Natural Spin Research Articles

The EPR D-parameter of 1,3-diarylcyclopentane-1,3-diyl triplet diradicals as a probe for steric substituent effects in benzyl-type radicals †

The zero-field splitting D parameter of a set of ortho-substituted 1,3-diarylcyclopentane-1,3-diyl triplet diradicals 3 has been determined by EPR spectroscopy in a 2-methyltetrahydrofuran glass matrix at 77 K. While for meta- and para-substituted triplet diradicals 3 the D parameter is only a function of the electronic nature (spin donor versus spin acceptor) of the substituent, for the ortho-substituted derivatives the steric influence must also be considered. We show herein that the steric effect of the ortho substituent diminishes delocalization of the cumyl spin density into the aromatic ring through twisting of the π system out of conjugation with the cumyl radical center; indeed, the steric effect may outweigh the electronic influence of the substituent. The computed (PM3) spin density of the minimum-energy conformer reproduces well the experimentally observed steric effect on the D parameter of the triplet diradicals 3. These results demonstrate that the EPR-spectroscopic D parameter serves as a sensitive probe for steric as well as electronic substituent effects.

Influence of the molding pressure on the magnetic properties of soft ferrite composite materials

Experimental studies on the complex susceptibility of soft magnetic composite materials (Ni and Ni-Zn spinels and YIG) in their region of spin resonance (SR) show that the SR frequency is a decreasing function of the magnetic concentration (demagnetising effects) which moreover tends to the natural spin resonance (NSR) frequency of the bulk material much before the unit concentration is reached. We take benefit of this remarkable property to show how powders could advantageously replace sintered materials to measure with no ambiguity (screening effects of domain wall oscillations) the NSR frequencies. Since composite materials are achieved by moulding processes under pressures, particular care of the technological stresses has been taken .

On the Magnetization Mechanisms in Polycrystalline Hexaferrites

An attempt was made to adapt the domain-wall size model to the explanation of magnetization mechanisms in real polycrystalline hard ferrites (SrFe 12 O 19 ). Some new data concerning the total anisotropy specifically for polycrystals were discussed and its value deduced from natural spin resonance measurements. It was observed that samples with different H eff grain size and shape, can present different hysteresis loops. To explain these differences, we applied the theoretical expressions for the magnetization curve and the hysteresis loop developed earlier.

Gyromagnetic microwave resonance of cobalt-aluminoborosilicate nanocomposites

The impetus for the study of electromagnetic wave- matter interactions in the microwave range is created mainly by the need to increase the stealthiness of aircraft, missiles and ships, in order to hinder their detection by energy radar receivers [1]. A significant decrease in the radar cross-section or radar equiva- lent surface (RES) may be obtained by optimization of their shape and by the ' use of absorbant materials. The phenomenon of absorption loss is a consequence of direct electromagnetic wave-matter interaction, i.e. of the partial transformation of electromagnetic energy into thermal energy by coupling with phonons, nuclei and/or electrons. Electromagnetic properties may be adjusted by adaptation of the permittivity and/or permeability. These complex values are written respectively e* = e'-je" and ~*=/x'-jkc" with e', /_~' (real parts) describing wave propagation inside the materials and e", ~" (imaginary parts) describing energy dissipation. Some studies deal with dielectric characterization in the microwave field for monoliths [2-4], fibres [5] and composites [6] although comparatively fewer investigations have been performed on scattering by magnetic loss [7, 8]. Absorption of an electromagnetic wave by mag- netic materials may be considered as occuring via relaxation, induced by the domain wall movement, or alternatively by a natural spin resonance mechan- ism. The possibility of obtaining absorption by natural spin resonance in ferromagnetic systems is presented in this letter. Indeed, the high magnetiza- tion density of iron and its alloys can lead to a significant absorption, and crystalline anisotropy induces a resonance frequency around the 1- 10 GHz range. In addition, iron and cobalt possess Curie temperatures which are compatible with their use in "hot" applications (500 °C), for example in coatings for vehicles operating at low altitude with high velocities. Furthermore, nanocomposites show many new properties, some of which may have applications in such diverse fields as catalysis, biological systems, and mineralogy [9]. In this letter we report initial results concerning the preparation of

Generalized overlap amplitudes using the extended Koopmans’ theorem for Be

Approximate generalized overlap amplitudes (GOAs), also called Feynman–Dyson amplitudes, have been calculated from a full configuration interaction (CI) wave function for the ground state of beryllium using the extended Koopmans’ theorem (EKT). The GOAs were also calculated from the ground state CI wave function for Be and the ground state and excited state CI wave functions for Be+. The EKT GOAs are nearly identical to the corresponding CI GOAs for the lower 2S states of Be+ and for the Be+ 1s2s2 state which has a large GOA occupation number. There are many CI GOAs for which there is not a corresponding EKT GOA. This may be due in part to the limited size of the basis set and to the inability to include natural spin orbitals with small occupation numbers in the EKT calculations.

Study of relaxation and spin resonance in magnetic liquids

Abstract The complex susceptibility of magnetic liquids is investigated in a wide frequency range (10 Hz-2 GHz) with a microwave-type technique. Three loss mechanisms can be identified: Brownian relaxation, Neel relaxation and natural spin resonance. The effect of the temperature on Neel relaxation and natural spin resonance is confirmed theoretically.

A pseudo threefold helical structure found in silk Langmuir-Blodgett films by electron diffraction

The high performance tensile properties of silkworm (Bombyx mori) silk fiber has resulted in a long standing interest in the semicrystalline morphology of this material. The properties of silk fiber depend not only on the chemical composition (primary protein structure) but also on the fiber spinning conditions present in the silk gland which induce the formation of a (β-sheet based crystalline morphology (secondary protein structure). Knowledge of the silk structure is essential for understanding how the natural spinning processes results in such excellent material properties, but surprisingly few experimental results are available concerning the detailed structures of silk proteins. Two β-sheet based silk fibroin crystalline structures (e.g. silk I and silk II) have been studied by many authors, but the silk I structure remains largely uncharacterized. Here we report results from thin silk films prepared by the Langmuir-Blodgett (LB) technique which display a new silk fibroin structure with a threefold helical chain conformation.

Mathematics in sport I

Newton's laws and coded sequences are used to explore sport mathematically. Track and field athletics, American, association, Gaelic and rugby football, golf, rowing, skating, snooker and hockey, among others, provide illustrations. Mathematical considerations include classical mechanics; linear and angular momentum, work, impulse, kinetic and gravitational energy, simple harmonic motion, friction, rolling, rotating frames, Euler angles and projectiles. Some mathematical tools used are differential and integral calculus, vectors and vector spaces, dot and cross products, multiple integrals, orthogonal coordinate systems, fluid mechanics, invariance principles, non‐Abelian algebra, matrices, variational principles, optimization, trigonometry, coordinate geometry, modelling, set theory, transverse and longitudinal waves, elementary statistics, dimensional analysis, combinatorics, partitions and prime numbers, parity, encryption and codes. It is concluded that at all levels of education, primary, secondary and tertiary, sport and games may be used to illustrate and motivate mathematics and for that matter, physics. In paper I, we concentrate on natural impulse and spin.

4985834 System and method employing pipelined parallel circuit architecture for displaying surface structures of the interior region of a solid body

Achieving homonuclear 1H decoupling remains one of the key challenges in liquid-state NMR. Such spectra would endow a variety of organic and analytical applications with an increased resolution, and would ideally do so even in a one-dimensional format. A number of parallel efforts aimed at achieving this goal using two-dimensional acquisitions have been proposed; approaches demonstrated over recent years include, among others, new modes for achieving purely-absorptive J spectroscopy, the use of spatially-selective manipulations, and exploiting the natural spin dilution afforded by heteronuclei. The present study relies on the latter approach, and explores the use of BIRD pulses distinguishing between protons bonded to 13C from those bonded to 12C, to achieve homonuclear decoupling in a continuous 1D scan. Studies on several representative compounds demonstrate that this goal can be implemented in a robust format, provided that suitable care is also taken to suppress unwanted coherences, of making all manipulations sufficiently broad-banded, and to provide adequate heteronuclear decoupling of the targeted protons. Dependable homonuclear decoupling performance can then be achieved, with minimal line width, fine-tuning, and sensitivity penalties.

EPR imaging with natural spin probes

Electron paramagnetic resonance imaging (EPRI) is a rapidly developing method for establishing spatial distribution of paramagnetic centers in a sample under study. This method has a wide field of potential applications, including biology and plant physiology. However, the concentration of native paramagnetic centers in such samples as a rule is insufficient for imaging experiments. The simple solution to this problem is to introduce into the object under study the necessary quantity of stable radicals as imaging substances. The most wide-spread type of such substance seems to be stable nitroxide radicals which have been applied recently to the study of spatial nonhomogeneity and diffusion in systems of chemical and biological interest (1-3). Nitroxide free radicals are very convenient imaging substances because the shapes of their EPR spectra are very sensitive to intermolecular conditions and can provide information about some important biological parameters, oxygen concentration, for example. In Ref. (4) the EPRI technique with nitroxide spin probes was used to investigate oxygen diffusion in models of biological systems. Nitroxide radicals were also used to study spatial characteristics of water penetration into wheat seeds during imbibition (5). In the latter work it was shown that nitroxide radicals may cause membrane destruction, thereby suppressing functions of cells. Much more attractive is the possibility of using some natural spin probes which demonstrate biological activity themselves and are compatible with biological tissues. In the present study we suggest the use, as natural probes, of the water-soluble fraction of soil humic substances (HS). These contain paramagnetic centers up to 10 ‘* spin/g, exhibiting at X band a single EPR signal with a linewidth about six gauss. The investigation of HS penetration into the seeds and their distribution is an important subject because of their biological effects on the seeds. It is well known that seeds are influenced by HS. It was reported that HS, which accelerated water uptake into the seeds (6)) could affect the rate of germination and initial seedling growth ( 7). Some observations indicate that humic substances act at the membrane level, but do not reveal the mode of action of these substances.

Maximum similarity orbitals for analysis of the electronic excited states

A new theoretical tool for analyzing the excited-state wave functions obtained from separate Hartree–Fock or configuration-interaction calculations on each state is proposed. Isopycnic transformations among the natural spin–orbitals of the ground and excited states yield the maximum similarity spin–orbitals (MSOs). These MSOs, which are a generalization of the method of corresponding spin–orbitals due to Amos and Hall, allow one to identify the initial and final spin–orbitals associated with the electronic excitation. The proposed formalism is equally valid in the cases of either identical or different geometries of the ground and excited states and makes it possible to assess magnitudes of the nuclear and electron relaxations accompanying the excitation. The methodology is illustrated using the example of the ROHF/6–31G** calculations on the keto and zwitterionic forms of the 3-hydroxypyrone molecule in their ground and two lowest-lying triplet excited states.

Gradient-induced water-suppression techniques for high-resolution NMR spectroscopy

Water-suppression techniques can be divided into two types, those that result in zero net excitation of the water resonance and those in which the water magnetization is randomized prior to observation of the spectrum of interest. The randomization mechanism may involve selective irradiation, use of natural spin relaxation, or application of a homospoil gradient pulse to dephase selectively excited transverse spin coherence. Destruction techniques considered in this paper will involve the latter of these processes. Nonexcitation methods usually involve a binomial-type pulse cluster which results in off-resonance excitation but zero net rotation on resonance ( 1). Although the water-suppression factor of these methods is usually high ( > 1000)) they suffer from the major limitation that the spectral response is either not flat and/ or there is a large linear phase distortion of the spectrum. Destruction techniques are attractive because the final read pulse can be a hard 90” pulse which ensures uniform excitation of the spectrum with minimal phase distortion. The aim of these methods is the establishment of (S,) = (3,) = (&) = 0 over some selective band in frequency space. For those that utilize a pulsed field gradient, selectivity is achieved by the application of a shaped RF pulse which generates transverse spin coherence over a narrow band and the coherence is dephased by the field gradient. In a previous paper (2) we discussed the difficulties of maintaining high-resolution conditions (Au < 1 Hz) during data acquisition following a field gradient pulse. The major problems can be overcome by preemphasis of the gradient pulse and by applying a correction current through the Z0 coil during data acquisition to ensure that the static magnetic field is stable. Decoupling of the gradient coils from the magnet, shim set, and bore tubes by shielding and isolation may also alleviate these induced eddy current problems. In this paper we consider the nature of the shaped pulse to achieve the optimum water suppression. As well, we introduce a new mechanism to achieve water suppression using a pulsed field gradient and a selective noise pulse. All spectra were obtained on a Bruker MSL-200 spectrometer interfaced to an Oxford Instruments 4.7 T, 13 cm vertical bore magnet. Gradient pulses and Z0 correction were controlled by a Bruker MSL preemphasis unit and were applied to the

Scalar field in a seven-dimensional manifold behaving as an SO(3)-covariant spinor field in space-time.

In a Kaluza theory with topology Minkowski-space\ifmmode\times\else\texttimes\fi{}three-sphere, a metrical field equivalent to the Pauli matrices introduces a natural spin structure. Other fields occurring naturally on this manifold behave then as spinor fields when viewed from their space-time projection.

Occupation function in extended systems

The occupation function in the sense of occupation numbers for the natural spin orbitals for a solid is analyzed from several points of view. The connection with the generalized overlap amplitudes is pointed out and the theory is developed for general spin orbitals both in position and momentum space. The symmetry properties of the occupation function and its Fourier coefficients are analyzed. General expressions for the gradient of the occupation function are derived. The special restricted Hartree-Fock case, where the occupation function is a step function is described as an illustration.

Density and density matrix from optimized linearly independent product basis functions for Be

The density matrix was computed from the density using basis orbitals which form linearly independent products (LIP). Calculations were performed on the Be atom using LIP bases containing several of the natural spin orbitals from an accurate nonrelativistic 650-term configuration interaction (CI) wavefunction. Calculations were also performed using a basis set of 6 s-type orbitals optimized so that the lowest eigenvalue, d1, of the overlap matrix of products is maximized and the energy of a small CI wavefunction is minimized. The value of d1 in the optimized basis is 1 × 10−7. The density and the potential energy obtained from the 650-term CI wavefunction were accurately reproduced by the density matrix expressed in the optimized LIP basis, but the kinetic energy was somewhat less accurate.

Hypergravitational field equations.

We extend Einstein's field equations and Hilbert's action for gravity to the hyperspace of dimension ${\mathrm{N}}^{2}$ that is built out of N-component spinors in the way that four-dimensional time-space is built out of two-component ones. The hypergravitational field equation is of differential order N in a chronometric tensor with N indices. The natural spin structures of the unitary groups U(N) define Einstein-type cosmologies, static, U(N)-invariant, and closed, that satisfy the extended equations with neither dust nor cosmological constant, except for N=2 which requires both.

The variation of the energy functional with respect to the density matrix of first order

The Hohenberg-Kohn theorem may be extended to prove the existence of an universal energy functional based upon the reduced density matrix of first order. We use a set of parameters representing these density matrices uniquely: the eigenvalues (occupation numbers), the projection operators on the corresponding eigenspaces and their dimensions. Without solving the corresponding variational equations explicitly, it is possible to show that, in the stationary case, the eigenfunctions of an "effective Hamiltonian" may serve as natural spin orbitals. Thus the connection to the familiar orbital representation is established. Because interacting many particle systems may lead to partial occupation numbers, the number of involved natural orbitals may exceed the number of particles in this scheme. The unique parametrization of the density matrix, the consideration of all necessary constraints and the possibility for partial occupation numbers consequently lead to more general and complicated variational equations. From these we derive an "average energy theorem" generalizing results of other authors.

ChemInform Abstract: Spin Probes as Mechanistic Inhibitors and Active Site Probes of Thymidylate Synthetase.

Chemischer InformationsdienstVolume 17, Issue 48 Natural Products ChemInform Abstract: Spin Probes as Mechanistic Inhibitors and Active Site Probes of Thymidylate Synthetase. P. D. DEVANESAN, P. D. DEVANESANSearch for more papers by this authorA. M. BOBST, A. M. BOBSTSearch for more papers by this author P. D. DEVANESAN, P. D. DEVANESANSearch for more papers by this authorA. M. BOBST, A. M. BOBSTSearch for more papers by this author First published: December 2, 1986 https://doi.org/10.1002/chin.198648336AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume17, Issue48December 2, 1986 RelatedInformation

A solvable model with an extreme AGP ground state: Relationships among fermion pairs, pairons, and natural spin geminals

A model many-fermion Hamiltonian is presented for which the ground state is asymptotically an Antisymmetrized Geminal Powers (AGP) wave function with largest possible greatest eigenvalue for its two-particle reduced density matrix. Closed analytical expressions and plane-wave expansions are presented for the generating geminal of the AGP ground state and for its one-particle reduced density matrix. The natural orbitals for this generating geminal are plane waves. The generating geminal shows intensely local character in its intracule and corresponds to the formation of a quasi-boson from two fermions. One may appropriately modify this generating geminal to introduce zero occupation numbers of its one-particle reduced density matrix and to make all the nonzero occupation numbers of its one-particle reduced density matrix equal, thus making this geminal a generator of an extreme AGP wave function, with an extreme large eigenvalue for its two-particle reduced density matrix. Closed analytical expressions are also given for this modified geminal and for its one-particle reduced density matrix. The modified generating geminal develops anomolously long “tails” in the intracule function. Nevertheless, both generating geminals introduced here reduce to a renormalized Dirac δ-function of the intracule coordinate, in the appropriate limit. In this limit, both of these generating geminals produce an absolute extreme AGP wave function with the absolute extreme value, N/2, for the Lowdin-normalized occupation number of the generating geminal, and vanishing occupation numbers for all other natural geminals, in the two-particle reduced density matrix of this wavefunction. A demonstration is given showing the formation of quasi-bosons in this ground state and showing their relationship to the generating geminal and to the other natural geminals of the two-particle reduced density matrix of the extreme AGP wave function. The similarities and differences of the features of this model and the accepted models of the superconducting ground state of electrons in metals, and the superfluid ground state of liquid He4 are mentioned.

Coupled-cluster method with optimized reference state

We propose a variant of the coupled-cluster (CC) method in which spin orbitals of the reference Slater determinant are optimized in a self-consistent way. This approach is a reformulation of the Brueckner–Hartree–Fock (BHF) method used in nuclear physics and known also as the exact SCF method. We discuss the use of the reference-state determinants built of HF, natural, and Brueckner spin orbitals, with relations among them investigated in terms of the many-body perturbation theory (MBPT). It is shown that the Brueckner spin orbitals emerge as a convenient basis set in the coupled-cluster method and equations that determine these spin orbitals are found. The Brueckner spin orbitals can be calculated as eigenvectors of a certain Hermitian one-electron operator which has a form of the Hartree–Fock operator plus a “correlation potential” depending linearly on two- and three- electron cluster amplitudes. The usual decoupling scheme in the coupled-cluster method leads to a hierarchy of approximations; in the first nontrivial one the three-electron cluster amplitudes are neglected, and the two-electron ones are determined by solving Cižek's CPMET equations. We also analyze the problem of spatial, spin, and time-reversal symmertry in the CC and BHF methods. If (and only if) the reference-state determinant belongs to a one-dimensional representation of a certain symmetry subgroup of the system, the CC operator and the BHF one-electron operator are invariant with respect to this subgroup. Thus the restricted (entirely symmetry-adapted) version of the BHF method can be formulated only for the closed-shell systems. This is done for the above-mentioned approximate BHF method. We discuss also the usefulness of the BHF method in application to extended metallic systems.