Radical recombination in interstellar ices, a not so simple mechanism.

Many complex organic molecules (hereafter COMs) have been detected in different regions of the interstellar medium (ISM). In each region, different energetic processes - UV irradiation, atom bombardments, etc. - that could be linked to the formation of detected COMs may occur depending on the environment. Several formation mechanisms were proposed but increasing attention is paid to radical recombination reactions. Previous studies showed that glycolaldehyde (HC(O)CH2OH) and ethylene glycol (HOCH2CH2OH) are formed by radical recombination between HC˙O and ˙CH2OH, and by ˙CH2OH dimerisation, respectively. Formyl (HC˙O), one of the most famous astrophysically-relevant radical species, has been detected as a gaseous component of the ISM. Its reactivity was already attributed to the formation of several COMs. This work aims to study the dimerisation of formyl radical HC˙O using a cryogenic matrix technique. The evolution of the chemical sample composition is monitored by infrared spectroscopy and by mass spectrometry during temperature programmed desorption (TPD) monitoring. Results indicate that the reaction of one HC˙O with another does not lead to the direct formation of glyoxal (HC(O)C(O)H) but yields H2CO and CO. Results are also compared with those for the reaction between two ˙CH2OH radicals and the recombination between HC˙O and ˙CH2OH. Also, glyceraldehyde was tentatively detected in our experiment using different spectroscopic techniques. A radical mechanism is proposed to explain its formation in our experiments. Complementary quantum chemical calculations provide an atomistic interpretation of the experimental findings.

Intramolecular Hydrogen Bond Reverting the Solvent Effect on Phosphorus Hyperfine Coupling Constants of ß-Phosphorylated Nitroxides.

Recently, we reported a dramatic solvent effect on the phosphorus hyperfine coupling constant aP of ß-phosphorylated six-membered ring nitroxides, that is, approximately 25 G of difference in aP from n-hexane to water (Org. Biomol. Chem. 2016, 14, -1228-1292). In this article, we report on the effect of intramolecular hydrogen bonding (IHB) in three nitroxides exhibiting IHB between the hydroxyl and diethylphosphoryl groups and one exhibiting IHB between the hydroxyl group and the nitroxyl moiety. It is observed that for the first three nitroxides, aP increases with increasing polarity/polarizability and hydrogen bond donor (HBD) properties of the solvent (π* and α, respectively)-in sharp contrast to the data reported in the literature-and for the last nitroxide, aP decreases with π* and α. In fact, the occurrence of IHB induces a large strain, its suppression by hydrogen bond acceptor (HBA) solvents affords an increase in aP .

The ß-phosphorus hyperfine coupling constant in nitroxides: 6. Solvent effects in non-cyclic nitroxides.

In two recent articles (Org. Biomol. Chem., 2015 and 2016), we showed that changes in the phosphorus hyperfine coupling constant aP at position ß in ß-phosphorylated nitroxides can be dramatic. Such changes were applied to the titration of water in organic solvents and conversely of organic solvents in water. One of the molecules tested was a non-cyclic nitroxide meaning that a thorough investigation of the solvent effect on the EPR hyperfine coupling constant is timely due. In this article, we show that the aP of persistent non-cyclic ß-phosphorylated nitroxides decrease with the normalized polarity Reichardt's constant E(N)T. The Koppel-Palm and Kalmet-Abboud-Taft relationships were applied to gain deeper insight into the effects influencing aN and aP: polarity/polarizability, hydrogen bond donor properties, and the structuredness of the cybotactic region.

Solvent effect in ß-phosphorylated nitroxides. Part 4: detection of traces of water by electron paramagnetic resonance.

For decades, the nitrogene hyperfine coupling constant aN of nitroxides has been applied to probe their environment using EPR. However, the small changes observed (≈2 G from n-pentane to water) with the solvent polarity allow only a qualitative discussion. A stable ß-phosphorylated nitroxide exhibiting a small change in aN (≈3 G from n-pentane to water) and a striking change (≈25 G from n-pentane to water) in phosphorus hyperfine coupling constant aP with the polarity of solvent was prepared and used to develop the first procedure for the titration of water in THF by EPR, down to 0.1% v/v.

The ß-phosphorus hyperfine coupling constant in nitroxide: part 3: titration of water by electron paramagnetic resonance.

Recently, we showed that the phosphorus hyperfine coupling constant aPß of persistent cyclic nitroxides decreased with the normalized polarity Reichardt's constant E. Thus, we investigated the changes in aPß in binary mixtures of solvents. The sensitivity of aPß to the solvent was high enough to allow us to perform water titration in THF, 1,4-dioxane, and acetonitrile by EPR. Accuracies of a few percent were achieved.

Intramolecular hydrogen bond in alkoxyamines. Influence on the C-ON bond homolysis.

The C-ON bond homolysis in alkoxyamines can be influenced by the presence of an intramolecular hydrogen bond (IHB) between the alkyl and the nitroxyl fragments, which leads to an 8-fold decrease in the homolysis rate constant k(d). When the IHB is disrupted by the solvent or by substitution of the hydrogen involved in the IHB by a protecting group (OMe, OAc, OBz, OBn, or OTBDMS), a higher homolysis rate constant k(d) is observed, as expected from the correlations developed by Marque (Bertin, D.; Gigmes, D.; Marque, S.; Tordo, P. Macromolecules 2005, 38, 2638-2650). Results were confirmed by DFT calculations at the B3LYP/6-31G(d,p) level.