Lutidyl Radical Photoelectron Spectra Reveal Additive Substituent Effects on Benzyl Derivatives’ Ionization Energy

Abstract

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 CH3 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.

Photoelectron spectroscopic study of 2-naphthylnitrene and its thermal rearrangement to cyanoindenes

“…S1 in ESI†), which shows a negligible background component proofing its high reactivity and quenching on the chamber walls to afford 6 . So far, only resonance stabilized and less reactive allyl, 59 phenoxy 60 , lutidyl 61 , or iodobenzyl 62 radicals could be detected after wall collisions and can thus produce room temperature cooled spectra. 59 At 1100 K, the peak at m / z 141 is still intense, while the related peak at m / z 143 does not appear anymore in the spectrum (Fig.…”