The CNEO-MD simulated IR spectra for both un-deuterated proline-D 0 and doubly deuterated proline-D 2 are shown in Figures ) and 2(b), respectively. For each species, we performed MD simulations for the lowest energy Type IIa conformer as well as the Type Ia and Type IIIa conformers. The Fourier transform is applied to the dipole autocorrelation functions of the MD trajectories to obtain their respective IR spectra. To compare with the experimental spectra, which are believed to be a mixture of mainly the Type Ia and Type IIa conformers, we also created mixed spectra for both proline-D 0 and proline-D 2 with a 50%-50% mixing ratio. In principle, if equilibrium is reached, the mixing ratio should follow a Boltzmann distribution. However, since the experiment involved first sublimating the amino acid-potentially breaking hydrogen bonds and allowing bond rotationand then cooling it down to a matrix environment, the relative ratio of different conformers formed and stabilized during the cooling process is uncertain. For qualitative investigation, we simply chose a 50% Type Ia and 50% Type IIa mixing ratio. Our peak assignments for the Type Ia and Type IIa conformers are shown in the spectra. CH stretch modes are of the main interest, we show the difference spectra focused in on the 2700 cm -1 to 3600 cm -1 range. For the Type IIa difference spectrum, a clear and broadened peak is observed with a maximum intensity at 3036 cm -1 , whereas the difference in this same region is essentially negligible for the Type Ia and Type IIIa conformers.