The nu1+nu5 high resolution infrared band of FClO3

The high resolution infrared spectra of the mono isotopic species (FClO3)-Cl-35-O-16, (FClO3)-Cl-37-O-16, (FClO3)-Cl-35-O-18 and (FClO3)-Cl-37-O-18 have been analysed in the region of the nu(1) + nu(5) perpendicular combination band, from 1540 to 1680 cm(-1). For all the isotopologues the nu(1) + nu(5) band is of medium intensity at the recording conditions. In total, about 2400, 2000, 2600 and 2700 transitions have been assigned for (FClO3)-Cl-35-O-16, (FClO3)-Cl-37-O-16, (FClO3)-Cl-35-O-18 and (FClO3)-Cl-37-O-18, respectively. The data for each band have been analysed on the basis of the usual Hamiltonian for a degenerate state of a molecule of C-3v symmetry. In addition, two interaction terms have been considered active within the v(1) = v(5) = 1 state, with Delta l = 0, Delta k = +/- 3 and Delta l = Delta k = +/- 2 selection rules. The ro-vibration parameters of the v(1) = v(5) = 1 states have been obtained, together with the band origins. Combining the nu(1) + nu(5) origins with those of the nu(1) and nu(5) fundamentals, the anharmonicity constant x(15) has been derived for the four molecules and compared to the corresponding value calculated ab initio.