Recent Research Developments

The strong and directional hydrogen-bonding interaction dominates the association of many molecules. Alcohols, having a hydroxyl head group attached to an organic tail, associate in both neat liquids and solutions with a variety of molecular aggregates formed based on molecular structure and thermodynamic state point (temperature, pressure, and composition). Alcohol aggregates in dilute non-polar solutions have found direct technological application due to their beneficial impact on the properties of the solution, such as enhancing a supercritical fluid's solvating power or solubilizing water contamination in fuel mixtures.

Fourier-transform infrared spectroscopy is a popular method for the experimental investigation of hydrogen-bonded aggregates, but linking spectral information to microscopic information on aggregate size distribution and aggregate architecture is an arduous task. A hydroxyl group participating as the donor in a hydrogen bond has a shifted vibrational frequency appearing as a broad band centered at 3350 cm^-1 as compared to a non-hydrogen-bonding hydrogen with a vibrational frequency of 3650 cm^-1. With the fraction of donating hydroxyl groups known through these quantitative spectral measures, various models must be employed to analyze the experimental results, the most prevalent being an assumption of an equilibrium between monomers and cyclic tetramer structures.

Graduate student John Stubbs John Stubbs (now on the faculty at Grinnell College) and Professor Ilja Siepmann have used a multi-scale modeling approach consisting of Car-Parrinello molecular dynamics (CPMD) simulations with explicit solvent molecules for specific aggregates selected from a large-scale Monte Carlo simulation to elucidate the vibrational spectrum of dilute solutions of 1-hexanol in n-hexane. In agreement with the experimental data, the computed spectra show three important features: a relative narrow “monomer-like” absorption peak, a very broad “polymer-like” absorption peak red-shifted by about 300 cm^-1 with a “dimer-like” shoulder that is red-shifted by about 150 cm^-1. These calculations demonstrate that the dangling hydrogen bonds found in all dimers and larger aggregates with linear or branched architecture absorb at the “monomer-like” frequency. The “dimer-like” shoulder arises mainly from non-cooperative hydrogen bonds formed by the next-to-last 1-hexanol molecule with an acceptor-only chain terminus of linear and branched aggregates consisting of four to six 1-hexanol molecules, whereas the absorption of the hydroxyl group donating the single hydrogen bond in “true” dimers is red-shifted to a smaller degree (only about 100 cm^-1) and does not contribute significantly to the overall absorption spectrum because of the relatively low abundance of dimeric aggregates. The CPMD simulations point to significantly different spectral features for aggregates of the same size but with different architecture, i.e. there is no simple way that would allow the deconvolution of experimental vibrational spectra to determine the contribution of specific aggregate sizes nor architectures. Thus, one needs to conclude that the distribution of alcohol aggregates in non-polar solvents is more complex than the often assumed equilibrium of monomers and cyclic tetramers.

A complete description of this research has appeared in J. Am. Chem. Soc. 127, 4722 (2005).

The development of advanced computational strategies for the most challenging problems in chemistry and chemical physics is a theme common to the research endeavors of the Minnesota Computational Chemistry Group, where research includes new theoretical formulations, the development of new computational algorithms, and use of state-of-the-art supercomputers to solve prototype problems to high accuracy and to predict chemically useful results for a wide range of system scales ranging from a few atoms to thousands of atoms.

Financial support from the National Science Foundation, Divisions of Chemical and Transport Systems and of Analytical and Surface Chemistry, and a DAAD Fellowship, a Frieda Martha Kunze Fellowship, a University of Minnesota Doctoral Dissertation Fellowship is gratefully acknowledged. Part of the computer resources were provided by the Minnesota Supercomputing Institute.

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