INFLUENCE OF INTERMOLECULAR INTERACTIONS IN THE CRYSTAL PHASE ON CONFORMATION OF FLEXIBLE DI- AND TETRAHYDROPYRIMIDINE RINGS. O.V.Shishkin, V.I.Shil'nikov, A.N.Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow Russia

The molecular and crystal structures of derivatives of the 7-aryl-4,7-dihydro-1,2,3-triazolo[1,5-a]pyrimidine and 4-aryl-1,2,3,4-tetrahydro-2-thiopyrimidine which contain flexible six-membered rings have been studied. Results of calculations by AM1 method show that substituents in the phenyl rings do not influence on equilibrium conformation of the di- and tetrahydropyrimidine rings (boat and half-chair respectively). However, experimental data, especially for 4,7-dihydroazolopyrimidine derivatives, indicate considerable change of endocyclic torsion angles. In the case of the nitro group presence in the dihydropyrimidine ring a chair conformation of dihydrocycle has been found in the crystal phase unlike results of quantum-chemical calculations. All these effects can be explained only by differences of intermolecular interactions in the crystals. Analysis of the crystal packing showed that strenghthening of some non-bonded interactions in the crystal (for example, hydrogen bond) causes a decrease of puckering of the pyrimidine ring. Dependence between energy of intermolecular interactions calculated by atom-atom potential method and magnitudes of torsion angles has been investigated. Analysis of the non-bonded interaction potential gradient agrees well with molecules deformation in the crystals as compared with the gase phase.