Molecular Simulations of Halomethanes at the Air/Ice Interface.

Halogenated organics are emitted into the atmosphere from a variety of sources of both natural and anthropogenic origin. Their uptake at the surface of aerosols can affect their reactivity, for example, in processes that take part in ozone destruction due to production of reactive chlorine, bromine, and iodine radicals. Classical molecular dynamics (MD) simulations are carried out to investigate the interaction of small halomethane molecules of atmospheric relevance with a crystalline ice surface. The following halomethanes were studied: CH3Cl, CH2Cl2, CHCl3, CH3Br, CH2Br2, and CHBr3. MD simulations provide an invaluable insight into the adsorption behavior of halomethanes species. The adsorption energy is increasing as the number of halogen atoms is increasing. Moreover, brominated methanes exhibit a stronger interaction with the ice than their chlorinated analogs. Implications for the atmospheric chemistry are discussed.

[Influence of a major decrease in the use of episiotomy applied to a high risk perineal situation: Occiput posterior presentation]. / Influence d'une forte diminution du recours à l'épisiotomie appliquée à une situation à haut risque périnéal : le dégagement en occipito-sacré.

OBJECTIVES: Occiput-posterior presentation represents 5% of all deliveries and is a high-risk situation for perineal tears requiring, for some authors, an episiotomy. The aim of this study was to evaluate the impact of an ultra-restrictive policy in the use of episiotomy on perineal tears in this high-risk situation. MATERIALS AND METHODS: This is a retrospective cohort study in a tertiary maternity comparing perineal outcome of occipito-posterior (OP) to occipito-anterior deliveries. We included all, spontaneous or assisted, live single pregnancies that delivered after 37 weeks in OP from January 2008 to December 2012. This group was then compared to a control group of occipito-anterior pregnancies. RESULTS: Episiotomy rate was similar in both groups (1.3 versus 1.8 in OP; p=0.5). We found more second degree perineal tears in the OP group (p<0.001) but severe perineal tears rate was similar. There was no increase in post-partum haemorrhage rate. Neonatal outcome was less favourable in the OP group. CONCLUSION: A restrictive use of episiotomy in OP does not increase the risk of severe perineal tears and does not worsen perineal prognosis.

Multiconfiguration time-dependent Hartree and classical dynamics studies of the photodissociation of HF and HCl molecules adsorbed on ice: extension to three dimensions.

The 3D photodissociation dynamics of HCl and HF molecules adsorbed on ice is studied by quantum and classical simulations. The quantum calculations are carried out with the multiconfiguration time-dependent Hartree (MCTDH) approach. Dynamical observables like angular distributions in the momentum space of the H fragments, absorption cross sections are computed. The results are compared with our previous 2D studies. As expected, less encapsulation of the H atom between the ice surface and the halogen atom is obtained in the 3D study, resulting in less pronounced interference structures in the photoabsorption cross sections and in a decrease of the classical rainbow peaks observed in the 2D scheme. Although the amplitudes of the oscillations corresponding to quantum interferences in the asymptotic angular distribution of the H fragment are different between the 2D and 3D results, the qualitative pattern of the oscillations is similar in the 2D and 3D approaches. In addition, a good agreement is observed for the angular distribution between the classical and the quantum calculations.

Interaction of methanol with amorphous solid water.

The interaction of methanol (MeOH) with amorphous solid water (ASW) composed of D2O molecules, prepared at 125 K on a polycrystalline Ag substrate, was studied with metastable-impact-electron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed desorption mass spectroscopy. In connection with the experiments, classical molecular dynamics (MD) simulations have been performed on a single CH3OH molecule adsorbed at the ice surface (T=190 K), providing further insights into the binding and adsorption properties of the molecule at the ice surface. Consistently with the experimental deductions and previous studies, MeOH is found to adsorb with the hydroxyl group pointing toward dangling bonds of the ice surface, the CH3 group being oriented upwards, slightly tilted with respect to the surface normal. It forms the toplayer up to the onset of the simultaneous desorption of D2O and MeOH. At low coverage the adsorption is dominated by the formation of two strong hydrogen bonds as evidenced by the MD results. During the buildup of the first methanol layer on top of an ASW film the MeOH-MeOH interaction via hydrogen-bond formation becomes of importance as well. The interaction of D2O with solid methanol films and the codeposition of MeOH and D2O were also investigated experimentally; these experiments showed that D2O molecules supplied to a solid methanol film become embedded into the film.

Classical and quantum studies of the photodissociation of a HX (X=Cl,F) molecule adsorbed on ice.

The photodissociation dynamics of a HX (X = Cl,F) molecule adsorbed on a hexagonal ice surface at T = 0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures.