Structurally planar

What is the MM3 enthalpy of formation at 298.15 K of styrene Use the option Mark all pi atoms to take into account the conjugated double bonds in styrene. Is the minimum-energy structure planar, or does the ethylene group move out of the plane of the benzene ring ... 

Figure 1,13 Expansion and/or compression effects on crystal structures (planar section). A = shared edge B = shared corner. From R. M. Hazen and L. W. Finger, Comparative Crystal Chemistry, copyright 1982 by John Wiley and Sons. Reprinted by permission of John Wiley Sons.

Squaric acid (H2SQ) has been chosen as a first test compound because it has a very simple molecular structure. Planar sheets of the squarate (C4O4) groups are linked to each other in a two-dimensional network through O - H...0 bonds (Fig. 1) with weak van der Waals forces [52,53]. The protons perform an order/disorder motion above the antiferroelectric phase transi-... 

There are several examples of complexes with type Ila structures (planar trans end-on bridging coordination). Vibrational spectroscopy indicates that this structure occurs in [(CN)5Co(S2)-Co(CN)5]6- (35). Trans arrangement of the metal atoms has been proved by X-ray structure determination for [(NH 3 )5 Ru(S2 )Ru(NH)3 )5 ]C14. 

A heuristic potential term to keep peptide bonds and aromatic ring structures planar is given by the improper dihedral term... 

Why is this structure planar and why are the bond lengths different from their standard values This sounds like the situation with benzene and again the answers lie in the molecular orbitals that can arise from the combination of the six p orbitals. Just as in benzene, these orbitals can all combine to give one big molecular orbital over the whole molecule. However, the p orbitals can overlap and combine only if the molecule is planar. 

Since the p orbitals on carbons 2 and 3 overlap, there is some partial double bond character in the central o bond, helping to keep the structure planar. This overlap means that it is slightly harder to rotate this formal single bond than might be expected—it requires about 30 kj mol"1 to rotate it whereas the barrier in propene is only around 3 kj mol-1. 

It is interesting to note that, due to its uniform and nearly straight pore structure, planar anodic alumina membranes have been used as the probes for monitoring catalyst deactivation at the single pore level [Nourbaksh et al, 1989a]. The nearly idealized structure is suitable for various surface analysis techniques such as scanning and transmission election microscopy and associated EDX and XPS to be applied to fresh as well as spent catalysts. In the case of hydrotreating of crude-derived heavy oils, catalyst... 

Enumerate all the possible symmetry groups for hypothetical molecules with the formula X2Y2, with a sketch of each structure. These will include (i) linear structures, (//) planar nonlinear structures and (Hi) notiplanar structures. 

Electronic excitation energies are shifted to longer wavelengths as the extent of molecular conjugation increases. The molecular symmetry that accompanies the structural planarity created by conjugation might... 

Figure 12. Local equilibrium and saddle structures of a four-atom Morse cluster, M4. The local equilibrium structures (regular tetrahedron) and the Hrst rank saddle structures (planar rhombus) have the energy —6.00s and —5.02s, respectively. cp1 cp2, and tp3 are the hyper-angles that specify each reaction channel. When tp2 = 3, (P and cp3 cannot be determined uniquely and only the summation cp3 +

Graphitized carbon black (GCB) has been reported to be a highly effective sorbent for sample clean-up but also removes structurally planar analytes, limiting its applications. Addition of HOAc to the extraction solvent may help to improve recovery of analytes, but it also inhibits the retention of acidic matrix compounds. Several papers have reported the use of Cl8 for dSPE in veterinary residue analysis. " The use of PSA, NH2, and silica has also been reported. While dSPE does not provide the same degree of clean-up as SPE, it does provide good recovery and reproducibility, coupled with practical and cost advantages. ... 

The ealculations demonstrate that equilibrium geometrical parameters of the eomplex change insignifieantly in the ealeulations by different methods. It should be noted that in the struetures obtained without electron correlation the atoms 3, 9, 10, and hydrogen of water molecule are in one plane. The structure planarity is distorted when considering electron correlation by the perturbation theory MP2. Even more considerable deviations... 

The structure planarity is broken with the inclusion of polarization functions into the basis set and electron correlation consideration. The semi-empirical calculation forecasts the planarity disturbance of the atom group considered. The hydrogen bond for j -naphthol complexes is longer than one for a-naphthol. These values are 0.1812 nm for cw-yff-naphthol and 0.1819 nm for trara-yS-naphthol calculated at HF/6-31G level. The values obtained at MP2/6-31G level oftheory are 0.1861 nm and 0.1862 nm. They are in accordance with the results of more accurate calculations 0.1849 nm and 0.1850 run [9]. 

Fig. 1.18 The face centered cubic and hexagonally close packed lattice are associated to two different stacking arrangements of hexagonally structured planar lattices. The dashed triangles indicated in the upper and lower layers of the fee lattice are centered on the atom indicated by a...

Smaller A components like Yb favour the formation of structures with tetrahedral or distorted tetrahedral coordination of the transition metal. This trend is obvious from the compoimds reviewed here, where representatives with AIB2 or ZrBeSi structures (planar TaXa hexagons) are scarce, but occur frequently in Eu7X compounds due to the larger europimn atoms (Pottgen and Johrendt 2000). 

The X-ray diffraction of a bundle of ten pieces of P(3HB) fibers (beam size 300 pm) as shown in Fig. 2c includes reflections simultaneously from both the a-structure (2/1 helix conformation Okamura and Marchessault 1967 Yokouchi et al. 1973) and the p-structure (planar zigzag conformation Orts et al. 1990) of P(3HB) that are drawn in Fig. 3. It is well known that P(3HB) crystallizes as an orthorhombic crystal system with unit cell parameters of a = 0.576 nm, ft = 1.320 nm, and c(fiber axis) = 0.598 nm and space group (a-structure ... 

More recently, we succeeded in revealing the crystal structure of PHB depolymerase from Penicillium funiculosum (Hisano et al. 2006). The trimer substrate of (/ )-3-hydroxybutyrate with a planar zigzag conformation was perfectly bound in a crevice of the active site. This result suggests that the P-structure (planar zigzag conformation) is degraded faster than the a-structure (2/1 heUx conformation), which was obtained from the enzymatic degradation of P(3HB) fibers with two kinds of molecular conformations. These results indicate that the rate of enzymatic degradation can be controlled by the molecular conformations of polymers. 

Dihydroxypyridinium chloride, crystal structure, planar cations, 744... 

The present model experiments are concerned with an important category of industrial hazards, i.e., with the problem of flame acceleration in atmospheric gas/air explosions due to obstacles in the flame path. As a system simulation of industrial structures, planar grids consisting of cylindrical parallel rods inserted in a square, cross-sectional channel were chosen for the experiments. The variation of the rod diameter and the number of rods allowed investigation of the influence of the turbulence properties (turbulence intensities and macro length scales) on flame acceleration. 

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