Conformation-specific spectroscopy of alkyl benzyl radicals: Effects of a radical center on the CH stretch infrared spectrum of an alkyl chain

Korn, Joseph A.; Tabor, Daniel P.; Sibert, Edwin L.; Zwier, Timothy S.

doi:10.1063/1.4963227

Title: Conformation-specific spectroscopy of alkyl benzyl radicals: Effects of a radical center on the CH stretch infrared spectrum of an alkyl chain

Journal Article · Wed Sep 28 00:00:00 EDT 2016 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4963227· OSTI ID:1468933

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Purdue Univ., West Lafayette, IN (United States). Dept. of Chemistry
Univ. of Wisconsin, Madison, WI (United States). Dept. of Chemistry and Theoretical Chemistry Inst.

An important initial step in the combustion of gasoline and diesel fuels is the abstraction of hydrogen from alkylbenzenes to form resonance-stabilized alkyl benzyl radicals. This work uses, for the first time, double resonance spectroscopy methods to explore the conformation-specific vibronic and infrared spectroscopy of the α-ethylbenzyl (αEtBz) and α-propylbenzyl (αPrBz) radicals. Local mode Hamiltonian modeling enables assignment of the alkyl CH stretch IR spectra, accounting for Fermi resonance that complicates aliphatic alkyl CH stretch IR spectroscopy. The ground state conformational preferences of the ethyl and propyl chains are changed from those in the alkylbenzenes themselves, with global minima occurring for an in-plane orientation of the alkyl chain (trans) about its first dihedral angle (Φ_f123, numbers are alkyl C atoms. C₁ is CH radical site). This in-plane structure is the only observed conformer for the α-EtBz radical, while two conformers, tt and tg' share this orientation at the first dihedral, but differ in the second (Φ₁₂₃₄) for the αPrBz radical. The in-plane orientation lowers the local site frequencies of the CH₂ group stretches immediately adjacent to the benzylic radical site by about 50 cm^-1 relative to those in pure alkyl chains or alkylbenzenes. This effect of the radical site is localized on the first CH₂ group, with little effect on subsequent members of the alkyl chain. In the D₁ excited electronic state, an out-of-plane orientation is preferred for the alkyl chains, leading to torsional mode Franck-Condon activity in the D₀-D₁ spectra that is both conformer-specific and diagnostic of the conformational change.

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Research Organization:: Purdue Univ., West Lafayette, IN (United States). Dept. of Chemistry

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division; National Science Foundation (NSF)

Grant/Contract Number:: FG02-96ER14656; CHE-1213449

OSTI ID:: 1468933

Alternate ID(s):: OSTI ID: 1328551

Journal Information:: Journal of Chemical Physics, Vol. 145, Issue 12; ISSN 0021-9606

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 15 works

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Modeling vibrational anharmonicity in infrared spectra of high frequency vibrations of polyatomic molecules Sibert, Edwin L. The Journal of Chemical Physics, Vol. 150, Issue 9 https://doi.org/10.1063/1.5079626	journal	March 2019

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