Spin-resolved electron waiting times in a quantum dot spin valve

We study the electronic waiting time distributions (WTDs) in a non-interacting quantum dot spin valve by varying spin polarization and the noncollinear angle between the magnetizations of the leads using scattering matrix approach. Since the quantum dot spin valve involves two channels (spin up and down) in both the incoming and outgoing channels, we study three different kinds of WTDs, which are two-channel WTD, spin-resolved single-channel WTD and cross-channel WTD. We analyze the behaviors of WTDs in short times, correlated with the current behaviors for different spin polarizations and noncollinear angles. Cross-channel WTD reflects the correlation between two spin channels and can be used to characterize the spin transfer torque process. We study the influence of the earlier detection on the subsequent detection from the perspective of cross-channel WTD, and define the influence degree quantity as the cumulative absolute difference between cross-channel WTDs and first passage time distributions to quantitatively characterize the spin flip process. The influence degree shows a similar behavior with spin transfer torque and can be a new pathway to characterize spin correlation in spintronics system.Comment: 9 pages, 7 figure

Multidimensional ¹⁹F NMR Analyses of Terpolymers from Vinylidene Fluoride (VDF)–Hexafluoropropylene (HFP)–Tetrafluoroethylene (TFE)

The use of multidimensional NMR methods for the characterization of polymer microstructure has been applied to terpolymers from vinylidene fluoride (VDF), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). By assembling the atomic connectivity information obtained from different multidimensional NMR experiments, selective ¹⁹F–¹⁹F COSY (correlation spectroscopy), ¹⁹F–¹⁹F gradient double-quantum COSY, and ¹⁹F–¹³C gradient heteronuclear single-quantum coherence (gHSQC), among others, the detailed monomer sequence arrangements in the terpolymer were obtained. Obtaining the resonance assignments of the terpolymer was greatly aided by the extrapolation of known resonance assignments from PVDF homopolymer, poly­(VDF-<i>co</i>-HFP) copolymer, and poly­(VDF-<i>co</i>-TFE) copolymer. A tabulated comparison of the microstructure assignment of resonances from PVDF homopolymer as well as poly­(VDF-<i>co</i>-HFP) and poly­(VDF-<i>co</i>-TFE) copolymers and the terpolymer is provided. Detailed comparisons of ¹⁹F spectra from 470 and 658.4 MHz spectrometers, revealing the AB patterns present in this terpolymer, are presented and discussed in this paper. The compositions of the comonomers in the terpolymers were calculated with different methods, all of which gave similar values. The percentages of VDF and HFP monomer inversions in the terpolymers were also calculated from the assigned NMR resonances

Characterization of Backbone Structures in Poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene) Copolymers by Multidimensional ¹⁹F NMR Spectroscopy

Advanced modern multidimensional solution NMR experiments have been used to deduce the sequence distribution in poly­(VDF-<i>co</i>-HFP) copolymers. Assignment of ¹⁹F resonances from different monomer- and regio-sequences in poly­(VDF-<i>co</i>-HFP) copolymer were identified. In addition to corroborating some assignments reported in earlier literature, this work provides assignments of some new resonances from the spectra of poly­(VDF-<i>co</i>-HFP) copolymer, which resulted from better dispersion of the resonances. These assignments are used for the monomer sequence analyses of two polymers using first-order Markovian statistics, and the relative reactivities of the monomers during polymerization are discussed. The results from this study provide insight into the polymerization chemistry for this fluoropolymer

Characterization of Backbone Structures in Poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene) Copolymers by Multidimensional ¹⁹F NMR Spectroscopy

Advanced modern multidimensional solution NMR experiments have been used to deduce the sequence distribution in poly­(VDF-<i>co</i>-HFP) copolymers. Assignment of ¹⁹F resonances from different monomer- and regio-sequences in poly­(VDF-<i>co</i>-HFP) copolymer were identified. In addition to corroborating some assignments reported in earlier literature, this work provides assignments of some new resonances from the spectra of poly­(VDF-<i>co</i>-HFP) copolymer, which resulted from better dispersion of the resonances. These assignments are used for the monomer sequence analyses of two polymers using first-order Markovian statistics, and the relative reactivities of the monomers during polymerization are discussed. The results from this study provide insight into the polymerization chemistry for this fluoropolymer

Use of ¹H/¹³C/¹⁹F Triple Resonance 3D-NMR to Characterize the Stereosequences in Poly(vinyl fluoride)

Tacticity has an enormous influence on the physical and chemical properties of polymers. There is considerable work using 1D NMR and empirical rules to study the stereosequences in polymers. This work shows that ¹H/¹³C/¹⁹F 3D NMR experiments can provide superior resolution and atomic connectivity information, so that unambiguous resonance assignments can be made for poly­(vinyl fluoride) (PVF). Compared to prior work on 3D NMR studies of stereosequence effects in fluoropolymers, the 3D NMR pulse sequence used in this work is based on single quantum coherence transfer, which eliminates the complicated splitting patterns resulting from evolution of multiple-quantum coherence. In addition, selective excitation of the ¹⁹F nuclei of interest significantly reduces the folding of peaks from other spectral regions. This greatly simplifies the spectra and makes the assignment of resonances much easier. Based on these results, it is possible to assign the ¹⁹F resonances to the pentad level. For example, consider the resonances of mm-centered sequences, which are not well resolved in ¹⁹F–¹⁹F COSY 2D NMR spectrum. ¹H/¹³C/¹⁹F 3D NMR data provide clear evidence for all of the three pentad structures: mmmm, mmmr, and rmmr. Examples showing the resonance assignments of head-to-tail sequences are presented

NMR Study of the Chain End and Branching Units in Poly(vinylidene fluoride-<i>co</i>-tetrafluoroethylene)

2D-NMR techniques were used to identify the detailed structures of chain end and branching units in poly­(vinylidene fluoride-<i>co</i>-tetrafluoroethylene), poly­(VDF-<i>co</i>-TFE). Atomic connectivity information was provided by selective ¹⁹F–¹⁹F COSY (correlation spectroscopy), ¹⁹F­{¹H} gHETCOR (gradient heteronuclear correlated), and ¹H­{¹³C} HSQC (heteronuclear single quantum correlation) experiments. Diffusion ordered spectroscopy (DOSY) and spin–lattice relaxation (<i>T</i>₁) data permitted distinction of backbone, short chain branch, and chain end resonances from one another. Quantitative data on these structures are reported; quantitation also supported assignments through the consistent relative intensities of resonances from the same structures. Possible reactions during the polymerization which could lead to these structures are discussed

Characterization of the Chain-Ends and Branching Structures in Polyvinylidene Fluoride with Multidimensional NMR

Multidimensional solution NMR (¹⁹F, ¹H, and ¹³C) has been used to determine chain-ends and backbone branching points and to obtain unambiguous ¹⁹F and ¹H resonances assignments from these chain-ends and branching structures in poly­(vinylidene fluoride) (PVDF). The multidimensional NMR methods employed in this study not only enabled the resonance assignments of the last monomer of the chain but also provided assignments for the last three monomer units of chain-end structures. The chain-end signals from PVDF were determined using spin–lattice relaxation measurements and 2D diffusion ordered spectroscopy (DOSY) analysis. 2D-NMR analyses were also used to assign resonances of chain branching points along the backbone of the polymer

2D-NMR Characterization of Sequence Distributions in the Backbone of Poly(vinylidene fluoride-<i>co</i>-tetrafluoroethylene)

NMR is a powerful tool to study the microstructures of poly­(vinylidene fluoride-<i>co</i>-tetrafluoroethylene), poly­(VDF-<i>co</i>-TFE). This study shows that the microstructures in this copolymer can be established completely on the basis of 2D-NMR, in which improved dispersion is achieved by the second dimension (¹⁹F or ¹³C chemical shifts). 2D-NMR has been proven to be extremely effective for identifying the carbon sequence distributions in the polymer main chain. For lower level sequences (3- or 5-carbon sequences), resonance assignments on the basis of one- and two-bond ¹⁹F­{¹³C} gradient heteronuclear single quantum coherence (gHSQC) experiments are in good agreement with assignments obtained by traditional methods. Higher level sequences (7- or 9-carbon sequences), which can not be assigned unambiguously by traditional methods, were determined by ¹⁹F–¹⁹F gradient double quantum correlation spectroscopy (gdqCOSY), which provides ¹⁹F–¹⁹F correlations over 3–5 bonds. A quantitative study was also conducted on the composition of this copolymer. Three different approaches were used to calculate the fraction of TFE and the inversion ratio of VDF units

Nonanthocyanin Secondary Metabolites of Black Raspberry (<i>Rubus occidentalis</i> L.) Fruits: Identification by HPLC-DAD, NMR, HPLC-ESI-MS, and ESI-MS/MS Analyses

Nonanthocyanin secondary metabolites potentially contributing to the antiproliferative bioactivity of black raspberry (Rubus occidentalis L.) fruits were extracted in ethyl acetate and isolated by semipreparative and analytical HPLC and analyzed by NMR, HPLC-ESI-MS, and ESI-MS/MS techniques. Here we present complete and partial structures of a variety of the chemical entities such as quercetin 3-glucoside, quercetin 3-rutinoside, myricetin glucoside, dihydrokaempferol glucoside, benzoic acid β-d-glucopyranosyl ester, 3,4-dihydroxybenzoic acid, epicatechin, caffeic acid, <i>p-</i>coumaric acid, <i>p-</i>coumaryl glucoside, <i>p-</i>coumaryl sugar ester, ellagic acid, methyl ellagic acid acetylpentose, methyl ellagic acid valerylpentose, <i>trans</i>-piceid, phloretin glucoside (phloridzin), dihydrosinapic acid, salicylic acid β-d-glucopyranosyl ester, a salicylic acid derivative without attached sugar, <i>p-</i>alkylphenyl glucoside, and a citric acid derivative. To our knowledge, 15 of these compounds were not previously reported in black raspberry fruits

Group 13 Superacid Adducts of [PCl₂N]₃

Irrespective of the order of the addition of reagents, the reactions of [PCl₂N]₃ with MX₃ (MX₃ = AlCl₃, AlBr₃, GaCl₃) in the presence of water or gaseous HX give the air- and light-sensitive superacid adducts [PCl₂N]₃·HMX₄. The reactions are quantitative when HX is used. These reactions illustrate a Lewis acid/Brønsted acid dichotomy in which Lewis acid chemistry can become Brønsted acid chemistry in the presence of adventitious water or HX. The crystal structures of all three [PCl₂N]₃·HMX₄ adducts show that protonation weakens the two P–N bonds that flank the protonated nitrogen atom. Variable-temperature NMR studies indicate that exchange in solution occurs in [PCl₂N]₃·HMX₄, even at lower temperatures than those for [PCl₂N]₃·MX₃. The fragility of [PCl₂N]₃·HMX₄ at or near room temperature and in the presence of light suggests that such adducts are not involved directly as intermediates in the high-temperature ring-opening polymerization (ROP) of [PCl₂N]₃ to give [PCl₂N]_n. Attempts to catalyze or initiate the ROP of [PCl₂N]₃ with the addition of [PCl₂N]₃·HMX₄ at room temperature or at 70 °C were not successful