Spectroscopy of structural isomers of pentanes: An experimental and theoretical study

Abstract

Vacuum ultraviolet photoabsorption spectra of pentane (C5H12) structural isomers (n-pentane and isopentane) are recorded in the 6–11.5 eV region using a synchrotron light source. The photoabsorption spectra for both the pentane isomers showed a continuum, beginning at about 7.5 eV and increasing in intensity up to 10.5 eV with few broad peaks devoid of discernible structures. Quantum chemical calculations using the DFT methodology predicted the optimized ground geometry and fundamental frequencies for neutral and cationic pentane isomers. The excited electronic state vertical energies are computed using time-dependent DFT and allied to construe the experimental spectra. From theory, the electronic spectra of pentane isomers encompass mostly valence-Rydberg mixed type transitions, converging to the first four ionization potentials while isopentane at 9.62 eV predicted a uni-molecular dissociation to C2H5• and C3H7• radicals. The plot of potential energy curves for the first few excited states facilitated understanding the spectral features UV region and its photodissociation mechanism for both pentanes. The broad continuum spectra in both isomers may be attributed to strong valence - Rydberg interactions along with a contribution from the presence of conformers, low torsional modes and Rydberg series converging to close-lying ionization potentials. Gas phase infrared spectra of pentane isomers in the 500–4000 cm−1 region were revisited and DFT calculations clarified few vibrational assignments. The present study provides a comprehensive understanding of the absorption spectra in the VUV and IR regions of n-pentane and isopentane.