Electronic properties of model quantum-dot structures in zero and finite magnetic fields

We have computed electronic structures and total energies of circularly confined two-dimensional quantum dots and their lateral dimers in zero and finite uniform external magnetic fields using different theoretical schemes: the spin-density-functional theory (SDFT), the current-and-spin-density-functional theory (CSDFT), and the variational quantum Monte Carlo (VMC) method. The SDFT and CSDFT calculations employ a recently-developed, symmetry-unrestricted real-space algorithm allowing solutions which break the spin symmetry. Results obtained for a six-electron dot in the weak confinement limit and in zero magnetic field as well as in a moderate confinement and in finite magnetic fields enable us to draw conclusions about the reliability of the more approximative SDFT and CSDFT schemes in comparison with the VMC method. The same is true for results obtained for the two-electron quantum dot dimer as a function of inter-dot distance. The structure and role of the symmetry-breaking solutions appearing in the SDFT and CSDFT calculations for the above systems are discussed. Received 16 October 2001 and Received in final form 17 January 2002     

Calculation of binary magnetic properties and potential energy curve in xenon dimer: second virial coefficient of (129)Xe nuclear shielding

Quantum chemical calculations of the nuclear shielding tensor, the nuclear quadrupole coupling tensor, and the spin-rotation tensor are reported for the Xe dimer using ab initio quantum chemical methods. The binary chemical shift delta, the anisotropy of the shielding tensor Delta sigma, the nuclear quadrupole coupling tensor component along the internuclear axis chi( parallel ), and the spin-rotation constant C( perpendicular ) are presented as a function of internuclear distance. The basis set superposition error is approximately corrected for by using the counterpoise correction (CP) method. Electron correlation effects are systematically studied via the Hartree-Fock, complete active space self-consistent field, second-order M?ller-Plesset many-body perturbation, and coupled-cluster singles and doubles (CCSD) theories, the last one without and with noniterative triples, at the nonrelativistic all-electron level. We also report a high-quality theoretical interatomic potential for the Xe dimer, gained using the relativistic effective potential/core polarization potential scheme. These calculations used valence basis set of cc-pVQZ quality supplemented with a set of midbond functions. The second virial coefficient of Xe nuclear shielding, which is probably the experimentally best-characterized intermolecular interaction effect in nuclear magnetic resonance spectroscopy, is computed as a function of temperature, and compared to experiment and earlier theoretical results. The best results for the second virial coefficient, obtained using the CCSD(CP) binary chemical shift curve and either our best theoretical potential or the empirical potentials from the literature, are in good agreement with experiment. Zero-point vibrational corrections of delta, Delta sigma, chi (parallel), and C (perpendicular) in the nu=0, J=0 rovibrational ground state of the xenon dimer are also reported.     

Computed positron lifetimes in vacancies and vacancy-iron clusters in gold

Annihilation characteristics are calculated for positrons trapped in clean and impurity decorated vacancy clusters in Au. The positron lifetime depends strongly on the structure of the clusters. In a strongly relaxed vacancy cluster, the lifetime can become smaller than the lifetime in a single vacancy. The substitution of some neighbour atoms of a vacancy cluster by Fe atoms has only a minor effect on the positron lifetimes.     

Evaluation and optimization of coherence transfer in high molecular weight systems

For very large proteins in the highest magnetic fields, the large chemical shift anisotropy (CSA) of carbonyl carbon deteriorates coherence transfer efficiency in experiments designed for unambiguous sequential backbone assignment. In this communication, coherence throughput of several TROSY experiments is evaluated. Two new experiments, MP-HNCA and HN(CO)CANH, are also introduced as attractive alternatives for sequential assignment purposes of large proteins with correlation time over 50 ns. Their theoretical coherence transfer efficiencies for the interresidual (13)C(alpha) correlations are significantly better than in recently introduced MP-CT-HNCA and sequential HNCA experiments. The improvement with the new experiments is observed already on 60.8 kDa homodimer of protein Cel6A at 800 (1)H MHz.     

Optimized pathway selection in intraresidual triple-resonance experiments

An optimized intraresidual pulse sequence element with better sensitivity and suppression of sequential cross peaks is presented. Concatenation of three magnetization transfer delays allows their independent setting, in accordance with the relaxation properties of the individual spins, without concomitantly prolonging the pulse sequence. Additionally, implementations of the scheme to HNCA, HNCACB, and the TROSY based triple-resonance experiments are proposed. The feasibility of the new element was verified by recording HNCA and HNCACB on the small 8.6 kDa protein ubiquitin. The corresponding HNCA-TROSY experiment was tested on a larger protein, the 30.4 kDa Cel6A from the thermophilic soil bacterium Thermobifida fusca at 800 (1)H MHz.     

Measurement of Homonuclear J-Couplings from Spin-State Selective Double-/Zero-Quantum Two-Dimensional NMR Spectra

¹H-detected two-dimensional double-/zero-quantum experiments are described for measurement of homonuclear ²J_HH-couplings of NH₂ or CH₂ groups in proteins. These experiments utilize multiple-quantum coherence for determination of the size and the absolute sign of the geminal scalar and dipolar couplings in the presence of broad lines. Spectra are simplified by gradient selection and spin-state selective filters.     

Synthesis and biomimetic mineralization of l‐proline substituted polyphosphazenes as bulk and nanofiber

This work presents the synthesis of polyphosphazenes bearing L ‐proline methyl ester (ProOMe) and 4‐hydroxy‐l ‐proline methyl ester (HypOMe), aiming for new bioactive polymers for bone repair. The polymers were characterized by ¹H and ³¹P NMR, FTIR, DSC, and TGA. Electrospun fibers were prepared using poly[bis(l ‐proline methyl ester)phosphazene] (PProP), and their potential for biomimetic mineralization, as well as the bulk material, were tested in simulated body fluid (1×SBF). Samples were analyzed between 24 h and 3 weeks of incubation using SEM/EDS and FTIR. After 24 h, spherical and flower‐like shapes of calcium phosphates (CaP) were crystallized on the bulk samples. The nanofibers presented spherical CaP crystals attached to them after 48 h of incubation. The Ca/P molar ratio of the crystals varied from 1.5 to 1.6. According to this study, PProP presents bioactivity in vitro, and its fibers offer sites for CaP nucleation like the collagen fibers in bone. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1318–1327