Gas-phase reactions between urea and Ca^2-: The importance of coulomb explosions

The gas-phase reactions between urea and Ca²⁺ have been investigated by means of electrospray mass spectrometry techniques. The MS/MS spectra of [Ca(urea)]²⁺ and [Ca(urea-H)]⁺ complexes show that both ions decompose by losing NH₃ and HNCO. However, for the [Ca(urea)]²⁺ system, additional intense peaks are observed at m/z 44, 56, and 82. Density functional theory calculations at the B3-LYP/cc-pWCVTZ level have been used to help rationalize these observations through an examination of the structures and bonding characteristics of the various stationary points on the [Ca(urea)]^2- and [Ca(urea-H)]⁺ potential energy surfaces (PESs). Analysis of the topology of these PESs allows mechanisms to be proposed for the loss of NH³ and HNCO. In addition, for [Ca(urea)]²⁺, the calculations suggest that the m/z 44, 56, and 82 peaks correspond to H²NCO^-, CaNH₂ ⁺, and [Ca, N, C, O]⁺, respectively, which are produced in Coulomb explosion processes. The unimolecular reactivity of [Ca(urea-H)] ⁺ differs from that of [Ca(urea)]^2- largely through the absence of the Coulomb explosion fragmentations. Urea behaves as an oxygen base with respect to Ca²⁺, the calculated binding energy being 453 kJ mol^-.