Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

Extensive ab initio calculations of the phenolâ‹…H₂O complex were performed at the Hartree–Fock level, using the 6â€�31G(d,p) and 6â€�311++G(d,p) basis sets. Fully energyâ€�minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H₂O plane orthogonal to the phenol plane, similar to (H₂O)₂; (b) the lowestâ€�energy transition state structure, which is nonplanar (C₁ symmetry) and has the H₂O moiety rotated by ±90°. The calculated MP2/6â€�311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intraâ€� and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenolâ‹…H₂O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β₂ intermolecular wag. Furthermore, the H₂O torsion Ï„ around the Hâ€�bond axis, and the β₂ mode are strongly anharmonically coupled, and a twoâ€�dimensional Ï„/β₂ potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of hâ€�phenolâ‹…H₂O and dâ€�phenolâ‹…D₂O employing twoâ€�color resonanceâ€�twoâ€�photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β₁ and β₂ wags of both isotopomers in the S₀ and S₁ electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S₀ state in conjunction with the excitation of the intermolecular σ and β₂ modes.

A combined experimental and theoretical study of the 2â€�naphtholâ‹…H₂O/D₂O system was performed. Two different rotamers of 2â€�naphthol (2â€�hydroxynaphthalene, 2HN) exist with the O–H bond in cisâ€� and transâ€�position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6â€�31G(d,p) basis set, fully energyâ€�minimized geometries were computed for both cisâ€� and transâ€�2HNâ‹…H₂O of (a) the equilibrium structures with transâ€�linear Hâ€�bond arrangement and C_s symmetry and (b) the lowestâ€�energy transition states for H atom exchange on the H₂O subunit, which have a nonplanar C₁ symmetry. Both equilibrium and transition state structures are similar to the corresponding phenolâ‹…H₂O geometries. The Hâ€�bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenolâ‹…H₂O at the same level of theory. Extension of the aromatic Ï€â€�system therefore hardly affects the Hâ€�bonding conditions. The barrier height to internal rotation around the Hâ€�bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6â€�31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and Mâ€�matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β₂ wag mode shows the largest anharmonic contributions. For the torsional mode Ï„ (H₂O Hâ€�atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonantâ€�twoâ€�photon ionization and dispersed fluorescence emission spectra of 2HNâ‹…H₂O and dâ€�2HNâ‹…D₂O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu²⁺ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.