A First-PrinciplesModel of Fermi Resonance in theAlkyl CH Stretch Region: Application to Hydronaphthalenes, Indanes,and Cyclohexane.

Theinfrared (IR) spectroscopy of the alkyl CH stretch region (2750–3000cm–1) of a series of bicyclic hydrocarbons and freeradicals has been studied under supersonic expansion cooling in thegas phase, and compared with a theoretical model that describes thelocal mode stretch–bend Fermi resonance interactions. The doubleresonance method of fluorescence-dip infrared (FDIR) spectroscopywas used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene,tetralin, indene, and indane using the S0–S1origin transition as a monitor of transitions. Resonant ion-dipinfrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN)and inden-2-yl methyl (I2M) radicals. The previously developed modelHamiltonian (J. Chem. Phys.2013, 138, 064308) incorporates cubic stretch–bend couplingwith parameters obtained from density functional theory methods. Fulldimensional calculations are compared to reduced dimensional Hamiltonianresults in which anharmonic CH stretches and CH2scissormodes are Fermi coupled. Excellent agreement between theoretical resultsis found. Scale factors of select terms in the reduced dimensionalHamiltonian, obtained by fitting the theoretical Hamiltonian predictionsto the experimental spectra, are found to be similar to previous work.The resulting Hamiltonian predicts successfully all the major spectralfeatures considered in this study. A simplified model is introducedin which the CH2groups are decoupled. This model enablesthe assignment of many of the spectral features. The model resultsare extended to describe the CH stretch spectrum of the chair andtwist-boat conformers of cyclohexane. The chair conformer is usedto illustrate the shortcomings of the CH2decoupling model. [ABSTRACT FROM AUTHOR]