The Theoretical Study On The Structure And Properties Of BH₃NH₃ In The External Electric Field

In recent years, the effect of an electric field is widely applied in the field of controlling and modulating the electric and magnetic properties of organic conductors, hydrogen bonding complexes, nanotubes, and graphene, and so on. In addition, the electron movement can be realized by the electric-field, and the nonlinear optical （NLO） responses can be enhanced by molulated the strength of external electric fields (E_ext). In present work, the external electric field are applied along the bond axis direction to investigate the electric field induced effect on the structure, characterization and bonding character and properties of BH₃NH₃ and the staggered biphenalenyl diradical π-dimer, respectively.The purpose of this paper is that:（1） to understand the effect of the external electric field to the structure and bonding characterization of BH₃NH₃. （2） to investigate the regularity of variation of the nonlinear optical properties of the staggered biphenalenyl diradical π-dimer.1. It is well known that ammonia borane （BH₃NH₃） is one of the simplest donor-acceptor complex. The donor-acceptor bond （N-B bond） is formed by sharing lone pair electrons between BH₃ and NH₃ groups. In present work, different strength of external electric fields (E_ext) are applied along bond axis direction to investigate the electric field induced effect on BH₃NH₃ structure and properties. Interestingly, we have found that the lone pair electrons can be gradually moved in the "channel" between the BH₃ and NH₃ groups by modulating E_ext.The donor-acceptor bond （N-B bond） is gradually elongated until it breaks when the E_ext range from 0.0000 to 0.0321 au. The N-B bond is the shortest at E_ext=-0.0519 au. Interestingly, the negative charge on BH₃ groups sharply decreases from 0.161 to 0.005 （for NH₃ groups decreases from 0.161 to 0.005） and the electron cloud of HOMO-2 exhibits an obvious transformation at "broking bonding" E_ext ranging from 0.0320 to 0.0321 au, indicating the electron movement induced by electric-field is the main reason to change the structure and stability of BH₃NH₃. Further, atoms in molecules （AIM） analysis shows that B…N interaction are similar to that of hydrogen bonds at E_ext=-0.0767 au and the iconicity of N-B bond in BH₃NH₃ are confirmed by its low electron density for N-B ρ（N-B） （0.103-0.073） in the region of E_ext （0～0.025 au）.2. In recent years, many studies have highlighted the biphenalenyl diradical π-dimer, specially for the fascinating 2-electron,12-center （2e-12c） bonding character. In present work, we applied external electric field along the bond axis direction and the strength of external electric fields (E_ext) change in the range of 0.0000～0.0217 au. The layer distance of the staggered biphenalenyl diradical π-dimer increases from 3.111 A to 3.553A. Meanwhile the interaction energies (E_ext) of the two fragments increase in the range of 16.83～41.74 kcal mol-1, which indicates the external electric field enhance the stabilities of the staggered biphenalenyl diradical π-dimer. The variations of NBO charge population are exactly accordance with the elongation of layer distance, which indicates that the electron movement along bond axis can be induced by external electric field. In addition, when the E_ext increases from 0.0000 au to 0.0217 au, the β_tot enhances from 1 au to 8521 au.