Why is the Lewis concept so often overlooked at the introductory level? Custom and historical perspective account for the traditional emphasis on the Arrhenius and Brønsted-Lowry concepts provided in textbook acid-base reaction chapters. These are certainly valuable concepts for inclusion in the teaching curriculum, but by themselves fall short of explaining how and why polar covalent reactions occur, which the Lewis concept can do. Any concern over confusing students by providing an alternate perspective seems inconsistent with the expectation in these same texts that students can deal with the far more challenging concepts of waveparticle duality and standing matter waves. In fact, applying the Lewis concept provides an opportunity for the student to use previous course concepts to predict and understand reaction behavior, increasing the students' confidence and retention of key concepts.To highlight this concept sequence, one of the most important recurring themes in introductory chemistry is that of structure-property relationships of atoms, ions, and molecules. A large part of a given text's coverage is devoted to leading the student through the methods of predicting the physical properties of substances. The general sequence takes the student through atomic structural theory, electronic configuration, periodicity, chemical bonding models (ionic, covalent, metallic), and more specifically for covalent compounds, the skills of drawing Lewis electron-dot structures and from this, the use of valence-shell electron-pair repulsion (VSEPR) theory to predict resultant properties related to polarity. Through this sequence of concepts, the central role of valence electrons is typically well emphasized and revisited, at first in the context of the chemical bonding models, and later with the finer details of molecular structure in VSEPR.With these skills in hand, students are ready to apply these to the realm of chemical properties and the "how" of chemical change, the reaction mechanism, at least for those reactions involving a simple change in covalent structure. In most texts, both at the full-year or one-semester level, the first major class of reaction to be studied in any detail is typically aqueous acid-base reactions, and the timing of this chapter sequence often closely follows that of molecular structure and properties. The fundamental reaction examples presented involve simple changes in covalent structure. These include the complete ionization of a strong acid in water, the net-ionic equation for neutralization of a strong acid and strong base,

(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-

Depicting the mechanism, the step-by-step path followed by a reaction, has evolved to also indicate the movement of electrons during each step through the use of curly arrows. The use of conventional curly arrows, as introduced by Robinson in the 1920s, can be ambiguous with regard to regiochemistry. Site-specific curly arrows, a refinement attributed to Woodward, remove this ambiguity. More importantly, our students indicate that they gain greater understanding and a clearer picture of the concepts under discussion through the use of site-specific arrows.

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