Hydrogen Bonding Induced Nonlinear ν(CD) Infrared Cross Sections

The Beer-Lambert law is commonly used to extrapolate the concentration of analytes confined in porous metal organic frameworks (MOFs). However, analytes which hydrogen bond to MOF moieties can induce nonlinear infrared cross sections, or, deviations from the Beer-Lambert law. Here, we employ a combination of Fourier transform infrared spectroscopy and Density functional theory (DFT) techniques to understand the nonlinear infrared intensity behavior of CD₃CN diffused into UiO-67 MOFs. Ultra-High Vacuum (UHV) methods are employed to eliminate atmospheric contaminants. We devise a new method to allow vibrationally active probe molecules to diffuse through the MOF and preferentially adsorb to internal sites. We show that the infrared intensities of all ν(CD) modes decrease following diffusion into UiO-67. In other words, the infrared ν(CD) cross sections of CD₃CN adsorbed on the exterior of UiO-67 is significantly larger than that for CD₃CN adsorbed on the interior of UiO-67. DFT calculations show that the infrared ν(CD) mode intensities decrease due to a significantly reduced dipole derivative following adsorption to µ₃-OH sites. These results have implications on the use of integrated infrared intensity for discerning molecular transport kinetics into and out of MOFs.