Knowledge Bank

With an interest in characterizing C$-$H$\cdots$$\pi$ interactions, CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$F$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\cdots$HCCH and CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ClF$\cdots$HCCH have been examined by Fourier-transform microwave (FTMW) spectroscopy. These interactions involve the $\pi$ bond in acetylene acting as a hydrogen bond acceptor to both hydrogen atoms of CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$FX. In addition, there is a secondary contact between one hydrogen atom from acetylene and the X atom in the halomethane (X=F in CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$F$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$, X=Cl in CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ClF). \ Initial assignments for the most abundant isotopologues of both species were completed using the chirped-pulse FTMW \hbox{spectrometers} at the University of Virginia (CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ClF$\cdots$HCCH) and at Eastern Illinois University (CH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$F$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$$\cdots$HCCH). Rotational constants \hbox{obtained} from experiment are in good agreement with those of the most stable orientations predicted by ab initio calculations at the \hbox{MP2/6-311++G(2d,2p)} level. Multiple isotopically substituted species for each complex were measured using a Balle-Flygare \hbox{cavity } FTMW spectrometer at Eastern Illinois University. Spectroscopic parameters for all observed isotopologues will be presented, and a comparison of the C$-$H$\cdots \pi$ interactions in these and related complexes will be discussed.