Ultra-violet absorption spectrum of isocyanic acid

Abstract

The absorption spectrum of HNCO between 2000 and 2800 Å has been photographed with high resolution and absorption paths of up to 8 m atm. None of the bands are completely sharp, but many show coarse rotational structure. Long progressions of bands are assigned to excitation of an NCO bending vibration. The rotational structure shows that the NCO chain is strongly bent in the excited state, and that either ∠NCO = 119° and r_NC+r_CO= 2.64 Å if the excited state is trans, or ∠NCO = 129° and r_NC+r_CO= 2.60 Å if the excited state is cis. Comparison of the observed band contours with computed asymmetric top band syntheses proves that the bands have type C polarization. The electronic and geometrical structure of HNCO are discussed with the aid of a diagram correlating the molecular orbitals of HABC and AB₂ molecules.