Magnetic Circular Dichroism of Molecules in Dense Media
1031
Magnetic C ~ r ~Dichroism ~ i ~ ~ ofr Molecules in Dense Media. I 11. Substituted
. H. Lin, and H. Eyring" Departmenf of Chemistry, University of Utah, Salt Lake City, Utah 847 72. (Received October 28, 7972) Pubiication COSTS assisted by The Nationai Institute of Health, The National Science Foundation, and the Army Research Office- i3urham
The magnetic circular dichroism (MCD) of benzene and 27 derivatives through the absorption region 1850-31000 A have been measured. Two different electronic transitions in the region 2000-2300 A (generally assigned to the benzene lBlu upper state) were observed in some of the compounds. The theory delBzu electronic veloped by Sklar, Platt, and Petruska for predicting the intensity of the benzene lA1, thansition is adapted to correlate the sign and intensity of the substituted benzene MCD in this region. The method predicts the correct MCD sign and gives approximate estimates of the magnitude of the zU MCD transition.
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Introduction The value of magnetic optical activity (the Faraday effect) as a tool in the sl:udy of molecular structure has been treated by several investigators. There are three general reviews,1-5 a general discussion of the t h e ~ r y and , ~ a review of organic chemical applications.5 We shall consider here the Faraday effect for the electronic transitions of benzene and its derivalives. The electronic spectra of benzene and its derivatives in dense media were studied extensively in the past. Various investigators including Sklar,a-8 Forster,9 Platt,lOJl Moffitt,lz Murrell and EIctnguet-Higgins,13and Petruska,l4J5 contributed their efforts toward developing a systematic phenomenological theory capable, to a degree, of explaining anti predicting the changes in the absorption wavelengths and intensities of the 2600-A transition. The 2600-A band of benzene was ascribed to the forbidden transition IAl, SB:!u in an extensive experimental and theoretical study.l6 The 1850-A band of benzene is generally assigned to the allowed transition lAlg lE1,.17 The argument as to wheth1.r the forbidden 2100-A transition of benzene should be assigned to a lA1, lBlu or a lA1, IE2, transition is not settled. Some recent calculationsl"-22 indicate that, the IB1, state lies at higher energies , Dunn and Ingold23 assigned the second than the ~ E z level. excited siqglet TT state of benzene to lEzg symmetry based on their gas-phase experimental data. Nelson and Simp~ 0 x found 1 ~ ~ a band in the 2350-A region in the vapor spectrum of hexamethylhenzene. They tentatively assigned lE2, transition. Petruska's the former band to thie ~ o r k , ~on~the J ~patterns of polysubstitutional shifts in the benzene singlets (which did not include the vibronic in electronic enereffects in the c a l ~ ~ u l ~of~ the t j ochanges ~ gies), indicated l B l U symmetry for the upper state. In their high-resolution study of electron-impact spectra on benzene Lassettre, et a1.,25 found two different electronic transitions, one a t 6.2 eV and another having peaks at 6.31, 6.41, and 6.53 eV, but they were not able to assign the symmetry of thle excited states. However, the recent experimental studies of the vibrational structures in the 2100-A spectral region of the benzene molecules in lowtemperature rare gas matrices,26 and of toluene, tolueneda, p-xylene, and 6%-rylerre an a solid krypton matrix,27
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provide strong evidence for the lB1, assignment. The reflection and absorption spectra o€ solid benzene and benzene-& a t low temperature reported by Brith, Lubart, and Steinberg28 also favored the assignment of the 2100-A band to the lA1, lBlu transition. The present investigation concerns the effects of chemical substitution in aromatic hydrocarbons on the electronic transitions associated with magnetic circular dichroism. The purpose is to find rules which will predict the sign of MCD of substituted aromatic hydrocarbons and describe the behavior of
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( I ) A. D. Buckingham and P. J. Stephens, Annu. Rev. Phys. Chem., 17,
399 (1966). (2) P. N. Schatz and A. J. McCaffery, Quart. Rev. Chern. SOC.,23, 552 (1969). (3) D. J. Caldwell, J. M. Thorne, and H. Eyring, Annu. Rev. Phys. Chem., 22,259(1971). (4) P. J. Stephens, J. Chem. Phys., 52,3489 (1970). (5) C. Djerassi, E. Bunnenberg, and D. L. Elder, Pure Appi. Chem., 25, 57 (1971). (6)A. L. Sklar, J. Chem. Phys., 7,984 (1939). (7) A. L. Sklar, J. Chem. Phys., 10, 135 (1944). (8) A. L. Sklar, Rev. Modern Phys., 14, 232 (1942). (9) T.Forster, 2. Naturforsch. A , 2, 149 (1947). (10) J. R. Platt, J. Chem. Phys., 17,484 (1949). (11) J. R. Platt, J. Chem. Phys., 19,263 (1951). (72) W. Moffitt, J. Chem. Phys., 22,320 (1954). (13) J. N. Murrell and H. C. Longuet-Higgins, Proc. Phys. Soc., London, Sect. A, 68,329,601,969(1955). (14) J. Petruska, J. Chem. Phys., 34, 1 1 1 1 (1961). (15) J. Petruska, J. Chem. Phys., 34, 1120 (1961). (16) G. Herzberg, "Electronic Spectra of Polyatomic Molecuies," Van Nostrand, Princeton, N. J., 1966,p 555-557. (17) G. Nordheim, H. Sponer, and E. Teller, J. Chem. Phys., 8, 455 (1 940). (le)J. E. Bioor, J . Lee, and S. Gartside, Proc. Chem. Soc., London, 413 (1960). (19) J. Koutecky, J. Cizek, J. Dubsky, and K. tllavaty, Theor. Ch:m. Acta, 2,462 (1965). (20)J. Koutecky, K. Hlavaty, and P. Hochrnann, Theor. Chirn. Acta, 3, 341 (1965). (21) J. Koutecky, "Modern Quantum Chemistry-istanbu! Lectures," Vol. 1 , 0. Sinanoglu, Ed., Academic Press, New York, N. Y., 1965,pp 215-220. (22) R. J. Buenker, J. L. Whitten, and J. D. Petke, J. Chem. Phys., 49, 2261 (1968). (23) T. M. Dunn and C. K. Ingold, Nature (London), 376, 65 (1955). (24) R . C. Nelson and W. T.Slmpson, J. Chem. Phys.. 23, 1146 (1955). (25) E. N. Lassettre. A. Skerbele, M. A. Dillon. and K. J. Ross. J. Chem. Phys., 48,5066 (1968). (26) B. Katz, M. Brith, B Sharf, and J. Jortner, J. Chen7 Phys., 52, 88 (I i.m _ -. 1_1, .
(27) E. Katz, M. Brith, 8. Sharf, and J. Jortner, J . Chem. Phys., 54, 3924 (1971). (28) M. Brith, R. Lubart, and I. T. Steinberg, J Chem. Phys, 54, 5104 (1971). The Journal of Physical Chemistry, Voi. 77, No. 8, 7973
D. J. Shieh, S. H. Lin, and H. Eyring
103%
the MCD intensity changes with the types and number of substituents and the position of substitution. For this purpose, we have measured MCD of some 27 substituted benzenes. To interpret and correlate the MCD data for these compounds, we shall adopt the theory developed by Sklar,6-8 Platt,Jl and Petruska14,15 for the absorption spectra of electronic transitions of aromatic hydrocarbons of MCD. A more detailed theoretical approach has been developed by Caldwell and EyringZ9which provides additional insight into these problems of symmetry.
Experimental Section The MCD spectra reported here were measured with a modified Cary Model 60 recording spectropolarimeter with a Varian superconducting solenoid system a t a magnetic field of 45 kG. The mechanical slit widths were preprogrammed as recommended by the data manual for the Cary Model 60 spectropolarimeter, with the assumption of a spectral slit width (half-band width) of about 50 A for the %le -* lBzu system and 100 A for the lA1, lElu systems. The molar ellipticity [@Ihl --- (@"/CIH)lOOodl dm-' mol-' G-'
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where 8" is the ellipticity in degrees, C is the concentration of the solution in mole liter1, 1 is the pathlength in centimeters, and 111 is the magnetic field in Gauss. The spectra were calibrated using a 1 mg/ml aqueous solution of c6lmphor-dl~-su~fonic acid in a 1-cm pathlength cell which gives forth a positive natural CD band centered at 2900 A with an ellipticity ( 8 " ) of about 0.31" (based on the Cary Model 6001 CD accessory instruction manual). The absorption spectra were obtained using a Cary 14 recording spectrophotometer. Benzene and n-heptane were Mallinckrodt spectrophotometric grade !golvents: toluene and benzonitrile were MCLB spectroquality reagent; fluorobenzene, chlorobenzene, bronnobenzene, phenol, nitrobenzene, benzaldehyde, o-dichlorobenzene, and hexafluorobenzene were MC/B reagent grade chemicals; o-xylene, m-xylene, p-xylene, mdichlorobenzene, 1,2,4-trimethylbenzene, 1,2,4-trichlorobenzene, and pentsimethylbenzene were Eastman Organic chemicals with boiling points or melting points ranging over 1 to 2"; iodobrmzene was Eastman Organic chemical grade, redistilled once (a colorless liquid at room temperature); aniliilie was MC/B reagent grade and was redistilled once with sonic zinc dust (an almost colorless liquid a t room temperature); 1,3,5-trimethylbenzene was Baker's reagent (bp 164-166"); 1,2,4$tetramethylbenzene was Bakclr's reagent ( m p 79-81') and was resublimed once; pentachlorobeenzene and benzenehexacarboxylic acid were obtained from Aldrich Chemical Co.; 1,2,3,5-tetramethylbenzene was purchased from M & K Laboratories Inc., and hexachlorobennene was Eastman Organic Chemical's practical grade and was resublimed twice (needle crystal, rnp 231-232"). All spectra except benzenehexacarbsxylic acid, which was dissolved in methyl alcohol, were obtained using n-heptane as the solvent. D a t a Redaction The method of monents has been used by several investigators in the past to extract the Faraday parameters from the experimental data.2.30-32 For a nondegerierate electronic transition the magnetic rotational strength is2,31 32
B = -(33.53)-'
f-[@I,A
d3,
The Journal of Physical Chemistry, Vol. 77, No. 8, 1973
We extract (or reduce) our data by using the following approximate equation
B = -(33.53Am)-"[8l,
dA
where ,X is the wavelength at the middle of the MCD band. Substituting the experimental magnetic molar ellipticity [e], = (8"/CIH)100 into the above equation we have B = -100(33.53/1,Clfp)-'~s8'o d3, The ordinate of the recording chart paper gives the degree of ellipticity and its abscissas records the wavelength. The area read from the planimeter is in square inches. Their degree cm so that $8" dX = relation is 1 in2 = 1.5 x area (in.2) X 1.5 X area(in?) B = -0.99 X 10-'2-DD2 /? 6m-I X,C1 where H = 4.5 x lo4 G was used. D i s the Debye unit and p the Bohr magneton. The experimental MCD rotational strength for the eleclBzu of benzene i s BO(1A1, tronic transition IAI, lBzu) = 0.14 x D2 p cm-l, and for all of the benzene derivatives with a transition derived from the 1A1, lBzu of benzene Bo becomes B = Bo -t- AB. The quantities in Table I are calculated as follows: e,g., 1,z-dimethylbenzene has an MCD rotational strength B = 2.35 x 10-4 D2 p cm-1 (Table 11). AB is obtained by ignoring AB(A --* B)M in eq 4, i e . , ILB(lA1, -* 1B2,) = B(IA1, 1B2,) BO(IA1, IBzu) = 2.21 X D2 p 6m-I Y AB(lA1, lBzJs AB(lA1, ~BZ,),. AB(IA1, IBz,)" is estimated according to the assumptions made in the text, e.g., for singly methylated benzene aB(IA1, IBz,), lBzu) of the 1,3,5-trimethylbenzene and N Y3B('A1g l/&lA1g lBzuf of hexamethylbenzene, i.e., -0.27 X D2 cm-I. Finally, the ABtlAlg 'BZ,,)~of 1,2dimethylbenzene is calculated from B('A1, --* 'Bz,) A B ( ~ A ~IBZ,)~ ~ = (2.21 -t- 2(0.27) X 10-5 = 2.75 X 10-5 ~2 p cm-1.
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- -
- + -
+
-
-+
Results and Discussion Foss and M ~ C a r v i l l ehave ~ ~ reported the correlation between the maximum molar MCD of the IAI, -+ lBzu band of benzene derivatives and the Hammett upara constant. Here we present some of our MCD spectra including the 2600-, 2100-, and 1850-p\ band (we pushed the instrument to the limit in this region, the baseline was not very stable). There is some evidence of two different electronic transitions in the 2100-A band, the spectral region from 2300 to 2000 A, in p-xylene (Figure 11, m-xylene (Figure 2), 1,2,4-trimethylbenzene (Figure 3), and 1,2,4,5-tetramethylbenzene (durene, Figure 4). It i s probable that we are observing both the IAl, lE2, and the lBzu transitions in these compounds. The zene derivatives are nondegenerate. serve A and B terms. The appearance of an A term in benzene and hexasubstituted benzenes, all having Den symmetry would prove the existence ol the transition to a
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(29) D. J. Caldwell and H. Eyring, J. Chern. Phys., to be submitted for publication. (30) C. H. Henry, S. E. Schnatterly, and C. P. Slichter, Phys. Rev. A, 137, 583 (1965). (31) P. J. Stephens, Chern. Phys. Letf., 2, 241 (1968). (32) J. P. Larkindale and D. J. Simkin, J. Chern. Phys., 55, 5668 (1971). (33) J. G. Foss and M. E. McCarville, J. Amer. Chem. Soc., 89, 30 (1967).
Magnetic Circular Dichroism of Molecules in Dense Media
1033
;\(nm)
180
200
220
240 260
280
180 200 220 240 260 I +
0 20 3 *
= ,a
4
5 u 6
-0
280
+
6
Q
5
IO *
0
x
I10
X
m
20 mx
22 15
CD
u
30
20
5
5
47 0
3:
180 200
3 ;
Z W
2 "
I
I
220 240 260 280 7%( n m )
Acnm)
Figure 1 Spectra of 1,4-clirnethylbenzene. 180
200
'AX(nm1 220 240
260
Figure 3. Spectra of 1,2,4-trimethylbenzene. 'X(nm)
280 2
0 m
50 x
E
IO
3
U
15
Q 3;
-0
2::
* 5 0 4 X
J
- 2 I
200 220 240 260 280 500
'Acnm)
'h
Figure 2. Spectra of I ,Mimethylbenzene.
(nm)
Figure 4. Spectra of durene. state having 'Eag symmetry. The reason we do not observe A, terms in these compounds (see Figures 5-7) could be due to the ovcrlap of the two transitions. p-Xylene, durene, and 1,2,4-trirnethylbenzene all show a positive peak around 2200 A and a negative peak around 2100 overlapped with A transition having 1A1, lElu benzene parentage. p-Xylene and durene belong to Dzh, m-xylene belongs to Czu, and 1,2,4-trimethylbenzene belongs to C, symmetry. The result of applying the symmetry selection rules indicates that the transition to the excited state lEzg of benzene for molecules having Dzh symmetry is forbidden
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but the transition to the lBlU excited state is allowed. The transitions to both of these excited states are allowed for compounds with CzU and C, symmetry (see Table I11 where (a) means symmetry allowed and (f) signifies a symmetry forbidden transition). It seems reasonable to speculate, by comparing the MCD line shapes in this region, that the positive peaks in the above-mentioned three compounds belong to the positive troughs of the sinusoidal A term of MCD bands and their negative trough overlaps with the negative B term MCD bands produced by the transition to the upper state derived from the lB1, of benThe Journal of Physical Chemistry, Vol. 77, N o . 8, 1973
D. J. Shieh, S. H. Lin, and H. Eyring
1034 wavenumber (cm-')
I
f
I I
83
w 40 N
S
--x
I '0
a
w 120
Left
i
Right
Scale
I
Scale
160
9 4
I E ; ~
zoo 220 240 wavelength (nm)
'z: 260
x
280
6
3
Figure 5. Specire of bt,wzene. (111)
zoo
250
250
200
3M)
1 (nm)
Figure 7 . Spectra of hexamethylbenzene.
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relative signs of the first and second T T* transitions of benzene derivatives are discussed by Caldwell and Eyring.29 It has been shown that the rotational strength of the MCD for a nondegenerate electronic transition A B is determined by B(A B), which is defined by4,35
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-
-+
(1) B (A B) = I ~ ( ~ A B * M B A ) where FARrep_resents the matrix element of the electric moment and MABis defined by the relation -+
in N-Heptane
7
6
5 4
N
'3
'0
i
3 2
1 250
300
a (m)
Figure 6. Spectra of 1 ,:3,5-trimethylbenzene. zene. We observed a\ large negative A term in the MCD in the spectral region of 1850 A for most of the benzene derivatives. The positive trough of almost all of the sinusoidal A terms are not recorded adequately (except for hexachlorobenzene which shows a clear A term). The MCD instrument is not very stable in this region, consequently we are not able at this time to make any definite comment on the Jahn-Teller splitting. The possibility of observing this effect has been brought out by Stephens, et aL3" The The Journal of Physical Chemistry, Vol. 77, No. 8, 1973
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By &(A B) we signify the diamagnetic term in the B transition. In eq 2, ( p m )and ~ ~ MCD of the A denote the matrix ,elements of the magnetic' moment. If we compare the expression for B ( A B) given in eq 1 with the rotational-strength of the natural optical activity, we can see that M B A in eq 1 plays the same role as the magnetic transition moment in natural optical rotation. Now let B ( A B) in eq 1 represent the MCD rotational strength before substitution. After substitution, when the wave functions are changed. by srndl amounts, and MBAare changed by A ~ A Band AMBA,respectively. B) that is AB(A B) caused by The changejn B(A AiAB and A M B A to the first order is given by
-
-
-
*
AB(A-BB)
-
*
= Im(ArAB.MBA) -I-+
.-
I
a.-
-+
Z&AB*A MBA) i- I , ( A ~ A B * L M B , ~(3) (34) P. J. Stephens, P. N. Schatz, A. €3. Richie, and A. d. McCaffery, J. Chem. Phys., 48, 132 (1968). (35) D. J. Shieh, S. t i . Lin, and ti. Eyring, J. Phys. Chem., 76, 1844 (1972).
Magnelic Circular Dichroism of Molecules in Dense Media
1035 TABLE I I : AB('A1,-
TABLE I: f3('Al, -* 'Bnu) and qm B('Alg+ ' B f u ) , D2p/cm-
x
Position
Substituents
1
1,s 1,4 ,2,4 i ,3,5 1.3,4,5 1,2,4,5
1.31 7.23 0.95 -1.35 3.38 4.26 3.72 11.58 3.27 - 3.45 1.16 - 1.34 3.38 2.80 - 14.61 16.22
1
1 2 1 3 1,2,4
Penla Hexa 1
tiexa Br I
1 1 1 1 1
CN OW COOH
tlexa 1 1 1
NO2 CHO " 2
2.01 2.21 1.20 5.58 4.74 -0.82 1.17 7.09
5.0
-0.68
Penta Hexa
F
q m x 10'0
2.15 2.35 1.34 5.72 4.88
72
CI
105
'B2")'
0.81 - 1.50 3.24 4.12 3.58 11.44 3.13 -3.59 ? .02 - 1.48 3.24 2.66 -14.75 16.08 -28.71 -5.43 -34.98 -36.20 46.41
6.0
12.5
6.0 8.0
-28.57
-11.0 20.0 .- 17.0
-5.2ga -34.84 -36.06 46.55
-20.0 24.0 (D*p/cm-l) X
-0.27 -0.54
-0.54 -0.82 -0.82 -1.08 - 1.08 - I .35
-1.50 -0.60
- 1.20
- 7.20 - 1.80 3.00 -3.59
-0.25 - 1.48
-0,91 -5.43
2.28 2.75 t .74 6.12 5.56
a 2.25
8.17 2.16
0 3.84 5.32 4.78 13.24 6.13 0 1.27
0
-27.80 0
lo5.
Merhanol solution.
The change in eiectnc transition moment ATABcaused by the introduction of substituents consists of two parts, LElf3 -- (5'AkJq f (AFAH)qrepresents the substituent induced component (due to the inductive and reso~,, induced comnance eff'ects) and ( L L < ~ ~ ~ ) the-vibronically ponen,t. The contri.bution to AMHAmay originate from the changes in the elect:ric transition moment, the magnetic transition moment, and in the energy levels. For convenience, we shall rewrite eq 3 as BB(A---,I3) = AE!(A-B), f 4B(A--+B)v + AB(A-B)M (4) which clearly leads to
-
-+
A B(A
"3,
-**
= f,[(A TAB),.(MBA 4
4B(A-B1v
-.I
+
-t
+
A MBA)]
(&AB) /23(2,2)1 > IB(1,3)1, for trisubstituted benzenes, [B(1,2,4)1 > [B(1,2,3)1 = IB(1,3,5)1, and for tetrasubstituted benzenes, the magnitude of B(1,2,4,5) is the largest among the three substituted benzenes as is verified experimentally. It should be noticed that for tetrasubstituted benzenes, the sign of B(1,2,3,5) might be opposite to that of 13(1,2,3,4), and B(1,2,4,5), if we choose m = 0,1,2,4 for B(1,2,3,5), but if we choose m = 0,2,3,4 ( i e . , rather than designating the compound 1,2,3,5-, it is designated 1,3,4,6- for the NICD) then the signs of B(195!,374),B('E,2,3,,5), and B(1,2,4,5) should be the same. B), described Using eq 10, the estimation of AB(A above and the spectroscopic moments qm for various substitutents, one can estimate B(A B) for the mixed substituted benzenes. For monosubstituted benzene AB,( 1) = bq and since q is given by P e t r u ~ k ab~can ~ , be ~ ~calculated. The average value is found to be b = 5.48 X lo4. This constant, as we have pointed out, is characteristic of benzene and its derivatives.
-
- -
-+
-
-
--
To show how well eq 12 correlates the experimental - of B(A B) of homosubstituted benzenes, we define data AB, = AB(A B),/ABq(l) and since a is a characteristic' constant of benzene itself, we can plot for various substitutents for the different types of substitution. Here B& is assumed to be negligible. In Figure 8 are AB(A shown the homosubstitution ABq patterns for the lAlg fBzu transition of benzene for' two different values of the parameters a , From Figure 8, we can see that the agreement between the theoretical predictions and the experimental observations are only fair. In other words, to obtain quantitative agreement, we will have to consider the B)M, as well contribution arising from changes in B ( A as improvements in the SPP theory. Since the performance of the SPP theory in the prediction of the spectral intensity itself is only fair, the results are as good as could be expected. The theoretical method for estimating B(A B) of substituted benzenes described in this paper thus B14 and to obtain apsuffice to predict the sign of B(A proximate estimations of its magnitude.
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Acknowledgment. We wish to thank Mr. Jack Adams for technical assistance, Mr. Alan Weeks for drawing the figures, and Mr. BC. Robertson for typing the manuscript. The authors wish to thank the National Institutes of Health, Grant No. GP 12862, National Science Foundation, Grant No. GP 28631, and the Army Research-Durham, Contract No. DA-ARO-D-31-124-;2-G15, for support of this work.
regate-Solvent Interaction Studied by Nuclear Magnetic Resonance H. E. Zaugg* and R . S. Egan Resea,vh D I V ~ S JAbbott O ~ , Laboratones, North ChJCago,///ino!s 60064 (Recewed November 8, 79721 Public~8ttoncosts asssted by Abbott Laboratories
The effect of varying proportions of diethyl sodio-n-butylmalonate on the chemical shifts of the CH2 and CHs protons of 1,2-dimethoxyethane (DME) in benzene and cyclohexane is examined. Upfield shifts for both groups with decreasing mole ratio of DME to sodium salt are observed in both solvents, the shifts being greater for the CH2 group. Despite these marked shielding effects the interaction is very weak and nonstoichiometric. It is concluded that the observed shielding is simply a consequence of adsorption of DME molecules on the negatively charged surface of the micellar particles known to exists in solutions of this salt in nonpolar media.
In their nmr exmiination of the interaction of donor molecules with sodium tetrabutylaluminate (NaAlBu4) in cyclohexane, Day and coworkers132 assumed that the complex produced by the donor (e.g., tetrahydrofuran (THF) or diethyl ether) was in equilibrium with an ion pair aggregate, the nature of which was not defined. Sedimentation studies have recently revealed3 that the sodium salt of dieLhy1 n-butylmalonate exists in benzene or cyclohexane solution as an inverse micellar system. In The Journal of Physical Chemistry, Vol. 77, No. 8, 1973
benzene the nearly spherical particles are composed of about 50 ion pair units (molecular weight 11,400) with the exterior lipophilic anions serving to insulate the hydrophilic sodium ions from the otherwise incompatible nonpolar environment. This note reports an nmr study of the (1) E. Schaschel and M. C . Day, J. Amer. Chem. SOC..90, 503 (1968). (2) C . N. Hammonds, T. D. WeStfnGrelafld, and M. C. Day, J. Phys. Chem., 73,4374 (19 6 9 ) .
