Pyrroles electronic structure

A comparative study of the electronic structures of A,A-diethyldithiocarbamate and pyrrole-A-carbodithioate has been undertaken.961 The enthalpy of formation of [Ni(S2CNMe2)2] (—146.1 10.9 kJmol-1) has been measured.962 The square planar dithiocarbamate complexes can be oxidized to the corresponding five-coordinate Ni111 dithiocarbamate complexes [Ni(S2CNR2)2X] (X = I, Br, C104) using Br2, I2, or (N0)C104.963,964... 

As a consequence of the smaller covalence of Cu(TPP), the pyrrole proton tensors are nearly axially symmetric and the Cu-H distances calculated with the entire unpaired electron at the Cu(II) ion are in excellent agreement with X-ray data. The difference in covalency of Ag(TPP) and Cu(TPP) is also reflected by the s-spin densities on the pyrrole protons which amount to PH(Ag) = 0.15% and pH(Cu) = 0.093%, respectively. A comparison with the corresponding data of an Xa calculation on Cu(II)-porphine1725 (oh(Cu) = 0.071%) indicates that the state-of-the-art electronic structure calculation underestimates the amount of unpaired spin density on the porphyrin ring. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

The whole series of pyrrol-l-ylborates, M[BH (NC4H4)4 ], has been synthesized (Tables 9 and 12). The mechanism of their formation and their hydrolysis kinetics have been studied in detail.100 The hydropyrazol-l-ylborates are very stable compounds and can be prepared in acid form.101 The di- and tri-pyrazol-l-ylborates are extensively used as complexing ligands (see Chapter 13.6). The photoelectron spectra of the sodium and thalliumfl) hydrotris(pyrazol-l-yl)borates and the electronic structure of the anion itself are reported.11 ... 

An alternative approach to the experimental estimation of REs utilizes equilibrium (protonation) data rather than thermochemical data, the idea being that comparisons of the basicities of pyrrole and its benzo fused analogues with those of non-aromatic systems which form cations of 7r-electron structure similar to the aromatic compounds should furnish a measure of the loss of RE accompanying protonation of the aromatic system (76T1767, 72CI(L)335). Thus, for the a-protonation of N-methylpyrrole, the model non-aromatic system was chosen as (20). Combination of pKa values for the protonation of the aromatic and non-aromatic molecules, taking into account the intrinsic resonance stabilization of the... 

This review, which covers the literature up to mid-1968, presents the knowledge, which is available to date, concerning the molecular and electronic structure of pyrrole. It also surveys the spectroscopic and nonspectroscopic physical properties of pyrrole and its simple derivatives. During the past 25 years, as a result of developments and improvements in instrument design, the literature on the physicochemical properties of organic compounds has expanded rapidly. Mainly as a result of the instability or nonavailability of many compounds, however, the study of pyrroles has lagged somewhat behind... 

Over the past 20 years and, in particular since 1955, many theoretical studies of the electronic structure of pyrrole using the molecular orbital approach with varying degrees of refinement have been reported. The 7r-electronic structure of pyrrole has been extensively discussed in terms of both the simple Hiickel molecular orbital (LCAO) theory37- 41-55-65 and the more sophisticated self-consistent field molecular orbital method (SCFMO method).18- 66-77 Extended... 

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. 

Indole is a ten-7t electron aromatic system. As with pyrrole, delocalisation of the lone pair of electrons from the nitrogen atom is necessary for aromaticity. The single overall electronic structure of indole is not completely described by structure 7.1, because this implies localisation of the lone pair on the nitrogen atom. Mesomeric representation 7.1a makes a contribution to the electronic structure of indole, as to a lesser extent do mesomeric representations where the negative charge occurs on the benzenoid ring. 

NMR spectroscopy is uniquely effective in probing the oxidation state, spin state, and ligation state of iron porphyrins (47, 48) and iron N-alkylporphyrins (22). For iron-tetraarylporphyrins the /3-pyrrole proton resonance is most indicative of the electronic structure of the metal... 

The pyrrole anion, C4H4N -, is a 6 n electron species that has the same electronic structure as the cyclopentadienyl anion. Both of these anions possess the aromatic stability of 6 n electron systems. 

The EPR spectra of the radical cations derived from pyrrole solutions can all be simulated by electronic structures in which the unpaired electron is located in an orbital with the nodal plane on the nitrogen and (2) showing large coupling constant values at the 2- and 5-positions and small values at the 3- and 4-positions <2000J(P2)905>. The EPR parameters of the radical cations from 2,5-dimethyl-l-phenylpyrroles and 3,4-bis(alkylthio)-2,5-dimethyl-l-phenylpyrroles (Table 30) denote a marked stabilization of the radical cations by the sulfanyl groups through mesomeric effects. 

Conformational and electronic structure of fused pyrrole systems and assemblies... 

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