lo] 2a : Crystal suitable for X-ray diffraction grown from dichloromethane/ ethanol under nitrogen. Triclinic P i , a = 1052.4(4), b = 1801.0(3), c = 1038.9(5) pm, a = 103.26 (3),y=76.11(2)", V = 1 . 7 0 5 6 2~ lo6 pm', 2 = 2 ; /I(MoK,)=71.069 pm; R=0.0398, R,, =0.0409 for 3455 unique reflections [2<20<46', 1>3o(I)], measured on a CAD4 diffractometer and solved using SHELX76. Further details of the crystal structure investigation are available on request from the Director of the tinue, albeit in a weaker form, in the case of 1,2-dihydrocyclobutabenzene 2aC2l and more markedly so in the case of 1,2,4,5-tetrahydrodicyclobuta(a,d)benzene 3.13. 41 So far X-ray structure analyses have been reported only for derivatives of 2a (2b-2d),[51 while in the case of 3 only one structure determination (from the year 1969)I6I exists, whose standard deviations d o not permit an interpretation of the bond lengths in the benzene ring. 2a, R' = ~2 = H 2b, R', R2 = 0 &:: 2 c , R' = H, R2 = CI, Br, I R' 2d. R' = H, R2 = ONO,The substituents in 2b-2d give rise to reductions in symmetry and electron shifts that are not observed in the parent compound (cf. also the situation in l"]). We have therefore carried out an X-ray structure analysis of 2a (m.p. ca. -75°C) at -170°C and calculated the X-X deformation electron density ; single crystals prepared by a Fig. 1. a) 2a with atomic labeling and important distances [A] and angles ["I (standard deviations 0.001 A and 0.1 resp.) averaged for mm2 symmetry (corrected values according to Schomaker and Trueblood [I31 in brackets): 86.5. b) X-X-deformation electron densities of 2a in the plane of the-molecule. Contour interval 0.025 eA', negative electron densities dashed, intervals 0.05 eA3. c) Bond lengths [A] and angles ["I of 2a, calculated in each case linearly via the maximum of the deformation electron densities (estimated standard deviations about I in the last decimal place):
