The structural non-rigidity of aromatic pyrimidine rings in adenine and selected related compounds has been investigated by quantum chemical non-empirical methods of calculation at the MP2 and DFT levels of theory. The results of the calculations demonstrate that the pyrimidine ring possesses a notable degree of conformational flexibility despite its aromatic character, The transition of the heterocycle from a planar equilibrium geometry to a non-planar structure with an endocyclic torsion angle of +/-20 degrees results in an energy increase of less than 2.8 kcal/mol. An analysis of the population of ground and excited vibrational levels for the lowest out-of-plane vibration of the ring indicates that in adenine 45% of the molecules have a non-planar pyrimidine ring with relevant torsion angle up to +/-17 degrees at any moment of time.