X-Ray crystal structures of two pyrethroid insecticides:cis-3-phenoxybenzyl 3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylate and the 3-(2,2-dichlorovinyl) analogue

摘要

1976 1231X-Ray Crystal Structures of two Pyrethroid Insecticides : cis-3-Phenoxy-benzyl 3- (22- Di bromovinyl) -2,2-dimethylcyclopropanecarboxylate andthe 3-(2,2- Dichlotovinyl) AnalogueBy John D. Owen," Molecular Structures Department, Rotharnsted Experimental Station, Harpenden, Hertford-shire AL5 2JQThe crystal structures of two isomorphous crystalline pyrethroid insecticides have been determined by X-raydiffraction analysis. Both compounds crystallise in the orthorhombic space group P2,2,2, with Z = 4. Thebromo-compound (I) has cell dimensions a = 12.177(4). b = 26.707(9), and c = 6.146(5) A. The structurewas solved by heavy-atom methods from diffractometer intensity data, and refined to R 0.044 for 721 observedreflections. The absolute configuration was determined and confirmed the chemical work.The chloro-compound(11) has cell dimensions a = 12.1 68(16), b = 26.577(22). and c = 6.006(4) A. Intensity data were collectedfrom films scanned on a microdensitometer, and the structure refined to R 0.069 for 545 observed reflections.The conformations of the two compounds are compared with each other and with those of other known pyrethroidstructures in the crystalline state.INSECTICIDAL activity in pyrethroids is related tomolecular shape,l and refinement of such concepts re-quires a good basis of reliable information on moleculardimensions, configuration, and conformation. As partof this work I report the determination of the structuresof (1R,3R) -cis-3-phenoxybenzyl3- (2,2-dibromovinyl) -2,2-dimethylcyclopropanecarboxylate (I) (NRDC 157), andthe 3-(2,2-dichlorovinyl) compound (11) (NRDC 167) inthe solid state by X-ray diffraction analysis.Compounds (I) and (11) are highly insecticidal mem-bers of a group of photostable pyrethroids.2 The com-pound (111) (NRDC 161) is a further member of thisgroup and its crystal structure has recently been re-p ~ r t e d .~ The three compounds (1)-(111) all crystallisein the same space group and have similar cell dimensions.However, (I) and (11) have similar diffraction patterns,whereas that of (111) is different. We decided to solvethe structure of (I), to compare and contrast it withthat of (111), and of (11), to show any differences which xamp;*p*o H bsol;H x Me M e 0 R(I) X = Br, R = H(111) X = Br, R = CN(11) X = C1, R = Hmay have been introduced by substituting chlorineatoms for bromine.The solution of these structures brings the number ofpyrethroids whose crystal structures have been deter-mined to four.The structure of the 6-bromo-2,4-di-nitrophenylhydrazone derivative of (4S)-2-(prop-2-enyl)-rethron-4-yl (lR,3R)-tran~-chrysanthemate (S)-bio-allethrin) 4 has been determined and was compared withthat of (111) in ref. 3.The crystal structure of (I) was solved by Patterson andFourier met hods and refined by least-squares techniques.M. Elliott, Chewz. and Ind., 1969, 776; Bull. World HealthOrganisation, 1971, 44, 315; M. Elliott, A. W. Farnham, N. F.Janes, P. H. Needham, and D. A. Pulman, Proc. Amer. Chem.SOC.Symposium Mechanism of Pesticide Action : Cellular andModel Systems, Los Angeles, California, April 1974, p. 80.The absolute configuration was obtained by calculatingstructure factors (using anomalous dispersion coefficientsfor the bromine atoms) for the hkl and hamp; Bijvoet pairs.The resulting atomic co-ordinates appear in Table 1 andthe atom numbering in Figure 1.FIGURE 1 Molecule of (I) showing atom numbering system used.Hydrogen atoms are numbered according to the carbon atomto which they are bondedThe final atomic co-ordinates for (I) were used as astarting point for the refinement of the structure of (11)by least-squares methods. The absolute configurationof (11) was not established by this structure determin-ation, but chemical and biological evidence indicates itto be the same as that of (I).2 Final atomic co-ordinatesappear in Table 2.The Structure of (I).-The crystal structure resultsfrom the packing of discrete molecules: the closest inter-molecular contacts are between hydrogen atoms and areall 2.68 A (Table 3).The packing diagram (Figure 2)shows there are no obvious strong interactions (such ashydrogen bonds) holding the molecules together in thecrystal.The isomer giving the lowest R (see Experimentalsection) also showed the same configuration about thetwo asymmetric centres on the cyclopropane ring as thatknown to be present from the method of synthesis.22 M. Elliott, A. W. Farnham, N. F. Janes, P. H. Needham,D. A. Pulman, and J. H. Stevenson, Nature, 1973,246, 169; P.E.Burt, M. Elliott, A. W. Farnham, N. F. Janes, and D. A. Pulman,Pesticide Sci., 1974, 5, 791; M. Elliott, A. W. Farnham, N. F.Janes, P. H. Needham, and D. A. Pulman, i b i d . , 1975, 6, 537.4 M. J. Begley, L. Crombie, D. J. Simmonds, and D. A.J. D. Owen, J.C.S. Perkin I , 1975, 1865.Whiting, J.C.S. Perkin I , 1974, 12301232 J.C.S. Perkin ITable 3 shows the bond lengths and angles. The onlyunusual feature of these is the short phenyl carbon-carbon distances, mean 1.363(8) A; the mean phenylTABLE 1Fractional co-ordinates for (I) ( x lo4) and isotropic vibrationparameters (Via x lo3 Hi2), with standard deviationsin parenthesesX-0 343(2)-0 723(1)-0 533(11)-0 541(11)-0 406(12)-0 944(12)-1 693(12)-1 323(14)0 266(13)0 823(13)0 482(9)1813(8)2 426(13)2 253(13)1626(12)1523(13)2 032(17)2 627(16)2 745( 16)0 928(10)0 034(13)-0 033(13)-0 927(16)-1 744(16)-1 719(15)-0 833(15)-0 617-0 361-2 389-1 373-1 856-0 823-2 052-1 3290 7202 2643 2011 26319892 9823 1470 565-0 958-2 365-2 365-0 812Y-1 106(1)-1 665(1)-0 630(5)-0 049(5)-0 974(5)0 429(6)0 429(5)0 773(6)0 382(6)0 325(6)0 133(4)0 566(3)0 600(6)1079(6)1462(6)1904(6)1978(7)1603(7)1163(6)2 306(4)2 223(6)2 425(7)2 128(7)1883(6)1912(6)2 475(5)-0 497-0 1400 2650 2440 7650 7550 68211150 5770 3250 5881 3942 29716590 8872 7072 6222 09516761723z9 843(3)5 193(3)6 846(25)6 994(22)7 216(24)6 531(26)4 649(28)8 329(29)6 245(24)4 131(25)2 664(19)4 228(16)2 298(29)1041(26)1980(27)0 867(27)-1 068(32)-1 966(36)-0 925(29)1724(21)3 092(27)4 914(36)6 272(33)6 686(33)3 780(31)2 400(30)4 4698 6434 9593 4144 1449 4968 7567 7607 1181 3682 6803 365-1 795-3 351-1 6005 2977 6056 6463 4111068* Anisotropic vibration parameters (Utj x 104Az) in theexp - 2n2( Ullh2a*2 + 2 U,?hka*b* + U,,k*2 + expression :2U13hZa*c* + U,,PC*~ + 2 U,,kZb*c*), with values:Atom Ull u,, us3 u,* u13 UZBBr(1) 1 OlO(14) 493(11) 376(ll) -4(9) -138(13) 83(10)Br(2) 799(12) 429(10) 425(11) -96(8) 35(12) -6(10)angles are 119.9(5)". The cyclopropane ring has a meanbond length of 1.51(1) A.The conformation of (I) is described by the torsionangles and mean planes in Table 4, and is very differentfrom that of (111), in spite of the similar cell dimensionsand identical space group.The Structure of (11) .-The crystal structure, molecularconformation, and bond lengths and angles are verysimilar to those of (I).The minimum intermolecularcontact is 2.61 A (Table 3) in this case. The absoluteconfiguration is known to be (lR,31i)ci~,~ the same asthat found for (I), although there is no crystallographicevidence for this in the present work.The bond lengths and angles have estimated standarddeviations which are slightly higher overall than thosefor (I), in line with the smaller number of observationsused in the least-squares refinement.Again the phenylcarbon-carbon bond lengths are short mean of 1.359(9)A; mean phenyl angles are 119.9(6)". The cyclo-propane ring has a mean bond length of 1.487(4) A.TABLE 2Fractional co-ordinates for (11) ( x lo4) and isotropicvibration parameters (Ui,, x lo3 A2)X-0 398(5)-0 746(4)-0 577(12)-0 651(13)-0 427(14)-0 964(15)-1 679(16)-1 331(16)0 249(16)0 828(16)0 536(10)1821(9)2 462(15)2 279(14)1612(14)1610(17)2 046(16)2 682( 17)2 797(16)0 902(12)0 OOO(l6)-0 023(18)-0 899(20)-1 737(20)- 1 722(18)-0 862(19)-0 642-0 410-2 354-1 329-1 760-0 781-2 019-1 3720 7082 2513 216123219663 0483 2490 672-0 932-2 352-2 327-0 854* Anisotropic thermalvalues :Y-1 104(2)-1 l58(2)-1 003(5)-0 631(6)-0 065(6)0 419(6)0 370(6)0 326(6)0 121(6)0 667(4)0 602(6)1 082(6)1466(7)1902(7)1614(8)1173(7)2 303(6)2 238(6)2 456(8)2 147(8)1886(7)1915(7)0 5010 1330 2840 2430 7760 7720 66011080 6520 3290 67414232 29316680 9182 7042 6502 111167017270 433(7)0 779(7)1973(7)2 495(7)parametersz9 720(9)5 348(8)6 930(26)6 106(27)7 339(27)6 633(28)4 642(33)8 488(34)6 457(28)4 266(32)2 652(22)4 392(19)2 376(33)1190(29)2 023(29)0 819(31)-1 071(33)-1 911(36)-0 825(34)1601(25)3 098(33)4 951(42)6 344(37)5 843(38)3 918(35)2 448(35)4 5238 8265 0533 9654 2269 6529 0127 8617 39414222 7553 421-1 835-3 308-1 4955 3747 6926 8543 5651 062(see TableAtom Ull u2* us, Ul, UlS u,,Cl(1) 1143(47) 565(31) 667(36) ll(31) -147(40) 81(28)Cl(2) 841(37) 420(23) 706(36) - 73(29) -5(34) -90(29)The conformation of (11) (Table 4) is not significantlydifferent from that of (I), in spite of the smaller size ofthe chlorine atoms1976 1233TABLE 3Bond lengths (A), angles ("), and intermolecular contacts (A)for (I) and (11)(a) Bond lengths, with standard deviations in parenthesesTABLE 3 (Continued)X(l)-C(1)XP)-c(l)C( 1)-C(2)C(2)-C(3)C(31-474)C(3)-C(7)C(4)-C(5)C(4)-C(6)C(4)-C(7)C(7)--C(f3)C(8)-0(9)0 (1 0)-C( 1 1)C( 1 1)-C( 12)C(8)-0(10)C(12)-C(13)C( 12)-C ( 1 7)C( 13) -C( 14)C( 14)-O( 18)C( 14)-C( 15)C( 15)-C( 16)C ( 1 6)-C (1 7)O( 1 8)-C( 19)C( 19)-C( 24)C(19)-C(20)C( 20)-C( 2 1)C(21)-C(22)C(22)-C(23)C (2 3)-C (2 4)(1)1.890( 15)1.891 (1 5)1.297(17)1.49 6 ( 1 8)1.496( 19)1.5 34 (20)1.622(21)1.509(20)1.488( 21)1.474( 21)1.172( 17)1.356(17)1.407(18)1.510(20)1.381 (19)1.363(21)1.394(19)1.399 (1 8)1.356(22)1.352 (23)1.365(22)1.39 3 ( 19)1.309( 23)1.409(21)1.378(24)1.323(23)1.342 (23)1.374(23)(11)1.7 1 1 (1 7)1.678 ( 15)1.350( 19)1.474(22)1.481 (21)1.493( 22)1.480(23)1.534(22)1.486(23)1.497(23)1.1 67 ( 1 9)1.3 69 ( 19)1.437( 21)1.47 6 (22)1.395(20)1.386(22)1.373 (22)1.3 82 (20)1.322 (23)1.328 (24)1.348(24)1.429 (20)1.307(25)1.41 0 (23)1.359(27)1.343(27)1.347 (2 6)1.371 (25)( b ) Bond angles, with standard deviations in parenthesesX( 1)-C(1)-X(2)X( 1)-C( 1)-C(2) x ( 2)-c ( 1) -c ( 2)C( 1)-C(2)443) c ( 2)-c ( 3)-CI 4) c (2)-C( 3)-C( 7)C(4)-C(3)-c(7) c (3) -C( 4)-C( 5)c 3)-c (4)-C( 6) c (3) -c (4)-c (7)C( 5)-C( 4)-C( 7)c (6)-c(4)-C(7)C( 3)-c (7)-c (4)C(5)-C(4)-C(6)C( 3) -C (7)-C (8)C(4)-C( 7)-C( 8)C( 7)-C( 8)-0 (9)C( 7)-C( 8)-0 ( 10)0 (9)-C( 8)-O( 10)C ( 8)-0 ( 1 O)-C( 1 1)C( 1 1 )-C( 12)-C( 13)C( 1 l)-C( 12)-C( 17)C ( 12)-C (1 3)-C( 14)C 1 3)-C ( 1 4)-0 ( 1 8)ClS)-C( 14)-0 ( 18)C( 14)-C( 15)-C( 16)C( 1 6)-C( 1 6) -C( 17)C( 12)-C(l7)-C(16)C( 14)-0 ( 1 8)-C( 19)O( 18)-C(19)-C(20)0(18)-C(19)(24)C (20)-C ( 1 9)-C ( 24)C( 19)-C( 2O)-C( 2 1)C (2 1 )-C (2 2)-C (23)O( 10)-C( 1 1)-C( 12)C( 1 3)-c ( 12)-C( 17)c(13)-c( 14)-c(15)C( 20)-c (2 1)-C( 22)c (2 2)-c (2 3)-C( 24) c (1 9)-C( 24)-C( 23)(1)11 2.6 (8)124.4( 11)123.1(12)125.6 ( 13)123.0 (1 2)120.6 (12)58.8 (9)11 9.1 (13)11 6.6( 12)6 1.9 (10)11 3.8 (1 2)1 19.9 (14)11 6.1 (14)09.3(9)120.7( 13)1 24.6 ( 14)127.7( 15)108.9(13)123.4( 14)1 1 8.2 ( 1 2)115.3 (1 3)1 13.3 (14)121.1 (1 6)120.6( 15)118.0(15)121.1 (1 7)12 1.8( 14)117.0( 16)119.3( 19)12 1.4 (1 9)119.6(17)120.6( 12)118.8( 15)1 19.8 (1 6)121 .O( 16)12 1.2 (1 6)1 19.1 (1 8)1 20.9 (20)1 2 2 .O( 1 8)1 15.8 (1 6)(11)1 16.2( 9)120.1 (13)123.6( 13)128.1 (1 6)122.7 ( 15)12 1.8 ( 15)60.0(11)120.9( 15)1 16.8 (14)60.4( 11)1 13.5 ( 14)12 1.9( 15)113.3( 15)59.6 (1 0)120.9 (1 5)1 2 2.5 (1 6)1 28.5 (1 8)109.3(15)122.2 ( 18)117.1 (14)1 12.7 (1 5)122.7( 17)1 20.5 (1 7)1 16.8( 17)1 18.7 (1 7)12 1.9 (20)12 1.6 (1 8)1 16.5 (19)120.7( 2 1)12 1.8 (20)122.1( 16)119.2(18)118.1(17)122.6(19)120.3( 21)1 19.7( 23)1 2 2.3 (2 2)1 15.0( 18)120.1 (20)120.0(20)(c) Selected intermolecular contacts(1)0(9) H(3I) 2.71O(10) HA(l1n) 2.922.812.91 H(2) HB(6IV)H(3) HA(5V) 2.88HA(5) HB(6IV) 2.72HA(6) H(24m) 2.76HC(6) 9 H(24vI) 2.68H(15) H(20I) 2.73H(20) H(22IU) 2.85O(18) H ( 2 2 9(11)2.662.902.792.892.852.702.682.612.622.90Roman numeral superscripts refer to the following equiva-lent positions, related to the reference molecules a t x , y , z :1 x , y , -1 + z IV -4 - x , -y, -amp; + z11 9 - x , -y, 9 + z v -4 - x , -y, 6 + I11 amp; + x , 9 - y , 1 - 2 VI x , y , z + 1FIGURE 2 Contents of two unit cells of (I), viewed parallelto the c axisDISCUSSIONComparison of the packing diagrams shows clearly thatthe molecules of (I) and (111) take up very differentconformations and also different positions and orient-ations in the unit cell.There seems to be no obviousreason why the cell dimensions should be so similar, oreven why the two compounds should crystallise in thesame space group, in view of the different shapes of themolecules which are packing together. The molecule of(I) is relatively more compact than that of (111).Thisis caused by major changes in the torsion angles aboutthe three unfixed bonds in the 3-phenoxybenzyl groups(see Table 4). It seems therefore, that there is no pre-ferred conformation for this end of the molecule in thesolid state.In (111), it is the hydrogen atom H(11) which is veryclose (0.05 A) to the plane of the benzyl group, whereasin (I), O(10) is close 0.07(1) A to this plane.The torsion angles about the three bonds in the estergroup linkage are similar in both (I) and (111); theC(8)-0(9) bond is pointing towards the axis of the cyclo-propane ring, with C(7) and C(11) disposed s-trans abou1234 J.C.S. Perkin IO(10)-C(l1) bond. This means that for (I), neither ofthe hydrogen atoms on C(11) are close to the ester plane,which was the case for (111).However, the plane of theester group does include both C(11) and the midpoint ofthe C(3)-C(4) bond in both (I) and (111).A further difference between (I) and (111) occurs atthe other end of the molecules. The plane of the di-bromovinyl group almost includes the midpoint of theC(4)-C(7) bond in (I), whereas in (111) the torsion angleabout C(Z)-C(3) is ca. 60" greater, to move the bromineatoms away from the ester group. This indicates somedegree of flexibility in this part of the molecule.The crystal structures of the pyrethroids determinedto date show that there is a certain degree of flexibilityat each end of the molecules, with the ester linkage inthe middle forming a fairly rigid entity.This supportsthe conclusions drawn from the chemical and biologicalstudies concerning the importance of the rigid esterlinkage in the pyrethr~ids.~Although more data is obviously needed, especiallyon the conformation in solution, the lack of close inter-molecular contacts in the crystal structures so far deter-mined, indicates that these features may carry over tothe dissolved insecticide molecules. 1-TABLE 4Torsion angles (") and mean planes for(a) Torsion anglesC( 1 )-c (2)-C( 3)-c (4) c ( 1 )-c (2)-c (3)-c (7) c (3)-c (7)-c (8)-0 (9) c (4)-c ( 7)-c (8)-0 (9)C( 8)-0 ( 1 0)-c ( 1 1 )-c ( 1 2)0 ( 9)-C (8) -0 ( 1 0) -C ( 1 1)C(8)-0(10)-C(1 l)-HA( 11)O(lO)-C(ll)-C(l2)-C(13)C( 13)-C( 14)-0 ( 18)-C( 19) c (14)-0 ( 18)-c ( 19)-c (24)(1)41(2)5(2)93P)7(2)- 148(1)141(2)- 30(3)- 30- 30( 2)-55(2)(I), (11), and (111)(11)36(3)6(2)95(2)3 ( 2 )- 146(2)141(2)- 36(3)- 26-27(3)- 56(2)(111)- 90(5) - 161(4)-36(6)39(6)7 (5)104(3)- 13- 106(3)- 136(3)21(5)(b) Deviations (A) from mean planes.Atoms italicised wereused to define the planeCCCI Plane ( 2 )(1)- O.OOO(2)-0.004(2)0.004( 14)0.004 ( 14)- 0.004( 15)-0.661(15)0.82 6( 16)0.004( 15)0.006(11)0.004(9)0.663 ( 14)0.155 ( 16)- 0.01 4( 15)- 0.832(13)-0.22- 0.331.576( 15)- 0.041 (17)0.025 ( 15)0.031(15)0.01 4( 16) - 0.02 1 (20)- 0.007 ( 2 0)- O.018( 12)0.016( 18)0.069 (9)0.64-0.910.56 7 (1 6)0.030 ( 1 2)0.001 (1 6)0.003( 19)0.001 (1 8)0.020( 17)0.014( 17)- 0.041 (1 5)- 0.880( 16)(11)0.004(6)-0.003(5) - 0.007 (1 5)- 0.009( 17)- 0.675( 18)0.01 6( 16)0.808 ( 1 9)-0.001(16)- 0.002(13)- 0.764( 17)-0.21- 0.420.005( 18)-0.001(11)0.716( 16)0.120( 18)1.5 12 (1 7)- 0.020( 19)O.OOS( 17)0.018(17)0.027 (20)- 0.006( 20)-0.009(21)-0.024(14)0.007(20)0.031(11)0.730.470(19)-0.830.02 1 (1 4)- 0.023(18)-0.011(21)O.Oll(21)0.001 (23)0.007(20)- 0.006(20)-0.909(19)(111)-0.004(4)0.006(4)0.006( 30)0.0 14( 3 2)-0.022(38)- 1.1 74( 29)-0.433(31)- O.OOl(30)-0.002(23)0.004(38)-0.001(20)0.786 (30)- 0.70 1 (27)- 0.167(28) - 0.040.836(29) *- 1.505(27)-0.079(27)0.049(28)0.063(31)0.0 10 ( 2 7) - 0.030(32)-0.026(29)- 0.027(24)- 1.445(20)0.040( 31)0.050.851 (29)*0.7281 28)- 0.024(25)0.015(29)0.024(33)0.006(36)0.014(35)0.346(27)- 0.021 (30)-0.014(36)( G ) Dihedral angles (") between planesPlanes (1) (11) (IW114 116 35134 90116 9280 7811 10 12673 72 124(1)-(2) 137(1143) 115(11-44) 81(2)-(3)(2144)(3)-(4)* Deviation of the carbon of the CN group bonded to C(11)in (111).the C(8)-O(l0) bond.However, there is a small butsignificant difference in the torsion angles about thet Note added in proof: The structure of the p-bromoanilidederivative of ( + )-trans-chrysanthemic acid has recently beenreported, and confirms the absolute configuration deduced fromchemical considerations (A.F. Cameron, G. Ferguson, and C .Hannaway, J.C.S. Perkin 11, 1975, 1567).EXPERIMENTALCvystal Data.-(I). C2,H2,03Br2, M = 480.2. Ortho-rhombic needles, a = 12.177(4), b = 26.707(9), c = 6.146(5)A, U = 1 999 A3, D, (flotation) = 1.57 g ~ m - ~ , 2 = 4,D, = 1.59 g ~ m - ~ , F(000) = 960. Space group P2,Z12,(D;, No. 19) uniquely determined. No molecular symmetryrequired. Mo-K, radiation, Zr filter, A = 0.7107 A;~(Mo-K,) = 40.32 cm-1. Xo absorption correction applied.(11). C21H2,03Cl,, M = 391.3. Orthorhombic needlesA3, D, (flotation) = 1.33 g CM-~, 2 = 4, D, = 1.34 g CM-~,F(000) = 816. Space group P2,212, (D;, No. 19) uniquelydetermined. No molecular symmetry required. Cu-Karadiation, Ni filter, A = 1.5418 A, ~(CU-K,) = 31.96 cm-1.No absorption correction applied.A crystal of dimensions 0.17 x0.17 x 0.76 mm was mounted on a Picker four-circle dif-fractometer.The orientation matrix and cell dimensionswere refined using the positions of 12 reflections. Intensitydata were collected by 8-20 scans at 0.5' min-l, fromZOcalc.(Kal) - 0.5 to 20calc.(H,2) + 0.6". Stationary-crystal-stationary-counter background counts were taken for 25 seach after the scans. The intensities of all reflections with20 36" and positive I were measured. A referencereflection (581) was measured every 25 reflections. Thevalue of its intensity was plotted against its sequence no.in the data collection, a smooth curve drawn through thepoints and the data then scaled to this line.This showeda mean reduction in intensity, I, of 4 over the entire datacollection. Lorentz and polarisation factors (LP) wereapplied and the standard deviations calculated from theformula: d ( F ) = 02(1)/4.LP.I, where ."I) = total count + (0.25 background) (scan time/background time)3. Of3 129 reflections measured, 361 were considered unobservedM. Elliott and N. F. Janes, ' Pyrethrum, the NaturalInsecticide,' ed. J. E. Casida, Academic Press, New York, 1973,u = 12.168(16), b = 26.577(22), c = 6.006(4) A, U = 1 942Data Collection.-(I).p. 551976 1235because either I 241) or I 80 counts, in which case itwas reset to 40. Averaging of equivalent reflections resultedin 1 564 reflections in the two octants hkl and hkl, of which1 342 were considered observed.The structure solutionand initial refinement were carried out on the 721 reflectionsconsidered observed in the hkE octant.A crystal of dimensions 0.02 x 0.11 x 0.48 mmwas used for the collection of intensity data on films. Thecrystal was mounted about c and Weissenberg photographsfor the hk0-4 layers obtained by the multiple-film equi-inclination method. Cell dimensions were calculated fromaccurately measured precession photographs. The Weis-senberg photographs were scanned by the S.R.C. Micro-densitometer Service at the Atlas Computer Laboratory,Chilton. Equivalent forms were averaged and the intensi-ties of 518 hkl reflections returned. Intensity values werethen checked qualitatively against the films.A few reflec-tions with measured values could not be seen on the topfilm of a pack, and were rejected. Several spots on thefilms did not have intensity values measured by the scannerowing to either bad interfilm agreement or misalignment ofthe crystal, the latter particularly leading to the omissionof some strong spots close to the centre line on upper layers.These spots were estimated visually and scaled to the otherreflections in the layer. All unmeasured reflections leftafter this were classed as unobserved. This resulted in atotal of 1 035 reflections, with 545 observed. Lorentz andpolarisation factors were applied but no interlayer scalingwas carried out at this stage.A three-dimensional Pat-terson map enabled location of the bromine atoms and aFourier map phased by these gave the positions of 10 otheratoms.These positions were refined and a further Fouriermap enabled location of the remaining 14 non-hydrogenatoms. A weighting scheme which gave a constant wA2for ranges of IFo gave satisfactory refinement. Thebromine atoms were allowed to refine with anisotropictemperature factors and this gave R 0.061 9 for the 721reflections used.At this stage the absolute configuration was sought bytwo structure-factor calculations on the 1 342 observed re-flections in both the Jtkl and hkl octants. Theisomer usedso far gave R 0.064 3 whereas the isomer obtained by revers-ing the signs of all three co-ordinates of each atom gave R0.055 4. The second isomer was thus taken as the correctone and refined to R 0.0549.The agreement betweenIFo and IFcbsol; was poorest for the low sin 0/A reflections; adifference map showed regions of high electron-density inpositions near those calculated for hydrogen atoms fromgeometrical considerations. The positions of all except themethyl hydrogen atoms were calculated and included in themodel. The non-hydrogen atoms were refined and a dif-ference-Fourier map was calculated. The methyl hydrogenatoms were located from this map, included in the model,and the non-hydrogen atoms refined.For the final refinement, all hydrogen-atom positions wererecalculated and given fixed isotropic temperature factorsof Uiso 0.05 Hi2 and the weighting scheme used was: IFo/ 9.5, w = 1/1.39, IFo 9.5, w = 1/(1.294 - 0.008 57 IFo +* See Notice to Authors No.7 in J.C.S. Perkin I , 1975, Indexissue.' World List of Crystallographic Computer Programs.'J . APPZ. Cryst., 1973, 6, 309.ICL 4/70 Programs adapted from the IBM 360 versions:NUCLS, R. J. Doedens and J. A. Ibers; ORFFE, W. R. Busing,K. 0. Martin, and H. A. Levy; ORTEP, C. K. Johnson.(11).Structure Determination.-(I) .0.001 98 Fo12). This refinement gave R 0.043 8 and R'0.057 1 for the 721 observed reflections in the hkE octant.The maximum change-to-error ratio on the last cycle was0.053. A weighting analysis showed no obvious trendsagainst ranges of sin ell, or IFo. The difference map afterthis refinement showed only peaks attributable to the in-adequate description of the thermal motion of the atoms.Measured and calculated structure factors for both (I) and(11) are listed in Supplementary Publication No.SUP21699 (10 pp., 1 microfiche).*Comparison of the diffraction photographs of (I)and (11) showed them to be isomorphous, so the final co-ordinates of the non-hydrogen atoms of (I) were used as astarting model for the solution of the structure of (11). Arefinement of this model with all atoms isotropic convergedto R 0.101 9, with unit weights. However, agreementbetween IFo and lFcl was poorest for largest IFo/, theobserved values being very much smaller than the calcu-lated. The high IFo reflections were therefore down-weighted using a three degree polynomial expression andthis led to satisfactory refinement later on.Since the fivelayer scale-factors were refined in the least-squares program,it was necessary to fix them in the cycles in which refine-ment of the chlorine atoms with anisotropic thermal para-meters was carried out. This refinement gave R 0.082 8 andwas followed by a difference Fourier map. This showedregions of high electron-density in positions near those forthe hydrogen atoms calculated from geometrical consider-ations. The positions of all except the methyl hydrogenatoms were calculated and included in the model and thenon-hydrogen atoms refined. A further difference mapthen showed the location of one hydrogen atom on eachmethyl carbon sufficiently well to allow location of the othertwo by geometrical considerations. The hydrogen atomswere included with fixed isotropic temperature factors ofUiso 0.05 A2, and refinement of the non-hydrogen atomsthen gave final values of R 0.069 0 and R' 0.081 6 for the 545observed reflections. The final weighting scheme used was :l.lllFoI - 0.052 8lFOl2 + 0.000 7841F013). The maximumchange-to-error in the last cycle was 0.065. A weightinganalysis showed no obvious trends against ranges ofsin euro;)/A or IFo. A final difference map showed no peaks 0.35 eA-3.Conzputing.-The orientation matrix and cell dimensionrefinement for (I) were carried out on an IBM 1130 computeras were the structure solution, refinement, and moleculargeometry calculations by use of the X-RAY ARC programs.6Final full-matrix least-squares refinement was done by useof NUCLS,' and bond lengths and angles, with errors fromthe variance-covariance matrix were calculated by use ofORFFE.' ORTEP' was used to draw the diagrams.Scattering factor curves for non-hydrogen atoms were takenfrom ref. 8, and for hydrogen from ref. 9; anomalous dis-persion coefficients for bromine were taken from ref. 10.(11).IFo 11.0, w = 1/4.0, 15'01 11.0, w = 1/(-3.843 +I thank Dr. M. Elliott for suggesting the work andsupplying the crystals, and Professor M. R. Truter and Dr.N. F. Janes for many helpful discussions.5/1733 Received, September loth, 19751D. T. Cromer and J. T. Waber, Acta Cryst., 1965, 18, 104.R. F. Stewart, E. R. Davidson, and W. T. Simpson, J .lo D. T. Cromer and D. Liberman, J . Chem. Phys., 1970, 53,Chem. Phys., 1965, 42, 3175.1891