Dahbia Talbi

Electronic and vibrational spectra of matrix isolated anthracene radical cations: Experimental and theoretical aspects

Radical cations of anthracene have been formed by vapor phase electron impact followed by trapping in an argon matrix at 12 K. Visible/ultraviolet and infrared absorption spectra of the anthracene cations in an argon matrix have been run. Significant differences in the infrared band intensities between neutral and cationic anthracene have been observed. The effects of photolysis and added CCl4 have been studied and their influence on the infrared band intensities correlated with known visible bands attributable to the anthracene cation. Theoretical calculations using Pariser–Parr–Pople and intermediate neglect of differential overlap methodologies with high level multireference perturbation configuration interaction, specifically modified for spectroscopic applications, have been performed. Both approaches predict the previously observed photoelectron spectrum well. For the optical absorption, the match with the experimental spectrum is also good, but there are notable differences between the two predicti...

Potential energy surfaces for the electronic dissociative recombination of HCNH+: astrophysical implications on the HCN/HNC abundance ratio

Abstract The two-dimensional potential energy surfaces for the electronic dissociative recombination reaction HCNH + xa0+xa0e − →HCNH * →HCNxa0+xa0H/HNCxa0+xa0H have been computed using large-scale CI calculations. The lowest dissociative states of 2 Σ + symmetry, resulting in the N–H or C–H bound scission yielding HCN or HNC, respectively are shown to cross the ionic HCNH + state at its minimum geometry, indicating that the direct mechanism of electronic dissociative recombination of HCNH + is an efficient process which should lead to an equal formation of HCN and HNC. The same conclusion can be drawn for the indirect mechanism: the crossing between the dissociative states and the lowest Rydberg state of 2 Σ symmetry occurs at the same energy for both NH and CH dissociations. The present calculations support the astrophysical hypothesis of an equal formation of HCN and HNC in the HCNH + xa0+xa0e − dissociative recombination process.

Isomerization versus hydrogen exchange reaction in the HNC HCN conversion

Abstract For the HNC/HCN interconversion we show that the push-pull hydrogen exchange reaction H+CNH⇌HCNH⇌HCN+H is favoured over internal isomerization; the formation of H2CN or CNH2 followed by rearrangement to HCNH and subsequent elimination are more energy demanding processes. Both push-pull forward and reverse reactions present activation barriers. However, the activation energy on the H+CNH entrance channel (4.2±1.0 kcal/mol) is four times smaller than on the HCN+H path. As a consequence, it can be anticipated that there will be a range of temperatures where the H+CNH reaction will be efficient while the reverse HCN+H process is still inhibited. This process, much less endothermic than internal isomerization, should become an important path for HNC/HCN conversion with increasing temperature in star forming regions.

Molecular fluorine chemistry in the early Universe

Some models of Big Bang nucleosynthesis suggest that very high baryon density regions were formed in the early Universe, and generated the production of heavy elements other than lithium such as fluorine F. We present a comprehensive chemistry of fluorine in the post-recombination epoch. Calculation of F, F- and HF abundances, as a function of redshift z, are carried out. The main result is that the chemical conditions in the early Universe can lead to the formation of HF. The final abundance of the diatomic molecule HF is predicted to be close to 3.75 10(-17) when the initial abundance of neutral fluorine F is 10(-15). These results indicate that molecules of fluorine HF were already present during the dark age. This could have implications on the evolution of proto-objects and on the anisotropies of cosmic microwave background radiation. Hydride of fluorine HF may affect enhancement of the emission line intensity from the proto-objects and could produce spectral-spatial fluctuations.

An extensive ab-initio study of the C+NH2 and N+CH2 reactions in relation to the HCN/HNC space abundance ratios

Abstract MP4SDTQ/6-311++G(3df,3pd) and CCSD(T)/6-311++G(3df,3pd) calculations have been performed for an extensive investigation of the C+NH 2 and N+CH 2 processes. This study reveals that reactions on the lowest doublet surfaces are strongly exothermic with no barrier to oppose the formation of HCN and HNC in their lowest singlet electronic state, suggesting that the formation of HCN( 1 Σ + ) and HNC( 1 Σ + ) from, respectively, N+CH 2 and C+NH 2 will be efficient and rather rapid. In regard to the present results, the problem of knowing to what extent N+CH 2 and C+NH 2 are minor processes not affecting the HCN and HNC abundances in the interstellar clouds is posed.

AN EXTENSIVE AB INITIO STUDY OF A PROCESS OF ASTROPHYSICAL INTEREST : THE N+(N) + CH3(CH3+) REACTION

Abstract The impact of the N + (N)+CH 3 (CH 3 + )→NH 2 C + +H process on the relative HNC and HCN space abundances is investigated in order to test the astrophysical hypothesis stating that, in cold interstellar clouds, this reaction may contribute to the observed HNC/HCN abundance ratio greater than unity. Our extensive ab initio study of the lowest surfaces of both doublet and quartet multiplicities shows that the HNCH + entity is formed in its linear arrangement, the dissociative recombination of which is known to lead to an equal amount of HCN and HNC. Therefore it can be inferred that this reaction cannot account for an HNC/HCN abundance ratio larger than unity and that the above hypothesis has to be reconsidered.

The ortho-to-para ratio of H2Cl+: Quasi-classical trajectory calculations and new simulations in light of new observations

Multi-hydrogenated species with proper symmetry properties can present different spin configurations, and thus exist under different spin symmetry forms, labeled as para and ortho for two-hydrogen molecules. We investigated here the ortho-to-para ratio (OPR) of H2Cl+ in the light of new observations performed in the z = 0.89 absorber toward the lensed quasar PKSu20091830−211 with the Atacama Large Millimeter/submillimeter Array (ALMA). Two independent lines of sight were observed, to the southwest (SW) and northeast (NE) images of the quasar, with OPR values found to be 3.15 ± 0.13 and 3.1 ± 0.5 in each region, respectively, in agreement with a spin statistical weight of 3:1. An OPR of 3:1 for a molecule containing two identical hydrogen nuclei can refer to either a statistical result or a high-temperature limit depending on the reaction mechanism leading to its formation. It is thus crucial to identify rigorously how OPRs are produced in order to constrain the information that these probes can provide. To understand the production of the H2Cl+ OPR, we undertook a careful theoretical study of the reaction mechanisms involved with the aid of quasi-classical trajectory calculations on a new global potential energy surface fit to a large number of high-level ab initio data. Our study shows that the major formation reaction for H2Cl+ produces this ion via a hydrogen abstraction rather than a scrambling mechanism. Such a mechanism leads to a 3:1 OPR, which is not changed by destruction and possible thermalization reactions for H2Cl+ and is thus likely to be the cause of observed 3:1 OPR ratios, contrary to the normal assumption of scrambling.