Hans J. Jakobsen

Broadband dipolar recoupling in the nuclear magnetic resonance of rotating solids: A compensated C7 pulse sequence

We introduce an improved variant of the C7 pulse-sequence for efficient recoupling of spin-1/2 pair dipolar interactions in magic-angle spinning solid-state NMR spectroscopy. The tolerance of C7 toward isotropic as well as anisotropic chemical shift offsets and rf inhomogeneity is improved considerably by replacing the original basic element Cφ44=(2π)φ(2π)φ+π with the cyclically permuted element Cφ143=(π/2)φ(2π)φ+π(3π/2)φ. The improved performance of this permutationally offset stabilized variant of C7 is analyzed by average Hamiltonian theory to fifth order, numerical simulations, and demonstrated by experiments on powder samples of doubly 13C-labeled barium oxalate hemihydrate and diammonium fumarate.

DOUBLE-QUANTUM HOMONUCLEAR ROTARY RESONANCE : EFFICIENT DIPOLAR RECOVERY IN MAGIC-ANGLE SPINNING NUCLEAR MAGNETIC RESONANCE

We describe an efficient method for the recovery of homonuclear dipole–dipole interactions in magic‐angle spinning NMR. Double‐quantum homonuclear rotary resonance (2Q‐HORROR) is established by fulfilling the condition ωr=2ω1, where ωr is the sample rotation frequency and ω1 is the nutation frequency around an applied resonant radio frequency (rf) field. This resonance can be used for double‐quantum filtering and measurement of homonuclear dipolar interactions in the presence of magic‐angle spinning. The spin dynamics depend only weakly on crystallite orientation allowing good performance for powder samples. Chemical shift effects are suppressed to zeroth order. The method is demonstrated for singly and doubly 13C labeled L‐alanine.

Satellite transitions in MAS NMR spectra of quadrupolar nuclei

Manifolds of spinning sidebands (ssbs) are observed for the satellite transitions of several half-integer quadrupolar nuclei (spin I12) using one-pulse magic-angle spinning (MAS) NMR experiments of crystalline powders. Numerical simulations of the variety of envelope patterns for the ssbs in such spectra show that the quadrupolar coupling parameters can be determined with high accuracy. The theory of the method is described and evaluated using average Hamiltonian theory to first and second order in the secular approximation. The influence of finite RF pulse excitation and quality factor of the MAS probes on the ssb intensities in the spectra is analyzed. Determination of quadrupole coupling constants and asymmetry parameters for the electric field gradient tensors from 17O, 23Na, and 27Al MAS NMR spectra of various solids illustrates the theory and high performance of the method.

A Series of Mixed‐Metal Borohydrides

Mix and match: A novel series of alkali-metal zinc borohydrides, LiZn 2(BH 4) 5 (see picture), NaZn 2(BH 4) 5, and NaZn(BH 4) 3, with fascinating structures are presented. An interpenetrated network structure, containing a [Zn 2(BH 4) 5] -. ion, is observed for the first time for a borohydride. A three-dimensional framework containing a polymeric [{Zn(BH 4) 3} n] n- ion is also identified.

Zeolites by confined space synthesis – characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and 27Al/29Si-MAS NMR spectroscopy

ZSM-5 zeolites prepared by confined space synthesis, i.e., by crystallization inside a mesoporous matrix, are characterized by X-ray powder diffraction (XRPD), ammonia temperature-programmed desorption (TPD), 27Al and 29Si-MAS NMR spectroscopy, and transmission electron microscopy (TEM). It is shown by XRPD that confined space synthesis can be used for the preparation of pure and highly crystalline ZSM-5 with a uniform crystal size distribution governed by the pore size of the inert matrix. Ammonia desorption demonstrates that the small zeolite crystals have the same number of acid sites as large crystals when the Si/Al ratio is similar. The number of acid sites have been determined by ammonia TPD and converted into framework Si/Al ratios. These ratios are higher than the bulk Si/Al ratios but agree with those obtained from 27Al/29Si-MAS NMR. Furthermore, the 27Al MAS NMR spectra show that the synthesis conditions are important in order to prevent the formation of non-framework Al and that the best result is obtained using NaAlO2 as the aluminum source. By proper control of the synthesis conditions, it is possible to produce a ZSM-5 zeolite with a framework Si/Al=50 and without non-framework aluminum. The relative numbers of Si*(OSi)4, HOSi*(OSi)3, and AlOSi*(OSi)3 units have been determined from 29Si-MAS NMR in combination with the Si/Al ratios from 27Al-MAS NMR. These results indicate a higher number of HOSi*(OSi)3 sites for ZSM-5 zeolites of the nm size when compared to those of the μm size. Transmission electron micrographs provide independent support for the formation of small zeolite crystals.

51V MAS NMR spectroscopy: determination of quadrupole and anisotropic shielding tensors, including the relative orientation of their principal-axis systems

Abstract 51 V MAS NMR spectra exhibiting the complete manifold of spinning sidebands (SSBs) from all seven 51 V ( I =7/2) transitions have been observed for NH 4 VO 3 and V 2 O 5 . These spectra are influenced by interactions from both 51 V quadrupole coupling and chemical-shielding anisotropy. A simulation and iterative-fitting program which allows the first determination of quadrupole and anisotropic shielding tensors, including the relative orientation of their principal-axis systems, from MAS NMR spectra is introduced. The 51 V parameters determined from the SSB intensities for NH 4 VO 3 and V 2 O 5 are compared with literature data from static NMR spectra.

High-resolution 13C NMR spectra and long-range 13C1H spin coupling constants in pyridine and 2-bromopyridine

Abstract High-resolution proton undecoupled 13 C NMR spectra (continuous wave mode) have been obtained for pyridine and 2-bromopyridine using the 13 C isotope in natural abundance. Analysis of these spectra has provided all possible long-range 13 CH coupling constants including their relative signs, all directly bonded 13 CH coupling constants, and in some cases differences in the 13 C isotope effects on the 1 H chemical shifts for these compounds. The 13 CH coupling constants in pyridine and the effects of a 2-bromo substituent on these parameters are discussed with respect to corresponding data in previously reported compounds and with values obtained from CNDO/2 and INDO calculations. Contributions from incomplete quadrupolar removal of the 13 C 14 N coupling constants to the 13 C line widths have been calculated and compared with experimental values.

29Si MAS NMR studies of portland cement components and effects of microsilica on the hydration reaction

29Si MAS NMR is shown to be a valuable tool for quantitative analysis of synthetic and natural cement minerals and for following the hydration of white Portland cement. Computer deconvolution of the alite/belite 29Si MAS NMR spectrum for white Portland cement allows the C3S and β-C2S content to be obtained with high accuracy. This composition differs significantly from that of a Bogue calculation. In the hydration of white Portland cement addition of microsilica has the effect of accelerating the reaction of C3S. Microsilica itself is consumed during the hydration, and an increased amount of polymer calcium silicate hydrates is formed. Preliminary high-speed 27Al MAS NMR investigations applied to the low aluminate content in white cement show great potential for this technique in studies of cement hydration reactions. An approximate value for the 27Al nuclear quadrupole coupling constant in hydrated white cement is reported.

Conformation of alamethicin in oriented phospholipid bilayers determined by (15)N solid-state nuclear magnetic resonance.

The conformation of the 20-residue antibiotic ionophore alamethicin in macroscopically oriented phospholipid bilayers has been studied using (15)N solid-state nuclear magnetic resonance (NMR) spectroscopy in combination with molecular modeling and molecular dynamics simulations. Differently (15)N-labeled variants of alamethicin and an analog with three of the alpha-amino-isobutyric acid residues replaced by alanines have been investigated to establish experimental structural constraints and determine the orientation of alamethicin in hydrated phospholipid (dimyristoylphosphatidylcholine) bilayers and to investigate the potential for a major kink in the region of the central Pro(14) residue. From the anisotropic (15)N chemical shifts and (1)H-(15)N dipolar couplings determined for alamethicin with (15)N-labeling on the Ala(6), Val(9), and Val(15) residues and incorporated into phospholipid bilayer with a peptide:lipid molar ratio of 1:8, we deduce that alamethicin has a largely linear alpha-helical structure spanning the membrane with the molecular axis tilted by 10-20 degrees relative to the bilayer normal. In particular, we find compatibility with a straight alpha-helix tilted by 17 degrees and a slightly kinked molecular dynamics structure tilted by 11 degrees relative to the bilayer normal. In contrast, the structural constraints derived by solid-state NMR appear not to be compatible with any of several model structures crossing the membrane with vanishing tilt angle or the earlier reported x-ray diffraction structure (Fox and Richards, Nature. 300:325-330, 1982). The solid-state NMR-compatible structures may support the formation of a left-handed and parallel multimeric ion channel.

Thermal Polymorphism and Decomposition of Y(BH4)(3)

The structure and thermal decomposition of Y(BH(4))(3) is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), (11)B MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH(4) and YCl(3) in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH(4))(3), denoted beta-Y(BH(4))(3), is discovered besides the Y(BH(4))(3) polymorph previously reported, denoted alpha-Y(BH(4))(3). beta-Y(BH(4))(3) has a cubic crystal structure and crystallizes with the space group symmetry Pm3m and a bisected a-axis, a = 5.4547(8) A, as compared to alpha-Y(BH(4))(3), a = 10.7445(4) A (Pa3). Beta-Y(BH(4))(3) crystallizes with a regular ReO(3)-type structure, hence the Y(3+) cations form cubes with BH(4)(-) anions located on the edges. This arrangement is a regular variant of the distorted Y(3+) cube observed in alpha-Y(BH(4))(3), which is similar to the high pressure phase of ReO(3). The new phase, beta-Y(BH(4))(3) is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from alpha- to beta-Y(BH(4))(3), at approximately 180 degrees C. Upon further heating, beta-Y(BH(4))(3) decomposes at approximately 190 degrees C to YH(3), which transforms to YH(2) at 270 degrees C. An unidentified compound is observed in the temperature range 215-280 degrees C, which may be a new Y-B-H containing decomposition product. The final decomposition product is YB(4). These results show that boron remains in the solid phase when Y(BH(4))(3) decomposes in a hydrogen atmosphere and that Y(BH(4))(3) may store hydrogen reversibly.