Ring strain integrity in the absorption spectrum of β-propiolactone: VUV spectroscopy in the photon energy 4.6–10.8 eV

Abstract

New absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements of β-propiolactone in the photon energy range 4.6–10.8 eV (268–115 nm). The assignment of the different vibronic features has been performed with the aid of quantum chemical calculations that provide vertical energies, oscillator strengths and harmonic frequencies. The joint experimental and theoretical methodology employed provides a comprehensive review of the electronic state spectroscopy of CH₂CH₂CO₂. A photoelectron spectrum has also been recorded from 10 up to 24 eV and compared to earlier data in the literature. A new value of (10.560 ± 0.002) eV for the ground ionic state adiabatic ionisation energy is recommended. The vibrational features in the spectral bands are assigned to C=O stretching, $v_3^{\prime }\left(a^{\prime }\right)$, CH₂ wagging, $v_6^{\prime }\left(a^{\prime }\right)$, ring C–O stretching, $v_{10}^{\prime }\left(a^{\prime }\right)$, and C=O in-plane bending, $v_{13}^{\prime }\left(a^{\prime }\right)$ modes. Potential energy curves for the lowest-lying electronic excited states, as a function of the C–O/C=O coordinates, have been obtained from time-dependent density functional theory (TD-DFT). Ring strain integrity is shown to be a fragile mechanism upon electronic excitation. The calculations have also shed light on the relevant internal conversion from the electronic excited states governing the nuclear dynamics with significant pre-dissociative character of the lowest-lying electronic states.