THE bond energy D0(IO) of the IO radical is not reliably known. An early estimate, D0(IO) = 44 kcalorie mol−1, was derived by Gaydon1 from the IO emission spectrum of the CH3I flame. Since this value was obtained by Birge–Sponer extrapolation of only five vibrational levels of the upper IO(2II) state, it is not expected to be very accurate. The same spectrum was studied by Ramsay2 in absorption by flash photolysis of O2/I2 mixtures. Birge–Sponer extrapolation of six bands gave D0(IO) = 42 ±5 kcalorie mol−1. But more accurate extrapolation2 for CIO and BrO using twenty bands showed that extrapolation from a small number of low lying levels leads to unreliable results. A rigorous upper bound D0(IO)<62.8 kcalorie mol−1 was determined from predissociation of the upper IO(2II) state.
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RADLEIN, D., WHITEHEAD, J. & GRICE, R. Bond energy of the IO radical from molecular beam reactive scattering measurements. Nature 253, 37 (1975). https://doi.org/10.1038/253037a0
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