Rotational and Vibrational Spectroscopy and Ideal Gas Heat Capacity of HFC 134a (CF3CFH2)

Abstract

The hydrofluorocarbon HFC 134a (CF3CFH2) is the primary replacement for the chlorofluorocarbon CFC 12 (CF2Cl2) in numerous applications, including automobile air conditioning and home and commercial refrigeration. Here we describe a comprehensive spectroscopic study of this molecule. Precise microwave frequencies and molecular constants have been obtained for the vibrational ground state with a pulsed-molecular-beam Fourier-transform microwave spectrometer. New isotopic ground-state microwave measurements have also been made to improve the ground-state structural determination. Infrared and Raman spectra have been obtained, and all 18 vibrations have been observed and assigned. A high-resolution (3 MHz) microwave-sideband CO2 laser and an electric-resonance optothermal spectrometer have been used to observe the molecular-beam infrared spectrum in the vicinity of the low-resolution gas-phase feature at 975 cm-1 assigned here and in some of the earlier studies as the ν15, A‘‘-symmetry, CH2 rock. Two nearly equal-intensity c-type bands 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 attributed to strong anharmonic mixing of the ν15 vibration with a nearby combination vibration. On the basis of our low-resolution infrared measurements, we identify the perturbing state as the 3ν18 + ν8 combination level. Finally, the experimental results are used to calculate the vibrational contribution to the heat capacity and are compared with the results of earlier experimental and theoretical work, including our own electronic-structure calculations using an HF/6-31G* basis set.