MICROWAVE SPECTRA OF THE 2,4-PENTADIYNE RADICAL, $H_{2}C-C\equiv C-C\equiv C-H$

The $H_{2}C-C\\equiv C-C\\equiv C-H$ radical was produced in a pulsed discharge supersonic jet and studied by Fourier transform microwave spectroscopy. Four rotational transitions of the radical from $N_{upper} = 2$ to 5 in the $K_{a} = 0$ ladder were observed between 8 and 21 GHz. A total of 36 hyperfine components were measured and assigned. Seven spectroscopic constants, including rotational and centrifugal distortion constants, the spin-rotation constant, and hyperfine coupling constants for the methylenic and acetylenic hydrogens, were determined. These spectroscopic constants of 2,4-pentadiyne will be compared with those of its isoelectronic analog, $H_{2}C-C\\equiv C-C\\equiv N$, and with those of the propargyl radical, $H_{2}C-C\\equiv C-H$.