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 o-phenylene sulfite and cooled in a supersonic expansion. The C-13 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 C-13 isotopomers with C-13 substitution at the C1, C2, and C3 positions. Microwave spectra were interpreted by CCSD(T) ab initio electronic structure calculations and an r(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.