Exploring molecular structure, electronic structure, vibrational analysis and thermal properties of Cucurbitine molecule using Density Functional Theory

Abstract

Density functional theory (DFT) calculations were performed to investigate the structural, electronic, spectroscopic, and thermodynamic properties of the Cucurbitine molecule. All computations were carried out using the B3LYP functional with the 6-311++G(d,p) basis set implemented in the Gaussian~09 package. Geometry optimization confirmed that the molecule attains a stable minimum-energy configuration with a total energy of -12425.81 eV. The obtained HOMO--LUMO energy gap of 6.094 eV indicates high chemical stability and a hard molecular nature, which is further supported by the density of states (DOS) spectrum, exhibiting a comparable energy gap of 6.089 eV. Global reactivity descriptors were evaluated, yielding a chemical hardness of 3.047 eV, electronegativity of 3.354 eV, chemical potential of -3.354 eV, softness of 0.329 eV^-1, and an electrophilicity index of 2.051 eV. These descriptors provide insight into the molecule's stability, polarizability, and electron-accepting ability. Molecular electrostatic potential (MEP), electrostatic potential (ESP), and electron density analyses were performed to elucidate the charge distribution and reactive regions within the molecule. Mulliken charge analysis revealed significant charge localization, with the C3 atom carrying the highest positive charge and the C4 atom exhibiting the highest negative charge. Vibrational analysis identified the characteristic vibrational modes corresponding to C--C, C--H, C--O, C=O, C--N, O--H, and N--H bonds, showing good agreement with standard reference values. Thermodynamic parameters, including heat capacity, internal energy, enthalpy, and entropy, were found to increase with temperature, whereas the Gibbs free energy decreased, reflecting the thermal stability and energetic behavior of the Cucurbitine molecule.