The Secret Behind Hydrogen Bond Blueshifts in Nitriles

Ever wondered why hydrogen bonds in nitriles cause strange shifts in their vibrational spectra? It turns out, the answer lies in the geometry and dynamics of these bonds. Scientists have been using a method called vibrational Stark effect (VSE) spectroscopy to study noncovalent, electrostatic interactions. Nitriles (C≡N) are often used as probes in this method, but they have a peculiar issue—an unusual hydrogen bond (HB) blueshift. This blueshift isn't explained by the current VSE framework. A new empirical model has been created to explain this anomaly. This model suggests that the blueshift happens due to the distance and angle of the HB with respect to the C≡N group. The model combines data from density functional theory and electrostatics from the AMOEBA force field. It shows that the blueshift is caused by multipolar and Pauli repulsion effects. When compared to experimental results, this model was spot on for rigid HBs, like those in proteins or chemical frameworks. However, in more dynamic environments, like protic solvents, the blueshift depends on more than just geometry. This is due to something called motional narrowing, which was demonstrated by simulating IR spectra. Overall, when both geometry and dynamics are considered, the observed and predicted blueshifts match perfectly. This means the model could help us understand HB blueshifts in various nitriles and HB donors.