Geometry optimization and vibrational frequencies of tetracene molecule in gas phase and in methanol based on Density Funtional Theory and Restricted Hartree-Fock

Abstract

Tetracene is an organic semiconductor with chemical formula C₁₈H₁₂ used in organic field effecttransistor (OFET) and organic light emitting diode (OLED). In this work, the molecular geometry (optimized bond lengths and bond angles), vibrational frequencies and intensities, HOMO-LUMO Energy gap and Atomic charge distribution of the Tetracene molecule in gas phase and in solution were calculated and reported. Restricted Hartree-Fock (RHF) and Density Functional Theory (DFT) with different basis sets were employed for the task. Windows version of Gaussian 03 software was used to perform all the calculations. The results obtained show that the bond length obtained using RHF has the lowest average value of 1.072Å and that obtained using DFT has the lowest average value of 1.085Å in gas phase. In Methanol, it is observed that at RHF level, the lowest average value was 1.075Å and at DFT level was 1.087Å. This shows that the values are a bit higher in methanol than in gas phase which implies that the bonds will be slightly stronger in gas phase than in methanol. The strongest bonds in tetracene molecule are those of C20-H28, C23-H29 and C24- H30 in both gas and methanol. The weakest bonds are those of C6-C12 and C6-C14.The bond angles were found to be so closed to 1200 at both levels of theory for all basis sets used suggesting that the molecule is planar benzene in which the C atoms are sp² hybridized. The calculated HOMOLUMO energy gap shows that the molecule will be slightly more stable in chemical reaction in gas phase than in methanol. DFT values of the energy gap appeared to be closer to the reported experimental value of 2.6eV than those obtained by RHF. The atomic charges distribution was found to be very sensitive to the basis sets which presumably occur due to polarization. From the results obtained for vibrational frequencies, it shows that tetracene molecule would be more stable in gas phase that in methanol as a result of no imaginary frequency found in gas phase. This confirms the stability of the molecule as stated in the results of HOMO-LUMO energy gap. The calculated vibrational frequencies show that the most intense frequency was obtained to be 924.9862cm^-1 at 146.7973KM/mole by RHF/3-21G in gas phase while at B3LYP/3-21G, it has the most intense frequency of about 474.1260cm^-1 at 390.1077.2845MK/mole in methanol.

Keywords: Optimixation, Methanol, Gas, Frequency, Tetracene