Detection of Tetrachlorobutadiene Isomers Using Density Functional Theory Methods, A Comparative Study of Hartree-Fock and Density Functional Theory Analysis

The study aims to build upon previous research by incorporating Density Functional Theory (DFT), specifically using the B3LYP functional, to improve the computational methodology for analyzing chlorobutadiene (TCBD) compounds. DFT is chosen for its ability to account for electron correlation effects beyond the mean-field approximation, a limitation found in earlier approaches such as the Hartree-Fock (HF) method. By incorporating electron correlation, DFT provides a more accurate description of molecular properties, making it highly suitable for analyzing complex molecular structures like those found in chlorobutadienes. The methodology adopted in the study comprises four key steps. First, the molecular structure of each isomer was created. Next, the geometry of the isomers was optimized using DFT methods to ensure the most stable configurations for further analysis. The third step involved computing the vibrational frequencies of the molecules using the B3LYP functional, with different basis sets applied depending on the isomer under study. Finally, the simulated infrared (IR) spectra generated through DFT were compared with existing data from the literature to validate the findings and assess the accuracy of the computational model. The study focuses on nine different Tetrachlorobutadiene (TCBD) isomers, each with unique configurations of hydrogen and chlorine atoms. These structures were visualized using Molden software, and the IR spectra for each isomer were obtained using DFT, specifically the B3LYP and B3LYP-D3BJ functionals. The analysis of the IR spectra revealed characteristic peaks corresponding to various functional groups within the TCBD molecules. Notable vibrational modes included C-Cl stretching, C=C stretching and bending, and C-H stretching and bending, which are essential in identifying the chemical composition of the isomers. A comparative analysis was conducted between DFT and the previously employed Hartree-Fock (HF) method. The results clearly showed that DFT was more efficient in identifying functional groups within the TCBD isomers. DFT provided IR spectra with well-defined peaks that were consistent across all isomers, offering a more detailed and accurate molecular analysis. In contrast, the HF method, while capable of detecting some functional groups, was less consistent and notably missed certain peaks, particularly in the C-H stretching region. This demonstrates the flexibility and superior accuracy of DFT, making it a more reliable tool for molecular structure analysis, especially in cases involving complex electron correlation effects. Despite DFT’s effectiveness, the overlapping of peaks in the IR spectra necessitates additional analytical techniques, such as nuclear magnetic resonance (NMR) or Raman spectroscopy, to enhance the differentiation of isomers.