Knowledge Bank

High-resolution infrared spectra of jet-cooled cyclopropyl radical are reported for the first time, specifically sampling the in-phase antisymmetric CH$2$ stretch ($\nu 7$) vibration. In addition to yielding first precise gas phase structural information for this radical, the spectra reveal doubling due to quantum tunneling of the lone $\alpha$-CH with respect to the CCC plane, lift the degeneracy and generating lower (+) and upper (-) tunneling states. The bands clearly reveal intensity alternation (6:10 for even:odd Ka+Kc levels in the lower $\leftarrow$ lower and 10:6 in the upper $\leftarrow$ upper) due to H atom nuclear spin statistics, confirming that the tunneling transition state for cyclopropyl radical is of C symmetry. However, in addition to the two predicted vibrational bands (i.e. lower $\leftarrow$ lower and upper $\leftarrow$ upper), a third band is observed due to IVR mixing of the upper tunneling component ($\nu 7$) with a nearly isoenergetic dark state. From fractional populations in the ground and excited state, the tunneling splittings for cyclopropyl radical in the ground and excited ($\nu 7$) state are estimated to be 3.2$\pm 0.3$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ and 4.9$\pm 0.3$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, respectively. This indicates that stereoracemization of the $\alpha$-CH radical center is a very fast process (k=2.0x10$\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$), and that the barrier decreases upon vibrational excitation of the CH$2$ in-phase antisymmetric stretch. The barrier height for $\alpha$-CH inversion through the cyclopropyl plane is also extracted from the analysis of the tunneling dynamics with simple 1D potential energy surface.