Research

Our research is aimed at understanding the fundamental chemical interactions between molecules. These interactions play important roles in solvation, purification, and separation processes. They can also alter reaction rates and product distributions, as well as impact the spectroscopic characteristics of molecules. We are interested in characterizing the interaction abilities of molecules both when they act as dilute solutes as well as bulk solvents. Thus, we are pursuing two major lines of research -- using gas chromatography to measure solute properties and using UV-visible spectroscopy to measure solvent properties.

Using Gas Chromatography to Measure Solute Properties:
In one are of our research, students are using gas chromatography (GC) to characterize the ability of individual solute molecules to participate in common intermolecular interactions such as dipole-dipole, London dispersion, and hydrogen bonding interactions. This work is based on the linear solvation energy relationship (LSER) model which is commonly used to study retention in gas chromatography:

Ix = pPx + dDx + vVx + aAx + bBx

where Ix is the Kovats retention index of solute X, and Px, Dx, Vx, Ax, and Bx are the solute’s polarizability, dipolarity, volume, hydrogen bond acidity, and hydrogen bond basicity, respectively. The coefficients p, d, v, a and b are sensitivity factors that reflect the relative importance of each type of interaction in determining the retention of solutes in GC columns with different stationary phase chemistries.
The goal of the research is to determine the parameters P, D, V, and A for common small organic compounds. This is done by choosing 3 reference compounds and measuring the parameters for other solutes relative to these reference compounds. Cyclohexane is used as a non-polar, non-hydrogen bonding solute. Benzonitrile is used as a polar, non-hydrogen bond donating reference compound, and phenol serves as the hydrogen bond donating reference solute. Since solute retention in the GC columns used in these studies is insensitive to solute hydrogen-bond basicity, and because good solute parameters for hydrogen bond basicity already exist, this parameter is not determined in these studies.
To determine the parameters for an individual solute, its Kovats retention indices on a minimum of four GC stationary phases of different composition (i.e. columns that are non-polar, polar, and hydrogen bond accepting) are measured and regressed against the Kovats retention indices for the reference compounds on the same columns according to the equation:

Ix = C1 + C2Icyclohexane + C3Ibenzonitrile + C4Iphenol

The coefficients C1, C2, C3, and C4 are determined by a multiparameter linear least squares regression and are mathematically related to the solute’s size, polarizability, dipolarity, and hydrogen bond acidity, respectively. Thus, from the coefficients it is possible to quantify a solute’s ability to interact with other molecules through fundamental chemical forces. By repeating the method for a variety of solutes, the solutes can be ranked relative to each other in terms of their interaction abilities. Most importantly, these parameters can be used in other linear solvation energy relationships to determine the role of the different interactions in determining solvent-dependent processes such as retention in chromatography, shifts in spectroscopic absorbance bands, and changes in reaction rates with changes in the reaction solvent.

Goals of the Research
1) This methodology has been used to determine parameters for fifty-two solutes using data collected on seven stationary phases at 80oC. Mathematically, a minimum of four columns is required. The use of seven columns thus represents an overdetermination of the system. While the use of additional columns will increase the reliability of the parameters, it also represents a significant increase in the amount of time required to obtain parameters. Thus, our first line of investigation is to identify a suite of judiciously selected GC columns comprised of the minimum number of columns that yields reliable parameters.

2) We are also investigating the effect of temperature on the parameters. We are interested in comparing parameters that have already been determined at 80oC to those determined at other temperatures.

3) Once the optimum GC conditions (temperature, columns, etc.) have been determined, our goal is to measure the solute parameters for the literally hundreds of compounds commonly used in LSER studies.

Aim of the Project
Ultimately, we aim to apply these parameters to understand solvation-dependent processes such as retention in chromatography, octanol-water partitioning, and gas-water partitioning. While LSERs already exist for these processes, they were determined using older, less illuminating parameters. Therefore, the chemical interpretation of these LSERs is not entirely complete. In fact, three of the five parameters used in these LSERs are chemically related and describe very similar interactions, ultimately meaning that they do not describe any one interaction distinctly. The newer parameters developed in this work are more chemically distinct, meaning that LSERs based on these new parameters will be more readily interpreted and more illuminating in terms of the chemical interactions governing the processes being studied.

Using UV-Visible Spectroscopy to Characterize Solvents
In a related line of research, we are investigating the changes in the UV-visible spectroscopy of organic dyes dissolved in water as a function of the temperature of the water. In this work we are using the Kamlet-Taft solvatochromic comparison method to determine the ability of water to interact through dipolarity/polarizability and hydrogen bond donating and accepting interactions. Since hydrogen bonding is temperature dependent, it is anticipated that the spectroscopy of organic dyes which can interact through hydrogen bonds will also change as a function of temperature when they are dissolved in water. These changes in the spectroscopic behavior of the dyes can be used to quantify the changes in the hydrogen bond ability of water as the temperature increases. This work is being supported through a grant from the Iowa Space Grant Consortium.
This line of research will be extended to the study of starburst dendrimers. In studies similar to those conducted on surfactant micelles, we will use the solvatochromic comparison method to probe the environment of organic dyes dissolved in solutions containing dendrimers. This work is being done in collaboration with Professor Lifang Sun at the University of Alberta.

Student Involvement:
These projects are being pursued with undergraduate students. Their involvement in the research familiarizes them with the chemical literature, chromatographic and spectroscopic techniques, critical data analysis, and the presentation of scientific results in both written and oral formats.