Chirped-Pulse Fourier Transform Microwave Spectroscopy Coupled with a Flash Pyrolysis Microreactor: Structural Determination of the Reactive Intermediate Cyclopentadienone

Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is combined with a flash pyrolysis (hyperthermal) microreactor as a novel method to investigate the molecular structure of cyclopentadienone (C₅H₄O), a key reactive intermediate in biomass decomposition and aromatic oxidation. Samples of C₅H₄O were generated cleanly from the pyrolysis of <i>o</i>-phenylene sulfite and cooled in a supersonic expansion. The ¹³C isotopic species were observed in natural abundance in both C₅H₄O and in C₅D₄O samples, allowing precise measurement of the heavy atom positions in C₅H₄O. The eight isotopomers include: C₅H₄O, C₅D₄O, and the singly ¹³C isotopomers with ¹³C substitution at the C1, C2, and C3 positions. Microwave spectra were interpreted by CCSD­(T) ab initio electronic structure calculations and an <i>r</i>_e molecular structure for C₅H₄O was found. Comparisons of the structure of this “anti-aromatic” molecule are made with those of comparable organic molecules, and it is concluded that the disfavoring of the “anti-aromatic” zwitterionic resonance structure is consistent with a more pronounced CC/CC bond alternation