The \u3bd 2\u2009+\u2009\u3bd 4 and 2\u3bd 2\u2009+\u2009\u3bd 5 high-resolution infrared bands 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(2) + nu(4) and 2 nu(2) + nu(5) perpendicular combination bands, from 1920 to 2060 cm(-1). Both bands are perturbed by the anharmonic interaction effective between the nu(2) = nu(4) = 1 and nu(2) = 2, nu(5) = 1 excited states of E symmetry. The nu(2) + nu(4) bands are of medium-strong intensity, while the 2 nu(2) + nu(5) bands are very weak. Since the origins of the two bands differ by about 10 cm(-1), the 2 nu(2) + nu(5) bands at lower wavenumbers are almost completely hidden by nu(2) + nu(4). In total, about 3400, 2400, 2800 and 3200 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 assignments of each dyad have been analysed simultaneously with the inclusion of the anharmonic resonance term in the model Hamiltonian. The ro-vibration parameters of the nu(2) = nu(4) = 1 and nu(2) = 2, nu(5) = 1 excited states have been obtained, together with the deperturbed band origins and the anharmonic resonance coefficients W-245. Combining the nu(2) + nu(4) origins with those of the nu(2) and nu(4) fundamentals, the anharmonicity constants x(24) have been derived for the four molecules and compared with the ab initio values calculated taking into account the Fermi resonance