Two-dimensional Double Quantum Research Articles

Magnetic field effect on the electronic states and the intraband optical absorption spectrum of a laser dressed double quantum dot molecule

The simultaneous effects of intense terahertz (THz) laser, a homogeneous magnetic fields, and the modification of the structural parameters on the electronic states, and the intraband optical absorption spectrum in a two-dimensional double quantum dot molecule are theoretically investigated. The crossing and anticrossing are observed in the energy dependence on the magnetic field induction between the third and the fourth energy levels. Additionally, it is shown that an intense THz laser field always shifts the energy spectrum to higher values. The variation of the structural parameters leads to the change of the positions of the energy levels and the anticrossing point. Finally, we have found that the intraband optical absorption spectrum, particularly the absorption intensity and the peak position, can be effectively regulated by an intense THz laser and a magnetic fields, as well as by the variation of the structural parameters of the double quantum dot molecule.

Reconfigurable quantum logic gates using Rashba controlled spin polarized currents

A reconfigurable logic gate is proposed in a two-dimensional double quantum wire system with a coupling window enabled by a Rashba field. Manipulating the spin states of incoming electrons several quantum logic gates (OR, AND, XOR, CNOT) can be implemented. The logic gate functionality can be switched by tuning the Rashba parameter only. In this context, we investigate suitable configurations of the device region by embedding a quantum point contact located in the coupling region to obtain all four logic gates. The ballistic spin polarized transmission functions are calculated using an effective mass scattering formalism in the framework of a multi-channel, multi-terminal system. Owing its versatility, the proposed logic gate can be integrated in programmable architectures, able to implement both classical and quantum algorithms.

Excited state electronic landscape of mPlum revealed by two-dimensional double quantum coherence spectroscopy.

Red fluorescent proteins (RFPs) are widely used probes for monitoring subcellular processes with extremely high spatial and temporal precision. In this work, we employed spectrally resolved transient absorption (SRTA) and two-dimensional double quantum coherence (2D2Q) spectroscopy to investigate the excited state electronic structure of mPlum, a well-known RFP. The SRTA spectra reveal the presence of excited state absorption features at both the low- and high-energy sides of the dominant ground state bleach contribution. The 2D2Q spectra measured at several excitation wavelengths reveal a peak pattern consistent with the presence of more than three electronic states (i.e., ground, excited, and doubly excited). Numerical modeling of this response suggests that the features are consistent with a 1-1-2 electronic structure. The two closely spaced (∼1500 cm(-1)) levels in the double quantum manifold appear at opposite anharmonicities relative to twice the energy of the lowest energy transition. These observations explain the excited state absorption contributions observed in spectrally resolved transient grating and transient absorption measurements and demonstrate the utility of multidimensional spectroscopy in unraveling congested spectra relative to conventional one-dimensional methods.

Sn surface-enriched Pt–Sn bimetallic nanoparticles as a selective and stable catalyst for propane dehydrogenation

A new one pot, surfactant-free, synthetic route based on the surface organometallic chemistry (SOMC) concept has been developed for the synthesis of Sn surface-enriched Pt–Sn nanoparticles. Bu3SnH selectively reacts with [Pt]–H formed insitu at the surface of Pt nanoparticles, Pt NPs, obtained by reduction of K2PtCl4 by LiB(C2H5)3H. Chemical analysis, 1H MAS and 13C CP/MAS solid-state NMR as well as two-dimensional double-quantum (DQ) and triple-quantum (TQ) experiments show that organo-tin moieties Sn(n-C4H9) are chemically linked to the surface of Pt NPs to produce, in fine, after removal of most of the n-butyl fragment, bimetallic Pt–Sn nanoparticles. The Sn(n-CH2CH2CH2CH3) groups remaining at the surface are believed to stabilize the as-synthesized Pt–Sn NPs, enabling the bimetallic NPs to be well dispersed in THF. Additionally, the Pt–Sn nanoparticles can be supported on MgAl2O4 during the synthesis of the nanoparticles. Some of the Pt–Sn/MgAl2O4 catalyst thus prepared exhibits high activity in PROX of CO and an extremely high selectivity and stability in propane dehydrogenation to propylene. The enhanced activity in propane dehydrogenation is associated with the high concentration of inactive Sn at the surface of Pt nanoparticles which “isolates” the active Pt atoms. This conclusion is confirmed by XRD, NMR, TEM, and XPS analysis.

Probing Hydrogen Bond Networks in Half-Sandwich Ru(II) Building Blocks by a Combined 1H DQ CRAMPS Solid-State NMR, XRPD, and DFT Approach

The hydrogen bond network of three polymorphs (1α, 1β, and 1γ) and one solvate form (1·H2O) arising from the hydration-dehydration process of the Ru(II) complex [(p-cymene)Ru(κN-INA)Cl2] (where INA is isonicotinic acid), has been ascertained by means of one-dimensional (1D) and two-dimensional (2D) double quantum (1)H CRAMPS (Combined Rotation and Multiple Pulses Sequences) and (13)C CPMAS solid-state NMR experiments. The resolution improvement provided by homonuclear decoupling pulse sequences, with respect to fast MAS experiments, has been highlighted. The solid-state structure of 1γ has been fully characterized by combining X-ray powder diffraction (XRPD), solid-state NMR, and periodic plane-wave first-principles calculations. None of the forms show the expected supramolecular cyclic dimerization of the carboxylic functions of INA, because of the presence of Cl atoms as strong hydrogen bond (HB) acceptors. The hydration-dehydration process of the complex has been discussed in terms of structure and HB rearrangements.

Structural Integrity of the B24 Site in Human Insulin Is Important for Hormone Functionality

Despite the recent first structural insight into the insulin-insulin receptor complex, the role of the C terminus of the B-chain of insulin in this assembly remains unresolved. Previous studies have suggested that this part of insulin must rearrange to reveal amino acids crucial for interaction with the receptor. The role of the invariant Phe(B24), one of the key residues of the hormone, in this process remains unclear. For example, the B24 site functionally tolerates substitutions to D-amino acids but not to L-amino acids. Here, we prepared and characterized a series of B24-modified insulin analogues, also determining the structures of [D-HisB24]-insulin and [HisB24]-insulin. The inactive [HisB24]-insulin molecule is remarkably rigid due to a tight accommodation of the L-His side chain in the B24 binding pocket that results in the stronger tethering of B25-B28 residues to the protein core. In contrast, the highly active [D-HisB24]-insulin is more flexible, and the reverse chirality of the B24C(α) atom swayed the D-His(B24) side chain into the solvent. Furthermore, the pocket vacated by Phe(B24) is filled by Phe(B25), which mimics the Phe(B24) side and main chains. The B25→B24 downshift results in a subsequent downshift of Tyr(B26) into the B25 site and the departure of B26-B30 residues away from the insulin core. Our data indicate the importance of the aromatic L-amino acid at the B24 site and the structural invariance/integrity of this position for an effective binding of insulin to its receptor. Moreover, they also suggest limited, B25-B30 only, unfolding of the C terminus of the B-chain upon insulin activation.

Dissecting biexciton wave functions of self-assembled quantum dots by double-quantum-coherence optical spectroscopy

Biexcitons feature prominently in various scenarios for utilization of quantum dots (QDs) for enhancing the efficiencies of solar cells, and for the generation of entangled photon pairs in single QD sources. Two-dimensional double quantum coherence (2D-DQC) nonlinear optical spectra provide novel spectroscopic signatures of such states beyond global intensity and lifetime characteristics which are available by more conventional techniques. We report the simulation of a prototype 2D-DQC optical experiment of a self-assembled InAs/GaAs dot. The simulations consider the QD in different charged states and are based on a state-of-the-art atomistic many-body pseudopotential method for the calculation of the electronic structure and transition dipole matrix elements. Comparison of the spectra of negatively charged, neutral, and positively charged QD reveals optical signatures of their electronic excitations. This technique directly accesses the biexciton ($XX$) energies as well as the projections of their wave functions on the single-exciton manifold. These signals also provide a unique tool for probing the charged state of the QD and thus the occupation of the quantum state. Signatures of Pauli blockade of the creation of certain single and two excitons due to charges on the particles are observed. For all quantum states of the QD, the spectra reveal a strong multiconfiguration character of the biexciton wave functions. Peak intensities can be explained by interference of the contributing Liouville space pathways.

Two-Dimensional Double-Quantum Spectra Reveal Collective Resonances in an Atomic Vapor

We report the observation of double-quantum coherence signals in a gas of potassium atoms at twice the frequency of the one-quantum coherences. Since a single atom does not have a state at the corresponding energy, this observation must be attributed to a collective resonance involving multiple atoms. These resonances are induced by weak interatomic dipole-dipole interactions, which means that the atoms cannot be treated in isolation, even at a low density of 10(12) cm(-3).

Optimal laser control of double quantum dots

Coherent single-electron control in a realistic semiconductor double quantum dot is studied theoretically. Using optimal-control theory we show that the energy spectrum of a two-dimensional double quantum dot has a fully controllable transition line. We find that optimized picosecond laser pulses generate population transfer at significantly higher fidelities (> 0.99) than conventional sinusoidal pulses. Finally we design a robust and fast charge switch driven by optimal pulses that are within reach of terahertz laser technology. Double quantum dots (DQDs), i.e., coupled twodimensional (2D) electron traps, have been under recent and extensive studies both experimentally 1,2,3 and theoretically. 4,7,8 The main interest in DQDs arises from their potential for solid-state quantum computation that could be achieved in principle by rapidly switching voltages of electrostatic gates. The gates permit to tune at will the system geometry and hence the electronic properties of DQDs. Coherent manipulation of a single charge 2 and coupled spins 1 has already been achieved, and recently a coherence time of ∼ 200 ns was obtained for a well isolated silicon DQD. 3 Theoretical studies on single-electron transport inside the DQD driven by linear switches and linearly polarized continuous waves (CWs) were reported very recently. 4 In the latter case the transport is rather sensitive to possible anharmonicity of the potential and limited to uncoupled dots far apart from each other. Electron control in DQDs has been studied also using genetic algorithms 5 as well as rotating-wave and resonant approximations leading to a reduction to a threelevel system. 6 To the best of our knowledge, however, a general N-level control scheme by using direct external electric fields has not been introduced for 2D-DQDs until now. In this paper we discuss the controllability criteria for single-electron states of DQDs by means of external laser pulses. We show that at certain interdot distances some of the single-electron states allow full population transfer from the ground state to those states. We apply quantum optimal-control theory (OCT) 9 which yields the optimal laser pulses for predefined transitions. We obtain high occupations (& 99%) of the target states in a realistic DQD in a few picoseconds, which is well in the coherent regime. If the initial and final states are chosen to have full localization of the electron in one or the other dot, this scheme enables rapid and controlled transport which is not sensitive to the interdot distance or to the inevitable anharmonicities in the confining potential.

A Solid-State NMR, X-ray Diffraction, and Ab Initio Investigation into the Structures of Novel Tantalum Oxyfluoride Clusters

A series of tantalum oxyfluoride materials containing the [Ta4F16O4]4− and [Ta8F24O12]8− anion clusters have been synthesized and characterized using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The structure of both tantalum oxyfluoride materials display octahedrally bonded tantalum atoms with bridging oxygen and terminal fluoride atoms. The [Ta4F16O4]4− cluster is an eight-membered ring, whereas the [Ta8F24O12]8- cluster forms a cagelike structure. Solid-state dynamics of these clusters were explored by monitoring the impact of temperature on the one-dimensional (1D) 19F magic angle spinning (MAS) NMR, 13C cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) 19F MAS NMR spectra. The DQ 19F NMR correlation experiments allowed the through space connectivity between the different resolved fluorine environments to be determined, thus aiding in the spectral assignment and structural refinement of these materials. Ab initio 19F NMR chemical shift calculations were used to assist in the interpretation of the 19F NMR spectra. The influence of scalar relativistic and Ta−F spin–orbit coupling on the 19F NMR shielding calculation arising from bonding to tantalum atoms is also addressed.

The Key Residue for Substrate Transport (Glu14) in the EmrE Dimer Is Asymmetric

Transport proteins exhibiting broad substrate specificities are major determinants for the phenomenon of multidrug resistance. The Escherichia coli multidrug transporter EmrE, a 4-transmembrane, helical 12-kDa membrane protein, forms a functional dimer to transport a diverse array of aromatic, positively charged substrates in a proton/drug antiport fashion. Here, we report (13)C chemical shifts of the essential residue Glu(14) within the binding pocket. To ensure a native environment, EmrE was reconstituted into E. coli lipids. Experiments were carried out using one- and two-dimensional double quantum filtered (13)C solid state NMR. For an unambiguous assignment of Glu(14), an E25A mutation was introduced to create a single glutamate mutant. Glu(14) was (13)C-labeled using cell-free expression. Purity, labeling, homogeneity, and functionality were probed by mass spectrometry, NMR spectroscopy, freeze fracture electron microscopy, and transport assays. For Glu(14), two distinct sets of chemical shifts were observed that indicates structural asymmetry in the binding pocket of homodimeric EmrE. Upon addition of ethidium bromide, chemical shift changes and altered line shapes were observed, demonstrating substrate coordination by both Glu(14) in the dimer.

Double Quantum 1H MAS NMR Studies of Hydrogen-Bonded Protons and Water Dynamics in Materials

Two-dimensional double quantum (DQ) 1H MAS NMR was used to investigate different proton environments in a series of alkali (Na, K, Rb, Cs) [Nb6O19]8- Lindqvist salts, with the water and hydrogen-bound intercluster protons being clearly resolved. Through the analysis of the DQ 1H NMR spinning sideband pattern, it is possible to extract both the mean and distribution of the motionally averaged intramolecular homonuclear 1H-1H dipolar coupling for the different water environments and the intercluster protons. Motional order parameters for the water environments were then calculated from the averaged dipolar couplings. The influence of additional intermolecular dipolar couplings due to multispin interactions were simulated and discussed.

Two-dimensional solid-state NMR studies of crystalline poly(ethylene oxide): Conformations and chemical shifts

The torsion angles of the OC–CO bonds in crystalline poly(ethylene oxide), PEO, were investigated by solid-state nuclear magnetic resonance (NMR). Two-dimensional double-quantum (DOQSY) spectra indicate that the OC–CO bonds are all gauche with an average torsion angle of ψ=74±4° and a narrow torsion-angle distribution, σ ψ <8°. This is contradictory to the wider range of gauche torsion angles in the distorted helical structure previously proposed based on X-ray fiber diffraction. The low-temperature magic-angle-spinning (MAS) 13C NMR spectrum of unlabeled PEO contains four maxima and several shoulders, over a range of 3.1 ppm. Deconvolution of this spectrum, together with two-dimensional 13C INADEQUATE NMR and exchange MAS spectra, suggests a possible assignment of chemical shifts to the 14 carbons in the 7 2 helical repeat unit. The small line widths of the individual peaks indicate that the helical repeat unit is accurately replicated throughout the crystals. The results show that packing effects or small conformational differences can change chemical shifts by amounts that had previously been ascribed only to trans/ gauche differences.

Coupling and dissociation in artificial molecules

We show that the spin-and-space unrestricted Hartree-Fock method, in conjunction with the companion step of the restoration of spin and space symmetries via Projection Techniques (when such symmetries are broken), is able to describe the full range of couplings in two-dimensional double quantum dots, from the strong-coupling regime exhibiting delocalized molecular orbitals to the weak-coupling and dissociation regimes associated with a Generalized Valence Bond combination of atomic-type orbitals localized on the individual dots. The weak-coupling regime is always accompanied by an antiferromagnetic ordering of the spins of the individual dots. The cases of dihydrogen (H$_2$, $2e$) and dilithium (Li$_2$, $6e$) quantum dot molecules are discussed in detail.

A 1H double-quantum magic-angle spinning solid-state NMR investigation of packing and dynamics in triphenylene and hexabenzocoronene derivatives

Solid-state 1H NMR methods employing very-fast magic-angle spinning (MAS) are applied to hexa-alkyl-substituted hexa- peri-hexabenzocoronenes (HBC) as well as an alkoxy-triphenylene derivative. Firstly for the HBC derivatives, the effect of changing the alkyl substituent from i-propyl to t-butyl on the structure adopted in the solid phase is investigated. From single-quantum MAS spectra, differences are apparent between the two cases, most notably in the aliphatic region, where for the t-butyl derivative, two peaks are resolved. The different structures adopted become much more apparent in rotor-synchronised two-dimensional double-quantum (DQ) MAS spectra, where definite proton–proton proximities can be identified, taking advantage of the significant resolution improvements afforded by the extension to a second frequency dimension. In both cases, different aromatic 1H resonances are resolved. This can be explained in terms of the differing degrees to which the protons experience the ring current due to the aromatic π-electrons of a nearby aromatic core. For the i-propyl derivative, the observed pattern of DQ peaks is the same as that observed in a previous study of an analogous hexa- n-dodecyl substituted HBC molecule, and thus the aromatic cores can be assumed to also pack as in unsubstituted HBC. For the t-butyl derivative, the steric bulk of the substituents leads to a different packing arrangement as identified in a separate X-ray single crystal investigation—the complicated arrangement of aromatic and aliphatic DQ peaks is shown to be in excellent agreement with this structure. For the triphenylene-based molecule, which exists in a liquid-crystalline phase at room temperature, the dynamics is investigated by an analysis of DQ MAS spinning-sideband patterns (recorded at a 1H Larmor frequency of 700.1 MHz), with, in particular, differential mobility along the alkyl chains being identified.

Conformational analysis of endothelin-1 analogs with indolizidinone amino acids incorporated at the C-terminus.

In light of the fact that a beta-turn conformation at the C-terminus of endothelin-1 (ET-1) could be responsible for activity at the ET(B) receptor, the incorporation of (3S, 6S, 9S)-2-oxo-3-amino-1-azabicyclo[4.3.0]nonane-9-carboxylic acid (IAA) should provide an elegant method to establish whether a formyl group on Trp21 plays a role in stabilizing a beta-turn. Eight linear ET-1 analogs, four formylated and four nonformylated, ET-1-(Leu17-Asp18-IAA-Trp21); ET-1-(Leu17-IAA-Ile20-Trp21); ET-1-(Leu17-Asp18-Pro19-Ile20-Trp21) and ET-1-(Leu17-Asp 8-Ile19-Pro20-Trp21) have been analyzed by high-resolution nuclear magnetic resonance (NMR) spectroscopy and molecular modeling. Two-dimensional double quantum filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhausen enhancement spectroscopy (NOESY) were resolved and analyzed for each molecule. Interspatial distance constraints were derived from the intensity of the NOESY connectivities, The formation of hydrogen bonding was monitored from the temperature-dependence of the NH chemical shifts. Molecular models calculated by means of distance geometry, simulated annealing and energy minimization, suggested a global elongated structure for the formylated analogs and a folded arrangement for the nonformylated derivatives, but no hydrogen bonding was detected at the C-terminus of ET-1 analogs.

Conformation of Poly(ethylene oxide) Intercalated in Clay and MoS2 Studied by Two-Dimensional Double-Quantum NMR

The conformation of poly(ethylene oxide), PEO, intercalated in layered clays and MoS2 has been characterized by double-quantum solid-state NMR. Using PEO with 13% 13C−13C labeled units, the conformational statistics of the OC−CO bonds have been determined. The OC−CO bonds of the polymer in narrow interlayer gaps of ∼0.8, ∼0.85, and ∼1.0 nm thickness consistently exhibit a high gauche content of approximately 90%. This rules out models with highly trans-containing chains as found in certain complexes of PEO with HgCl2 or p-nitrophenol.

Novel opportunities for studying the short and medium range order of glasses by MAS NMR, 29Si double quantum NMR and IR spectroscopies

The glass structure in two ternary systems was investigated with respect to their applicability as electrode glasses. For Na 2O–Al 2O 3–SiO 2 glasses, a good pNa electrode signal is observed if (i) AlO − 4 units exist, linked mainly to Q 4 m units (Q n m : silicate tetrahedra, n: number of Si–O–Si bridges, m: number of Si–O–Al bridges), (ii) some other AlO − 4 units are bonded to mainly Q 3 m units and (iii) the condition 2>Q 4 m /Q 3 m >1 holds. In Na 2O–B 2O 3–SiO 2 glasses, a pNa signal is measured if the ratio of the BO − 4 units linked to BO 3 or to Q 4 is less than 1. Two-dimensional double quantum nuclear magnetic resonance (DQ NMR) spectroscopy has been applied to 29Si enriched sodium silicate glasses for measuring the connectivities of the Q n units ( m=0), enabling improved studies of the medium range order. We are able, for the first time, to distinguish between the various Q 3 and Q 4 units bonded to different numbers of Q 3 and/or Q 4 groups.

A double quantum coherence transfer proton NMR spectroscopy technique for monitoring steady‐state tumor lactic acid levels in Vivo

If proton nuclear magnetic resonance (1H NMR) spectroscopy is to provide a clinically useful modality for monitoring tumor growth and treatment, the technique must be able to unambiguously detect steady-state metabolite concentrations in human tumors and differentiate these from normal tissue levels. To address this problem, a two-dimensional double quantum coherence transfer spectroscopy (2DDQCT) method was developed and tested in a series of tumor cell lines implanted in mice. Lactate-edited proton NMR spectra were determined from a roughly 1-cm3 region of interest in EMT6, RIF-1, and fibroma. In two-dimensional data matrix representations of the 2DDQCT experiments (double quantum frequency on the vertical axis and chemical shift on the horizontal axis) the lactate signal (330 Hz with the transmitter set at the water resonance) was well-resolved from lipid (480 Hz, 600 Hz). The resolution in the double quantum dimension was also sufficient to conclude that a detectable level of alanine, which would reside at 358 Hz, was not present in the three tumor types. Following the NMR experiment, tumors were chemically assayed for lactate giving 8.17, 9.1, and 6.73 mumols/g wet wt for RIF-1, EMT6, and fibroma, respectively. This technique is likely to provide a noninvasive method for monitoring the steady-state lactic acid levels in small tumors before and after therapy, as well as in tissues with impaired oxygen delivery using clinical and research NMR systems.

Investigation of the adducts formed by reaction of malondialdehyde with adenosine

Malondialdehyde (MDA) forms oligomeric adducts with DNA bases. It has been proposed that the 2:1 MDA-guanosine (M2G) and 3:1 MDA-adenosine (M3A) adducts result from sequential addition of MDA molecules to the nucleoside base. Reaction of 1:1 MDA-guanosine (M1G) and 1:1 MDA-adenosine (M1A) adducts with MDA does not produce the multimeric adducts. This suggests they arise by reaction of the nucleoside bases with oligomers of MDA. If so, the proposed structure for M3A is inconsistent with the addition of an MDA trimer to adenosine. Therefore, we investigated the structure of this molecule. Two-dimensional double quantum filtered COSY and hetero-COSY NMR experiments were performed, and a series of insensitive nuclei assignment by polarization transfer (INAPT) NMR spectra were also recorded. The results of these experiments revealed the presence of a propano group and two alpha,beta-unsaturated aldehydes. The UV spectrum of M3A displayed a maximum at 326 nm, similar to that of N6-[3-oxo-1(E)-propenyl]adenosine (M1A). The adducts were reduced with sodium borohydride for comparison of the UV and NMR spectra. On the basis of our results, a new structure for M3A is proposed which is tentatively named 6(5*,7*-diformyl-2*H-3*,6*-dihydro-2*,6*-methano-1*,3*-oxazocin -3*-yl)-9-beta-D-ribofuranosylpurine.