Gibbard J A, Castracane E, Krylov A I, Continetti R E
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0340, USA.
Phys Chem Chem Phys. 2021 Sep 14;23(34):18414-18424. doi: 10.1039/d1cp02972j. Epub 2021 Aug 23.
Photoelectron-photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p-coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO + CH + e, with a very small amount of stable CHCO showing that the radical ground state is stable and the excited states are dissociative. For p-coumarate (p-CA) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO + HOCHCHCH + e, characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOCH + HCC + CO. The ion beam contained both the phenolate and carboxylate isomers of p-CA, but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.
利用光电子-光碎片符合光谱法研究了苯甲酸和对香豆酸共轭碱的解离动力学。在266 nm（4.66 eV）处进行光解离时，两种芳香羧酸盐都会发生脱羧反应，并形成稳定的羧基自由基。使用高水平电子结构方法计算了关键的能量学数据。苯甲酸盐的解离动力学主要由导致CO + CH + e的双体双光子解离（DPD）通道主导，只有极少量稳定的CHCO，这表明自由基基态是稳定的，而激发态是解离的。对于对香豆酸盐（p-CA），主要通道是光解离，产生一个稳定的自由基和一个电子动能（eKE）<2 eV的光电子。我们还观察到一个次要的双体解离光解离（DPD）通道，导致CO + HOCHCHCH + e，其特征是eKE <0.8 eV。还发现了一个三体离子光解离通道，产生HOCH + HCC + CO。离子束中包含p-CA的酚盐和羧酸盐异构体，但DPD仅发生在羧酸盐形式上。对于这两种物质，DPD都出现在自由基的第一激发态和第二激发态，从第一激发自由基态进行脱羧反应需要振动激发。
