b-Dipole Transitions in X ˜ 2A' t-HOCO Observed by FIR Laser Magnetic Resonance

The HOCO radical is crucially important in combustion chemistry as the intermediate in the reaction between hydroxyl radicals and carbon monoxide. Studies of the temperature dependence of the rate constant for this reaction implied the existence of the radical intermediate1 but only very recently was HOCO detected in the gas phase2,3. Both the infrared spectrum3, which is due to the perturbed C=O stretching fundamental (v2), and the observed rotational spectrum2 consist only of a-dipole transitions. Such transitions obey the selection rule ΔKa = 0 in this near prolate top rotor and as such contain no direct information on the spacings between energy levels of different Ka. The high precision of the millimeter wave data allowed an approximate value for the A rotational constant to be derived2 however the accuracy with which it is known is much less than for B and C. Nonetheless, when combined with data for DOCO, it was possible to unequivocally attribute the spectra as due to the trans- geometrical isomer of the radical2.