Monday, August 04, 2008

Al's Update

Quantitative prediction of gas-phase 19F nuclear magnetic shielding constants
J. Chem. Phys. 128, 244111 (2008)
Michael E. Harding, Michael Lenhart, Alexander A. Auer, and Jürgen Gauss
Benchmark calculations of 19F nuclear magnetic shielding constants are presented for a set of 28 molecules. Near-quantitative accuracy (ca. 2 ppm deviation from experiment) is achieved if (1) electron correlation is adequately treated by employing the coupled-cluster singles and doubles (CCSD) model augmented by a perturbative correction for triple excitations [CCSD(T)], (2) large (uncontracted) basis sets are used, (3) gauge-including atomic orbitals are used to ensure gauge-origin independence, (4) calculations are performed at accurate equilibrium geometries [obtained from CCSD(T)/cc-pVTZ calculations correlating all electrons], and (5) vibrational averaging and temperature corrections via second-order vibrational perturbation theory (VPT2) are included. For the CCSD(T)/13s9p4d3f calculations corrected for vibrational effects, mean and standard deviation from experiment are −1.9 and 1.6 ppm, respectively. Less elaborate theoretical treatments result in larger errors. Consideration of relative shifts can reduce the mean deviation (through an appropriately chosen reference compound), but does not change the standard deviation. Density-functional theory calculations of absolute and relative 19F nuclear magnetic shielding constants are found to be, at best, as accurate as the corresponding Hartree–Fock self-consistent-field calculations and are not improved by consideration of vibrational effects. Molecular systems containing fluorine-oxygen, fluorine-nitrogen, and fluorine-fluorine bonds are found to be more challenging than the other investigated molecules for the considered theoretical methods.

Structural and Spectroscopic Impact of Tuning the Stereochemical Activity of the Lone Pair in Lead(II) Cyanoaurate Coordination Polymers via Ancillary Ligands
Inorg. Chem., 47 (14), 6353–6363, 2008
Michael J. Katz, Vladimir K. Michaelis, Pedro M. Aguiar, Renante Yson, Haiyan Lu, Harini Kaluarachchi, Raymond J. Batchelor, Georg Schreckenbach, Scott Kroeker, Howard H. Patterson, and Daniel B. Leznoff
The reaction of Pb(ClO4)2·xH2O, an ancillary ligand L, and two equivalents of Au(CN)2− gave a series of crystalline coordination polymers, which were structurally characterized. The ligands were chosen to represent a range of increasing basicity, to influence the stereochemical activity (i.e., p-orbital character) of the Pb(II) lone pair. The Pb(II) center in [Pb(1,10-phenanthroline)2][Au(CN)2]2 (1) is 8-coordinate, with a stereochemically inactive lone pair; all 8 Pb−N bonds are similar. The Au(CN)2− units propagate a 2-D brick-wall structure. In [Pb(2,2′-bipyridine)2][Au(CN)2]2 (2), the 8-coordinate Pb(II) center has asymmetric Pb−N bond lengths, indicating moderate lone pair stereochemical activity; the supramolecular structure forms a 1-D chain/ribbon motif. For [Pb(ethylenediamine)][Au(CN)2]2 (3), the Pb(II) is only 5-coordinate and extremely asymmetric, with Pb−N bond lengths from 2.123(7) to 3.035(9) Å; a rare Pb−Au contact of 3.5494(5) Å is also observed. The Au(CN)2− units connect the Pb(ethylenediamine) centers to form 1-D zigzag chains which stack via Au−Au interactions of 3.3221(5) Å to yield a 2-D sheet. 207Pb MAS NMR of the polymers indicates an increase in both the chemical shielding span and isotropic chemical shift with increasing Pb(II) coordination sphere anisotropy (from δiso = −2970 and Ω = 740 for 1 to δiso = −448 and Ω = 3980 for 3). The shielding anisotropy is positively correlated with Pb(II) p-character, and reflects a direct connection between the NMR parameters and lone-pair activity. Solid-state variable-temperature luminescence measurements indicate that the emission bands at 520 and 494 nm, for 1 and 2, respectively, can be attributed to Pb → L transitions, by comparison with simple [Pb(L)2](ClO4)2 salts. In contrast, two emission bands for 3 at 408 and 440 nm are assignable to Au−Au and Pb−Au-based transitions, respectively, as supported by single-point density-functional theory calculations on models of 3.

Cupric Siliconiobate. Synthesis and Solid-State Studies of a Pseudosandwich-Type Heteropolyanion
Inorg. Chem., 47 (17), 7834–7839, 2008

Travis M. Anderson, Todd M. Alam, Mark A. Rodriguez, Joel N. Bixler, Wenqian Xu, John B. Parise, and May Nyman
The Na+ and [Cu(en)2(H2O)2]2+ (en = ethylenediamine) salt of a pseudosandwich-type heteropolyniobate forms upon prolonged heating of Cu(NO3)2 and hydrated Na14[(SiOH)2Si2Nb16O54] in a mixed water−en solution. The structure [a = 14.992(2) Å, b = 25.426(4) Å, c = 30.046(4) Å, orthorhombic, Pnn2, R1 = 6.04%, based on 25869 unique reflections] consists of two [Na(SiOH)2Si2Nb16O54]13− units linked by six sodium cations, and this sandwich is charge-balanced by five [Cu(en)2(H2O)2]2+ complexes, seven protons, and three additional sodium atoms (all per a sandwich-type cluster). Diffuse-reflectance UV−vis indicates that there is a λmax at 383 nm for the CuII d−d transition and the 29Si MAS NMR spectrum has two peaks at −78.2 ppm (151 Hz) and −75.5 ppm (257 Hz) for the two pairs of symmetry-equivalent internal [SiO4]4– and external [SiO3(OH)]3− tetrahedra, respectively. Unlike tungsten-based sandwich-type complexes, the [Na(SiOH)2Si2Nb16O54]13− units are linked exclusively by Na+ instead of one or more d-electron metals.

On the Structure of Amorphous Calcium Carbonate—A Detailed Study by Solid-State NMR Spectroscopy
Inorg. Chem., 47 (17), 7874–7879, 2008

Holger Nebel, Markus Neumann, Christian Mayer, and Matthias Epple
The calcium carbonate phases calcite, aragonite, vaterite, monohydrocalcite (calcium carbonate monohydrate), and ikaite (calcium carbonate hexahydrate) were studied by solid-state NMR spectroscopy (1H and 13C). Further model compounds were sodium hydrogencarbonate, potassium hydrogencarbonate, and calcium hydroxide. With the help of these data, the structure of synthetically prepared additive-free amorphous calcium carbonate (ACC) was analyzed. ACC contains molecular water (as H2O), a small amount of mobile hydroxide, and no hydrogencarbonate. This supports the concept of ACC as a transient precursor in the formation of calcium carbonate biominerals.

Binding Properties of Solvatochromic Indicators [Cu(X)(acac)(tmen)] (X = PF6− and BF4−, acac− = Acetylacetonate, tmen = N,N,N′,N′-Tetramethylethylenediamine) in Solution and the Solid State
ASAP Inorg. Chem., ASAP Article, 10.1021/ic800685z Web Release Date: July 25, 2008
Shin-ichiro Noro, Nobuhiro Yanai, Susumu Kitagawa, Tomoyuki Akutagawa, and Takayoshi Nakamura
The solvatochromic indicator [Cu(acac)(tmen)(H2O)]·PF6 (1·H2O) has been synthesized and crystallographically characterized. 1·H2O binds an H2O molecule at the Cu(II) axial site, while the PF6− anion is coordination free. The binding properties of [Cu(PF6)(acac)(tmen)] (1) and [Cu(BF4)(acac)(tmen)] (2) have been investigated in solution and the solid state. The donor number of the PF6− anion (DNPF6) was determined from the UV−vis spectra of 1 in 1,2-dichloroethane. The value of DNPF6 of the PF6− anion is slightly larger than that of the tetraphenylborate anion (BPh4−), which is known as a noncoordinating anion. In the solid state, 1 and 2 reversibly bind and release H2O molecules at the Cu(II) axial sites. The coordinated H2O molecules in 2 are more easily removed than those in 1 because of the strong Lewis basicity of the BF4− anion compared to the PF6− ion. The lower melting point of 1 versus 2 is attributed to the loose binding of the PF6− anions to the Cu(II) centers, which induces the dynamic nature of the crystal.

17O Solid-State NMR and First-Principles Calculations of Sodium Trimetaphosphate (Na3P3O9), Tripolyphosphate (Na5P3O10), and Pyrophosphate (Na4P2O7)
Inorg. Chem., 47 (16), 7327–7337, 2008

Filipe Vasconcelos, Sylvain Cristol, Jean-Francois Paul, Grégory Tricot, Jean-Paul Amoureux, Lionel Montagne, Francesco Mauri, and Laurent Delevoye
The assignment of high-field (18.8 T) 17O MAS and 3QMAS spectra has been completed by use of first-principles calculations for three crystalline sodium phosphates, Na3P3O9, Na5P3O10, and Na4P2O7. In Na3P3O9, the calculated parameters, quadrupolar constant (CQ), quadrupolar asymmetry (ηQ), and the isotropic chemical shift (δcs) correspond to those deduced experimentally, and the calculation is mandatory to achieve a complete assignment. For the sodium tripolyphosphate Na5P3O10, the situation is more complex because of the free rotation of the end-chain phosphate groups. The assignment obtained with ab initio calculations can however be confirmed by the 17O{31P} MAS-J-HMQC spectrum. Na4P2O7 17O MAS and 3QMAS spectra show a complex pattern in agreement with the computed NMR parameters, which indicate that all of the oxygens exhibit very similar values. These results are related to structural data to better understand the influence of the oxygen environment on the NMR parameters. The findings are used to interpret those results observed on a binary sodium phosphate glass.

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