Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Biofuel additives like γ-valerolactone (GVL) and methyl butyrate (MB) are crucial for sustainable energy.
• Understanding their photoionization dynamics is essential for predicting their behavior under irradiation.
• Previous studies lacked detailed insights into the fragmentation pathways and energetics of these molecules upon photoionization.

Purpose of the Study:

• To investigate the photoionization and dissociative fragmentation of GVL and MB using high-resolution spectroscopy.
• To determine the appearance energies of fragment ions and calculate dissociation barriers.
• To elucidate the nonstatistical dissociation mechanisms and compare them with theoretical predictions.

Main Methods:

• Utilized imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the Swiss Light Source (SLS) VUV beamline.
• Performed Franck-Condon simulations to analyze vibrational fine structure in photoelectron spectra.
• Employed quantum chemical calculations to determine ionization energies and fragmentation energetics.

Main Results:

• Determined the appearance energy for the 1-butene fragment ion from GVL via CO2 loss (10.35 ± 0.01 eV).
• Established the reverse barrier height for CO2 loss in GVL to be 66.6 ± 4.3 kJ mol⁻¹.
• Observed McLafferty rearrangement in methyl butyrate, leading to ethylene loss and formation of the methyl acetate enol ion (10.24 ± 0.04 eV).

Conclusions:

• Both GVL and MB exhibit low-energy, nonstatistical dissociative ionization channels.
• GVL shows nonstatistical competition between CH3 and CO2 loss, while MB exhibits inhibited McLafferty rearrangement on excited states.
• The study provides critical energetic data and mechanistic insights into the photoionization of potential biofuel additives.