Pyrrole orbital structure

The question of the correctness of such an explanation of the nonplanarity of 2-azirine and the planarity of pyrrole was analyzed in Mo et al. [89JMS(201)17]. The evolution of the MOs as a function of the pyramidalization angle in nitrogen was traced. For both 2-azirine and pyrrole nitrogen pyramidalization leads to the stabilization of all 7t-MOs of the planar structure and, conversely, to the destabilization of several [Pg.368]

Potts and McKeough81 have recently obtained several interesting mesoionic azapentalenes 83, 84 by reaction of the appropriate 1,4-dicarbonyl compound with phosphorus pentasulfide. Compounds 83 and 84 can be written as dipolar structures, but participation of sulfur d-orbitals allows nonpolar forms to be envisaged. MO calculations neglecting rf-orbital participation predict the thieno[3,4-c]pyrrole system (84) to be very unstable.430 A related system 86 has been prepared by Japanese workers63 from the diamino-r-triazole (85) and sulfur dichloride. 

Pyrrole (1) has a pentagonal structure of C2v symmetry having all the atoms in a single plane. The orbital hydridization18 of the nitrogen atom approximates to sp2, although the possibility of a pyrimidal hybridization, i.e., sp2 sp3, was originally considered. Dipole moment studies allow the N-H bond an out-of-plane latitude of 7° (see Section... 

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 valence bond method has not been used as widely as the molecular orbital approach. With the inclusion of polar structures, however, the valence bond method gives correct orientation for electrophilic substitution and a calculated dipole moment close to the experimental value.100 An application of the one-center method of the 7r-electron system of pyrrole gives electron densities of 1.612, 1.167, and 1.028 on the nitrogen atom and the a- and /3-carbon atoms, respectively.101 Transition energies and the dipole moment by this method are in accord with the observed values. 

Electrophilic Aromatic Substitution. The 7t-excessive character of the pyrrole ring makes the indole ring susceptible to electrophilic attack. The reactivity is greater at the 3-position than at the 2-position. This reactivity pattern is suggested both by electron density distributions calculated by molecular orbital methods and by the relative eneigies of the intermediates for electrophilic substitution, as represented by the protonated structures (7a) and (7b). Structure (7b) is more favorable than (7a) because it retains the benzenoid character of the carbocydic ring (12). 

The pi bonding structure of pyrrole. The pyrrole nitrogen atom is sp2 hybridized, with a lone pair of electrons in the p orbital. This p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring. Counting the four electrons of the double bonds and the two electrons in the nitrogen p orbital, there are six pi electrons. 

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