Effect of Crystal Packing on the Solid-State Spectral Properties of Vicinal Diketones

Abstract

X-ray crystal structures are reported for six substituted benzils: 4,4′-dimethoxy (1), 4-ethoxy (7), 4,4′-diethoxy (6), and the meso (4b), racemic (4a), and LL chiral (4c) bis-4,4′-(2,2′-dimethyl-l,3-dioxacyclopentyl-4-methoxy) benzils. The syntheses of 4a, 4b, and 4c are described as well as the separation of 4a and 4b. Crystal structure data follow. 1: a = 21.934 Å, b = 4.064 A, c = 15.208 Å, β = 102.11°, C2/c, Z = 4. 6: a = 14.512 Å, b = 4.795 A, c = 22.089 Å, β = 98.82°, C2/c, Z = 4. 7: a = 7.577 Å, b = 14.845 A, c = 12.153 Å, β = 97.63°, P2/n, Z = 4. 4a: 10.044 Å, b = 5.339 Å, c = 22.83 Å, β = 95.84°, P2/n, Z = 2. 4b: a = 22.826 Å, β = 11.067 Å, c = 9.506 Å, β = 97.03°, P21/c, Z = 2. 4c: a = 5.390 Å, 6 = 10.487 Å, c = 43.985 Å, P2121, Z = 4. The packing modes tend to be dominated by van der Waals close packing considerations and the preference for aligning substituted phenyl rings parallel to each other at about 3.45-3.70 A separation. Our results afford semiquantitative support for the proposition that the solid-state color of dicarbonyl compounds is red-shifted by increased overlap and blue-shifted by decreased overlap of the carbonyls. In the case of sheets of stacked substituted benzils, the increased size of the remote substituent is accommodated in the stack by increased twisting of the carbonyls while maintaining close packing of the phenyl rings. © 1983, American Chemical Society. All rights reserved.