High-pressure raman study of hydrogen bonds in crystalline formamide

Abstract

The high-pressure Raman spectra of hydrogen-bonded crystalline formamide (HCONH2 and HCOND,) have been measured up to 10 GPa at 300 K in a gasketed diamond anvil cell. The frequencies of the intermolecular modes which are characteristic of the hydrogen-bond along the zig-zag chain increase with increasing pressure with a slope (dv/dP) of 11.1 cm'/GPa, and its slope changes steeper, to 19.2 cm^-1/GPa, at about 5.0 GPa (= P_c). The frequencies of the intramolecular ND2 stretching mode decrease with increasing pressure at all pressure range except around P_c. where they show an increase of 20cm^-1 It turns out from these results that the hydrogen-bonds along the chain direction become stronger in response to the pressure, and their increasing strength is more developed above P_c, and the other type of hydrogen bonds which bridge the chains to make their planar hydrogen bonded network is weakened around P_c.