Gas-phase ion chemistry of cyanamide. A mass spectrometric and ab initio study of gaseous [H2N-CN].+, [H2N-CN]H+, and [HN-CN]- ions

The gas-phase ion chemistry of cyanamide (1) has been investigated by EI, positive- and negative-ion CI, ICR, MIKE, and CAD mass spectrometry, as well as by theoretical calculations at the Gaussian-1 level. Attention has been focused on the CH2N2'+ molecular ion, on the structure and the relative stability of the conjugate acids of 1, and on the HNCN- ion. The experimental proton affinity (PA) of 1, 194.5 f 2 kcal mol-', deduced from ICR equilibrium experiments is consistent with the Gaussian-1 value, 191.7 f 2 kcal mol-', which refers to formation of the most stable protomer, identified as the HzN-CNH+ ion having the structure of cyanamide protonated at the N atom of the cyano group. Calculations performed at the MP4/6-3 1 lG**//MP2/6-3 lG* + ZPE (RHF/6-3 1G*) level of theory have identified two additional protomers, having the structure of cyanamide protonated at the amino group and at the C atom, less stable by 22.4 and 54.0 kcal mol-', respectively, than the most stable HzN-CNH+ species. From the stability difference between cyanamide and carbodiimide, the PA of the latter is estimated to be 198.1 f 2 kcal mol-'. The results of experiments aimed at evaluating the acidity of 1 by the ICR equilibrium method give a gas-phase PA of the [HNCNI- anion of 349.8 f 4 kcal mol-', in excellent agreement with the Gaussian-1 theoretical value of 347.8 * 2 kcal mol-'.