Aromatic molecules polyenes

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

We first consider benzene, 17, the prototypical aromatic molecule. From the entries in Table 3.21 and comparisons analogous to Example 3.15, one can recognize that conjugative stabilizations in benzene are significantly stronger than those of comparable species in Table 3.19. Thus, on a per-pi-bond basis, the estimated stabilizations in benzene are 40.8 kcal mol-1, more than three times those of diene 2 (12.8 kcal mol-1), twice those of the acyclic triene 11 (20.3 kcal mol-1), and about 58% greater than the most strongly stabilized polyene, 16 (25.8 kcal mol-1). 

In 1937, Coulson s mentor, Lennard-Jones, wrote a series of papers on molecular orbitals applied to polyenes and aromatic molecules, discussing the variations in bond lengths in conjugated and aromatic molecules from a theoretical point of view. 92 He involved his student in the work. Coulson defined... 

Whereas the benzene molecule possesses a structure of D6A symmetry with equal lengths of the CC bonds, for acyclic polyenes alternation of bond lengths is a characteristic [87JCS(P2)S1]. For antiaromatic molecules, alternation is even more pronounced and unlike the aromatic molecules, a high-symmetry structure of the lowest singlet state of the antiaromatic molecules does not correspond to a minimum on the PES. For... 

Unsaturated hydrocarbons, such as alkenes, polyenic material, as well as aromatic molecules, are our target. Carbonyl compounds are considered in Chapter 16. 

Results of MNDO calculation of lA-azonine (35 X=NH) are in agreement with experimental evidence that this is a planar, aromatic molecule. The calculated geometry of oxonin (35 X = 0), as a buckled, unsymmetrical polyenic heterocycle, is also in agreement with its known properties. The MNDO calculations on thionin (35 X = S) indicate that this molecule is planar, which should allow effective tt delocalization, and at least some aromatic character (86MI927-OI). The topological resonance energy model also predicts lA-azonine and thionin to be aromatic, and oxonin nonaromatic (84JHC273). 

In 1969, Suppan reviewed experimental data about dipole moment changes in excited states of substituted aromatic molecules and suggested a theoretical approach according to which charge transfer occurs if the lowest vacant orbitals are very close in energy.41 In 1978, Birks introduced the term horizontal radiationless transition, which was applied to intramolecular rotation in stilbene and polyene derivatives.37 In this... 

G. W. Wheland, Proc. R. Soc. London, Ser. A 159, 397 (1938). The Electronic Structure of Some Polyenes and Aromatic Molecules. V—A Comparison of Molecular Orbital and Valence Bond Methods. 

In contrast to polyenes the aromatic molecules exhibit not only the Ti — So absorption, but also the longlived T — So emission, which gives rise to phosphorescence phenomena of rigid solvents and crystals. This is another important field of applications of spin-orbit quadratic response theory. Such calculations refer to lifetimes, transition moments, oscillator strengths and polarization directions for the radiative decay of molecular triplet states. These quantities may either be averaged over the triplet levels or refer to specific triplet spin sublevels depending on the conditions for the relevant experimental measurements. 

Guaratini, T. Lopes, N.P. Pinto, E. Colepicolo, P. Gates, P.J. 2006. Mechanism for the elimination of aromatic molecules from polyenes in tandem mass spectrometry. Chem. Comm. 39 4110 112. 

In contrast to compounds of aromatic and polyene-like electronic structure, polymethines are conjugated chain molecules with equal bond lengths and charge alternation along the methine chain [18, 19], They exhibit the following common structural features ... 

Coulson, C.A. (1938) The electronic structure of some polyenes and aromatic molecules. IV. The nature of the links of certain free radicals. Proc. R. Soc. London Ser. A, 164, 383-396. 

Bond-bond interactions are important in doubly bonded or 7t systems, such as polyenes and aromatic molecules, and even in partially conjugated... 

At present, then, aromaticity is best defined in terms of stability derived from the delocalization of bonding electrons. An aromatic molecule is characterized by appreciable stabilization relative to a noncyclic polyene. An antiaromatic molecule is one that is destabilized relative to a polyene model, and the term nonaromatic can be applied to molecules for which the calculated energy and energy of the polyene model are comparable. Cyclobutadiene, with an estimated destabilization energy of 15-20 kcal/mol, is a good example of an antiaromatic species. 

A good set of models will show this strain easily. Construct the planar, aU-cis cyclodecapentaene and the strain will be apparent. If you are lucky, the plastic will mimic the molecule and the model will snap into a pretzel shape, which approximates the energy minimum form of the molecule. These molecules are best described as strained polyenes. They fail the test of planarity for aromatic molecules because the planar structures are much more strained than the nonplanar forms. In this case, strain trumps aromaticity. 

Coulson, C. A. 1939. The electronic stmcture of some polyenes and aromatic molecules. VII. Bonds of fractional order by the molecular orbital method. Proceedings of the Royal Society of London. Series A 169 413-428. 

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