5. Specific Intermolecular Interactions of Nitrogen Containing Six-Membered Heterocycles

Piperidine molecules and their derivatives with methylene groups and hetero cyclic nitrogen atoms, connected with the hydrogen atom, are influenced by the latter by the shifting of the electron density in comparison with pyridines. The nitrogen atom of piperidine gets electron density from the hydrogen atom of the amino group and contacting by carbon atoms С(2)Н₂ and С(6)Н₂, reducing its negative charge and reaching the increased electron density at its own 3p_z-orbital [1–5]. However, the further formation by the same nitrogen atom of the reverse dative bond is accompanied by the transmission of part of the electron density from this orbital to the essentially unshared 2s² electron pair of the carbon atoms. As a result, there appears a reduction of its own negative charge and relative enrichment of the electron density of the carbon atoms С(2) and С(6) of the methylene groups, of which charges differ from the charges of the carbon atoms С(3), С(5) and, of course, С(4). It follows that the nitrogen atom contributes to a definite change in the charges of all five carbon atoms of the piperidine molecule. Thus, each of the five free bond vacancies of the essentially unshared 2s² electron pair of the carbon atoms of the methylene group of piperidine forms the specific interaction D–H₂C → CH₂, contributing to the vaporization enthalpy being equal to the contribution of the СН₂ group, the energy value (5.70 kJ mol⁻¹) of which is identical to the energy of the realized similar specific interaction in liquid cyclopentane (Table 3.​1). This value of the energy of the specific interaction formed by the methylene group is slightly higher than the energy of the specific interaction formed by the benzene СH-group (5.63 kJ mol⁻¹). It directs attention to the fact that differences of the structure of МО pyridine С₅Н₅N as regards symmetry positions are insignificant compared to the electron structure of the benzene molecule and, consequently, cyclohexane, splitting e(σ) –MO at a ₁ + b ₂, e(π) – at а ₂ + b ₁ and transforming one а ₁ (σ) –orbital of radial type to the n-orbital [6, 7]. It follows that the accepted assumption of the energy contribution by the methylene group of cyclohexane for conducting thermodynamic calculations of the energies of the specific interactions of piperidine, piperazine, and their derivatives is correct and grounded. The rule of using the energy contributed by the СН group of benzene to the enthalpy characteristics of the vaporization process of pyridine and its derivatives is also correct.