Orientation of H-bonds revealed by cryogenic FTIR spectroscopy

We propose a new way to reveal structural arrangement of hydrogen bond network using FTIR spectroscopy. When KBr pellet and fluorolube mull methods are used to prepare samples for IR measurements, the resultant spectra sometimes show significant differences, sometimes not. The reason is unknown. We use cryogenic FTIR spectra to prove pressure effect accounts for spectral variation in different sampling process. The sensitivity of the FTIR spectra to external pressure is related to the three-dimensional arrangement of hydrogen bonding network in the sample. When hydrogen bonds are in one plane, they can not resist external pressure applied perpendicular to that plane. Consequently, large differences appear between the spectra obtained using KBr pellet and fluorolube mull methods. On the other hand, when hydrogen bonds orientate toward various directions, the hydrogen bonding system can resist pressure from any directions. Thus, the FTIR spectra exhibit little variation using different sampling methods. We propose that the spectral variation obtained using above methods is a criterion to judge whether the hydrogen bonds orientate toward various directions in three-dimensional space or not.