Bond-Breaking Isomerization in HCN  HNC Robert W. Field Department of Chemistry, MIT, Cambridge, MA 02139 Bending dynamics revealed through hyperfine structure The use of electronic properties as reporters of large amplitude motion has stimulated applications to other systems, such as acetylene  vinylidene. First laboratory measurements of HNC hyperfine structure Important astrophysical implications for measurements of HCN / HNC abundance ratios. First observation of CNCN electronic spectrum Excited state is bent and predissociative, in agreement with ab initio calculations. Millimeter-wave spectra of HCN and HNC in bend-excited vibrational levels. The splitting is determined by the hyperfine coupling constant (eQq). Ab initio calculations indicating evolution of (eQq) with bend angle. These calculations are in quantitative agreement with experimental values.