Stabilizing Effect of a 4c/6e Hypervalent Bond in Dinitrodiphenyl Disulfides and Their Thermochemical Properties: Experimental and Computational Approach.

Thermochemical properties and intramolecular interactions of 2,2'-dinitrodiphenyl disulfide (2DNDPDS) and 4,4'-dinitrodiphenyl disulfide (4DNDPDS) were determined and analyzed. Their standard molar formation enthalpies in the gas phase (ΔfHm°(g)'s) were experimentally determined; theoretically, they were computed using the G4 composite method and atomization reactions. Specifically, ΔfHm°(g)'s were obtained by combining formation enthalpies in the condensed phase and enthalpies of phase change. Formation enthalpies in the condensed phase were determined experimentally through combustion energies, which in turn were found by means of a rotatory bomb combustion calorimeter. Sublimation enthalpies were derived from thermogravimetric experiments, measuring the rate of mass loss and using Langmuir and Clausius-Clapeyron equations. Fusion enthalpies and heat capacities of the solid and liquid phases were measured as functions of temperature by differential scanning calorimetry, and the heat capacities of the gas phase were calculated via molecular orbital calculations. Theoretical and experimental ΔfHm°(g)'s differed by <5.5kJ·mol-1, and isomerization enthalpies are discussed. In addition, using theoretical tools [natural bond orbitals (NBO) and quantum theory of atoms in molecules (QTAIM)], intramolecular interactions were analyzed. An uncommon hypervalent four-center six-electron interaction of type O···S-S···O was found in 2DNDPDS. This hypervalent interaction, in addition to the extent of conjugation between the aryl and NO2 moieties and the formation of intramolecular C-H···S hydrogen bonds, counteracts the repulsion caused by steric repulsions. Hydrogen bonding was confirmed through geometric parameters as well as QTAIM.

Can an n(O) → π* Interaction Provide Thermodynamic Stability to Naturally Occurring Cephalosporolides?

The stereocontrolled synthesis of naturally occurring products containing a 5,5-spiroketal molecular structure represents a major synthetic problem. Moreover, in a previous work, the stereocontrolled synthesis of cephalosporolide E (ceph E), which presumably was obtained from its epimer congener (ceph F) through an acid-mediated equilibration process, was reported. Consequently, we performed a theoretical investigation to provide relevant information regarding the title question, and it was found that the higher thermodynamic stability of ceph E, relative to ceph F, is caused by an n → π* interaction between a lone electron pair of the oxygen atom of the spiroketal ring (nO) and the antibonding orbital of the carbonyl group (π*C=O). Although similar stereoelectronic interactions have been disclosed in other molecular structures, its presence in ceph E, and very likely in other related naturally occurring products, represents a novel nonanomeric stabilizing effect that should be introduced into the chemical literature.