High-pressure Raman study of the hydrogen-bonded crystalline formamide

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Abstract
The Raman spectra of hydrogen‐bonded crystalline HCONH2 and HCOND2 have been measured up to 10 GPa at 300 K in a gasketed diamond anvil cell. The frequencies of the in‐plane intermolecular stretching mode which characterizes the hydrogen bond along the zig–zag chain increase with increasing pressure with a slope (dν/dP) of 11.1 cm−1/GPa, and its slope becomes steeper, to 19.2 cm−1/GPa, at about 5.0 GPa (=Pc). The frequencies of the intramolecular ND2 stretching mode decrease with increasing pressure at all pressure range except around Pc where they show an increase of 20 cm−1. It turns out from these results that (1) the hydrogen (deuteron) bonds along the chain direction become stronger in response to the pressure, and their increasing strength are more developed above Pc, and (2) the other type of hydrogen (deuteron) bonds which bridge the chains to make their planar hydrogen‐bonded network are weakened around Pc. The pressure‐induced phase transition at Pc is discussed by focusing on the behavior of two types of hydrogen bondings and the effect of the interplanar interaction.

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