Benzenoid PHs

Most benzenoid PHs actually can be characterized by a closed-shell electronic configuration accommodating their % electrons only in bonding orbitals. However, researchers faced difficulties for certain types of PHs due to their high reactivity. An important work by Bendikov et al. came into spotlight in 2004 when their computational study on oligoacenes supported that the longer acenes... 

The synthesis and study of open-shell PHs have become a rising hot topic nowadays, so we aim to provide a brief overview on recent advancements including theoretical studies and experimental characterizations of a series of benzenoid PH-based diradicaloids comprising higher order acenes, bis(phenalenyl)s. 

Fig. 30. The range of potentials in which the half-wave potentials of monosubsti-tuted and of most of the disubstituted benzenoid compounds can be expected, provided that the reduction of the substituted compound follows the same path as that for the parent compound of the particular reaction series. For pH 5—8 and unbuffered media...

The ultraviolet spectra of aspartame, obtained under various conditions, are shown in Figure 4. The spectral features up to about 225 nm are attributed to end absorption, primarily from the amide and carboxylate carbonyl groups. The benzenoid group is responsible for absorption in the range of 245 - 270 nm, exhibiting maxima at 252.2,257.6 (e = 752, anion s = 727, zwitterion and cation) and 263.0 nm, as well as shoulders at about 247 and 267 nm. There is minimal dependence of absorption on pH, as would be expected for a primary unconjugated amine, except in the end absorption region below 210 nm. 

Pauson obtained his Ph.D. from the University of Sheffield in 1949 and then moved to the Duquesne University in Pittsburgh, where he was offered the post of a temporary Assistant Professor without interview. Following his interests in non-benzenoid systems, he attempted to prepare fulvalene (see Scheme 5.1) but, as mentioned above, isolated ferrocene instead. After he finished his 2-years term at Duquesne, he spend the academic year 1951-1952 at the University of Chicago as a postdoctoral fellow working on peroxide chemistry with Morris S. Kharasch, in those days the pope in this field. In 1953, he was appointed to a DuPont... 

At a pH value substantially below its pKa, the indicator is largely protonated, and at pH values above its pKc, it is largely dissociated. The protonation-deprotonation of the indicator usually involves the conversion of a benzenoid moiety to its quinoid form, and it is this structural... 

Part of the folklore of nonbenzenoid hydrocarbons suggests fulvenes are on the nonaro-matic/aromatic border. It is thus not obvious whether these species really belong in this chapter. Yet, because their aromaticity is so much less than that found for their isomeric benzenoid derivatives we feel confident to proceed. Other than the parent hydrocarbon species 103 [i.e. 127 wherein (R, R ) = (H, H) most of the other thermochemically char-acterized fulvenes have substimtion on the exomethylene carbon cf (R R ) = (H, Me), (Me, Me) and (Ph, Ph) for reference, the suggested enthalpies of formation of the (H, H), (H, Me), (Me, Me) and (Ph, Ph) species are 224, 185, 144 and 402 kJmor respectively. Were all differences in steric interactions and contributions from the dipolar resonance structures of the generic type 128 negligible, then AH (127, R R ) and A//f(CH2=CR R ) would be linearly related. We find that a nearly perfect straight line... 

In this article we mainly focus on the central issue of vibronic coupling in the benzenoid systems viz., the phenyl radical (Ph ) and phenylacetyleneradical cation (PA +) and the lowest members of the family of the PAH radical cations viz., naphthalene (N +) and anthracene (AN +) radical cations. Consequences of this coupling for the nuclear dynamics of these systems are studied at length. The difficulties faced in the quantum mechanical treatments of these large systems are also discussed. Dynamical observables like the rich vibronic spectrum are calculated and assigned. The ultrafast nonradiative dynamics of the excited states is also studied. These observables are compared with the available experimental data to validate the established theoretical model [19-22]. 

The complexity in the assignment of molecular spectra is addressed by showing recent results on four representative examples viz., Ph, PA +, N + and AN +. The first two are directly derived from the JT active benzene system. Manifestation of the JT activity in these substituted benzenoid systems is also discussed. The mechanistic details of the observed photostability in the PAH radical cations, N + and AN + are examined. The discussions in this article reveal the need of understanding the complex vibronic coupling mechanisms while dealing with the electronically excited molecules in particular, and the recent advancements in the experimental and theoretical techniques to observe and treat them. 

It has previously been shown that in the triplet state a nitro group on a benzene ring is highly electron withdrawing at the 3- and 4- positions and induces decarboxylations, retro-Aldol type processes and a novel intramolecular redox-type reaction by way of nitrobenzyl carbanion-type intermediates in all cases. In this context, the irradiation of the three 4-nitrobiphenyl derivatives (217), (218) and (219) has been examined in order to assess the potential of photoexcited nitro groups to induce such processes across the biphenyl system.Indeed analogous reactions are observed in these biphenyls but with enhanced quantum efficiencies to those for the reactions in benzenoid compounds. Thus (217) yields the aldehydes (220) and (221) at pHs of 2 and 7, respectively, and (218) gives (222) and (223) under similar conditions, whereas (223) is also formed from (219) at pH 13 but no reaction is observed in neutral solution. The authors note that these results provide more evidence that the... 

Ozonolysis of benzo[h]thiophene yields mainly the aldehydes 46 (34%) and 47 (22%).204 Aldehyde 46, which may also be obtained from benzenoid precursors, reacts with active methylene compounds of the type RCH2C02H (e.g, R = C02H, CN, Ph) or RCH3 (e.g R = N02), to give a 2-(R-substituted) benzo[b] thiophene. The reaction is of synthetic value in cases where the substituent R cannot easily be introduced by more conventional routes (e.g., R = N02). 

Redox reaction PANI-like benzenoid —> quinoid, pH-dependent transformations [694-696],... 

As depicted in Fig. 2.7, PANI is found under a variety of forms differing in chemical and physical properties [41]. The structure consists of reduced (benzenoid diamine) and oxidized (quinoid diamine) repeated units. The form under which it is commercially available and stable under ambient conditions is Emeraldine Base (EB), possessing equal amounts of reduced and oxidized repeating units. The conductivity of this form is rather poor, but it can be easily converted into a more conductive form by lowering the pH of the solution to obtain the relevant fully protonated structure, namely Emeraldine Salt (ES). Eor this reason, PANI shows significant conduction in acidic solutions, namely below pH 2.5-3.0. 

Fig. 38. The ranges of half-wave potentials in which the reductions of monosubstituted benzenoid compounds at pH 5-8 and in unbuffered solution take place, provided that the reduction follows the same mechanism as that for the parent compound.

Mallion, R. B. Nuclear Magnetic Resonance a Theoretical and Experimental Study of the Spectra of Condensed, Benzenoid Hydrocarbons, Ph.D. Thesis University of Wales (University College, Swansea), U.K., 1969. 

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