The Structure of Aromatic Molecules

In the foregoing discussion of the structure of benzene the stability and characteristic aromatic properties of the substance have been attributed to resonance of the molecule between the two Kekul6 structures. A similar treatment, which provides a similar explanation of their outstanding properties, can be given the condensed polynuclear aromatic hydrocarbons. 

For naphthalene the conventional valence-bond structure is the Er-lenmeyer structure  

There are two other structures that differ from this only in a redistribution of the bonds  

These three structures, the most stable valence-bond structures that can be formulated for naphthalene, are seen to have about the same energy and to correspond to about the same molecular configuration. It is to be expected then that they will be combined to represent the normal state of the naphthalene molecule, to which they should contribute about equally. Resonance among these three stable structures should stabilize the molecule to a greater extent than does the Kekul resonance in benzene, involving two equivalent structures it is seen from Table 6-2 that the resonance energy of naphthalene, 75 kcal/mole, is indeed much greater than that of benzene. 

For anthracene four stable valence-bond structures can be formulated, 

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The error in Hiickel s treatment lies not in the quantum mechanical calculations themselves, which are correct as far as they go, but in the oversimplification of the problem and in the incorrect interpretation of the results. Consequently it has seemed desirable to us to make the necessary extensions and corrections in order to see if the theory can lead to a consistent picture. In the following discussion we have found it necessary to consider all of the different factors mentioned heretofore the resonance effect, the inductive effect, and the effect of polarization by the attacking group. The inclusion of these several effects in the theory has led to the introduction of a number of more or less arbitrary parameters, and has thus tended to remove significance from the agreement with experiment which is achieved. We feel, however, that the effects included are all justified empirically and must be considered in any satisfactory theory, and that the values used for the arbitrary parameters are reasonable. The results communicated in this paper show that the quantum mechanical theory of the structure of aromatic molecules can account for the phenomenon of directed substitution in a reasonable way. 

The agreement of the two treatments with each other and with the empirical resonance-energy values makes it probable that the point of view presented above regarding the structure of aromatic molecules will not need extensive revision in the future, although it may be subjected to further refinement. 

During his researches into the history of the theory of the relationship between colour and constitution of organic compounds,167 168 Dahne has examined the contributions of W. A. Ismailsky (1885-1973), whose career (mainly in Moscow) spanned both the closing years of pre-revolutionary Russia and the Soviet period.169170 Ismailsky appears to have been one of those who anticipated the theory of resonance in connection with the structures of aromatic molecules. This was in his thesis at the Technical University of Dresden in 1913, where he had worked under the direction of Walter Koenig. 

A close examination of the structures of aromatic molecules shows that they all share two common features. 

Together with singly charged ions doubly and multiply charged ions may also arise in the ionization process. However, the number of doubly charged and especially of multiply charged species is much lower. The yield of these ions depends on the structure of a molecule and on the experimental conditions. For example, polycyclic aromatic hydrocarbons give more ions of these types compared to aliphatic or monoaromatic compounds. 

In biological systems, H-bond donors and acceptors are predominantly nitrogen and oxygen atoms. However, the n electrons of aromatic systems can also act as acceptors, and H-bonds involving sulfur groups or metallic cofactors are also known. The presence of individual H-bonds in biomacromolecular structures is usually derived from the spatial arrangement of the donor and acceptor groups once the structure of a molecule has been solved by diffractive or NMR techniques. More detailed information about H-bonds... 

The theory of molecular structure based on the topology of molecular charge distribution, developed by Bader and co-workers (83MI2 85ACR9), enables certain features to be revealed that are characteristic of the systems with aromatic cyclic electron delocalization. To describe the structure of a molecule, it is necessary to determine the number and kind of critical points in its electronic charge distribution, i.e., the points where for the gradient vector of the charge density the condition Vp = 0 is fulfilled. 

The elemental and binding state information obtained from the XPS analysis is in good agreement with, and sometimes complementary to, the information from the TOFSIMS spectra. The information that can be extracted from the SIMS spectra is, however, more detailed in terms of the structure of surface molecules and is also more surface-sensitive. For instance, the high degree of aromaticity in the surface of the film, as seen very clearly in the SIMS spectra (e.g. at +117 amu), can be confirmed only with difficulty by XPS, because the only feature indicating aromaticity is the ji-ji transition, which is not clearly seen in Fig. 1. 

Fluorescence is greatly affected by the structure of a molecule. Usually only aromatic compounds fluoresce although some aliphatic and alicyclic molecules are known to fluoresce. Electron-donating groups such as -OH and -OCH3, that can increase the electron flow of an aromatic system usually increase the fluorescence while other groups that contain hetero atoms with n-electrons that can absorb the emitted energy, will usually quench the fluorescence. However, it is always difficult to predict whether or not, or to what extent, a compound will fluoresce. 

Interactions between 7t-systems have long been observed in crystal structures of aromatic molecules and, for example, play a role in stabilising DNA through vertical base-pair interactions. They are also involved in the intercalation of drugs into the grooves of DNA. Such interactions have also been exploited in many synthetic host-guest systems. Previous rationales for the occurrence of k-k interactions have included explanations based on solvophobic,electron donor-acceptor as... 

A molecule that does not contain polar or ionizable groups is not likely to be soluble in water. The structures of biological molecules that arc not water soluble generally contain only aromatic or aliphatic (alkane-like) components. These groups do not interact well with water and are said to be lipophilic. If an alkane (e.g., octane) is added to a container of water it will not associate with the water but will form a separate layer on top of the water. (If the molecule were more dense than water, it would sink to the bottom to form a separate layer)... 

If a piece of fat is added lo the container, it will float on the water layer and absorb some of the alkane. Some of the alkane will "dissolve" into the fat. If an aromatic liquid is added e.g., ben2ene), it also will associate with the piece of fat. These matedais are fat soluble or lipophilic. Fat-soluble nutrients associate with fat, not because they are forced away from water by some repulsive force but because their moJecules are attracted by those in the fat Water does not "repel" fat. Figure 1.13 shows the structures of two molecules that are not water soluble. The structure at the left is octane, an alkane. The structure in the center is benzene, an aromatic molecule, which usually is simplified as shown on the dght. 

Excimer formation is observed quite frequently with aromatic hydrocarbons. Excimer stability is particularly great for pyrene, where the enthalpy of dissociation is A// = 10 kcal/mol (Fbrster and SeidI, 1965). The excimers of aromatic molecules adopt a sandwich structure, and at room temperature, the constituents can rotate relative to each other. The interplanar separation is 300-350 pm and is thus in the same range as the separation of 375 pm between the two benzene planes in 4,4 -paracyclophane (13), which exhibits the typical structureless excimer emission. For the higher homologues, such as 5,5 -paracylophane, an ordinary fluorescence characteristic of p-dialkyl-benzenes is observed (Vala et al., 1965). 

We report an in situ IR spectroscopic study performed during sorption and kinetic measurements to show that these adsorbate structures consequently predetermine the chemical selectivity. The importance of the individual surface species (and their relative concentrations) on the activity and selectivity of the catalyst for the alkylation of aromatic molecules is investigated. 

In Chap. 10 we discussed the structure of aromatic compounds. An aromatic molecule is flat, with cyclic clouds of dclocali/ed n electrons above and below the plane of the molecule. We have just seen, for benzene, the molecular orbitals that permit this delocalization. But delocalization alone is not enough. For that special degree of stability we call aromaticity, the number of n electrons must conform to HuckeFs rule there must he a total of 4n -f 2) rr electrons. 

The structure of hydrotrope molecules, as mentioned earlier, is characteristic (Figure 2.1). One finds a short, predominantly aromatic hydrophobic chain and in most cases an ionized polar group. With this structure in mind, it is not surprising that the association structures of the hydrotrope molecules in water have attracted some interest over the years, even if these may not be the decisive feature in the practical applications of these compounds. 

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