Density Functional Theory Predictions of Noncovalent Hydrogen Isotope Effects during Octane Sorption to a Kaolinite Surface

Abstract

Interactions with mineral surfaces are crucial to the short- and long-term survival of organic compounds in the natural environment. The weak interactions that drive sorption of organic phases to mineral surfaces have the potential to impart H isotope effects that could complicate the interpretation of 2H/1H signatures in modern and ancient organic materials. The influence of hydrophobic interactions on H isotope signatures of n-octane during sorption to a kaolinite surface was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). These experimental results were compared to both harmonic and anharmonic vibrational frequencies calculated with density functional theory (DFT) to determine the role of anharmonicity in predicting isotope effects during noncovalent interactions of the molecule and mineral surface. The experimental results showed a relatively minor (<2‰) 2H-depletion of the sorbed octane, indicating that hydrophobic interactions do not impart significant isotope effects during sorption. Comparisons of the experimental and computational results highlighted the importance of anharmonic contributions to the direction of noncovalent isotope effects. Calculations that incorporated anharmonicity predicted a 3.5‰ depletion of sorbed octane, whereas purely harmonic calculations predicted a 0.9‰ enrichment. Differences in C–1H and C–2H bond lengths influence the molecular polarizability, surface area, and molar volume. The longer C–1H bonds are more hydrophobic and interact with the kaolinite surface more strongly, leading to a slight 2H depletion in sorbed octane. Harmonic approximations cannot account for differences in bond length and are insufficient for predicting noncovalent isotope effects caused by hydrophobic intermolecular interactions.