Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part I. Spectral Overviews and Vibronic Analysis

Abstract

We report vibronically resolved laser-induced fluorescence (LIF) spectra of jet-cooled C5−C10 secondary alkoxy radicals. The LIF spectra demonstrate vibronic structures similar to smaller (C3−C4) secondary alkoxies. For 2-pentoxy and 2-hexoxy, rotationally resolved LIF spectra have also been recorded. Two types of rotational structures have been observed in vibronic bands of each molecule. Extensive quantum chemistry calculations have been performed on 2-pentoxy and 2-hexoxy. The computed results include the relative energies of conformers, their geometries, and the energy separations between the nearly degenerate A ̃and X ̃electronic states (ΔE A ̃−X ̃). Based on the similarity between the vibronic structures of different secondary alkoxies and calculated molecular parameters, including the relative energies of conformers, the B ̃← X ̃transition frequencies, and the vibrational frequencies, strong vibronic bands in the LIF spectra are assigned to the origin bands and CO stretch bands of the two lowest-energy conformers of each secondary alkoxy radical. The distinct rotational structures of the two different conformations of 2-pentoxy and 2-hexoxy will be simulated and analyzed in Part II of this series (J.

CH3O Substitution Effect Revisited in the Vibrationally Resolved Laser-Induced Fluorescence Spectra of Methoxycyclohexoxy Radicals

“…The vertical excitation energies of the three lowest excited states are depicted in Figure . Natural transition orbital (NTO) analysis shows that same as previously reported alkoxy radicals, ,, the à ← X̃ transition of both radicals corresponds to the orientation change of the half-filled O p orbital (Figure ). For 4- trans (e, e) methoxycyclohexoxy, the B̃ ← X̃ transition maintains the nature of the excitation of one electron from the C 1 –O σ bond to the half-filled O p orbital, which is the carrier of the LIF spectra of alkoxy radicals .…”

“…For 3-methoxycyclohexoxy, the trans (e, a) conformer has the lowest energy, in which the CH 3 O group is at the axial position. Generally, the conformation structure with two substituents both on the equatorial position of the cyclohexane six-carbon ring, has the lowest energy. − For 3-methoxycyclohexoxy, the lower energy of trans (e, a) compared to cis (e, e) conformer might be due to the gauche effect between the axial oxygen of CH 3 O and the axial H atoms on meta C. This makes the attribution of the spectrum more difficult because the lowest-energy conformer is the premier candidate of the spectral carrier. ,,, Fortunately, the cis –3-methoxycyclohaxanol isomer was available to synthesize the nitrite precursor. The LIF experiment of the cis –3-methoxycyclohexoxy radical was conducted, and the same spectrum as the cis–trans mixture was observed (Figure S4 in the Supporting Information), indicating that the LIF spectrum of 3-methoxycyclohexoxy comes from the cis isomer.…”

“…Alkoxy radicals have been extensively studied due to their significance in ozone chemistry. − They are intermediate products produced during the oxidation of organic compounds in the presence of nitrogen oxides (NO x ). − The competition between the three reaction pathways of alkoxy radicals in the atmosphere, i.e., isomerization, β C–C bond scission, and reaction with O 2 , may result in large uncertainties in the rate of ozone production. ,, In 1997, Atkinson proposed an estimation method for the calculation of alkoxy reaction rates in the troposphere on the basis of the experimental observation that the alkyl leaving group influenced the energetics of the alkoxy radical decomposition . Structure–activity relationship associated with alkoxy reaction barrier heights and reaction pathway competition was further investigated and discussed by Orlando, Vereecken, , Carrasquillo, and other researchers .…”

A combined experimental and computational study on the transition of the calcium isopropoxide radical as a candidate for direct laser cooling

“…Going from ethoxy to isopropoxy, symmetries of the two lowest electronic states are interchanged. Upon alkyl substitution, the upper (X ˜2E 1/2 ) and lower (X ˜2E 3/2 ) SO components of the methoxy radical are coupled by the zero-point-energy (ZPE)-corrected ''difference potential'' [43][44][45][46][48][49][50][51][52][53][54][55][56][57][58][59][60][61] between the A 0 and A 00 states DE 0 = E(A 0 ) À E(A 00 ) (see ref. 43 for detailed discussion on spin-vibronic coupling between two nearly degenerate electronic states).…”

Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part II. Rotational and Fine Structure

“…In the preceding paper called Part I, both the moderate-resolution (vibronically resolved) and high-resolution (rotationally and fine-structure resolved) laser-induced fluorescence (LIF) spectra of 2-pentoxy and 2-hexoxy are reported. Conformational identifications and vibronic assignments were made based on the comparison between the experimental LIF spectra and the ab initio calculated B̃–X̃ electronic transition frequencies and B̃-state vibrational frequencies.…”