Pyridine electrostatic potentials

Identify the more basic of the two nitrogens of 4 (N,N dimethylamino)pyndine and suggest an explanation for its enhanced basicity as compared with pyridine and N N dimethylanihne Refer to Learning By Modeling and compare your prediction to one based on the calculated charge and electrostatic potential of each nitrogen... 

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 map for pyridine shows negatively-charged regions (in red) and positively-charged regions (in blue). 

Draw and compare Lewis structures for benzene and pyridine. How many 7C electrons does each molecule have Where are the most accessible electrons in each Display the electrostatic potential map for pyridine and compare it to the corresponding map for benzene. Would you expect electrophilic attack on pyridine to occur analogously to that in benzene If so, should pyridine be more or less susceptible to aromatic substitution than benzene If not, where would you expect electrophilic attack to occur Explain. 

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>. 

A surface for which the electrostatic potential is negative (a negative potential surface) delineates regions in a molecule which are subject to electrophilic attack, for example, above and below the plane of the ring in benzene, and in the ring plane above the nitrogen in pyridine. 

Hydrogen bond basicity is of much relevance to the problem of drug design. Hydrogen bond basicity was shown to correlate with the location of the electrostatic potential local minimum along the axis of the nitrogen lone pair in a series of heterocycles (94JCS(P2)199). The experimental and calculated basicities for oxazole, 2,4,5-trimethyloxazole, and pyridine are shown in Table 2. 

The extension of these studies to zeolites has yielded some interesting results. In an early study Egerton, Hardin and Sheppard (27) showed, in agreement with Ward s previous infrared results (28,29), that pyridine adsorbs mainly to the cation in a series of cation exchanged Y zeolites but that there is a linear shift of v] to higher frequency with the electrostatic potential (or charge to radius ratio, e/r, of the exchange cation). 

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. 

The increase in the electrostatic potential of the exchange cation, e/r, causes an increase in the shift of the absorption band (Figure 1). The difference in this dependence for p-phenylenediamine, aniline, and pyridine is owing mainly to the difference in the properties of the lone electron pair of nitrogen in the molecules of these compounds. 

Figure 1. The dependence of the shift of the absorption bands (AX) on the electrostatic potential of the exchange cations (e/r), A, in the case of adsorption of (1) aniline, (2) p-phenyl-enediamine, (3) pyridine, (4) nitrobenzene on LiX, NaX, KX, RbX, and CsX zeolites...

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). 

We have at present electrostatic potential maps only for pyridine (XX) and pyrazine (XXI)... 

The electrostatic potential of pyridine outside the ring plane presents large negative regions with small minima (nearly equivalent) in correspondence with atoms C3-C5 and C4. It is not easy to find clear-cut examples of reactions involving the free base. Indirect evidence indicates that atoms C3-C5 are the most reactive. Nucleophilic substitution... 

Recently Pack, Wang and Rein51) published a convergence analysis of analytical expansions of the electrostatic potential on parallel lines to the present one. These authors compare with the exact expansion the one-center one and a segmental atomic expansion (centered at the nuclei). Convergence is tested on pyridine (semiempirical iterative extended Hiickel wave function) along the symmetry axis with expansion truncated after the octopole term. Their results are comparable to those reported here in particualr, the segmental expansion appears quite reasonable)). 

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

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. 

An alternative approach to the rationalization of reactivity patterns, involving the three-dimensional visualization of molecular electrostatic potential surfaces, has had some success in explaining the results of the nitration of substituted imidazo[l,2-fl]pyridines <85EJM50l>. 

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