Infrared Diffusion‐Ordered Spectroscopy Reveals Molecular Size and Structure**

Inspired by ideas from NMR, we have developed Infrared Diffusion-Ordered Spectroscopy (IR-DOSY), which simultaneously characterizes molecular structure and size. We rely on the fact that diffusion coefficient of a molecule is determined its size through Stokes-Einstein relation, achieve sensitivity to creating concentration gradient tracking equilibration in an IR-frequency resolved manner. Analogous NMR-DOSY, two-dimensional IR-DOSY spectrum has IR frequency along one axis (or equivalently, size) other, so chemical compound are characterized simultaneously. In mixture, molecules with different sizes nicely separated into distinct sets peaks. Extending this idea higher dimensions, also perform 3D-IR-DOSY, combine conformation femtosecond multi-dimensional spectroscopy sensitivity.