ROTATIONAL AND VIBRATIONAL SPECTROSCOPY AND IDEAL-GAS HEAT-CAPACITY OF HFC 134A (CF3CFH2)

The hydrofluorocarbon HFC 134a (CF3CFH2) is the primary replacement fo r the chlorofluorocarbon CFC 12 (CF2Cl2) in numerous applications, inc luding automobile air conditioning and home and commercial refrigerati on. Here we describe a comprehensive spectroscopic study of this molec ule. Precise microwave frequencies and molecular constants have been o btained for the vibrational ground state with a pulsed-molecular-beam Fourier-transform microwave spectrometer. New isotopic ground-state mi crowave measurements have also been made to improve the ground-state s tructural determination. Infrared and Raman spectra have been obtained , and all 18 vibrations have been observed and assigned. A high-resolu tion (3 MHz) microwave-sideband CO2 laser and an electric-resonance op tothermal spectrometer have been used to observe the molecular-beam in frared spectrum in the vicinity of the low-resolution gas-phase featur e at 975 cm-l assigned here and in some of the earlier studies as the v(15), A ''-symmetry, CH2 rock. Two nearly equal-intensity c-type band s are observed under high resolution with origins at 974.35 and 974.87 cm(-1) The presence of two vibrational bands of A '' symmetry is attr ibuted to strong anharmonic mixing of the v(15) vibration with a nearb y combination vibration. On the basis of our low-resolution infrared m easurements, we identify the perturbing state as the 3v(18) + v(8) com bination level. Finally, the experimental results are used to calculat e the vibrational contribution to the heat capacity and are compared w ith the results of earlier experimental and theoretical work, includin g our own electronic-structure calculations using an HF/6-31G basis s et.