The vibrationless Ã←X̃ transition of the jet-cooled deuterated methyl peroxy radical CD3O2 by cavity ringdown spectroscopy

Calculated and Empirical Values of Vibronic Transition Dipole Moments of Reactive Chemical Intermediates for Determination of Concentrations

Absorption spectroscopy has long been known as a technique for making molecular concentration measurements and has received enhanced visibility in recent years with the advent of new techniques, like cavity ring-down spectroscopy, that have increased its sensitivity. To apply the method, it is necessary to have a known molecular absorption cross section for the species of interest, which typically is obtained by measurements of a standard sample of known concentration. However, this method fails if the species is highly reactive, and indirect means for attaining the cross section must be employed. The HO₂ and alkyl peroxy radicals are examples of reactive species for which absorption cross sections have been reported. This work explores and describes for these peroxy radicals the details of an alternative approach for obtaining these cross sections using quantum chemistry methods for the calculation of the transition dipole moment upon whose square the cross section depends. Likewise, details are given for obtaining the transition moment from the experimentally measured cross sections of individual rovibronic lines in the near-IR Ã-X̃ electronic spectrum of HO₂ and the peaks of the rotational contours in the corresponding electronic transitions for the alkyl (methyl, ethyl, and acetyl) peroxy radicals. In the case of the alkyl peroxy radicals, good agreement for the transition moments, ≈20%, is found between the two methods. However, rather surprisingly, the agreement is significantly poorer, ≈40%, for the HO₂ radical. Possible reasons for this disagreement are discussed.

A uniform flow-cavity ring-down spectrometer (UF-CRDS): A new setup for spectroscopy and kinetics at low temperature

The UF-CRDS (Uniform Flow-Cavity Ring Down Spectrometer) is a new setup coupling for the first time a pulsed uniform (Laval) flow with a continuous wave CRDS in the near infrared for spectroscopy and kinetics at low temperature. This high resolution and sensitive absorption spectrometer opens a new window into the phenomena occurring within UFs. The approach extends the detection range to new electronic and rovibrational transitions within Laval flows and offers the possibility to probe numerous species which have not been investigated yet. This new tool has been designed to probe radicals and reaction intermediates but also to follow the chemistry of hydrocarbon chains and PAHs which play a crucial role in the evolution of astrophysical environments. For kinetics measurements, the UF-CRDS combines the CRESU technique (French acronym meaning reaction kinetics in uniform supersonic flows) with the SKaR (Simultaneous Kinetics and Ring-Down) approach where, as indicated by its name, the entire reaction is monitored during each intensity decay within the high finesse cavity. The setup and the approach are demonstrated with the study of the reaction between CN (v = 1) and propene at low temperature. The recorded data are finally consistent with a previous study of the same reaction for CN (v = 0) relying on the CRESU technique with laser induced fluorescence detection.

Room-Temperature Cavity Ring-Down Spectroscopy of Methylallyl Peroxy Radicals

We report room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of 1-, 2-, and 3-methylallyl peroxy (MAOO^•) radicals produced by 193 nm photolysis of methyl-substituted allyl chlorides in the presence of O₂. Vibronic structure of experimentally observed spectra was simulated using calculated relative populations of MAOO^• conformers, their electronic transition frequencies and oscillator strengths, as well as their vibrational frequencies and Franck-Condon factors of the à ← X̃ electronic transition. The reaction intermediate for the production of 1- and 3-MAOO^• radicals, CH₃CHCHCH₂, is a resonance-stabilized free radical. CRD spectra of 1- and 3-MAOO^• radicals obtained using different precursors suggest that allylic rearrangement between the two resonance structures (CH₃CH=CHCH₂^• and CH₃CH^•CH=CH₂) is significantly faster than oxygen addition. Branching ratio between terminal and nonterminal oxygen addition was predicted to be 52:48 on the basis of calculated spin densities, which agrees qualitatively with the experimental CRD spectra of 1- and 3-MAOO^• radicals.

tert-Butyl peroxy radical: ground and first excited state energetics and fundamental frequencies

The lowest adiabatic electronic transition origin and fundamental vibrational frequencies are computed, with high accuracy, for the tert-butyl peroxy radical.

Direct Observation of Tetrahydrofuranyl and Tetrahydropyranyl Peroxy Radicals via Cavity Ring-Down Spectroscopy

Room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of tetrahydrofuranyl peroxy (THFOO^•) and tetrahydropyranyl peroxy (THPOO^•) radicals were recorded. The peroxy radicals were produced by Cl-initiated oxidation of tetrahydrofuran and tetrahydropyran. Quantum chemical calculations of the lowest-energy conformers of all regioisomers of these two peroxy radicals have been carried out to aid the spectral simulation. Conformational identification and vibrational assignment were achieved by comparing the experimentally obtained spectra to the simulated ones. The absence of α-THPOO^• absorption peaks in the CRD spectrum is attributed to ring opening due to its weak C_α'O bond.

Axis-switching in the vibrationless Ã←X̃ transition of the jet-cooled deuterated methyl peroxy radical CD3O2

The jet-cooled high resolution spectrum of the vibrationless Ã←X̃ transition of the deuterated species of the methyl peroxy radical has been recently published in this journal (S. Wu, P. Dupré, P. Rupper, and T. A. Miller, J. Chem. Phys. 127, 224305 (2007)). The spectrum was analyzed using a rigid-rotor model with quadratic spin-rotation coupling. The analysis was based on the fit of ∼350 partially resolved line positions and was quite satisfactory. However, the full simulation of the spectral intensity clearly identifies a lack of ability to reproduce relatively small line clumps ("extra" lines) located between the two main central Q branches. This is indicating of an incomplete initial analysis. In the present paper we reanalyze this electronic transition by considering a reference-frame axis-switching resulting from the nuclear rearrangement associated to the electronic transition (spectra obtained at two different temperatures are considered). The potential energy hypersurfaces of the two electronic states are sufficiently dissimilar to induce changes in the molecule geometry, particularly, the angle COÔ, which induces a rotation (∼1.7°) of the principal axes of inertia located in the molecule symmetry plane. The present analysis is supported by a global fitting of the spectrum intensity and gives rise to a slightly different set of molecular constants. Attention is paid to the wavefunction symmetry assignment of a non-orthorhombic molecule. Couplings due to the torsion of the methyl group are discussed in the following paper (P. Dupre, J. Chem. Phys. 134, 244309 (2011)).

Internal rotation: single diagonalization approach versus standard approaches, application to the methyl peroxy radical Ã←X̃ transition

A new approach avoiding double (two-step) diagonalization is proposed to deal with internal rotation. The development of this method was stimulated by the jet-cooled high resolution spectrum of the vibrationless Ã←X̃ transition of the deuterated species of the methyl peroxy radical. This spectrum, originally analyzed with a rigid rotor Hamiltonian including spin-rotation but neglecting internal rotation, has been revisited in the previous paper (P. Dupré, J. Chem. Phys. 134, 244308 (2011)) and a determinable yaw of the molecular principal axes of inertia about the c-axis (axis-switching) during the electronic transition was established. The spectral resolution of the jet-cooled data of the vibrationless transition (∼7355 - 7390 cm(-1)) does not allow the observation of splitting due to internal rotation of the methyl top, but when these data are combined with the low resolution room temperature data (∼7200 - 8000 cm(-1)) accurate fits or simulations of the two sets of data are possible. A recent study of the room temperature data has been reported in this journal (G. M. P. Just et al., J. Chem. Phys. 127, 044310 (2007)) showing evidence of the internal rotation coupling by analyzing the intensity of the torsional mode energy progression. That investigation combined ab initio quantum chemistry calculations and the rho-axis-method (RAM) to model the internal rotation. Here, a comparison of full spectral intensity analyses based on both the usual RAM and on the new approach requiring a single-diagonalization principal-axis-method is presented. The comparison favors the single-diagonalization approach. Axis-switching and spin-rotation coupling are incorporated in the analysis, in which the use of the principal axes of inertia is maintained. Symmetries, energy levels, and advantages are carefully discussed for all methods.

High-resolution cavity ringdown spectroscopy of the jet-cooled ethyl peroxy radical C2H5O2

We have recorded high resolution, partially rotationally resolved, jet-cooled cavity ringdown spectra of the origin band of the A-X electronic transition of both the G and T conformers of the perproteo and perdeutero isotopologues of the ethyl peroxy radical, C(2)H(5)O(2). This transition, located in the near infrared, was studied using a narrow band laser source (< or approximately 250 MHz) and a supersonic slit-jet expansion coupled with an electric discharge allowing us to obtain rotational temperatures of about 15 K. All four spectra have been successfully simulated using an evolutionary algorithm approach with a Hamiltonian including rotational and spin-rotational terms. Excellent agreement with the experimental spectra was obtained by fitting seven molecular parameters in each ground and the first excited electronic states as well as the band origin of the electronic transition. This analysis unambiguously confirms the assignment of the lower frequency origin band to the G conformer and the higher frequency one to the T conformer.