To kink or not: A search for long-chain cumulenones using microwave spectral taxonomy

A systematic search for carbon-chain cumulenones beyond H₂C₄O has been undertaken using microwave spectral taxonomy in combination with a pulsed jet discharge source. No evidence was found for the C_2υ isomer of H₂C₅O or its longer derivatives, but HC(O)C₄H, the longer variant of propynal, HC(O)CCH, was identified instead. Its rotational and leading centrifugal distortion constants have been derived to high accuracy from detection of both a- and b-type lines; those below 40 GHz were measured using a Fabry-Perot cavity, while lines between 40 and 72 GHz were recorded by double resonance techniques. Overwhelming evidence for the identification is provided by detection of HC(O)C₄D, DC(O)C₄H, and HC(¹⁸O)C₄H at the expected frequencies using isotopically enriched samples. Because HC(¹⁸O)C₄H is produced with comparable abundance when using either ¹⁸O₂ or C¹⁸O as the source of oxygen, and because H¹³C(O)C₄H is not preferentially formed when starting from ¹³CO, atomic oxygen appears to be a key reactant in formation, plausibly via O insertion, e.g., H₂CC_2n+2H + O → HC(O)C_2n+2H + H. Under the same experimental conditions, HC(O)CCH is more than 10 times more abundant than H₂C₃O, regardless of the source of oxygen, and no evidence is found for cyclopropenone, c-C₃H₂O. Taken together, these observations indicate that propynal and longer chains with an odd number of carbon atoms are either energetically more stable than cumulenones of the same size, are kinetically favored products, or both. On the basis of the HC(O)C₄H discovery, searches for the isovalent sulfur species, HC(S)C₄H, and HC(O)C₆H have been conducted. Guided by new quantum chemical calculations, the rotational spectra of both were observed in the centimeter-wave band with the same spectrometer.