The Structure of Liquid Formamide Studied by Means of X-Ray Diffraction and ab Initio LCGO-MO-SCF Calculations

The molecular and liquid structures of formamide have been studied by means of X-ray diffraction. Relative stabilities of aggregates of formamide molecules have been calculated by using the ab initio LCGO-MO-SCF procedure and minimal basis sets. The atomic distances within a formamide molecule and the intermolecular N···O distance between the hydrogen–bonded molecules are determined by the X-ray diffraction experiment as follows: C=O: 1.24(1) Å, C–N: 1.33(1) Å, N···O (intramolecular): 2.25(2) Å, and N···O (intermolecular): 3.05(5) Å. The intermolecular (Remark: Graphics omitted.) bond angle (Θ) is about 120°. It is concluded from the X-ray diffraction experiment that liquid formamide mainly consists of the chain-like hydrogen–bonded structure of formamide molecules by combining through –NH2···O=CH– interactions. The conclusion is supported by the ab initio calculations. Formation of ring-dimers in the liquid formamide has not been confirmed by the X-ray diffraction study, although a possibility of the formation of the ring-dimer structure as another constituent of liquid formamide is suggested from the ab initio calculations.