchange processes through NQR nutation spectra recorded after the mixing interval. The response is analysed of a system of spins I = 3/2 in zero applied field, experiencing electric quadrupole couplings, to the three-pulse sequence with incrementing pulse widths. The systems investigated here were hexachloroethane and tetrachloroethylene, which show threefold and twofold reorientational jumps about the carbon-carbon axis, respectively. It is shown that the 2D nutation exchange NQR spectrum exhibits characteristic ridges, which reveal the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read directly from elliptical ridges in the 2D spectrum, which are also sensitive to the value of asymmetry parameter of the electric field gradient tensor.

Application of Nutations to Studying the Exchange Processes by the Method of Nuclear Quadrupole Resonance Spectroscopy

A method of direct determining the discontinuous rotational reorientation angles from NQR exchange spectra based on recording of two-dimensional nutations of magnetization after the mixing period is suggested. The possible modification of the Jeener pulse sequence with independently varying duration of pulses for obtaining a 2D exchange nutation spectrum is described. Shapes of singularities in the 2D nutation spectrum of CCl2- and CCl3-group reorientations, which depend on the angle θS between C–Cl bonds and on the asymmetry parameter for nonequivalent positions of nuclei participating in the exchange are examined. The 2D exchange NQR nutation spectra of 35Cl in powders prepared from C2Cl4 and C2Cl6 molecular crystals are modeled. The advantages and capabilities of the method are discussed.

Electronic and Molecular Structures of Dendrimeric High-spin Polycarbenes as Studied by CW and Pulsed ESR-based Electron Spin Transient Nutation Spectroscopy

Highly symmetric high-spin polycarbenes of π-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.

Electronic and Molecular Structures of Dendrimeric High-spin Polycarbenes as Studied by CW and Pulsed ESR-based Electron Spin Transient Nutation Spectroscopy

Highly symmetric high-spin polycarbenes of π-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.

The study of the influence of Jahn-Teller coupling and low symmetry strain on the anomalous electron paramagnetic resonance spectrum of titanium(III) doped CsAl(SO4)2⋅12H2O

The controversial EPR spectrum of titanium(III) as an impurity in cesium aluminum sulphate alum has been re-examined using conventional CW- and a novel pulsed-EPR technique, which monitors the nutation frequency as a function of field. The CW-spectra display a high degree of structure, which is interpreted as arising from chemically distinct titanium(III) species. The two-dimensional nutation spectrum maps the g∥ vs g⊥ relation from just one crystal orientation, and to far greater precision than available from CW-EPR. This novel technique shows that the origin of the linewidths observed for some of the EPR lines is inhomogeneous broadening, the nature of which can be described adequately only in the two-dimensional nutation spectrum. Calculations of g∥ vs g⊥ have been undertaken by numerical diagonalization of the vibronic Hamiltonian. It is found that the relationship between g∥ and g⊥ can be modeled only by assuming that the titanium(III) ions are subject to both dynamic Jahn-Teller coupling and low sym...

Solid-State 93Nb NMR and 93Nb Nutation Studies of Polycrystalline Pb(Mg1/3Nb2/3)O3 and (1 - x)Pb(Mg1/3Nb2/3)O3/xPbTiO3 Solid-Solution Relaxor Ferroelectrics

Solid-state 93Nb static and MAS NMR and 93Nb nutation studies of polycrystalline Pb(Mg1/3Nb2/3)O3, (PMN) and (1 − x)Pb(Mg1/3Nb2/3)O3/xPbTiO3 (x = 0.05 to 0.50) solid-solutions are reported. The 93Nb static and MAS NMR spectra of PMN measured at 14.1 T have two major resonances due to the central transition (1/2 ↔ −1/2), a sharp peak at −902 ppm (1950 Hz fwhh) and a broad resonance (12 800 Hz fwhh) centered at −980 ppm. Two-dimensional 93Nb nutation spectra measured at 9.4 T, by contrast, have three different resonances, a sharp peak due to niobiums with a CQ 62 MHz, respectively, that are associated with the broad 93Nb peak centered at −980 ppm. The sharp 93Nb peak for PMN is assigned to Nb(V) B-sites of cubic or high local symmetry in Mg(II)-rich regions with Nb(OMg)6 configurations. The broad 93Nb resonance at −980 ppm is assigned to a range of Nb(ONb)6-x(OMg)x site co...