Knowledge Bank

We perform dynamic rotational spectroscopy on vibrationally excited 4-fluorobut-1-yne using our state-of-the-art, chirped-pulse broadband spectrometer. This new spectrometer allowed us to record 11 GHz of spectrum (7.5-18 GHz) with a single microwave pulse to study the energy-dependence of the isomerization kinetics, dramatically reducing data acquisition times. 4-Fluorobut-1-yne is an asymmetric top found in one of two conformers in a supersonic expansion, with fluorine either {\it trans} or {\it gauche} with respect to the ethynyl group. The {\it trans- gauche} isomerization barrier is approximately 1377 $\backslash wn$, and the {\it trans} form is more stable by 492 $\backslash wn$. Infrared laser excitation was used to prepare 4-fluorobut-1-yne in a region of the potential surface where isomerization can occur ( 3000 $\backslash wn$). Using our broadband spectrometer, the rotational spectrum of the infrared excited molecule was recorded from 7.5-18 GHz, covering the $\Delta$J = 1$\rightarrow$2 and $\Delta$J = 2$\rightarrow$3regionsofthespectrum.Theoveralllineshapeoftherotationalspectrumwasfitwithathree-stateBlochmodel,modifiedforchemicalexchange,yieldinganisomerizationrateofk$_iso$ = 2.7x10$\mathrm{<mrow\; data-mjx-texclass="ORD">10</mrow>}$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, much slower than the rate predicted by RRKM theory: k = 5.9x10$\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The single eigenstate rotational spectra recorded with our newly developed spectrometer agree with previously measured single eigenstate rotational spectra, recorded in the acetylenic CH-stretch at 3330 $\backslash wn$. Using a combination of double (infrared-microwave) and triple resonance (infrared-microwave-microwave) techniques, single eigenstate rotational spectra were recorded for several IR bands. Infrared excitation of the various CH-stretching bands allows us to observe the isomerization dynamics over a broad energy range (2900-3330 $\backslash wn$). Observed changes on the rotational spectrum will be discussed.