Resonance energy estimation

The dearomatization of benzene requires that a resonance energy estimated at 36 kcal mol-1 be overcome [14]. For this reason, when benzene attacks an electrophile, the end result is typically a substitution product rather than an addition product. The initial cyclohexadienyl cation is unstable to deprotonation by even extremely poor bases, and it therefore loses a proton and returns to an aromatic state. 

G(J,/)) level [48], and by 7.8 kcal/mol based on the difference of their empirical resonance energies estimated from heats of combustion [128]. The classical explanation for this difference usually stops at the resonance energy argument. 

The extent to which benzene is more stable than either of the Kekule structures is its resonance energy, which is estimated to be 125-150 kJ/mol (30-36 kcal/mol) from heats of hydrogenation data... 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

Resonance energies of ca. 90, 182 and 330 kJ moF have been estimated for pyrrole, indole and carbazole respectively by comparing their protonation constants with those for selected model compounds (72C1(L)335, 72TL5019). 

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

The thermodynamic properties of thiophene,2-methylthiophene, ° and 3-methylthiophene have been computed from careful measurements of the heat capacity of the solid, liquid, and vapor states, the heat of fusion, the heat of vaporization, and the heat of combustion. From the heat of combustion of thiophene and from thermochemical bond energies, the resonance energy of thiophene has been re-estimated to be only 20 kcal/mole. 

C O , that is, the normal covalent carbon-oxygen double bond,- the estimated bond energy 6.60 v.e., then the ketones would show a resonance energy of 1.11 v.e. arising from the + —... 

The values of and can be estimated from their internal consistency to be accurate to about 0.1 v.e., the value of 1.71 v.e. for the resonance energy being accurate to about 0.15 v.e. The quantum mechanical discussion of resonance in benzene and naphthalene is given in the preceding paper.1... 

Other congeners of phosphinins—arsenin, antimonin, and bismin—have been shown to be definitely less aromatic than benzene by diverse theoretical treatments that have been reviewed.236 For instance, the Bird aromaticity index for arsenin was found to be 67, compared to 100 for benzene.123 Table 4 summarizes a few parameters used to estimate the aromaticity of heterobenzenes resonance energies... 

Chapter 9 is devoted to resonance energy transfer and its applications in the cases of donor-acceptor pairs, assemblies of donor and acceptor, and assemblies of like fluorophores. In particular, the use of resonance energy transfer as a spectroscopic ruler , i.e. for the estimation of distances and distance distributions, is presented. 

Time-dependent fluorescence measurements have been made on tyrosine in calf thymus nucleosome core particles by Ashikawa et al. S7) Based on the salt dependence of the decay data, the tyrosines were divided into two classes. At 20 to 400 mM salt, about half of the tyrosine residues appear to be partially quenched, possibly by resonance energy transfer to DNA bases. The other half are thought to be statically quenched, possibly by hydrogen bonds this quenching is partially eliminated at about 2 M salt. In view of the number of tyrosines per nucleosome core particle (estimated at 30), it is impossible to make a more detailed analysis of the decay data. 

As well as the major causes listed above, a number of more specific reasons have resulted in much work in some special cases. These include studies resulting directly or indirectly from an interest in the kinetics and thermochemistry of the reactions of methylene. In some cases, interest in reaction mechanisms of some alicyclic compounds in the liquid phase has led to gas phase work. Finally in some instances the pyrolytic studies have allowed estimates of resonance energy and strain energy to be made. 

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