Magic Angle Spinning and Truncated Field Concept in NMR

In order to thoroughly comprehend and adequtely interpret NMR data, it is necessary to perceive the complex structure of spin Hamiltonian. Although NMR principles have been extensively discussed in a number of distinguished introductory publications, it still remains difficult to find illustrative graphical models revealing the tensorial nature of spin interaction. Exposure of the structure standing behind mathematical formulas can clarify intangible concepts and provide a coherent image of basic phenomena. This approach is essential when it comes to hard to manage, time-dependent processes such as Magic Angle Spinning (MAS), where the anisotropic character of the spin system interactions couple with experimentally introduced time evolution processes. The presented work concerns fundamental aspects of solid state NMR namely: the uniqueness of the tetrahedral angle and evolution of both dipolar D and chemical shield σ coupling tensors under MAS conditions.

The Structure and Properties of Water

Water is one of the most complex fluids on Earth. Even after intense study, there are many aspects regarding the structure, properties, and chemistry of water that are not well understood. In this chapter, we highlight the attributes of water that dictate many of the reactions that take place between water and oxides. We start with a single water molecule and progress to water clusters, then finally to extended liquid and solid phases. This chapter provides a baseline for evaluating what happens when water encounters simple ions, soluble oxide complexes called hydrolysis products, and extended oxide phases. The primary phenomenon highlighted in this chapter is hydrogen bonding. Hydrogen bonding dominates the structure and properties of water and influences many water–oxide interactions. A single water molecule has eight valence electrons around a central oxygen anion. These electrons are contained in four sp3-hybridized molecular orbitals arranged as lobes that extend from the oxygen in a tetrahedral geometry. Each orbital is occupied by two electrons. Two of the lobes are bonded to protons; the other two lobes are referred to as lone pairs of electrons. The H–O–H bond angle of 104.5° is close to the tetrahedral angle of 109.5°. The O–H bond length in a single water molecule is 0.96 Ǻ. It is important to recognize that this bond length is really a measure of the electron density associated with the oxygen lone pair bonded to the proton. This is because a proton is so incredibly small (with an ionic radius of only 1.3·10−5 Ǻ) that it makes no contribution to the net bond length. The entire water molecule has a hard sphere diameter of 2.9 Ǻ, which is fairly typical for an oxygen anion. This means the unoccupied lone pairs are distended relative to the protonated lone pairs, extending out to roughly 1.9 Ǻ. The unequal distribution of charges introduces a dipole within the water molecule that facilitates electrostatic interactions with other molecules.

Crystal structure of dibenzyldimethylsilane

In the title compound, C16H20Si, a geometry different from an ideal tetrahedron can be observed at the Si atom. The bonds from Si to the benzylic C atoms [Si—C = 1.884 (1) and 1.883 (1) Å] are slightly elongated compared to the Si—Cmethylbonds [Si—C = 1.856 (1) and 1.853 (1) Å]. The Cbenzyl—Si—Cbenzylbond angle [C—Si—C = 107.60 (6)°] is decreased from the ideal tetrahedral angle by 1.9°. These distortions can be explained easily by Bent's rule. In the crystal, molecules interact only by van der Waals forces.

Crystal structure oftrans-1,4-bis[(trimethylsilyl)oxy]cyclohexa-2,5-diene-1,4-dicarbonitrile

The asymmetric unit of the title compound, C14H22N2O2Si2, contains one half of the molecule, which is completed by inversion symmetry. The cyclohexa-2,5-diene ring is exactly planar and reflects the bond-length distribution of a pair of located double bonds [1.3224 (14) Å] and two pairs of single bonds [1.5121 (13) and 1.5073 (14) Å]. The tetrahedral angle between thesp3-C atom and the two neighbouringsp2-C atoms in the cyclohexa-2,5-diene ring is enlarged by about 3°.

1-(4-Chlorophenyl)-2-[tris(4-methylphenyl)-λ5-phosphanylidene]butane-1,3-dione

In the title ylide, C31H28ClO2P [common name α-acetyl-α-p-chlorobenzoylmethylenetri(p-tolyl)phosphorane], the dihedral angle between the 4-chlorophenyl ring and that of the ylide moiety is 66.15 (10)°. The geometry around the P atom is slightly distorted tetrahedral [angle range = 105.22 (8)–115.52 (9)°] and the carbonyl O atoms aresyn-oriented with respect to the P atom. The ylide group is close to planar [maximum deviation from the least-squares plane = 0.006 (2) Å] and the P—C, C—C and C=O bond lengths are consistent with electron delocalization involving the O atoms.

Modulation of quartz-like GeO2structure by Si substitution: an X-ray diffraction study of Ge1−xSixO2(0 ≤x< 0.2) flux-grown single crystals

The spontaneous nucleation by the high-temperature flux method of GeO2and SiO2-substituted GeO2(Ge1−xSixO2) compounds was improved to give single crystals free of hydroxy groups. The crystal structure and quality of these α-quartz-like piezoelectric materials were studied by single-crystal X-ray diffraction at room temperature. The refinements gave excellent final reliability factors, which are an indication of single crystals with a low level of defects. A good correlation was found between the silicon content in Ge1−xSixO2crystals determined through extrapolation from the inter-tetrahedral bridging angle and that found from energy-dispersive X-ray spectroscopy. The effect of germanium replacement by silicon on the distortion of the α-quartz-type GeO2structure was followed by the evolution of the intra-tetrahedral angle and other structural parameters. TheTO4(T= Si, Ge) distortion was found to be larger in α-GeO2than in α-SiO2and, as expected, the irregularity of theTO4tetrahedra decreased linearly as the substitution of Si for Ge increased.

(2,2-Dimethyl-α,α,α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanolato-κ2 O 4,O 5)ethyl(tetrahydrofuran-κO)aluminium(III)

The title compound, [Al(C2H5)(C31H28O4)(C4H8O)], has a slightly distorted tetrahedral geometry around the AlIII metal centre. The bidentate TADDOLate ligand and the AlIII metal atom form a seven-membered ring, with an O—Al—O angle of 111.19 (10)°, which is close to the ideal tetrahedral angle. The Al—O—C(alkoxide) angles of 139.17 (17) and 137.95 (17)° are larger than the sp 3 bond angle, suggesting substantial π-donation of the alkoxide O atom to the AlIII metal centre.

The crystal structure of natural 33R moissanite from Tibet

The crystal structure of the natural 33R moissanite, recovered as inclusions in chromitites from Luobusa harzburgite in Tibet, was determined from X-ray diffraction data collection by a Nonius CAD-4 four-circle diffractometer. The crystal is trigonal in symmetry with a space group ofThe bond angles and tetrahedral angle variance of the present crystal structure show that the tetrahedra are quite regular in geometry. The distance between two adjacent

Crystal chemistry of tetraradial species. Part 6. Embarras de richesses: lithium cation coordinated by eight O—H … phenyl bonds

Lithium tetraphenylborate tetrahydrate, [Li(OH2)4]BPh4 (LiTBw; tetragonal, I41/a, a = 27.566(2) Å, c = 12.228(2) Å, Z = 16) is remarkable in that the four H2O molecules coordinating the Li+ ion all form O—H …π hydrogen bonds to the phenyl groups of the anion. LiTBw thus appears to be the first reported example of such an exhaustive O—H …π coordination and can be described as a 3-dimensional, completely H-bonded polymer in which all the H2O hydrogens are bonded to phenyl groups and all the phenyl groups are involved in O—H … phenyl bonds. Six of the O—H … phenyl bonds are essentially normal and two are highly bent, possibly bifurcated. The existence of the H-bonds has been corroborated from variable-temperature FT-ir spectra of weakly deuterated LiTBw. The O—Li—O angles in the LiO4 coordination tetrahedron (of symmetry C1) exhibit large departures from the tetrahedral angle, two of the angles bisected by a quasi-S4 axis being 118.3° and 122.5°, respectively. An ab initio (6-31G*) investigation of the Li(OH2)4+ and Be(OH2)42+ cations has shown that such large O—M—O angles are to be expected even in the free ions and are thus not necessarily the result of packing effects. A detailed comparison with several Li(OH2)4+ and Be(OH2)42+ salts provides a rationale for the observed M(OH2)4n+ (M = Li, Be) geometries.