Axis Switching in the Transition of HCO: Determination of Molecular Geometry

The [Formula: see text] band systems of HCO and DCO have been reinvestigated with higher resolving power than in earlier work. Some weak lines have been assigned to K′ = 0 – K″ = 0 and K′ = 0 – K″ = 2 subbands; these lines derive their intensity from axis switching. The new data give values for the A rotational constants of HCO and DCO and permit a more reliable determination of the ground state geometry, viz, r 0 (CH) = 1.125(5) Å, r 0 (CO) = 1.175(1) Å, [Formula: see text]. The sign of the spin–rotation constant ε aa has been shown to be positive.