There has recently been a huge renaissance in the study of the magnetism of 4f-coordination complexes.[1] There have been remarkable results, such as slow relaxation of magnetization in the "single-ion magnets" (Bu 4 N)[Tb(Pc) 2 ] (H 2 Pc = phthalocyanine), for which the thermal energy barrier for relaxation is 330 K.[2] Equally remarkable has been the slow relaxation brought about by the toroidal arrangement of local magnetization vectors in a {Dy 3 } triangle ("spin chirality"). [3] In parallel, studies of polymetallic dysprosium cages have shown slow relaxation in a variety of cages with energy barriers as high as 200 K, [4a] and showing magnetic hysteresis to 8 K.[4b]Much of the fascinating physics of (Bu 4 N)[Tb(Pc) 2 ][2] and other single-ion magnets, such as Na 9 [Er(W 5 O 18 ) 2 ], [5] is associated with their fourfold symmetry. Equally, the toroidal magnetism of the {Dy 3 } cage is associated with the triangular array of 4f-ions.[3] Therefore, we targeted a molecule that had fourfold symmetry and metal triangles. The obvious polyhedron is a square-based pyramid. Oxo-centered {Ln 5 } pyramids of general formula [Ln 5 (m 5 -O)(m 3 -OR) 4 (m 2 -OR) 4 (OR) 5 ] are known for around half of the lanthanoids (R = iPr, Ln = Nd, [6,7] Eu, [8] Gd, [6] Er, [6,9] Yb;[10] R = tBu, Ln = La, [11] Nd [11,12] ) but not with dysprosium. The iso-propoxide-bridged dysprosium square-based pyramid [Dy 5 O(OiPr) 13 ] (1) is made by the reaction of freshly generated KOiPr with DyCl 3 in iPrOH/toluene with a stoichiometric amount of H 2 O (see the Experimental Section). Crystals of 1 form in two different crystal systems: one is isostructural with the previously reported [9] {Er 5 } cage whereas the second has a new unit cell.[13] Both polymorphs have essentially identical magnetic behavior. There is no evidence, either visual or by X-ray diffraction, [14] that samples ever contain a mixture of polymorphs, that is, we have studied pure samples of each. Polymorphs have been previously reported for lanthanide alkoxides. [6,9] In the new structure the square-based pyramid is disordered, with four of the Dy sites common to both disorder forms. The final Dy site is 50:50 disordered over two positions, however it is clear that the square-based pyramid is slightly elongated (Figure 1), with the average distance between the apical dysprosium and the basal dysprosia 3.43 , while the average distance between the adjacent dysprosia within the basal plane is 3.37 in one of the two models and 3.40 in the second. The structure has no crystallographic symmetry.All Dy sites are six-coordinate. The geometry at each site is based on octahedral, but with the Dy shifted towards the terminal alkoxide and away from the central m 5 -oxide-thus each Dy site has local, but non-crystallographic, C 4v symmetry. The distances of the central oxide to the Dy sites fall in the range 2.25-2.60 . The thirteen alkoxides fall into three groups: there is a terminal alkoxide on each metal site; a second group of four alkoxides bridges on each of the four triangul...
Recent studies suggested that X-ray photoelectron spectroscopy (XPS) sensitively determines the protonation state of nitrogen functional groups in the solid state, providing a means for distinguishing between co-crystals and salts of organic compounds. Here we describe how a new theophylline complex with 5-sulfosalicylic acid dihydrate was established as a salt by XPS prior to assignment with conventional methods. The presence of a C=NH(+) (N9) N1s peak in XPS allows assignment as a salt, while this peak is clearly absent for a theophylline co-crystal. The large low frequency shift for N9 observed by (15)N solid-state nuclear magnetic resonance spectroscopy (ssNMR) and corresponding density functional theory (DFT) calculations confirm that protonation has occurred. The crystal structure and further analytical studies confirm the conclusions reached with XPS and ssNMR. This study demonstrates XPS as an alternative technique for determining whether proton transfer has occurred in acid-base complexes.
Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antiferromagnetic octanuclear rings where a finite total spin of the ground state is obtained by substituting one Cr 3+ ion (s = 3/2) with Zn (s = 0), Mn (s = 5/2) or Ni (s = 1) di-cations. Energy and intensity measurements for several intra-multiplet and inter-multiplet magnetic excitations allow us to determine the spin wavefunctions of the investigated clusters. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the energy and intensity of magnetic excitations in the neutron spectra. On the contrary to what is observed for the parent homonuclear Cr8 ring, the symmetry of the first excited spin states is such that anticrossing conditions with the ground state can be realized in the presence of an external magnetic field. Heterometallic Cr7M wheels are therefore good candidates for macroscopic observations of quantum effects.
Proposals for systems embodying condensed matter spin qubits cover a very wide range of length scales, from atomic defects in semiconductors all the way to micron-sized lithographically defined structures. Intermediate scale molecular components exhibit advantages of both limits: like atomic defects, large numbers of identical components can be fabricated; as for lithographically defined structures, each component can be tailored to optimise properties such as quantum coherence. Here we demonstrate what is perhaps the most potent advantage of molecular spin qubits, the scalability of quantum information processing structures using bottom-up chemical self-assembly. Using Cr 7 Ni spin qubit building blocks, we have constructed several families of two-qubit molecular structures with a range of linking strategies. For each family, long coherence times are preserved, and we demonstrate control over the inter-qubit quantum interactions that can be used to mediate two-qubit quantum gates. INTRODUCTIONAn information processing device whose elements are capable of storing and processing quantum superposition states (a quantum computer) would support algorithms for useful tasks such as searching 1 and factoring 2 that are much more efficient than the corresponding classical algorithms, 3 and would allow efficient simulation of other quantum systems. 4 One of the key challenges in realizing a quantum computer lies in identifying a physical system that hosts quantum states sufficiently coherently, and provides appropriate interactions for implementing logic operations.5 Among the molecular spin systems that have been proposed as qubit candidates are N@C 60, 6-9 organic radicals
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