Ultrafast Hopping from Band to Band: Assigning Infrared Spectra based on Vibrational Energy Transfer

摘要

Infrared spectroscopy is widely used in fundamental and applied chemical research. Applications cover tasks in analytical chemistry, such as process monitoring or product identification,as well as cutting-edge studies elucidating reaction mechanisms or resolving ultrafast biomolecular dynamics and functions. Typically, analysis of an IR spectrum starts with assignment, that is, with establishing a connection between the bands observed in the spectrum and the moieties of the molecule that are involved in the corresponding vibrations. For small molecules, vibrations often can be assigned based on experience, aided by databases that list functional groups and the typical wavenumber range of their absorption bands. However, the database approach quickly becomes unfeasible if several groups are present that cause vibrations in the same wavenumber range or if a certain group is present several times in a molecule. A way to obtain assignment information in such cases is isotope labeling, which might involve complex synthesis. Very frequently, quantum-chemical computations are used for assignment. However, even for seemingly simple cases, contradicting results can be obtained from different computational methods, as illustrated by the results presented herein. Furthermore, there are cases where the computation of vibrational modes can be intrinsically difficult, such as excited states or reaction mixtures containing unknown species. A broadly applicable experimental approach to aid assignment is therefore highly desirable.