Ethane molecular shape

Given the diversity of different SCRF models, and the fact that solvation energies in water may range from a few kcal/mol for say ethane to perhaps 100 kcal/mol for an ion, it is difficult to evaluate just how accurately continuum methods may in principle be able to represent solvation. It seems clear, however, that molecular shaped cavities must be employed, the electiostatic polarization needs a description either in terms of atomic charges or quite high-order multipoles, and cavity and dispersion terms must be included. Properly parameterized, such models appear to be able to give absolute values with an accuracy of a few kcal/mol." Molecular properties are in many cases also sensitive to the environment, but a detailed discussion of this is outside the scope of this book. ... 

When all the rotations are possible in the solid state the symmetry increases to hexagonal. This form corresponds to the close packing of spheres or cylinders and the molecule is in a rotational crystalline state, characterized by rigorous order in the arrangement of the center (axes) of the molecules and by disordered azimuthal rotations [118]. If the chain molecules are azimuthally chaotic (they rotate freely around their axes), their average cross sections are circular and, for this reason, they choose hexagonal packing. The ease of rotation of molecules in the crystal depends merely on the molecular shape, as in molecules of an almost spherical shape like methane and ethane derivatives with small substituents, or molecules of a shape close to that of a cylinder (e.g., paraffin-like molecules). 

Many molecules, especially those in living systems, have more than one central atom. The shapes of these molecules are combinations of the molecular shapes for each central atom. For these molecules, we find the molecular shape around one central atom at a time. Consider ethane (CH3CH3 molecular formula C2Hg), a component of natural gas (Figure 10.11 A). With four bonding groups and no lone pairs around each of the two central carbons, ethane is shaped like two overlapping tetrahedra... 

Three-dimensional models of ethane, propane, and butane. The ball-and-stick models at the left show the way in which the atoms are connected and depict the correct bond angles. The spacefilling models at the right are constructed to scale and give a better idea of the molecular shape, though some of the hydrogens may appear hidden. 

Ethane (CH3CH3 molecular formula C2Hg) is a component of natural gas. With four bonding groups and no lone pairs around the two central carbons, ethane is shaped like two overlapping tetrahedra (Figure lO.lOA). 

Zeng et al. have examined the coordination of ZnPc with three bipyridines, namely l,2-bis(4-pyridyl)ethane(61), Irans-1,2-bis(4-pyridyl)cthcnc (62), and 1,3-bis(4-pyridyl)propane (63) [63], The former two bipyridines are linear molecules favoring the formation of H-shaped supramolecular complexes ZnPc 61 ZnPc and ZnPc 62 ZnPc, while the last bipyridine adopts a V-shaped conformation leading to the formation of a T-shaped 1 1 complex (ZnPc 63). The molecular structures of all these complexes have been determined by X-ray diffraction analyses. The... 

Flexibility in ligands can lead to subtle or dramatic changes in architecture. For example, l,2-bis(pyridyl)ethane, dipy-Et, can readily adapt gauche or anti conformations. In the case of [Co(dipy-Et), 5(N03)2]n, which contains a T-shape node, infinite molecular ladders which contain six molecules of chloroform per cavity exist as the most commonly encountered architecture (Figure 3A).45b In such a situation all spacer ligands are necessarily anti. However, under certain crystal-... 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

The full structural formula of methane is often written as though it was flat. Such formulae are called projection or displayed formulae, but by using a molecular modd kit and remembering that covalent bonds are directed in space, you can construct the more correct 3-D shapes for methane, ethane, propane and butane. 

Make molecular models of ethane and ethene. Notice the tetrahedral shape and 109° bond angles around each carbon of ethane and the trigonal shape and 120° bond angles around each carbon of ethene. Also notice that you can rotate the single bond of ethane but not the double bond of ethene. 

Ethene is chemically more interesting than ethane because of the n system. As you saw in Chapter 5, alkenes can be nucleophiles because the electrons in the n bond are available for donation to an electrophile. But remember that when we combine two atomic orbitals we get two molecular orbitals, from combining the p orbitals either in phase or out of phase. The in-phase combination accounts for the bonding molecular orbital (w), whilst the out-of-phase combination accounts for the antibonding molecular orbital (w ). The shapes of the orbitals as they were introduced in Chapter 4 are shown below, but in this chapter we will also represent them in the form shown in the brown boxes—as the constituent p orbitals. 

Addition of an anti-solvent to a polymer solution causes the polymer solution to split into a polymer-rich phase and a solvent-rich phase. When a non-solvent is added the overall density of the original solvent becomes lower, which decreases the Lower Critical Solution Temperature (LCST) of the solution. A liquid-liquid phase-split thus occurs without raising the temperature. A low-molecular weight anti-solvent like CO2, propane or ethane can effectively decrease the LCST of the polymer solution (6) and thus induce a liquid-liquid phase-split. It is due to this effect that Gas Anti-Solvent precipitation of polymers has focused on the production of polymer particles with a specific size, structure or shape such as micro tubes (7) or micro balloons (8). Phase separation phenomena in PPE solutions during the formation of polymer membranes by the addition of a conventional anti-solvent have been described by (9). 

Deviations from the aU-anti conformations occur even when only one C-X bond is present in a 1,2-disubstituted ethane moiety. For example, solutions of dopamine provide mixtures of trans and gauche conformers in the protonated forms whereas the isolated molecules prefer the gauche conformation. The conformational equilibrium in such species often depends on a number of factors but understanding their interplay is important because the shape of neurotransmitters influences their transport properties and plays a key role in the molecular recognition at the receptor site. 

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