Computational Chemistry Exercise: cis, trans Temperature Dependence of a Copper(II) glycine Complex
Part 1: Reaction Pathway analysis via semi-empirical calculations
Chemical reactions forming chiral complexes often have different energy pathways. The two stable chiral products of the bis-Copper(II) glycine complex formation produces an intermediate that can form two different transition states, leading to the formation of two different chiral products. The amount of product formed depends on the energy relationship between all of the species in the reaction pathway.
Figure 1. Reaction pathways for Cu(II) glycine cis and trans products
The two products are labeled K (kinetic) and T (thermal) and have a common intermediate, but has an Ea for T that is higher than for K. Furthermore, The total energy change of the product T is much higher than for K so that the backward reaction energy barrier for K is less (to the intermediate, I) than for T. This means that at moderate temperatures, K forms more readily than T, producing this particular product in abundance. This occurs even though the product K is higher in energy than T (final energy).
If the reaction is run at a temperature where there is an energy Et(K) available, but not Et(T), then there is enough energy for the back reaction of K to occur, but not for T. Since Et(K) is larger than the EaT, then any K that forms can back-react to the intermediate and re-form as T, but T cannot back react, and a preponderance of T forms at the expense of K.
Since Et(K) and Et(T) are not known, the reaction cannot be driven to a particular product. However, if the reaction is run at sufficiently high temperatures, so that at least Et (T) is available, then all reaction pathways can be accessed ( both T and K can back-react to form the intermediate) and the product distribution depends on the thermodynamic stability of the products K and T. Since T is lower in final energy than K, the thermodynamic product is the predominant one under these conditions.
Procedure: Open ChemDraw on the PC and open the file cis_glycine.cd3. This is the beginning structure for the complex of one steric conformation. Select the Analyze menu bar and then select compute properties. There will be a set of parameters displayed on a menu:
select a minimum of Total energy, torsional energy, bend energy, stretch energy, and the dipole moment as parameters to calculate.
Run the computation and record the results. Repeat the experiment with the second compound (trans) and then compare the values for each parameter determined. Determine the stability (at room temperature) of the isomers from the tabulated data.
Predict the predominant isomer produced at room temperature.
Determine the symmetry point group of each isomer. From the appropriate symmetry table, determine the number of IR bands for each complex. Assign the predicted bands asymmetric and symmetric for the cis isomer (M-N and M-O), and asymmetric for the trans isomer. Assign the peaks on the IR spectra provided and compare them with your predicted assignments. Can you differentiate between the complexes based on the IR spectra?
cis and trans structures