Electrostatic potential maps of pyridine

The electrostatic potential map of pyridine on Learning By Modeling clearly shows its decreased Ti electron density. 

Electrostatic potential maps of pyridine and pyrrole. The color range is the same for both. In pyridine the unshared electron pair is responsible for the concentration of electron density red) near nitrogen. In pyrrole the corresponding electron pair is delocalized into the tt system of the ring. 

Figure 25-2 (A) Orbital picture of pyridine. The lone electron pair on nitrogen is in an sp -hybridized orbital and is not part of the aromatic 7T system. (B) The electrostatic potential map of pyridine reveais the location of the lone electron pair on nitrogen (red) in the molecular plane and the electron-withdrawing effect of the electronegative nitrogen on the aromatic tt system (green compare to the electrostatic potential map of pyrrole in Section 25-3).

Another example of electron delocalization can be seen in a comparison of the electrostatic potential maps of pyrrole and pyridine in Figure 18.16. The lone pair of pyrrole is delocaUzed due to its participation in estabhshing aromatic stabilization. As a result, pyrrole is an extremely weak base because protonation of pyrrole would result in the loss of aromaticity. In contrast, the lone pair in pyridine is locahzed and does not participate in estabhshing aromaticity. As a result, pyridine can function as a base without destroying its aromatic stabilization. For this reason, pyridine is a stronger base than pyrrole. In fact, inspection of the pA values in Table 23.1 reveals that pyridine is five orders of magnitude ( 100,000 times) more basic than pyrrole. 

Examine the models of ammonia and pyridine on your Learning By Modeling CD. Are the calculated charges on nitrogen consistent with their relative basicities What about their electrostatic potential maps ... 

Electrostatic potential maps, shown in Figure 17.7 for pyridine and pyrrole, confirm that the nonbonded electron pair in pyridine is localized on N, whereas the lone pair in pyrrole is part of the delocalized n system. Thus, a fundamental difference exists between the N atoms in pyridine and pyrrole. 

Electronic structure of pyridine, a six-jr-eiectron, nitrogen-contai ning analog of benzene. The electrostatic potential map shows that the nitrogen is the most negative atom (red). 

The dipole moment of pyridine is 1.57 D. As the resonance contributors and the electrostatic potential map indicate, the electron-withdrawing nitrogen is the negative end of the dipole. 

Physicochemical properties rather than reactivities were also explored. Molecular electrostatic potential (MEP) was calculated for the [l,2,4]triazolo[4,3- ]pyridine fragment 23, according to the CHELPG algorithm. This afforded a prediction of its H-bond acceptor ability in view of the synthesis of p38 MAP kinase inhibitors <2005JME5728>. Tautomerism was also examined for compound 24, also postulated as two possible acyclic structures. The ab initio self-consistent field (SCF)-calculated energies support 24a as the most stable tautomer <1999MRC493>. 

MO Calculations and Photoelectron Spectroscopy. Some all-valence-electron CNDO/2 SCF-MO calculations on fluorobenzene, hexafluorobenzene, pentafluoro-anisole, and some derived Wheland intermediates have been reported in a paper which is mainly concerned with derivatives of pyridine and the diazines (see p. 467). An MO-LCAO-SCF study of the electronic structure of fluorobenzene has yielded the electrostatic molecular potential and isopotential maps which are consistent with a poru-directing influence of fluorine in electrophilic substitution, ... 

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