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

Abstract

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

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

“…Using Δ p S° = 6.0 J · mol −1 · K −1 , the resulting experimental proton affinity is PA(NH 2 CN) = 800.9 kJ/mol (Table ), a value in correct agreement with our G4MP2 computation which give PA(NH 2 CN) = 803.9 kJ/mol (Table ). Computation also show that protonation at the cyano nitrogen is strongly favored against protonation at the amino nitrogen; this work). The difference amount to ∼105 kJ/mol at the G4MP2 level (Table ).…”

“…Note that protonation at the carbon atom of cyanamide would produce a cyclo-NH 2 CHN + species situated ∼220 kJ/mol above the linear protonated form NH 2 CNH + . 109 Thus the experimentally determined proton affinity of cyanamide unambiguously corresponds to a cyano-N protonation. As a consequence, the observation that PA(NH 2 CN) is larger than PA(CH 3 CN) by ca.…”

“…The difference amount to ∼105 kJ/mol at the G4MP2 level (Table ). Note that protonation at the carbon atom of cyanamide would produce a cyclo‐ NH 2 CHN + species situated ∼220 kJ/mol above the linear protonated form NH 2 CNH + . Thus the experimentally determined proton affinity of cyanamide unambiguously corresponds to a cyano ‐N protonation.…”

“…Moreover, it has been also detected in interstellar media and is suspected to be a key intermediate in prebiotic chemistry . Its gas phase ion chemistry has been investigated by various mass spectrometry techniques by Cacace and co‐workers . The authors were able to determine GB(NH 2 CN) by using the equilibrium method with isopropyl cyanide and o‐xylene as reference bases in an ICR apparatus.…”

“…108 Its gas phase ion chemistry has been investigated by various mass spectrometry techniques by Cacace and co-workers. 109 The authors were able to determine GB(NH 2 CN) by using the equilibrium method with isopropyl cyanide and o-xylene as reference bases in an ICR apparatus. Considering the presently accepted GB values of these two reference molecules, 6 an average value of GB(NH 2 CN) = 771.2 ± 2.3 kJ/mol is obtained.…”