X-ray crystallographic
analysis has revealed that the light-sensitive form (A) and the light-stable
form (A') of tetrabenzoylethylene are not valence tautomers but are two
crystalline modifications of the same substance. The difference in the
stability of these forms to light appears to be a consequence of their
different molecular conformations and packing in the solid state. The molecular
conformation of the A form is represented by (8) and that of the A' form by
(9). The related tetraacetylethylene has the
conformation (10) in the solid state. X-ray crystallographic analysis has also
revealed that the photolysis product (B) of tetrabenzoylethylene is
4-phenoxy-3,4,6-triphenyl-1H,4H-furo[3,4-c]-furan-1-one
(11). ��� The crystal structures of (8), (9), (10)
and (11) were determined by X-ray diffraction: diffractometer data at 295 K
were refined by least-squares techniques to residuals of 0.064 (2161 'observed'
reflections) for (8), 0.057 (1310) for (9), 0.049 (1295) for (10) and 0.061
(1639) for (11). Crystals of (8) are monoclinic, P21/n, a 8.775(5),
b 17.352(9), c 30.985(8)Ǻ, β 93.01(3)°, Z 8. Crystals of (9) which
contain carbon disulfide are monoclinic, A2/a, a 21.21(2), b 6.181(4), c
20.35(2) Ǻ, β 102.77(7)°, Z 4. Crystals of (10) are monoclinic, P21/n,
a 7.540(2), b 4.392(2), c 14.770(4) Ǻ, β 96.29(2)°, Z 2. Crystals of
(11) are monoclinic, P1/n, a 14.820(4), b 15.000(3), c
10.819(4)Ǻ, β 106.02(2)°, Z 4.
Tautomerism in acyl-pyrazolones and in a novel photolysis product—importance and impact of the accurate localization of hydrogen atoms in crystal structures
