May, 1962
N.M.R. S T U D I E S OF T H E
P31 NUCLEUS IS
PHOSPHORUS COMPOUNDS
90 1
Application of the Modified Equation.-It is not chloroform could not be explained by the Hartley and Crank equation unless it was modified by as- possible at present to assign a definite value to signing intrinsic diff usivities to the three species fix since not enough is known about the diffusion present in solution, &e., the two monomers and the mechanism. However, a qualitative check of the ether-chloroform complex. For the system methyl validity of eq. 4 may be obtained by determining ethyl ketone-carbon tetrachloride, hydrogen bond- a single value of fll empirically from the data of ing can OCCULT between ketone molecules. These Fig. 1 and then using eq. 4 to see how well the interactions cause positive deviations from Raoult’s entire diff usivity-mole fraction curve is predicted. The parameters fl and fi as calculated from the law as opposed to the ether-chloroform system where the cross interaction causes negative devia- intercepts ,Of the experimental curve were 29.0 tions. and 34.7 A., respectively. These correspond to For the present investigation, it is assumed that Stokes-Einstein radii of 1.54 and 1.84 A.d respecthe ketone exists in solution as monomers and tively. Using the empirical value of 2.44 A. for the dimers only and that the concentrations of these radius of the dimer, the curve species can be related by an equilibrium constant Stokes-Einstein shown in Fig. 1, which fits the experimental data K = :x -11 very closely, was calculated. XI2 The system acetic acid-carbon tetrachloride where XI1and X1 are the true mole fractions of the also provides an interesting test of eq. 4. Although dimer and monomer, respectively. The methods it is not possible to estimate the degree of associaoutlined by Hildebrand and Scott5 were used along tion of the acetic acid in concentrated solutions, with activity data4 to estimate an equilibrium it dimerizes very strongly in dilute solution. constant of 5.6 for the methyl ethyl ketone in Davies, et al., have found? the dimerization cong.-mole/l. at 25°,8 so that stant to be 2.07 X carbon tetrachloride. Intrinsic diffusivities were assigned to the over 96% of the acid is associated to dimers at a dimer as well as to both monomers, in solution and mole fraction of acetic acid of 0.04. Calculations a modified form of the Hartley and Crank equation based on eq. 4 indicated that a very sharp increase in the diffusivity should be observed a t low conwas derived6 centrations of acetic acid. Data taken at concentrations as low as possible are shown in Fig. 2. It is seen that the diffusivity does increase sharply near the carbon tetrachloride intercept as predicted. This fact is not predictable from eq. 3. It would be of interest to have values at even lower concentrations. Additional experimental data are being obtained Equation 4 reduces to eq. 3 for the case when no association occurs { ( i e . , XIo = X A and Xl10 = 0). for systems with known degrees of association and Values of XIQand Xl10 can be calculated from the will be reported in future communications. Acknowledgment.-This work was supported by equilibrium constant. the U. S. Army Research Office (Durham). (6) J. H. Hildebrand and R. L. Scott, “The Solubility of Nonelectrolytes,” 3rd. ed., Reinhold Publ. Corp., New York, N. Y., 1950. (6) D. K. Anderson, P h D. Thesis, University of Washington, 1960, Dissertation Ab&., XXI, No. 6, 1388 (1960).
(7) M. Davies, P. Jones, D. Patnaik, and E. A. Moelwyn-Hughes, J . Chem. Soc.. 1249 (1961). (8)Their equilibrium constant was defined as CI~/CII.
NUCLEAR MAGNETIC RESONANCE STUDIES OF THE P31 NUCLEUS I N PHOSPHORUS COMPOUNDS BY LEOC. D. GROENWEGHE, LUDWIG MAIER,AND KURTMOEDRITZER M m s a n t o Chemical Company, Research Department, Inorganic Chemicals Division, St. Louis, Missouri Received November $8. 1961
A study of two hundred new Pal nuclear magnetic resonance chemical shifts shows that the Pal relative shift contributions of common organic ligands directly bonded to a quadruply connected phosphorus atom lie in the order CaH6>/ CH&l > CHa > CzHs. Since these contributions are relatively unaffected by other groupings connected to the phosphorus, their value can be used in the estimation of chemical shifts-a fact of considerable interest in the structural analysis by n.m.r. of mixtures of organic phosphorus compounds. Consecutive substitution of one organic ligand for another on triply connected phosphorus leads to approximately equal stepwise changes in the PS1chemical shift. However, the effect of other ligands bonded to the phosphorus is so large that characteristic shift contributions cannot be assigned readily.
Since the discovery of the chemical shift in the nuclear magnetic resonance (n.m.r.) of the P31nucleus by Knie;htl Some ten years ago, about 400 n.m.r. spectra of individual phosphorus compounds have been reported in the literature. Various aut,hor$2-7 (1) W. D. Knight, Phys. Rev., 76, 1269 (1949).
have published impressive lists of P3* chemical shifts and have shown that P31 n.m.r. spectros(2) H. 8. Gutowsky, D. W. McCall, and c. P. Sliohter, J . Chern. P h w . 21.279 (1953). (3) He 6. Gutowsky and D*W. McCall, i b d , 22, 162 (1964). (4) N. Muller, P. C. Lauterbur, and J. Goldenson, J. Am. Chem. SOC.,78, 3657 (1956).
902
L. C. D. GROESWEGHE, L. -I~AIER, AND I (.MOEDRITZER
copy is an extremely valuable tool for structure proofs and for qualitative as well as quantitative analysis of mixtures of phosphorus compounds.* This paper is based on previously unreported chemical shifts on 200 phosphorus compounds, published elsewhere by these author^.^ With these data, empirical shift correlations have been carried out to a precision not obtained in previous publications. Such shift correlations are of great importance in the practical application of n.m.r. to the analysisof mixturesof phosphorus compounds. Experimental
Vol. AB
connected compounds, only a sometimes useful estimation could be obtained. Complications which one could encounter in estimating chemical shifts are illustrated in the following example. The chemical shifts of P0Cl3 and POBr3 are -2.2 and +103.4 p.p.m., respectively. This would mean that on the average this shift becomes 35.2 p.p.m. more positive each time a bromine is substituted for a chlorine. This corresponds within h3.4 p.p.m. to the experimental findings (POC12Br = $29.6 p.p.m. and POClBr2 = +64.8 p.p.m.8). However, the chemical shifts of PSCls, All n.m.r. measurements were made with a Varian PSC12Br, PSClBr2, and PSBr3 are -28.8, $14.5, $61.4, and $111.8 p.p.m., respectively,8 averaging Model V-4300B high-resolution spectrometer with a radiofrequency of 16.2 Mc. and a magnetic field of approximately an increase of 46.9 p.p.m. for each bromine sub9395 gauss, using a Varian magnet, Model V-4012-HR. stituted. Although here too, an agreement with Chemical shifts are reported in p.p.m. of the applied field using 85% H8PO4as a standard (zero shift). Upfield shifts the experiment of rithin k3.6 p.p.m. was obaTe denoted by a plus sign and downfield shifts by a minus served. the findings in the first series of compounds sign. The chemical shifts were determined by the con- do not seem to be related to those of the second centric-tube technique, whereby a narrow tube containing series. Furthermore, the correlation becomes worse the reference compound is inserted in the sample giving when one compares the effect of substituting sulfur an accuracy depending on the broadness of the peak of ea. k 0 . 5 p.p.m. for well-resolved resonances. The few data for oxygen in P0Cl3 and POBr3. A change in obtained by the tube-interchange technique are accurate chemical shift of -26.6 and $8.4 p.p.m., respecrvithin ea. 5 1 . 5 p.p.m. Most of the samples were con- tively, then is obtained. tained in 15 mm. 0.d. Pyrex tubes. When only a small Obviously, changes in chemical shift as a result quantity (as little as 100 mg.) of sample was available, either a 5 mm. 0.d. tube was used with the proper probe of substitution may be very much dependent on insert or the tube in the sample holder was adjusted to a other atoms connected to the phosphorus. Condepth at rhich maximum response from the coil was ob- sequently, the prediction of the chemical shift of a tained. Most of the n.m.r. shifts have been obtained with pure compound in which all but one ligand is different samples. However, some of the compounds or phosphorus from a compound with known chemical shift is structure-building units have not been separated. I n only possible when such dependency does not occur almost every case, compounds which were not isolated to an appreciable extent, unless one would be able Rere contained in the reaction products of random reorganito include the proper corrections for this effect. zation reactions8,10,11 carried out in sealed Pyrex tubes. At the present time, however, such corrections Results and Discussion do not seem possible. The chemical shift of phosphonic acids is deQuadruply-Connected Phosphorus Compounds. pendent on the pH; therefore, the shifts of 16 -The change in chemical shift due to substituting sodium phosphonates have not been included one organic ligand for another is shown in Table I. in the published list. It was found that, in con- Cases studied comprise substitution of either a trast with the case of phosphoric acid,12the chemi- methyl, chloromethyl, ethyl, or phenyl group cal shift for phosphonates becomes more positive directly connected to the phosphorus by another with increasing p H . The chemical shifts of the group of this series. The table gives average monosodium and disodium phosphonates were, values for changes due to the substitution of one respectii-ely, from 2 to 4 and from 3 to 8 p.p.m. specific ligand for another one as computed for a t more positive than the shifts of the corresponding least three structures which are identical except acids. for different organic ligand(s) (R) and/or a broContribution of Atoms and Ligands Connected to mine atom instead of a chlorine directly attached the Phosphorus.-In the past, one could calculate to the phosphorus. I n such related structures, with a certain broad approximation the chemical identical substitution results in a change in chemishift of a phosphorus compound from additive cal shift which is the same within relatively close “shift constants” attributed to the individual limits as shown by the reported standard deviations. ligands attached to the phosphorus atom. The Furthermore, from the data in this table, it is shift of quadruply-connected compounds could apparent that such substitutions also result in the be estimated roughly through the knoxledge of same change in chemical shift, although with the shift of similar compounds; whereas, for triply- somei7rhat broader limits, if one disregards the ( 5 ) J. R. Van Wazer, C. F. Callis, J. N. Shoolery, and R. C. Jones, nature of the other ligands. As a matter of fact, J . Am. Chem. Soc., 78, 5715 (1956). it appears possible to assign a fixed relative con( 6 ) H. Finegold, Ann. N. Y . Acad. Scz., 70, 875 (1958). tribution for each of these ligands, such as $7 (7) 1%’. A. Henderson, Jr., and S. A. Buckler 3. Am. Chem. SOC.,82, p.p.m. for methyl, +12 p,p.m. for chloromethyl, 5794 (1960) (8) L. C. D. Groenweghe and J. H. Payne, zbid., 81, 6357 (1959). and $15 p.p.m. for phenyl, if one assigns a con(9) K. Moedritzer, L. Maier, and L. C. D. Groenweghe, 3. Chem. tribution of 0 p.p.m. for ethyl. It is highly probCng. D a t a , 7, 307 (1962). able that simjlar relationships can be written for (10) E. Fluck, J. R. Van Wazer, and L. C. D. Groenueghe, J . Am. other organic ligands when a sufficient amount of Cham. S o c , 81, 6363 (1959); L. C. D. Groenweghe, J. H. Payne, and J. R. Van Wazer, zbzd., 82, 5305 (1960). data becomes available. (11) L. C. D. Groenweghe and J. H. Payne, zbzd.. 83, 1811 (1961). Table I1 shows the change in chemical shift as a (12) R. A. Y. Jones and A. R. Katrizky, J. Inoro. & iVzlcZeur Chem., result of substitutions other than one organic ligand 15, 193 (1960).
S.M.R. STUDIESOF
May, 1962
THE
903
PnlSIJCLEVS IN PHOSPHORUS COMPOUNDS
compound generally within 5 p.p.m., using the TABLEI known chemical shift of a compound having the EFFECTO N N.M.R. CHEMICAL SHIFT BY SUBSTITUTION OF same structure except for one ligand on the phosONE ORGANIC LIGANDBY ANOTHERIN QUADRUPLY-CONphorus. Such predictions are valuable in the strucNECTED PHOSPHORUS COMPOUNDS tural analysis of mixtures of organic phosphorus ( R = any alkyl or aryl group, X = either C1 or Br) Original structure
Substituted structure
Av. change in shift, p.p.m.
Stand. No. of substitutions dev., p.p.ni. considered
C8Hereplaced by another organic ligand 16,l 2.1 CaHsRPSCl CzH6RPSCl - 16. 7 0 . 9 C J M i l " C ~ CzHsROPCl CHiClRPSCl - 1.1 3.3 CeHbRPSCl - 3.7 1.7 CeHsRPOCII CHzClRPOCl CH3 repliwed by another organic ligand CHaRPOCl CzH[bROPCl - 7.0 1 . 8 CHaRzPS CzH[sRzPS 1.5 1.3 CzH6RPSX -12.8 4.5 CHaRPSX CHiClRPOCl 7.2 0.8 CHaRPOCl CH3RPSX CHzClRPSX 2.9 1.0 10.8 1. 3 CHaRPOCl C8H:sRPOCl CaHjRPSX 5.2 2.5 CHsRPSX
-
compounds by n.m.r. technique. Triply-Connected Phosphorus Compounds.-The changes in chemical shift due to consecutive substitution of one organic ligand for another one are shown in Table 111. It is apparent from this table
TABLEI11 EFFECTO N N.M.R. CHEMICAL SHIFTOF SUBSEQUENT EXCHANGES OF HALOGENS OR R-GROUPSIN TRIPLY-CONNECTED PHOSPHORUS COMPOUNDS
+ + + +
+
for another one. Once again, jt appears that the same substitution results in the same change in chemical shift within relatively close approximation when carried out in structures which are identical except for different other organic ligands. TABLE I1
Original compound
Subatituted compound
--Change 1st substitution
(CH3)aP (CzH6)aP (CH3)sP (CH3)zPH (CH,)2PH (CH3)ZPH (CH3)zPCl (CH3)iPCl (C2Hs)zPCl (CH&PBr (CH&PBr
(CZHs)3P (CeH5)sP (C,&,)sP (CzHs)J'H (C4Hs)zPH (CsH5)iPH (CzIIj)zPCl (C&,)iPc1 (C6H6)zPCl (C2H&PBr (c&)~pBr
-13 5 5 3 -15 0 -22.5 -19 8 -27 2 -13 8 86 +22 0 -10 6 4-10 7
-
+
in chemical shift for2nd sub3rd substitution stitution
-14 5 16 -19 0 -21 5 -10 3 -31 2 -13 2 19 +15 5 -17.7 f6 2
-
-13 6 6 5 -21 O
-
+
EFFECTON N X R . CHEMICAL SHIFTBY SUBSTITUTION OF OXE I'VORGANIC LIGAND BY ANOTHER, O R G a N I C OR INORthat all the changes studied fall within 5 p.p.m. of GANIC, I N Q CJADRUPLY-CONNECTED PHOSPHORUS COMPOUNDSthe average change obtained by dividing the change in chemical shift due to substitution of all the (R and I t ' = any alkyl ar aryl group)
AV.
Original structure
Substituted structure
change in shift, p p.m.
0 Replaced by S -70 1 (R0)rPS RP( S)Clz -375
(RO)rPO RP( 0)Clg RR'P(0)Cl in CCll RR'P( S)C1 -31 0 R,PO in CHC13 R3PS in CHCh - 89 Mi,scellaneoussubstitutions RP(O)(OH)z +19 1 RP( 0 ) C l ~ 0 8 RP(O)(OH)i RP(O)(OCzHa)2 RP(O)(OC1Hs)z $18 9 RP( 0)Clz RR'P( 0)Cl RR'P( O)OH $126 RR'P(S)Cl RR'P(S)Br f182 RzP( O)CH&l RzP(0)Cl +12 3 RzP(0)Cl R~P(O)-O-P(O)RJ +11 5 RzP(O)CH&l RzP(0)-0-P(O)R* - 4 2 RiP(0)OH RzP(O)-O-P(O)Rz - 1 3 RzP(0)Cl Rap( O)OH 4-12 6
+
(ILPOL), RP(O)(OH)i RP( 0)Clz (FLPOZ), RP(O)(OCzHs)2 (RPOi),
+I70 4-304 +l73
Stand. dev., p.p.m.
No. of pairs of structures considered
1 0 2 8
3 4
3 9 5 9
8 3
49 14 2 7 5 1 46 6 6 16 5 8 6 1 4 4 1 3 5 7
4 4 6 5 4
0 8
4
5 3 5 6 3 3 3
It should be noted that these relationships also have been tested on similar substitutions for which only two examples are available, and that disagreement with the above conclusions could not be found. Consequently, it appears possible to predict the chemical shift of a quadruply-coiinected phmphorus
ligands by the number of ligand sites involved. Predictions of chemical shifts made on the basis of these findings can be expected to be relatively accurate, especially when one considers the wide range of chemical shifts covered by the triplyconnected phosphorus compounds ( -230 p.p.m. to +240 p.p.m.). I n certain cases, relationships as in quadruplyconnected phosphorus compounds also can be found. For instance, substitution of a bromine for a chlorine in compounds of the type RR'PCI results in an average change of $6 f 2.7 p.p.m. as calculated for five examples. Substitution of two bromines for two chlorines in structures of the type RPClz increases the chemical shift by 6.6 3.8 p.p.m. (three example^).^ However, one cannot generalize these findings, since substitutioii of a phenyl group for a methyl group in (CH3)2PH and in (CH3)zPCl causes a change in chemical shift of -27.2 and $8.6 p.p.m., respectively. For the same substitution in CH,(C6H5)PH and CH3(CsH5)PBr, a respective change of -31.2 and +6.2 p.p.m. is obtained. An interesting phenomenon can be seen in the family of the aminophosphines. If one replaces all the dimethylamino groups by diethylamino groups, the chemical shift increases between 4 and 8 p.p.m., regardless of the number of amino groups connected to the phosphorus atom. It also is of interest to note the surprising fact that the ethyl group takes a separate place in the aliphatic series of ligands. It can bs seen from the datag that certain ethyl-substituted triply-conhected phosphorus compounds have a much mora
*
904
C. L. LEE, J. SXID,ASD 34. SZWARC
negative chemical shift than either the methyl or the higher alkyl-substituted compounds (e.g.,
(CzH6)2PH,CH3(C2€16)PH,and (CzHs)3P6). Even in the quadruply-connected compounds, the ethyl group takes a separate place in the aliphatic series of ligands, but then only in a few cases does this ligand exhibit a more negative contribution than either methyl or higher alkyl (e.g., (C*H&PSBr and CH8(C&s)PSBr. Spin-Spin Coupling.-Table I V shows some averTABLEIV RANGEOF SPLIT MAGNITUDE AS
A
RESULTOF SPIN-SPIN
IXTERACTION WITH HYDROGEN AND FLUORINE IN VARIOUS STRUCTURES
X = halogen, Y = either halogen or hydrogen, 2 = any ligand, n = integer from 1 to 3.
Structure of interacting ligand(s)
(ZCHrS)nPXa- It Y CH2-PX2 XCHg-P(O)Zs H-PR2 F-P( 0)RX
Av. coupling constant, C.P.S.
Stand. dev., 0.p.s.
No. of com-
3 5 3 14 89
7 3
11 19 13 198 1110
pounds checked
6
8 3
age coupling constants resulting from interaction of hydrogen and fluorine in various structures.
T'ol. 66
Once again, averages of less than three examples have not been reported. It is interesting to note that the hydrogen directly attached to the phosphorus in phosphines gives an average coupline; constant of 197 C.P.S. (standard deviation = 14 c.P.s.), whereas this value becomes 606 C.P.S. (standard deviation = 85 c.P.s.) for the same hydrogen in quadruplyconnected compounds.6 Coupling constants for hydrogen connected to phosphorus over more than one bond do not seem to exceed 25 C.P.S. and depend on the number of bonds xv-hich separate the two atoms, the kind of atoms through which they are linked, and the other atoms connected to the phosphorus. Splits resulting from hydrogen connected to the phosphorus through two intermediate carbon atoms (-C-C-P) have not been observed at 9393 gauss fields, which means that the corresponding constants are less than ca. 5 C.P.S. Ho.cvever, coupling over a carbon and a sulfur, a carbon and an oxygen, or a carbon and a nitrogen seem8 to give constants of about 10-15 C.P.S. in both triply- and quadruply-connected compounds. Acknowledgment.-The authors wish to thank Dr. J. R. Van Wazer for helpful discussions and John Yoder for taking some n.m.r. spectra, as well as John Chupp and Steven Fitch for some samples.
THE MECHANISM OF' FORMATION OF LIVING a-METHYLSTYRENE DIMER AND TETRAMER BY
c. L. LEE,J. S M I D , A N D M. S Z W A R C
Department of Chemistry, State University College of Forestry at Syracuse University, Syracuse 10, A'ew York Received November 14, 1961
The mechanism of formation of living a-methylstyrene dimer and tetramer a-as investigated. These oligomers result from an electron transfer reaction: alkali metal aMS -., aMS-, alkali+. It was shown that the monomeric radical-ions (aMS-) do not dimerize into dimeric dianons ( -aMS-aMS-); their reaction with the monomer yielding dimeric radical-ions ( -aMS.aMS.) is preferred. In the absence of a large alkali metal surface, the dimeric radical-ions dimerize into tetrameric dianions, but in systems having a large metal surface, e.g., in the presence of sodium emulsion, the dimeric radical-ion acquires an electron and is transferred into dimeric dianion. The structure of the dimer and the tetramer was discussed. The kinetics of dissociation of the living tetramer into living dimer was investigated.
+
It was recognized by Dainton and Ivin' that the conversion of monomer to high molecular weight polymer is restricted by certain thermodynamic conditions. Their treatment of polymerization processes leads t o the concept of ceiling temperature which is determined by the heat and entropy of polymerization. This temperature becomes particularly lorn when a strained polymer is formed, the polymerization of a-methylstyrene being an example. Indeed, at 0' the conversion of this monomer to a high molecular weight polymer is possible only if its concentration exceeds 0.7 M , and a still higher concentration is needed for the polymerization to proceed at room temperature.%* The thermodynamic restrictions outlined by Dainton and Ivin do not apply to processes yield(1) F. S. Dainton and K. J. Ivin, NatuT6, 162, 705 (1948); Quart. Beus. (London). 12, 61 (1958). (2) H. W. MeCormick, J . Polymer Sei., 26, 488 (1957). (3) D. J. W@rsfo,ldand B' Bywater, ibid.. 26, 299 (1967).
ing low molecular weight polymers,' and it is indeed possible to form a low molecular weight polya-methylstyrene under conditions which forbid the formation of a high molecular weight material. It is the purpose of this paper to discuss such reactions, to investigate their mechanisms, and to establish the structure of the resulting oligomers. Dimerization of a-Methylstyrene.-A slow addition of a-methylstyrene to a vigorously stirred emulsion of sodium in tetrahydrofuran produces dimeric dianions which yield 2,5-diphenyl-2,5dimethyladipic acid on carboxylation.4 The dimerization is quantitative, and the addition of aromatic hydrocarbons such as naphthalene seems to catalyze the process. It was recognized by Paul, Lipkin, and Weissman5 that the reaction of alkali metals with aro(4) C. F. Frank, et al., J . Org. Chena., 26, 307 (1961). (5) D. E. Paul, D. Lipkin, and 9. I. Weisswan, J . Am. Chem. Soc., 78, 116 (1956).
