Furan electronic structure

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

This technique links several others, especially mass spectrometry and UV absorption spectrophotometry, and its results often illuminate them. A relatively new technique, it has been used extensively to probe the electronic structure of furan and its allies a general review is available (80PAC1509). 

The nomenclature of peri-naphthalene heterocycles does not follow a common principle. In many original papers, the names of heterocyclic systems are derived from the corresponding peri-annelated hydrocarbon derivatives (1,2-diazaacenaphthylene, 1-oxaphenalene, etc.), from monoheterocycles with an indication of linked positions (naphtho[l,8-6c]furan, naphtho[l,8-de]azepine, etc.), and from benzoannelated heterocycles (benzo[o/]indole, benzo[heterocyclic systems and some compounds have trivial names, for instance, perimidine, naph-thostyryl, and naphtholactone. Moreover, it is necessary to remember some peculiarities in the electronic structure of peri-annelated heterocycles, namely the absence of independent existance of the 7r-closed-loop monoheterocycles which could be a fragment of peri-annelated heterocyclic systems. Therefore, the separation of a heterocycle from the united 7r-system is impossible. In this case, the simplest structure and the tt-electron unit is the whole peri-heterocyclic nucleus. 

The basis and extent of their aromaticity is discussed in Chapter 1. In summary, the capacity for the lone pair on a particular heteroatom to be delocalised is inversely related to the electronegativity of the heteroatom. For instance, furan is the least aromatic of the trio because oxygen has the greatest electronegativity and hence mesomeric representations 2.4b-e make relatively less of a contribution to the electronic structure of furan than they do in the cases of pyrrole and thiophene. The order of aromaticity is furan < pyrrole < thiophene. We shall see later how this variation in aromaticity affects the reactivities of these three related heterocycles. 

In contrast, 2- and 4-pyrones are considered to have relatively little aromatic character. Whereas in an analogous nitrogen series 4-pyridone 5.23 has significant aromatic character (mesomeric representation 5.23a making a considerable contribution to the overall electronic distribution), aromatic mesomeric representation 9.3a makes less of a contribution to the overall electronic structure of 4-pyrone. As with furan, the higher electronegativity of oxygen leads to heterocycles of little aromaticity in cases where delocalisation of electron density from the heteroatom is a prerequisite for that aromaticity. 

If the heteroatom contributes two electrons to the tr-electronic structure of the molecule (e.g., nitrogen in pyrrole or indole, oxygen in furan, sulfur in thiophene), it essentially represents a charge-transfer donor-type substituent. Its effect on the electronic structure and spectra will be dominated by its ability to donate electrons and to a lesser extent by considerations of electronegativity. Thus, the interactions of nitrogen, oxygen, and sulfur in, say, indole, benzofuran, and benzothiophene, with their aromatic systems, are similar to the interactions of amino, hydroxy, and mercapto exocyclic groups with their aromatic systems. 

Accurate investigation of the valence ionization spectra is important subject to elucidate the electronic structure of molecules. Ionization spectra of five-membered aromatic compounds have also been intensively studied. The high-resolution synchrotron radiation photoelectron spectra (SRPES) of furan and thiophene were measured and analyzed with asymmetry parameter up to about 40 eV [63,64]. The electron momentum spectroscopy (EMS) was also applied to furan up to 30-40 eV [65]. The ionization spectra of these molecules were also studied by several theoretical methods. However, there were some controversial assignments even for the outer-valence region, in particular for the peak position of Ibi(TTi) state and the inner-valence spectra have not been theoretically reproduced. 

A description of the electronic structure of the furan molecule is based on the assumption that all ring atonis are sp -hybridized (see Fig. 5.2a). The overlap of the five 2pz atomic orbitals yields delocalized r-MOs, three of which are bonding and two antibonding. 

Thiophene is aromatic. Its electronic structure follows from Fig. 5.2 (see p 53). Thiophene is a n-excessive heterocycle, i.e. the electron densitiy on each ring atom is greater than one. The value of the empirical resonance energy of thiophene is approximately 120 kJ moT the Dewar resonance energy is quoted as 27.2 kJ mol The aromaticity of thiophene is thus less than that of benzene but greater than that of furan. There are two possible explanations to account for the difference between thiophene and furan ... 

The differences in the bond lengths, especially between the bonds N/C-2 and N/C-4 indicate that the delocalization of the r-electrons is affected by the heteroatoms. As in the case of furan, the structural formula with two r-bonds is a good representation of the electronic structure of the molecule. The ionization energy of oxazole is 9.83 eV and its dipole moment is 1.5 D. 

Bis(cyclopentadienyl) complexes CP2M (M = Ge, Sn, Pb) have been prepared by reaction of cyclopentadienylsodium and an appropriate dihalide in tetrahydro-furan. Electron diffraction shows that in the vapour these molecules have angular structures on account of the electron pair on the metal. In the solid state the germanium compound polymerizes within 3 h at ambient temperature, the tin compound within five days. The cyclopentadienyl groups are coordinated in a symmetrical pentahapto fashion (p. 191). Association through the rings occurs in the crystalline lead compound to give a polymeric chain structure (Fig. 4.8). 

Any 4m -I- 2 system is aromatic, and hence cyclopropenyl cation, cyclopentadienyl anion, cycloheptatrienyl cation (called tropylium ion), pyrrole, and furan are all aromatic. They are all unusually stable, as described for cyclopropenyl cation in the following Connections highlight. Furthermore, they are all planar, like benzene. One would hope for a good theoretical justification for aromaticity, unifying the similar character of all these compounds. Indeed, aromaticity has been studied extensively with electronic structure theory methods. But, as we discuss in Section 14.5.1, there is still significant debate as to the origin of aromaticity. We leave a detailed presentation of the theory to that section of the book. 

Figure 5.2 Electronic structure of furan (a) sp -hybridization of the ring atoms, (b) energy level scheme of the jt-MO (qualitative) and occupation of electrons, (c) jt-MO (the O-atom is situated at the lowermost corner of the pentagon), and (d) Ji-electron densities calculated by ab initio MO...

The experimental data, also in gas phase, is from Ref. [41]. In the region below 7 eV, furan shows a series of Rydberg states over-imposed to a broad band. The nuclear-ensemble method provides a good qualitative prediction of the spectrum. The intensity and the shape of the broad band are in very good agreement with the experiment. The energy shift is caused by the electronic structure method (see Sect. 5.1), rather than by the spectrum simulation method itself. 

The electronic structures of benzo[c]furan (85) and its sulphur, selenium, and nitrogen analogues have been discussed in the light of their n.m.r. spectra for each system, bond fixation in the six-membered ring is indicated, independent of the nature of the heteroatom. Treatment of the rhodium complex (86) with m-chloroperbenzoic acid, sulphur, selenium, or nitrosobenzene leads to the quinones (87 X = O, S, Se, or NPh, respectively)/ ... 

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