Multidimensional Description of the Aromaticity in Planar Five-Membered NSNS Ring Systems

Density functional theory studies of boron clusters with exotic properties in bonding, aromaticity and reactivity

Atomic clusters are unique in many perspectives because of their size and structure features and are continuously being applied for different purposes. To unveil their unconventional properties, in this work, using neutral tetraboron clusters as illustrative examples, we study their exotic behaviors in bonding, aromaticity, and reactivity. We show that both double and triple bonds can be formed, ring current patterns can be totally different, and both electrophilic and nucleophilic reactivities can coexist simultaneously. These features are often in contrast with our conventional chemical wisdom and could enrich the possibility for their potential applications. The methodologies employed in this work can be readily applied to other systems. Our studies should help us better appreciate atomic clusters with many atypical properties and henceforth yield novel applications.

Formation of S-alkyl thiophenium ionic liquids: mechanistic rationale and structural relationships

The quaternization of thiophenes through S-alkylation reactions with iodoalkanes in the presence of silver salts opens the door to a new family of room-temperature ionic liquids, yet relatively unexplored in terms of chemical reactivity and applications. This computational study provides a mechanistic rationale that accounts for their formation. The results are consistent with the calculated energy barriers of the corresponding transition structures. Calculations indicate that the geometry at the sulfur atom goes from flat to tetrahedral during salt formation, and the electron delocalization of thiophene is greatly reduced, if not lost, as inferred from aromaticity indexes. Moreover, the rationale explains the influence of polarizable anions on S-alkylation and why alkyl substitution at the α-position of thiophenes gives rise to more stable species than unsubstituted derivatives.

1,5,2,4,6,8-Dithiatetrazocine. Synthesis, computation, crystallography and voltammetry of the parent heterocycle

The prototypal 1,5,2,4,6,8-dithiatetrazocine has been synthesized for the first time by two routes: reaction of 1,2,3,5-dithiadiazolium chloride with N,N,N'-tris(trimethylsilyl)formamidine in acetonitrile and reaction of 1,2,3,5-dithiadiazolyl radical with dioxygen in solution. Yields are low but single crystals could be obtained for an X-ray crystal structure determination which shows it to have the planar delocalized structure predicted by B3LYP/6-311+G(2d,p) hybrid DFT calculations. The crystal structure is strongly reminiscent of that of benzene in the same Pbca space group. Aromaticity is demonstrated by a (1)H NMR chemical shift of +9.70 ppm indicative of diamagnetic ring shielding and an intense low-energy optical absorption with λ(max) = 349 nm (MeOH). The voltammetric behaviour of the title compound is compared with that of 1,3λ(4)δ(2),5,2,4-trithiadiazepine; both resist electrochemical oxidation and reduction over a wide potential range as is typical for aromatic heterocycles.

Structure, bonding, aromaticity and reactivity of Roesky's sulfoxide

The molecular and crystal structure of 1-oxo-1,2,4,3,5-trithiadiazole () has been studied experimentally by the determination of the crystal structure and theoretically at the DFT/B1B95/aug-cc-pVTZ level of theory. The combination of the geometrical data with a number of properties of the title compound, consisting of orbital topologies, Hirshfeld charges and bond orders, aromaticity parameters and Fukui functions, led to a description of its structure, aromaticity and reactivity. In addition, the nature of the long sulfur-sulfur bond has been investigated. The bands in newly recorded infrared and Raman spectra have been assigned to the normal vibrations of the molecule, based on calculated vibrational data. The results of theoretical calculations of the 14N NMR chemical shifts have been compared to the experimentally obtained shifts.

Neural Networks as a Tool To Classify Compounds According to Aromaticity Criteria

Aromaticity is a fundamental concept in chemistry, with many theoretical and practical implications. Although most organic compounds can be categorized as aromatic, non-aromatic, or antiaromatic, it is often difficult to classify borderline compounds as well as to quantify this property. Many aromaticity criteria have been proposed, although none of them gives an entirely satisfactory solution. The inability to fully arrange organic compounds according to a single criterion arises from the fact that aromaticity is a multidimensional phenomenon. Neural networks are computational techniques that allow one to treat a large amount of data, thereby reducing the dimensionality of the input set to a bidimensional output. We present the successful applications of Kohonen's self-organizing maps to classify organic compounds according to aromaticity criteria, showing a good correlation between the aromaticity of a compound and its placement in a particular neuron. Although the input data for the training of the network were different aromaticity criteria (stabilization energy, diamagnetic susceptibility, NICS, NICS(1), and HOMA) for five-membered heterocycles, the method can be extended to other organic compounds. Some useful features of this method are: 1) it is very fast, requiring less than one minute of computational time to place a new compound in the map; 2) the placement of the different compounds in the map is conveniently visualized; 3) the position of a compound in the map depends on its aromatic character, thus allowing us to establish a quantitative scale of aromaticity, based on Euclidean distances between neurons, 4) it has predictive power. Overall, the results reported herein constitute a significant contribution to the longstanding debate on the quantitative treatment of aromaticity.

5,6,7,8-Tetrafluoro-3λ4δ2,1,2,4-benzothiaselenadiazine, 5,6,7,8-Tetrafluoro-1,3λ4δ2,2,4-benzodithiadiazine, and Their Hydrocarbon Analogues: Molecular and Crystal Structures

5,6,7,8-Tetrafluoro-3lambda4delta2,1,2,4-benzothiaselenadiazine (1) is prepared by the intramolecular nucleophilic cyclization of C6F5SeN=S=NSiMe3 (2) mediated by CsF. According to an X-ray diffraction analysis, the heterocycle of 1 is bent along the Se(1)...N4 line by 6.0(2) degrees in the crystal. Despite the obvious similarities between 1 and its 1,3-dithia analogue (7) with respect to molecular composition and shape, the crystal packing of 1 is substantially different from that of 7. An interesting consequence of this is the inclusion of atmospheric N2 in the crystal lattice of the selenium derivative 1. The molecular structure and bonding of 1 have been investigated using quantum-chemical calculations at the DFT/B3LYP/6-311+G level of theory, and the results have been compared to those of 5,6,7,8-tetrafluoro-1,3lambda4delta2,2,4-benzodithiadiazine (7) and their hydrocarbon analogues (5 and 8).