Processes In Organic Molecules Research Articles

Calculating free energies of organic molecules on insulating substrates.

The challenges and limitations in calculating free energies and entropies of adsorption and interaction of organic molecules on an insulating substrate are discussed. The adhesion of 1,3,5-tri(4'-cyano-[1,1'-biphenyl]-4-yl)benzene (TCB) and 1,4-bis(4-cyanophenyl)-2,5-bis(decyloxy)benzene (CDB) molecules to step edges on the KCl(001) surface and the formation of molecular dimers were studied using classical molecular dynamics. Both molecules contain the same anchoring groups and benzene ring structures, yet differ in their flexibility. Therefore, the entropic contributions to their free energy differ, which affects surface processes. Using potential of mean force and thermodynamic integration techniques, free energy profiles and entropy changes were calculated for step adhesion and dimer formation of these molecules. However, converging these calculations is nontrivial and comes at large computational cost. We illustrate the difficulties as well as the possibilities of applying these methods towards understanding dynamic processes of organic molecules on insulating substrates.

Howard E. Zimmerman (1926-2012)

Howard E. Zimmerman (1926-2012)

Terrestrial atmosphere, water and astrobiology

Primitive life, defined as a chemical system capable to transfer its molecular information via self-replication and also capable to evolve, originated about 4 billion years ago from the processing of organic molecules by liquid water. Terrestrial atmosphere played a key role in the process by allowing the permanent presence of liquid water and by participating in the production of carbon-based molecules. Water molecules exhibit specific properties mainly due to a dense network of hydrogen bonds. The carbon-based molecules were either home made in the atmosphere and/or in submarine hydrothermal systems or delivered by meteorites and micrometeorites. The search for possible places beyond the earth where the trilogy atmosphere/water/life could exist is the main objective of astrobiology. Within the Solar System, exploration missions are dedicated to Mars, Europa, Titan and the icy bodies. The discovery of several hundreds of extrasolar planets opens the quest to the whole Milky Way.

Estimation of the past and present Martian water-ice reservoirs by isotopic constraints on exchange between the atmosphere and the surface

The discovery of high concentrations of water-ice just below the Martian surface polar areas made by Mars Odyssey has strengthened the debate about the search for life on Mars. Generally it is believed that life on Earth emerged in liquid water from the processing of organic molecules. Thus, the possible origin of life on early Mars should have been related to the evolution of the planetary water inventory, consequently it is important to know the amount of water-ice stored below the planetary surface. The search and mapping of the present subsurface water and ice reservoirs will be carried out experimentally by Mars Express with its Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) ground-penetrating radar in the near future. We estimate the present and past water-ice reservoirs, which are and were in exchange with the atmosphere by using the observed D/H ratio in the atmospheric water vapour, measured D/H ratios in Martian SNC meteorites and D/H isotope ratios based on a study by Lunine et al. (2003) regarding asteroid and cometary water delivery to early Mars. Using the results of this study with initial D/H ratios of about 1.2–1.6 times the terrestrial sea water (TSW) ratio and the assumption that these ratios were not fractionated by XUV-driven hydrodynamic escape due to a more active young Sun prior to 3.5 Ga, one finds a present water-ice reservoir, which can exchange with the Martian atmosphere, equivalent to a global ocean layer with a thickness of about 3.3–15 m. By assuming that hydrodynamic escape fractionated the D/H ratio to a value that is stored in the old Martian SNC meteorites with a measured average enrichment of about 2.3 times the TSW ratio we estimate a present water-ice reservoir equivalent to a global layer with a thickness of about 11–27 m. From the obtained range of the estimated present water-ice deposit, we estimate a water-ice reservoir exchangeable with the atmosphere on Mars 3.5 Ga equivalent to a global ocean with a thickness of between 17 and 61 m. All the estimated reservoirs depend on the escape of water from Mars since 3.5 Ga equivalent to a global ocean with a thickness of about 14 m (minimum) to 34 m (maximum). The main uncertainties in the estimate of the minimal and maximal water-ice reservoir is related to the present uncertainties in the efficiency of atmospheric escape rates triggered by plasma instabilities and momentum transfer effects between the solar wind and the ionosphere. However, these uncertainties will be reduced in the near future, since both loss processes will be studied in detail by the Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA-3) on-board Mars Express. The obtained results combined with the discovery of the present water-ice subsurface reservoirs by the MARSIS radar and isotope studies as presented in this work, will also give us an idea of how enriched the atmosphere was in D compared with H after the heavy bombardment corresponding to a better understanding of the efficiency of the hydrodynamic escape process due to the young Sun.

On the performance of approximate spin–orbit Hamiltonians in light conjugated molecules: the fine-structure splitting of HC6H+, NC5H+, and NC4N+

Spin–orbit calculations have been performed on the carbon-rich radical cations HC6H+, NC5H+, and NC4N+ using the full one-and two-electron no-pair spin–orbit Hamiltonian and more approximate approaches. It is found that the full operator can be replaced by an effective one-electron mean-field spin–orbit Hamiltonian almost without loss of accuracy. An analysis of the approximations reveals that multi-center integrals are large, but one- and two-electron terms tend to cancel. The one-center spin–orbit mean-field approach yields fine-structure splittings within 5% of the full results and thus appears to be a favorable means to reliably predict rates of spin-forbidden processes in organic molecules at low cost.

Photoprocesses in complex organic compounds

We present the results of investigations of photoprocesses in organic compounds. We have done a theoretical and experimental study of the relationship of photophysical and photochemical processes in organic molecules to the characteristics features of intramolecular and intermolecular interactions and the exciting electromagnetic field. We have developed quantum chemical procedures for theoretical prediction of the structure of organic compounds with specified physicochemical and optical properties. We have designed new active media for lasers with enhanced photostability and increased lifetime.

Why exobiology on Mars?

Processing of organic molecules by liquid water was probably an essential requirement towards the emergence of terrestrial primitive life. According to Oparin's hypothesis, organic building blocks required for early life were produced from simple organic molecules formed in a primitive reducing atmosphere. Geochemists favour now a less reducing atmosphere dominated by carbon dioxide. In such an atmosphere very few building blocks are formed. Import of extra-terrestrial organic molecules may represent an alternative supply. Experimental support for such an alternative scenario is examined in comets, meteorites and micrometeorites. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars attesting the presence of an atmosphere capable of deccelerating C-rich micrometeorites. Therefore, primitive life may have developed on Mars as well. Liquid water disappeared from the surface of Mars very early, about 3.8 Ga ago. The Viking missions did not find, at the surface of the Martian soil, any organic molecules or clear-cut evidence for microbial activities such as photo-synthesis, respiration or nutrition. The results can be explained referring to an active photochemistry of Martian soil driven by the high influx of solar UV. These experiments do not exclude the existence of organic molecules and fossils of micro-organisms which developed on early Mars until liquid water disappeared. Mars may store below its surface some well preserved clues of a still hypothetical primitive life.

Investigation of electron transfer processes in organic molecules and metal complexes by means of electrolysis-EPR

Kinetics of electrochemical reduction of trialkylphosphates, slow homogeneous electron transfer reaction with the participation of paramagnetic intermediates and multispin Co(II)-, Cu(II)-, Zn(II),-nitroxyde radical systems reduction processes are studied with the help of combining electrochemical method and EPR.

Fast transfer processes in organic molecules†

In organic compounds constituted of several smaller molecules it is possible that the subunits keep their 'identity’. This behaviour results in absorption lines observed both in the subunits as well as unchanged in the whole molecule and can be explained by the existence of states being localized predominantly on the subunits. The coupling of these states to molecular vibrations may lead to an intersection of adiabatic potential curves of states with totally different localization behaviour and therefore to a fast transition between these states. Using such a transfer mechanism allows experimentally observed excitation spectra to be explained and the time constant to be predicted for energy transfers from initial states localized on one end of the molecule to final states localized on the other side of the same molecule.

Study of elementary photophysical and photochemical processes in organic molecules using the optoacoustical effect

Study of elementary photophysical and photochemical processes in organic molecules using the optoacoustical effect

The Triplet State and Molecular Electronic Processes in Organic Molecules

ADVERTISEMENT RETURN TO ISSUEPREVArticleThe Triplet State and Molecular Electronic Processes in Organic MoleculesS. K. Lower and M. A. El-SayedCite this: Chem. Rev. 1966, 66, 2, 199–241Publication Date (Print):April 1, 1966Publication History Published online1 May 2002Published inissue 1 April 1966https://doi.org/10.1021/cr60240a004RIGHTS & PERMISSIONSArticle Views4715Altmetric-Citations777LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (5 MB) Get e-Alerts Get e-Alerts

Mechanism of the Primary Photodissociation Processes of Organic Molecules

IN his comments on our communication1 under this title, Prof. R. G. W. Norrish2 has raised certain questions which we naturally would like to answer. But as this discussion would certainly require more space than NATURE can devote to these questions, we prefer to deal with the matter in the detailed paper which is now in course of completion.