Wechselwirkungen in kristallen, 95. - Formyl(2-pyridyl)amin: Monomer in der gasphase, dimer im kristall

The title molecule [HC]₄NC-NH-CHO (2), an α-aminopyridine derivative, forms a dimer via an hydrogen bridge pair (NH⋯N)₂ in the crystal. In the gasphase, however, it remains a monomer as proven by the well-resolved He(I) photoelectron spectrum and its Koopman's assignment, IE_n^v = -ε_J^SCF, of the radical cation state pattern. The enthalpy difference for dimerization is approximated by AM1 calculations to be rather small. Nevertheless, on crystallization either from an aprotic solution or by sublimation through the gasphase, due to the cooperative effect and to the density-dependent increase in lattice energy, a self-organization to the dimer is observed.