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Broadband Microwave Spectroscopy of 2‑Furanyloxy Radical: Primary Pyrolysis Product of the Second-Generation Biofuel 2‑Methoxyfuran

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journal contribution
posted on 31.07.2018, 00:00 by Chamara Abeysekera, A.O. Hernandez-Castillo, John F. Stanton, Timothy S. Zwier
Broadband microwave spectra over the 2–18 GHz range have been recorded for the resonance-stabilized 2-furanyloxy radical, formed in the first step of pyrolysis of the second-generation biofuel 2-methoxyfuran by methyl loss. Using a flash pyrolysis source attached to a pulsed valve, a 0.7% mixture of 2-methoxyfuran in argon was pyrolyzed at a series of temperatures ranging from 300 to 1600 K. Subsequent cooling in a supersonic expansion produced rotational temperatures of ∼2 K in the interrogation region. Using chirped-pulse Fourier transform microwave (CP-FTMW) methods, combined with strong-field coherence breaking (SFCB), a set of transitions due to the radical were identified and assigned. The experimental rotational constants (A = 8897.732(93), B = 4019.946(24), C = 2770.321(84)), centrifugal distortion constants, and spin-rotation coupling constants have been determined for the radical and compared with ab initio predictions at the CCSD­(T) level of theory. Compared to the 2-methoxyfuran precursor, the 2-furanyloxy radical has allylic C–C bond lengths intermediate between single and double bonds, a shortened C(5)–O(6) bond characteristic of partial double-bond character, and an O(1)–C(5)–O(6) bond angle of 121°, which resembles the O–C–O angle of an ester. Atomic spin densities extracted from the calculations confirm that the 2-furanyloxy radical is best viewed as a carbon-centered allylic lactone radical, with 80% of the spin density on the two allylic carbons and 20% on the pendant O(6) atom.