Furan ionization potentials

Benzo[b]furan, 2,3-dihydro-2-phenyl-synthesis, 4, 697 Benzo[b]furan, 2,3-dihydroxy-tautomerism, 4, 37 Benzo[6]furan, 4,6-dimethoxy-acylation, 4, 606 Benzo[6]furan, 2,3-dimethyl-acetylation, 4, 606 bromination, 4, 605 photooxygenation, 4, 642 Benzo[b]furan, 5,6-dimethyl-2,3-diphenyl-applications, 4, 709 Benzo[b]furan, 1,3-diphenyl-vertical ionization potential, 4, 587 Benzo[b]furan, 2-ethoxy-5-hydroxy-synthesis, 4, 127... 

The ionization potentials, using mass spectrometry, for both 2-hydroxy-and 3-hydroxythiophenes have been compared with data for compounds derived from either tautomeric form in order to analyze the tautomeric composition.124 125 In the 2-hydroxy-substituted system the enol isomer could not be detected. Of the two possible unsaturated lactones the oc,/l-unsaturated form was the major isomer. In the 3-hydroxy-substituted case both the oxo form and the enol form are important. The position of the equilibrium was compared with those of the corresponding furan and sele-nophene systems for both isomers. 

The potentially tautomeric side-chain thiol systems exist mainly in the thiol form in liquid solution and in the gas phase, as found by IR and NMR spectroscopy and by a study of ionization potentials.126 Upon alkylation using the ion-pair extraction method, only the S-alkylated compounds were obtained. The synthesis, reactions, and properties of some selenides of thiophene, furan, and selenophene have been reviewed.127... 

Fio. 19. Comparison of the two lowest adiabatic ionization potentials in benzene with the three lowest in pyrrole and furan. The values arranged as an energy level diagram were obtained by photoelectron spectroscopy. (T, N. Badwan and D. W. Turner, unpublished work.)... 

Photoelectron spectra have been reported for 2,4 and A-methylisoindole ° and the ionization potentials (IP) assigned in the light of nonempirical calculations using Koopmans theorem. Linear correlations of the type IPobs = I Peak + b were obtained in all three cases. As was noted, extended Hiickel, PPP, and other semiempirical calculations also led to satisfactory correlations of the first three IPs, but the scatter was generally larger. The first IP of 4 lies in the order of 7.9 eV (Fig. 1 of Palmer and Kennedy ) a value of 7.91 eV has been reported by other authors. In comparison, the first IP of 1,3-diphenylbenzo[c]furan is 7.09 eV. ... 

The TT-electron excess of the five-membered rings is accompanied by a high rr-donor character. The best measure of rr-donation is the value of first ionization potential, IP, which for all aromatic heterocycles with one heteroatom of pyrrole type reflects the energy of highest occupied rr-orbital. IP, values decrease in the sequence pyrrole > indole > carbazole furan > benzo[/ ]furan > dibenzofuran thiophene > benzo[/ ]thiophene (Section 2.3.3.9, Tables 21 and 23). Thus, the more extensive the rr-system, the stronger is its electron donor ability. Furan and thiophene possess almost equal rr-donation, which is considerably lower than that of pyrrole. 

The ab initio techniques have also been employed to estimate other molecular properties such as electronic spectra and ionization potentials. These studies have been performed on furan (84CPH(90)231, 85JCP(83)723, 89JCS(P2)263, 93JA6184), in comparison with other five-membered rings and (83JST(105)375) with benzoheterocyclic derivatives. These ab initio calculations provide values for molecular properties in accordance with experimental trends, but it is necessary to consider the effects of electron correlation for the calculations to quantitatively reproduce experimental values. 

In CT complexes such as that between furan and tetracyanoethylene split maxima are often observed in the electronic absorption spectrum, since inner, as well as the outermost, occupied orbitals of the (donor) heterocycle are involved. However, the absorption maxima run parallel with the first ionization constants and can be used to predict them (75JCS(F1)2045, 68MI31000). Again, furan is thought to quench the fluorescence of 1-cyanonaphthalene because it forms an exciplex intermediate with a fluorescence of its own to the red side of the original one the exciplex has CT character and the frequency of the new fluorescence correlates with the first ionization potentials in a series of furans (75CC203). 

Table 36 Vertical Ionization Potentials (eV) for Furan Derivatives...

Most of the information presently available has been obtained with a view to comparing selenophene and tellurophene with furan and thiophene, and has already been discussed in Chapter 3.01. The resulting ionization potentials are in good agreement with those deduced from the spectra of charge transfer complexes with tetracyanoethylene (82CS(20)214, 75JCS(F1)2045). 

The lack of reactivity of the aryloxazolinones (65) in photocycloaddition to many of the olefins other than 1,1-dimethoxyethene and furan probably results from efficient decay of E2 or D. Exciplex E2 and diradical D are proposed as intermediates in these cases for several reasons. Exciplex formation is most likely dependent on olefin ionization potential, and the ionization potential of many of the un-reactive olefins are intermediate between the ionization potential of furan and 1,1-dimethoxyethene as determined from the maxima of tetracynoethylene olefin charge transfer bands60 66,67. Although ds-2-butene does not form a cycloadduct with 2-phenyl-2-oxazolin-4-one (65a), ds-2-butene is isomerized to rram-2-butene during the irradiation52. Cis-trans isomerization is expected from decay of a triplet diradical. Decay of the exciplex and diradical intermediates in competition with reaction presumably results from steric hindrance from the aryl substituent. The olefins which give cycloadducts, furan and 1,1-dimethoxyethene, are expected to produce low steric hindrance with the aryl substituent in an exciplex or diradical. 

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

Figure 14 shows a plot of the ionization potentials of substituted furans against those of the corrresponding thiophenes the slope is, remarkably, greater than unity (1.28). 

Fin. 14. Plot of ionization potentials (I.P.) of substituted furans vs. ionization potentials of substituted thiophenes (P. Linda, G. Marino, and S. Pignataro, J. Chem. Soc. B, in press). 

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