Experimental and Theoretical Investigation of the Pyrolysis Products of Iminodiacetonitrile, (NCCH₂)₂NH

The gas-phase FTIR study of the pyrolysis products of iminodiacetonitrile, (NCCH₂)₂NH has revealed the existence of C-cyanomethanimine, NCCHNH, and ketenimine, CH₂CNH. The former has two isomers: Z and E; while the later readily taumerizes to acetonitrile, CH₃CN. A trapping/revaporization system has been used to purify C-cyanomethanimine. The analysis of the rotational structures of the IR medium resolution C-type CNH bend, ν₆, and CN torsional mode, ν₁₀, has led to a conformational characterization of these isomers. The Z-isomer was shown to be the major product. This conjecture was supported by ab initio MO calculations that confirmed the relative total energy stability of the Z-isomer over its E-counterpart by 0.173 to 2.326 kJ/mol. The K values indicated that the equilibrium concentration of Z-C-cyanomethanimine amounts to up to three times that of E-C-cyanomethanimine. A further investigation using NBO technique proved the predilection of the Z-isomer. In addition it relates its provenance of preference to the remote n_N6 → σ*_C4–N5 interaction that stabilized it by 1.10 kcal/mol. A thorough theoretical investigation of the tautomerization reaction between ketenimine and acetonitrile will be published in a separate contribution.