Observation of a core-excited dipole-bound state ∼1 eV above the electron detachment threshold in cryogenically cooled acetylacetonate.

Dipole-bound states in anions exist when a polar neutral core binds an electron in a diffuse orbital through charge-dipole interaction. Electronically excited polar neutral cores can also bind an electron in a diffuse orbital to form Core-Excited Dipole-Bound States (CE-DBSs), which are difficult to observe because they usually lie above the electron detachment threshold, leading to very short lifetimes and, thus, unstructured transitions. We report here the photodetachment spectroscopy of cryogenically cooled acetylacetonate anion (C5H7O2-) recorded by detecting the neutral radical produced upon photodetachment and the infrared spectroscopy in He-nanodroplets. Two DBSs were identified in this anion. One of them lies close to the electron detachment threshold (∼2.74 eV) and is associated with the ground state of the radical (D0-DBS). Surprisingly, the other DBS appears as resonant transitions at 3.69 eV and is assigned to the CE-DBS associated with the first excited state of the radical (D1-DBS). It is proposed that the resonant transitions of the D1-DBS are observed ∼1 eV above the detachment threshold because its lifetime is determined by the internal conversion to the D0-DBS, after which the fast electron detachment takes place.

The temperature variation of the CH+ + H reaction rate coefficients: a puzzle finally understood?

CH+ was the first molecular ion identified in the interstellar medium and is found to be ubiquitous in interstellar clouds. However, its formation and destruction paths are not well understood, especially at low temperatures. A new theoretical approach based on the canonical variational transition state theory was used to study the H + CH+ reactive collisions. Rate coefficients for formation of C+ ions are calculated as a function of temperature. We considered the participation of a direct path and an indirect path in which the reactants should overcome an entropic barrier to form a van der Waals complex or pass through a CH2+ intermediate complex, respectively. We show that the contribution of both pathways to the formation of C+ has to be taken into account. The new reactive rate coefficients for the title reaction, complemented by reactive data for CH+/CH2+ in the H/H2/He mixture, have been used to simulate the corresponding kinetics experimentally measured using an Atomic Beam 22 Pole Trap apparatus at low temperature. A good agreement with the experimental findings was found at 50 K. At a lower temperature, the model overestimates the formation of C+. This shows that secondary reactions are not responsible for the weak C+ production in the experiments at such temperature. Then, we discuss the possible impact of non-adiabatic effects in the study of the H + CH+ reactive collisions and we found that such effects can be responsible for the decrease of the H + CH+ rate coefficients at low temperature. This study offers an explanation for the disagreement between H + CH+ theoretical and experimental rate coefficients which has been going on for 20 years and highlights the need for performing non-adiabatic studies for this simple chemical reaction.

Understanding the active role of water in laboratory chamber studies of reactions of the OH radical with alcohols of atmospheric relevance.

In this work, we studied the reactions of three cyclic aliphatic alcohols with OH at room temperature, atmospheric pressure and different humidities in a Teflon reaction chamber. It was determined that the lower the solubility of the alcohol in water, the larger the effect of the humidity on the acceleration of the reaction. This experimental evidence allows suggesting that the acceleration is due to the reaction of the co-adsorbed reactants at the air-water interface of a thin water film deposited on the Teflon walls of the reaction chamber, instead of between co-reactants dissolved in the water film or due to gas phase catalysis as previously suggested. Therefore, formation of thin water films on different surfaces could have some implications on the tropospheric chemistry of these alcohols in the tropical regions of the planet with high humidity.

Photodetachment of Deprotonated R-Mandelic Acid: The Role of Proton Delocalization on the Radical Stability.

The photodetachment and stability of R-Mandelate, the deprotonated form of the R-Mandelic acid, was investigated by observing the neutral species issued from either simple photodetachment or dissociative photodetachment in a cold anions set-up. R-Mandalate has the possibility to form an intramolecular ionic hydrogen-bond between adjacent hydroxyl and carboxylate groups. The potential energy surface along the proton transfer (PT) coordinate between both groups (O- …H+ …- OCO) features a single local minima, with the proton localized on the O- group (OH…- OCO). However, the structure with the proton localized on the - OCO group (O- …HOCO) is also observed because it falls within the extremity of the vibrational wavefunction of the OH…- OCO isomer along the PT coordinate. The stability of the corresponding radicals, produced upon photodetachment, is strongly dependent on the position of the proton in the anion: the radicals produced from the OH…- OCO isomer decarboxylate without barrier, while the radicals produced from the O- …HOCO isomer are stable.

Inter- and Intramolecular Proton Transfer in an Isolated (Cytosine-Guanine)H+ Pair: Direct Evidence from IRMPD Spectroscopy.

The collision-induced dissociation of the protonated cytosine-guanine pair was studied using tandem mass spectrometry (MS3) coupled to infrared multiple photon dissociation spectroscopy with the free electron laser at Orsay (CLIO) to determine the structure of the CH+ and GH+ ionic fragments. The results were rationalized with the help of electronic structure calculations at the density functional theory level with the B3LYP/6-311++G(3df,2p) method. Several tautomers of each fragment were identified for the first time, some of which were previously predicted by other authors. In addition, two unexpected and minor tautomers were also found: cytosine keto-imino [CKI(1,2,3,4)H+] and guanine keto-amino [GKA(1,3,7)H+]. These results highlight the importance of the DNA base tautomerization assisted by inter- and intramolecular proton or hydrogen transfer within the protonated pairs.

Rate Coefficient and Mechanism of the OH-Initiated Degradation of 1-Chlorobutane: Atmospheric Implications.

In this work, we investigate the degradation process of 1-chlorobutane, initiated by OH radicals, under atmospheric conditions (air pressure of 750 Torr and 296 K) from both experimental and theoretical approaches. In the first one, a relative kinetic method was used to obtain the rate coefficient for this reaction, while the products were identified for the first time (1-chloro-2-butanone, 1-chloro-2-butanol, 4-chloro-2-butanone, 3-hydroxy-butanaldehyde, and 3-chloro-2-butanol) using mass spectrometry, allowing suggesting a reaction mechanism. The theoretical calculations, for the reactive process, were computed using the BHandHLYP/6-311++G(d,p) level of theory, and the energies for all of the stationary points were refined at the CCSD(T) level. Five conformers for 1-chlorobutane and 33 reactive channels with OH radicals were found, which were considered to calculate the thermal rate coefficient (as the sum of the site-specific rate coefficients using canonical transition state theory). The theoretical rate coefficient (1.8 × 10-12 cm3 molecule-1 s-1) is in good agreement with the experimental value (2.22 ± 0.50) × 10-12 cm3 molecule-1 s-1 determined in this work. Finally, environmental impact indexes were calculated and a discussion on the atmospheric implications due to the emissions of this compound into the troposphere was given.

Dissociative photodetachment vs. photodissociation of aromatic carboxylates: the benzoate and naphthoate anions.

The competition between dissociative photodetachment and photodissociation of cold benzoate and naphthoate anions was studied through measurement of the kinetic energy of the neutral fragments and intact parent benzoyloxy and naphtoyloxy radicals as well as by detecting the anionic fragments whenever they are produced. For the benzoate anion, there is no ionic photodissociation and the radical dissociation occurs near the vertical photodetachment energy. This is in agreement with DFT calculations showing that the dissociation energy in CO2 and C6H5˙ is very low. The dissociation barrier can be deduced from experimental results and calculations to be (0.7 ± 0.1) eV, which makes the benzoyloxyradical C6H5COO˙ very unstable, although more stable than the acetyloxy radical. In the case of naphthoate, the observation of negative fragments at low excitation energies demonstrates the opening of the ionic photodissociation channel in the excited state of the naphthoate anion, whose yield decreases at higher energies when the dissociative photodetachment channel opens.

Water catalysis of the reaction between hydroxyl radicals and linear saturated alcohols (ethanol and n-propanol) at 294 K.

The rate coefficients for the reactions of OH with ethanol and n-propanol were determined by a relative method in a smog chamber at 294 K, 1 atm of air or N2 and a wide range of humidity. The rate coefficients for both reactions show a quadratic dependence on the water concentration as in the case of the reaction of OH with methanol (Jara-Toro et al. Angew. Chem., Int. Ed., 2017, 56, 2166). The detailed mechanism responsible for the reaction acceleration was studied theoretically at the uMP2/aug-cc-pVDZ level of theory while the electronic energies of all the structures were refined at the uCCSD(T)/aug-cc-pVDZ level. From these results it is suggested that the catalytic effect of two water molecules is due to two cooperative effects in the reactions between the ROH(H2O) and OH(H2O) equilibrium complexes: (1) an enhanced capture cross-section as a consequence of the larger dipolar moment of the ROH(H2O) and OH(H2O) complexes as compared to those of the free reactants ROH and OH and (2) a strong stabilization of the TSs below the energy of the reactants that leads to a very fast decomposition of the pre-reactive complexes to products with an extremely low probability of dissociation back to the reactants. The tropospheric lifetime of these alcohols is also shown to strongly depend on the humidity, suggesting the need to incorporate this dependence in global atmospheric models.

Water Catalysis of the Reaction between Methanol and OH at 294†K and the Atmospheric Implications.

The rate coefficient for the reaction CH3 OH+OH was determined by means of a relative method in a simulation chamber under quasi-real atmospheric conditions (294 K, 1 atm of air) and variable humidity or water concentration. Under these conditions, a quadratic dependence of the rate coefficient for the reaction CH3 OH+OH on the water concentration was found. Thus the catalytic effect of water is not only important at low temperatures, but also at room temperature. The detailed mechanism responsible of the reaction acceleration is still unknown. However, this dependence should be included in the atmospheric global models since it is expected to be important in humid regions as in the tropics. Additionally, it could explain several differences regarding the global and local atmospheric concentration of methanol in tropical areas, for which many speculations about the sinks and sources of methanol have been reported.