We report calculated ab initio energies and barriers to internal rotat
ion for ONOO-anion and ONOO-radical species at the 6-31G* level. The
anion is 48 kJ/mol more stable than the radical in its lowest (2)A'' (
trans) state. The difference between the more stable trans and less st
able cis conformations of the ONOO-anion and ONOO-radical is small, am
ounting to 4.6 and 3.9 kJ/mol, respectively. However, the energy of th
e 90 degrees shew form is 68 kJ/mol above the trans form in the ONOO-a
nion, but only 40 kJ/mol in ONOO-radical. Several ways to produce the
ONOO-anion for FTMS analysis and determination of its thermodynamic pr
operties are presented.