We have studied a model compound for oxyhemoglobin and oxymyoglobin, the iron-dioxygen complex of "picket fence porphyrin" (5,10,15,a,a,porphyrinato)iron (II) ((1-Melm)02), as well as oxymyoglobin and oxyhemoglobin themselves, by using 170 solid-state nuclear magnetic resonance spectroscopy. For the model picket fence porphyrin, the principal components of the chemical shift tensors for both bridging and terminal oxygens in the Fe-02 unit have been determined, and the isotropic chemical shifts occur at 1200-1600 and 2000 ppm, respectively, somewhat deshielded from the -1750 and ~2500 ppm values found by Gerothanassis et al. in solution {J. Am. Chem. Soc. 1989, III, 7006-7012).The anisotropies of the shift tensors are very large for both oxygens ( = ~2200 ppm for the bridging oxygen and = 3350 ppm for the terminal oxygen, at 77 K). From partial averaging of the shift tensors at room temperature, due to fast axial rotation of the dioxygen ligand, an Fe-O-O bond angle of ~140°has been derived for the model system. Temperature dependence studies indicate essentially no change in the isotropic chemical shift of the terminal oxygen down to 4.2 K, while there is an apparent low-frequency shift of the bridging oxygen on cooling to 77 K, possibly due to the freezing in of one conformational substate. Spectra of oxymyoglobin and oxyhemoglobin, at 77 K, are very similar to those of the model compound at low temperature. Our results indicate that the 170 nuclear quadrupole coupling constants must be relatively small for both oxygens (55 MHz) in all systems, much smaller than the 8.5and 20-MHz values found for ozone, suggesting extensive ir-delocalization in the Fe-O-O fragment. Our results are also consistent with an overwhelmingly spin paired configuration, both in the model system, and in oxyhemoglobin and oxymyoglobin themselves.
We have obtained ' 0 nuclear-magnetic-resonance spectra of powder samples of YBa2Cu307and EuBa2Cu30& -, as well as magnetically ordered YBa2Cu307-". Two major features are observed in YBa2Cu307 -. 'a sharp resonance at =1800 ppm (from external H2' 0, International Union of Pure and Applied Chemistry 8 scale) and a broader series of features centered at =400 ppm. The 1800-ppm feature undergoes a diamagnetic shift of =800 ppm on cooling to 77 K, and a similar magnitude shift on Eu substitution, suggesting assignment to the plane oxygens, O(2, 3). Measurements on magnetically ordered samples at 8.45 and 11. 7 T give the magnitude of the diagonal terms of the electric-field-gradient tensor, which are 2.3, 3.5, and 5.8 MHz. For the column oxygen, O(1), we find e q2g/h 7.7 MHz, with a chemical shift anisotropy of =660 ppm.All of the new high-T, superconducting oxides' are, in principle, excellent candidates for study using ' 0 nuclear-magnetic-resonance (NMR) spectroscopy. We show in this Rapid Communication that such studies are indeed possible, and by using magnetically aligned materials, ' we are able to investigate both chain and plane sites in ' 0-labeled YBa2Cu307 . We observe large changes in the ' 0 resonance frequency shifts (of the plane oxygens) upon formation of the superconducting state. Additionally, we have determined the nuclear quadrupole coupling constants (e qQ/h), electric-field-gradient tensor asymmetry parameters (rl), and chemical shift anisotropies (t5, 8) for selected oxygen sites. Given the ubiquitous nature of oxygen in high-T, superconductors, such atomic level probes of both magnetism and structure oN'er considerable hope for elucidating the details of superconductivity in these systems. Unlabeled YBa2Cu307 -, (and EuBa2Cu 307 -") was prepared from Y203(Eu203), BaCO3, and CuO (99.99%,
360ChemInform Abstract (50 refs.).
Low temperature diffractometer below 1 K by a 3He4He dilution refrigerator used for synchrotron radiation xray diffraction Rev.We report an NMR probe that we use in ultra low temperature experiments in a top-loading dilution refrigerator. The probe is thermally anchored to the 1.2 K pumped 4He pot and is thermally isolated from the sample located inside a Kel-F cup containing liquid 3He. The probe is adapted from a standard double resonance probe, using air trimmer capacitors, a i1/4 cable, a homemade saddle coil, and metal film resistors. The double resonance design breaks the probe's tuning ranges into two bands. The high frequency band tunes from 100 to 150 MHz covering all the nuclei with large gyromagnetic ratios, 3H, 19F, 'H, 3He. The low frequency band tunes for 17 to 54 MHz covering all nuclei with gyromagnetic ratios between 133Cs and '19Sn. The air trimmer capacitors have an open structure that allows efficient pumping and the metal film resistors are used to reduce the Q of the probe. This probe has been successfullv used for a wide range of nuclei including 'H, 19F, 3He, '19Sn, '17Sn, "'Sn, "B, 13C, 29Si, 27A1; and 2H.
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