New investigation of the ν3 C-H stretching region of 12CH4 through the analysis of high temperature infrared emission spectra.

The ν3 C-H stretching region of methane was reinvestigated in this work using high temperature (620-1715 K) emission spectra recorded in Rennes at Doppler limited resolution. This work follows our recent global analysis of the Dyad system Δn = ±1 (1000-1500 cm-1), with n being the polyad number [B. Amyay et al., J. Chem. Phys. 144, 24312 (2016)]. Thanks to the high temperature, new assignments of vibration-rotation methane line positions have been achieved successfully in the Pentad system and some associated hot bands (Δn = ±2) observed in the spectral region 2600-3300 cm-1. In particular, rotational assignments in the cold band [Pentad-ground state (GS)] and in the first related hot band (Octad-Dyad) were extended up to J = 30 and 27, respectively. In addition, 1525 new transitions belonging to the Tetradecad-Pentad hot band system were assigned for the first time, up to J = 20. The effective global model used to deal with the new assignments was developed to the 6th order for the first three polyads (Monad, Dyad, and Pentad), and to the 5th order for both the Octad and the Tetradecad. 1306 effective parameters were fitted with a dimensionless standard deviation σ = 2.64. The root mean square deviations dRMS obtained are 4.18 × 10-3 cm-1 for the Pentad-GS cold band, 2.48 × 10-3 cm-1 for the Octad-Dyad, and 1.43 × 10-3 cm-1 for the Tetradecad-Pentad hot bands.

Low-Temperature Reactivity of C2n+1N(-) Anions with Polar Molecules.

Following the recent discovery of molecular anions in the interstellar medium, we report on the kinetics of proton transfer reactions between cyanopolyynide anions C2n+1N(-) (n = 0, 1, 2) and formic acid HCOOH. The results, obtained from room temperature down to 36 K by means of uniform supersonic flows, show a surprisingly weak temperature dependence of the CN(-) reaction rate, in contrast with longer chain anions. The CN(-) + HCOOH reaction is further studied theoretically via a reduced dimensional quantum model that highlights a tendency of the reaction probability to decrease with temperature, in agreement with experimental data but at the opposite of conventional long-range capture theories. In return, comparing HCOOH to HC3N as target molecules suggests that dipole-dipole interactions must play an active role in overcoming this limiting effect at low temperatures. This work provides new fundamental insights on prototypical reactions between polar anions and polar molecules along with critical data for astrochemical modeling.

High temperature reaction kinetics of CN(v = 0) with C2H4 and C2H6 and vibrational relaxation of CN(v = 1) with Ar and He.

The investigation of the chemical complexity of hot environments, ranging from combustion flames to circumstellar envelopes of evolved stars, relies on the determination of the reaction kinetics and product branching ratio. We have designed a chemical reactor for the exploration of high temperature chemistry. This apparatus is employed in the present study to measure the reaction kinetics of the CN radical with C2H4 and C2H6 over the 300-1200 K temperature range. In our setup and in some environments, the CN radical is partially produced in a vibrationally excited state, before relaxing by collision with the surrounding gas. We complement the experimental kinetic studies of hydrocarbons reactions with CN(v = 0) with a theoretical study of vibrational relaxation of CN(v = 1) by He and Ar atoms, the main collisional partners in our apparatus. Calculations are carried out to determine the collisional elastic and inelastic cross sections versus the kinetic energy as well as the corresponding vibrationally elastic and inelastic rate coefficients. The results are compared with empirical calculations and with a few experimental observations. The range of validity of the empirical model is discussed and potential applications sketched.

Direct infrared absorption spectroscopy of benzene dimer.

The direct infrared (IR) absorption spectrum of benzene dimer formed in a free-jet expansion was recorded in the 3.3 µm region for the first time. This has led to the observation of the C-H stretching fundamental mode ν(13) (B(1u)), which is both IR and Raman forbidden in the monomer. Moreover, the IR forbidden and Raman allowed ν(7) (E(2g)) mode has been observed as well. These two modes were found to be red-shifted along with the IR allowed ν(20) (E(1u)) mode, as previously reported by Erlekam et al. [Erlekam; Frankowski; Meijer; Gert von Helden J. Chem. Phys.2006, 124, 171101], using ion-dip spectroscopy, contrary to the blue-shift predicted earlier by theoretical studies. The observation of the ν(13) band indicates that the symmetry is reduced in the dimer, confirming the T-shaped structure observed by Erlekam et al. Our experimental results have not provided any direct evidence for the presence of the parallel displaced geometry, the main objective of the present work, as predicted by theoretical calculations.

Understanding reactivity at very low temperatures: the reactions of oxygen atoms with alkenes.

A remarkable number of reactions between neutral free radicals and neutral molecules have been shown to remain rapid down to temperatures as low as 20 kelvin. The rate coefficients generally increase as the temperature is lowered. We examined the reasons for this temperature dependence through a combined experimental and theoretical study of the reactions of O(3P) atoms with a range of alkenes. The factors that control the rate coefficients were shown to be rather subtle, but excellent agreement was obtained between the experimental results and microcanonical transition state theory calculations based on ab initio representations of the potential energy surfaces describing the interaction between the reactants.

Laboratory measurements of the recombination of PAH ions with electrons: implications for the PAH charge state in interstellar clouds.

Laboratory measurements of the recombination of polycyclic aromatic hydrocarbon (PAH) ions with electrons are presented. Experimental data have been obtained at room temperature for azulene (C10H8) and acenaphthene (C12H10) cations by the Flowing Afterglow with PhotoIons method. The results confirm that the recombination of PAH ions is fast although well below the geometrical limit. The set of our recent and present measurements reveal a definite trend of increasing rate with the number of carbon atoms of the PAH. This behaviour that needs further characterization is potentially of great interest for charge state models as recombination is a dominant mechanism of PAH ion destruction in the interstellar medium. The design of experiments to measure the recombination of larger PAHs and their temperature dependence is discussed.

Reaction of anthracene with CH radicals: an experimental study of the kinetics between 58 and 470 K.

The rate coefficient of the reaction of the methylidine radical CH with anthracene has been studied over a wide temperature range (58-470 K) in a dedicated "Cinétique de Réaction en Ecoulement Supersonique Uniforme" (Reaction Kinetics in Uniform Supersonic Flow) apparatus. The reaction exhibits a slight positive temperature dependence, which can be fitted to the expression k(T) = (3.32 +/- 1.00) x 10(-10)(T/298)((0.46+/-0.14)) cm3 molecule(-1) s(-1). Even at the lowest temperature, the reaction remains very fast indicating that the kinetics are probably driven by a capture process.