MOLECULAR-BEAM SPECTRUM OF THE 970 CM-1 FERMI TRIAD OF CF3CH3

A tunable microwave-sideband CO2 laser is used with an electric-resona nce optothermal spectrometer to investigate the infrared spectrum of C F3CH3 near 970 cm-1. A Fermi-coupled triad of states is observed, resu lting from the interactions of 2nu6+nu11 and nu5+nu12 with the fundame ntal vibration, nu10, which is assumed to carry the oscillator strengt h in this region. The high resolution (approximately 3 MHz) of the spe ctrometer allows the observation of tunneling splittings associated wi th the nu6 torsional vibration. These splittings are used to identify the torsional character of the states observed. At the normal-mode lev el the nu10 and nu5 + nu12 states are found to be nearly degenerate an d interacting by an anharmonic matrix element of approximately 3 cm-1. The lower-energy component of this diad exhibits torsional splittings of up to 400 MHz due to an anharmonic coupling of 0.70 cm-1 with the lower energy 2nu6+nu11 state which has an intrinsic tunneling splittin g of approximately 800 MHz. A fourth state, 3nu6 + nu12, which has a s till larger zeroth-order tunneling splitting, may also be affecting th e torsional splittings of the observed states. The present investigati on illustrates the utility of using resolved torsional splittings to u nravel complex vibrational couplings in molecules.