Thiadiazole, electronic structure

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

Little has been added to the firm body of evidence that 1,2,5-thiadiazole (2) is a planar, cyclic molecule of C2, symmetry. The previous edition of this series <84CHEC-I(6)513> details the molecular geometry, molecular calculations, and electronic structures of the ring systems. A few recent efforts,... 

While it may be intellectually unsatisfying, the electronic structure of 1,2,5-thiadiazole cannot be depicted in a single drawing. The physical and chemical data as they relate to the position and nature of the double bonds can best be represented by a series of canonical forms (Scheme 1) <84CHEC-I(6)513>. 

As expected from the electronic structure, the 1,2,5-thiadiazoles readily undergo nucleophilic attack (see also Section 4.02.1.6). The site of attack, however, is variable and can take place either at carbon, sulfur or a ring proton (Scheme 1 also see Section 4.26.3.3.6). 

In an extensive investigation of the r-electron structure of N- and S-hetero-cycles, the application of fundamental PPP MO calculations and n.m.r. and u.v. spectral measurements have provided information concerning the changes in the TT-electron distribution due to 2-amino- and 2-formyl-substituents in 1,3,4-thiadiazole derivatives. Energy barriers to rotation of the dimethylamino-group in 1,3,4-thiadiazoles have been determined (see p. 427). 

Fig. 1-6). The structure obtained for thiazoie is surprisingly close to an average of the structures of thiophene (169) and 1,3,4-thiadiazole (170) (Fig. 1-7). From a comparison of the molecular structures of thiazoie, thiophene, thiadiazole. and pyridine (171), it appears that around C(4) the bond angles of thiazoie C(4)-H with both adjacent C(4)-N and C(4)-C(5) bonds show a difference of 5.4° that, compared to a difference in C(2)-H of pyridine of 4.2°, is interpreted by L. Nygaard (159) as resulting from an attraction of H(4) by the electron lone pair of nitrogen. 

A reliable calculation of polarizabilities requires an adequate description of the outer part of the electron density. For this reason Kassimi and Lin [98JPC(A)9906] used augmented basis sets of triple- quality to study polarizabilities and dipole moments of thiazoles and thiadiazoles. They expect their results to be reliable within 5%. In addition, the authors provide MP2/6-31G geometries for most of their structures. Hyperpolarizabilities for substituted thiazoles obtained from calculations at lower levels are also provided [99MI2]. 

The electronic nature of the NSN fragment was studied, using both ab initio and DFT methods, for a series of 1,2,5-thiadiazoles and compared to the established zwitterionic structure of naphtha[l,8-rz/][l,2,6]thiadiazine 6 (Figure 1). 

The first single-crystal structure was reported for 6-methyl-3-phenyl-.r-triazolo[3,4-3]-l,3,4-thiadiazole in which the nucleus of the triazolothiadiazole system was planar confirming the aromatic character of the lOn-electron system <1974CSC7>. This is generally the case for the entire series of 5,5-fused heterocycles. 

Thiadiazole and selenodiazole rings are isoelectronic with benzene and form a fully conjugated, essentially planar structure when fused to porphyrazines at the p-pyrrolic position (168). Compared to pc, the incorporation of sulfur and selenium in addition to nitrogen atoms to the peripheral heterocyclic rings affects the electronic charge density and influences the interactions between adjacent molecular stacks. 

Reactions of 1,2,4-thiadiazoles with radicals and electron-deficient species are virtually unknown. Catalytic and dissolving metal reductions usually cleave the nucleus at its N—S bond by a reaction that may be regarded as the reverse of its synthesis by the oxidative cyclization of amidinothiono structures (Section 4.08.9.4). For example, reduction of the diamino compound (37) gives the amidinothiourea (38) from which it may be prepared by oxidation (Equation (8)). 

The 1,3-dipolar cycloaddition reaction of nitrile sulfides with nitriles yields 3,5-disubstituted 1,2,4-thiadiazoles of unequivocal structure. This method has received considerable attention in recent years. Electron deficient nitriles such as tosyl cyanide afford high yields of 5-tosyl derivatives (341) (Equation (53) see also Scheme 61) <93JHC357). 

In attempting to approximate (CNDO/2) the nature of the 1,2,5-thiadiazole ring (2a vs. 2b), a nontraditional view has been proposed <83IJC(B)802> with two C=N bonds and one three-center two-electron 7t-bond delocalized over the N—S—N system. This would account for the electron-rich N—S—N nature of the molecule. Classical structures (2a, 2b) are viewed as unlikely versus a hybrid structure (2c). 

The structural indices of aromaticity, I, of oxadiazoles (145-148), thiadiazoles (150-153) and selenadiazoles (155, 156) are compared with that of the parent furan (144), thiophene (149) and selenophene (154) (Scheme 11). 1,2,3-Oxadiazole (145) is the least stable among them since all attempts to synthesize this compound were unsuccessful, most likely because of its easy isomerization to the acyclic isomer. At the same time its sulfur analogue (150) possesses good stability and has been synthesized. Its 2,4-diaza- (151), 3,4-diaza- (152) and 2,5-diaza-(153) isomers demonstrate even more the extent of n-electron delocalization. There exists a well-known tendency of decreasing aromaticity depending on the type of pyrrole-like heteroatom S > Se > O. However, there is no uniformity in the change in aromaticity in the horizontal rows, i. e., dependence on heteroatom disposition. 

A series of CNDO (complete neglect of differential overlap) calculations have been performed on the structure, formation, and nucleophilic reactivity of thiatriazole.2 Figures 1 and 2 indicate the lowest energy structures, with optimal geometry and electron distribution, for thiatriazole (2) and the related 1,3,4-thiadiazole (I). 

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