Structural and spectroscopic characterization of 4-methylumbelliferone: a combined experimental and computational study

The FTIR and FT-Raman spectra of 4-methylumbelliferone (4MUB) have been recorded in the region 4000–400 and 3500–100 cm-1, respectively. The optimized geometry, frequency and intensity of the vibrational bands of 4MUB were obtained by the density functional theory (DFT) using6-31G(2d,3p) and 6-311++G(2d,3p) basis sets. The harmonic vibrational frequencies were scaled and compared with experimental values. The observed and the calculated frequencies are found to be in good agreement. The UV–visible spectrum was also recorded and compared with the theoretical values. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. The first order hyperpolarizability (0), related properties (, 0 and ) and the Mulliken charges of the molecule were also computed using DFT calculations. Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital (NBO) analysis. The results show that charge in electron density (ED) in the * and * antibonding orbitals and second order delocalization energies (E2) confirms the occurrence of intramolecular charge transfer (ICT) within the molecule. Information about the charge density distribution of the molecule and its chemical reactivity has been obtained by mapping molecular electrostatic potential surface.