Study of substituent effects for aliphatic CH₃–X compounds by resonant Auger spectroscopy

In this study linear free energy relationships (LFER) are applied to the relaxation of core-excited states in aliphatic CH₃X compounds. There are at least two valence orbitals involved in Auger decay following C_CH31s →σ*_C–X excitation, which may as well be localized on the substituents in this series. Therefore, the observed property cannot be strictly attached to the so-called reaction center. The behavior of substituent constants in the description of core-hole processes in aliphatic compounds is also checked on C1s ionization potentials for the extended series of substituted methanes. The role of the inductive effect for both resonant Auger kinetic energies (E_k) and C1s binding energies (E^core_b) is very important. C_CH31s →σ*_C–X excitation energies do not show any significant correlations with inductive substituent constants. On the contrary, Taft’s inductive parameters alone give good correlations for E_k. A simple model with ‘Z + 1’ approximation is developed to describe E_k and explain the observed correlations:E_k = E^core_b− (E^v(Z)_b + E^ea(Z+1)+),where E_b^v(Z) is the valence ionization potential for CH₃X and E^ea(Z+1)+ is the electron affinity for [NH₃X]⁺.