Gas-phase acidity of urea

Abstract

The kinetic method was applied to the estimation of the gas-phase acidity of urea by generating proton-bound dimeric anions comprised of urea and various reference compounds using negative ion desorption chemical ionization and examining their dissociation upon collision in a triple quadrupole mass spectrometer. The individual mass-selected dimers fragment to yield the two deprotonated anions as the only product ions. From the relative product ion abundances, ΔHacid is determined to be 1513.8±12 kJ mol-1, ΔSacid is 85.8±8 J mol-1 K-1 and ΔGacid is estimated to be 1488.2±14 kJ mol-1 at 298 K by using ΔGacid=ΔHacid-TΔSacid, where the ΔHacid and ΔSacid values are determined directly from the kinetic method plots. The experimental ΔHacid value is in good agreement with a value of 1520.5 kJ mol-1 obtained from G2 molecular orbital calculations. © 1998 John Wiley & Sons, Ltd.