ABSTRACT
A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.
ABSTRACT
Quasielastic neutron scattering (QENS) at two energy resolutions (1 and 14 microeV) was employed to study high-frequency cholesterol motion in the liquid ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatidylcholine at three temperatures: T = 20 degrees C, T = 36 degrees C, and T = 50 degrees C. We studied two orientations of the bilayer stack with respect to the incident neutron beam. This and the two energy resolutions for each orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separately at two different time scales in the GHz range. We find a surprisingly high, model-independent motional anisotropy of cholesterol within the bilayer. The data analysis using explicit models of molecular motion suggests a superposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined motion within the bilayer plane. The rather high amplitude of the out-of-plane diffusion observed at higher temperatures (T >/= 36 degrees C) strongly suggests that cholesterol can move between the opposite leaflets of the bilayer while it remains predominantly confined within its host monolayer at lower temperatures (T = 20 degrees C). The locally confined in-plane cholesterol motion is dominated by discrete, large-angle rotational jumps of the steroid body rather than a quasicontinous rotational diffusion by small angle jumps. We observe a significant increase of the rotational jump rate between T = 20 degrees C and T = 36 degrees C, whereas a further temperature increase to T = 50 degrees C leaves this rate essentially unchanged.
Subject(s)
1,2-Dipalmitoylphosphatidylcholine/chemistry , Cholesterol/chemistry , Lipid Bilayers/chemistry , Anisotropy , Diffusion , Neutrons , Scattering, Radiation , TemperatureABSTRACT
Six albino rats were exposed in three groups to different values of T (a repetitive time cycle at the end of which an electric shock occurs with a specified probability in absence of a first response), (V.g.: P(E-/R) = .75 and .10) on different weeks. When occurs at least one response on each T cycle it has a probability to delete the shock equal to P(E-/R) that changed on different weeks from .50 to .10. In this way, as the Bayesian probability system indicate for exclusive events, Pk = P(E-/R) - [P(E-/R) x P(E-/R)] was the effective probability of shocks when at least one response occur on each T cycle and Pm = P(E-/R) - [P(E-/R) x P(E-/R) x P(1rst Response)] is the modulated probability of shocks when a first response occur on some T cycles. The results indicated that T/Pk (the mean interval between shocks before the experimental session) nor T/Pm (The mean interval between shocks after the experimental session when the subject responds), or its inverse, are reductional continua that explains monotonically ordered changes of the response rate or the mean total number of responses. As a consequence T and P must be dissociated. On the other hand, corrected response rate (Total responses minus Total first responses/minute) is a direct and monotonically increased function of Pk and of 1/T. In this sense is the increased probability with which noxious stimuli impinge on the behavior stream and the time reduction between stimuli, that separated, sustain responding.
Subject(s)
Avoidance Learning , Electroshock , Escape Reaction , Animals , Male , Probability , Rats , Rats, Inbred StrainsABSTRACT
Four pigeons were trained to peck a key under different values of a temporally defined independent variable (T) and different probabilities of reinforcement (p). Parameter T is a fixed repeating time cycle and p the probability of reinforcement for the first response of each cycle T. Two dependent variables were used: mean response rate and mean postreinforcement pause. For all values of p a critical value for the independent variable T was found (T=1 sec) in which marked changes took place in response rate and postreinforcement pauses. Behavior typical of random ratio schedules was obtained at T 1 sec and behavior typical of random interval schedules at T 1 sec.
