The Microwave Spectrum and Molecular Constants of Trifluoromethyl Acetylene

Pure rotational transitions, $J=4\mathrm{}\to 5,6\to 7, \mathrm{and} 8\to 9$ have been measured for C${\mathrm{F}}_3$CCH and $J=5\mathrm{}\to 6 \mathrm{and} 8\to 9$ for C${\mathrm{F}}_3$CCD. These yield for C${\mathrm{F}}_3$CCH, $B_0=2,877.948\mathrm{}$ Mc/sec, $D_J={0.2}_4$ kc/sec, $D_{\mathrm{JK}}=6.3\mathrm{}$ kc/sec, and for C${\mathrm{F}}_3$CCD, $B_0=2,696.073\mathrm{}$ Mc/sec, $D_J={0.2}_6$ kc/sec, $D_{\mathrm{JK}}=6.2\mathrm{}$ kc/sec. If $\angle \mathrm{FCF}$ is tetrahedral, $d\left(C\equiv C\right)$ and $d\left(\mathrm{CH}\right)$ are assumed to be 1.207A and 1.056A, respectively, then the bond lengths $d\left(\mathrm{CF}\right)=1.330\mathrm{}\mathrm{A}$ and $d(\mathrm{C}-\mathrm{C})=1.493\mathrm{}\mathrm{A}$ are obtained. Rotational frequencies of molecules in excited bending vibration modes have been measured and found to agree with Nielsen's theory of $l$-type doubling.