Computational Chemistry Practical exercise 1 1 Quantification of aromaticity Introduction Aromaticity is a crucial aspect of a huge number of compounds, and until the concept of aromaticity was developed, the inert nature of compounds such as benzene in comparison to many other unsaturated compounds such as alkenes and alkynes that were much more reactive was extremely puzzling to chemists. It was not until around the 1950s that a general theory to determine which molecules would exhibit aromaticity was developed by HÃ¼ckel. The quantification of the energy provided by resonance is an important aspect of aromatic compounds. Isodesmic reactions (which are often hypothetical, and in which the number and type of chemical bonds are identical in both the reactants and products) are a useful method used in computational chemistry to assess relative energies of different structures. An example of an isodesmic reaction designed to determine resonance energy is shown in Figure 1. The cyclic unit in the reactant of this reaction cannot be aromatic, whereas the product can be aromatic (try drawing the resonance structrues to prove this), but there are no differences in the numbers and types of formal chemical bonds within the structures. Hence, any difference in energies between these two compounds may be considered to correspond to the resonance energy of the product, i.e. the lowering of energy resulting from the aromatic character of the species. Figure 1 Example of an isodesmic reaction scheme to determine resonance energy. In this exercise you will quantitatively assess the aromatic nature of a range of polyunsaturated cyclic structures by constructing suitable isodesmic reactions and calculating molecular energies using electronic structure calculations. This exercise demonstrates the quantitative use of quantum chemical methods, providing detailed information that is not readily gained from drawing formal structures and applying empirical methods. Methods Compounds may typically be classified as either aromatic, non-aromatic, or anti-aromatic. From the structures shown in Figure 2, select one for which you already know its classification in order to verify the method. Construct an isodesmic reaction for this compound comparable to that given in Figure 1, and optimise structures of the reactant and product of your reaction using the PM3 method in Hyperchem. Then calculate the single point energies of the reactant and product of your reaction, using the HF (ab-initio) method with the 6-21G* basis-set, and hence determine the resonance energy of the compound. Classify the molecule according to the categories listed above using your calculated resonance energy, and check it matches your expectation.Computational Chemistry Practical exercise 1 2 1 2 3 4 5 6 Figure 2 Structures and numbering for the compounds to be studied in this investigation Once you have verified the method, carry out a similar procedure using the same computational methods for each of the compounds in Figure 2. In each case, tabulate the number of Ï€ electrons the compound has, whether it is planar or non-planar, and the resonance energy you calculate from your chosen isodesmic reaction. Hence, classify them in terms of their aromaticity. In some cases there may be multiple choices of isomer when constructing the isodesmic reactions; in these cases just pick one, but consider how your choice may affect the calculated resonance energy. For charged species remember to specify the molecular charge before running the calculations. Construct qualitative MO diagrams of the Ï€ electrons to see if MO theory provides consistent results with the classifications you have made from the resonance energies. Write-up The report should be less than 2500 words in total (and may well not need to be near this number). Include a brief introduction outlining the background and relevance of the exercise. A short methods section should include sufficient information for a reader to replicate the experiment. A results and discussion section should set out your main results, including figures showing each of the isodesmic reactions you used, tabulated data for each of the compounds in Figure 2, and your qualitative MO diagrams. Make sure that all tables and figures have a number and caption. Finish with a short conclusions section outlining the significance and implications of the results, along with ideas for how the work could be extended. Questions to consider: 1. What are some of the limitations of using isodesmic reactions to determine quantities such as resonance energies? 2. Consider reasons for differences in resonance energies for reactions with a choice of isomer. Why might some isomers provide more accurate resonance energies than others?