Monday, July 26, 2010

J Phys Chem A

Triple-Decker Sandwiches and Related Compounds of the First-Row Transition Metals Containing Cyclopentadienyl and Benzene Rings

Haibo Liu, Qian-shu Li, Yaoming Xie, R. Bruce King and Henry F. Schaefer

J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp104689r
Publication Date (Web): July 21, 2010

The triple-decker sandwich compound trans-Cp2V2(η6:η6-μ-C6H6) has been synthesized, as well as “slipped” sandwich compounds of the type trans-Cp2Co2(η4:η4-μ-arene) and the cis-Cp2Fe2(η4:η4-μ-C6R6) derivatives with an Fe−Fe bond (Cp = η5-cyclopentadienyl). Theoretical studies show that the symmetrical triple-decker sandwich structures trans-Cp2M2(η6:η6-μ-C6H6) are the global minima for M = Ti, V, and Mn but lie 10 kcal/mol above the global minimum for M = Cr. The nonbonding M···M distances and spin states in these triple decker sandwich compounds can be related to the occupancies of the frontier bonding molecular orbitals. The global minimum for the chromium derivative is a singlet spin state cis-Cp2Cr2(η4:η4-μ-C6H6) structure with a very short CrCr distance of 2.06 Å, suggesting a formal quadruple bond. A triplet state cis-Cp2Cr2(η4:η4-μ-C6H6) structure with a predicted Cr≡Cr distance of 2.26 Å lies only 3 kcal/mol above this global minimum. For the later transition metals the global minima are predicted to be cis-Cp2M2(η6:η6-μ-C6H6) structures with a metal−metal bond, rather than triple decker sandwiches. These include singlet cis-Cp2Fe2(η4:η4-μ-C6H6) with a predicted Fe═Fe double bond distance of 2.43 Å, singlet cis-Cp2Co2(η3:η3-μ-C6H6) with a predicted Co—Co single bond distance of 2.59 Å, and triplet cis-Cp2Ni2(η3:η3-μ-C6H6) with a predicted Ni—Ni distance of 2.71 Å.

J. Phys. Chem. C, vol.114, Issue 29

Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the Templated Synthesis and Characterization of the Aluminophosphate Molecular Sieve STA-2

Maria Castro†, Valerie R. Seymour†, Diego Carnevale†, John M. Griffin†, Sharon E. Ashbrook*†, Paul A. Wright*†, David C. Apperley‡, Julia E. Parker§, Stephen P. Thompson§, Antoine Fecant and Nicolas Bats

J. Phys. Chem. C, 2010, 114 (29), pp 12698–12710
DOI: 10.1021/jp104120y
Copyright © 2010 American Chemical Society

Abstract: Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO4 form of STA-2, bis-diazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state 13C and 15N MAS NMR, supported by 14N and 1H-15N HMQC solution NMR and density functional calculations. As-prepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al12P12O48(OH)2·BDAB, P1, a = 12.3821(2) Å, b = 12.3795(2) Å, c = 12.3797(3) Å, α = 63.3585(8)°, β = 63.4830(7)°, γ = 63.4218(7)°) with the distortion from rhombohedral R symmetry resulting from the partial order of hydroxide ions in bridging Al−OH−Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO4 STA-2 with a pore volume of 0.22 cm3 g−1 (R, Al36P36O144, a = 12.9270(2) Å, c = 30.7976(4) Å). Dehydrated calcined AlPO4 STA-2 has two crystallographically distinct P and Al sites: 31P MAS NMR resolves the two distinct P sites, and although 27Al MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D 27Al → 31P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via Al−O−P connections in a 3:1 ratio (and vice versa for P linked to different Al). The 27Al and 31P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO4(SAT) framework.

Propane Aromatization on Zn-Modified Zeolite BEA Studied by Solid-State NMR in Situ

Anton A. Gabrienko†, Sergei S. Arzumanov†, Dieter Freude‡ and Alexander G. Stepanov*†
J. Phys. Chem. C, 2010, 114 (29), pp 12681–12688
DOI: 10.1021/jp103580f
Copyright © 2010 American Chemical Society

Abstract:The conversion of propane (propane-1-13C and propane-2-13C) on Zn/H-BEA zeolite at 520−620 K has been studied by 1H and 13C (CP) MAS NMR. Propene adsorption complex with zinc sites (π-complex) and σ-allylzinc species as intermediates have been identified in the course of propane conversion to aromatics. The mechanism leading to the formation of methane and ethane, which are constituents of an undesirable route in propane conversion, has been examined by kinetic modeling of the expected reaction network based on in situ 1H MAS NMR kinetic measurements of the reaction performance. The pathways for propane aromatization and hydrogenolysis have been proposed. Hydrogenolysis of propane has been concluded to occur with the involvement of both Brønsted acid sites and Zn sites.

13C Chemical Shift of Adsorbed Acetone for Measuring the Acid Strength of Solid Acids: A Theoretical Calculation Study

Hanjun Fang†‡, Anmin Zheng*†, Yueying Chu†‡ and Feng Deng*†
J. Phys. Chem. C, 2010, 114 (29), pp 12711–12718
DOI: 10.1021/jp1044749
Copyright © 2010 American Chemical Society

Abstract: Adsorption of basic probe molecules is one of the widely used methods to characterize the acid strength of solid acids. In this contribution, the adsorptions of acetone on various Brønsted and Lewis acid sites (from weak acid to superacid) are theoretically studied, in order to elucidate the quantitative relationships between 13C chemical shifts of acetone and intrinsic acid strength of solid acids. The Brønsted acid sites are represented by a series of 8T zeolite models with varying terminal Si−H bond lengths, and the different extents of acidic proton transfer from these acid sites to acetone are revealed explicitly. We found that three adsorption conformations (hydrogen-bonded, proton-shared, and ion-pair) exist for acetone, and concurrently, a correlation of three-broken lines is obtained for the 13C chemical shift of acetone versus the deprotonation energy (DPE). The correlation can be used as a scale for quantitatively measuring the Brønsted acid strength of solid acids. A threshold of 245 ppm is determined for superacidity, in good agreement with the experimental value (244 ppm). The Lewis acid sites are modeled by tricoordinate framework aluminum species and various extra-framework aluminum cations or neutral species such as Al3+, AlO+, AlOH2+, Al(OH)2+, Al(OH)3, and AlOOH. We found that acetone is coordinately adsorbed on the aluminum atoms of Lewis acid sites and that the 13C chemical shift of acetone is almost linear to the lowest unoccupied molecular orbital (LUMO) energy of the acid sites.

Tuesday, July 20, 2010

J. Phys. Chem C. v. 114, issue 28

45Sc Spectroscopy of Solids: Interpretation of Quadrupole Interaction Parameters and Chemical Shifts

Mara D. Alba*†, Pablo Chain†, Pierre Florian‡ and Dominique Massiot‡

J. Phys. Chem. C, 2010, 114 (28), pp 12125–12132
Publication Date (Web): June 28, 2010

Abstract: The aims of the present study is to describe for the first time the 45Sc MAS NMR spectra of X2-Sc2SiO5 and C-Sc2Si2O7, to combine the spectroscopic information with the structures published from diffraction data, and to propose a rational interpretation of the chemical shifts and quadrupolar parameters. For that purposed, we have correlated the experimental quadrupole coupling parameters of 45Sc determined for a number of scandium compounds to those found by a simple electrostatic calculation and we have found that the isotropic chemical shift of the 45Sc is linearly correlated to the shift parameter, calculated by bond-valence theory. We also show that a simple point charge calculation can approximate the electric field gradient to a sufficiently good approximation that it provides a valuable mean to assign the NMR spectra.

Nuclear Magnetic Resonance Study of Reorientational Motion in α-Mg(BH4)2
Alexander V. Skripov*†, Alexei V. Soloninin†, Olga A. Babanova†, Hans Hagemann‡ and Yaroslav Filinchuk§
J. Phys. Chem. C, 2010, 114 (28), pp 12370–12374
Copyright © 2010 American Chemical Society

Abstract: To study the reorientational motion of BH4 groups in the low-temperature (α) phase of Mg(BH4)2, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin−lattice relaxation rates in this compound over wide ranges of temperature (82−443 K) and resonance frequency (14−90 MHz for 1H and 14−28 MHz for 11B). It is found that the thermally activated reorientational motion in α-Mg(BH4)2 is characterized by a coexistence of at least three jump processes with strongly differing activation energies. Taking into account the anisotropy of the local environment of BH4 groups in α-Mg(BH4)2, these jump processes can be attributed to different types of reorientation. The nearly linear coordination of BH4 groups by two Mg atoms suggests that the fastest jump process corresponds to the rotation around the 2-fold axis connecting B and two Mg atoms, whereas the slowest process is associated with the rotation around two other 2-fold axes perpendicular to the Mg−B−Mg line.

Thursday, July 15, 2010

Journal of Magnetic Resonance ASAP

A bit of shameless self-promotion:

Journal of Magnetic Resonance
Article in Press

The application of frequency swept pulses for the acquisition of nuclear quadrupole resonance spectra
Aaron J. Rossinia, Hiyam Hamaeda and Robert W. Schurko, a,

Abstract:The acquisition of nuclear quadrupole resonance (NQR) spectra with wideband uniform rate and smooth truncation (WURST) pulses is investigated. 75As and 35Cl NQR spectra acquired with the WURST echo sequence are compared to those acquired with standard Hahn-echo sequences and echo sequences which employ composite refocusing pulses. The utility of WURST pulses for locating NQR resonances of unknown frequency is investigated by monitoring the integrated intensity and signal to noise of 35Cl and 75As NQR spectra acquired with transmitter offsets of several hundreds kilohertz from the resonance frequencies. The WURST echo sequence is demonstrated to possess superior excitation bandwidths in comparison to the pulse sequences which employ conventional monochromatic rectangular pulses. The superior excitation bandwidths of the WURST pulses allows for differences in the characteristic impedance of the receiving and excitation circuits of the spectrometer to be detected. Impedance mismatches have previously been reported by Marion and Desvaux [D.J.Y. Marion, H. Desvaux, J. Magn. Reson. (2008) 193(1) 153–157] and Muller et al. [M. Nausner, J. Schlagnitweit, V. Smrecki, X. Yang, A. Jerschow, N. Muller, J. Magn. Reson. (2009) 198(1) 73–79]. In this regard, WURST pulse sequences may afford an efficient new method for experimentally detecting impedance mismatches between receiving and excitation circuits, allowing for the optimization of solids and solution NMR and NQR spectrometer systems. The use of the Carr–Purcell Meiboom–Gill (CPMG) pulse sequence for signal enhancement of NQR spectra acquired with WURST pulses and conventional pulses is also investigated. Finally, the utility of WURST pulses for the acquisition of wideline NQR spectra is demonstrated by acquiring part of the 63/65Cu NQR spectrum of CuCN.

Monday, July 12, 2010

Magn. Reson. Chem. - July 2010

New perspectives in the PAW/GIPAW approach: JP-O-Si coupling constants, antisymmetric parts of shift tensors and NQR predictions

from Magnetic Resonance in Chemistry by Christian Bonhomme, Christel Gervais, Cristina Coelho, Frédérique Pourpoint, Thierry Azaïs, Laure Bonhomme-Coury, Florence Babonneau, Guy Jacob, Maude Ferrari, Daniel Canet, Jonathan R. Yates, Chris J. Pickard, Siân A. Joyce, Francesco Mauri, Dominique Massiot

In 2001, Pickard and Mauri implemented the gauge including projected augmented wave (GIPAW) protocol for first-principles calculations of NMR parameters using periodic boundary conditions (chemical shift anisotropy and electric field gradient tensors). In this paper, three potentially interesting perspectives in connection with PAW/GIPAW in solid-state NMR and pure nuclear quadrupole resonance (NQR) are presented: (i) the calculation of J coupling tensors in inorganic solids; (ii) the calculation of the antisymmetric part of chemical shift tensors and (iii) the prediction of 14N and 35Cl pure NQR resonances including dynamics. We believe that these topics should open new insights in the combination of GIPAW, NMR/NQR crystallography, temperature effects and dynamics. Points (i), (ii) and (iii) will be illustrated by selected examples: (i) chemical shift tensors and heteronuclear 2JP[bond]O[bond]Si coupling constants in the case of silicophosphates and calcium phosphates [Si5O(PO4)6, SiP2O7 polymorphs and [alpha]-Ca(PO3)2]; (ii) antisymmetric chemical shift tensors in cyclopropene derivatives, C3X4 (X = H, Cl, F) and (iii) 14N and 35Cl NQR predictions in the case of RDX (C3H6N6O6), [beta]-HMX (C4H8N8O8), [alpha]-NTO (C2H2N4O3) and AlOPCl6. RDX, [beta]-HMX and [alpha]-NTO are explosive compounds.

Received: 31 March 2010; Revised: 17 May 2010; Accepted: 20 May 2010
Digital Object Identifier (DOI)

Computation and NMR crystallography of terbutaline sulfate
from Magnetic Resonance in Chemistry by Robin K. Harris, Paul Hodgkinson, Vadim Zorin, Jean-Nicolas Dumez, Bénédicte Elena-Herrmann, Lyndon Emsley, Elodie Salager, Robin S. Stein

This article addresses, by means of computation and advanced experiments, one of the key challenges of NMR crystallography, namely the assignment of individual resonances to specific sites in a crystal structure. Moreover, it shows how NMR can be used for crystal structure validation. The case examined is form B of terbutaline sulfate. CPMAS 13C and fast MAS 1H spectra have been recorded and the peaks assigned as far as possible. Comparison of 13C chemical shifts computed using the CASTEP program (incorporating the Gauge Including Projector Augmented Wave principle) with those obtained experimentally enable the accuracy of the two distinct single-crystal evaluations of the structure to be compared and an error in one of these is located. The computations have substantially aided in the assignments of both 13C and 1H resonances, as has a series of two-dimensional (2D) spectra (HETCOR, DQ-CRAMPS and proton-proton spin diffusion). The 2D spectra have enabled many of the proton chemical shifts to be pinpointed. The relationships of the NMR shifts to the specific nuclear sites in the crystal structure have therefore been established for most 13C peaks and for some 1H signals. Emphasis is placed on the effects of hydrogen bonding on the proton chemical shifts.

Received: 26 March 2010; Revised: 20 May 2010; Accepted: 24 May 2010
Digital Object Identifier (DOI)

Prediction of NMR J-coupling in solids with the planewave pseudopotential approach
from Magnetic Resonance in Chemistry by Jonathan R. Yates
We review the calculation of NMR J-coupling in solid materials using the planewave pseudopotential formalism of Density Functional Theory. The methodology is briefly summarised and an account of recent applications is given. We discuss various aspects of the calculations which should be taken into account when comparing results with solid-state NMR experiments including anisotropy and orientation of the J tensors, the reduced coupling constant, and the relation between J and crystal structure. Copyright © 2010 John Wiley & Sons, Ltd.

Received: 20 April 2010; Revised: 2 June 2010; Accepted: 4 June 2010
Digital Object Identifier (DOI)

Comparing quantum-chemical calculation methods for structural investigation of zeolite crystal structures by solid-state NMR spectroscopy
from Magnetic Resonance in Chemistry by Darren H. Brouwer, Igor L. Moudrakovski, Richard J. Darton, Russell E. Morris

Combining quantum-chemical calculations and ultrahigh-field NMR measurements of 29Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum-chemical calculations have been performed on 'molecular fragments' extracted from the zeolite crystal structure using Hartree-Fock methods (as implemented in Gaussian). Using recently acquired ultrahigh-field 29Si NMR data for the pure silica zeolite ITQ-4, we report the results of calculations using recently developed quantum-chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites, we report the completion of the NMR crystallography of the zeolite ITQ-4, which was previously solved from NMR data. We compare three options for the 'refinement' of zeolite crystal structures from 'NMR-solved' structures: (i) a simple target-distance based geometry optimization, (ii) refinement of atomic coordinates in which the differences between experimental and calculated 29Si CS tensors are minimized, and (iii) refinement of atomic coordinates to minimize the total energy of the lattice using CASTEP quantum-chemical calculations. All three refinement approaches give structures that are in remarkably good agreement with the single-crystal X-ray diffraction structure of ITQ-4.

Received: 31 March 2010; Revised: 27 May 2010; Accepted: 2 June 2010
Digital Object Identifier (DOI)

Prog. in NMR Spectrosc. - July 2010 - Floquet theory

Progress in Nuclear Magnetic Resonance Spectroscopy
Floquet Theory in Solid-State Nuclear Magnetic Resonance
Michal Leskes, P.K. Madhu, Shimon Vega

Received 12 April 2010; accepted 11 June 2010. Available online 30 June 2010.

Keywords: Solid-state NMR; Floquet Theory; van Vleck transformation; Average Hamiltonian Theory