Why 1,2‑quinone derivatives are more stable than their 2,3‑analogues?

In this work, we have studied the relative stability of 1,2- and 2,3-quinones. While 1,2-quinones have a closed-shell singlet ground state, the ground state for the studied 2,3-isomers is open-shell singlet, except for 2,3-naphthaquinone that has a closed-shell singlet ground state. In all cases, 1,2-quinones are more stable than their 2,3-counterparts. We analyzed the reasons for the higher stability of the 1,2-isomers through energy decomposition analysis in the framework of Kohn–Sham molecular orbital theory. The results showed that we have to trace the origin of 1,2-quinones’ enhanced stability to the more efficient bonding in the π-electron system due to more favorable overlap between the SOMOπ of the ·C4n−2H2n–CH·· and ··CH–CO–CO· fragments in the 1,2-arrangement. Furthermore, whereas 1,2-quinones present a constant trend with their elongation for all analyzed properties (geometric, energetic, and electronic), 2,3-quinone derivatives present a substantial breaking in monotonicity.European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme. O.A. S., H. S. and T.M. K

Crystal structure of 3,6-bis­­(pyridin-2-yl)-1,4-di­hydro-1,2,4,5-tetra­zine

The structure of the title compound, C12H10N6, at 100 K has monoclinic (P21/n) symmetry. Crystals were obtained as a yellow solid by reduction of 3,6-bis­(pyridin-2-yl)-1,2,4,5-tetra­zine. The structure displays inter­molecular hydrogen bonding of the N—H⋯N type, ordering mol­ecules into infinite ribbons extending along the [100] direction.Funding for this research was provided by: Narodowe Centrum Nauki (grant No. 2015/19/B/ST4/01773); EFRD in Operational Programme Development of Eastern Poland 2007–2013, the Oxford Diffraction SuperNova DualSource diffractometer (award No. POPW.01.03.00-20-004/11)

(3+2)‐Cyclization Reactions of Unsaturated Phosphonites with Aldehydes and Thioketones

By exploiting the unique reactivity of ethynyl‐phosphonites we obtain novel P(V)‐containing five‐membered heterocycles via (3+2)‐cyclization reactions with aldehydes or cycloaliphatic thioketones in satisfactory to excellent yields. Whereas reactions with thioketones to yield 1,3‐thiaphospholes‐3‐oxides occur smoothly at room temperature with equimolar amounts of the starting materials in absence of any catalyst, the analogous conversions with aldehydes to generate 3‐oxides of 1,3‐oxaphospholes require addition of triethylamine as a base. We postulate a step‐wise (3+2)‐cyclization mechanism for the formation of the 1,3‐thiaphosphole ring based on DFT quantum chemical calculations. With this study, we introduce new cyclization reactions originating from unsaturated phosphonites as central synthetic building blocks to yield previously inaccessible stable phosphorus‐containing heterocycles with unexplored potential for the molecular sciences.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Alexander von Humboldt-Stiftung http://dx.doi.org/10.13039/100005156Peer Reviewe

Oxidation of 2-mercaptopyridine N-oxide upon iodine agent: structural and FT-IR studies on charge-assisted hydrogen bonds CAHB(+) and I…I halogen interactions in 2,2′-dithiobis(pyridine N-oxide) ionic cocrystal

2-Mercaptopyridine N-oxide (I) undergoes spontaneous dimerization to the disulfide form due to reaction with iodine acting as an oxidizing reagent. As a result, a di-N-oxide disulfide derivative of pyridine is obtained. During the process of crystallization, one of N-oxide groups undergoes protonation and a cation form of disulfide moiety cocrystallizes with I3 − counterion forming a salt structure. Therefore, in the crystalline state, the 2,2′-dithiobis(pyridine N-oxide) molecule exists in a not observed previously form of monocation. Interestingly, the protonated N-oxide group does not form hydrogen-bonded salt bridges (of the CAHB(±) type with I3 − anions) but prefers to be involved in intermolecular interactions with the unprotonated N-oxide group of the adjacent molecule This results in formation of intermolecular CAHB(+) hydrogen bridges finally linking molecules into infinite chains. The crystal structure is also stabilized by other various noncovalent interactions, including iodine...iodine and sulfur...iodine contacts

Temperature-dependent polymorphism of N-(4-fluorophenyl)-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide: experimental and theoretical studies on intermolecular interactions in the crystal state

X-ray analysis of N-(4-fluorophenyl)-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide reveals the temperature-dependent polymorphism associated with the crystallographic symmetry conversion. The observed crystal structure transformation corresponds to a symmetry reduction from I41 /a (I) to P43 (II) space groups. The phase transition mainly concerns the subtle but clearly noticeable reorganization of molecules in the crystal space, with the structure of individual molecules left almost unchanged. The Hirshfeld surface analysis shows that various intermolecular contacts play an important role in the crystal packing, revealing graphically the differences in spatial arrangements of the molecules in both polymorphs. The N-oxide oxygen atom acts as a formally negatively charged hydrogen bonding acceptor in intramolecular hydrogen bond of N–H…O− type. The combined crystallographic and theoretical DFT methods demonstrate that the observed intramolecular N-oxide N–H…O hydrogen bond should be classified as a very strong charge-assisted and closed-shell non-covalent interaction