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

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Isomerism significantly impacts photoelectron spectra, complicating analysis of reactive mixtures, particularly heterocycles.
• Lutidyl radicals, nitrogen-containing benzyl derivatives with methyl substituents, present a complex case for spectral assignment due to multiple isomers.

Purpose of the Study:

• To investigate the influence of isomerism on the electronic structure of lutidyl radicals.
• To determine experimental ionization energies and compare them with theoretical calculations.
• To understand substituent effects on adiabatic ionization energies (AIE) and charge distribution.

Main Methods:

• Recording threshold photoelectron spectra (TPES) of lutidyl radical isomers using vacuum ultraviolet synchrotron radiation.
• Producing radicals via flash pyrolysis of aminomethyl methylpyridine precursors.
• Performing composite method calculations and Franck-Condon simulations for spectral assignment and analysis.

Main Results:

• Experimental ionization energies for 2,4-, 2,6-, and 3,5-lutidyl radicals were determined as 7.54, 7.50, and 7.45 eV, respectively, showing excellent agreement with calculations.
• Substituent effects (nitrogen heteroatom and methyl group) on AIE were found to be additive and position-dependent (para > ortho ≳ meta).
• Franck-Condon simulations highlighted potential artifacts when large-amplitude motions like methyl internal rotation are included in the double harmonic approximation.

Conclusions:

• The study provides accurate ionization energies for lutidyl radical isomers, aiding spectral assignment.
• Substituent effects on AIE are predictable and additive, offering insights into charge distribution upon ionization.
• The findings enhance the understanding of isomerism in photoelectron spectroscopy and radical chemistry, with implications for predicting substituent effects in analogous systems.