High resolution FTIR spectroscopic study of the v₄ band of CH₃CHF₂ enclosed in a flow of cold N₂ gas

An enclosive flow cooling (EFC) cell has been constructed, and coupled to a Brüker IFS 120HR high resolution Fourier transform spectrometer to record rotationally cold absorption spectra of gases of atmospheric interest at high spectral resolution. The new system has been characterized using N₂O, revealing that rotational temperatures as cold as 110K are readily attainable using liquid nitrogen as a cryogen. Infrared spectra of the v₄ band of 1,1-difluoroethane (R152a), CH₃CHF₂, cooled in the EFC cell have been measured at a resolution of 0.0019 cm^-1. Eight hundred and twenty rovibrational transitions of the weak v₄ band with 2≤ J′ ≤46 and K′_c ≤ 16 were assigned and fitted to Watson's A-reduced Hamiltonian. The v₄CH₃ symmetric deformation (alc-type) was found to be coupled to the v₁₃ asymmetric deformation (b-type) via an a-axis Coriolis interaction. In the ensuing analysis, values of spectroscopic constants were obtained for both the v₄ and dark v₁₃ states. Supporting ab initio calculations up to the MP2/TZV+(3df.3p) level are presented.