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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Broadband Microwave Spectroscopy of 2-Furanyloxy Radical: Primary Pyrolysis Product of the Second-Generation Biofuel 2-Methoxyfuran Chamara Abeysekera, Alicia O. Hernandez-Castillo, John F. Stanton, and Timothy S. Zwier J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05102 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018
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The Journal of Physical Chemistry
Broadband Microwave Spectroscopy of 2-Furanyloxy Radical: Primary Pyrolysis Product of the Second-Generation Biofuel 2-Methoxyfuran Chamara Abeysekera1, A.O. Hernandez-Castillo1, John F. Stanton2*, and Timothy S. Zwier1* 1
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
2
Department of Chemistry, University of Florida, PO Box 117200, Gainesville FL 32611, USA
ABSTRACT 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 300-1200 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, 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(2)-O(6) bond characteristic of partial double-bond character, and an O(1)C(2)-O(6) bond angle of 121 degrees, 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.
*
To whom correspondence should be addressed. E-mail:
[email protected];
[email protected] 16 pages, 2 Tables, 4 Figures
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I. Introduction The steady rise in world energy consumption, and the shortage of fossil fuels demand renewable, cheaper alternatives. Biomass is considered an economical renewable energy source due to its abundance and lack of residual impurities containing sulfur, nitrogen, and metals1-2. Recent advances in converting lignocellulosic biomass to biofuels have stimulated the interest in second generation biofuels such as alkylated furans, furanic ethers and lactones, due to their added benefits of consuming waste residues while circumventing a direct impact on human food supply.3-5 However, due to the chemical complexity of these second-generation biofuels, much remains to be learned about their pyrolysis and combustion. Developing accurate combustion models requires experimental data on the kinetics and product branching ratios of their individual reaction steps. New spectroscopic tools and methods are needed to characterize this widening array of fuel components and reactive intermediates.6 Rotational spectroscopy has the inherent characteristics of a near-ideal molecular shape detector. The broadband capability and the rapid spectral velocity at high resolution (
