Hamiltonian of the V15 spin system from first-principles density-functional calculations

We report first-principles all-electron density-functional-based studies of the electronic structure, magnetic ordering, and anisotropy for the V15 molecular magnet. From these calculations, we determine a Heisenberg Hamiltonian with five antiferromagnetic and one ferromagnetic exchange couplings. We perform direct diagonalization to determine the temperature dependence of the susceptibility. This Hamiltonian reproduces the experimentally observed spin S = 1/2 ground state and low-lying S = 3/2 excited state. A small anisotropy term is necessary to account for the temperature independent part of the magnetization curve.     

Fourth-order magnetic anisotropy and tunnel splittings in Mn(12) from spin-orbit-vibron interactions

From density-functional-theory based methods, we calculate the vibrational spectrum of the Mn(12)O(12)(COOH)(16)(H(2)O)(4) molecular magnet. Calculated infrared intensities are in accord with experimental studies. There have been no ab initio attempts at determining which interactions account for the fourth-order anisotropy. We show that vibrationally induced distortions of the molecule contribute to the fourth-order anisotropy Hamiltonian and that the magnitude and sign of the effect (-6.2 K) is in good agreement with all experiments. Vibrationally induced tunnel splittings in isotopically pure and natural samples are predicted.     

Backbone dynamics in the DNA HhaI protein binding site

The dynamics of the phosphodiester backbone in the [5'-GCGC-3'] 2 moiety of the DNA oligomer [d(G 1A 2T 3A 4 G 5 C 6 G 7 C 8T 9A 10T 11C 12)] 2 are studied using deuterium solid-state NMR (SSNMR). SSNMR spectra obtained from DNAs nonstereospecifically deuterated on the 5' methylene group of nucleotides within the [5'-GCGC-3'] 2 moiety indicated that all of these positions are structurally flexible. Previous work has shown that methylation reduces the amplitude of motion in the phosphodiester backbone and furanose ring of the same DNA, and our observations indicate that methylation perturbs backbone dynamics through not only a loss of mobility but also a change of direction of motion. These NMR data indicate that the [5'-GCGC-3'] 2 moiety is dynamic, with the largest amplitude motions occurring nearest the methylation site. The change of orientation of this moiety in DNA upon methylation may make the molecule less amenable to binding to the HhaI endonuclease.     

Semiconducting TlSr2RCu2O7 (R=rare earth) and its superconducting derivatives

Semiconducting TlSr₂RCu₂O₇ (R=Pr or Er) with a 1212-type structure has been synthesized in the single-phase form. Partial substitution of Sr²⁺ for R³⁺ converts this semiconductor to a 90 K superconductor TlSr₂(R_1–y Sr _y )Cu₂O₇. A combination substitution, Sr²⁺ for R³⁺ and Pb⁴⁺ for Tl³⁺, leads to the Ca-free 100 K superconductor (Tl, Pb)Sr₂(R, Sr)Cu₂O₇. The results are explained in the framework of the mixed Cu²⁺/Cu³⁺ valence.