Knowledge Bank

C--H$ \cdots $Xhydrogenbondswithsystematicvariationofhalogens(X=F,Cl,Br)havebeenexaminedusingFourier-TransformMicrowave(FTMW)spectroscopy.Rotationalconstantsfortrifluoromethane--vinylfluoride(TFM$\cdots$VF) were consistent with a $Cs$ symmetry structure that exhibited both bifurcated and single C--H$ \cdots $FinteractionsbetweentheTFMandVF.Thisnearprolateasymmetrictopexhibitedthree-foldinternalrotationoftheCF$_3$ group causing characteristic doubling in its spectra. Initial assignments were completed using chirped-pulse FTMW spectroscopy with additional measurements made using a resonant-cavity FTMW spectrometer. Rotational constants from ab initio calculations at the MP2/6-311++G(2d,2p) level were in agreement with preliminary experimental values ($A=4828$ MHz, $B=1049$ MHz, $C=1018$ MHz). XIAM, nderline{\textbf {51a}}, (1996), 923-932.} was used to provide the barrier to internal rotation ($25(5)$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) and other spectral information. Spectroscopic parameters for the normal isotopic species as well as preliminary structural results on the C--H$ \cdots $F interactions in this complex will be presented.