Bruce K. Janousek

Electron photodetachment of thiomethoxyl and deuterothiomethoxyl anions: Electron affinities, vibrational frequencies, and spin–orbit splitting in CH3S⋅̄ and CD3S⋅̄

The cross section for electron photodetachment has been measured for the thiomethoxyl anion (CH3S−) and the deuterothiomethoxyl anion (CD3S−) using an ion cyclotron resonance spectrometer in conjunction with a tunable dye laser. After accounting for the effective rotational broadening, these spectra yield the electron affinities, EA(CH3S) =1.861±0.004 eV and EA(CD3S) =1.858±0.006 eV. A doubling of features is observed in the photodetachment spectra which arises from transitions to the 2E3/2 and 2E1/2 spin–orbit states of the final‐state radicals. From these spacings we obtain a spin–orbit coupling constant (A) in CH3S of −280±50 cm−1 and in CD3S of −260±50 cm−1. The spectra also contain transitions to excited vibrational states of the radicals for which we obtain vibrational frequencies of 770±50 cm−1 for the symmetric C–S stretching motion in CH3S (660±60 cm−1 in CD3S) and 1360±70 cm−1 for the methyl umbrella motion (1100±50 cm−1 in CD3S).

An experimental and theoretical determination of the electron affinity of the ethynyl radical, HC2⋅

The cross section for electron photodetachment has been measured for the acetylide anion (HC2−) using an ion cyclotron resonance spectrometer in conjunction with a xenon arc lamp. Calculation of the photodetachment behavior near threshold and an estimate of the Franck–Condon factors for the anion→neutral transition allow us to determine EA (HC2⋅) = 2.94±0.10 eV. A theoretical determination using eighth‐order perturbation theory gives an adiabatic electron affinity of 3.18±0.25 eV, in good agreement with the experimental result. The use of a thermochemical cycle with the experimental electron affinity and gas‐phase acidity data gives a C–H bond dissociation energy in acetylene of 132±5 kcal/mol.