Nitrogen molecule electron distribution

C6o solubility in pyridine is identical to that in benzene. Pyridine has a pronounced "aromatic" nature. zi-electron distribution in a pyridine molecule is identical to that in benzene. Pyridine has six mobile 7i-bonds, one of them is formed by an unshared pair of -electrons of a nitrogen atom. Pyridine can be nitrated. A nitro group enters the P-position. Because carbon with the highest electron density is a center for electrophilic substitution, one can make a logical assumption that the reaction center for charge-transfer interaction between pyridine molecules and C6o is also in the P-position or, what is equivalent, in the ortho-position relative to a nitrogen atom (Table 6). 

Whichever name it is given, the origin of this attraction is the mushy electron cloud that surrounds the nitrogen molecule. Because the electrons can be considered mobile in the electron cloud, they can be pictured as congregating momentarily at one end of the molecule or the other. This momentary uneven distribution of electrons is termed a temporary dipole, but it acts in the same manner as a permanent dipole. It is attracted to other dipoles, temporary or otherwise. The redistribution of electrons may be spontaneous, or if there is an ion or a molecule with a permanent dipole in the vicinity, this species might induce a momentary dipole, too. This situation is shown in figure 1.8.2. 

In the application of theoretical studies to the azole field many of these have attempted to achieve comparisons within the range of azole molecules. Thus, calculations of electron densities, dipole moments, and energies of formation give values that reflect the decrease in azole stability as the number of nitrogen atoms increase. ° Good correlations between a and total electron densities and and chemical shifts have been obtained. " Calculations (SCF) of n-electron distributions for the ground state of imidazole do not take into account the tautomeric equivalence of the 4-and 5-positions, but predict the order of electrophilic substitution as 5 > 2 > 4242,243 Various other quantum-mechanical calculations have... 

Density contour maps, like those of Fig. 1 (Sect. 2.3) for the nitrogen molecule or Fig. 2 (Sect. 4.2) for ethylene give a complete picture of the electronic distribution in a molecule. However, it is more convenient, especially for comparative studies, to describe the electronic structure by a set of single numbers rather than by maps, even if this involves the loss of much information. This is why indices summarizing the form of the electron distribution in the neighborhood of an atom or a bond have been defined by quantum chemists. Following Mulliken, the assignment of a set of such indices to a molecule may be called its population analysis. 

To conclude this review, several important aspects should be highhghted. The nitrogen atoms of the purine skeleton are the centers for the intermolecular interactions between the purine molecule and its environment. The changes in the electron distribution induced by these interactions are reflected in changes of several NMR parameters which can be obtained from the samples at natural and " N abundance, namely the C and " N NMR chemical shifts and the and scalar coupling constants. All these... 

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