RESUMO
ConspectusDark molecular clouds have low temperatures of approximately 10 K and experience very little UV irradiation. These clouds are the birthplace of new stars and consist of gas and dust particles. The latter can act as a meeting place to facilitate surface chemistry to form saturated molecules such as formaldehyde, methyl formate, and dimethyl ether. These complex organic molecules or COMs become encapsulated in the ice that forms on the dust grains, and these ices are the precursor for cometary ices and other icy bodies. They likely played a role in bringing material to the early earth.Although these COMs are likely formed on the surfaces of dust grains, several of them have been detected in the gas phase. This means that they have desorbed from the grain under these cold, dark conditions where thermal desorption and photodesorption are negligible. It has been speculated that reactive, or chemical, desorption is responsible for the high gas-phase abundance. After a surface reaction, its products might be vibrationally, translationally, and/or rotationally excited. Dissipation of the excess energy to translational energy can briefly increase the desorption rate, leading to chemical desorption. Astrochemical modellers have added terms to their rate equations to account for this effect. These terms, however, have had little experimental or theoretical verification.In this Account, we use classical molecular dynamics (MD) simulations to give adsorbed molecules a fixed amount of energy as a proxy for excess energy and to record whether this leads to desorption. The excitation energy can be varied freely while keeping all other variables constant. This allows for the study of trends rather than being limited to a single reaction and a single system. The focus is on the dependence of the chemical desorption on the excitation energy, excitation type, and binding energy. Rotational and vibrational excitation was explicitly taken into account. An analytical expression for the chemical desorption probability was obtained in this way. It depends on the binding energy and reaction enthalpy. This expression was then implemented in a gas-grain astrochemical code to simulate the chemical evolution of a dark molecular cloud, and the results were compared against observational abundances of COMs in three different molecular clouds. The results with our new expression based on the MD simulations show good agreement for all species except H2CO, which has both gas-phase and surface-formation routes. This is a significant improvement over models without chemical desorption or with other expressions for chemical desorption, as frequently used by other authors. It is encouraging to see that a general description with a firmer theoretical basis leads to a significant improvement. Understanding chemical desorption can help to explain the unexpectedly high gas-phase abundance of some COMs, and chemical desorption also provides a link between the gas phase and the ice mantle, and its understanding might help in creating a diagnostic tool to learn more about the ice composition.
RESUMO
A charge-transfer (CT) interaction between 1,3,5-trinitro-2,4-dimethylbenzene (TNX) and anionic phosphate is evaluated, yielding a high band electronic transfer interaction that can be observed as a distinct color change when phosphate is present in solution. The induced interaction was studied using 1H NMR, UV-visible, and Fourier transform infrared spectroscopies. The stoichiometric determination of the interaction was divined by means of continuous variation, applying the Schaeppi-Treadwell method to calculate the binding constant (k). Furthermore, the effect of the polarity of solvents toward the generation of the CT interaction was examined, with multiple solvents considered. Complex deconstruction studies were undertaken, examining the effects of water on complex destruction and understanding the volumes needed to hinder the CT interaction potency. Specificity and selectivity of the CT interaction were also studied against other biologically relevant species (CH3CH2OH, Na+, K+, Ca2+, Cl-, HCO3 -, F-, CH3COO-, and SO4 2-), assessing the capabilities of the assay to differentiate anionic species and counter cations that could act as interferences. The role of TNX concentration in CT formation was also analyzed, aiming to optimize the phosphate-sensing assay and improve its limit of detection. The sensing platform was subsequently used to study phosphate concentrations in urine samples to further understand its potential application in biomedical research. To validate the developed technique, urine samples were analyzed for their phosphate content with both the developed sensor and a validated vanadate-molybdate reagent. The results indicate that the sensing method is capable of accurately reporting elevated phosphate levels in urine samples in a rapid and sensitive manner, illustrating that the colorimetric test could be used as a prescreening test for conditions such as hyperphosphatemia or chronic kidney disease.
