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Fotoreaktywność związków heterocyklicznych w warukach zmieniającego się otoczenia chemicznego w świetle chemii obliczeniowej

Janicki Mikołaj J., Szabla Rafał, Góra Robert W.

Wiadomości Chemiczne

2023

[Z] 77, 11-12

995--1024

The natural environment and living organisms that surround us are made up of chemical compounds, called chromophores, which can absorb photons coming from UV light from the Sun. The one-photon absorption process leads to ultrafast alteration in the electron density of chromophores, resulting in the population of short-lived excited states. These UV-induced electronic states can be responsible for performing high-energy chemical reactions, which cannot be observed in the chemistry of the ground state. Consequently, photochemical processes could damage the initial structure of a chromophore or allow molecules to undergo the chemical transformation to photoproducts. Therefore, understanding the photochemical and photophysical properties of essential chemical molecules for biology, medicine, renewable energy, and other fieldsis crucial to improve and find applications of light-sensitive systems in daily life. To scrutinize UV-induced chemistry, time-resolved spectroscopy is widely used as an experimental tool to investigate photochemical events. However, the experimental approach cannot provide detailed information about the molecular mechanisms of photochemical processesthat occur in the excited states. Therefore, experimental methods in conjunction with computational photochemistry are used to elucidate the behaviour of UV-excited chemical molecules. Only the synergistic approach can comprehensively describe the photochemical picture of UV-induced molecules. This concise review contains a short introduction to the applications of computational chemistry in the studies of the photochemical properties of chromophores, and major radiationless deactivation pathways occurring in heteroaromatic compounds are briefly discussed. Furthermore, two very recent achievements of joint experimental and theoretical photochemistry studies are outlined, demonstrating how solvent or solute molecules can actively participate in photorelaxation channels of chromophores, allowing for an excited-state intermolecular electron transfer mechanism. The selected and discussed research results show that computational chemistry plays an invaluable role in answering questions about molecular mechanisms in the excited states and enables prediction of unexpected chemical processes.

Chemia radiacyjna w eksploracji Marsa

Zagórski Z. P.

Wiadomości Chemiczne

2005

[Z] 59, 5-6

469--488

The discussion of chemical reactions caused by ionizing radiation is started, as usual, with the nature of the object which absorbs the energy. First, the composition (CO2 = 95.3%) and density of martian atmosphere is discussed; the latter is low, 120 times smaller than over the Earth atid does not protect the surface of Mars from ionizing radiations. The atmosphere over the Earth secures the shield equivalent to 3 m of concrete, with many positive consequences for the Life on Earth. In addition, high energy protons from the Sun are diverted magnetically around the Earth, and that is not the case around the Mars. The radiolysis of Martian atmosphere starts with formation of CO2 as the primary product of single ionization spurs. The multi-ionization spurs can yield exotic carbon-oxygen compounds, not explored yet. Anyway, the radiolysis of martian atmosphere is completely different from the case of Earth, where the primary product is N* ion-radical. The lack of water vapor over Mars prevents the formation of an analogue to acid rains, and of creation of many other compounds. The most interesting feature of Martian regolith is the possibility of the presence of hydrated minerals, which could have been formed milliards years ago, when (probably) water was present on Mars. Water present in the crystalline lattice undergoes only limited radiolysis, as it is known from the case of concrete, produced as biological shield build on Earth, around the sources of ionizing radiation. Formation of natural hydrated silicates on Mars was possible, therefore survival of traces of H2O on Mars is possible. However, this kind of water cannot be recovered easily, to be used by Mars explorers. The interface of the atmosphere and the regolith is probably the site of many chemical reactions. Very intensive UV, which includes part of the vacuum UV, can cause reduction of carbon dioxide to methane, recently discovered in traces over Mars, hopefully, but erroneously connected, in the mode of wishful thinking, to the Life. Minerals like sodalite, discovered on Mars can contribute as reagents in the mentioned reaction, and could be the source of hydrogen. Conclusions are dedicated to questions of the live organisms connected with exploration of Mars; from microorganisms, comparatively resistant to ionizing radiation, to human beings, considered not to be fit to manned flight, survival on Mars and return to Earth. The genius of Mankind which is able to create effective means of exploration over the distance of millions of kilometers, should be a satisfaction more important than the extremely expensive presence of man or woman on Mars.