Reactions of Urea with Cu⁺in the Gas Phase: An Experimental and Theoretical Study

Abstract

The gas-phase reactions between Cu+ and urea have been investigated by means of mass spectrometry techniques. The primary products formed in the ion source correspond to [urea−Cu]+, [(urea)2−Cu]+, and [Cu+,C,N2,H2] complexes. The MIKE spectrum of [urea−Cu]+ complex shows several spontaneous losses, namely, NH3 and HNCO. A very weak peak corresponding to the loss of H2O is also observed, as well as a minor fragmentation of the adduct ion to yield Cu+. The structures and bonding characteristics of the different complexes involved in the urea−Cu+ potential energy surface (PES) were investigated using density functional theory (DFT) at the B3LYP level of theory and a valence triple-ζ. Attachment of Cu+ takes place preferentially at the carbonyl oxygen atom, while attachment at the amino group is 12.4 kcal/mol less exothermic. Insertion of the metal cation into the C−N bonds of the neutral is predicted to be slightly exothermic, in contrast with what was found for formamide and guanidine. The estimated urea−Cu+ binding energy (62.3 kcal/mol) is 6.0 kcal/mol greater than that of formamide. The exploration of the PES indicates that there are several reaction paths leading to the loss of ammonia yielding as product ions HNCOCu+ complexes where the metal cation is attached either to the oxygen or the nitrogen of the HNCO species. Also several reaction paths can be envisaged for the loss of HNCO, in which bisligated [HNCO−Cu−NH3]+ and [OC(NH)−Cu−NH3]+ complexes play an important role.