Reports: UNI455504-UNI4: Cation-pi Interactions Involving Aromatic Hydrocarbons: A Quantitative Study of Substituent and Solvent Effects

Bright U. Emenike, PhD, State University of New York at Old Westbury

Cation-¹ interactions are applied in many areas of material science including the materials composed from petroleum products. Grant support from ACS-PRF has enabled my group to investigate the effects of solvation on the strengths of cationic methyl/aromatic interactions. In addition to measuring these interactions in solution, we successfully established quantitative relationships between the interaction energy and the properties of the local environment. In the first year of the grant-funding period, we successfully published one peer-reviewed manuscript, and in the second year, we have also published a peer-reviewed article and we have a third manuscript currently under review. These manuscripts directly relate to the objectives of the work described in the grant proposal. Undergraduate research students who were supported financially by the grant funds are included as co-authors on these articles.

To measure cation-¹ interactions in solution, we utilized synthetic N-arylimide molecular torsion balances, which are capable of adopting folded and unfolded conformations, as shown in the selected examples below. The ratio of the conformers provides a quantitative measure of the interaction energy (ΔG) as a function of solvation. In the folded state, the cationic CH donor forms an interaction with the naphthalene aromatic ring due to their close proximity. By analyzing the proton NMR spectra of the balances, the energy of the CH-¹ interaction (ΔG) was measured in various organic solvents, and the data was correlated to multivariate linear solvation energy relationship (LSER) with aims of establishing a mathematical model that could predict the observed experimental values.

We tested two LSER models: the Kamlet-Taft and the Hunter solvation models. In each model, the observed ΔG was correlated as a linear combination of solvent α and β parameters, where α = hydrogen-bond acceptor and β = hydrogen-bond donor. These models predicted the experimental ΔG values with high accuracy. That is to say, if the α and β values of a given local environment are known, then the interaction energy of the CH-¹ interaction associated with each of the balances can be predicted.

(a) The CH-¹ interaction as a function of solvation: In our first report, we utilized balance 1 to show that CH-¹ interaction can vary systematically between -0.90 kcal/mol to -0.22 kcal/mol by changes in the solvation. To this end, Kamlet-Taft's model provided an equation that predicted the relationship between ΔG and solvent parameters:

                                      ΔG = −0.24 + 0.23α − 0.68β − 0.1¹* + 0.09δ     

(b) Cationic CH-¹ interaction as a function of solvation: To study the effects of solvation in cation-¹ interaction, we measured the conformational preferences of the balance 2 in various solvents. Balance 2 is structurally equivalent to balance 1, but balance 2 is cationic. Therefore, relative to balance 1, changes in the conformational preferences of balance 2, is a direct measure of cationinteraction in solution. The measured ΔG was correlated to Hunter solvation model in which a linear relationship was established: ΔG = −0.69 + 0.12αs − 0.09βs.

(c) Solvent modulation of aromatic substituent effects: CH-p aromatic interactions are ubiquitous in nature and are capable of regulating important chemical and biochemical processes. Solvation and aromatic substituent effects are known to perturb aromatic interactions. However, the nature by which the two factors influence one another is relatively unexplored. Here we demonstrated that there is a quantitative correlation between substituent effects in CH-p interactions and the hydrogen-bond acceptor constant of the solvating molecule. Reliable measurements of the CH-p interaction energies were accessed by the conformational study of a series of aryl-substituted molecular balances. The observed conformational energy as a function of substituents and solvation was subsequently interpreted with the aid of Hunter's solvation model, which revealed a linear relationship between the rho values ρ (derived from the Hammett plot) and the hydrogen-bond acceptor propensity (βs) of the solvent molecule: ρ = 0.06βs − 0.04.

As result of these investigations, we have established collaboration with Dr. Matthew Zeller (in the Chemistry department at Purdue University) for solving the X-ray crystal structures of the balance system. During the second year of this grant, the PRF funds provided for materials, supplies, and undergraduate research stipends. Two undergraduate student researchers have been supported through funded positions during the academic year and summer months. These students presented their work at national ACS meetings, at local seminars at SUNY Old Westbury, and were co-authors on peer-reviewed journal manuscripts. Ultimately, the funding of the PI's and students' salaries in the summer enabled us to spend an extensive amount of time on this project that would not have been possible during the academic year.