Resonance energy compounds

Mosshauer effect The resonance fluorescence by y-radiation of an atomic nucleus, returning from an excited state to the ground state. The resonance energy is characteristic of the chemical environment of the nucleus and Mossbauer spectroscopy may be used to yield information about this chemical environment. Used particularly in the study of Fe. Sn and Sb compounds. 

The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

The existence of the XeCHg [34176-86-8] cation has been estabtished ia the gas phase. The Xe—C bond energy of the XeCHg cation has been estimated to be 180 A 33 kJ/mol (112) and more recently, 231 A 10 kJ/mol (113) by ion cyclotron resonance. The compound Xe(CF3)2 [72599-34-9] is reported to be a waxy white sotid having a half-life of ca 30 min at room temperature (114). The synthesis iavolved the addition of XeF2 to a tritiuoromethyl plasma, but the characterization of this compound is limited and has not been iadependently confirmed. 

One of the more useful predicative applications of the relatively crude Hiickel method has been to illustrate quantitatively the effect of benzenoid annelation on the resonance energies of furan and thiophene. The results are summarized in Figure 1. As expected, thiophenes are more stable than the corresponding furans and 3,4-fusion results in less stable compounds than 2,3-fusion (77CR(C)(285)42l). 

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). 

Benzene rings can also be fused in angular fashion, as in phenanthrene, chrysene, and picene. These compounds, while reactive toward additions in the center ring, retain most of the resonance energy per electron (REPE) stabilization of benzene and naphthalene. ... 

Earlier studies of 4-aminopyridine 1-oxide were less conclusive. The solid-state infrared spectrum could be interpreted to indicate the existence of both the imino structure and/or, more probably, the amino structure. Comparison of the actual pKa value of 4-aminopyridine 1-oxide wdth the value calculated using the Hammett equation was considered to indicate that the compound existed as such or as an equilibrium mixture with l-hydroxypyrid-4-onimine, the latter possibility being considered the less likely on the basis of resonance and bond energies/ Resonance energy and ultraviolet spectral considerations have been advanced to support the 4-aminopyridine 1-oxide structure/ The presence of an infrared absorption band at... 

It is understandable that dihydro adducts should be formed by polycyclic compounds and not by benzene or pyridine, because the loss of aromatic resonance energy is smaller in the former than in the latter process, (c) When dibenzoyl peroxide is decomposed in very dilute solution (0.01 Af) in benzene, 1,4-dihydro biphenyl is produced as well as biphenyl, consistent with addition of the phenyl... 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

In the following paper of this series6 a value of about 1.7 v.e. has been found from thermochemical data for the resonance energy of benzene. Equating the negative of this quantity to 1.1055a, we calculate the value of a to be about —1.5 v.e. This value may not be very reliable, however, since it is based on the assumption that values of bond energies obtained from aliphatic compounds can be applied directly to aromatic compounds. 

From Table III we see that the difference between the free radical resonance energies of tribiphenylmethyl and triphenylmethyl is 0.07a. Hence X]/X2 = 37 = 2.2 X103. Ziegler and Ewald8 found that at 20°C the value of the dissociation constant for hexaphenylethane in benzene solution is 4.1 X10-4 and consequently we calculate for hexabiphenylethane a value of X = 2.2X103 X4.1 X 10 4 = 0.90. This value is probably too low as the compound is reported to be completely dissociated the error may not be large, however, since a dissociation constant of 0.90 would lead to 91 percent dissociation in 0.05M solution. 

Resonance Energies of Aliphatic Acids, Esters, Amides, and Related Compounds... 

Data are given in Table IV for heterocyclic compounds. For piperidine there is no difference between E and E, showing that the bond energies used are applicable to saturated heterocyclic molecules. Pyridine and quinoline differ from benzene and naphthalene only by the presence of one N in place of CH and, as expected, the values 1.87 v.e. and 3.01 v.e., respectively, of the resonance energy are equal to within 10 percent to the values for the corresponding hydrocarbons. 

The data given in Table VII for the quinones show large extra resonance energies of 0.57 v.e. in quinone, 1.42 v.e. in anthraquinone, and 1.4 v.e. in phenanthraquinone, in addition to the resonance energy of two benzene rings in the last two compounds. These large values we at-... 

Table V. Resonance energies of biphenyl and related compounds.

Compound Formula Structure E E Reso- nance energy Extra resonance energy... 

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