Allenes planar

There are a number of questions about the reaction. Is the 3,3-shift reversible Is the allene planar or chiral What is the stereochemistry of the dimer(s) ... 

In conclusion, whether or not this species crystallizes in an allene-like or a trans-planar structure may depend on the crystal packing.43... 

Fig. 4 Ground-state and excited-state energies of the TICT complexes thioflavin T (a) and 9-(dicyanovinyl)-julolidine (DCVJ) (b) as a function of the intramolecular rotation angle (data from Stsiapura et al. [13] and Allen et al. [14]). In both cases, energy levels were determined by quantum mechanical simulations. For thioflavin T, the energy difference between Si and S0 corresponds to approximately 400 nm in the planar state and 470 nm in the twisted state. In the case of DCVJ, the energy differences correspond to 310 and 960 nm, respectively. The DCVJ energy levels reflect a rotation around the vinyl double bond...

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

These reactions point to the polar character of 179-Za and 293-Za. Already the first calculations resulted in a virtually planar geometry and thus in the conclusion that 179-Zj and likewise 293-Zj are the ground states [144]. In consequence, the allene structures 179 and 293 are not energy minima. Utilizing a higher level of theory,... 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Structure C is usually written with a pentavalent phosphorus this description, however, should merely denote that the phosphorus lone pair is delocalized and no allenic bond with two perpendicular jr-systems has been formed. H2C=P(H)=CH2, a typical representative of structure C phosphorus, can be described by an allyl-like r-system, ° with the p -type lone pair of a planar tricoordinate phosphorus participating in the 4-electron-3-center jT-bonding, without invoking d-orbital participation. " Thus, the C -type bonding of phosphorus involves tricoordination, with a planar bonding environment, whereby the lone pair is available for jT-bonding. 

The allene VCD spectra can be interpreted in terms of the ring current mechanism (114). As shown in 46 and 47, for a conformation that maintains a planar orientation due to conjugation with the allene, for both the phenyl and carboxyl groups relative to the adjacent portion of the allene group, an intramolecular hydrogen bond is possible between the carboxyl hydrogen and a x orbital of the central allene carbon atom. The antisymmetric C=C=C stretch can be de-... 

The molecular geometries of oximes in crystals are quite similar to those in the gas phase. The central carbons are planar. The C=N double bonds are primarily between 1.27 and 1.29 A in length (Figure 12) and the C—N—O angles are mostly 110°-114° (Figure 13). These histograms, which combine aldoximes and ketoximes, are based on data taken from Bertolasi and coworkers . For comparison, Allen and coworkers give the mean C=N distance in a collection of 67 crystalline oximes as 1.281 A, with a median of 1.280 A and standard deviation 0.013 A. ... 

A hypothetical doubly-bridged allene 123 of D2 symmetry was first formulated in Cahn, Ingold, and Prelog s classical paper 11 > in which they summarized their novel proposal for specifying various molecular chiralities. Despite the close structural resemblance between 123 and [m.m]betweenanene, Cahn, Ingold and Prelog s rules shows that the chirality of these two classes of compounds are to be specified as axial and planar respectively. 

Because the central carbon atom in allene has two charge clouds (two double bonds), it has a linear geometry and is sp-hybridized. Because the two terminal carbon atoms have three charge clouds each (one double bond and two C-H bonds), they have trigonal planar geometry and are sp2-hybridized. The central carbon uses its sp orbitals to form two cr bonds at 180° angles and uses its two unhybridized p orbitals to form... 

A series of nickel triad complexes ML3(allene) (Table VII) were prepared (122, 123, 139) and studied in solution by means of H NMR spectroscopy. Consistent with a planar molecular structure,... 

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