Knowledge Bank

From x-ray diffraction studies $HCrO2$ is $reported1$ to belong to the trigonal system and to have one molecule per unit cell with a hydrogen bond of length 2.54 \AA. Its infrared spectrum is consistent with the proposed structure and with the short hydrogen bond However, the spectrum of $DCrO2$ dues not correspond to an $HCrO2$ spectrum which has been modified simply by a mass change. We find: (1) the ratio of the stretching frequencies $\nu OH/$OD is near unity (theoretical 1.38);(2) the ratio of the bending frequencies $\nu \angle OHO/\nu \angle ODO$ exceeds considerably the theoretical value (observed 1.47: theoretical 1.39); (3) the O-H stretching band is a singlet but the O-D stretching hand is a doublet; (4) in the lattice mode region (700-$300cm-1$) there are two bands for $HCrO2$ but there are five for $DCrO2$). The above findings become explicable if we assume that the potential energy curve for the hydrogen bond is altered in going from $HCrO2$ to $DCrO2$. Specifically we require for $HCrO2$ that the potential barrier separating the two minima be lower than the ground vibrational level of the proton; for $DCtO2$ we require that the Lanier be considerably higher than the ground level for the deuterium Such a change in the height of the hairier is reasonable if the oxygens of the hydrogen bond are separated more in $DCrO2$ than in $HCrO2$. We have measured the lattice constants of $HCrO2$ and $DCrO2$ and find significant differences which if interpreted as arising solely from a change in the O-O distance, indicate a 0.04 expansion in that distance in going from $HCrO2$ to $DCrO2$, This expansion appears sufficient to lead to a significantly weaker hydrogen bond in $DCrO2$.