Remarkably strong polarization of amidine fragment in the crystals of 1-imino-1H-isoindol-3-amine

Results of the X-ray diffraction studies of 1-imino-1 H -isoindol-3-amine and its derivatives reveal unusual distribution of bond lengths within the NH 2 –C=N amidine fragment in the considered crystals. In the crystal phase, single C–N bond is shorter or at most has the same length as double C=N bond, while in isolated molecules both these bonds have expected lengths, according to the calculations by MP2/6-311G(d,p) and MP2/cc-pvtz methods. It is demonstrated that so unusually strong polarization of the amidine fragment is induced by polar environment in solid state. This effect stabilizes the amino-imino tautomer of 1-imino-1 H -isoindol-3-amine, which becomes more stable than the diimino tautomer possessing the lowest energy in the gas phase.