Adamowicz and co-workers originally proposed that this signal appears in the C-H stretching region and is thus obscured by the C-H stretch peaks. This assignment was based on the experimental difference spectrum between proline (proline-D 0 ) and doubly deuterated proline (proline-D 2 , with N-H and O-H deuterated), which shows a broad peak centered at 3025 cm -1 . Computationally, density functional theory (DFT)-based harmonic analysis with an empirical scale factor of 0.92 yielded a frequency of 3031 cm -1 , which further supported this assignment. However, pure DFT-based harmonic analysis is unable to capture anharmonicity and NQEs, and is of questionable reliability when using empirical scale factors to account for these effects.