Professor Ward Thompson of the University of Kansas is receiving an award from the Macromolecular, Supramolecular and Nanochemistry Program. Theoretical and computational approaches are used in the awarded project for inquiring what vibrational spectroscopic techniques can reveal about the properties of liquids and solutions confined in nanoscale silica pores of varying size (~2.4 to 4.5 nm in diameter) and surface functionality (hydroxyl- and alkyl-terminated). Dramatic changes in the molecular-level liquid structure and dynamics occur upon such nanoscale confinement, yet these effects are not readily observable in the linear infrared (IR) spectra. By analyzing vibrational spectroscopic probes, this project sheds light on the complex structure and dynamics of nanoconfined liquids. The project focuses on confined acetonitrile and related systems. The application choice is based on the presence of a dramatic blue shift in the CN stretching frequency upon hydrogen bonding and the relatively slow vibration relaxation, which permits examination of longer timescale dynamics. A combination of molecular dynamics, grand canonical Monte Carlo simulations, electronic structure calculations, and mixed quantum-classical MD are the chosen methodologies. The proposed properties to be studied include: i) determination of the relative intensities of hydrogen bonded and non-hydrogen bonded peaks in the linear infrared spectra of nitriles and isonitriles, ii) prediction of the IR pump-probe spectroscopy of CH3CN confined in silica pores of varying surface chemistry, iii) prediction of the IR photon echo spectroscopy of CH3CN confined in silica pores of varying surface chemistry, and iv) simulation of the spectroscopy of solutes in nanoconfined CH3CN and of a CH3CN solute in other nanoconfined liquids.
Porous silica materials are of interest in catalysis, separations, and sensing and are part of a wider class of porous oxide materials similarly important in a variety of applications. Moreover, nanoconfined liquids are present in a number of other systems including supramolecular assemblies, templated materials, reverse micelles, biological systems, hydrogels, membranes, fuel cell electrodes, and nonlinear optical materials. The broader impact aim of this project is to gain deeper mechanistic understanding of how liquids move and interact within nanoconfined structures and how these mechanisms can be probed via spectroscopy to assist in the design and characterization of applications that exploit the unique physical properties of liquids in confined environments. Graduate and undergraduate students are involved in this research, providing them with training in theoretical and computational techniques and a broad background in physical chemistry. The participation of underrepresented groups continues to be encouraged within this research group.
This project used computational modeling to improve our understanding of liquids confined within nanoscale pores, that is ones that are roughly five to ten molecules across. There are now many ways to synthesize such mesoporous materials; the focus of this work was on porous glass (silica) which are of interest for many applications such as catalysis, water purification, and drug delivery. In rationally designing a porous silica for a particular use it is important to know how the behavior of the liquid inside the pores is changed compared to the normal, or "bulk," liquid. Typically spectroscopy is used to probe the properties of the confined liquid through the changes in its interaction with light of varying wavelengths. A particularly important example of this is infrared spectroscopy, which is sensitive to the vibrations of the molecules in thie liquid that are themselves sensitive to both the environment around and the motions of the molecule. The aims of this project were to understand what infrared spectroscopy can (and cannot) tell us about the nanoconfined liquid as well as how such information about the liquid can be extracted from measured spectra. The studies focused primarily on liquid acetonitrile (CH3CN), which is an important organic solvent but also has useful vibrational properties. (A picture of acetonitrile confined inside a silica pore, taken from one of the simulations in this work, is shown in the accompanying figure.) In particular, the stretching of the CN, or "nitrile," chemical bond is often used to probe the environment of this molecule and related ones with the same group. In this work the diffusion and rotation of acetonitrile molecules confined inside nanoscale silica pores were examined using computer simulations. These motions can be probed by different infrared spectroscopy techniques. The results predicted mechanisms for these motions as well as time scales. The latter can be quite long. For example, rotation of an acetonitrile molecule in a silica pore can take as much as 1000 times longer than in the normal liquid. Some of the features of this slower rotational motion should be observable using a particular version of infrared spectroscopy. Rotational and translational (diffusive) motions of liquid molecules are key components of the function of porous materials in all of the applications mentioned above. Thus, this work has provided insight into not only how those mtions are changed due to the confinement but also how they might be best measured. In addition, the behavior of other molecules dissolved in confined acetontrile were examined. There the aim was to understand where those molecules prefer to reside in the silica pore (which might be probed with infrared spectrocopy) and how that depends on the properties of the molecule. This is important in understanding how porous materials would function in applications that required a dissolved molecule reach the pore surface, for example, in a catalyst to undergo a chemical reaction. The results show that where a molecule spends time in the confined acetonitrile depends strongly on its interactions with the pore surface and the contributions of several different factors were determined. Because the pore surface can have different chemical groups on its surface depending on how the material is synthesized, this can provide guidance for controlling molecule positions.
