Structural Studies Of Bilirubin: Molecular Structure, Fourier Transform Infra-Red, Ultraviolet-Visible, And Vibrational Frequency Analysis

Molecular structure, harmonic vibrational frequencies, Mulliken charges and temperature dependence of thermodynamic properties of bilirubin molecule have been investigated by Density Functional Theory (DFT) using standard B3LYP/6-311G basis set in the ground state. The Fourier Transform Infra-red (FTIR) spectrum of bilirubin in the spectral region 4000-400 cm(-1) and its electronic absorption spectrum in the range 190-1400 nm has been recorded. The results of observed FTIR and absorption spectrum are compared with the calculated one. A good agreement between experimental and calculated vibrational frequencies has been observed. Hyper-conjugative interactions and charge delocalization have been analyzed using natural bond orbital (NBO) analysis which is responsible for the stability of the molecule. The thermodynamic properties of the bilirubin molecule at different temperatures were calculated, revealing the correlations between the standard heat capacity, standard entropy and standard enthalpy changes with temperature. The study HOMO-LUMO energy is extended to calculate the energy gap and other related molecular properties such as ionization potential, electron affinity, global hardness, electron chemical potential and global electrophilicity of bilirubin. The electrostatic potential (ESP), molecular electrostatic potential (MEP) and total electron density (ED) map of bilirubin were also studied.