Mass spectrometric study of molecular and ionic sublimation of lanthanum triiodide

The molecular and ionic composition of saturated vapor over lanthanum triiodide was studied by Knudsen effusion mass spectrometry. The (LaI3)n molecules (n = 1–3) and the [I(LaI3)n]− ions (n = 0–4) were observed. The partial pressures of the molecules were determined and the enthalpies of sublimation, ΔsH° (298.15 K) in kJ mol−1, in the form of monomers (304 ± 7), dimers (428 ± 25), and trimers (455 ± 50) were obtained by the second and third laws of thermodynamics. The enthalpy of formation, ΔfH° (298.15 K) in kJ mol−1, of the LaI3 (−376 ± 10), La2I6 (−932 ± 25), La3I9 (−1585 ± 50) molecules and the LaI4− (−841 ± 24), La2I7− (−1486 ± 32) ions were determined. The electron work function, φe = 3.5 ± 0.3 eV, for the LaI3 crystal was calculated from the thermochemical cycle.

The Contribution of the Activation Entropy to the Gas-Phase Stability of Modified Nucleic Acid Duplexes

Tricyclo-DNA (tcDNA) is a sugar-modified analogue of DNA currently tested for the treatment of Duchenne muscular dystrophy in an antisense approach. Tandem mass spectrometry plays a key role in modern medical diagnostics and has become a widespread technique for the structure elucidation and quantification of antisense oligonucleotides. Herein, mechanistic aspects of the fragmentation of tcDNA are discussed, which lay the basis for reliable sequencing and quantification of the antisense oligonucleotide. Excellent selectivity of tcDNA for complementary RNA is demonstrated in direct competition experiments. Moreover, the kinetic stability and fragmentation pattern of matched and mismatched tcDNA heteroduplexes were investigated and compared with non-modified DNA and RNA duplexes. Although the separation of the constituting strands is the entropy-favored fragmentation pathway of all nucleic acid duplexes, it was found to be only a minor pathway of tcDNA duplexes. The modified hybrid duplexes preferentially undergo neutral base loss and backbone cleavage. This difference is due to the low activation entropy for the strand dissociation of modified duplexes that arises from the conformational constraint of the tc-sugar-moiety. The low activation entropy results in a relatively high free activation enthalpy for the dissociation comparable to the free activation enthalpy of the alternative reaction pathway, the release of a nucleobase. The gas-phase behavior of tcDNA duplexes illustrates the impact of the activation entropy on the fragmentation kinetics and suggests that tandem mass spectrometric experiments are not suited to determine the relative stability of different types of nucleic acid duplexes.

The new insight into dynamic crossover in glass forming liquids from the apparent enthalpy analysis

One of the most intriguing phenomena in glass forming systems is the dynamic crossover (T(B)), occurring well above the glass temperature (T(g)). So far, it was estimated mainly from the linearized derivative analysis of the primary relaxation time τ(T) or viscosity η(T) experimental data, originally proposed by Stickel et al. [J. Chem. Phys. 104, 2043 (1996); J. Chem. Phys. 107, 1086 (1997)]. However, this formal procedure is based on the general validity of the Vogel-Fulcher-Tammann equation, which has been strongly questioned recently [T. Hecksher et al. Nature Phys. 4, 737 (2008); P. Lunkenheimer et al. Phys. Rev. E 81, 051504 (2010); J. C. Martinez-Garcia et al. J. Chem. Phys. 134, 024512 (2011)]. We present a qualitatively new way to identify the dynamic crossover based on the apparent enthalpy space (H(a)(') = dlnτ/d(1/T)) analysis via a new plot lnH(a)(') vs. 1∕T supported by the Savitzky-Golay filtering procedure for getting an insight into the noise-distorted high order derivatives. It is shown that depending on the ratio between the "virtual" fragility in the high temperature dynamic domain (m(high)) and the "real" fragility at T(g) (the low temperature dynamic domain, m = m(low)) glass formers can be splitted into two groups related to f < 1 and f > 1, (f = m(high)∕m(low)). The link of this phenomenon to the ratio between the apparent enthalpy and activation energy as well as the behavior of the configurational entropy is indicated.

LakeMIP Kivu: evaluating the representation of a large, deep tropical lake by a set of one-dimensional lake models

The African great lakes are of utmost importance for the local economy (fishing), as well as being essential to the survival of the local people. During the past decades, these lakes experienced fast changes in ecosystem structure and functioning, and their future evolution is a major concern. In this study, for the first time a set of one-dimensional lake models are evaluated for Lake Kivu (2.28°S; 28.98°E), East Africa. The unique limnology of this meromictic lake, with the importance of salinity and subsurface springs in a tropical high-altitude climate, presents a worthy challenge to the seven models involved in the Lake Model Intercomparison Project (LakeMIP). Meteorological observations from two automatic weather stations are used to drive the models, whereas a unique dataset, containing over 150 temperature profiles recorded since 2002, is used to assess the model’s performance. Simulations are performed over the freshwater layer only (60 m) and over the average lake depth (240 m), since salinity increases with depth below 60 m in Lake Kivu and some lake models do not account for the influence of salinity upon lake stratification. All models are able to reproduce the mixing seasonality in Lake Kivu, as well as the magnitude and seasonal cycle of the lake enthalpy change. Differences between the models can be ascribed to variations in the treatment of the radiative forcing and the computation of the turbulent heat fluxes. Fluctuations in wind velocity and solar radiation explain inter-annual variability of observed water column temperatures. The good agreement between the deep simulations and the observed meromictic stratification also shows that a subset of models is able to account for the salinity- and geothermal-induced effects upon deep-water stratification. Finally, based on the strengths and weaknesses discerned in this study, an informed choice of a one-dimensional lake model for a given research purpose becomes possible.

Adsorption interaction of carrier-free thallium species with gold and quartz surfaces

The adsorption interactions of thallium and its compounds with gold and quartz surfaces were investigated. Carrier-free amounts of thallium were produced in nuclear fusion reactions of alpha particles with thick gold targets. The method chosen for the studies was gas thermochromatography and varying the redox potential of the carrier gases. It was observed that thallium is extremely sensitive to trace amounts of oxygen and water, and can even be oxidized by the hydroxyl groups located on the quartz surface. The experiments on a quartz surface with O2, He, H2 gas in addition with water revealed the formation and deposition of only one thallium species – TlOH. The adsorption enthalpy was determined to be Δ HSiO2ads(TlOH) = −134 ± 5 kJ mol−1. A series of experiments using gold as stationary surface and different carrier gases resulted in the detection of two thallium species – metallic Tl (H2 as carrier gas) and TlOH (O2, O2+H2O and H2+H2O as pure carrier gas or carrier gas mixture) with Δ HAuads(Tl) = −270 ± 10 kJ mol− and Δ HAuads(TlOH) = −146 ± 3 kJ mol−1. These data demonstrate a weak interaction of TlOH with both quartz and gold surfaces. The data represent important information for the design of future experiments with the heavier homologue of Tl in group 13 of the periodic table – element 113 (E113).