Molecule aromatic

Aromatic Molecules.—Much interest in the photophysics of large polyatomic molecules, such as the aromatics, has been generated by experimental efforts to characterize the intramolecular vibrational energy redistribution (IVR) of 

Castiglione, G. Freddi, and P. Morales, Chern. Phys. Lett., 1983, 97, 319. 

Jones collision frequencies, was found to reproduce the experimentally observed behaviour for all but the smallest collision partners, viz. He, H2, and D2. 

It is interesting to compare two papers which explore the dynamics of exciplex formation between electronically excited 1-cyanonaphthalene and triethylamine. Chewter et al. obtained exciplex fluorescence lifetimes as a function of temperature over the range 453—593 K. A kinetic scheme developed by Davis et was used to interpret the kinetic data and was 

The electronic transitions of organic molecules with the Jt-system are located in the UV region and sometimes also in the visible region with transition energies below 3 eV. The photophysics of Jt-systems is of great general interest. For example, natural photosynthesis is built around molecules related to porphyrin (see Chapters 14 and 15). 

FIGURE 3.13 Orbital energy diagram for x-systems and types of excitations. 

The calculations are best made following the strategy explained for the dienes, in the atom-by-atom mode. 

All this is best illustrated by an example 2-methylnaphthalene. The presence of the CH3 group offers the opportunity of using Eq. (11.17). The charges of the 

Example 14.1 Atom-by-Atom Calculation of 2-Methylnaphthalene. Using the A c values calculated from the shifts, counting 13.2 me for each aromatic carbon and 35.1 me for the sp reference, we get Xqc = 162.88 me and SA h = 162.88 10 X ( 11.7) = 45.88 me. Note the contribution of carbon-2, 

In the absence of experimental results, predicted A/// values are indicated in parentheses. Two A results fisted in the column reporting experimental values are indicated in parentheses these are theoretical results offered for comparison, deduced from enthalpies of formation calculated by Dewar and de Llano [269]. 

The unsigned average deviation between calculated and experimental energies is 0.36 kcal/mol for a collection of 35 benzenoid molecules. This result does not include 7,12-dimethylbenz[a]anthracene (36) the discrepancy of 16 kcal/mol between theory and experiment is in all likelyhood due in part to an error in the latter. Although certainly real, steric interactions involving the methyl group in position 12 are probably not so severe as to cause a destabilization exceeding that found in 1,8-dimethylnaphthalene and 4,5-dimethylphenanthrene—molecules that are discussed further below. 

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Birks J B 1970 Photophysics of Aromatic Molecules (London Wiley)... 

Add the necessary control lines to obtain the full MM3 minimal input file and run the file under the MM3 force field to obtain the enthalpy of formation of the aromatic molecule benzene. 

In both media a limit was reached beyond which the introduction of further activating substituents did not increase the rate of nitration this limit was identified as the rate of encounter of the nitronium ions and the aromatic molecules. 

For nitrations in sulphuric and perchloric acids an increase in the reactivity of the aromatic compound being nitrated beyond the level of about 38 times the reactivity of benzene cannot be detected. At this level, and with compounds which might be expected to surpass it, a roughly constant value of the second-order rate constant is found (table 2.6) because aromatic molecules and nitronium ions are reacting upon encounter. The encounter rate is measurable, and recognisable, because the concentration of the effective electrophile is so small. 

Under these first-order conditions the rates of nitration of a number of compounds with acetyl nitrate in acetic anhydride have been determined. The data show that the rates of nitration of compounds bearing activating substituents reach a limit by analogy with the similar phenomenon shown in nitration in aqueous sulphuric and perchloric acids ( 2.5) and in solutions of nitric acid in sulpholan and nitro-methane ( 3.3), this limit has been taken to be the rate of encounter of the nitrating entity with the aromatic molecule. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

However, the electronic theory also lays stress upon substitution being a developing process, and by adding to its description of the polarization of aromatic molecules means for describing their polarisa-bility by an approaching reagent, it moves towards a transition state theory of reactivity. These means are the electromeric and inductomeric effects. 

In this model, reaction is considered to occur preferentially at that position in the aromatic molecule to which the approach of the electrophile causes the smallest increase in zero energy. In molecules possessing polar or dipolar groups, long range electrostatic forces will initially be the most important. 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

Another difficulty is that the extent to which hydrogen bonded association and ion-pairing influence the observed kinetics has yet to be determined. However the high order of the reaction in the stoichiometric concentration of nitric acid would seem to preclude a transition state composed only of a nitronium ion and an aromatic molecule. 

In Table 1-9 we have collected only the 7r-bond orders calculated by allvalence-electrons methods and compared their values with those deduced from experimental bond lengths. Both data are indicative of an aromatic molecule with a large dienic character. The 2-3 and 4-5 bonds especially present a large double-bond character, whereas both C-S bonds are relatively simple. 

Organic aromatic molecules are usually sweet, bitter, a combination of these, or tasteless, probably owing to lack of water solubiUty. Most characteristic taste substances, especially salty and sweet, are nonvolatile compounds. Many different types of molecules produce the bitter taste, eg, divalent cations, alkaloids, some amino acids, and denatoirium (14,15). 

Catalytic Reforming. Worldwide, approximately 30% of commercial benzene is produced by catalytic reforming, a process ia which aromatic molecules are produced from the dehydrogenation of cycloparaffins, dehydroisomerization of alkyl cyclopentanes, and the cycHzation and subsequent dehydrogenation of paraffins (36). The feed to the catalytic reformer may be a straight-mn, hydrocracked, or thermally cracked naphtha fraction ia the... 

At pressures of 13 GPa many carbonaceous materials decompose when heated and the carbon eventually turns into diamond. The molecular stmcture of the starting material strongly affects this process. Thus condensed aromatic molecules, such as naphthalene or anthracene, first form graphite even though diamond is the stable form. On the other hand, aUphatic substances such as camphor, paraffin wax, or polyethylene lose hydrogen and condense to diamond via soft, white, soHd intermediates with a rudimentary diamond stmcture (29). 

Methylation of aromatic molecules can also be achieved ia the vapor phase, over a soHd catalyst. 

A.mina.tlon. Amination describes the introduction of amino groups into aromatic molecules by reaction of ammonia or an amine with suitably substituted halogeno, hydroxy, or sulfonated derivatives by nucleophilic displacement. Although reaction and operational conditions vary, the process always involves the heating of the appropriate precursor with excess aqueous ammonia or amine under pressure. 

Pyrazolediazonium salts (448) couple with activated aromatic molecules, like naphthols (79KGS805), and can be reduced to hydrazines (452) with tin(II) chloride (74MI40406). 

Isothiazole behaves as a typical stable aromatic molecule. Thermolysis of substituted isothiazoles at 590 °C leads to the formation of thioketenes (80MI41700) and phenyl-isothiazoles undergo photoisomerism (Section 4.17.6.2) (73BSF1743, 81T3627). 1,2-Benzisothiazole boils at 220 °C without appreciable decomposition, and the 2,1-isomer... 

J. B. Birks, in, (Wiley and Sons, London, 1970). A general review of the molecular spectroscopy of aromatic molecules. 

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