Conformational analysis of strained bis-(trimethylsilyl) derivatives by molecular mechanics calculations and 29Si NMR

Journal of Molecular Structure, 3 19 (1994) 167-l 75 0022-2860/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved

167

~onformational analysis of strained bis-(trimethylsi~yl) derivatives by molecular mechanics calculations and 29Si NMR Michel Laguerrea, Micheline Grignon-Duboisb3* aLaboratoire de chimie analytique, Fact&e’ de Pharmacie, Universite’ Bordeaux II, 3, Place de Ia Victoire, 33076 Bordeaux-Cedex, France ~~boratoire de chimie org~ique et orga~om~tal~ique, Wniversit~ Bordeaux I, 351, Cours de la Li~~rati~n, 33405 Talence-Cedex, France

(Received 23 September 1993) Abstract This paper describes a ~nfo~ational study of hindered gem-disilyl derivatives using “5 NMR and molecular mechanics calculations (MM2 and MM3). Angle valence deformations and conformational preference due to steric hindrance between the two silyl groups are demonstrated. Good agreement between the results obtained by these two approaches is observed. They show their comp~emen~~ty and their usefulness for confo~ational study.

Introduction Organosilicon compounds are useful tools for the introduction of functionalities (see ref. 1 for reviews), and therefore it is impo~ant to be able to assign their geometry. We have previously described stereochemical dete~inations using spectroscopic methods like *‘Si NMR [2,3] or IR and Raman spectroscopies [4]. We are now involved in conformational determinations, especially in strained polysilyl derivatives. In the case of medium-size molecules, such information can be efficiently obtained using molecular mechanics (MM) calculations. The literature contains a few reports of MM calculations for organosili~on compounds [5]. However, according to these authors, these geometries can be correctly

* Corresponding

author.

SSDI 0022-2860(93)07926-N

described using parameters defined by Allinger in the MM2 force field [6]. In this paper, we report a conformational study of three gem-bis(t~methylsilyl) derivatives l-3 using MM calculations and 29Si NMR. Values of 3J(29Si,‘H), which have been shown to be a function of the Si-C-C-H dihedral angle [2,3], could provide info~ation about the conformational preference of compounds 1 and 2, which are sterically hindered. Moreover, the study of 29Si,‘H coupling constants is of particular interest in the case of symmetric molecules like 1. Indeed, all protons being chemically equivalent, ‘H,‘H coupling constants data leading to the molecule’s confo~ational preference cannot be extracted from the ‘H NMR spectra. In contrast, in 29Si NMR, approximately 95% of the signal is due to dissymmetric molecules containing only one silicon atom of isotope 29, the others being the more abundant isotope 28.

168

M. Laguerre, M. Grignon-Dubois/J. Mol. Struct. 319 (1994) 167-175

SiMea

SiMe,

SiMe,

MqSi

iMe,

Me&3 xs

iMq

H&CO

1

Me,Si

\

d

3

2

Si*

Si* -%.

Si* Si* t : I

Si*ti 5

Si+

I

i Si* Si* 6

Si*= SiMe,

Experimental

29Si NMR measurements All the 2gSiNMR experiments were performed on a Bruker AC 200 spectrometer (*H: 200.132 MHz; 2gSi:39.761 MHz) fitted with an Aspect 3000 data system using a 1Omm broad-band probe (29Si 90 pulse-width: 15 ps; ’ H 90” pulp-~dth through the decoupler coil: 29 ps). 2gSi spectra were recorded at room temperature as a 30% (w/v) solution in CbD6 using a sweep width of 355Hz digitized with 8K data points (0.087Hz per point) (polarization transfer delay, r = 0.0378 s, refocusing delay, A = 0.01495 s). 2gSi chemical shifts were determined using INEPT with refocusing for decoupled spectra [7]. They are referenced to TMS (internal standard) and expressed as Gppm. The (2gSi,‘H) coupling constants were obtained from spectra recorded during a proton selective decoupling of the methylsilyl groups by refocused INEPT [8] and from SPT spectra [9]. The following parameters were used: r = 0.0378 s, A = 0.018 s, decoupling power S2 = 30 L for the INEPT experiment and ‘H SPT, 180” pulse 0.1 s, 2gSi 90” pulse 15 ps, recycle delay = 10 s for the SPT experiment.

All the SPT spectra were simulated by calculation on a Microvax 3100 computer using an in-house program [lo]. Generally, width at halfheight was taken close to 0.6 Hz. ~~le~~lar modelling Calculations were performed on a local network consisting of 3100-80 and 3100-20 DEC station computers and an INDIGO 4000 Elan Silicon Graphics workstation running the MACROMODEL program (versions 3.0 and 3.5 respectively) [1 11. Conformational minima were found with the multiconformer submode using the modified MM2 (87) or MM3 (91) force fields implemented in the program [12]. In the case of 1, the H-C-CH torsion angle was explored in 15” increments over the full 360” circle, while the four C-Si torsion angles were explored in 60” increments. One hundred and thirty-seven reasonable conformers were thus generated and then fully minimized to a final r.m.s. gradient < 0.005 kJ A$-’mol-* using 250 cycles of the truncated Newton conjugate gradient method (TNCG) [ 131, followed by 30 cycles of the full-matrix Newton-Raphson method (FMNR). All unique conformers within 40 kJ of the global minimum were reported

M. Laguerre, M. Grignon-Dubois/J.

Mol. Struct. 319 (1994)

and classed in ascending energy, resulting in 63 “different” conformers, which, due to equivalencies of the methyl hydrogens, fell, in fact, into only two classes with respective energies of 5.05 and 9.04 kJ mol-‘. The same approach used while choosing the MM3 force field resulted in only one class of conformers, with an energy of 64.54 kJmol_‘. 3 and 4 were studied in the same way. On account of the lack of several parameters in the MM3 force field, 2 was only studied using MM2. The C-X(=0) and C-C(Cl)(SiMe& torsional angles were simultaneously explored in 15” increments over the full 360” circle. Ninety-two reasonable conformers were thus generated and then fully minimized to a final r.m.s. gradient G 0.005 kJ A-’ mol-’ using the same methods as above, resulting in the ten different conformers listed in Table 3. With 1 and 2, several attempts were made using a molecular dynamics approach (leap-frog molecular dynamics algorithm according to ref. 14). The following parameters were used: time step = 1.5 fs, total time = 20 ps, initial tem~rature = 300 K, final temperature = 400 K (thermal bath option with 0.2~s coupling time constant), translational and rotational momentum periodically set to zero (momentum ON), time sampling or geometry sampling (r.m.s. geometry difference = 0.2 A). In all cases, using MM2 or MM3 (only for l), the calculations resulted in the only lowest-energy conformer for 1 and for 2, in the two lowest-energy conformers previously found by multiconformer search.

Table 1 _^ ??+i NMR data for compounds Compound

1-3 (coupling constants

d *‘Si

169

167-175

Results and discussion 29SiA’MR study 29Si SPT spectra with polarization transfer from the trimethylsilyl protons [9] and INEPT spectra with selective decoupling of the trimethylsilyl protons [15] of compounds 1 and 2 were recorded. Their complete analysis using an inhouse program [lo] has been performed, allowing access to all the coupling constants (Table 1). The 29Si NMR spectra of 1 and 2, as well as the ‘H and “C NMR spectra, show two signals for the silicon groups (Fig. 1). The IH chemical shifts of the trimethylsilyl groups have been unambiguously assigned, taking advantage of the frequency selectivity of the 29Si SPT sequence which allows correlation of 2gSi and ‘H chemical shifts of polysilyl derivatives. It is interesting to note that the more shielded the ‘H NMR signal is, the more deshielded the 2qSi NMR signal is, as we observed in the cyclopropylsilane series [2]. Analysis of the silicon resonances of 1 and 2 leads to two different sets of coupling constants. This is consistent with a non-anti conformation of 1 and steric hindrance preventing free rotation of the disilylmethyl group at room temperature in both cases. In contrast, the unique signal in the 29Si as well as the ‘H and 13C NMR spectra of 3 prove this compound to be in free rotation about the carbon u bond. Therefore, examining the values of the coupling constants and the 29Si chemical

are given to 10.2 Hz)

*‘Si,‘H coupling constants

(Hz)

6’H SiMe3 groups

@pm) *J(9H)

2J’(1H)

‘J( 1H)

(ppm)

1

SiA 1.1 Sia 4.1

6.2 6.2

12.2 12.4

11.2 9.6

0.00 0.07

2

SiA 5.4 Sia 6.2

6.4 6.4

_

10.5 6.2

-0.32 0.00

3

0.8

6.4

8.8

0.9

0.00

170

M. Laguerre, h4. Grignon-Dubois/J.

Mol. Struct. 319 (1994) 167-175

_ s --+====-

M. Laguerre.

M. Grignon-Dubois/J.

Mol. Struct. 319 (1994)

167-175

shifts gives access to some interesting clues concerning the conformational preference of these compounds. (1) The 29Si,1H coupling with the Q proton (2J’), expected to be the same for each set of silicons in compound 1, is found to be respectively equal to 12.2 and 12.4 Hz. It is interesting to compare these values with the 8.8 Hz coupling measured in compound 3, which is not sterically hindered. We had previously observed such a difference in magnitude between the endo and exo isomers of trimethylsilyl bicyclocycloalkane derivatives [2(a),3], the values of which were respectively close to 3 and OHz. We had attributed this difference to a distortion of the H-C-Si angle in the endo isomer. The values found in this paper are in accordance with this hypothesis and show the sensitivity with which 29Si techniques demonstrate valence angle deformation due to steric hindrance. It is worth noting that the 2J’ value found in 3 is very close to the 8.4Hz constant measured for the allylic silicon in 1,3-bis(t~methylsilyl)propene [2(b)], confirming an undistorted Si-C-H angle in 3. (2) The magnitude of the 3J coupling constants in the case of 1 (9.6 and 11.2 Hz) leads to dihedral Si-C-C-H angle values close to 15 and 155”, implying an almost eclipsed conformation. Indeed, a gauche conformation with normal dihedral angles of 60 and 180” should lead to one 3J approximately twice as great as the other. This is almost the case with compound 2, for which the measured 3J values (10.5 and 6.2 Hz) are in agreement with a gauche confo~ation having the H-l proton between chlorine and one of the silicon groups. (3) The difference in magnitude of the two 29Si chemical shifts in 1 clearly reflects a geometry which requires the two kinds of silicon to be in a different environment. Such a difference is not observed in 2, the chemical shifts of which are similar to those observed with cY-chlorosilylcyclopropanes [2(c)]. According to our previous results for gem-bis(trimethylsily1) derivatives [2(a),2(c),3], we can assume that the resonance at 1.1 ppm is due to the more hindered silicon (i.e. S&J and that at

171

4.1 ppm to Sin, which is less hindered. Brownstein et al. [16], on the basis of a dynamic ‘H and 13C NMR study, previously discussed the conformational preferences of 1. They concluded that 1 adopts a gauche conformation with a distortion at the tertiary carbons, assuming the Si-C-Si angle to be considerably greater than a tetrahedral angle on account of repulsion between the geminal silyl group (but neither angle value nor signal attribution were given). In fact, we will see from the MM calculations that the more distorted angle is the CC-SiA one. Pentacoordination of silicon due to an intramolecular Si +- 0 bond has been reported [17] in the cases of dimethyl(N-diacetylaminomethyl) chlorosilane and N,N-dialkyl-N’[(dimethylchlorosilyl)methyl]ureas. Such a coordination, theoretically possible in 2, can be rejected on the basis of the 29Si chemical shifts and the normal IR frequency of the carbonyl group that has the same 1710 cm-’ value for the solid and in solution (CC0

Structural data have been previously reported for CHz(SiMes), (4) using both gas electron diffraction (GED), semiempirical SCF methods (MNDO technique) and molecular mechanics (BIGSTRN-2). The value found for the Si-C-Si angle by GED is 123.2” [18] while the values calculated by MNDO and MM are 125.5 and 121.2” respectively [19]. Good overall agreement between experimental and calculated parameters was also reported for HC(SiMe)3, showing the validity of the MM approach in this kind of compound: 117.2” by GED [20], 116.0” using MNDO [19] and 115.3” with MM [19]. These observations show that the Si-C-Si angle is substantially distorted in bis(trimethylsilyl)methane but not in tris(trimethylsilyl)methane, and prompt us to correlate our NMR results with geometrical data obtained by calculation. In our case, we have performed a molecular mechanics study using MM2 (1987 parameters)

172

Table 2 Geometrical

M. Laguerre, M. ~rig~~n-~boislJ.

parameters

Compound

1 3 4

(A, deg) for compounds

1,3 and 4 from MM2 and MM3 calculations

MM2

MM3

C-Si

Si-C-Si

%-C-H

C-Si

Si-C-Si

Si-C-H

1.91 1.89 1.89

116.1 118.5 119.2

96.1 104.4 107.2

1.92 1.89 1.88

114.8 120.7 120.4

100.1 106.3 107.1

and MM3 (1991 parameters) force fields [ 121. Geometrical parameters for compounds l-3 are reported in Tables 2-4 along with those we found for bis(trimethylsilyl)methane 4 taken as reference. Examination of Table 2 suggests the following comments. In compound 1, which is the most hindered, we noticed a lengthening of the C-Si bond (1.91 to 1.92A) compared to compounds 3 and 4 (1.89A) and the 1.888, unperturbed bond length. We have previously observed the same phenomenon in severely hindered polysilyl derivatives like 5-7, for which crystal structures reveal C-Si bonds ranging from 1.884 to 1.936A 1211. Comparison Table 3 Conformational Conformer

Mol. Struct. 319 (1994) 167-175

study of compound

of the calculated valence angles with the 109.8” normal value shows an opening of the Si-C-Si angle and an important closing of the two Si-CH angles. Attempts to build the molecule in a classical anti conformation provoked a dramatic increase of van der Waals repulsion. As a result, a spontaneous rotation of the CH(Sij2 group about the C-C bond leads to a distorted eclipsed conformation. The final stable conformer (Fig. 2) is characterized by: (i) a lengthening of the C-C bond (1.59A with MM3 and 1S6 A with MM2), (ii) a weak SiA-C-C-SiA dihedral angle (52.5” with MM3 and 60.2” with MM2) counterbalanced

2: energy levels (kJ mol-‘), bond lengths (&tgstriims)

and geometrical dataa

Energy

C-SiA

C-Sia

C-C(Si)s

C-Cl

C6 cycle geometry

C=O position

1 2 3 4

45.44 48.11 50.19 50.24

1.915 1.915 1.915 1.912

1.924 1,929 1.926 1.929

1.568 1.566 1.566

1.561

1.814 1.814 1.812 1.814

twist twist twist twist

towards towards towards towards

5

65.66

1.917

1.921

1.574

1.820

sofa

6

70.84

1.915

1.926

1.577

1.824

twist

7

73.33

1.920

1.921

1.574

1.821

sofa

8

77.11

1.914

1.927

1.577

1.824

twist

9

78.83

1.923

1.933

1.573

1.817

twist

10

82.22

1.922

1.932

1.574

1.815

“sofa”

towards Si Si.e.0 = 3.77 towards Si Si . s. 0 = 3.38 towards Si Si . I. 0 = 4.28 towards 2 Si Si...O=3.98 Si . . 0 = 4.05 towards Si Si . . . 0 = 3.66 towards Si Si . . .O = 3.52

a SiA and Sin are respectively the silicon in anti and gauche positions relative to the hydrogen atom. For conformers there is no silicon in anti position but one in gauche position and one in syn position.

Cl Cl Ci Cl

9 and 10

M. Laguerre. M. Grignon-Dubois/J.

Table 4 Conformational

Mol. Struct. 319 (1994)

study of compound

173

f67-175

2: valence and torsional angles (degla

C-C-S&

c-c-Sia

Cl-C-S&

Cl-C-sis

Si-C-Si

H-C-C-SiA

H-C-C-SiB

H-C-C-H

115.9 117.1 116.1 116.9

110.0 110.9 110.2 111.2

102.8 103.4 103.5 104.2

102.0 102.3 101.5 101.3

114.9 113.3 114.6 113.5

162.9 177.5 165.3 174.5

30.5 45.3 32.9 53.0

163.4 165.4 168.8 176.8

Mean value

116.5f0.6

110.6f0.6

103.5f0.7

101.8ztO.5

114.1f0.8

170.01fr7.5

40.4f12.5

168.6 f 8.2

5 6 7 8 9 10

115.4 116.3 116.4 116.5 118.6 118.1

112.8 114.5 112.6 114.6 111.0 110.3

104.2 103.1 103.8 102.5 100.7 101.1

101.1 98.0 99.8 97.7 106.5 106.7

114.3 114.5 113.7 114.9 110.3 110.3

168.5 175.6 171.3 170.9 89.1 90.1

57.6 38.4 37.5 32.7 40.0 38.0

135.8 174.0 137.7 176.4 161.2 162.0

Conformer 1 2 3 4

a SiA and Sia are respectively the silicon in anti and gauche positions with respect to the hydrogen atom. For conformers 10 there is no silicon in anti position but one in gauche position and one in syn position.

by a large SiA-C-C valence angle (119.9”) due to repulsion of the two SiA atoms. In contrast, the Siu-C-C-Sia dihedral angle is found equal to 126.4” with MM3 and 118.1” with MM2, and a 116.5” Sin-C-C angle showing a weak repulsion Sia-Sia in accordance with the greater Sia . . . Sin distance (SiA . . I SiA = 3.81 A and Sia . . . Sia = 4.41 A). (iii) H-C-C-Si dihedral angles measured as 13” (Sia) and 154.4” (SiA) using MM3 and 11” (Sin) and 160. lo @iA) using MM2. (iv) a Si-C-Si valence angle smaller than in compounds 3 and 4 showing that, contrary to

a

9 and

the predictions of Brownstein et al, 1161,this angle cannot be considerably increased in highly strained molecules. These calculated parameters agree perfectly with the measured 29Si NMR data. In particular, the dihedral Si-C-C-H angle values correlate well with our prediction based on the ma~itude of 3J (29Si,‘H) coupling constants, i.e. 15” (Sin, 9.6Hz) and 155” (SiA, 11.2 Hz). Alternatively, replacing SiA or Sia by a hydrogen atom results in a calculated ‘J coupling constant (see ref. 22, as implemented in MACROMODEL [ll]) 20% greater with SiA than with Sia honeying both our 2%i

3.24 f 0.01

b

c

Fig. 2. Geometry of the lowest-energy conformer for compound 1: (a) Newman projection; (b) above view; (c) side view showing deviations of the silicon atoms from the standard position (dotted lines).

174

AU. Laguerre. M. Grignon-Dubois/J. Mol. Struet. 319 (1994) 167-175

S-based attribution and the existence of the 3J (29Si,1H) Karplus-type relationship as we had previously assumed [2,3]. For compound 3, in which the two CH(Si)* groups cannot interact, as expected, we found geometrical parameters close to those of disilylmethane 4 (Table 2). It is interesting to note the almost normal value (compared to 4) for the Si-C-H valence angle as expected from the ‘J’ magnitude. This fact correlates well with the large ‘5’ values (12.2 and 12.4 Hz) measured for compound I, for which we observed a dramatic closing of the valence angle. Similar calculations with compound 2 leads to ten different conformers (Tables 3-4). Examination of the energy shows that only the first four can really exist in solution. These four nearly identical conformers (Fig. 3) are characterized by an Si-C-Si valence angle close to 114” and Si-C-C-H dihedral angles close to 170 and 40” respectively. These last values closely agree with the geometry we assumed on the basis of 29Si NMR data. Examination of Tables 3 and 4 shows a different behaviour of the two silicon groups as

E = 48.77 kJ/mole

E = 45.44 kJ/mole

c---

0

H

-3 /

AL

E = 50.19 kJ/mole

E = 50.24 W/mole

Fig. 3. Geometries of the four lowest-energy conformers of compound 2 (Newman projection along the C-C(Si)2 bond).

observed with 1. With the gauche silicon atom versus H-l (Sin), we observed an almost normal C-C-Si angle but a long C-Si bond (1.927 A mean value), whereas with the anti silicon atom @iA) the large C-C-Si angle is associated with a shorter Si-C bond (1.914A mean value). The opening of the C-C-Si valence angles leads to a noticeable closing of the Cl-C-Si angle. It appears that the chlorine atom plays in this case a similar role to the hydrogens in compound 1. As expected, we noticed a lengthening of the C-C(Si*) bond (1.567A mean value). These results agree well with calculations reported by Profeta et al. [S(c)]. This study also confirms that none of the reasonable conformers, as expected from 29Si NMR data, can lead to an intramolecular silicon-oxygen interaction. The shorter Si . . ’ 0 distance (3.38A in conformer 6, Table 3) is too long to allow an interaction, as shown by comparison with the 2.077 A Si f . +0 distance [17] and the 2.781 A Sn. ’ .O distance [23] measured in similar compounds by X-ray diffraction. Conclusion

This work shows that the repulsion between the two silicon atoms in a bis(trimethylsily1) group leads to an important opening of the Si-C-Si valence angle only with non-strained molecules like 4. In contrast, steric hindrance leads to opening of the C-C-Si angle(s) and lengthening of the Si-C bonds. It is worth noting that the larger CC-Si angle deformation is always associated with the weaker C-Si bond lengthening. Moreover, we observed in all cases a lengthening of the C-C bond adjacent to the C-Si bond(s). These results demonstrate that molecular mechanics studies can provide an accurate and detailed description of the conformational preference of organosilicon compounds that is in good agreement with 2qSi NMR experimental data. The association of these two techniques constitutes a tool of choice for structural analysis of silyl compounds, the knowledge of which is necessary for the understanding of their reactivity.

M. Laguerre, M. Grignon-Dubois/J. Mol. Struct. 319 (1994)

167-175

175

Acknowledgements We are grateful to M. Pktraud and B. Barbe (Cesamo, UniversitC Bordeaux I) for recording the NMR spectra. The NMR spectrometer was purchased with funds from the CNRS and Conseil Regional d’Aquitaine.

9

10 11

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W.P.

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