CAN ISOTOPIC-SUBSTITUTION CHANGE A BRIGHT STATE INTO A DARK STATE - THE CASE OF THE NU(3)=1 STATE OF FCLO3

High-resolution infrared spectra of monoisotopic samples of (FClO3)-Cl -35-O-18 and (FClO3)-Cl-37-O-18 have been recorded with the purpose of analyzing the nu(3) fundamental at 535 cm(-1). However, this band cou ld not be observed whereas it had been seen and studied earlier in (FC lO3)-Cl-35-O-16. To determine the parameters of the upsilon(3) = 1 sta te, indirect methods were used. Hot bands nu(n) + nu(3) - nu(3) (n = 1 or 2) were first analyzed and their LSCD (Lower State Combination Dif ferences) yielded rotational parameters of nu(3). Then, with the help of nu(1) + nu(3) all rovibrational parameters of nu(3) were obtained. Similar methods were applied to spectra of (FClO3)-Cl-35-O-16 and (FCl O3)-Cl-37-O-16 to prove that the parameters of nu(3) obtained in this fashion are identical to those determined directly for these isotopome rs and are even more comprehensive. It is shown that the different cha racter of nu(3), in the two O-18 and in the two O-16 isotopomers is du e to the fact that the former are much closer to a spherical top molec ule ((A(0) - B-0)/A(0) = 0.015). This is not only reflected in intensi ties different by two orders of magnitude but also in the very differe nt values of alpha(3)(B) in these two pairs. (C) 1997 Academic Press.