Fig. 3.

Spectroscopic signatures of proton activity using isotopic scrambling. (Left) Isotopic exchange from surface proton hopping is reflected by loss (D₂O: 2,725 cm⁻¹) and gain (HDO: 2,712 cm⁻¹) of intensity of the bands of unique surface-dangling O–D bonds. (Right) Interior proton motion causes a much slower conversion of the isolated-D₂O doublet (2,448 and 2,370 cm⁻¹) to bands of an HDO pair having two coupled OD bonds in tandem. The prominent HDO singlet (2,420 cm⁻¹) is produced by subsequent HDO rotation from passage of an orientational defect that decouples the O–D bonds (17). For the ice nanocrystal interior, the passage of orientational defects is fast compared with proton hopping (17) so the bands of the coupled HDO (2,442 and 2,400 cm⁻¹) are hidden throughout by the more intense isolated HDO band at 2,420 cm⁻¹. Less than a minute is required to complete the surface exchange at 140 K, whereas 30 min at 145 K is required to convert most of the interior D₂O to HDO.