In financial time series there are time periods in which market indices values or assets prices increase or decrease monotonically. We call those events "price runs", "elementary uninterrupted ...trends" or just "uninterrupted trends". In this paper we study the distribution of the duration of uninterrupted trends for the daily indices DJIA, NASDAQ, IPC and Nikkei 225 during the period of time from 10/30/1978 to 08/07/2020 and we compare the simple geometric statistical model with p = 1 2 consistent with the EMH to the empirical data. By a fitting procedure, it is found that the geometric distribution with parameter p = 1 2 provides a good model for uninterrupted trends of short and medium duration for the more mature markets; however, longest duration events still need to be statistically characterized. Estimated values of the parameter p were also obtained and confirmed by calculating the mean value of p fluctuations from empirical data. Additionally, the observed trend duration distributions for the different studied markets are compared over time by means of the Anderson-Darling (AD) test, to the expected geometric distribution with parameter p = 1 2 and to a geometric distribution with a free parameter p, making possible to assess and compare different market geometric behavior for different dates as well as to measure the fraction of time runs duration from studied markets are consistent with the geometric distribution with p = 1 2 and in parametric free way.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures ...involves synthesizing weak-field clusters featuring close M–M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund’s rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χMT ≈ 53 cm3 K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field 57Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm–1 and a magnetic blocking temperature of 2.9 K (0.059 K/min).
Chemical oxidation and reduction of the all-ferrous (HL)2Fe6 in THF affords isostructural, coordinatively unsaturated clusters of the type (HL)2Fe6 n : (HL)2Fe6BArF24 (1, n = +1; where BArF24− = ...tetrakis(3,5-trifluoromethyl)phenylborate), Bu4N(HL)2Fe6 (2a, n = −1), P(HL)2Fe6 (2b, n = −1; where P+ = tributyl(1,3-dioxolan-2-ylmethyl)phosphonium), and Bu4N2(HL)2Fe6 (3, n = −2). Each member of the redox-transfer series was characterized by zero-field 57Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox-directed trends are observed when comparing the structural metrics within the Fe6 core. The metal octahedron Fe6 decreases marginally in volume as the molecular reduction state increases as gauged by the Fe–Feavg distance varying from 2.608(11) Å (n = +1) to 2.573(3) (n = −2). In contrast, the mean Fe–N distances and ∠Fe–N–Fe angles correlate linearly with the Fe6 oxidation level, or alternatively, the changes observed within the local Fe–N4 coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state (S) increases, the Fe–N(H)avg distances also increase. The structural metric perturbations within the Fe6 core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for (HL)2Fe6. Chemical reduction of the (HL)2Fe6 cluster results in an abrupt increase in spin ground state from S = 6 for the all-ferrous cluster, to S = 19/2 in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 provides further evidence of the fully delocalized ground state observed by 57Fe Mössbauer spectroscopy for all species examined (1–3). For each of the clusters examined within the electron-transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = 19/2 in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe–N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund’s rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe–Fe orbital overlap within the Fe6 cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm–1.
Here we report on the ion conductance through individual, small diameter single-walled carbon nanotubes. We find that they are mimics of ion channels found in natural systems. We explore the factors ...governing the ion selectivity and permeation through single-walled carbon nanotubes by considering an electrostatic mechanism built around a simplified version of the Gouy–Chapman theory. We find that the single-walled carbon nanotubes preferentially transported cations and that the cation permeability is size-dependent. The ionic conductance increases as the absolute hydration enthalpy decreases for monovalent cations with similar solid-state radii, hydrated radii, and bulk mobility. Charge screening experiments using either the addition of cationic or anionic polymers, divalent metal cations, or changes in pH reveal the enormous impact of the negatively charged carboxylates at the entrance of the single-walled carbon nanotubes. These observations were modeled in the low-to-medium concentration range (0.1–2.0 M) by an electrostatic mechanism that mimics the behavior observed in many biological ion channel-forming proteins. Moreover, multi-ion conduction in the high concentration range (>2.0 M) further reinforces the similarity between single-walled carbon nanotubes and protein ion channels.
Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric Ar3NX initiated a nitride coupling reaction to N2, a major step toward catalytic ammonia oxidation ...(salen=N,N′‐bis(salicylidene)‐ethylenediamine dianion; Ar=p‐bromophenyl; X=SbCl6− or B(C6F5)4−). N2 production was confirmed by mass spectral analysis of the isotopomer, 1‐15N, and the gas quantified. The metal products of oxidation were the reduced MnIII dimers, (salen)MnCl2 (2) or (salen)Mn(OEt2)2B(C6F5)42 (3) for X=SbCl6− or B(C6F5)4−, respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed‐valent MnV/MnIII bridging nitride, (salen)MnV(μ‐N)MnIII(salen)B(C6F5)4 (4), was isolated, and its oxidation‐state assignment was confirmed by X‐ray diffraction (XRD) studies, perpendicular and parallel‐mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re‐oxidized to 1 in the presence of NH3 and NaOCl closing a pseudo‐catalytic “synthetic” cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH3 oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.
Store that H2! Oxidation of a simple (salen)Mn≡N resulted in the 6e− nitride coupling reaction, central to NH3 oxidation to N2. A complete synthetic cycle was established with implications for catalytic NH3 oxidation for H2‐storage applications. An unusual mixed‐valent MnV/MnIII species was also isolated and characterized (see scheme).
Conjugated Molecular Nanotubes Mirzaei, Saber; Castro, Edison; Hernández Sánchez, Raúl
Chemistry : a European journal,
June 16, 2021, Letnik:
27, Številka:
34
Journal Article
Recenzirano
Molecular compounds with permanent tubular architectures displaying radial π‐conjugation are exceedingly rare. Their radial and axial delocalization presents them with unique optical and electronic ...properties, such as remarkable tuning of their Stokes shifts, and redox switching between global and local aromaticity. Although these tubular compounds display large internal void spaces, these attributes have not been extensively explored, thus presenting future opportunities in the development of materials. By using cutting‐edge synthetic methodologies to bend aromatic surfaces, large opportunities in synthesis, property discovery, and applications are expected in new members of this family of conjugated molecular nanotubes.
Synthetic efforts to bend aromatic surfaces have had a remarkable evolution during the last dozen years. Conjugated nanorings and nanobelts are among the most well‐known examples. However, this progress has also led to the synthesis of molecular compounds with permanent tubular shapes with radial and axial π surfaces, which are the highlight of this Minireview.
We report the synthesis and characterization of conjugated, conformationally rigid, and electroactive carbon-based nanotubes that we term tubularenes. These structures are constructed from a resorcin
...n
b
arene base. Cyclization of the conjugated aromatic nanotube is achieved in one-pot eight-fold C-C bond formation
via
Suzuki-Miyaura cross-coupling. DFT calculations indicate a buildup of strain energy in excess of 90 kcal mol
−1
. The resulting architectures contain large internal void spaces >260 Å
3
, are fluorescent, and able to accept up to 4 electrons. This represents the first scaffolding approach that provides conjugated nanotube architectures.
First scaffolding approach to obtain tubular-shaped contorted aromatic architectures.
Maximizing Electron Exchange in a [Fe3] Cluster Hernández Sánchez, Raúl; Bartholomew, Amymarie K; Powers, Tamara M ...
Journal of the American Chemical Society,
02/2016, Letnik:
138, Številka:
7
Journal Article
Recenzirano
Odprti dostop
The one-electron reduction of (tbsL)Fe3(thf)1 furnishes M(tbsL)Fe3 (M+ = (18-C-6)K(thf)2+ (1, 76%) or (crypt-222)K+ (2, 54%)). Upon reduction, the ligand tbsL6– rearranges around the triiron ...core to adopt an almost ideal C 3-symmetry. Accompanying the (tbsL) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe–Fe distances within the trinuclear cluster contract by ∼0.13 Å in 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 11/2 spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χM ″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field 57Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (U eff) of 22.6(2) cm–1, nearly matching the theoretical barrier of 38.7 cm–1 calculated from the axial zero-field splitting parameter (D = −1.29 cm–1) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (H ext = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions.
A Helicene Nanoribbon with Greatly Amplified Chirality Schuster, Nathaniel J; Hernández Sánchez, Raúl; Bukharina, Daria ...
Journal of the American Chemical Society,
05/2018, Letnik:
140, Številka:
20
Journal Article
Recenzirano
We report the synthesis and characterization of a chiral, shape-persistent, perylene-diimide-based nanoribbon. Specifically, the fusion of three perylene-diimide monomers with intervening naphthalene ...subunits resulted in a helical superstructure with two 6helicene subcomponents. This π-helix-of-helicenes exhibits very intense electronic circular dichroism, including one of the largest Cotton effects ever observed in the visible range. It also displays more than an order of magnitude increase in circular dichroism for select wavelengths relative to its smaller homologue. These impressive chiroptical properties underscore the potential of this new nanoribbon architecture in the context of chiral electronic materials.
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