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The featured molecules this month are from the paper by Bernard, Gernigon, and Britz-McKibbin exploring trans to cis photoisomerization in resveratrol.. Examination of Figure 1 in that paper, where the hydrogen atoms have been omitted, might lead one to conclude that the structures are relatively straightforward. These isomers provide students an excellent opportunity to test their ability to take a two-dimensional representation and envision the three-dimensional structure of the molecule and to consider the competing factors that might lead to the three-dimensional structures being non-planar. The two-dimensional models focus attention on the possibility of extended pi-electron delocalization. Addition of the hydrogen atoms clearly suggests that delocalization will compete with non-bonded H–H repulsions in the cis isomer. Further examination of the trans isomer shows that such non-bonded interactions are, in what one might call a first-order approximation, like those in biphenyl—interactions that lead biphenyl to be non-planar in both the gas phase and in a variety of solvents. The backbone of the trans isomer of resveratrol, trans-stilbene, has been the subject of a number of theoretical and experimental investigations (1, 2). In general, Hartree–Fock calculations predict a non-planar geometry for this molecule while Density Functional Calculations, using the same basis sets, predict an essentially planar structure. Spectroscopic evidence supports a temperature-dependent structure for trans-stilbene with the molecule being planar at low temperature and non-planar at high temperatures. Our calculations on trans-resveratrol produce similar results. Hartree–Fock calculations using the 6-31G** (6- 31G(d,p)) basis set predict a dihedral angle of approximately 24 degrees between each ring and the central carbon–carbon double bond. This result is consistent with the reported value of 23 degrees using the 6-31G* basis set. We also find that DFT calculations using the B3LYP functional and the 6- 31G** basis set, lead to a planar configuration. We include several versions of trans-stilbene and trans-resveratrol in the molecule collection so that students can explore these structural questions in more detail. For each molecule, structures obtained from PM3, HF(6-31G**), and DFT(B3LYP/6-31G**) calculations are included, as well as planar and non-planar structures of biphenyl. Measurement of the various bond and torsion angles using Jmol will help students develop a sense of the distance dependence of the non-bonded interactions and their importance in determining the actual structure. They might also wish to consider what additional degree(s) of freedom resveratrol and stilbene have that biphenyl does not, allowing the trans-form of the former molecules to remain planar under certain conditions, while minimizing the effect of the non-bonded repulsions. Students can examine all these structures along with other molecules in the collection, at the JCE Digital Library Web site. Literature Cited- Catalán, J. Chem. Phys. Let. 2006, 421, 134–137.
- Choi, C. H.; Kertesz, M. J. Phys. Chem. A 1997, 101, 3823–3831.
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