The Effects Of Noncovalent Interactions Studied By Ultrafast Two-Dimensional Infrared Spectroscopy
Understanding how noncovalent interactions affect dynamics and energetics is essential for predicting and describing chemical reactivity and stability. In this thesis I investigate the impact of noncovalent interactions on two different types of reactive systems: fluxional metal carbonyl complexes and biological cofactors. Through varying the local environment, by changing solvents or encapsulation cages, I focus on characterizing the effects of noncovalent interactions on the ultrafast solvation dynamics, anharmonicities, and intramolecular vibrational energy redistribution (IVR) of these different types of systems by applying ultrafast two-dimensional infrared (2DIR) spectroscopy and ultrafast polarization dependent mid-IR pump-probe spectroscopy. Bis(cyclopentadienyl ruthenium dicarbonyl) dimer and bis(cyclopentadienyl iron dicarbonyl) dimer are two fluxional metal carbonyl complexes that exist as multiple isomers in dynamic equilibrium in solution. However, when encapsulated by self-assembled cages, the dynamic equilibrium is disrupted, and only one isomeric form is stabilized. Though the static picture of the trapped complexes is known from x-ray crystallography, the ultrafast equilibrium dynamics of the trapped complexes have not been well characterized. A better understanding of the equilibrium dynamics of these systems could lead to additional insight into the mechanism by which hosts tune the reactivity of trapped guest molecules. Using polarization dependent mid-IR pump-probe spectroscopy I characterized the population relaxation and molecular reorientation of the trapped complexes. I find that encapsulation results in spatial restriction – with the metal carbonyl complexes undergoing hindered molecular reorientation. In addition, encapsulation results in a faster timescale for vibrational population relaxation. Applying ultrafast 2DIR spectroscopy to the diiron complex I explore the effects of encapsulation on the intramolecular vibrational energy redistribution and the energetics of the terminal carbonyl stretching modes. The timescale for IVR is extracted from the waiting time dependent growth of crosspeaks in the 2DIR spectra. Through comparative studies, I find that IVR is faster for the iron carbonyl complex trapped by the cage. In addition, the cage acts to alter the energetics with the trapped metal carbonyl complexes exhibiting larger anharmonicities. In addition to metal carbonyl complexes, I also investigated the effects of hydrogen bonding on phylloquinone, a biological cofactor that plays an important role in photosynthetic electron transfer. Previous studies have shown that the local protein environment can tune the redox properties of phylloquinone; however, the exact mechanism and role of different noncovalent interactions in this process is not well understood. To obtain a better understanding of how hydrogen bonding alters the properties of phylloquinone I applied 2DIR spectroscopy to the carbonyl stretching modes of phylloquinone in protic and aprotic solvents. From the 2DIR spectra, the potential energy surface, population relaxation, and system-bath interactions of PhQ in different solvent environments are characterized. The results are interpreted through molecular dynamics simulations and DFT calculations. I find that hydrogen bonding acts to decouple the carbonyl stretching modes and decreases the vibrational anharmonicity.
Joseph E. Subotnik