Kanayama Keisuke, Fernholz Christin, Nakamura Hisashi, Maruta Kaoru, Bodi Andras, Hemberger Patrick
Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute, CH-5232, Villigen PSI, Switzerland.
Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba, 980-8577, Sendai, Miyagi, Japan.
Chemphyschem. 2023 Oct 17;24(20):e202300359. doi: 10.1002/cphc.202300359. Epub 2023 Aug 11.
Understanding how isomerism influences photoelectron spectra helps in the assignment and analysis of reactive mixtures, especially for heterocycles with numerous isomers. Threshold photoelectron spectra of lutidyl radical isomers, i. e., benzyl derivatives with a nitrogen heteroatom and a methyl substituent, are recorded using vacuum ultraviolet synchrotron radiation. The radicals are produced by flash pyrolysis from aminomethyl methylpyridine precursors. Experimental ionization energies are determined to be 7.54, 7.50, and 7.45 eV for 2,4-, 2,6- and 3,5-lutidyl, respectively, in excellent agreement with composite method calculations. Franck-Condon simulations aid the TPES assignment but are also shown to exhibit artifacts if large-amplitude motions, notably the methyl internal rotation are assumed to be active in the double harmonic approximation. Based on calculated adiabatic ionization energies (AIE) of benzyl, picolyl, and xylyl radicals, the N and CH substituent effects are found to be additive, position-dependent and decrease in the para>ortho≳meta order in magnitude with the nitrogen heteroatom increasing and the methyl substituent decreasing the AIE. These effects are discussed in light of the charge distribution upon ionization. The additivity of the substituent effects also helps predict the influence of substituents on the binding energy of the unpaired electron in analogous radicals.
了解异构现象如何影响光电子能谱有助于对反应性混合物进行归属和分析，特别是对于具有众多异构体的杂环化合物。使用真空紫外同步辐射记录了卢替啶基异构体（即带有氮杂原子和甲基取代基的苄基衍生物）的阈值光电子能谱。这些自由基通过氨基甲基甲基吡啶前体的快速热解产生。实验测得的2,4-、2,6-和3,5-卢替啶基的电离能分别为7.54、7.50和7.45 eV，与复合方法计算结果非常吻合。弗兰克-康登模拟有助于阈值光电子能谱的归属，但如果在双谐波近似中假设大幅度运动（特别是甲基内旋转）是活跃的，也会显示出伪像。基于苄基、皮考啉基和二甲苯基自由基的计算绝热电离能（AIE），发现N和CH取代基的效应是可加的、位置依赖的，并且随着氮杂原子增加和甲基取代基减少AIE，其效应在对位>邻位≳间位顺序上逐渐减小。根据电离时的电荷分布对这些效应进行了讨论。取代基效应的可加性也有助于预测取代基对类似自由基中未成对电子结合能的影响。
