Comprehensive Solid-State NMR Characterization of Electronic Structure in Ditechnetium Heptoxide Herman Cho,* ,† Wibe A. de Jong, ‡ Alfred P. Sattelberger, §,| Fre ´de ´ ric Poineau, | and Kenneth R. Czerwinski | Fundamental and Computational Sciences Directorate and EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, Energy Engineering and Systems Analysis Directorate, Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry and Harry Reid Center for EnVironmental Studies, UniVersity of NeVada Las Vegas, 4505 Maryland Parkway, Las Vegas, NeVada 89154 Received June 28, 2010; E-mail: hm.cho@pnl.gov Abstract: A relativistic density functional theory description of the electronic structure of Tc 2 O 7 has been evaluated by comparison with solid-state 99 Tc and 17 O NMR spectroscopic data (the former isotope is a weak  emitter). Every site in the molecule can be populated by a nucleus with favorable NMR characteristics, providing the rare opportunity to obtain a comprehensive set of chemical shift and electric field gradient tensors for a small molecular transition-metal oxide. NMR parameters were com- puted for the central molecule of a (Tc 2 O 7 ) 17 cluster using standard ZORA-optimized all-electron QZ4P basis sets for the central molecule and DZ basis sets for the surrounding atoms. The magnitudes of the predicted tensor principal values appear to be uniformly larger than those observed experimentally, but the discrepancies were within the accuracy of the approximation methods used. The convergence of the calculated and measured NMR data suggests that the theoretical analysis has validity for the quantitative understanding of structural, magnetic, and chemi- cal properties of Tc(VII) oxides in condensed phases. The solid-state structures of Tc 2 O 7 and Re 2 O 7 feature a distinctive linear oxo bridge and pyramidal -MO 3 end groups. 1-3 These heptavalent oxides are also notable for displaying weak temperature- independent paramagnetism (TIP) despite having d 0 electronic configurations. 4 In the case of Tc 2 O 7 , the favorable properties of the 100%-abundant 99 Tc isotope (I ) 9 / 2 ; γ ) 9.583 × 2π × 10 6 rad s -1 T -1 ) and the ability to synthesize samples with 17 O enrichment make it possible to measure the electric field gradient (EFG) and chemical shift tensors of every atom in the molecule by solid-state NMR spectroscopy. Such data provide an excellent basis for determining the electron distributions and states of this molecular transition-metal oxide in its entirety. Room-temperature spectra of polycrystalline Tc 2 O 7 with and without 17 O enrichment were recorded in a magnetic field of 7.04 T[ν L ( 99 Tc) ) 67.565 MHz; ν L ( 17 O) ) 40.722 MHz], and the results were compared to theoretical predictions (Figures 1 and 2). The dominant feature in both sets of spectra is the + 1 / 2 T - 1 / 2 powder line shape. The simulated line shapes are functions of the nuclide’s EFG and chemical shift tensors (Table 1) and the three Euler angles relating the principal axes of the two tensors. Methods for computing spectra given these inputs have been described previ- ously. 7 Shielding and EFG tensors and scalar coupling constants were computed with the Amsterdam Density Functional (ADF version 2008.01) software 8 using the spin-orbit zeroth-order relativistic approximation (ZORA) 9 and the Perdew91 generalized gradient approximation density functional. 10 Calculations with a simple local density functional yielded results similar to Perdew91. The modeled system was a cluster of 17 molecules (153 atoms) based on the structure data of Krebs. 3 NMR properties were calculated for the central molecule of the cluster (Figure S2 in the Supporting Information) using standard ZORA-optimized all-electron QZ4P basis sets, while the surrounding atoms were described by DZ basis sets with a frozen core up to 1s and 3d for O and Tc, respectively. The numerical integration accuracy parameter was set to 6.0 throughout. Shielding values for nuclei in the reference compounds were obtained with the same Perdew91 and QZ4P basis sets, and the aqueous solution environment was approximated using the COSMO model 11 with a dielectric constant of 78.8 and radii † Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory. ‡ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory. § Argonne National Laboratory. | University of Nevada Las Vegas. Figure 1. (a) Predicted and (b, c) experimental 99 Tc powder spectra of Tc 2 O 7 with natural oxygen isotope levels. Spectrum (a) was computed with nonideal pulse parameters matching those used in the experiment. The arrows in (c) point to features assigned to the extreme shoulders of the ( 3 / 2 T ( 1 / 2 powder line shapes. Published on Web 08/31/2010 10.1021/ja105687j  2010 American Chemical Society 13138 9 J. AM. CHEM. SOC. 2010, 132, 13138–13140