RESUMO
The use of the electron-capture detector (ECD) to measure molecular electron affinities and kinetic parameters for reactions of thermal electrons is reviewed. The advances of the past decade are emphasized and include the multistate electron-capture detector model and the use of semi-empirical self-consistent field quantum mechanical calculations and half wave reduction potential values to support gas phase experimental results. A procedure for the evaluation of the adiabatic electron affinities of the main group elements and the homonuclear diatomic molecules is presented. Potential excited states are identified for the magnetron (MGN) values for quinones, thermal charge transfer (TCT) values for CS2, C6F6, SF6 and photoelectron spectroscopy (PES) values for O2, NO, nitromethane, and the nucleic acids. Literature electron affinities are then evaluated. The temperature dependence of the electron-capture detector can be calculated using values for kinetic rate constants and electron affinities to optimize response and temperature sensitivity in analytical procedures. The temperature dependence for adenine, guanine, thymine and cytosine are predicted for reactions with thermal electrons. Using the recent advances, the new adiabatic electron affinities are: all in electron volts (eV), 4-F-benzaldehyde (0.57 +/- 0.05) and acetophenones (APs) 4-F-AP (0.52 +/- 0.05); 2-CF3-AP (0.79 +/- 0.05); 3-CF3-AP (0.79 +/- 0.05); 4-CF3-AP (0.89 +/- 0.05); 3-CI-AP (0.67 +/- 0.05); and 4-Cl-AP (0.64 +/- 0.05). The adiabatic electron affinities of chloro and fluorobenzenes range from 0.17 to 1.15 eV and 0.13 to 0.86 eV.
