Raman studies of cyanate: Fermi resonance, hydration and hydrolysis to urea

Abstract

Raman spectra were measured for potassium cyanate in the solid phase and as aqueous solutions in H2O and D2O for freshly prepared and for aged solutions. The results indicated that the assignment of the Fermi doublet, ν1 and 2ν2, for solid potassium cyanate was reversed from the assignment for the aqueous solution. The Fermi doublet has an associated pair of hot bands at 1191 and 1315 cm−1 which originate from the 638 cm−1 ν2 state, 010. Assignment of the hot bands was confirmed by studies of solid potassium cyanate at liquid-N2 temperature, room temperature, and at 473 K. Raman spectra of aged aqueous potassium cyanate revealed that the cyanate ion hydrolyzed slowly and spontaneously at room temperature (even without added ammonium) to produce urea and a carbamate, carbonate equilibrium mixture in parallel reactions. Hydrolysis of cyanate in aqueous ammonium chloride solution resulted in almost total conversion of cyanate to urea. The reaction was not reversible under ambient conditions. Differences in peak frequencies and half-widths were observed for the cyanate dissolved in H2O compared to solutions in D2O. The results provide evidence for strong hydrogen bonding of cyanate to water and are consistent with greater structure in the D2O solution. Theoretical ab initio calculations indicated that the water molecules hydrogen bond well at both the oxygen and nitrogen atoms of cyanate although the hydrogen bond to nitrogen was found to be slightly stronger.