Four-center six-electron interaction versus lone pair lone pair interaction between selenium atoms in naphthalene peri positions

The X-ray crystallographic analysis of bis[8-(phenylselanyl)naphthyl]-1, 1' -diselenide (1) and 1-(methylselanyl)-8-(phenylselanyl)naphthalene (2) show ed that the four selenium atoms in 1 aligned linearly, while the Se-C(Me) a nd Se-C(Ph) bond in 2 declined by about 50 degrees and 40 degrees from the naphthyl plane, respectively. Ab initio molecular orbital calculations were performed on the models of 1 and 2, model a ((HbHcSe)-Se-2 ... (HaSeSeHa)- Se-1-Se-3...(SeHbHc)-Se-4) and model b ((HaHbSe)-Se-1 ...(SeHa'Hb')-Se-2), respectively, with the 6-311++G(3df,2pd) basis sets at the HF and MP2 level s, and the calculations reproduced well the observed structures and reveale d the nature of the bonds constructed by the selenium atoms containing nonb onded interactions. The bond with four linearly aligned selenium atoms in m odel a can be analyzed with the 4c-6e model constructed with the nonbonded interaction between the two p-type lone pairs on the outside selenium atoms and the sigma*(Se-Se) orbital of the inside Se-Se bond, which results in c harge transfer from the outside Se atoms to the inside Se atoms. The nonbon ded interaction between the two p-type lone pairs at the Se atoms in model b is analyzed as a pi-type 2c-4e bond. The bent structure in H-b-Se-1 ...Se -2-H-b' was demonstrated to be the result of the requirement to avoid the s evere exchange repulsion between the filled p-type lone pairs at the two se lenium atoms. The calculations on the other models, PhSeH ... HSeSeH ... HS ePh and PhSeH ... HSeMe, with the 6-311+G(d,p) basis sets at the DFT (B3LYP ) level showed that the pi-orbitals of the phenyl groups of the former inte racted effectively with the 4c-6e orbitals but the pi-orbitals of the latte r did little with the 2c-4e orbitals due to the orthogonality of the two sy stems.