Conformational dynamics of some short-chain biradicals in solutions

Intramolecular electron spin exchange, as a function of temperature and the solvent nature, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in five short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in short flexible biradicals has some peculiarities in comparison with that in long-chain molecules.     

Effect of solvent nature on spin exchange in rigid nitroxide biradicals

Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation only. Temperature variations of the isotropic hyperfine splitting constanta and exchange integral value |J/a| were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments of nitroxide groups led to a slight decrease of the |J/a| value with the increase of temperatureT. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable increase of |J/a| vs.T. In the last case, a coverse relationship between the values of |J/a| and the hyperfine splitting constanta has been observed for solvents with different polarity.     

Regularities of the spin exchange coupling through a bridge in nitroxide biradicals

The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.     

Spin relaxation in acyl radicals measured using spin correlated radical pair (SCRP) polarization in flexible biradicals

Time resolved electron paramagnetic resonance spectra and the decay kinetics of spin correlated radical pair (SCRP) polarization in an acyl-benzyl biradical were measured over a wide temperature range (180–274 K). The major mechanism of intersystem crossing in this biradical is the spin rotation induced relaxation of the acyl moiety, which is associated with the rotation of the carbonyl group about the neighbouring CC bond axis. This relaxation determines the decay rate of the polarization. The relaxation time is largely viscosity independent; it changes by a factor of less than two going from room temperature (60 ns) to 180 K (110 ns) in 2-propanol.     

The study of the electron spin exchange efficiency in liquid solutions of radicals at various EPR frequencies

The electron spin exchange efficiency in toluene solutions of stable nitroxide radicals at different EPR frequencies has been studied. The experimental results are in good agreement with the theory. The spin exchange constantsK _e measured at X-band and 2-mm band coincide. TheK _e values measured from the exchange broadening of the EPR lines do not depend on the value of the nitrogen nuclear spinI _N     

Sound propagation and viscosity in water short-chain amphiphiles solutions,evidence of percolation phenomena

Summary A water/butoxyethanol solution has been investigated by means of several techniques (viscosity, ultrasound and hypersound) as a function of concentration and temperature. The butoxyethanol molar fraction range investigated was situated between 0.015 and 0.09, while temperature ranged between −10°C and +45°C. We observed the building-up of large structures lowering the temperature, in particular below +15°C. All data coming from different techniques seem to confirm such a picture and indicate that this aggregation phenomenon is a percolation-like process. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Features of spin exchange in nitroxide biradicals in the ionic liquid bmimPF<Subscript>6</Subscript>

The intramolecular electron spin exchange has been studied by electron paramagnetic resonance (EPR) spectroscopy in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) for various nitroxide biradicals as a function of temperature and the nature of the connecting bridge between two >NO^· centers. Temperature variations of the isotropic nitrogen hyperfine splitting constant a and exchange integral values |J/a| were measured from EPR spectra and analyzed. Thermodynamic parameters of the conformational rearrangements were obtained. The spin exchange in rigid and flexible biradicals dissolved in the ionic liquid bmimPF₆ was compared with that in toluene solutions. Interesting features of the spin exchange in biradicals in ionic liquid were observed and explained as a result of the specific intramolecular conformational transitions. The first example of a rather rigid biradical molecule becoming flexible under the influence of an ionic liquid is reported.     

An investigation of the efficiencies of various buffer gases in Na-Xe spin exchange

Measurements of the efficiencies of He, Ne and Xe as buffer gases in Na-Xe spin exchange are presented, and earlier values determined for N₂ revised slightly. The spin-rotation coupling of Na-Xe van der Waals molecules is also measured and found to be consistent with recent theoretical work.     

Time-resolved EPR study of electron spin polarization and spin exchange in mixed solutions of porphyrin stable free radicals

The time-resolved electron paramagnetic resonance (EPR) spectra are studied in the temperature range of 110–300 K for two mixed solutions of porphyrins, ZnTPP and H₂TPP, in toluene and the stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). The EPR spectra and their kinetic behavior were studied for concentrations of TEMPO varied in the interval from 0.51 to 7.68 mM, while the porphyrin concentration was fixed as 1 mM. The EPR spectra of triplet-state porphyrins and free radicals manifest the chemically induced spin polarization. For the relatively short-lived radical-triplet pairs, with the perturbation theory up to the fourth order, the theoretical expressions are obtained for the triplet and radical spin polarization induced by the enhanced intersystem crossing (ISC) due to the interaction of excited singlet-state porphyrins with free radicals and by the triplet quenching by free radicals. The time-dependent EPR spectra of the triplets are simulated taking into account the spin-lattice relaxation. It is shown that the variation of the triplet EPR spectra shape, when the time of observation increases, arises from the spin-lattice relaxation kinetics. The kinetic behavior of the TEMPO EPR spectrum was simulated on the basis of the kinetic scheme suggested earlier in the literature. The triplet spin-lattice relaxation time, the rate of the ISC and the lifetime of the excited singlet state were estimated by fitting the kinetic curves for the triplet EPR spectra intensity. For the mixed porphyrin-TEMPO solutions, a possible set of the rate constants of important bimolecular processes were determined. For this set of parameters, it turns out that the spin polarization transfer has a smaller rate constant than the rate constant of the diffusion collisions of the triplet and radical. It appears that the rate constant of the ISC catalyzed by radicals is relatively high in the solutions close to the melting point of the solvent and in the soft-glassy state. In the triplet porphyrins the initial spin polarization induced by the spin-selective ISC was found to exceed the equilibrium spin polarization by up to two orders of magnitude.     

Kawasaki dynamics with infinite-range spin exchange

The essential feature of the Kawasaki model is the conserved order parameter, which places the model in class B of the Halperin, Hoheberg, and Ma classification. We have studied the energy relaxation of this model in one and two dimensions with the added feature that spin exchange may take place between any pair of sites within the system. Our results for the dynamic exponentz are indistinguishable from those for class A models, in which the order parameter is not conserved.     

Quantum spin chains with various defects

Using the density matrix renormalization group (DMRG) method, we study the quantum coherence in one‐dimensional disordered spin chains and Fermi systems. We consider in detail spinless fermions on a ring, and compare the influence of several kinds of impurities in a gapless and a dimerized, gapped system. In the translation‐invariant system a so‐called site‐impurity, which can be realized by a local potential or a modification of one link, increases for repulsive interaction, and decreases for attractive interaction, upon renormalization. The weakening of two neighbouring bonds, which is a realization of a so‐called bond‐impurity, on the other hand, is healed for repulsive interaction, but enhanced for intermediate attractive interactions. This leads to a strong suppression of the quantum coherence measured by the phase sensitivity, but not to localization. Adding a local distortion to a dimerized system, we find that even the presence of a single site‐impurity increases the metallic region found in the dimerized model. For a strong dimerization and a high barrier, an additional sharp maximum, is seen in the phase sensitivity as a function of interaction, already for systems with about 100 sites. A bond‐impurity in the dimerized system also opens a small metallic window in the otherwise isolating regime.     

Exact solutions in Brans-Dicke theory with bulk viscosity

The effect of bulk viscosity on the evolution of the homogeneous and isotropic cosmological models in the Brans-Dicke theory of gravitation is studied. Solutions are found, with a baratropic equation of state, a time-independent bulk viscosity, the gravitational constant inversely proportional to the age of the universe, and the mass of the universe (in the closed model) proportional to the square of its age; the expansion factor is a linear function of the cosmological time. For flat space, power law expansions are found, among them one that is related to extended inflation.     

Order-order transitions in spin systems with double electron exchange

Spin systems in the presence of isotropic single and double electron exchange are investigated at nonzero temperature. We use a generalized mean field approximation that allows for a variable local axis of quantization. This leads to the introduction of an angular parameter Θ giving the angle between nearest neighbor quantization axes. It is shown that order-order transitions can occur between phases of partially aligned spins (general Θ) and pure ferro- (Θ = 0) or antiferromagnetic (Θ = π) spin structures for physically reasonable values of the exchange ratio J₂/J₁ of double to single exchange when J₂ > 0. The order-order critical temperature τ₀ is determined as a function of J₂/J₁ for the particular cases of atomic spin $\text{s = 1,}\text{3}\text{2}\text{, 2,}\text{5}\text{2}$, and the corresponding phase diagrams are presented. Biquadratic and three-atom double electron exchange effects are considered separately. Expressions for the paramagnetic transition temperature and for the spontaneous magnetization M_s for general atomic spin s are given, and the discontinuous change in M_s at τ₀ is also found as a function of J₂/J₁.     

Muon charge exchange and muonium spin exchange in gases

The spin dynamics of the positive muon undergoing cyclic charge exchange (μ⁺⇌Mu) while it is slowing down is compared with that in the case of repeated muonium spin exchange after thermalization. The expectation value of the spin polarization at timet aftern spin exchange collisions (att ₁,t ₂, ...,t _n ) are calculated explicitly from time dependent wave functions, and the quantity is averaged over the statistical distribution of the times of collisions and over all possible numbers of collisions betweent=0 and the observation timet. This result is complementary and equivalent to the conventional density matrix formalism, but offers an insight into the roles of spin flip and spin non-flip processes. The neutral fraction during slowing down is also discussed.     

Mu spin exchange in polarized media

A study has been made of highly nitrogenated pure iron and iron-aluminum alloy powder containing 2 wt.% Al (Fe-2Al) using Mössbauer spectroscopy and X-ray diffraction. The samples were prepared by high-temperature, high-pressure diffusion of nitrogen. They were prepared in a hot-isostatic-pressure (HIP) furnace at 1000 °C at nitrogen pressures up to 200 MPa. The alloy powders contained up to 8 at.% nitrogen as determined by vacuum fusion analysis. XRD analysis indicates that for the pure iron powders, the lattice spacing remained independent of nitrogen concentration, but that for the Fe-2Al powders, it decreased with increasing nitrogen concentration. Mössbauer analysis showed that for the pure iron powders, most of the nitrogen was associated with the formation of the iron nitride Fe₄N. For the alloy powders containing less than 5 at.% nitrogen, all of the nitrogen was contained interstitially. The presence of nitrogen completely eliminated the conduction-electron spin-density oscillations observed in many ferromagnetic alloys.     

Changing exchange bias in spin valves with an electric current

We show that a high-density electric current, injected from a point contact into an exchange-biased spin valve, systematically changes the exchange bias. The bias can either increase or decrease depending upon the current direction. This observation is not readily explained by the well-known spin-transfer torque effect in ferromagnetic metal circuits, but could be evidence for the recently predicted current-induced torques in antiferromagnetic metals.     

Spin-chirality duality in a spin ladder with four-spin cyclic exchange

Effect of four-spin cyclic exchange on magnetism is studied in the two-leg S=1/2 ladder. We develop an exact spin-chirality duality transformation, under which the system is self-dual when the four-spin exchange J4 is half of the two-spin exchange. Using the density-matrix renormalization-group method and the duality relation, we find that the four-spin exchange makes the vector-chirality correlation dominant. A "chirality short-range resonating-valence-bond" phase is identified for the first time at large J4.     

Phase transition in spin systems with various types of fluctuations

Various types ordering processes in systems with large fluctuation are overviewed. Generally, the so-called order–disorder phase transition takes place in competition between the interaction causing the system be ordered and the entropy causing a random disturbance. Nature of the phase transition strongly depends on the type of fluctuation which is determined by the structure of the order parameter of the system. As to the critical property of phase transitions, the concept “universality of the critical phenomena” is well established. However, we still find variety of features of ordering processes. In this article, we study effects of various mechanisms which bring large fluctuation in the system, e.g., continuous symmetry of the spin in low dimensions, contradictions among interactions (frustration), randomness of the lattice, quantum fluctuations, and a long range interaction in off-lattice systems.     

Temperature dependence of viscosity in macromolecular solutions

Summary The viscosity of BSA water solutions has been measured as a function of temperature, for different values of concentration. Measurements are performed both with a capillary-flow viscometer and a rotating-cylinder one, in order to investigate the thyxotropic behaviour of the system. The data can be discussed in terms of the Einstein-Mooney-Padova equation. In such a case it can be shown that both the attractive and the repulsive interaction, between BSA molecules, take place, the latter being mainly responsible for the thyxotropic behaviour. It is also shown that between 35°C and 40°C the solutions exihibit a singular behaviour, the interaction attaining a minimum value.

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Riassunto La velocità di soluzioni in acqua di BSA è stata misurata in funzione della temperatura, per differenti concentrazioni. Le misure sono state fatte sia con un viscosimetro a capillare che con uno a cilindri rotanti, in modo da indagare sul comportamento tixotropico del sistema. I dati possono essere discussi nell’ambito di un’equazione di Einstein-Mooney-Padova. Si può vedere che hanno luogo interazioni sia attrattive che repulsive tra molecole di BSA, le prime essendo responsabili del comportamento tixotropico. Si mostra anche che fra 35° e 45° si ha un comportamento singolare con un minimo nelle interazioni.

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Резюме Измеряется вязкость водяных растворов BSA, как функция темпе-ратуры, для различных значений концентрации. Измерения проводились с помощью каполлярноло вискозиметра и с помощью вискозиметра с вращаюимся цилиндром, чтобы исследовать тиксотропное поведение системы. Полученные данные обсуж-даются с помощью уранвения Эйнштейна-Муни-Падова. В рассматриваемом случае можно показать, что имеют место взаимодействия притяжения и отталкивания между молекулами ВЅА, причем, отталкивание, в основном, ответственно за тиксотвопное поведение. Также показывается, что в интервале между 35°C и 40°C растворы обнаруживают особое поведение, взаимодействие доститает минимального значения.

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