Vibration-rotation Hamiltonian for asymmetric Cs molecules adapted to very strong Coriolis resonance : Application to the microwave reinvestigations of the ν7 and ν9 states of HCOOH and DCOOH

Formic acid is a C_s asymmetric top molecule exhibiting an exceptionally strong Coriolis resonance between its ν₇ and ν₉ vibrational states. The usual molecular model composed of two Watson Hamiltonians coupled by linear and quadratic vibration-rotation coupling terms does not allow satisfactory interpretations of such rotational spectra by microwave spectroscopy. In this case, it is necessary to perform a more complete development of the vibration-rotation coupling part of the standard Hamiltonian operator. The first part of this paper gives details of these developments, yielding a new molecular model adapted to very strong Coriolis resonance for C_s asymmetric top molecules. This new model consists of two Watson Hamiltonians developed up to the sextic centrifugal distortion coefficients and linked by 10 coupling constants. In the second part of this paper, this model has been successfully tested on H¹²COOH and D¹²COOH. From careful microwave reinvestigations of the ν₇ and ν₉ states of these two molecules, numerous new important rotational lines of various μ_b type and intervibrational transitions of μ_c type have been assigned. Various tests are performed to estimate the quality of the results. A critical discussion of the numerical investigation revealed the limits of the new molecular model proposed for strong Coriolis resonance.