Monoterpene flash pyrolysis reveals weakly coupled mechanistic domains and fleeting biradical intermediates

The monoterpenes α-pinene, β-pinene, and limonene were in-vacuum flash pyrolyzed at 950 °C in a microreactor and the pyrolysis products were ionized and detected using vacuum ultraviolet synchrotron radiation and double imaging photoelectron photoion coincidence spectroscopy. Photoion mass-selected threshold photoelectron spectra identify the major pyrolysis products, including isoprene and cyclopentadiene (limonene and β-pinene), methylbenzenes (α-pinene), and the propargyl (β-pinene) and methyl radicals (α-pinene). Based on band intensities and photoionization cross sections, semi-quantitative product abundances were obtained. Non-negative matrix factorization shows that the pyrolysis mechanisms of α-pinene and limonene are distinct and that of β-pinene is strongly coupled to that of limonene at this temperature. This is rationalized by potential energy surface calculations that account for the main fragmentation paths. Monoterpene interconversion and limonene fragmentation to isoprene take place on the closed-shell singlet surface, while ring-opening reactions involve open-shell singlet transition states. Biradicals with quasi-degenerate triplet and singlet states, the assumed central intermediates in monoterpene decomposition, are high in energy. Although they may drive racemization and hydrogen randomization, they are not the crucial nodes previously proposed in the literature.