2417
Bond Orientation in 1,3,2-Dithiaphosphorinanes chapter 1 (1975); (b) D. A. Torchia and J. R. Lyeria, Jr., Biopolymers, 13, 97 (1974); (c) R. A. Komoroski, I. R. Peat, and G. C. Levy, Blochem. Biophys. Res. Commun., 65, 272 (1975). (20) R. Deslauriers, 2. Grzonka, K. Schaumburg, T. Shiba, and R. Walter, J. Am. Chem. SOC.,97, 5093 (1975). (21) A. Allerhand and R. A. Komoroski, J. Am. Chem. SOC., 95, 8228 (1973). (22) R. A. Komoroski, Ph.D. Thesis, Indiana University, Bloomington, Ind., 1973. (23) J. Grandjean and P. Lasio, Mol. Phys., 30, 413 (1975). (24) (a) "Tables of Interatomic Distances and Configuration in Molecules and Ions", Supplement, Special Publication No. 18, The Chemical Society, London, 1965; (b) "Molecular Structures and Dimensions", Vol. AI, 0. Kennard et al., ed., International Union of Crystallography, 1972. (25) K. C. Cole and D. F. R. Gilson, J. Chem. Phys., 60, 1191 (1974).
(26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37)
G. C. Levy and I. R. Peat, J. Magn. Reson., 18,500 (1975). A. Gierer and K. Wlrtz, Z.Naturforsch. A, 8 , 532 (1953). J. A. Glasel, J. Am. Chem. SOC.,91, 4571 (1969). J. T. Edwards, J. Chem. €doc., 47, 261 (1970). A. Bondi, J. Phys. Chem., 68, 441 (1964). J. H. Noggle and R. E. Schirmer, "The Nuclear Overhauser Effect", Academic Press, New York, N.Y., 1971, pp 26-31. H. G. Hertz, Ber. Bunsenges. Phys. Chem., 75, 183 (1971). H. G. Hertz and M. D. Zeidler, personal communication. R. E. D. McClung and D. Kivelson, J. Chem. Phys., 49, 3380 (1968). R. A. Assink, J. DeZwaan, and J. Jonas, J. Chem. Phys., 56, 4975 (1972). C. M. Hu and R . Zwanzig, J. Chem. Phys., 60,4354 (1974). D. R. Bauer, J. I. Brauman, and R. Pecora, J. Am. Chem. SOC.,96,6840 (1974).
Axial and Equatorial Bond Orientation around Phosphorus in 1,3,2=Dithiaphosphorinanes. Use of J(31P1H)and J(31P13C)for Stereochemical Assignments J. Martin, J. B. Robert,' and C. Taieb Laboratoire de Chimie Organique Physique, Mpartement de Recherche Fondamentale, Centre d'Etudes Nucleaires de Grenoble, 85 X 3804 1. Grenoble Cedex, France (Received April 19, 1976) Publication costs assisted by C.E.A. France
The total NMR spectral analysis (lH, l3C, and 3lP) of ten 2-R-1,3,2-dithiaphosphorinanesbearing different R groups (CH3,l; C6H5,2; OCH3, 3; C1,4; C - C ~ N5;, N(H)tBu, 6;c-CbN, 7; t-Bu, 8; N(iPr)z, 9; and N(iPr)tBu, 10) have been analyzed. For compounds 1-5, the P-R bond is axial, and for compounds 9 and 10 it is equatorial. In the case of compound 6-8,there exists a t room temperature a conformational equilibrium. The NMR coupling constants 3J(PH), 4J(PH), and 4J(PC) are highly sensitive to bond orientation around the phosphorus and can be used as a tool for stereochemical assignment.
, t-Bu
Introduction In 2-R-1,3,2-dioxaphosphorinanes, the preferred orientation of the P-R bond has been established for various R groups from dipole moment measurements,la NMR chemical shifts,lb deductive reasoning concerning the stereochemical course of exchanging one R group for another, and x-ray worklcon the corresponding four coordinated phosphorous compounds. Electronegative R groups (Cl, F, OCH3, OC6H5) have a strongly preferred axial orientation. For R = alkyl, there exists an equilibrium which shows an increasing amount of the equatorial form as the bulk of the substituent R In the case of a tervalent nitrogen atom directly bonded to the phosphorus atom, the extracyclic P-N bond can adopt either an axial or an equatorial orientation, depending upon the nature of the groups attached to the nitrogen a t ~ m . ~ - ~ In 2-R-1,3,2-dithiaphosphorinanes, the very few conformational studies support a chair conformation for the ring with axial orientation of the extracyclic P-R bond.8-10 We report here the full NMR spectral analysis (1H, l3C, 31P) of several 2-R-1,3,2-dithiaphosphorinanes (1-10) in which the ring carbon atoms bear only hydrogen atoms, thus obtaining as much stereochemical information as possible from the NMR data. Depending upon the nature of the R group, the P-R bond is shown to adopt an axial or an equatorial orientation.
c!P-R
l,R=CH,
6,R=N H'
Z,R=CsH,
7,R=N
" 3, R = OCHB 43,2-dithiaphosphorinane 4,R=C1 5,R =
N3
3
8, R = t-Bu 9, R = N( i-Pr)2
,t-Bu 10,R = N 'i-R
NMR Spectral Analysis Proton NMR spectra of compounds 1-10 were recorded a t 100 and 250 MHz. They display similar multiplets for the ring
protons representing AA'BB'CDX patterns (X phosphorus). The spectral analyses are considerably more difficult than those of the corresponding 1,3,2-dioxaphosphorinanes due to the smaller low field shift induced by a sulfur atom as compared to that induced by an oxygen atom. 31P decoupled spectra were recorded in order to obtain simpler spectra from which good approximate values for the chemical shifts and coupling constants could be extracted. The final parameters have been obtained using the iterative NMR program LAOCOON 3 (Figure 1).The NMR data (lH, 13C, and 31P)are presented in Tables I (lH) and I1 ( 13C,31P). The Journal of Physical Chemistry, Vol. 80, No. 2 1, 1976
2418
-: J.
Martin, J. B. Robert, and C. Taieb
-t .l"
Figure 1. Observed and calculated proton NMR spectrum (250MHz) of 2-fert-butylamino-l,3,2-dithiaphosphorinane(6).
Discussion of the Results The NMR results concerning compounds 1-5 are quite comparable and are characterized by the following main features: (i) A large 3J(H4aHga)value of about 12 Hz, indicative of axial-axial coupling, which allows the assignment of axial and equatorial protons on C4, CS,and Cg. All of the 3J(HH)values are close to the ones measured on 2-phenyl-1,3-dithiane1° which shows in the solid state a somewhat flattened chair conformation with the phenyl group in the equatorial position.ll (ii) Two small 3J(PSCH) cupling constants with 3J(PSCH4a) > 3J(PSCHde). In contrast to the dioxaphosphorinanes, the 3J(PH) coupling constant corresponding to the axial proton is higher than the one corresponding to the equatorial proton. (iii) A large 4J(PSCCH5,) coupling ranging from 7.7 to 10 Hz. It is noteworthy that in the 1,3,2-dioxaphosphorinanes and in the 1,3,2-diazaphosphorinanes12 the two V ( P H ) values The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
are smaller than the 3J(PH) values. The difference observed between the two 4J(PSCCH) values indicates that such a coupling is highly dependent upon geometrical parameters. (iv) A near zero value for 3J(PSCCg). From previous studies on 2-R-1,3,2-dioxaphosphorinanes which exist in a rigid chair conformation, the 3J(POCCg) NMR coupling is larger for an equatorial P-R bond than for an axial one.l3J4 A typical value is 10 Hz for a P-R equatorial bond and 5 Hz for an axial P-R one. (v) A chemical shift difference 6(H4a) - B(H4e) larger than 0.5 ppm, for all the spectra recorded in the same solvent. This chemical shift difference which ranges from 1.19 ppm for 5 to 0.52 ppm for 2 is highly dependent upon the nature of the R group, and it correlates well with the chemical shift difference observed on the corresponding dioxaphosphorinanes6J5 which are known to exist in a rigid chair conformation with an axial P-R bond. The full set of the 3J(HH) values for compounds 1-5 and mainly the large 3J(H4aH5a)coupling as compared with the values measured in 1,3-dithianes1° indicate that in these
Bond Orientation in 1,3,2-Dithiaphosphorinanes
2419
TABLE I: ' H NMR Spectral Data for 2-R-1,3,2-Dithiaphosphorinanes
J(HH) coupling constantsb
Chemical shifts 6 a Compd 1 2
Solvent
H(4a)c
H(4e)C
H(5a)
H(5e)
4a-4e
4a-5e
CSD, C,D,
2.78 2.51 2.97 3.23 3.45 2.71 2.60 2.96 2.89 2.74
2.14 1.99 2.02 2.36 2.26 2.93 2.72 3.12 2.93 2.70
1.85 1.91 1.88 1.82 2.05 1.64 1.40 1.77 1.51 1.30
1.46 1.28 1.36 1.68 1.70 1.76 1.43 2.08 1.89 1.60
-14.0 -13.9 -13.7 -14.0 -13.7 -14.4 -14.3 -14.5 -14.1 -14.1
2.1 2.1 2.5 2.2 2.2 3.7 3.1 2.0 2.7 2.2
C6D6 C6D6
5
C,D, C6D6
4a-5a 12.0 12.3 12.0 12.5 12.0 7.6 10.3 11.5 12.2 12.4
4e-5e 6.0 5.4 5.5 5.2 5.5 8.6 6.6 3.5 4.7 4.5
4e-5a 2.5 2.5 2.7 2.6 2.7 3.6 2.6 2.6 2.6 2.6
5a-5e -14.5 -14.7 -14.2 -15.0 -14.5 -14.1
ind. C,D, CDC1,d -14.8 CDC1,d -14.4 -15.0 C,D, a Chemical shifts in ppm downfield from TMS as internal reference. b Values in Hz. RMS error calculated line position 0.20;Jgem values are assumed to be negative, cFor compounds existing in a rigid conformation (1-5, 9, 1 0 ) a refers to axially oriented protons, and e to equatorially oriented protons, dTh6 ' H NMR spectrum of compounds 8 and 9 have been recorded in CDC1, because of a very small difference 6(H,, - 6(H,,) in C,D, which makes an accurate spectral analysis extremely difficult, The NMR couplings are similar in both solvent. 7 8 9 10
TABLE 11: l 3 C and 31PNMR Parameters for 2-R-1,3,2-Dithiaphosphorinanes
Phosphorus-proton and phosphorus-' 'Cb coupling constants
Chemical shiftsa Compd 1 2 3 4 5 6 7 8 9 10
3'P 23.4 40.0 153.0 139.7 117.5 87.5 122.9 97.3 121.0 125.0
'"(
4) 22.6 24.1 24.8 24.2 23.5 25.8 26.4 29.8 28.7 32.7
5)
13C(
26.8 25.8 27.1 24.7 26.3 26.1 24.1 28.8 24.2 28.1
P-4ac
P-4e
3.5 3.1 2.5 4.2 2.2 11.6
0.0
0.0 0.1 0.5 1.4 0.0 0.1 14.7 16.5 21.1 26.0
P-5a 2.5 2.6 2.7 4.2 2.7 1.3
ind.
P-5e 8.0 7.9 8.0 10.0 7.7 3.1 1.1 1.0 0.7 1 .o
P-C, 12.0 12.5 14.0 14.4 13.7 10.0 10.0 9.1 10.5 15.6
P-c, 0.0 0.0 0.0 1.5 0.0 3.8 6.1 8.4 10.8 12.1
1.6 2.1 1.2 0.9 4.0 1.0 a 31Pchemical shifts in ppm downfield from external 85% H,PO, ; chemical shifts in ppm downfield from TMS as internal reference. b Values in Hz. cFor a and e designation of protons, see Table I.
molecules, the S C H ~ C H ~ C H fragment ZS adopts a rigid C, conformation. The R ratio defined as R = Jtrans/Jcis = ( J a a Jee)/(Jae + allows a good estimate of the torsional angle JI and of the various H-C-C-H dihedral angles (aaa= 120 JI; 9,,= 120 - 9;9,, = aea= 9). The 9 angles for compounds 1-5 are within a 1.5" range with a mean value of 65". A conformational study of 2-R-2-thiono-1,3,2-dithiaphosphorinanes (R = C1, R = CH3)I7 by use of x-ray structure analysis and NMR data showed us that the R ratio gives very reliable values for the 1,3,2-dithiaphosphorinanerings (63' as compared to 62 and 61.5" from x-ray data). On the basis of the previous stereochemical studies on 1,3,2-dithiapho~phorinanes~ and from the similar trends observed on compounds 1-5, the NMR results obtained on these compounds are best explained in terms of a fixed chair conformation and of an axial preference for the various substituents on phosphorus. The NMR parameters of molecules 9 and 10 are characterized by a large 3J(H4aH5a)value as observed for compounds 1-5 but conversely one notes:(i) A large 3J(PSCHde)coupling constant (21.1 and 26.0 Hz for 9 and 10, respectively) as compared with the values measured in compounds 1-5. Such an increase of the 3J(PH4e)value has already been observed in the 1,3,2-dioxaphosphorinaneswhen the P-N bond is changed from axial to equatorial ~ r i e n t a t i o n . (ii) ~ , ~A small 4J(PSCCH5e) coupling constant. (iii) A large 3J(PSCC5)value as compared to the near zero value measured in compounds 1-5. As already mentioned in previous studies on 2-R-1,3,2dioxaphosphorinanes, it was established that the 3J(POCC5)
+
+
NMR coupling is larger for an equatorial P-R bond than for an axial 0ne.13J4 (iv) A fairly small chemical shift difference 6(H4a)- b(H4e)which is smaller than 10 Hz for a spectrum recorded in benzene at 250 MHz. Such a small 6(H4a) - d(Hde) difference is also observed in 2-amino-1,3,2-dioxaphosphorinanes with an equatorially oriented P-N bond.5,6 The 3J(HH) NMR coupling values obtained on compounds 9 and 10 show that these molecules also exist in a rigid C, conformation. The R values, 3.19 and 3.31, respectively, indicate a smaller puckering of the ring in the SCH2CH2CHzS fragment than for compounds 1-5. Comparison of the 3J(PH4,), 4J(PH5e), 3J(PC5), and F(H4e) - 6(H4a) values obtained in compounds 9 and 10 and in the set of compounds 1-5 in which the P-R bond is axially oriented lead us to the conclusion that in compounds 9 and 10, the P-R bond lies in the equatorial orientation. Such an assignment is supported by the upfield shift of several parts per million observed on the I3C4 (y effect)18 in going from compound 9 and 10 to the set of compounds 1-5. In the molecules 6-8, the 3J(PSCH), 3J(PSCC5), and 4J(PSCCH) coupling constants have intermediate values between the corresponding ones observed in molecules 9 and 10 and the set of molecules 1-5. The 3J(HH)coupling constants clearly indicate that molecules 6-8 do not exist in a rigid unique C, conformation (chair or boat) which would give a 3J(HH) value of 12 Hz instead of the 7.6-, 10.3-,and 11.5-Hz values obtained. These results can be interpreted in terms of a chair-chair equilibrium with the P-R bond occupying alternately the axial and equatorial orientation. We exclude a The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
2420
flexible pseudorotating twist form for two reasons. First, it should give more identical 3J(HH) and 3J(PSCH) NMR couplings, and second the 3J(H4a, Hba) and 3J(H4e, Hse) couplings show an opposite trend of variation in going from 6 to 8, a result which is in agreement with the existence of an equilibrium between conformers of different energy. An additional answer to this question, chair-chair equilibrium or twist conformation, could be given by low-temperature NMR experiments. However, the complexity of the proton spectra (AA'BB'CDX) and some solubility problems have thus far precluded such studies. As the 3J(HH) values are not known for 6-8 locked in a rigid conformation, it is not possible to calculate an exact value for the two conformer populations, only approximate values can be obtained. Moreover, the examination of 3J(HH) does not allow the determination of the predominant conformation, P-R axial or P-R equatorial. However, such a question can he answered by use of the bond orientation at phosphorus. Typical values for the two orientations, taken as the mean values obtained on the compounds showing a strongly biased equilibrium (1-5, 9, lo), are presented in Table 111. Thus, the examination of the coupling constants appear to be a good tool for stereochemical assignment. Since such coupling values are not only dependent upon the bond orientation around phosphorus but also, to a lesser extent, upon the nature of the R group attached to the phosphorus, they do not allow precise population analysis determination. However, combining the values of Table I11 and the 3J(HH) couplings, the following stereochemical conclusions may be drawn: the equatorial orientation is predominant in compound 7 (-60%) and 8 (-8O%), and the equilibrium close to 50% for 6. The results presented here show that as previously observed for 2-R-1,3,2-dioxaphosphorinanes, the extracyclic P-R bond of the 2-R-1,3,2-dioxaphosphorinanes can adopt axial or equatorial orientation. For an alkyl group, the tendency to adopt the axial orientation is more pronounced for the 1,3,2-dithiaphosphorinanesthan for the 1,3,2-dioxaphosphorinanes. When R is methyl or phenyl the 2-R-1,3,2dithiaphosphorinanes are biased with the P-R bond in the axial orientation, while the corresponding 1,3,2-dioxaphosphorinanes show an equilibrium between the two conformations P-R axial or P-R equatorial.'-3 The same trend is observed in the solid state conformation of the corresponding 2-thiono derivatives. For example, 2-methyl-2-thiono1,3,2-dioxaphosphorinaneexists in the solid state in chair conformation with an equatorial PCH3 bond19 but 2methyl-2-thiono-1,3,2-dithiaphosphorinane shows a chair form with an axial P-CH3 bond.17 The extracyclic P-N bond is axial for a nonplanar nitrogen atom (aziridine) in which the P-N bond is longer than in the case of a planar nitrogen.20 Bulky groups attached to the nitrogen give rise to severe 1,3 nonbonded interactions as a consequence of the rotational preference of the rotameric conformation around the P-N bond21 which displaces the equilibrium toward the P-N equatorial orientation. Experimental Section The synthesis of the 1,3,2-dithiaphosphorinanes used in this study were conducted in an inert gas atmosphere using rigorously anhydrous solvents. It is noteworthy that pure samples of compounds 1, 3, and 5-10 kept under vacuum in sealed tubes show additional 3lP NMR peaks which appear with time. These extra peaks are due to three coordinated phosphorus species as shown by the 3lP chemical shifts, and are now under investigation. The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
J. Martin, J. E. Robert, and C. Taieb
TABLE 111: Stereochemical Dependence of the NMR Coupling Constants (Hz)in 1,3,2-Dithiaphosphorinanesa P-R bond orientation
3J
(PH,,)
4J (PH,,)
3J (PC,)
8.0 1.0
11.5
9 -R
H
Axial Equatorial
0.5 23.5
0.5
aThe values reported in this table correspond to mean values.
2-Methyl-1,3,2-dithiaphosphorinane (1). The synthesis was accomplished by use of the procedure described for the corresponding 1,3,2-dithiapho~pholane.~~ A t room temperature, methylphosphonous dichloride (5.25 g, 0.045 mol) was added dropwise to a solution of 1,3-propanedithiol (4.75 g, 0.045 mol) in benzene. After addition, the solution was heated for 2 h at 60 "C. After evaporation of benzene, the compound was purified by sublimation to obtain 2.6 g of 2-methyl1,3,2-dithiaphosphorinane(38% yield): mp 50 "C. Anal. Calcd. for C4HgPS2: C, 31.56; H, 5.96; P, 20.35; S, 42.13. Found: C, 31.75; H, 5.97; P, 20.25; S, 42.25. 2-Phenyl-1,3,2-dithiaphosphorinane (2). This compound was prepared by the same procedure as described for 1 using phenylphosphonous dichloride instead of methylphosphonous dichloride. 2 was purified by crystallization in methanol or acetonitrile (75%yield); mp 48 "C. Anal. Calcd. for CgHllPS2: C, 50.51; H, 5.18; P, 14.47; S, 29.97. Found: C, 50.94; H, 4.87; P, 14.13; S, 28.70. 2-Methoxy-l,3,2-dithiaphosphorinane(3). To a benzene solution of methanol (2.30 g, 0.071 mol) and triethylamine (8 g, 0.079 mol) at 0 "C was added dropwise 2-chloro-1,3,2dithiaphosphorinane (12.4 g, 0.071 mol). After 24 h, the mixture was filtered and concentrated. Molecular distillation (70 "C, 1mm) gave a colorless liquid which becomes viscous with time. This compound can also be purified by chromatography on a silica column, using benzene as eluent. Anal. Calcd. for CdHgPS20: C, 28.56; H, 5.39; P, 18.41; S, 38.12. Found: C, 28.70; H, 5.33; P, 18.56; S, 38.02. 2-Chloro-1,3,2-dithiaphosphorinane (4). This compound was prepared in a 90% yield by reaction of phosphorus trichloride on 1,3-propanedithiol according to the procedure of Arbuzov and Zoroastrova for the 2-chloro-1,3,2-dithiaphosp h 0 1 a n e . ~It~was purified by sublimation: mp 45 OC. Anal. Calcd. for C3H6PS2C1: C, 20.87; H, 3.50; P, 17.94. Found: C, 20.53; H, 4.19; P, 17.98. 2-Amino-1,3,2-dithiaphosphorinanes (5-7, 9, 10). All of these compounds were prepared by the same experimental procedure. 2-Chloro-l,3,2-dithiaphosphorinane (1mol) was added dropwise to a benzene solution of the amine (2 mol) at ca. 0 "C. The solution was stirred for 24 h and the amine hydrochloride filtered off. After evaporation of the solvent, purification was done by molecular distillation. The identification of each compound was performed by NMR spectral analysis (1H, 13C, 31P) and mass spectrometry. The symmetry of the spectrum, the integration of the peak area, and the 31P chemical shift demonstrate without any ambiguity the formula of each compound. The mass spectra show peaks corresponding to the molecular weights. 2-tert-Butyl-1,3,2-dithiaphosphorinane (8). 1,3-Propanedithiol(5.31 g, 0.049 mol) was added dropwise to a benzene solution (100 cm3) of tert-butylphosphonous dichloride (7.84 g, 0.049 mol) and pyridine (7.76 g, 0.098 mol). The mixture is stirred for 2 weeks at 50 "C and the pyridinium salt is
Bond Orientation in 1,3,2-Dithiaphosphorinanes filtered off. A 31PNMR spectrum of the benzene solution shows four peaks at 97.3,104.0,115.5, and 121.0 ppm respectively, with respect to a 85% H3P04solution as external reference. Column chromatography over silica and performed under nitrogen using as eluent a 3:l mixture of hexane-benzene, allows isolation of compound 8 (6 31P97.3 ppm). The compound is identified by NMR spectroscopy lH, 13C,3lP, and the mass spectrum which Shows a peak at mle 194. The fact that the isolated compound is a six-membered ring is shown by the identification of its 2-thiono derivative. The extra peaks observed in the benzene solution correspond to cyclic polymeric species which will be described in a forthcoming paper. Anal. Calcd. for C7H15PS2: C, 43.27; H, 7.78; P, 15.94; S, 33.00. Found: C, 43.74; H, 7.51; P, 15.80; S, 32.17.
Acknowledgments. The NMR spectra were recorded at the “Laboratoire Grenoblois de RBsonance MagnBtique NUclBaire”, on a Cameca 250-MHz spectrometer (lH) and a Varian-Informatek Spectrometer (13C, 31P). We thank Mr. R. Nardin and Mr. H. Reutenauer for skillful assistance in recording the NMR spectra. References and Notes (1) (a) D. W. White, G. K. McEwen, and J. G. Verkade, Tetrahedron Lett., 5369 (1968);C. Bodkin and J. P. Simpson, J. Chem. SOC.0,829 (1969);B. A. Arbouzov and R. P. Archinova, Dokl. Akad. Nauk SSSR, 195,835(1970); B. A. Arbouzov, R. P. Archinova, A. N. Verechaguin, S. G. Voulfson, 0. N. Nouredinova, and L. 2 . Nikonova, Khim. Keferocycl. Soedin., 10, 1324 (1971);(b) W. G. Bentrude, K. C. Yee, R. D. Bertrand, and D. H. Grant, J. Ai??.Chem. Soc.. 93,797 (1971);W. G. Bentrude and J. H. Hargis, ibid., 92, 7136 (1970);W. G. Bentrude and K. G. Yee, Tetrahedron Lett., 3999 (1970);K. Bergensen and P. Albriktsen, Acfa Chem. Scand.. 26, 1680 (1972);(c) M. G. B. Drew, J. Rodgers, D. W. White, and J. G. Verkade, J. Chem. soc. 0,227 (1971);J. Rodgers, D. W. White, and J. G. Verkade, J. Chem. Soc. A, 77 (1971);M. G. B. Drew and J. Rodgers, Acfa Clysfallogr,,
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The Journal of Physical Chemistry, Vol. 80, No. 21, 1976
