Structures of the E,Z (cis–trans) isomer of diacetamide and the 1:1 complex with acetamide at 123 K. Ab-initio molecular orbital calculations on the Z,Z (trans–trans), E,Z (cis–trans) and E,E (cis–cis) isomers of diacetamide

Abstract

The crystal structure of the E,Z (cis-trans) isomer of diacetamide, N-acetylacetamide, C4HvNO 2, has been refined using X-ray diffractometer data at 123 K. The crystal data are P2l/n, Z=4, a=4.069 (1), b= 16.818 (5), c = 7.604 (2) A, fl = 94.10 (3) °. The final agreement factors are R(F) = 0-050, wR(F) = 0.041 and S= 1-40 with 1384 observed reflections. The structure is disordered with an 8.2% minor component in which the molecules are appoximately related to those in the major site by a 180 ° rotation about the N1-C2 bond followed by a 51 ° rotation in the mean molecular plane about an axis through the central N atom. The crystal structure of the 1:1 complex of (E,Z)-diacetamide with acetamide, C4HvNO2.C2HsNO, is monoclinic, P2~/m, Z=2, a=7.695(9), b=6.443(8), c=8.918(9)A, fl= 108.90 (9) ° at 123 K. The structure was solved by direct methods and refined to R(F) = 0.059, wR(F) = 0.049, S = 1.61 with 1275 observed reflections. Ab-initio molecular orbital calculations at the HF/3-21G level gave energies for the Z,Z (trans-trans) and E,E (cis-cis) isomers of diacetamide of 24.6 and 45.8kJmol -I respectively higher than that of the most stable E,Z isomer. The largest difference in calculated bond lengths between the isomers was 0-014 A between the C-N bonds in the E,Z and Z,Z isomers. The theoretical calculations indicate that the significant differences in the C-N and C=O bond lengths observed in the crystal structures of diacetamide and acetamide are due, in part, to the differences in intermolecular hydrogen bonding of the NH, NH 2 and C=O groups.