Thermodynamic and kinetic characterization of nitrogen-containing polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

A series of five nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs) was studied on polymeric octadecylsilica using methanol and acetonitrile as the mobile phase. The thermodynamic and kinetic behavior was examined as a function of ring number, annelation structure, and position of the nitrogen atom. The retention factors for the NPAHs are smaller than those for the parent PAHs in methanol, while the converse is true in acetonitrile. The changes in molar enthalpy are relatively comparable in both mobile phases with 1-aminopyrene having values of −5.0 ± 0.2 kcal/mol in methanol and −6.3 ± 0.7 kcal/mol in acetonitrile (1 cal = 4.184 J). However, the rate constants from mobile to stationary phase (ksm) and from stationary to mobile phase (kms) demonstrate large differences as a function of mobile phase. For example, the rate constants kms for 1-aminopyrene and 4-azapyrene are 675 and 62 s−1, respectively, in methanol at 303 K. In contrast, the same solutes demonstrate rate constants of 3.47 and 3.9 × 10−3 s−1, respectively, in acetonitrile. The activation energies for transfer from mobile phase to transition state (ΔE‡m) and from stationary phase to transition state (ΔE‡s) also differ as a function of mobile phase. For example, the activation energies ΔE‡s for 1-aminopyrene are 21 and ∼0 kcal/mol, whereas those for 4-azapyrene are 19 and 23 kcal/mol, in methanol and acetonitrile, respectively. Based on these thermodynamic and kinetic results, the relative contributions from the partition and adsorption mechanisms are discussed.