Bond lengths, thiazole

Similarly, using another model based on heteroatom increments proposed by Dischler (267), it was possible to calculate a very satisfactory empirical geometry for thiazole (122) with the aid of Shoolery s correlation (268) between and C-H bond length. 

Sulfur. Thiophene and benzo[ >] thiophene are both aromatic heterocycles, as discussed earlier in this review. Isothiazole is a planar molecule with an aromaticity comparable with those of thiazole and pyrazole, and higher than those of isoxazole and oxazole,122 140 as evaluated on the basis of Bird s aromaticity index A, based upon the statistical degree of uniformity of the bond orders of the ring periphery. Theoretical calculations and experimental data in connection with the aromaticity of isothiazole have been reviewed.141 Thiazole is also viewed as an aromatic molecule, similar to thiophene. It lacks an experimental aromaticity value, but the heat of formation together with bond lengths and angles have been calculated by various computational meth-... 

Details of bond lengths and bond angles for the X-ray structures of heterocyclic compounds through 1970 are listed in Physical Methods in Heterocyclic Chemistry , volume 5. This compilation contains many examples for five-membered rings containing two heteroatoms, particularly pyrazoles, imidazoles, isoxazoles, oxazoles, isothiazoles, thiazoles, 1,2-dithioles and 1,3-dithioles. Further examples of more recent measurements on these heterocyclic compounds can be found in the monograph chapters of CHEC and CHEC-II. 

Figure 4 Molecular structure of thiazole bond lengths (A) and bond angles (°)...

Rg.I-6. Molecular structure of thiazole bond lengths in A (left), bond angles in degrees (right). 

The molecular structure of thiazole has been refined by the joint analysis of data obtained from gas-phase electron diffraction (GED), microwave (MW) spectroscopy, and ab initio molecular orbital (MO) calculations. The combined approach, making use of the structure analysis restrained by ab initio calculations for electron diffraction (SARACEN) method, has led to a very precise structure in which all independent geometric parameters are well defined <1999PCP2421>. The refined bond lengths and angles are listed in Table 1, and these agreed well with experimental data previously obtained (see Section 4.06.3.2 and Section 3.06.02 of CHEC-II(1996)). 

Table 1 Calculated bond lengths and bond angles of thiazole...

A variety of computational methods have been compared in the prediction of bond lengths, bond angles, and the heat of formation of thiazole <93JCC75>. Among these methods are the semiempirical techniques AMI, PM3, and MNDO, and the thermochemical basis sets of Benson. 

Microwave spectral analysis of thiazole in the gas-phase was carried out to determine bond lengths and bond angles <71JST(8)225). The results are listed in Tables 1 and 2. These experimental values were compared with those obtained from theoretical calculations (see Section 3.06.2) by... 

A comparison with the bond lengths of oxazole (see Fig. 5.9, p 123) leads to the conclusion that the delocalization of the r-electrons in thiazole is greater. Thus, the aromaticity of thiazole is greater than that of oxazole. The ionization potential is 9.50 eV and its dipole moment 1.61 D. UV and NMR data are listed in the following table ... 

Figure 5.13 Structure of thiazole (bond lengths in pm, bond angles in degrees).

Thiazol-2-ylidenes exhibit spectroscopic and structural properties similar to those of saturated imidazolin-2-ylidenes 86. Upon deprotonation of the thiazolium salts to thiazol-2-ylidenes the N-C-S angle becomes smaller and the endocyclic N-C2 bond length increases slightly within an essentially planar... 

The formation of (149) involves nucleophilic addition of sulfamide (136) across the carbon-nitrogen triple bond of (150), followed by tautomerisation of the initial iminoproduct (Scheme 61). A series of analogous N-sulfamoylamidines in which the length of the chain between the thiazole nucleus and the amidine moiety was varied also inhibited gastric juice secretion.7... 

A] are comparable with the dimensions of sulphathiazole (97), rather than with the dimensions of either (94), (95), or (96), thus suggesting limited delocalization within the five-membered ring. However, the influence of the thiazole moiety on the benzene ring is evidenced in particular by the length of the C(8)-C(9) bond [1.353(6) A], which parallels the effects observed in the furazan derivatives (89), (90), (91), and (92). Rather similar distortions... 

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