Bimolecular reactions of S2+ with Ar, H2 and N2: reactivity and dynamics

The reactivity, energetics and dynamics of bimolecular reactions between S2+ and three neutral species (Ar, H2 and N2) have been studied using a position-sensitive coincidence methodology at centre-of-mass collision energies below 6 eV. This is the first study of bimolecular reactions involving S2+, a species detected in planetary ionospheres, the interstellar medium, and in anthropogenic manufacturing processes. The reactant dication beam employed consists predominantly of S2+ in the ground 3P state, but some excited states are also present. Most of the observed reactions involve the ground state of S2+, but the dissociative electron transfer reactions appear to exclusively involve excited states of this atomic dication. We observe exclusively single electron-transfer between S2+ and Ar, a process which exhibits strong forward scatting typical of the Landau-Zener style dynamics observed for other dicationic electron transfer reactions. Following collisions between S2+ + H2, non-dissociative and dissociative single electron-transfer reactions were detected. The dynamics here show evidence for the formation of a long-lived collision complex, [SH2]2+, in the dissociative single electron-transfer channel. The formation of SH+ was not observed. In contrast, the collisions of S2+ + N2 result in the formation of SN+ + N+ in addition to the products of single electron-transfer reactions

A new study of an old sink of sulfur in hot molecular cores: the sulfur residue

Sulfur appears to be depleted by an order of magnitude or more from its elemental abundance in star-forming regions. In the last few years, numerous observations and experiments have been performed in order to to understand the reasons behind this depletion without providing a satisfactory explanation of the sulfur chemistry towards high-mass star-forming cores. Several sulfur-bearing molecules have been observed in these regions, and yet none are abundant enough to make up the gas-phase deficit. Where, then, does this hidden sulfur reside? This paper represents a step forward in our understanding of the interactions among the various S-bearing species. We have incorporated recent experimental and theoretical data into a chemical model of a hot molecular core in order to see whether they give any indication of the identity of the sulfur sink in these dense regions. Despite our model producing reasonable agreement with both solid-phase and gas-phase abundances of many sulfur-bearing species, we find that the sulfur residue detected in recent experiments takes up only ~6 per cent of the available sulfur in our simulations, rather than dominating the sulfur budget.Comment: 13 pages, 6 colourful figures, accepted by MNRA

Electron ionisation of cyanoacetylene: Ionisation cross sections and dication formation

Cyanoacetylene (HC3N) is an important trace species in the atmosphere of Titan. We report, for the first time, absolute partial electron ionisation cross sections and absolute precursor-specific partial electron ionisation cross sections for cyanoacetylene, following an experimental and computational investigation. Our methodology involves using 2D ion-ion coincidence mass spectrometry to generate relative cross sections, over the electron energy range 50 – 200 eV. These relative values are then normalised to an absolute scale, using a binary encounter-Bethe (BEB) calculation of the total ionisation cross section. The BEB calculation agrees well with previous determinations in the literature. The mass spectrometric observations of HC2N+ and HCN+, ions with a connectivity markedly different to that of the neutral molecule, point towards a rich cationic energy landscape possessing several local minima. Indeed, [HC3N]2+ minima involving a variety of cyclic configurations are revealed by a preliminary computational investigation, along with two minima with linear and bent geometries involving H atom migration (CCCNH2+). Determination of the energy of a transition state between these local minima indicates that the dication is able to explore the majority of this rich conformational landscape at our experimental energies. This investigation of the energetics also determines an adiabatic double ionisation energy of 30.3 eV for the lowest lying singlet state of HCCCN2+, and 30.1 eV for the lowest-lying triplet state. The bulk of the cation pairs detected in the coincidence experiment appear to originate from markedly excited dication states, not the ground state. We observe 5 two-body dissociations of HCCCN2+, and subsequent decay of one of the ions generated in such two-body processes accounts for the majority of three-body dissociations we observe

Diet Composition: A Proximate Mechanism Explaining Stream Salamander Declines in Surface Waters with Elevated Specific Conductivity

Changes in land use, such as mountaintop removal mining with valley fills (MTR-VF), often results in headwater streams with elevated specific conductivity (SC). Stream salamanders appear to be particularly sensitive to elevated SC, as previous studies have shown occupancy and abundance decline consistently among all species and life stages as SC increases. Yet, the proximate mechanism responsible for the population declines in streams with elevated SC have eluded researchers. We sampled salamander assemblages across a continuous SC gradient (30–1966 μS/cm) in southeastern Kentucky and examined the diet of larval and adult salamanders to determine if the ratio of aquatic to terrestrial prey (autochthony), total prey volume, aquatic prey importance (Ix), and body condition are influenced by SC. Further, we asked if threshold points for each diet component were present along a gradient of SC. Larval salamanders experienced a 12–fold decline in autochthony at 153 μS/cm, a 4.2–fold decline in total prey volume at 100 μS/cm, a 2.2-fold decline in aquatic Ix at 135 μS/cm, and a rapid decline in body condition as SC increased. Adult salamanders experienced a 3–fold decline in autochthony at 382 μS/cm, no change in prey volumes, a 2-fold decline in aquatic Ix at 163 μS/cm, and a decline in body condition as SC increased. Our results indicate that SC indirectly affects stream salamander populations by changing the composition of diet, which suggests that food availability is a proximate mechanism that leads to reduced population occupancy, abundance, and persistence in streams with elevated SC

Occupancy and Abundance of Stream Salamanders along a Specific Conductance Gradient

In the Central Appalachians (USA), mountaintop-removal mining accompanied by valley fills often leads to streams with elevated specific conductivity (SC). Thus, the ionic composition of freshwaters in this region is hypothesized to be a driver of the distribution and abundance of freshwater taxa, including stream salamanders. We examined the association between SC and stream salamander populations by conducting salamander counts in 30 southeastern Kentucky streams across a continuous gradient of SC that ranged from 30 to 1966 μS/cm. We counted 2319 salamanders across 5 species and, using a hierarchical Bayesian version of the N-mixture model, found a negative association between SC and salamander occupancy rates. This finding was consistent across adults and larvae of the 5 species we examined. Furthermore, we found that most salamander species and life stages showed reduced abundances given occupancy at greater SC levels. For example, estimated mean abundance given occupancy of larval Southern Two-lined Salamanders (Eurycea cirrigera) was 67.69 (95% credible interval 48.31–98.25) ind/10 m at 250 μS/cm and 2.30 (95% credible interval 1.46–3.93) ind/10 m at 2000 μS/cm. The consistent negative association across all species and life stages supports the hypothesis that salamander distributions and abundances are negatively associated with elevated SC of streams in southeastern Kentucky, even though physical and chemical environmental attributes, such as forest cover within stream catchments, were correlated with SC. Restoration of streams affected by mountaintop-removal mining should focus on restoring the ionic compositions that naturally occur in this region

Household Clustering of Escherichia coli Sequence Type 131 Clinical and Fecal Isolates According to Whole Genome Sequence Analysis

Background. Within-household sharing of strains from the resistance-associated H30R1 and H30Rx subclones of Escherichia coli sequence type 131 (ST131) has been inferred based on conventional typing data, but has been assessed minimally using whole genome sequence (WGS) analysis. Methods. Thirty-three clinical and fecal isolates of ST131-H30R1 and ST131-H30Rx, from 20 humans and pets in six households, underwent WGS analysis for comparison with 52 published ST131 genomes. Phylogenetic relationships were inferred using a bootstrapped maximum likelihood tree based on core genome sequence polymorphisms. Accessory traits were compared between phylogenetically similar isolates. Results. In the WGS-based phylogeny, isolates clustered strictly by household, in clades that were distributed widely across the phylogeny, interspersed between H30R1 and H30Rx comparison genomes. For only one household did the core genome phylogeny place epidemiologically unlinked isolates together with household isolates, but even there multiple differences in accessory genome content clearly differentiated these two groups. The core genome phylogeny supported within-household strain sharing, fecal-urethral urinary tract infection pathogenesis (with the entire household potentially providing the fecal reservoir), and instances of host-specific microevolution. In one instance the household\u27s index strain persisted for 6 years before causing a new infection in a different household member. Conclusions. Within-household sharing of E. coli ST131 strains was confirmed extensively at the genome level, as was long-term colonization and repeated infections due to an ST131-H30Rx strain. Future efforts toward surveillance and decolonization may need to address not just the affected patient but also other human and animal household members