GAS-PHASE ION CHEMISTRY OF CYANAMIDE - A MASS-SPECTROMETRIC AND ABINITIO 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 +/- 2 kcal mol-1, deduced from ICR equilibrium experiments is consistent with the Gaussian-IX value, 191.7 +/- 2 kcal mol-1, which refers to formation of the most stable protomer, identified as the H2N-CNH+ ion having the structure of cyanamide protonated at the N atom of the cyano group. Calculations performed at the MP4/6-311G**//MP2/6-31G* + ZPE(RHF/6-31G*) 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-1, respectively, than the most stable H2N-CNH+ species. From the stability difference between cyanamide and carbodiimide, the PA of the latter is estimated to be 198.1 +/- 2 kcal mol-1. The results of experiments aimed at evaluating the acidity of 1 by the ICR equilibrium method give a gas-phase PA of the [HNCN]- anion of 349.8 +/- 4 kcal mol-1, in excellent agreement with the Gaussian-IX theoretical value of 347.8 +/- 2 kcal mol-1.