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  1. 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 occurringmore » for an in-plane orientation of the alkyl chain (trans) about its first dihedral angle (Φ f123, numbers are alkyl C atoms. C 1 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 (Φ 1234) for the αPrBz radical. The in-plane orientation lowers the local site frequencies of the CH 2 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 CH 2 group, with little effect on subsequent members of the alkyl chain. In the D 1 excited electronic state, an out-of-plane orientation is preferred for the alkyl chains, leading to torsional mode Franck-Condon activity in the D 0-D 1 spectra that is both conformer-specific and diagnostic of the conformational change.« less
  2. In this paper, conformation-specific UV-IR double resonance spectra are presented for ethyl, n-propyl, and n-butylbenzene. With the aid of a local mode Hamiltonian that includes the effects of stretch-scissor Fermi resonance, the spectra can be accurately modeled for specific conformers. These molecules allow for further development of a first principles method for calculating alkyl stretch spectra. Across all chain lengths, certain dihedral patterns impart particular spectral motifs at the quadratic level. However, the anharmonic contributions are consistent from molecule to molecule and conformer to conformer. This transferability of anharmonicities allows for the Hamiltonian to be constructed from only a harmonicmore » frequency calculation, reducing the cost of the model. Finally, the phenyl ring alters the frequencies of the CH 2 stretches by about 15 cm -1 compared to their n-alkane counterparts in trans configurations. Conformational changes in the chain can lead to shifts in frequency of up to 30 cm -1.« less

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