Single-crystal samples of the semi-organic compounds mono-L-alaninium nitrate and monoglycine nitrate have been studied by Raman spectroscopy in a diamond-anvil cell up to 5.5 GPa, in order to observe the behavior of the deformation mode of NH 3 units. It was observed for these semi-organic crystals that increasing pressure produces a decrease in the wavenumber of the band associated with the deformation vibration, differently from most of the modes. Comparatively, mono-L-alaninium has a higher dν/dP than monoglycine nitrate, for the band associated with the deformation vibration. The anomalous behavior is explained in terms of the effect of high pressure in the short and linear intermolecular hydrogen bonds.

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