Pyridine orbital structure

Problem 10.4 The molecules of pyridine CsHsN, are flat, with all bond angles about 12(P. All carboir-carbon bonds are 1.39 A long and the two cai bon nitrogen bonds are 1.36 A long. The measured heat of combustion is 23 kcal lower than that calculated by the method of Problem 10.2 on page 323. Pyridine undergoes such substitution reactions as nitration and sulfonation (Sec. 10.4). (a) Is pyridine adequately represented by formula I (b) Account for the properties of pyridine by both valence-bond and orbital structures. (Check your answer in Sec. 31.6.)... 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

More advanced semiempirical molecular orbital methods have also been used in this respect in modeling, e.g., the structure of a diphosphonium extractant in the gas phase, and then the percentage extraction of zinc ion-pair complexes was correlated with the calculated energy of association of the ion pairs [29]. Semiempirical SCF calculations, used to study structure, conformational changes and hydration of hydroxyoximes as extractants of copper, appeared helpful in interpreting their interfacial activity and the rate of extraction [30]. Similar (PM3, ZINDO) methods were also used to model the structure of some commercial extractants (pyridine dicarboxylates, pyridyloctanoates, jS-diketones, hydroxyoximes), as well as the effects of their hydration and association with modifiers (alcohols, )S-diketones) on their thermodynamic and interfacial activity [31 33]. In addition, the structure of copper complexes with these extractants was calculated [32]. 

The pyrylium cation is isoelectronic with pyridine it has the same number of electrons and, therefore, we also have aromaticity. Oxygen is normally divalent and carries two lone pairs. If we insert oxygen into the benzene ring structure, then it follows that, by having one electron in a p orbital contributing to the aromatic sextet, there is a lone pair in an sp orbital,... 

These structures are all aromatic, the nitrogen atoms functioning in the same way as the pyridine nitrogen, each contributing one p electron to the aromatic sextet, with a lone pair in an sp orbital. The... 

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

Problem 11-4. Determine and sketch the nodal structure of the pyridine molecular orbitals. 

Pyridine has tt electron structure similar to that of benzene. Each of the five 5 p -hybridized carbons has a p orbital perpendicular to the plane of the ring. Each p orbital has one tt electron. The nitrogen atom is also sp -hybridized and has one electron in the p orbital. So, there are six tt electrons in the ring. The nitrogen lone pair electrons are in an sp orbital in the plane of the ring and are not a part of the aromatic tt system. 

The Au-Au distances are 3.1882(1) A and they are considered to be responsible for the emission band that appears at 460 nm at room temperature. When the anion in the carbene complex is BF4, the structure is similar although the Au-Au distances are substantially longer (3.4615(2) A). The different distance leads to a different emission band that is blue-shifted. This indicates a greater orbital interaction in the former, consistent with its shorter Au-Au distance. Nevertheless, the behavior in solution is similar for both. Thus, at room temperature in solution they lose their emissive properties but they recover them in frozen solutions at 77 K. Interestingly, the emission differs in color, depending on the solvent, ranging from orange (acetone) to blue (pyridine), which would seem to result from the self-association... 

Qualitatively, the resonance picture is often used to describe the structure of molecules, but quantitative valence-bond calculations become much more difficult as the structures become more complicated (e.g., naphthalene, pyridine, etc.). Therefore the molecular-orbital method is used much more often for the solution of wave equations.5 If we look at benzene by this method (qualitatively), we see that each carbon atom, being connected to three other atoms, uses sp1 orbitals to form a bonds, so that all 12 atoms are in one plane. Each carbon has a p orbital (containing one electron) remaining and each of these can overlap equally with the two adjacent p orbitals. This overlap of six orbitals (see Figure 2.1) produces six new orbitals, three of which (shown) are bonding. These three (called it orbitals) all occupy approximately the same space.6 One of the three is of lower energy than... 

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