Friday, May 23, 2008

Al's Journal Update

Synthesis, NMR and Vibrational Spectroscopic Characterization, and Computational Study of the cis-IO2F32− Anion
Inorg. Chem., 47 (8), 3243–3247

Johnathan P. Mack, Jerry A. Boatz, and Michael Gerken
The N(CH3)4+ salt of the cis-IO2F32− anion was synthesized from [N(CH3)4][IO2F2] and excess [N(CH3)4][F] in CH3CN solvent. The [N(CH3)4]2[IO2F3] salt was characterized by Raman, infrared, and 19F solid-state MAS NMR spectroscopy. Geometry optimization and calculation of the vibrational frequencies at the DFT level of theory corroborated the experimental finding that the IO2F32− anion exists as a single isomer with a cis-dioxo and mer-trifluoro arrangement. The fluorine atom in IO2F32− that is trans to one of the oxygen atoms is weakly bound with a calculated bond length of 228.1 pm. The IO2F32− anion is only the second example of an AEO2F3 species after XeO2F3−.

DFT Calculation of 1J(119Sn,13C) and 2J(119Sn,1H) Coupling Constants in Di- and Trimethyltin(IV) Compounds
Inorg. Chem., 47 (11), 4796–4807, 200
Girolamo Casella, Francesco Ferrante, and Giacomo Saielli
We have tested several computational protocols, at the nonrelativistic DFT level of theory, for the calculation of 1J(119Sn,13C) and 2J(119Sn,1H) spin−spin coupling constants in di- and trimethyltin(IV) derivatives with various ligands. Quite a good agreement with experimental data has been found with several hybrid functionals and a double-ζ basis set for a set of molecules comprising tetra-, penta-, and hexa-coordinated tin(IV). Then, some of the protocols have been applied to the calculation of the 2J(119Sn,1H) of the aquodimethyltin(IV) ion and dimethyltin(IV) complex with D-ribonic acid and to the calculation of 1J(119Sn,13C) and 2J(119Sn,1H) of the dimethyltin(IV)−glycylglycine and glycylhistidine complexes in water solutions. Solvent effects have been considered in these cases by including explicit water molecules and/or the solvent reaction field, resulting in a good agreement with experimental data. The proposed protocols constitute a helpful tool for the structural determination of di- and triorganotin(IV) derivatives.

67Zn Solid-State NMR Spectroscopy of {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3]. The Importance of the Anion [HOB(C6F5)3]−
Inorg. Chem., 47 (12), 5184–5189, 2008
Andrew S. Lipton, Melissa M. Morlok, Gerard Parkin, and Paul D. Ellis
One of the paradigms of Zn2+ metallobiochemistry is that coordination of water to Zn2+ provides a mechanism of activation that involves lowering the pKa by approximately 7 pH units. This idea has become central to the development of mechanisms of action for zinc metalloproteins. However, the direct measurement of the pKa of water bound to Zn2+ in a metalloprotein has yet to be accomplished. Developing models for Zn2+−OH2 species has been a significant challenge, but we have utilized solid-state 67Zn NMR spectroscopy as a means to characterize one of the few examples of water bound to mononuclear tetrahedral Zn2+: {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3]. The measured quadrupole coupling (Cq) constant is 4.3 MHz with an asymmetry parameter of ηq of 0.6. Likewise, due to the small value of Cq, anisotropic shielding also contributed to the observed 67Zn NMR lineshape. As expected, the computed values of the magnetic resonance parameters depend critically on the nature of the anion. The predicted value of Cq for {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3] is –4.88 MHz. We discuss the results of these calculations in terms of the nature of the anion, the local electrostatics, and its subsequent hydrogen bonding to [TpBut,Me]Zn(OH2)+.

NMR implementation of adiabatic SAT algorithm using strongly modulated pulses
J. Chem. Phys. 128, 124110 (2008)

Avik Mitra, T. S. Mahesh, and Anil Kumar
NMR implementation of adiabatic algorithms face severe problems in homonuclear spin systems since the qubit selective pulses are long and during this period, evolution under the Hamiltonian and decoherence cause errors. The decoherence destroys the answer as it causes the final state to evolve to mixed state and in homonuclear systems, evolution under the internal Hamiltonian causes phase errors preventing the initial state to converge to the solution state. The resolution of these issues is necessary before one can proceed to implement an adiabatic algorithm in a large system where homonuclear coupled spins will become a necessity. In the present work, we demonstrate that by using “strongly modulated pulses” (SMPs) for the creation of interpolating Hamiltonian, one can circumvent both the problems and successfully implement the adiabatic SAT algorithm in a homonuclear three qubit system. This work also demonstrates that the SMPs tremendously reduce the time taken for the implementation of the algorithm, can overcome problems associated with decoherence, and will be the modality in future implementation of quantum information processing by NMR.

Spin dynamics in the modulation frame: Application to homonuclear recoupling in magic angle spinning solid-state NMR
J. Chem. Phys. 128, 124503 (2008)

Gaël De Paëpe, Józef R. Lewandowski, and Robert G. Griffin
We introduce a family of solid-state NMR pulse sequences that generalizes the concept of second averaging in the modulation frame and therefore provides a new approach to perform magic angle spinning dipolar recoupling experiments. Here, we focus on two particular recoupling mechanisms—cosine modulated rotary resonance (CMpRR) and cosine modulated recoupling with isotropic chemical shift reintroduction (COMICS). The first technique, CMpRR, is based on a cosine modulation of the rf phase and yields broadband double-quantum (DQ) 13C recoupling using >70 kHz ω1,c/2π rf field for the spinning frequency ωr/2=10–30 kHz and 1H Larmor frequency ω0,H/2π up to 900 MHz. Importantly, for p ≥5, CMpRR recouples efficiently in the absence of 1H decoupling. Extension to lower p values (3.5 ≤ p <>

Analyzing NMR shielding tensors calculated with two-component relativistic methods using spin-free localized molecular orbitals
J. Chem. Phys. 128, 164112 (2008)

Jochen Autschbach
A recently developed analysis method [J. Chem. Phys. 127, 124106 (2007)] for NMR spin-spin coupling constants employing two-component (spin-orbit) relativistic density functional theory along with scalar relativistic natural localized molecular orbitals (NLMOs) and natural bond orbitals (NBOs) has been extended for analyzing NMR shielding tensors. Contributions from a field-dependent basis set (gauge-including atomic orbitals) have been included in the formalism. The spin-orbit NLMO/NBO nuclear magnetic shielding analysis has been applied to methane, plumbane, hydrogen iodide, tetracholoplatinate(II), and hexachloroplatinate(IV).

Measuring distances between half-integer quadrupolar nuclei and detecting relative orientations of quadrupolar and dipolar tensors by double-quantum homonuclear dipolar recoupling nuclear magnetic resonance experiments
J. Chem. Phys. 128, 204503 (2008)
Gregor Mali, Venčeslav Kaučič, and Francis Taulelle

We studied the possibility of using double-quantum homonuclear dipolar recoupling magic angle spinning nuclear magnetic resonance experiments for structural analysis of systems of half-integer quadrupolar nuclei. We investigated symmetry-based recoupling schemes R221 and R221R22-1 and showed that the obtained double-quantum filtered signals depend substantially on magnitudes and relative orientations of dipolar and quadrupolar tensors. Experimental results measured on aluminophosphate molecular sieve AlPO4-14, containing dipolar-coupled spin-5/2 aluminum nuclei, were compared to results of time-consuming numerical simulations. The comparison for short mixing times allowed us to roughly measure internuclear Al–Al distances, if constraints about relative tensor orientations were available. Inspection of relative orientations of dipolar and quadrupolar tensors, using known distances between nuclei, required experimental and simulated data for long mixing times and yielded less accurate results. Two experimental protocols were employed for measuring double-quantum filtered curves, the symmetric protocol, in which excitation and reconversion periods are incremented simultaneously, and the asymmetric protocol, in which only the length of the excitation period is incremented and the length of the reconversion period is kept constant. The former experimental protocol was more convenient for the detection of internuclear distances, and the latter one was more appropriate for the inspection of relative orientations of interaction tensors.

Structural ordering in CdxPb1−xF2 alloys: A combined molecular dynamics and solid state NMR study
J. Chem. Phys. 128, 224705 (2008)

Adalberto Picinin, Rashmi R. Deshpande, Andrea S. S. de Camargo, José Pedro Donoso, José Pedro Rino, Hellmut Eckert, and Maurício A. P. Silva
Molecular dynamics (MD) simulations of binary CdxPb1−xF2 alloys have been carried out, using a two-body Buckingham interaction potential, leading to a correct description of structural properties as a function of composition and pointing towards an understanding of the eutectic phenomenon. The simulation data can be analyzed in terms of five local fluorine environments Q(n) (4≥n≥0), where n is the number of Pb nearest-neighbor environments. The results suggest a highly nonstatistical population distribution, suggesting an intrinsic phase segregation tendency in the undercooled melt, during the cooling process. This prediction has been tested experimentally for six representative compositions (0.2 ≤ x ≤0.7) on the basis of high-resolution 19F solid state NMR data, revealing important similarities between theory and experiment. While the NMR data confirm that the population distribution is, indeed, nonstatistical for all compositions, the results are only found to be consistent with an intrinsic segregation tendency of PbF2-rich domains. This tendency manifests itself in substatistical populations of Q(3) units, resulting in preferred Q(2) and Q(4) formations.

A study of the phase transitions and proton dynamics of the superprotonic conductor Cs5H3(SO4)4·0.5H2O single crystal with 1H and 133Cs nuclear magnetic resonance
Journal of Solid State Chemistry Volume 181, Issue 4, April 2008, Pages 796-801
Ae Ran Lim
The phase transitions and proton dynamics of Cs5H3(SO4)4·0.5H2O single crystals were studied by measuring the NMR line shape, the spin-lattice relaxation time, T1, and the spin–spin relaxation time, T2, of the 1H and 133Cs nuclei. The “acid” protons and the “water” protons in Cs5H3(SO4)4·0.5H2O were distinguished. The loss of water protons was observed above TC1, whereas the content of water protons was found to recover above TC2. Therefore, the water protons play a special role in the stability of the superprotonic phase at high temperatures. The mechanism of fast proton conduction was found to consist of hydrogen-bond proton transfer involving the breakage of the weak part of the hydrogen bond and the formation of a new hydrogen bond. Thus, these structural phase transitions probably involve significant reorientation of the SO4 tetrahedra and dynamical disorder of the hydrogen bonds between them.

Local structure and disorder in crystalline Pb9Al8O21
Journal of Solid State ChemistryVolume 181, Issue 5, May 2008, Pages 1087-1102
Alex C. Hannon, Emma R. Barney, Diane Holland and Kevin S. Knight

Crystalline Pb9Al8O21 is a model compound for the structure of non-linear optical glasses containing lone-pair ions, and its structure has been investigated by neutron powder diffraction and total scattering, and 27Al magic angle spinning NMR. Rietveld analysis (space group
(No. 205), a=13.25221(4) Å) shows that some of the Pb and O sites have partial occupancies, due to lead volatilisation during sample preparation, and the non-stoichiometric sample composition is Pb9−δAl8O21−δ with δ=0.54. The NMR measurements show evidence for a correlation between the chemical shift and the variance of the bond angles at the aluminium sites. The neutron total correlation function shows that the true average Al–O bond length is 0.8% longer than the apparent bond length determined by Rietveld refinement. The thermal variation in bond length is much smaller than the thermal variation in longer interatomic distances determined by Rietveld refinement. The total correlation function is consistent with an interpretation in which AlO3 groups with an Al–O bond length of 1.651 Å occur as a result of the oxygen vacancies in the structure. The width of the tetrahedral Al–O peak in the correlation function for the crystal is very similar to that for lead aluminate glass, indicating that the extent of static disorder is very similar in the two phases.

Exploring Helical Folding of Oligoureas During Chain Elongation by High-Resolution Magic-Angle-Spinning (HRMAS) NMR Spectroscopy

Chem. Eur. J. Volume 14 Issue 13, Pages 3874 - 3882

Aude Violette, Dr., Nathalie Lancelot, Dr., Alexander Poschalko, Dr., Martial Piotto, Dr., Jean-Paul Briand, Dr., Jesus Raya, Dr., Karim Elbayed, Dr., Alberto Bianco, Dr., Gilles Guichard, Dr. Abstract:
The development of novel folding oligomers (foldamers) for biological and biomedical applications requires both precise structural information and appropriate methods to detect folding propensity. However, the synthesis and the systematic conformational investigation of large arrays of oligomers to determine the influence of factors, such as chain length, side chains, and surrounding environment, on secondary structure can be quite tedious. Herein, we show for 2.5-helical N,N-linked oligoureas (-peptide lineage) that the whole process of foldamer characterization can be accelerated by using high-resolution magic-angle-spinning (HRMAS) NMR spectroscopy. This was achieved by monitoring a simple descriptor of conformational homogeneity (e.g., chemical shift difference between diastereotopic main chain CH2 protons) at different stages of oligourea chain growth on a solid support. HRMAS NMR experiments were conducted on two sets of oligoureas, ranging from dimer to hexamer, immobilized on DEUSS, a perdeuterated poly(oxyethylene)-based solid support swollen in solvents of low to high polarity. One evident advantage of the method is that only minute amount of material is required. In addition, the resonance of the deuterated resin is almost negligeable. On-bead NOESY spectra of high quality and with resolution comparable to that of liquid samples were obtained for longer oligomers, thus allowing detailed structural characterization.

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