Tetrahedral molecular structure hydrogen bonds

The initiating step in the oxidation of methane is the first abstraction of a hydrogen atom. However, because of the tetrahedral molecular structure with comparatively high C-H bond energies, the methane molecule is extremely stable, and at lower temperatures the initiation step may be rate limiting for the overall conversion. In methane-oxygen systems, the chemistry is generally initiated by reaction of CH4 with O2,... 

The non-complementarity between the ellipsoidal 33-6H+ and the spherical halides results in much weaker binding and appreciable distortions of the ligand, as seen in the crystal structures of the cryptates 35 where the bound ion is F , Cl-, or Br-. In these complexes, F- is bound by a tetrahedral array of hydrogen bonds whereas Cl- and Br- display octahedral coordination (Fig. 4). Thus, 33-6H+ is a molecular receptor for the recognition of linear triatomic species of a size compatible with the size of the molecular cavity [3.11]. 

Koltun molecular model of carrier 2 NH cation can be encapsulated, "tetrahedrally", donating two hydrogen bonds to two diametric ring nitrogen atoms as well as two furan oxygen atoms. Similar complex structure has been reported in the compljj of 1,4,7,10-tetrahydroxyethyl-1,4,7,10-tetraazacyclododecane H O. 

Chapter 9, on entropy and molecular rotation in crystals and liquids, is concerned mostly with statistical mechanics rather than quantum mechanics, but the two appear together in SP 74. Chapter 9 contains one of Pauling s most celebrated papers, SP 73, in which he explains the experimentally measured zero-point entropy of ice as due to water-molecule orientation disorder in the tetrahedrally H-bonded ice structure with asymmetric hydrogen bonds (in which the bonding proton is not at the center of the bond). This concept has proven fully valid, and the disorder phenomenon is now known to affect greatly the physical properties of ice via the... 

Fig. 6.77. Calculations done using the statistical mechanical theory of electrolyte solutions. Probability density p(6,r) for molecular orientations of water molecules (tetrahedral symmetry) as a function of distance rfrom a neutral surface (distances are given in units of solvent diameter d = 0.28 nm) (a) 60H OH bond orientation and (b) dipolar orientation, (c) Ice-like arrangement found to dominate the liquid structure of water models at uncharged surfaces. The arrows point from oxygen to hydrogen of the same molecule. The peaks at 180 and 70° in p(0OH,r) for the contact layer correspond to the one hydrogen bond directed into the surface and the three directed to the adjacent solvent layer, respectively, in (c). (Reprinted from G. M. Tome and G. N. Patey, ElectrocNm. Acta 36 1677, copyright 1991, Figs. 1 and 2, with permission from Elsevier Science.

The compound Ru4(jt-H)4(CO)12 is obtained as a yellow air-stable powder, which is soluble in most organic solvents, but insoluble in water. The IR spectrum contains v(CO) bands at 2081 (s), 2067 (vs), 2030 (m), 2024(s), and 2009(w)cm-1 (cyclohexane solution) the HNMR spectrum has a resonance at <5 — 17.98 (CDC13 solution). The molecular structure of Ru4(/i-H)4(CO)12 has been determined by X-ray diffraction the four hydrogen atoms bridge the edges of the tetrahedral Ru4 core in a D2d arrangement, while three CO ligands are terminally bonded to each ruthenium.8 The deuterated complex Ru4(/i-D)4(CO)12 can be prepared in the same way if D2 is used in place of H2.6... 

Furthermore, the sulfonamide bond is expected to possess enhanced metabolic stability with structural similarities to the tetrahedral transition state involved in amide bond enzymatic hydrolysis, thus making sulfonamide peptides interesting candidates in the development of protease inhibitors and new drugs. The oligomers and polymers should also be interesting molecular scaffolds, with specific secondary structures enforced by hydrogen bonding)100,101 ... 

The most common solid form of water is known as ice Ih (hexagonal ice), with the molecular structure as shown in Figure 2.1 from Durrant and Durrant (1962). In ice each water molecule (shown as a circle) is hydrogen bonded (solid lines) to four others in essentially tetrahedral angles (Lonsdale, 1958). A description of... 

The three-dimensional network structure of diamond can be considered as constructed from the linkage of nodes (C atoms) with rods (C-C bonds) in a tetrahedral pattern. From the viewpoint of crystal engineering, in a diamondoid network the node can be any group with tetrahedral connectivity, and the linking rods (or linker) can be all kinds of bonding interactions (ionic, covalent, coordination, hydrogen bond, and weak interactions) or molecular fragment. 

B. von Ahsen, M. Berkei, G. Henkel, H. Willner, and F. Aubke, The Synthesis, Vibrational Spectra, and Molecular Structure of [Ir(CO)6][SbF6]3-4HF - The First Structurally Characterized Salt with a Tripositive, Homoleptic Metal Carbonyl Cation and the First Example of a Tetrahedral Hydrogen-Bonded (HF)4 Cluster, J. Am. Chem. Soc. 124, 8371-8379 (2002). 

An ab initio molecular-orbital calculation for the NH3-HC1 dimer was performed and the existence in it of a hydrogen bond was predicted [98-700]. This result was confirmed experimentally when it was found that three N-H bonds are shorter than the fourth one [707], NH4C1 crystallizes in the CsCl-type structure, where NH4 has tetrahedral symmetry, because of the stabilization effect of the Madelung energy... 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

From Fig. 1 we propose that the water molecule has temporarily tetrahedral-like structure in a short time, because if the water has been constructed by a simple H2O (C2v) molecule there should be only three molecular vibration modes (vi, V2, V3). In Fig. 1 we can see that between 1600 cm l and 4000 cm"l more than three molecular vibrations. They can be classiHed into essentially four kinds molecular vibrations (vi, V2, V3, V4). Besides three or four vibration components in the viAts modes region there exists an extra broad mode at about 2200 cm i. We had better to interpret this spectral pattern as the molecular vibradons of tetrahedral-like C2v symmetry which is composed by two O-H bonds and two 0---H hydrogen bonds in each oxygen. Although the conventional explanation of 2200 cm mode is the combination mode between the molecular vibration V2 and the lattice vibration v, there is no direct experimental evidence. Rather the tetrahedral-like C2v local structure can produce the four molecular vibration modes (Ai, Ai, Bi, B2) in the viA S frequency region and three molecular vibration modes (Ai, Bi, B2) which are bundled in the V4 frequency region. This latter modes correspond to the broad 2200 cm l mode. The above picture is consistent with the pentamer model of liquid water which is stressed in the interpretation of the low-frequency Raman specnal pattern. 

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