On the anharmonic XCN bending modes of the quasilinear molecules BrCNO andClCNO

The millimeter-wave spectra of the unstable halofulminates BrCNO arid ClCNO were recorded at room temperature in several frequency intervals between 5 2 and 230 GHz. Besides rotational transitions in the vibrational ground sta te, transitions in numerous thermally, excited states of the XCN bending mo de (X = Br or Cl) could be observed. The irregular sequence of these satell ites indicated that in both molecules, the XCN bending mode is highly anhar monic. Indeed, the XCNO molecules exhibit truly quasilinear behavior. From semirigid bender analyses of the rotational data, the effective potential f unctions and their barriers to linearity were determined. The barrier heigh ts were found to be 130.82 (56) cm(-1) for BrCNO and 166.86 (84) cm(-1) for ClCNO, resulting in quasilinearity parameters gamma (0) of +0.362 and +0.4 16, respectively.