Introduction. The structure of the title compound (1) was determined in continuation of our programme of work to relate diacyl peroxide structure and rates of thermolysis in solution and also to assist in the interpretation of polarized NMR spectra (CIDNP) by providing geometric data from which electron-nuclear hyperfine coupling constants in the related styryl radical might be estimated.~O~o~ (1) Experimental. (1) (2) 2506 (8) 3511 (22) 1541 (26) 75 (10) C (3) 2755 (14) 2935 (26) 3306 (39) 37 (15) C (4) 2247 (12) 2342 (30) 5287 (34) 64 (14) C (5) 1345 (14) 2257 (26) 5076 (30) 61 (14) C (6) 742 (14) 1587 (27) 6844 (34) 63 (14) C (7) -182 (15) 1658 (28) 6240 (31) 73 (17) C (8) -774 (12) 994 (30) 7649 (39) 68 (15) C (9) -461 (16) 311 (31) 9546 (41) 72 (17) C (10) 435 (20) 186 (28) 10177 (36) 77 (17) C (11) 1040 (13) 908 (28) 8789 (34) 64 (14) O (12) 4247 (7) 3100 (21) 2140 (22) 62 (9) O (13) 3879 (9) 1083 (23) 186 (25) 72 (10) C (14) 4332 (12) 2093 (33) 384 (37) 57 (14) C (15) 5103 (11) 2546 (27) -967 (31) 54 (13) C (16) 5343 (12) 1809 (26) -2761 (31) 55 (13) C (17) 6099 (12) 2022 (26 . C~H,201). Discussion. 1 and 2 respectively, and the atomic numbering, bond lengths, bond angles and torsion angles (not involving H) are in Fig. 3.The molecule adopts an extended conformation in the crystal with the two cinnamoyl moieties essentially planar; the mean angles between the nodal planes of the carbonyl and phenyl groups in the two halves of the molecule are 4.5 and 11.5 o. The two cinnamoyl groups lie in roughly orthogonal planes, however, since the We call attention to two other features of the present structure, namely the bond lengths in and the conformation of the a,fl-unsaturated acyloxy groups. The carbonyl bond length is substantially shorter [mean value 1.15 (2)A] than the average for the 13 compounds containing the cinnamoyl moiety for which structures having R < 0.10 are available [ 1.228 (25) A]. On the other hand it is little less than the average for the eight diacyl peroxides [1.186 (28)/~] which is itself less than is typical for unconjugated acyloxy groups [1.212 (4) .A] (Allen, 1981). The C=C bond lengths are close to the average for a cinnamoyl group. The overall picture is thus of strong conjugative interaction between the phenyl groups and their adjacent C--C bonds, but there seems to be little extension of the conjugation to the carbonyl groups despite the planarity.In trans-cinnamoyl peroxide the aft-unsaturated carbonyl group adopts the s-cis conformation. A review of gas-phase electron diffraction studies has led to the conclusion that the s-trans (or antiplanar) conformation is favoured for sterically unhindered compounds at normal temperatures (Bohn, 1977), and this is also the case in almost half of the a,fl-unsaturated acids for which crystal structures have been scrutinized (Einspahr & Donohue, 1973). Nevertheless, the s-cis conformation occurs in the majority of cases, especially in cinnamoyl compounds (see, for example, Hanson, 1975). In the present case there seems to be no obviou...

Abstract. C20HlaO4, M r = 322.34, monoclinic, C2/c or Cc, a = 14.65 (2), b = 11.21 (2), c = 11.10 (2)A, fl= 106.6(1) ° , U= 1747.1A3, Z=4, D,,,= 1.25, Dx= 1.26 Mg m -3, Cu Ka radiation, 2 = 1.5418/~, R = 0.060 for 765 unique reflexions. The title compound decomposes in solution at a rate significantly greater than those of related peroxides: the molecular structure has been determined to explore possible steric reasons for this.

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