Electronic Structure and IR Spectra Analysis of Tetrathiafulvelene (TTF) Using RHF and DFT Quantum Mechanical Methods

Abstract

Tetrathiafulvalene (C6H4S4) is an organosulfur compound used in the manufacture of organic optoelectronic materials. Restricted Hartree-Fock (RHF) and Density Functional Theory (DFT) were employed for the study of the optimized molecular structure of TTF using different basis sets. All computations were performed using Gaussian 03 package. Parameters such as minimum energy, bond lengths and bond angles, HOMO-LUMO energy gap, chemical reactivity descriptors, Non-linear optical properties (such as isotropic polarizability (α), anisotropy of polarizability (∆∝), and total first hyper-polarizability (βtot)), density of states, vibrational frequencies, and intensities were computed and reported in order to determine the relative stability as well as chemical reactivity of the molecule. The results obtained show that RHF has the lowest average value of bond length of 1.0729Å while that obtained using DFT has the lowest average value of 1.0812Å using the same basis set 6-311++G (d, p). This shows that the value is a bit higher using DFT than RHF which implies that the bonds of TTF molecule will be slightly stronger when optimized using RHF than DFT. The bond angles were found to be slightly higher by using DFT than RHF. The calculated HOMO-LUMO energy gap shows that the molecule will be slightly more stable in chemical reaction using RHF than DFT. The DFT values, 3.59 eV and  3.60 eV obtained for the energy gap are closer to the reported value of 3.63eV compared to those obtained by RHF. From the results obtained for vibrational frequencies using both methods, TTF is stable due to the absence of imaginary frequencies. 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 752.6293 cm-1  at corresponding intensity of 145.9063KM/mole by RHF/6-31++G (d, p) while at B3LYP/6-31+G(d), the most intense frequency is about 635.0243 cm-1 with corresponding intensity of 138.5738MK/mole. By and large, the studyunveil the potential of Tetrathiafulvalene for optoelectronic applications.