Monday, March 31, 2008

Al's Journal Update

Linear-scaling method for calculating nuclear magnetic resonance chemical shifts using gauge-including atomic orbitals within Hartree-Fock and density-functional theory
J. Chem. Phys. 127, 054103 (2007)
Jörg Kussmann and Christian Ochsenfeld
Details of a new density matrix-based formulation for calculating nuclear magnetic resonance chemical shifts at both Hartree-Fock and density functional theory levels are presented. For systems with a nonvanishing highest occupied molecular orbital–lowest unoccupied molecular orbital gap, the method allows us to reduce the asymptotic scaling order of the computational effort from cubic to linear, so that molecular systems with 1000 and more atoms can be tackled with today's computers. The key feature is a reformulation of the coupled-perturbed self-consistent field (CPSCF) theory in terms of the one-particle density matrix (D-CPSCF), which avoids entirely the use of canonical MOs. By means of a direct solution for the required perturbed density matrices and the adaptation of linear-scaling integral contraction schemes, the overall scaling of the computational effort is reduced to linear. A particular focus of our formulation is to ensure numerical stability when sparse-algebra routines are used to obtain an overall linear-scaling behavior.

Analyzing molecular properties calculated with two-component relativistic methods using spin-free natural bond orbitals: NMR spin-spin coupling constants
J. Chem. Phys. 127, 124106 (2007)

Jochen Autschbach
An analysis method for static linear response properties 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 developed. The spin-orbit NLMO/NBO analysis has been applied to study the indirect spin-spin coupling (J-coupling) constants in Tl–I, PbH4, and a dinuclear Pt–Tl bonded complex with a very large Pt–Tl coupling constant (expt.: 146.8 kHz). For Tl–I it is shown that the analysis scheme based on scalar relativistic NLMOs is applicable even if spin-orbit coupling is responsible for most of the coupling's magnitude. For PbH4 it is shown that electron delocalization plays a much larger role for the Pb–H coupling than it is the case for the C–H coupling in methane. For the Pt–Tl complex the analysis clearly demonstrates the strong influence of the ligands on the Pt–Tl coupling constant and quantifies the effect of the delocalization of the Pt–Tl bond on the Pt–Tl coupling constant.

The appearance of an interval of energies that contain the whole diamagnetic contribution to NMR magnetic shieldings
J. Chem. Phys. 127, 154115 (2007)

Alejandro Maldonado and Gustavo A. Aucar
Working within relativistic polarization propagator approach, it was shown in a previous article that the electronic origin of diamagnetic contributions to NMR nuclear magnetic shielding, d, are mostly excitations that fit in a well defined interval of energies such that 2mc2 ( i− [overline s]">)<4mc2. x="Br," x="O," x="N," x="Sn" x="Br">

Theory of damped quantum rotation in nuclear magnetic resonance spectra. II. Numerical simulations for the benzene rotor
J. Chem. Phys. 127, 184504 (2007)
T. Ratajczyk and S. Szymański
In Part I of this series of papers, the damped quantum rotation (DQR) theory, formulated originally for hindered threefold molecular rotors in solids, was generalized to the N-fold case. The stochastic dynamics of such objects, evidenced in NMR line shapes, was shown to be more complicated than in the standard model of classical jumps between the wells of the N-fold torsional potential. Actually, it comprises certain quantum rate (i.e., coherence-damping) processes subject to the requirements of the Pauli principle. The jump picture is recovered only when the quantum rates fit specific patterns. In this work, one of the ways of approaching such a classical limit is identified for the benzene rotor. This is inferred from a quantum mechanical model whose validity was earlier confirmed for a methyl group. Based on that model, theoretical calculations for the benzene ring dynamics in a clathrate crystal, 1-(9-anthryloxy)anthraquinone/benzene-d6, confronted with the pertinent literature data, point to possible deviations from the classical limit. However, the predicted DQR effects are too small to be observed in solid echo 2H NMR spectra of the C6D6 isotopomer. The chances of detecting the effects are improved when Carr-Purcell echo 1H spectra of a single crystal of the isotopomer including C6H6 as a guest are considered. The substantial differences in the sensitivity to the DQR effects of the spectra of protonated and deuterated benzene are concerned with different magnitudes of the intramolecular dipolar spin couplings. The dynamic isotope effect (C6D6 vs C6H6), which is small in this case, is only of secondary importance. Legitimacy of the use of the jump model in 2H NMR line shape studies of benzene-d6 is fully confirmed by the present considerations. However, the physical significance of the dynamic parameters extracted from such studies is shown from a new perspective.

A first principles theory of nuclear magnetic resonance J-coupling in solid-state systems
J. Chem. Phys. 127, 204107 (2007)
Siân A. Joyce, Lee Maltings, Jonathan R. Yates, Chris J. Pickard, Francesco Mauri
A method to calculate NMR J-coupling constants from first principles in extended systems is presented. It is based on density functional theory and is formulated within a planewave-pseudopotential framework. The all-electron properties are recovered using the projector augmented wave approach. The method is validated by comparison with existing quantum chemical calculations of solution-state systems and with experimental data. The approach has also been applied to the silicophosphate, Si5O(PO4)6, giving 31P–29Si-couplings which are in excellent agreement with experiment.

Theoretical predictions of nuclear magnetic resonance parameters in a novel organo-xenon species: Chemical shifts and nuclear quadrupole couplings in HXeCCH
J. Chem. Phys. 127, 234314 (2007)
Michal Straka, Perttu Lantto, Markku Räsänen, and Juha Vaara
We calibrate the methodology for the calculation of nuclear magnetic resonance (NMR) properties in novel organo-xenon compounds. The available state-of-the-art quantum-chemical approaches are combined and applied to the HXeCCH molecule as the model system. The studied properties are 129Xe, 1H, and 13C chemical shifts and shielding anisotropies, as well as 131Xe and 2H nuclear quadrupole coupling constants. The aim is to obtain, as accurately as currently possible, converged results with respect to the basis set, electron correlation, and relativistic effects, including the coupling of relativity and correlation. This is done, on one hand, by nonrelativistic correlated ab initio calculations up to the CCSD(T) level and, on the other hand, for chemical shifts and shielding anisotropies by the leading-order relativistic Breit-Pauli perturbation theory (BPPT) with correlated ab initio and density-functional theory (DFT) reference states. BPPT at the uncorrelated Hartree-Fock level as well as the corresponding fully relativistic Dirac-Hartree-Fock method are found to be inapplicable due to a dramatic overestimation of relativistic effects, implying the influence of triplet instability in this multiply bonded system. In contrast, the fully relativistic second-order Møller-Plesset perturbation theory method can be applied for the quadrupole coupling, which is a ground-state electric property. The performance of DFT with various exchange-correlation functionals is found to be inadequate for the nonrelativistic shifts and shielding anisotropies as compared to the CCSD(T) results. The relativistic BPPT corrections to these quantities can, however, be reasonably predicted by DFT, due to the improved triplet excitation spectrum as compared to the Hartree-Fock method, as well as error cancellation within the five main BPPT contributions. We establish three computationally feasible models with characteristic error margins for future calculations of larger organo-xenon compounds to guide forthcoming experimental NMR efforts. The predicted 129Xe chemical shift in HXeCCH is in a novel range for this nucleus, between weakly bonded or solvated atomic xenon and xenon in the hitherto characterized molecules.

Triple oscillating field technique for accurate distance measurements by solid-state NMR
J. Chem. Phys. 128, 015103 (2008)
Navin Khaneja, Niels Chr. Nielsen
We present a new concept for homonuclear dipolar recoupling in magic-angle-spinning (MAS) solid-state NMR experiments which avoids the problem of dipolar truncation. This is accomplished through the introduction of a new NMR pulse sequence design principle: the triple oscillating field technique. We demonstrate this technique as an efficient means to accomplish broadband dipolar recoupling of homonuclear spins, while decoupling heteronuclear dipolar couplings and anisotropic chemicals shifts and retaining influence from isotropic chemical shifts. In this manner, it is possible to synthesize Ising interaction (2IzSz) Hamiltonians in homonuclear spin networks and thereby avoid dipolar truncation—a serious problem essentially all previous homonuclear dipolar recoupling experiments suffer from. Combination of this recoupling concept with rotor assisted dipolar refocusing enables easy readout of internuclear distances through comparison with analytical Fresnel curves. This forms the basis for a new class of solid-state NMR experiments with potential for structure analysis of uniformly 13C labeled proteins through accurate measurement of 13C–13C internuclear distances. The concept is demonstrated experimentally by measurement of C –C , C –C , and C –C internuclear distances in powder samples of the amino acids L-alanine and L-threonine.

Satellite transitions acquired in real time by magic angle spinning (STARTMAS): “Ultrafast” high-resolution MAS NMR spectroscopy of spin I=3/2 nuclei
J. Chem. Phys. 128, 034507 (2008
Michael J. Thrippleton, Thomas J. Ball, and Stephen Wimperis
The satellite transitions acquired in real time by magic angle spinning (STARTMAS) NMR experiment combines a train of pulses with sample rotation at the magic angle to refocus the first- and second-order quadrupolar broadening of spin I=3/2 nuclei in a series of echoes, while allowing the isotropic chemical and quadrupolar shifts to evolve. The result is real-time isotropic NMR spectra at high spinning rates using conventional MAS equipment. In this paper we describe in detail how STARTMAS data can be acquired and processed with ease on commercial equipment. We also discuss the advantages and limitations of the approach and illustrate the discussion with numerical simulations and experimental data from four different powdered solids.

A fully relativistic method for calculation of nuclear magnetic shielding tensors with a restricted magnetically balanced basis in the framework of the matrix Dirac–Kohn–Sham equation
J. Chem. Phys. 128, 104101 (2008)
Stanislav Komorovský, Michal Repiský, Olga L. Malkina, Vladimir G. Malkin, Irina Malkin Ondík, and Martin Kaupp
A new relativistic four-component density functional approach for calculations of NMR shielding tensors has been developed and implemented. It is founded on the matrix formulation of the Dirac–Kohn–Sham (DKS) method. Initially, unperturbed equations are solved with the use of a restricted kinetically balanced basis set for the small component. The second-order coupled perturbed DKS method is then based on the use of restricted magnetically balanced basis sets for the small component. Benchmark relativistic calculations have been carried out for the 1H and heavy-atom nuclear shielding tensors of the HX series (X=F,Cl,Br,I), where spin-orbit effects are known to be very pronounced. The restricted magnetically balanced basis set allows us to avoid additional approximations and/or strong basis set dependence which arises in some related approaches. The method provides an attractive alternative to existing approximate two-component methods with transformed Hamiltonians for relativistic calculations of chemical shifts and spin-spin coupling constants of heavy-atom systems. In particular, no picture-change effects arise in property calculations.

Stimulated echoes and two-dimensional nuclear magnetic resonance spectra for solids with simple line shapes
J. Chem. Phys. 128, 114506 (2008)
Burkhard Geil, Gregor Diezemann, and Roland Böhmer
Nuclear magnetic resonance (NMR) experiments on ion conductors often yield rather unstructured spectra, which are hard to interpret if the relation between the actual translational motion of the mobile species and the changes of the NMR frequencies is not known. In order to facilitate a general analysis of experiments on solids with such spectra, different models for the stochastic evolution of the NMR frequencies are considered. The treated models involve random frequency jumps, diffusive evolutions, or approximately fixed frequency jumps. Two-dimensional nuclear magnetic resonance spectra as well as stimulated-echo functions for the study of slow and ultraslow translational dynamics are calculated for Gaussian equilibrium line shapes. The results are compared with corresponding ones from rotational models and with experimental data.

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 1,C/2 rf field for the spinning frequency r/2=10–30 kHz and 1H Larmor frequency 0,H/2 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<5) 2 ="750">

31P Magic Angle Spinning NMR Spectroscopy for Probing Local Environments in Paramagnetic Europium-Substituted Wells-Dawson Polyoxotungstates
Inorg. Chem., 46 (19), 7861 -7869, 2007
Wenlin Huang, Lynn C. Francesconi, and Tatyana Polenova
A series of europium-substituted Wells-Dawson polyoxotungstates were addressed by 31P magic angle spinning (MAS) NMR spectroscopy. The electron-nuclear dipolar interaction dominates the 31P spinning-sideband envelopes. The experimental electron-nuclear dipolar anisotropies were found to be in good agreement with those calculated based on the known crystallographic coordinates and effective magnetic moments and assuming a point-dipole approximation. These electron-nuclear dipolar anisotropies directly report on the anion stoichiometry and on the positional isomerism, indicating that 31P MAS NMR spectroscopy may be a useful and quick analytical probe of the local environment in Wells-Dawson solids containing localized europium paramagnetic centers.

Probing Lead(II) Bonding Environments in 4-Substituted Pyridine Adducts of (2,6-Me2C6H3S)2Pb: An X-ray Structural and Solid-State 207Pb NMR Study
Inorg. Chem., 46 (21), 8625 -8637, 2007
Glen G. Briand, Andrew D. Smith, Gabriele Schatte, Aaron J. Rossini, and Robert W. Schurko
The effect of subtle changes in the -electron donor ability of 4-substituted pyridine ligands on the lead(II) coordination environment of (2,6-Me2C6H3S)2Pb (1) adducts has been examined. The reaction of 1 with a series of 4-substituted pyridines in toluene or dichloromethane results in the formation of 1:1 complexes [(2,6-Me2C6H3S)2Pb(pyCOH)]2 (3), [(2,6-Me2C6H3S)2Pb(pyOMe)]2 (4), and (2,6-Me2C6H3S)2Pb(pyNMe2) (5) (pyCOH = 4-pyridinecarboxaldehyde; pyOMe = 4-methoxypyridine; pyNMe2 = 4-dimethylaminopyridine), all of which have been structurally characterized by X-ray crystallography. The structures of 3 and 4 are dimeric and have -trigonal bipyramidal S3N bonding environments, with the 4-substituted pyridine nitrogen and bridging sulfur atoms in axial positions and two thiolate sulfur atoms in equatorial sites. Conversely, compound 5 is monomeric and exhibits a -trigonal pyramidal S2N bonding environment at lead(II). The observed structures may be rationalized in terms of a simple valence bond model and the -electron donor ability of the 4-pyridine ligands as derived from the analysis of proton affinity values. Solid-state 207Pb NMR experiments are applied in combination with density functional theory (DFT) calculations to provide further insight into the nature of bonding in 4, 5, and (2,6-Me2C6H3S)2Pb(py)2 (2). The lead chemical shielding (CS) tensor parameters of 2, 4, and 5 reveal some of the largest chemical shielding anisotropies (CSA) observed in lead coordination complexes to date. DFT calculations using the Amsterdam Density Functional (ADF) program, which take into account relativistic effects using the zeroth-order regular approximation (ZORA), yield lead CS tensor components and orientations. Paramagnetic contributions to the lead CS tensor from individual pairs of occupied and virtual molecular orbitals (MOs) are examined to gain insight into the origin of the large CSA. The CS tensor is primarily influenced by mixing of the occupied MOs localized on the sulfur and lead atoms with virtual MOs largely comprised of lead 6p orbitals.

Investigating the Vanadium Environments in Hydroxylamido V(V) Dipicolinate Complexes Using 51V NMR Spectroscopy and Density Functional Theory
Inorg. Chem., 46(22); 9285-9293, 2007
Kristopher J. Ooms, Stephanie E. Bolte, Jason J. Smee, Bharat Baruah, Debbie C. Crans, and Tatyana Polenova Using 51V magic angle spinning solid-state NMR, SSNMR, spectroscopy and quantum chemical DFT calculations we have characterized the chemical shift and quadrupolar coupling parameters of a series of eight hydroxylamido vanadium(V) dipicolinate complexes of the general formula VO(dipic)(ONR1R2)(H2O) where R1 and R2 can be H, CH3, or CH2CH3. This class of vanadium compounds was chosen for investigation because of their seven-coordinate vanadium atom, a geometry for which there is limited 51V SSNMR data. Furthermore, a systematic series of compounds with different electronic properties are available and allows for the effects of ligand substitution on the NMR parameters to be studied. The quadrupolar coupling constants, CQ, are small, 3.0-3.9 MHz, but exhibit variations as a function of the ligand substitution. The chemical shift tensors in the solid state are sensitive to changes in both the hydroxylamide substituent and the dipic ligand, a sensitivity which is not observed for isotropic chemical shifts in solution. The chemical shift tensors span ~1000 ppm and are nearly axially symmetric. On the basis of DFT calculations of the chemical shift tensors, one of the largest contributors to the magnetic shielding anisotropy is an occupied molecular orbital with significant vanadium dz2 character along the V=O bond.

Structure of (NH4)3GaF6 Investigated by Multinuclear Magic-Angle Spinning NMR Spectroscopy in Comparison with Rietveld Refinement
Inorg. Chem., 47 (2), 663 -670, 2008
Thoralf Krahl, Mike Ahrens, Gudrun Scholz, Detlef Heidemann, and Erhard Kemnitz
The structure of ammonium gallium cryolite (NH4)3GaF6 was investigated by 19F and 69,71Ga magic-angle spinning (MAS) NMR in comparison with X-ray powder diffraction followed by Rietveld refinement. In agreement with previous thermodynamic measurements, NMR experiments on (NH4)3GaF6 support the model of rigid GaF6 octahedra. At high spinning speeds (30 kHz), the scalar coupling between the six equivalent 19F nuclei and 69,71Ga can be directly observed in the powder spectra. The coupling constants are J19F69Ga = 197 Hz and J19F71Ga = 264 Hz. To explain the 71Ga spectra recorded at 3 kHz a small distribution of quadrupolar frequencies has to be included. The spread of the spinning sidebands hints to a largest Q value of 28 kHz for 71Ga. This can be explained by the occurrence of highly symmetric GaF6 octahedra, which are tilted against the surrounding atoms. In addition, the incomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy to those of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver proper results.

Lead and Aluminum Bonding in Pb-Al Metaphosphate Glasses
Inorg. Chem., 47 (2), 690 -698, 2008
J. E. Tsuchida, J. Schneider, P. S. Pizani, and S. L. Oliveira
The bonding properties of cations in phosphate glasses determine many short- and medium-range structural features in the glass network, hence influencing bulk properties. In this work, Pb-Al-metaphosphate glasses (1 - x)Pb(PO3)2·xAl(PO3)3 with 0 x 1 were analyzed to determine the effect of the substitution of Pb by Al on the glass structure in the metaphosphate composition. The glass transition temperature and density were measured as a function of the Al concentration. The vibrational and structural properties were probed by Raman spectroscopy and nuclear magnetic resonance of 31P, 27Al, and 207Pb. Aluminum incorporates homogeneously in the glass creating a stiffer and less packed network. The average coordination number for Al decreases from 5.9 to 5.0 as x increases from 0.1 to 1, indicating more covalent Al-O bonds. The coordination number of Pb in these glasses is greater than 8, showing an increasing ionic behavior for compositions richer in Al. A quantitative analysis of the phosphate speciation shows definite trends in the bonding of AlOn groups and phosphate tetrahedra. In glasses with x <> 0.48 more than one nonbridging O can be linked to AlOn polyhedra. There is no corner sharing of O between AlOn and PbOn polyhedra nor between AlOn themselves throughout the compositional range. The PbOn coordination polyhedra show considerable nonbridging O sharing, with each O participating in the coordination sphere of at least two Pb. The bonding preferences determined for Al are consistent with the behavior observed in Na-Al and Ca-Al metaphosphates, indicating this may be a general behavior for ternary phosphate glasses.

Cs4P2Se10: A new compound discovered with the application of solid-state and high temperature NMR
Journal of Solid State Chemistry, Volume 180, Issue 10, October 2007, Pages 2877-2884
Matthew A. Gave, Christian G. Canlas, In Chung, Ratnasabapathy G. Iyer, Mercouri G. Kanatzidis
and David P. Weliky
The new compound Cs4P2Se10 was serendipitously produced in high purity during a high-temperature synthesis done in a nuclear magnetic resonance (NMR) spectrometer. 31P magic angle spinning (MAS) NMR of the products of the synthesis revealed that the dominant phosphorus-containing product had a chemical shift of −52.8 ppm that could not be assigned to any known compound. Deep reddish brown well-formed plate-like crystals were isolated from the NMR reaction ampoule and the structure was solved with X-ray diffraction. Cs4P2Se10 has the triclinic space group P-1 with a=7.3587(11) Å, b=7.4546(11) Å, c=10.1420(15) Å, α=85.938(2)°, β=88.055(2)°, and γ=85.609(2)° and contains the [P2Se10]4− anion. To our knowledge, this is the first compound containing this anion that is composed of two tetrahedral (PSe4) units connected by a diselenide linkage. It was also possible to form a glass by quenching the melt in ice water, and Cs4P2Se10 was recovered upon annealing. The static 31P NMR spectrum at 350 °C contained a single peak with a −35 ppm chemical shift and a 7 ppm peak width. This study highlights the potential of solid-state and high-temperature NMR for aiding discovery of new compounds and for probing the species that exist at high temperature.

MAS-NMR study of lithium zinc silicate glasses and glass-ceramics with various ZnO content
Journal of Solid State Chemistry, Volume 181, Issue 2, February 2008, Pages 269-275
Madhumita Goswami, Govind P. Kothiyal, Lionel Montagne
and Laurent Delevoye
Lithium zinc silicate glasses of composition (mol%): 17.5Li2O–(72−x)SiO2–xZnO−5.1Na2O−1.3P2O5−4.1B2O3, 5.5 x 17.7, were prepared by conventional melt-quenched technique and converted to glass-ceramic by controlled crystallization process. 29Si and 31P MAS-NMR was used to characterize the structure of both glass and glass-ceramic samples. Despite the complex glass composition, Q2, Q3 and Q4 sites are identified from 29Si MAS-NMR, which relative intensities are found to vary with the ZnO content, indicating a network depolymerization by ZnO. Moreover, well separated Q3 and Q4 resonances for low ZnO content indicates the occurrence of phase separation. From 31P MAS-NMR, it is seen that phosphorus is mainly present in the form of ortho-(Q0) and pyro-phosphate (Q1) structural units and variation of ZnO content did not have much effect on these resonances, which provides an additional evidence for phase separation in the glass. On conversion to glass-ceramics, lithium disilicate (Li2Si2O5), lithium zinc ortho-silicate (Li3Zn0.5SiO4), tridymite (SiO2) and cristobalite (SiO2) were identified as major silicate crystalline phases. Using 29Si MAS-NMR, quantification of these silicate crystalline phases is carried out and correlated with the ZnO content in the glass-ceramics samples. In addition, 31P spectra unambiguously revealed the presence of crystalline Li3PO4 and (Na,Li)3PO4 in the glass-ceramics.

Self-Assemblies Based on [Cp2Mo2(CO)4( , 2-P2)] - Solid-State Structure and Dynamic Behaviour in Solution
Chem. Eur. J., Volume 14, Issue 1, Date: December 28, 2007, Pages: 282-295
Manfred Scheer, Prof. Dr. , Laurence J. Gregoriades, Dr. , Manfred Zabel, Dr. , Junfeng Bai, Prof. Dr. , Ingo Krossing, Prof. Dr. , Gunther Brunklaus, Dr. , Hellmut Eckert, Prof. Dr.
Reaction of complex [Cp2Mo2(CO)4( , 2-P2)] (Cp=C5H5 (1)) with CuPF6, AgX (X=BF4, ClO4, PF6, SbF6, Al{OC(CF3)3}4) and [(Ph3P)Au(THF)][PF6] (THF=tetrahydrofuran), respectively, results in the facile formation of the dimers 3 b-h of the general formula [M2({Cp2Mo2 (CO)4( , 2: 2-P2)}2)({Cp2Mo2(CO)4 ( , 2: 1: 1-P2)}2)][X]2 (M=Cu, Ag, Au; X=BF4, ClO4, PF6, SbF6, Al{OC(CF3)3}4). As revealed by X-ray crystallography, all these dimers comprise dicationic moieties that are well-separated from the weakly coordinating anions in the solid state. If 1 is allowed to react with AgNO2 and LAuCl (L=CO or tetrahydrothiophene), respectively, the dimer [Ag2{Cp2Mo2 (CO)4( , 2: 1: 1-P2)}2( 2-NO2)2] (5) and the complex [AuCl{Cp2Mo2(CO)4( , 2: 1-P2)}] (6) are formed, which have also been characterised by X-ray crystallography. In compounds 5 and 6, the anions remain coordinated to the Group 11 metal centres. Spectroscopic data suggest that the dimers 3 b-h display dynamic behaviour in solution and this is discussed by using the comprehensive results obtained for 3 g (M=Ag; X=Al{OC(CF3)3}4) as a basis. The interpretation of the experimental results is facilitated by density functional theory (DFT) calculations on 3 g (structures, energetics, NMR shielding tensors). The 31P magic angle spinning (MAS) NMR spectra recorded for the dimers 3 b (M=Cu; X=PF6) and 3c (M=Ag; X=BF4) as well as that of the previously reported one-dimensional (1 D) polymer [Ag2{Cp2Mo2(CO)4( , 2: 1: 1-P2)}3( , 1: 1-NO3)]n[NO3]n (4) are also discussed herein and the strong dependence of the chemical shift of the phosphorus atoms within each compound on subtle structural differences in the solid state is demonstrated. Furthermore, the X-ray crystallographic and 31P MAS NMR spectroscopic characterisation of a new polymorph of 1 is reported.

Self-Assembled Amphotericin B Is Probably Surrounded by Ergosterol: Bimolecular Interactions as Evidenced by Solid-State NMR and CD Spectra
Chem. Eur. J., Volume 14, Issue 4, Date: January 28, 2008, Pages: 1178-1185
Yusuke Kasai, Dr., Nobuaki Matsumori, Dr., Yuichi Umegawa, Shigeru Matsuoka, Dr., Hiroyuki Ueno, Hiroki Ikeuchi, Tohru Oishi, Prof. Dr., Michio Murata, Prof. Dr.
Amphotericin B (AmB) is thought to exert its pharmacological effects by forming a barrel-stave assembly with ergosterol in fungal membranes. To examine the interaction between AmB and ergosterol (Erg) or cholesterol (Cho), 13C- and 19F-labelled covalent conjugates were prepared as reported previously (N. Matsumori et al. Chem. Biol. 2004, 11, 673-679). The CD spectra of the conjugates in a membrane-bound form suggested that the distance between the heptaene moieties of the ergosterol conjugates AmB-C2-(6-F)Erg 2 and AmB-C2-Erg 3 is similar to that of AmB in ergosterol-containing membranes, but significantly larger than that of AmB in nonsterol or cholesterol-containing membranes. These observations suggest that, as is the case with ergosterol-containing membranes, the conjugated sterol moiety prevents the close contact between the heptaene moieties within the membrane that would reduce channel conductivity of the AmB assemblies. To further investigate this bimolecular interaction, we recorded the solid-state NMR spectra of conjugates 2 and AmB-C2-(6-F)Cho 4, which are composed of uniformly 13C-labelled AmB and 6-fluorinated ergosterol or cholesterol; the conjugates were expected to facilitate the estimation of distances between the fluorine and carbon atoms. By using rotor-synchronous double resonance (rotational echo double resonance of X cluster; RDX) experiments, we deduced the distance between the fluorine atom and its nearest carbon atom in the heptaene moiety of 2 to be less than 8.6 Å. This indicates that the B ring of ergosterol comes close to the AmB polyene moiety. A conformational search of the AmB-ergosterol conjugate by using distance constraints derived from the RDX results suggested that ergosterol molecules possibly surround the AmB assembly, which is in contrast with the conventional image in which ergosterol is inserted into AmB molecules.

Octahedral Adducts of Dichlorosilane with Substituted Pyridines: Synthesis, Reactivity and a Comparison of Their Structures and 29Si NMR Chemical Shifts
Chem. Eur. J., Volume 14, Issue 10 , Pages 3164 - 3176
Gerrit W. Fester, Jörg Wagler, Dr. , Erica Brendler, Dr., Uwe Böhme, Dr. , Gerhard Roewer, Prof. Dr. , Edwin Kroke, Prof. Dr.
H2SiCl2 and substituted pyridines (Rpy) form adducts of the type all-trans-SiH2Cl2 2 Rpy. Pyridines with substituents in the 4- (CH3, C2H5, H2C CH, (CH3)3C, (CH3)2N) and 3-positions (Br) give the colourless solids 1 a-f. The reaction with pyrazine results in the first 1:2 adduct (2) of H2SiCl2 with an electron-deficient heteroaromatic compound. Treatment of 1 d and 1 e with CHCl3 yields the ionic complexes [SiH2(Rpy)4]Cl2 6 CHCl3 (Rpy=4-methylpyridine (3 d) and 4-ethylpyridine (3 e)). All products are investigated by single-crystal X-ray diffraction and 29Si CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the Si N distances vary between 193.3 pm for 1 c (4-(dimethylamino)pyridine complex) and 197.3 pm for 2. Interestingly, the pyridine moieties are coplanar and nearly in an eclipsed position with respect to the SiH2 units, except for the ethyl-substituted derivative 1 e, which shows a more staggered conformation in the solid state. Calculation of the energy profile for the rotation of one pyridine ring indicates two minima that are separated by only 1.2 kJ mol-1 and a maximum barrier of 12.5 kJ mol-1. The 29Si NMR chemical shifts ( iso) range from -145.2 to -152.2 ppm and correlate with the electron density at the Si atoms, in other words with the +I and +M effects of the substituents. Again, compound 1 e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the 29Si NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent Si H bonds and very polar Si Cl bonds, with the highest bond polarisation being observed for the Si N interaction, which must be considered a donor-acceptor interaction. An analysis of the topological properties of the electron distribution (AIM) suggests a Lewis structure, thereby supporting this bonding situation.

No comments: