The Mystery of a Band Vanishing upon Isotopic Substitution: The Case of the v(3) = 1 State of FClO(3)

F. Meguellati, G. Graner, K. Burczyk, and H. Bürger [J. Mol. Spectrosc. 185, 392-402 (1997)] reported in their paper on nu(3)(A(1)) bands of the (35,37)Cl and (16,18)O isotopomers of FClO(3) that the nu(3) bands, which, although weak, could be well observed for the (16)O isotopomers, disappear almost completely in the spectra of the (18)O isotopomers. Because the A and B values for the (18)O isotopomers are so similar that these molecules are very close to spherical tops, much closer than the values for the (16)O species, disappearance of the band was ascribed to selection rules for tetrahedral molecules, the A(1) vibrations of which are inactive. Alternative explanations are proposed in this paper and analyzed. The most likely explanation is that a coincidence among the intensity parameters is responsible for a very low value of the dipole moment derivative; a weak Fermi resonance with the (v(6) = 2, l = 0) state may also participate in the final total wipeout of the band. It is believed that the last mechanism may be of more general interest. Copyright 2000 Academic Press.