Photoionization mass spectrometric studies of free radicals

Abstract

Free radicals have been fascinating to chemists for decades, partly because of their important but emphemeral nature as intermediates in chemical reactions, e.g., combustion and atmospheric processes. For example, the reaction OH + CO {yields} H + CO{sub 2} is the dominant source of CO{sub 2} in the oxidation of hydrocarbons and is important in the chemistry of the upper atmosphere and in the formation of chemical smog. This reaction is believed to proceed through a COOH intermediate, but until very recently, this species had not been detected in the gas phase. The depletion of ozone in the stratosphere is attributed to the reaction of the free radical ClO with O{sub 3}. The chemical vapor deposition method of producing solid-state devices based on Si or Ge employs a plasma containing free-radical hydrides of Si and Ge. We have shown that the direct application of eq 1 and 2 to determine the dimerization energy of BH{sub 3} by measuring its ionization potential and its appearance potential from B{sub 2}H{sub 6} is inappropriate for this system. However, a rather lengthy analysis of the data leads to a dimerization energy of 34.3-39.1 kcal/mol, in good agreement with ab initio calculations. Preliminary results frommore » our B{sub 2}H{sub 5} and B{sub 2}H{sub 4} studies indicate that D{sub 0}(B{sub 2}H{sub 5}-H) {le} 102.7 kcal/mol, but D{sub 0}(B{sub 2}H{sub 4}-H) is much weaker, {approx} 40.1 kcal/mol.« less