Comparison of computational methods applied to oxazole, thiazole, and other heterocyclic compounds

Abstract

(A variety of computational methods, including the semiempirical techniques AMI, PM3, and MNDO, and the thermochemical basis sets of Benson and Stine, was used to calculate and compare heats of formation M (A-/i) data for optimized geometries of a variety of aromatic and nonaromatic heterocycles. Detailed analyses, including 6-31G' and MP2/6-31G* ab initio calculations, were performed for the oxazole and thiazole heterocycles. The results indicate a scatter among the methods sensitive to the nature of the heterocycle. This was in particular evident in the oxazole molecule, where AMI gave a singularly high value of WI; consistent with longer calculated bond lengths, particularly about the oxygen atom. Aromatic stabilization energy appears to be addressed differently among the employed methods. Implications of this contrast applied to calculation of macromolecular systems containing heterocyclic units are discussed. INTRODUCTIONthe paraphenylene unit to be a major contributor to the observed differences. Organic heterocycles are becoming increasingly imThis persistently large contrast in heat of formaportant in materials chemistry. Several promising tion results led us to suspect a significant difference • conducting polymers, nonlinear optical polymers, in these computational methods applied to the oxbiomaterials, and high-strength polymers contain azole heterocycle. We thus concentrated our further heterocyclic moieties. In the field of high-strength efforts on the oxazole and the related thiazole het-* polymers, oxazole and thiazole heterocycles are inerocycles to determine which method is most apcorporated into two advanced high-strength polymer propriate for studying these polymers. (The applifibers (Fig. 1).1-4 These materials exhibit extremely cation of these calculations to polymeric systems high mechanical strength in the direction of the polywill be addressed in a subsequent article.) mer backbone due to the overall rod-like fiber orientation.RESULTS AND DISCUSSION There is a great deal of interest in determining the relationship between computationally derived paHeat of Formation Calculations rameters (such as the heat of formation) with ultiHeats of formation data were computed for the oxSmate strength and elastic modulus properties of azole heterocycle using MOPAC 6.0 and an eigenthese polymeric materials. In this context, we pervector following routine for geometry optimization." formed comparative quantum chemical calculationsThe thiazole heterocycle, (the next homologue in the on the cis-PBO structure using the three most commonly employed semiempirical methods (MNDO, chalcogen family) was also examined due to the im-AMI, 6 and PM3) . 7 ' The RHFe optimized geometries portance of thiazole based high-ordered polymers.' o eon rieasiThe results are summarized below in descending or-

Ligands for the Central Benzodiazepine Receptor: Structure - Affinity Relationship Studies on Imidazo[1,2-b]pyridazines

“…ref. 36). comparison of the AM^ geometries, potential derived charges and dipole moments with ab initio data for some imidazo [l,2-b] pyridazines revealed a good agreement, particularly for the charge values.]…”

“…Some imidazo[l,2-blpyridazines listed in Tables 1-5 possessed the structural features outlined above (apparently in the proposed spatial arrangements) but showed only poor affinities for the receptor [e.g. (5), (8), (lo), (32), (33), (36)l. This indicated that all substituents and their effects should be considered, but not in isolation.…”

A Computational Study on the Mechanism of the Formose Reaction Catalyzed by the Thiazolium Salt.

“…A counter-anion may approach the S atom, since the space surroundings the S atom is sterically open. AM1 calculations usually give the poorest results for most five-membered aromatic heterocycles with one nitrogen [14] and thus, the N3 atom of thiamine may have positive charge. The heat of formation was calculated to be 130.8 kcal/mol.…”

“…For instance, the computational analyses of the structures (bond length, bond angle, and partial charge distribution) using semi-empirical methods (extended-Huckel calculation for thiamine derivatives [12] and AM1 calculation for thiamine [13]) have been reported. For thiazole and pyrimidine that are the component of thiamine, the most suitable calculation method has been discussed by the comparison between the results of the semi-empirical methods (MNDO, AM1, PM3) [14] or those of the non-empirical ones (RHF/STO-3G, 3-21G, 3-21G*, and 6-31G* levels) [15]. As a former example, Shaffer and Wierschke reported that the PM3 program has given the best semi-empirical results for 4-methylthiazole in comparison with MNDO, AM1, and the Benson method [14].…”

“…For thiazole and pyrimidine that are the component of thiamine, the most suitable calculation method has been discussed by the comparison between the results of the semi-empirical methods (MNDO, AM1, PM3) [14] or those of the non-empirical ones (RHF/STO-3G, 3-21G, 3-21G*, and 6-31G* levels) [15]. As a former example, Shaffer and Wierschke reported that the PM3 program has given the best semi-empirical results for 4-methylthiazole in comparison with MNDO, AM1, and the Benson method [14]. In terms of the geometry of the positive species, an ab initio study has been made for protonated heteroatomic rings such as imidazole, oxazole and thiazole [16].…”