A comparison of sensitivities to substituent effects of five-membered heteroaromatic rings in gas phase ionization

Abstract

The molecular ionization potentials of a number of substituted furans, thiophens, selenophens, and pyrroles have been determined by the electron-impact technique. The p value for this reaction is more negative than that for the most selective electrophilic substitutions. The sensitivity to substituent effects appears to vary in the reverse order to that of the ' ground-state aromaticity ' of the rings (furan > pyrrole > thiophen > benzene).THE relative magnitudes of substituent effects in fivemembered heteroaromatic rings vary according to the type of reaction. For reactions in the side-chain, such as the ionization of carboxylic acids 293 and the polarographic reduction of nitro-derivatives+ the order of sensitivity is pyrrole > furan > thiophen > benzene.A different sequence is observed in electrophilic substitutions. Quantitative data which permit a direct comparison among the three heteroaromatic rings are available for trifluoroacetylation by trifluoroacetic anhydride in di~hloroethane.~ In this reaction the furan ring is the most and the pyrrole ring the least sensitive to substituent effects.Other data available seem to confirm that, so far as Preliminary communication on the ionization potentials of 2-substituted thiophens, S.

Experimental and computational energetic study of two halogenated 2-acetylpyrrole derivatives: 2-Trichloroacetylpyrrole and 2-trifluoroacetylpyrrole

“…The experimental value obtained by Linda et al [59], (885.7 ± 4.8) kJ Á mol À1 , differs by 16.8 kJ Á mol À1 from the result computed in this work {(868.9 kJ Á mol À1 )}. Similar enthalpic differences between the experimental and calculated data obtained with the same G3(MP2)//B3LYP approach were found for 2-and 3-fluoroanilines [60], as well as for 2-acetylpyrrole [9].…”

2-methyl furan: An experimental study of the excited electronic levels by electron energy loss spectroscopy, vacuum ultraviolet photoabsorption, and photoelectron spectroscopy

“…The sequence of the highest occupied molecular orbitals of the molecule has been studied by UV photoelectron spectroscopy [17][18][19][20][21][22] and semiempirical calculations. 19,21,23 The ordering of the outer valence molecular orbitals is the same as that of the unsubstituted molecule: 24 2 (b 1 ) 2 3 (a 2 ) 2 , in the C 2v notation, giving an X 1 A 1 electronic ground state.…”

Absolute photoionization cross sections of furanic fuels: 2‐ethylfuran, 2‐acetylfuran and furfural

“…The overall shape of the photoionization cross section of 2EF is very similar to that of 2MF. The literature ionization energies of 2EF and 2MF are 8.45 ± 0.05 and 8.38 ± 0.03 eV, respectively, which are very comparable with each other. These values are also very similar to our experimentally observed ionization energies for 2EF and 2MF of 8.43 ± 0.05 and 8.40 ± 0.05 eV, respectively.…”

“…Figure shows the photoionization spectrum of 2EF obtained from this experiment, with the corresponding photoionization cross section values listed in Table . From these data, it was identified that the threshold for 2EF ionization at 8.43 ± 0.05 eV agrees very well with the literature value of 8.45 ± 0.05 eV, and the CBS‐QB3 calculated AIE of 8.40 ± 0.05 eV. 2EF exhibits a shoulder at 10.0 ± 0.1 eV.…”

“…The first spectral band is a result of the removal of an electron from the bonding π highest occupied molecular orbital (HOMO) localized on the CC bonds shown in Fig. and also observed by Linda et al . This is also evident in the change of the CC bond lengths between the neutral and molecular cation.…”