Urea's match in the hydrogen-bond network? A high pressure THz study

We present results of the measurement of the low frequency spectrum of solvated urea. The study revealed a blue shift of the intramolecular mode of urea centered at 150 cm^-1 of Δν= 17 cm^-1 upon increasing the pressure up to 10 kbar. The blue shift scaled linearly with the increase in density and was attributed to a stiffening of the water-urea intermolecular potential. We deduced an increase in the number of affected water molecules from 1 to 2 up to 5-7, which corresponds to the sterical coordination number of urea. The increase in hydration number can be explained by an suppression of the NH₂ inversion and the hydrogen bond switching around the NH₂ group. Pressure induced sterical constraints are proposed to hinder the rapid switching of hydrogen bond partners and make the water around urea less bulk-like than under ambient conditions.