Study on the ground state properties and excitation properties of C<sub>18</sub> under different external electric fields

Abstract

In this work, density functional theory method with the ωB97XD/def2-TZVP level is carried out to investigate the ground state structures, energy, electronic structures, aromaticity, infrared and Raman spectra of cyclo[18]carbon under different external electric field in the <i>x</i>, <i>y</i> and <i>z</i> direction of cyclo[18]carbon molecule. The excitation properties (the first 48 excited states containing excited energies, excited wavelengths and oscillator strengths) of cyclo[18]carbon are calculated by the time-dependent density functional theory method (TD-ωB97XD) with the def2-TZVP basis set under the same external electric field. The results show that cyclo[18]carbon can be elongated in the <i>x</i> or <i>y</i> direction under the electric field, and some C-C bond lengths can be elongated or shortened under the electric field. Meanwhile, the calculated results show that electric dipole moment is proved to be increasing with the increase of the external field intensity, but the total energy and LUMO-HOMO gap are proved to decrease with the increase of external field intensity. Moreover, addition of electric field can modify the electron delocalization and molecular aromaticity, such as external electric field in <i>z</i> direction can lower the electron delocalization and molecular aromaticity and external electric field in <i>x</i> or <i>y</i> direction can enhance the electron delocalization and molecular aromaticity. The addition of electric field can modify the infrared spectra, such as shift of vibrational frequencies and strengthening of infrared peaks. Furthermore, the calculated results indicate that the external electric field has significant effects on the excitation properties of cyclo[18]carbon. The increase of the electric field intensity can lead to the redshift of transition wavelengths (such as the first excited state). With the change of the electric field intensity, the stronger excited state (with the bigger oscillator strength) can become weak (with the small oscillator strength) or optically inactive (with the oscillator strength of zero). Meanwhile, the weak or optically inactive excited state can become stronger excited state by the external field. The ground state properties and excitation properties of cyclo[18]carbon can be modified by the external electric field. Our works can provide theoretical guidance for the application of cyclo[18]carbon in the nanotechnology such as molecular device.