Comparative study of aromaticity in five‐membered rings containing S, SO and SO₂ groups

Abstract

Owing to the importance of the concept of aromaticity, different indices have been developed to try to quantify this property. The possible π delocalization through an XSOnX group (X = C, N; n = 0, 1 or 2) could explain heteroaromaticity in rings containing the moiety. For that reason, the aromaticity of five‐membered sulphurcontaining rings with different oxidation numbers (S, SO and SO2) and including no (thiophene), one (isothiazole) or two (1,2,5‐thiadiazole) adjacent atoms was investigated. Ab initio calculations were carried out to determine geometrical parameters (planarity of the ring, bond length and bond order), electronic structure (charge distribution and dipolar moment) and the participation of d‐orbitals of sulphur. According to these calculations, only compounds with S(II) can be considered to be aromatic, whereas compounds bearing S(IV) or S(VI) are better described as ylides.

Local and nonlocal density functional calculations of the molecular structure of isomeric thiadiazole monoxides

“…For the 1,2,5‐thiadiazole derivatives, the B3LYP dipole moment changes from 1.5876 to 5.1466 D on going from the reduced molecule to the corresponding monoxide (I). The same effect has been observed in the past 31 and interpreted as a consequence of the loss of aromaticity when passing from the S(II) to the S(IV) and S(VI) molecules. However, the opposite result has been found in this work, for the 1,3,4‐thiadiazole derivative.…”

“…Although experimental gas‐phase data for the compounds studied is not available, a comparison with the theoretical results can be done by resorting to the structures of compounds with related moieties that have been characterized by X‐ray diffraction studies. For the moiety that is related to the 1,2,5‐thiadiazole 1‐oxide molecule, some statistical analysis has been presented and used to confirm geometrical results provided by the theoretical calculations 31, 40. The results are shown as statistical average (SA) in Table I.…”

“…Condensed softness indices allowing the comparison of reactivity between similar atoms of different molecules can be easily calculated starting from the relation between the Fukui function f ( r ), and the local softness s ( r ): It is well known that the use of basis sets including polarization functions is essential to obtain reliable results for compounds with hypervalent sulfur atoms 27. It has also been shown 28–31 that the STO‐3G* basis set reproduces the geometry of hypervalent sulfur compounds reasonably well. Nevertheless, as a higher accuracy is needed for a good description of properties that depend on the electronic structure, the condensed Fukui functions and condensed local softness indices, the electronic density maps, the molecular electrostatic potentials, and the gross orbital charges were obtained from HF and local and nonlocal DFT using the 6‐31G** basis set through single‐point calculations on the fully optimized at the same level neutral molecule and the singly ionized positive and negative molecules with the same geometric structure of the neutral molecule.…”

Reactivity Study of Pyridyl‐Substituted 1‐Metalla‐2,5‐diaza‐cyclopenta‐2,4‐dienes of Group 4 Metallocenes

“…The valueso ft he bond lengths of the five-membered cycle( N1ÀS1 1.625(2), C1ÀN1 1.331(3), C1À C1A 1.436 (5) ) and the fact that it is nearly planar (meand eviation from the best plane 0.007 ) indicates ome electron delocalisation,a sa lready described for other thiadiazoles. [35] The pyridyl substituent and the five-membered cycle are tilted by 42.77(9)8 (angle between the planesd efined by S1, N1, C1, C1A, N1A and C2, C3, N2, C4, C5, C6).…”

Supportless Silver Nanowires as Oxygen Reduction Reaction Catalysts for Hydroxide‐Exchange Membrane Fuel Cells

“…Among these, the heteroaromatic ring systems are the most interesting and the quantitative assessment of the aromaticity of heterocycles has been considered 13–28. The aromaticity of five‐membered sulfur containing rings with different oxidation numbers (S, SO, and SO 2 ) and including no (thiophene), one (isothiazole), or two (1, 2, 5‐thiadiazole) adjacent heteroatoms has been previously investigated 29. Also, the purpose of most research was to quantify the substituent effect on some chemical or physicochemical property of aromatic systems that provided information about the mechanism of chemical reactions.…”