Pulsed discharge nozzle Fourier transform microwave spectroscopy of the propargyl radical (H2CCCH)

The rotational spectrum of the propargyl radical (H2CCCH) was recorded with Fourier-trans-form microwave (FTMW) spectrometer coupled with a pulsed discharge nozzle (PDN). Fine and hyperfine components belonging to the 101−000, 202−101, 212−111, and 211−101 rota-tional transitions in the 2B1 ground vibronic state were observed. The analysis of the FTMW spectra resulted in the following ground-state molecular constants, with 3σ uncertainties given in parentheses. The rotational and centrifugal distortion constants are B0=9523.6775(60) MHz, C0=9206.8805(60) MHz, ΔN=3.44(63) kHz, and ΔNK=0.3753(28) MHz. The spin–rotation coupling constants are εaa=−529.386(60), εbb=−11.524(30), and εcc=−0.520(32) MHz. The hyperfine coupling constants are aF=−36.323(24), Taa=17.400(24), and Tbb=−17.220(37) MHz for the acetylenic (CH) proton, and aF=−54.21(11) and Taa=−14.121(19) MHz for the methylenic (CH2) proton. The derived hyperfine coupling constants indicate compar-able contributions of the allenyl (H2C=C=CH) and propargyl (H2C–C≡CH) forms to the resonance structure [H2Ċ–C≡CH↔H2C=C=ĊH].