Observation of Fluorescence Excitation Spectra of tert-Pentoxy and 3-Pentoxy Radicals

Intramolecular charge transfer excitation induced by CH3O substitution in the 3-methoxy-1-propoxy radical

The oxy-substituted alkoxy radicals are generated from the oxidation of ethers. Their degradation path affects ozone production and the formation of the secondary organic aerosol in the atmosphere. In this work, three alkoxy radicals with methoxy (CH3O) substitution at β, γ, and δ carbon are studied using laser-induced fluorescence (LIF) spectroscopy and theoretical calculation methods. A charge transfer (CT) excited state induced by the CH3O substitution is identified to be because of the intramolecular electron transfer from the C-O-C p orbital to the radical O p orbital. Comparison of the structure and CT transition strength between GGt and TTt conformers of the 3-methoxy-1-propoxy radical (CH3OCH2CH2CH2O) suggests that this long-range charge transfer effect is mainly a through-bond interaction. The CT excited state of CH3OCH2CH2CH2O has a conical intersection with the CO σ → O p excited state, which, hence, changes the LIF spectrum of the radical. Only the decomposition product HCHO was observed in the LIF spectrum of β substituted radical CH3OCH2CH2O. For δ substituted radical CH3OCH2CH2CH2CH2O, the substitution effect on the radical stability is negligible and its LIF spectrum is close to that of unsubstituted alkoxy radicals. The results provide information for understanding the degradation chemistry of oxygenated hydrocarbon molecules in the atmosphere.

Revealing Long-Range Substituent Effects in the Laser-Induced Fluorescence and Dispersed Fluorescence Spectra of Jet-Cooled CHxF3-xCH2O (x = 1, 2, 3) Radicals

The B̃-X̃ laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the atmospherically important β-monofluoro ethoxy (MFEO), β,β-difluoro ethoxy (DFEO), and β,β,β-trifluoro ethoxy (TFEO) radicals were recorded with vibronic resolution under jet-cooled conditions. To simulate the spectra, Franck-Condon factors were obtained from quantum chemical computations carried out at the CAM-B3LYP/6-311++G(d,p) level of theory. The simulations reproduce well both the LIF and DF spectra. Both conformers (G and T) of MFEO and one (G) of the two conformers of DFEO contribute to the LIF spectrum. A comparison between the experimental and calculated spectra confirms the expected long-range field effects of the CH_xF_3-x group on electronic transition energies and bond strengths, especially in the excited electronic (B̃) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the -CF₃ internal rotation.

Laser-induced fluorescence of isobutoxy in competition with ground state decomposition

Spectroscopic detection is an important method to monitor alkoxy radicals in atmospheric photochemistry studies. In this work, we report the first observation of the laser-induced fluorescence (LIF) spectra of isobutoxy, 2-methyl-1-butoxy, and 3-methyl-1-butoxy in supersonic jet-cooled condition. Ground state unimolecular decomposition and isomerization as well as excited state relaxation dynamics of isobutoxy were discussed in combination with the theoretical calculations. Analysis of the experimental and theoretical results showed that methyl substitution on the β carbon of the alkoxy radicals changed the LIF spectra of alkoxy radicals significantly. The competition between the ground state reactions and the photoexcitation process depended strongly on the radical structure and hence affected the involvement of alkoxy radicals in the photochemical reactions in the upper troposphere. This study will help to understand the dynamic role of alkoxy radicals in the atmosphere.

High resolution spectra and conformational analysis of 2-butoxy radical

We have recorded five high resolution (200 MHz), rotationally resolved, vibrational bands of the B-X electronic transition of 2-butoxy. Two bands of the 2-butoxy spectrum have been rotationally analyzed and assigned to two different geometrical conformers of the molecule. The analyses allow the determination of the six experimental rotational constants defined by the geometry of the species in the ground (X) and excited (B) electronic states and also four spin-rotation constants for the X electronic state of the conformers. Comparison of the experimental rotational constants with the results of ab initio computations provides unambiguous conformational assignment of these bands. This approach can be extended to assign two other spectral bands to the third 2-butoxy conformer.

Laser-Induced Fluorescence Spectra of 4-Methylcyclohexoxy Radical and Perdeuterated Cyclohexoxy Radical and Direct Kinetic Studies of Their Reactions with O2

The laser-induced fluorescence (LIF) excitation spectra of the 4-methylcyclohexoxy and d11-cyclohexoxy radicals have been measured for the first time. LIF intensity was used as a probe in direct kinetic studies of the reaction of O(2) with trans-4-methylcyclohexoxy and d11-cyclohexoxy radicals from 228 to 301 K. Measured rate constants near room temperature are uniformly higher than the Arrhenius fit to the lower-temperature data, which can be explained by the regeneration of cyclic alkoxy radicals from the product of their beta-scission and the effect of O(2) concentration on the extent of regeneration. The Arrhenius expressions obtained over more limited ranges were k(O2) = (1.4(+8)(-1)) x 10(-13) exp[(-810 +/- 400)/T] cm(3) molecule(-1) s(-1) for trans-4-methylcyclohexoxy (228-292 K) and k(O2) = (3.7(+4)(-1)) x 10(-14) exp )[(-760 +/- 400) /T] cm(3) molecule(-1) s(-1) for d11-cyclohexoxy (228-267 K) independent of pressure in the range 50-90 Torr. The room-temperature rate constant for the reaction of trans-4-methylcyclohexoxy radical with O2 (obtained from the Arrhenius fit) is consistent with the commonly recommended value, but the observed activation energy is approximately 3 times larger than the recommended value of 0.4 kcal/mol and half the value previously found for the reaction of normal cyclohexoxy radical with O2.

Jet-cooled laser spectroscopy of the cyclohexoxy radical

The laser-induced fluorescence and laser-excited dispersed fluorescence spectra of the cyclohexoxy radical has been observed under two sets of free-jet-cooling conditions, characterized by rotational temperatures of approximately 1 and 100 K. Although five conformers of cyclohexoxy are possible, it appears that all presently observed spectral bands can be accounted for by a single one. All cold spectral bands are assigned to the B-X electronic transition of the cyclohexoxy radical. Transitions to both a' and a" B state vibrational levels are observed and allowed due to a substantial pseudo-Jahn-Teller effect in the X state. Hot bands are also observed, which we attributed to transitions to the B state from the low-lying A electronic state. Analysis of the spectra yields vibrational frequencies for the X, A, and B states as well as the energy separations of their vibrationless levels.

Reactions of the alkoxy radicals formed following OH-addition to alpha-pinene and beta-pinene. C-C bond scission reactions

The atmospheric degradation pathways of the atmospherically important terpenes alpha-pinene and beta-pinene are studied using density functional theory. We employ the correlation functional of Lee, Yang, and Parr and the three-parameter HF exchange functional of Becke (B3LYP) together with the 6-31G(d) basis set. The C-C bond scission reactions of the beta-hydroxyalkoxy radicals that are formed after OH addition to alpha-pinene and beta-pinene are investigated. Both of the alkoxy radicals formed from the alpha-pinene-OH adduct possess a single favored C-C scission pathway with an extremely low barrier (approximately 3 kcal/mol) leading to the formation of pinonaldehyde. Neither of these pathways produces formaldehyde, and preliminary computational results offer some support for suggestions that 1,5 or 1,6 H-shift (isomerization) reactions of alkoxy radicals contribute to formaldehyde production. In the case of the alkoxy radical formed following OH addition to the methylene group of beta-pinene, there exists two C-C scission reactions with nearly identical barrier heights (approximately 7.5 kcal/mol); one leads to known products (nopinone and formaldehyde) but the ultimate products of the competing reaction are unknown. The single C-C scission pathway of the other alkoxy radical from beta-pinene possesses a very low (approximately 4 kcal/mol) barrier. The kinetically favored C-C scission reactions of all four alkoxy radicals appear to be far faster than expected rates of reaction with O2. The rearrangement of the alpha-pinene-OH adduct, a key step in the proposed mechanism of formation of acetone from alpha-pinene, is determined to possess a barrier of 11.6 kcal/mol. This value is consistent with another computational result and is broadly consistent with the modest acetone yields observed in product yield studies.