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

Structure and Conformation of DAB Dendrimers in Solution via Multidimensional NMR Techniques

NOESY−HSQC 3D-NMR and NOESY 2D-NMR techniques have been used on a 750 MHz spectrometer to study the chain conformations of different generation DAB dendrimers (poly[propylene imine] dendrimers) in chloroform and benzene solutions. The high-field multidimensional NMR techniques provided the chemical shift dispersion needed to resolve all of the unique resonances in the dendrimers. By studying the NOE interactions among dendritic chain protons, information about through space interactions between protons on different parts of the dendrimer chain is obtained, which is directly related to the conformation of the dendrimer. These experiments also give further proof of the chemical shift assignments obtained from the HMQC-TOCSY 2D and 3D NMR experiments. The concentration effects on chemical shifts have also been observed, revealing information about the interactions between solvent and different parts of dendrimer molecules. These studies clearly show for DAB dendrimers, that folded chain conformations can occur in nonpolar solvents such as benzene and extended chain conformations are predominant in polar solvents such as chloroform

2D NMR Studies of Poly(ethylene-<i>co</i>-vinyl acetate-<i>co</i>-carbon monoxide)

A series of poly(ethylene-co-vinyl acetate-co-carbon monoxide) terpolymer samples with varying monomer compositions were studied using a variety of one- and two-dimensional nuclear magnetic resonance (NMR) techniques at 750 MHz. Samples were characterized in deuterated 1,4-dichlocrobenzene-d4 at high temperature (120 °C) to overcome the signal loss and broadening due to T2 relaxation. Identification of major triads formed due to the addition of the three monomers in different permutations was facilitated by the use of different 2D techniques such as gradient assisted heteronuclear single-quantum correlation (gHSQC), gradient selected heteronuclear multiple-bond correlation (gHMBC), and gradient assisted HSQC-TOCSY. Detection of unusual structures formed due to the addition of carbon monoxide to vinyl acetate or structures due to the inversion of vinyl acetate was also possible. Because of improved dispersion the study was not restricted to the triad level, but in some cases pentads could also be identified. Distinct resonances of chain ends and short-chain branches were also observed

Characterization of Isomeric Structures in a Mixture of Organosilanes Using Multidimensional NMR

Triple resonance 1H/13C/29Si three-dimensional nuclear magnetic resonance methods were used to characterize the isomeric structures in a mixture of isomeric disilylated 4-vinyl-1-cyclohexene. Triple resonance techniques when used together with pulsed field gradient coherence selection permit selective detection of the structure fragments of 1H−13C−29Si spin systems. Three-frequency correlation provides a method of separating the resonances from a complex mixture of isomers without physical separation of the components. Many signals that are not resolvable in 1D NMR or 2D NMR spectra can be seen with the aid of triple resonance 3D NMR techniques. The experiment was done in three ways to selectively detect correlations between 29Si resonances and CHn groups which are one, two, and three bonds away. The combined data from the three experiments provide unambiguous atomic connectivity information

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

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

3D NMR Studies of C-Centered <i>n</i>-ad Structures in Poly(ethylene-<i>c</i><i>o</i>-<i>n</i>-butylacrylate-<i>c</i><i>o</i>-¹³C-carbon monoxide) (PolyEBC*)

Three-dimensional HCACX and HCACX−HH−TOCSY NMR experiments (J. Magn. Reson. 2004, 168, 352) at 750 MHz have been used to investigate the resonance assignments and monomer sequence distributions of C-centered triads and pentads in poly(ethylene-co-n-butyl acrylate-co-13C-carbon monoxide), poly(EBC*) terpolymer prepared from 13C-labeled carbon monoxide. These experiments provide spectra with correlations among mutually coupled atoms with δCO plotted along the f1 dimension and δCA and δH of adjoining groups plotted along the f2 and f3 dimensions, respectively. The correlation among signals in three planes in a 3D NMR spectrum facilitates selective detection of the resonances from the structure fragments near the 13C labeled ketone carbonyl groups. Moreover, these experiments provide enormous spectral resolution, permitting the identification of numerous unresolved signals that are overlapped in 2D 1H−13C HMBC NMR spectrum. The correlation of resonances involving three different nuclei (δH, δCA, and δCO) provides unambiguous atomic connectivity information in the polymer backbone that facilitates analysis of comonomer composition, stereo-sequence distribution and branching structures. Occasionally, these experiments provide additional proof of the methods adopted to perform chemical shift assignments from 2D HMBC NMR experiments

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

One-Step Synthesis and Characterization of Difunctionalized N-Confused Tetraphenylporphyrins

Three disubstituted N-confused porphyrins (2−4) were prepared in ca. 4% yield using a one-pot synthesis. These porphyrins bear 3,5-di-tert-butylphenyl groups substituted at the C5 and C20 meso positions and para-substituted (Br, NO2, ethynyl) phenyl groups at the C10 and C15 meso positions. The specific orientation of the aryl rings around the macrocycle in porphyrin 2 was definitively determined using a combination of 1D (1H and 13C) and 2D (gHMQC and gHMBC) NMR spectroscopy. The absorption spectra of 2−4 in CH2Cl2 and dimethylacetamide are similar to those of N-confused tetraphenylporphyrin in the same solvents but have Soret and Q-bands that are shifted to lower energies. Steady-state fluorescence measurements revealed Qx(0,0) and Qx(0,1) bands similar in energy to the unsubstituted NCPs 1i and 1e. The fluorescence quantum yield results for two of these NCPs (2, 4) are atypical of porphyrin behavior and are being further investigated by time-resolved spectroscopy

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