9625
J. Phys. Chem. 1993,97, 9625-9629
Structure and Conformation of 4,4,5,5-Tetrafluoro-1,3,2-dithiazolidine,S-NH-S-CF2-CF2. Gas Electron Diffraction, X-ray Diffraction, and ab Initio Study
A
Roland b e , ' . Heinz Oberhammer,'Jb Peter Pulay,lCand Alfred Waterfeldld Institut f i r Anorganische Chemie, Universitht GH Essen, 4300 Essen, Germany; Institut f i r Physikalische und Theoretische Chemie, Univeristht Tubingen, 7400 Tubingen, Germany; Department of Chemistry, University of Arkansas, Fayettville, Arkansas 72701; Lehrstuhl f i r Anorganische Chemie II, Ruhr- Universitat. 4630 Bochum. Germany Received: March I I , I993
-
The geometric structure and conformation of the saturated five-membered ring 4,4,5,5-tetrafluoro-1,3,2dithiazolidine, S-NH-S-CFrCFz, have been determined in the gas and solid phases by electron and X-ray diffraction, respectively. The equilibrium structure and the total energy along the pseudorotational path have been calculated by ab initio methods (HF/4-21G(*) and HF/TZP). In the gas phase, the five-membered ring adopts a near envelope conformation with one sulfur atom bent out of the CCSN plane and with the N-H bond in the axial direction. The ab initio calculations predict this conformation to be the only stable structure along the pseudorotational path. Two molecules with different conformations are present in the crystal. Whereas one molecule adopts an envelope conformation which is very similar to the gas-phase structure, the other molecule possesses an envelope conformation with the nitrogen atom bent out of the SCCS plane. According to the a b initio calculations, the latter conformation corresponds to a distorted structure and is 1 kcal/mol higher in energy. The two molecules are linked by a nearly linear No-H-N hydrogen bond with N-H = 2.30 A and N*-N = 3.099 A.
Introduction The conformation of saturated five-membered rings depends on a delicate balance between angle strain and torsional strain around the individual bondsa2 In rings containing atoms with electron lone pairs, such as nitrogen, oxygen,or sulfur, their mutual interactions or the anomeric effect can also exert a big influence on theringstructure.3 Sinceverylittleis knownabout therelative magnitude of these different effects, a reliable prediction of the preferred conformation or conformations of a certain fivemembered ring is impossible, and experimental determinations are required for each individualring. Such conformational studies should preferably be performed in the gas phase, since intermolecular interactionsin condensed phases, such as packing forces in the crystal, may affect the conformation. On the other hand, comparison between gas-phase and crystal structures provides interesting information about the effects of intermolecular interactions on molecular conformations. In general, saturated five-membered rings can adopt 10 different envelope (E) and ten different twist (T) conformations (Figure 1) along the pseudorotational coordinate @, depending on which ring atom is bent out of the plane formed by the four remaining atoms (envelope) or around which bond the ring is twisted.45
envelope
-
twist
E8 180"
T6
E6
T5
Figure 1. Envelope (E) and twist (T) conformations of five-membered rings along the pseudorotational angle a. The dotted line indicates the C, plane in the El and E6 conformation or 'pseudo" C, plane in all other E conformations (all E conformers possess a C,plane only in the case of cyclopentane). For the T conformations, the dotted line corresponds to the twist axis.
our knowledge, no structural or conformational studies for fivemembered rings of this type have been reported so far.
Intermediate conformationscan occur as well. In this paper, we report a structural and conformational studyof 4,4,5,5-tetrafluoro-
Ab Initio Calculations
1,3,2-dithiazolidine, S-NH-S-CFAF2 (1)by gas-phase electron diffraction (GED) and X-ray crystallography. The experimental studies are supplemented by ab initio calculations. Derivatives of this ring system with organic substituents at nitrogen (Me, Ph, and SiMe3) are described in the literature.'j To
H F calculations were performed with the gradient program TX90,7 using the 4-21 G(*) basis set, which includes d-functions for S and N,8 and TZ+P basis sets (d-orbital exponents s(C,N,F) = 0.8 and s(S) = 0.65). Earlier calculations for pyrrolidines had demonstrated that polarization functions on N are crucial for
0022-3654/93/2QSI-9625%04.QQ~Q0 1993 American Chemical Society
9626
Boese et al.
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
H
n I I - 180
- 120 - 60
0 60 pseudorotational angle
120
180
(0
Figure2. Total energy (HF/4-21G(*)) of 1along pseudorotationalpath.
predicting the correct ring geometry. The conformation of the ring is described by four coordinates: two combinations of angles and two combinations r and f of torsions around ring bonds.8 The two latter coordinates are related to the pseudorotational coordinates r and 9 by r = r sin 9 and r' = r cos 9. For the envelope conformations with N out of the SCCS plane (9= Oo, N-H bond axial and 9 = 180°, N-H bond equatorial) and for the structure corresponding to the energy mimimum near 9 = f36' (Etot= -1318.1034 au), all internal coordinates were optimized simultaneously. For other conformations along the pseudorotational angle 9,one of the two coordinates r or T' was fixed. The choice of the fixed coordinate depends on which one is more sensitive at this particular conformation. With this procedure, the pseudorotational potential was calculated in steps of approximately 30° (Figure 2). The geometry at the energy minimum was optimized also with a TZ+P basis set (Etot= -1323,72405 au). The geometric parameters for both basis sets are given in Table I together with the experimental values, and the pseudorotational potential is discussed below.
CED Analysis A preliminary molecular model was derived on the basis of the radial distribution curve (Figure 3). Different ring conformations were considered. Neither envelope conformations with C, symmetry, Le., N bent out of the SCCS plane and the N H bond axial (9= Oo) or equatorial (9= 180°), nor a twist conformation with rotation around the C-C bond (C,symmetry disregarding the N-H bond) fits the experimental curve reasonably well. Only a combination of both ring distortions, puckering of the SNS plane and torsion around the C-C bond, leads to satisfactory agreementbetween calculated and experimenal radial distribution functions. In the subsequent least-squares refinement, the molecular intensities were modified with a diagonal weight matrix, and known scattering amplitudes and phases were used.9 The GED analysis was performed with the program package EB1.10 The following parameters were chosen for defining the ring geometry: N S , S-C, and C-C bond lengths; SNS bond angle; puckering angle a between SNS and SCCS planes; and torsional angle r around the C-C bond. Starting with a planar ring structure of C, symmetry, puckering and torsion keep the two SCC angles equal but lead to different NSC angles. Inspection oftheabinitioresultsreveals that theNSCangles in theoptimized structure indeed differ by ca. 5 O , whereas the SCC angles are very similar. Thus, thegeometricmodel used in the GED analysis appears to be adequate. Initially, Cb symmetry was assumed for
the CF2 groups, Le., equal CCF and SCF angles. In the final refinement, the ab initiovalues for rocking,wagging, and twisting angles of the CF2 groups were introduced as constraints. The GED intensities are not sensitivetoward the position of the amino hydrogen (axial or equatorial), and N-H bond length and SNH angle were assumed according to the ab initio calculations. Constraints for thevibrational amplitudes are evident from Table 11. With these assumptions, eight geometric parameters (C-F and FCF in addition to the six ring parameters given above) and 10 vibrational amplitudes (uk) were refined simultaneously.The following correlation coefficientshad values larger than 10.71(the full correlation matrix is available as supplementary material): pI/p2 = 0.70, pl/u2 = 0.82, p2/u2 = -0.83, p2/us = 0.75, and ps/us = -0.91. Numbering of geometric parameters pi and amplitudes uk and the final results of the GED analysis are collected in Table I and 11. For the puckering angle a which is related to the dihedral angle S - N S - C , a value of 36.6(28)O was obtained.
X-ray Diffraction Analysis Becauseofthelow melting point of 1(238 K), thecrystallization had to be performed in a sealed glass capillary which was cooled by a gas stream on the diffractometer. Even when cooled to 90 K, no crystals were formed, and a supercooled liquid or glassy solid remained in the capillary due to poor nucleation tendency. A technique which helps very often in such cases led to success also in this situation:ll attaching the piezo crystal of an old ultrasonic remote control from a TV set to the capillary produced some crystal seeds from which polycrystalline material emerged. This could be used to grow a single crystal at 225 K by the use of a computer-controlled miniature zone melting procedure, utilizing focused infrared light from a halogen lamp.12 The data were collected after further cooling to 125 K. The unit cell contains eight pairs of molecules which are linked by N-H-N hydrogen bonds. The two molecules a and b of each pair possess different conformations (Figure 4). The atomic coordinates, isotropic displacement coefficients, bond distances, and angles for both structures are listed in Table 111.
Discussion of Results The conformational properties of 1 are best discussed on the basis of the pseudorotational potential (Figure 2). The two minima near 9 f34O correspond to two equivalent nearenvelope conformations with one of the two sulfur atoms bent out of the CCSN plane and the N-H bond axial. The exact envelope conformation occurs at 9 = f36'. The near envelope character is also evident from the small C5-C4-S3-N2 torsional angles (0.3O and 0.8O from ab initio and 543)' from GED). The small potential barrier at 9 = 0' (AE= 1.0 kcal/mol) which separates these two equivalent minima corresponds to the envelope conformation with the nitrogen out of the SCCS plane and N-H in axial direction. This conformation was found to be thegroundstate structure for pyrr~lidine.~ In 1, this envelope conformation is destabilized by repulsions between the two eclipsed CF2 groups which cause torsion around the C-C bond by about 30° (30.1O and 28.4O from ab initio and 32(2)" and GED). Thus, the conformation of 1 can also be viewed as an envelope structure with N our of plane and with additional torsion around the C-C bond. This description of the structure has been used in the GED analysis. The pseudorotational potential gives a straightforward explanation of the X-ray results, where two molecules with different conformationsoccur in theunit cell (Figure 4). Theconformation of molecule a (envelope with S bent out of the plane and CSC4-S3-N2 = -1 .So) is close to the structures derived by GED and ab initio calculations, and molecule b adopts a near envelope conformation with nitrogen out of plane (@ Oo, Sl-C5-C4S3 = 7.4O). According to the ab initio potential function, this
-
-
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9627
Conformation of Saturated Five-Member Rings
TABLE I: Geometric Parameters for 1 from GED, X-ray, and ab Initio Calculations* X-rayd
GEDb
molecule a 1.350(3) 1.560(3) 1.722(7) 1.807(5) 0.7 l(3) 1l0.6(1) 91.4( 1) 96.3(1) 108.1(1) 108.8(1) 515.2(3) 109.8(27) 106.0(2) 107.0(2) 1.8 -2.4 -1.3 45.1 -28.5 -1.8 33.8 -47.7
1.346(2) (PI) 1.540(7) (Pz) 1.705(6) @3) 1.801(6) @4) 1.03e 1 1 0 3 7 ) @SI 93.4( 14) 99.2(12)
C-F C-C S-N
s-c
N-H Sl-N2-S3 N2Sl-C5 N24344 Sl-C5-C4 s3-C4-c5
107.6(11) 518(3) 11o.w
zaru$
S-N-H Fl-C4-F2 F3-C5-F4 rock (CF# wag (CFzP twist (CF$ N241-C5-C4 Sl-CS-C443 C5-C443-N2 C443-N341 S3-N24l-C5
1 0 6 3 4 ) (P6) 3.0 -2.1
4.9 45(4) - W 2 ) (P7) 5(3) 26(4) (Pa) -42m
ab initiod molecule b
4-21G(*)
TZ+P
1.350(2) 1.582(3) 1.708(2) 1.797(4) 0.81(4) 109.6(1) 95.7(1) 94.1( 1) 109.4(1) 109.6(1) 518.4(3) 106.3(34) 106.5(2) 106.0(2) 0.3 -2.2 -1.8 20.0 7.4 -31.1 46.6 -43.0
1.372(5) 1.547 1.704(5) 1.800(9) 1.011 111.3 90.8 95.9 107.9 108.4 514.3 110.0 107.6 109.4 3.O -2.1 4.9 46.1 -30.1 0.3 33.4 -48.1
1.328(3) 1.555 1.707(6) 1.8 16(8) 1.ooo 111.5 91.9 96.4 107.9 108.6 516.3 110.8 106.6 107.9 1.9 -1.7 -1 .o 44.3 -28.4 0.8 32.6 -46.7
Distances in A and angle in degrees. pi are the independent parameters which were refined in the GED analysis. For atom numbering, see Figure 4. r, distances and 4 angles. Error limits are 3a values and refer to the last digit. r, parameters, error limits are a values. For parameters which are not unique (C-F, S-N, S-C, and S-N-H), the mean value and average deviation from this value are given. The individual parameters of the X-ray study are given in Table 111. Not refined. /Sum of ring bond angles. E Rock = 1/2(j31 - 8 2 + 63 -a);wag = 1/2(@1+ 8 2 - @3 - j34); twist = 1/2(p1 - 8 2 - @3 + @4), @I C5-C4-F1; 8 2 C5-C4-F2; j33 S344-Fl; 8 4 S3-C4-F2.
TABLE E Interatomic Distances and Vibrational Amplitudes from GED Analysis. (without Distances Involving Hydrogen) distance C-F C-C
1.35 1.54
F-F C-F S-C N..C
2.16 2.33-2.39 2.70 2.55-2.67
amplitude 0.049(2) 0.052b
1
( ~ 1 )
0.061(6) (242)
1
0.055(4) 0.063(5) 0'10(3)
(143) (144)
S-F
distance 2.80 2.59-2.64
N.*F N-F F-F F-F
3.09 3.61-3.76 2.50 2.99-3.46
S-S
::
::;:
amplitude 0.074(7) (u6) 0.084(4) ( ~ 7 ) 0 . 2 ~ 4 )(us) 0.11(1) (uo) 0.306 0.21(5) ( ~ 1 0 ) 0.206 0.15b
Values in A, error limits are 3a values and refer to the last digit. Not refined.
L
1
1
I
' 3 4 5 RIA Figure 3, Experimental GED radial distribution function of 1 and difference curve. The positions of important interatomic distances are indicated by vertical bars. 0
1
2
is a distorted structure, but the low distortion energy of only ca. 1kcal/mole can easily be provided by packing effects. The energy increase is mainly due to the nearly eclipsed position of the two CF2 groups which leads to short intramolecular F-F contacts (Fl-.F3 = 2.425 A and F2-.F4 = 2.513 A). The investigation of intermolecular distances reveals a short contact between thenitrogen atomof moleculeawith the hydrogen atom of molecule b: N 2 w H l b = 2.30 A, N 2 w N 2 b = 3.099 A, N Z w H l b N 2 b = 172'. For idealized H positions (N-H = 1.008 A, S-N-H = 125'), the NZa-sHlb distance is 2.44 A. This hydrogen bonding leads to a distorted tetragonalcoordination at N2a (Figure 5 ) with H l b - N 2 a S l a = 120°, Hlb-NZaS3a = logo, Sla-NZaS3a = 110.6', and Hlb-N2a-Hla = 97'. Simultaneously, this causes the S-N-H angles of molecule b to be smaller than those of molecule a, which has no close contact from the nitrogen atom to a hydrogen atom of surrounding molecules. No further intermolecular contacts could be found
in the molecular packing. The anisotropic displacement parameters of the atoms in the two molecules are not equal. Whereas molecule a behaves close to a rigid body, molecule b has relatively high displacements out of the molecular plane (see Figure 4). This indicates the flexibilityof the molecule, which has the nitrogen atom in an unsatisfactory situation where the lone pair has no contact to a hydrogen atom. The ab initio calculations predict the maximum of the pseudorotational path at Q, = f18O' (Q, = 180' in Figure l), which corresponds also to an envelope conformation with N out of plane, but with an equatorial N-H bond. This conformation is predicted to be 13.5 kcal/mol above the energy of the groundstate structure. The reason for this large difference between axial and equatorial hydrogen bonds, which is only about 1 kcal/ mol in pyrrolidine,s is the interaction of the lone pairs at sulfur with the lone pair at nitrogen. The S and N lone pairs are approximately perpendicular in the axial conformation and parallel in the equatorial conformation. The latter position is highly unfavorable. Considering systematic differences in structural parameters derived by the three different methods, the agreement for the bond lengths is very close, with two exceptions. The X-ray value for the C-C distance in molecule b (1.582(3) A) is significantly larger than gas-phaseor ab initiovalues. This can be rationalized
4
9628
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
Boese et al.
TABLE IIk Atomic Coordinates ( X l W ) and Equivalent Isotropic Displacement Coefficients Ulr atom X Y z Ui#olUCq S(1A) N(2A) H(1A) S(3A) C(4A) C(5A) F( 1A) F(2A) F(3A) F(4A) S( 1B) N(2B) H(1B) S(3B) C(4B) C(5B) F( 1B) F(2B) F(3B) F(4B)
8884(1) 10129(2) 10564(32) 9277(1) 7721(2) 7856(2) 7564(2) 6493(2) 8535(2) 6546(2) 13842(1) 12698(2) 11976(39) 12340(1) 11995(2) 12948(2) 10601(2) 12296(2) 12086(2) 13929(2)
6246(1) 6911(1) 7175(22) 7694(1) 7962(2) 7360(2) 8945(1) 7683( 1) 7948(1) 7141(1) 5804(1) 5855(2) 6097(29) 4671(1) 4192(2) 4775(2) 4293(1) 3204(1) 5075(1) 4154(1)
6700(1) 6196(1) 6455(18) 5568(1) 6147(1) 6888(1) 6304(1) 5796( 1) 7389(1) 7166(1) 6161(1) 5416(1) 5592(19) 5110(1) 6026(1) 6614(1) 6230(1) 6055(l) 7178(1) 6921(1)
16(1) 16(1) 18(7) 17(1) 15(1) l6(l) 24(1) 26( 1) 23(1) 25(1) 30(1) 20(1) 35(9) 26(1) 17(1) 16(1) 29(1) 24(1) 28(1) 26(1)
F30
linked by a hydrogen bond. Views perpendicular to the plane of molecule a (left) and of molecule b (right).
bond lengths (A) and angles (deg)
S(1A)-N(2A) N( 2 A ) S ( 3A) C(4A)-C( 5A) C(4A)-F(2A) C(5A)-F(4A) S( 1B)-C(SB) S(3B)-C(4B) C(4B)-F( 1B) C( 5B)-F( 3B) N(ZA)S(lA)-C(5A) N( 2 A ) S ( 3A)-C(4A) S(3A)-C(4A)-F(lA) S(3A)-C(4A)-F(2A) F(lA)-C(4A)-F(ZA) S(lA)-C(SA)-F(3A) S(lA)-C(SA)-F(4A) F(3A)-C( 5A)-F(4A) S(1B)-N( 2 B ) S ( 3B) S(3B)-C(4B)-C(5B) C(SB)-C(4B)-F(lB) C(5B)-C(4B)-F(2B) S( lB)-C(SB)-C(4B) C(4B)-C( 5B)-F(3B) C(4B)-C( 5B)-F(4B)
1.7 15(2) 1.729(2) 1.560(3) 1.351(3) 1.344(3) 1.801(2) 1.793(2) 1.348(3) 1.349(3) 91.4(1) 96.3( 1) 113.4(2) 110.4(1) 106.0(2) 111.2(1) 111.4(1) 107.4(2) 109.6(1) l09.6( 1) 107.6(2) 110.0(2) 109.4( 1) 108.5(2) 1 10.0(2)
S( 1A)-C(SA) S(3A)-C(4A) C(4A)-F(lA) C(5A)-F(3A) S(lB)-N(2B) N( 2 B ) S (3B) C(4B)-C(5B) C(4B)-F(2B) C(5B)-F(4B) S(lA)-N(2A)S(3A) S(3A)-C(4A)-C( 5A) C(SA)-C(4A)-F(lA) C(SA)-C(4A)-F(ZA) S(lA)-C(SA)-C(4A) C(4A)-C(SA)-F(3A) C(4A)-C(5A)-F(4A) N ( 2 B ) S ( 1B)-C( 5B) N( 2 B ) S ( 3B)-C(4B) S(3B)-C(4B)-F( 1B) S(3B)-C(4B)-F(2B) F(lB)-C(4B)-F(ZB) S( lB)-C(SB)-F(3B) S( 1B)-C(SB)-F(4B) F(3B)-C( 5B)-F(4B)
1.802(2) 1.8 1l(2) 1.355(3) 1.351(3) 1.706(2) 1.71O( 2) 1.582(3) 1.353(2) 1.349(3) 110.6(1) 108.8( 1) 109.4(2) 108.8(2) 108.1(1) 107.7 (2) 110.9(2) 95.7(1) 94.1( 1) 112.6(2) 110.4( 1) 106.5(2) 112.5(2) 110.3(1) 106.0(2)
Equivalent isotropic U defined as one-third of the t.race of the orthogonalized U, tensor.
I
I
I
I
0
5
10
15
I
20
I
I
I
25
30
35
slA-’
Figure 6. Experimental (dots) and calculated (full line) molecular intensities of 1 and differences. differences between the different methods are especially large for the dihedral angles. Whereas ab initio calculations derive equilibrium values, GED determines vibrational averages which can deviate appreciably in molecules which perform large amplitude motions, such as ring out-of-plane vibrations. In the solid phase, these dihedral angles are most affected by intermolecular interactions such as N-H-N bridges and packing effects. Despite these differences, the dihedral angles for the gas phase, crystal (molecule a) and ab initio structures are very close. Also, the sum of the ring bond angles (&xw), which measures the degree of aplanarity of the five-membered ring (&, = 540’ for planar structures), is nearly equal for the gas-phase, crystal, and ab initio structures.
Experimental Section
Figure 4. Conformations of 1 in the gas phase (above)and in the crystal (below).
by the eclipsed position of the CFz groups in this molecule. The ab initio calculations predict C-F bonds too long in the absence of polarizationfunctions and too short with polarizationfunctions. The N S - C bond angles derived in the gas phase are about 3O larger than the solid-state and ab initio values. The systematic
The title compound 1 was obtained by the reaction of tetrafluoro- 1,Zethanedisulfenyl dichloride with ammonia at low temperature. A detailed description of the synthesis and of the chemistry of this compound will be published elsewhere.13 The purity of the sample was checked by the melting point (mp = -35 “C); by elemental analysis; and by infrared, mass, and NMR spectroscopies. The GED intensities were recorded with a Gasdiffraktograph KD-G214 at two camera distances (25 and 50 cm) and with an accelerating voltage of ca. 60 kV. The electron wavelength was calibrated by ZnO powder diffraction. The sample reservoir was cooled to 9 OC, and the inlet system and nozzle were maintained at room temperature. Two photographic plates (Kodak Electron Image plates, 13 X 18cm) for each camera distance were analyzed with the usual procedures.1s Averaged molecular intensities in the s ranges 2-18 and 8-35 A-l in intervals As = 0.2 A-1 are shown in Figure 6, and numerical values for total scattering intensities are available as supplementary material. X-ray Study of C2HFdNS2: mol. weight 179.15. Crystal dimension and color: cylindric, 0.3-mm diameter, colorless, orthorhombic; T = 125 K. Nicolet R3/mV diffractometer, cell dimensions [A]: a = 9.256(2), b = 13.389(3), c = 17.977(3); V
Conformation of Saturated Five-Member Rings
= 2223.8(7) A', 2 = 16, p d c = 2.007 g/cm3, space group Pbca (no. 61), Mo Ka radiation, graphite monochromator, 29,, = So, = 0.89 "-1. Number of reflections collected 3822 (-9 Ih I12,O Ik I17,O II I23); 2542 independent reflections (Rmerg= 0.0248), of which 2276 were treated as observed (FoL 4u(F)). LP, extinctioncorrectionand correctionfor a cylindrical crystai,I6 structure solution by direct methods and refinement on F(ful1 matrix, all atoms anisotropicexcept hydrogen atoms which were refined without constraints and with isotropic u's.) with SHELXTL-Plus on a Micro-Vax IIa, 172parameters, R = 0.044, R, = 0.045 (wl = u2(Fo) O.O0056F0*, maximum residual electron densitv 0.50 e A-3.
+
Acknowledgment. We thank the Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie for financial support. SupplementaryMaterial Available: Intensitiesfor 1, correlation coefficients, checklist for crystal structures, atomic coordinates, anisotropic temperature factors, bond distances, and angles (8 pages); structure factor tables (10 pages). Ordering information is given on any current masthead page.
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9629
References and Notes (1) (a) Essen. (b) Tilbingen. (c) Arkansas. (d) Bochum. (2) Pitzer, K.;Donath, W. E. J. Am. Chem. SOC.1959,81,3213. (3) Kirby,A.J. TheAnomericEffect andRelatedSterecchemicalEffects 1983* " Oxygen; Springer: (4) For a review on conformation of five-membered rings, see: Legon, A. c. Chem. R ~ V1980.80.231. . ( 5 ) Pfafferot, G.; Oberhammer, H.; Bow,J. E.; Caminati, W. J. Am. Chem. Soc. 1985,107,2305. (6) Roesky, H.W.; Thiel, A.; Noltemeyer, M.; Sheldrick, G. M. Chem. Ber. 1985,118,2811. (7) Pulav, P. TX 90, Fayettville, AR, 1990. Pulav. P. Theor. Chim. Acta i979,50, 299. (8) Pulay, P.; Fogarasi, G.; Pang, F.; Bogga, J. E. J. Am. Chem. SOC. 1979.101. 2550. (9) Haase, J. Z.Naturforsch. A 1970,25, 936. (10) Oberhammer, H. EBI program package for GED analysis. (1 1) Boese, R.; Nussbaumer, M. In Situ Crystallization Techniques. In OWnic Crystal Chemistry;IUCr CrystallographicSymposia, Vol. 6; Oxford, 1993,in press. (12) Brodalla, D.;Mootz, D.; Boese, R.; Osswald, W. J. Appl. Cryst. 1985. 18. 316. -(13) Haas, A.; Waterfeld, A., unpublished results. (14) Oberhammer, H.Molecular Structure by Diffraction Methods; The Chemical Society: London, 1976;Vol. 4,p. 26. 70,(15) 273.Oberhammer, H.; Gombier, W.; Willner, H. J. Mol. Struct. 1981,
,---.----
--.--.
(16) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, England, 1972;Vol. 11, p. 291.
