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We describe a new calorimeter for measuring heat capacity in the range 1–35 K, using a silicon chip bolometer as sample holder, temperature sensor, and sample heater. The apparatus is capable of measuring very small samples (1–500 mg), and may be used with a number of different experimental methods.

PROMETHEUS is a Fortran IV program system for investigation of anharmonic thermal vibrations in crystals, using elastic X-ray or neutron data. Three different anharmonictemperature-factor equations can be used, one of them up to sixth-order terms. The corresponding probability densities and mean effective atomic potentials can be calculated. Programs for correction of isotropic thermal diffuse scattering and of isotropic and anisotropic extinction are included. Fast averaging of symmetrically equivalent reflections combined with extended reflection statistics is possible. Calculations of Fourier syntheses and their errors and of Patterson functions can be carried out and the corresponding contour maps can be plotted.

Different pulse Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR) techniques are used to study the electronic and geometric structure of (tetraphenylporphyrinato)cobaltate(II) with pyridine as an axial ligand (cobaltous tetraphenylporphyrin(pyridine), CoTPP(py)). This complex is considered as a model for the heme group in deoxygenated hemoglobin and myoglobin. For the first time, the small hyperfine and nuclear quadrupole interactions of the porphyrin nitrogens of CoTPP(py) are determined using three-pulse electron spin−echo envelope modulation (ESEEM), hyperfine sublevel correlation (HYSCORE), and double nuclear coherence transfer (DONUT)−HYSCORE spectroscopy as well as Davies-ENDOR, and hyperfine correlated ENDOR (HYEND). The assignment of the different cross-peaks in the HYSCORE spectra is shown to be considerably facilitated by the DONUT−HYSCORE experiment. Furthermore, the hyperfine interactions of the surrounding protons are investigated by Davies- and Mims-ENDOR and HYSCORE spectroscopy at X- and S-band microwave frequencies. For the first time, the potential of proton HYSCORE spectroscopy at S-band is demonstrated. Assignment of the observed couplings is facilitated using deuterated pyridine as the axial ligand. The interactions with the ortho and meta protons of pyridine and with the protons of the porphyrin ligand are determined. From the Davies-ENDOR spectra of CoTPP(py) and CoTPP([15N]py), the hyperfine and nuclear quadrupole couplings of the pyridine nitrogen are evaluated. From the hyperfine and nuclear quadrupole parameters, structural (internuclear distances) and electronic information is derived. Comparisons are made with known EPR and ENDOR studies on a number of porphyrin systems with different metal ions. The similarities and differences are discussed in detail.

The P450 enzyme model 1 is a high-spin system. EPR and ENDOR spectra reveal the coordination of water to the Fe center. This is the first experimental proof that coordination of water is not the single determining factor in the stabilization of the low-spin character of the cytochrome P450 resting state.

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Cyclic delocalization of five and six electrons, respectively, in the plane of four N centers was explored by one-/two-electron reduction of more or less rigid, parallel bisdiazenes (1-7), with NdN/NdN distances (d ππ ) ranging from ∼2.8 to 5.0 Å, interorbital angles (p,p; ω) from ∼l75°to 90°, and with more or less kinetic protection. For the "proximate" substrates 1-5 (d ππ ≈ 2.8-3.2 Å, ω ≈ 175-156°), short contact with alkali metals (Li, Na, K, Cs) generates turquoise to deeply green radical anions (λ max (DME) ≈ 700-900 nm). The character of these radical anions as cyclically in-plane delocalized bishomoconjugated 4N/5e species with a high concentration of spin density between the NdN units is established by extensive UV/vis, electrochemical, and EPR measurements (CW, pulsed) at temperatures down to 8 K, and by DFT calculations (B3LYP/6-31G*). After longer exposure to the metals, the three most persistent 4N/5e radical anions (M + 1 •-, M + 2 •-, M + 5 •-) are further reduced to the red dianions (2M + 1 2-, 2M + 2 2-, 2M + 5 2-, λ max (THF) ≈ 360-430 nm). Of the nine ion pair combinations with Li + , Na + , and K + all except one (2K + 1 2-) are thermally highly persistent. For the dianions the l H, l3 C, and 7 Li NMR analyses assisted by DFT calculations confirm the 4N/6e σ-bishomoaromatic electronic structure. The reduction potentials determined (CV) for dialkyl-diazenes (8-10) and bisdiazenes (1, 2, and 5) allow an estimate of the thermodynamic stabilization of the respective radical anions and of the gain in electron delocalization energy in the dianions (σ-bishomoaromaticity).

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