Electronic spectroscopy of an isolated halocarbocation: the iodomethyl cation CH2I+ and its deuterated isotopomers

Building upon our recent observation of the gas-phase electronic spectrum of the iodomethyl cation (CH2I+), we report an extensive study of the electronic spectroscopy of CH2I+ and its deuterated isotopomers CHDI+ and CD2I+ using a combination of fluorescence excitation and single vibronic level (SVL) emission spectroscopies. The spectra were measured in the gas phase under jet-cooled conditions using a pulsed discharge source. Fluorescence excitation spectra reveal a dominant progression in nu3 (C-I stretch), the frequency of which is markedly smaller in the upper state. Rotational analysis shows that, while the A constant is similar in the two states, the excited state has significantly smaller B and C constants. These results indicate a lengthening of the C-I bond upon electronic excitation, consistent with calculations which show that this transition is analogous to the well-known pi-pi* transition in the isoelectronic substituted formaldehydes. SVL emission spectra show progressions involving four of the six vibrational modes; only the C-H(D) stretching modes remain unobserved. The vibrational parameters determined from a Dunham expansion fit of the ground state vibrational term energies are in excellent agreement with the predictions of density functional theory (DFT) calculations. A normal-mode analysis was completed to derive a harmonic force field for the ground state, where resonance delocalization of the positive charge leads to partial double bond character, H2C+-I <--> H2C=I+, giving rise to a C-I stretching frequency significantly larger than that of the iodomethyl radical.     

The pH dependent interaction between nicotine and simulated pulmonary surfactant monolayers with associated molecular modelling

Pulmonary surfactant is an endogenous material that lines and stabilises the alveolar air–liquid interface. Respiratory mechanics can be compromised by exposure to environmental toxins such as cigarette smoke, which contains nicotine. This study aims to determine the influence of nicotine on the activity of simulated lung surfactant at pH 7 and pH 9. In all cases, the addition of nicotine to the test zone caused deviation in surfactant film performance. Importantly, the maximum surface pressure was reduced for each system. Computational modelling was applied to assess key interactions between each species, with the Gaussian 09 software platform used to calculate electrostatic potential surfaces. Modelling data confirmed either nicotine penetration into the two‐dimensional structure or interfacial/electrostatic interactions across the underside. The results obtained from this study suggest that nicotine can impair the ability of pulmonary surfactant to reduce the surface tension term, which can increase the work of breathing. When extrapolated to gross lung function, alveolar collapse and respiratory disease (e.g. chronic airway obstruction) may result. The delivery of nicotine to the (deep) lung can cause a deterioration in lung function and lead to reduced quality of life. Copyright © 2017 John Wiley & Sons, Ltd.     

Stationary analysis of the shortest queue problem

In this paper, we derive an approximation for throughput of TCP Compound connections under random losses. Throughput expressions for TCP Compound under a deterministic loss model exist in the literature. These are obtained assuming that the window sizes are continuous, i.e., a fluid behavior is assumed. We validate this model theoretically. We show that under the deterministic loss model, the TCP window evolution for TCP Compound is asymptotically periodic and is independent of the initial window size. We then consider the case when packets are lost randomly and independently of each other. We discuss Markov chain models to analyze performance of TCP in this scenario. We use insights from the deterministic loss model to get an appropriate scaling for the window size process and show that these scaled processes, indexed by p, the packet error rate, converge to a limit Markov chain process as p goes to 0. We show the existence and uniqueness of the stationary distribution for this limit process. Using the stationary distribution for the limit process, we obtain approximations for throughput, under random losses, for TCP Compound when packet error rates are small. We compare our results with ns2 simulations which show a good match and a better approximation than the fluid model at low p.     

Crystal-Packing-Driven Enrichment of Atropoisomers

Crystal-packing forces can have a significant impact on the relative stabilities of different molecules and their conformations. The magnitude of such effects is, however, not yet well understood. Herein we show, that crystal packing can completely overrule the relative stabilities of different stereoisomers in solution. Heating of atropoisomers (i.e. “frozen-out” conformational isomers) in solution leads to complex mixtures. In contrast, solid-state heating selectively amplifies minor (<25 mole %) components of these solution-phase mixtures. We show that this heating strategy is successful for compounds with up to four rotationally hindered σ bonds, for which a single stereoisomer out of seven can be amplified selectively. Our results demonstrate that common supramolecular interactions—for example, [methyl⋅⋅⋅π] coordination and [C−H⋅⋅⋅O] hydrogen bonding—can readily invert the relative thermodynamic stabilities of different molecular conformations. These findings open up potential new avenues to control the folding of macromolecules.     

Unleashing the Potential of 17O NMR Spectroscopy Using Mechanochemistry

¹⁷O NMR spectroscopy has been the subject of vivid interest in recent years, because there is increasing evidence that it can provide unique insight into the structure and reactivity of many molecules and materials. However, due to the very poor natural abundance of oxygen-17, ¹⁷O labeling is generally a prerequisite. This is a real obstacle for most research groups, because of the high costs and/or strong experimental constraints of the most frequently used ¹⁷O-labeling schemes. Here, we show for the first time that mechanosynthesis offers unique opportunities for enriching in ¹⁷O a variety of organic and inorganic precursors of synthetic interest. The protocols are fast, user-friendly, and low-cost, which makes them highly attractive for a broad research community, and their suitability for ¹⁷O solid-state NMR applications is demonstrated.     

Synthesis and reactivity of {Ru(p-cymene)}2+ derivatives of [Nb6O19]8-: a rational approach towards fluxional organometallic derivatives of polyoxometalates

Three {Ru(p-cym)}(2+) (p-cym = p-cymene) derivatives of [Nb(6)O(19)](8-)-[Nb(6)O(19){Ru(p-cym)}](6-) (Nb(6)Ru(1)), trans-[Nb(6)O(19){Ru(p-cym)}(2)](4-) (t-Nb6Ru2), and [Nb(6)O(19){Ru(p-cym)}(4)] (Nb(6)Ru(4))--have been synthesized in water by reaction between [Ru(p-cym)Cl(2)](2) and the hexaniobate. In the solid state, Nb(6)Ru(1) and Nb(6)Ru(4) have been characterized by IR and EDX spectroscopies, whereas t-Nb(6)Ru(2) has been characterized by IR spectroscopy and single-crystal X-ray diffraction (crystal data for K(4)-trans-[Nb(6)O(19){Ru(p-cym)}(2)].14H(2)O (K(4)-t-Nb(6)Ru(2).14H(2)O). In solution, all compounds were characterized by (1)H NMR and ESI mass spectrometry analyses, and Nb(6)Ru(1) was also analyzed by (17)O NMR. These studies allowed a comparison of the differences in behaviour of the three complexes in water: Nb(6)Ru(1) is particularly stable, Nb(6)Ru(4) decomposes by loss of {Ru(p-cym)}(2+) fragments, and trans-[Nb(6)O(19){Ru(p-cym)}(2)](4-) isomerizes into cis-[Nb(6)O(19){Ru(p-cym)}(2)](4-). A rational mechanism for the isomerisation of t-Nb(6)Ru(2) is proposed on the basis of a kinetic study.     

Assessing the detectability of antioxidants in two‐dimensional high‐performance liquid chromatography

This paper explores the analytical figures of merit of two‐dimensional high‐performance liquid chromatography for the separation of antioxidant standards. The cumulative two‐dimensional high‐performance liquid chromatography peak area was calculated for 11 antioxidants by two different methods—the areas reported by the control software and by fitting the data with a Gaussian model; these methods were evaluated for precision and sensitivity. Both methods demonstrated excellent precision in regards to retention time in the second dimension (%RSD below 1.16%) and cumulative second dimension peak area (%RSD below 3.73% from the instrument software and 5.87% for the Gaussian method). Combining areas reported by the high‐performance liquid chromatographic control software displayed superior limits of detection, in the order of 1 × 10^?6 M, almost an order of magnitude lower than the Gaussian method for some analytes. The introduction of the countergradient eliminated the strong solvent mismatch between dimensions, leading to a much improved peak shape and better detection limits for quantification.     

A Comparison of Solar Ultraviolet Radiation Exposure in Urban Canyons in Venice,Italy and Johannesburg,South Africa

Urban environments can have high-risk spaces that can provide excess personal sun exposure, such as urban or street canyons, and the spaces between buildings, among others. In these urban spaces, sun exposure can be high or low depending on several factors. Polysulphone film (PSF) was used to assess possible daily solar ultraviolet radiation (UVR) exposure in urban canyons in Venice, Italy and, for the first time in Africa, in Johannesburg, South Africa. The photodegradation of PSF upon solar exposure was monitored at a wavelength of 330 nm by ultraviolet-visible spectrophotometry, and the resultant change was converted to standard erythemal dose (SED) units (1 SED = 100 J m⁻²). Mean daily ambient solar UVR exposure measured for Venice and Johannesburg ranged between 20–28 SED and 33–43 SED, respectively. Canyon-located PSF exposures were lower in Venice (1–9 SED) than those in Johannesburg (9–39 SED), depending mainly on the sky view factor and orientation to the sun. There was large variation in solar UVR exposure levels in different urban canyons. These preliminary results should be bolstered with additional studies for a better understanding of excess personal exposure risk in urban areas, especially in Africa.     

ESIPT-mediated photocycloadditions of 3-hydroxyquinolinones: development of a fluorescence quenching assay for reaction screening

Irradiation of 1,2-dimethyl-3-hydroxyquinolinone (DMQ) leads to excited state intramolecular proton transfer (ESIPT) generating a 3-oxidoquinolinium species which undergoes [3 + 2] photocycloaddition with dipolarophiles. A parallel, fluorescence quenching assay using a microplate format has been developed to evaluate fluorescence quenching of this species with a range of dipolarophiles.