H-bonding in amorphous acetamide CH_3CONH_2 as studied by x-ray scattering

摘要

Noncrystalline solid acetamide (CH_3CONH_2) can be prepared by vapor deposition onto precooled substrates by 77 K. The structure factor S_M(Q) derived from x-ray diffraction patterns is compared to that recently obtained for liquid acetamide near athe melting point. The data are analyzed to yield the molecular structure factor F_1(Q) and the distinct pair correlation function g_L(r). Our measurements clearly show that the hydrogen bonded network occurs in amorphous acetamide between C= O and N-H groups. Two H-bonds per molecule are found in average. The mean N…O distance is about 3.01 A. The results could be explained in terms of an open-chain structure which resembles preferentially the one existing in the metastable crystalline form. The crystallization of the glassy film which occurs on heating is also studied.