Ab initio and density functional investigation of the molecular geometries and Ch...N interactions in diamido chalcogenides, Ch(NR_2)_2 (Ch = S, Se, Te; R = H, CH_3)

摘要

The molecular structures of diamido chalcogenides, Ch(NR_2)_2 (Ch = S, Se, Te; R = H, Me) as well as their radical cations have been optimized by means of ab initio calculations at HF, MP2 level of theory and density functional theory (B3LYP). All molecules contain V-shaped ChN_2 units and relatively flat ChNR_2 pyramidal units. The calculated geometrical parameters are in very good agreement with the experimental data, particularly at the MP2 and B3LYP level. The N-Ch-N angle decreases from S to Te (S: 112.8 deg, Se: 111.2 deg, Te: 105.1 deg at B3LYP for Ch(NMe_2)_2). The theoretical study confirms also the tendency of Te to form secondary bonds. Thus dimeric and trimeric structures of the type [Te(NH_2)_2]_x, (x = 2, 3) have been optimized where the monomer units are held together via Te...N secondary bonds (2.88 A at HF). The calculated binding energy of 43.93 kJ/mol for the Te dimer has been analyzed by means of the Morokuma energy decomposition scheme. The binding energy decreases in the corresponding Se dimer (20.50 kJ/mol), whereas no important association has been found in the S molecules.