3010
E. B. WHIPPLE,J. H. GOLDSTEIN AKD LEONMANDELL
and gl. Unfortunately AI and A2 are not known since the optical absorption spectra of the ring pi electrons probably masks the A1 and A2 transitions. 2 transition corresponding to A2 would be weak in any case. However graphs of 0 us. 41 and dl D S . A? are given in Figs. 3a and 5b. The values obtained for & I and B are, using the value obtained for a’ A = zt1.88 X lo-’ R = k 0 . 3 9 X lO-’cin.-’
The sign of .1 is taken as negative; apparently all copper compounds studied so far have negative -1’s. For $1 = I , equations 13 solve to give P = 0.030 c m - l and K = 0.36. If, in equation 14, we replace +1(O)@.a(O) by y’lS(O) ? and take IS(Uj12= 23 X l o z 4c111.-~ d o n g with ycU = 1.53, K = 0.36 and I’ = 0.030 c ~ i i . - we ~ obtain the result that
fif*=
-0.048
This shows t h a t only a sinal1 admixture of \k2 is necessary to explain the copper hyperfine structure. The negative sign obtained for f is due to the promotion of an electron from a bonding orbital to an antibonding orbital. Conclusions Some insight has been gained on the nature of the molecular orbital which involves the odd electron. Since this bond has a fair amount of covalent character it is expected that the other signia orbitals involving electrons from lower copper d
Vol. s2
orbitals are also appreciably covalent in character. This is in agreement with the fact that Cu Etio 11, as well as other metallo etioporphyrins, is very stable toward acids.Ig Although knowledge of 0 and PI cannot be obtained without prior knowledge of AI and &, we can do some speculating. As is pointed out by GoutermanZO the similarity of the pi spectra of the metal porphyrins seems to indicate that d orbitals do not appreciably affect the pi spectrum. This is equivalent to saying t h a t the d orbitals of 7 s y n metry do not interact with the ring orbitals. It seems then that the conclusion that P1 = 1 can be drawn. For 13, = 1 the expressioti for g l is satisfied if 1 2 is taken to be 20,700 c1n-l. The optical spectrum of Cu Etio IIIs suggests taking 1S,O50 cm.-’ for 1 2 which leads to = 0.99. If we further suppose that the probability of an electron being found near the nitrogens is the same for the excited BtBand E, states then [P= 2812 - I For PI = 0.99 we obtain the results t h a t /j = 0.9s and A, = 27,000 c i i ~ . -2 ~ 1,2’ The authors arc indebted to Professor A. H. Corwin for the high purity sample of copper etioporphyrin 11. (19,
W. S. Caughey and A .
11. Corwin, THISJ O U R N A I . , 77, 1509
(193,;). (20) M.Goutcrman, Thesis, T h e Univcrsity of Chicago, 1958. (21) J. G. E r d m a n :1nd .ficcta complete analysis including the vinyl proton assignments. Chemical shifts and spin-coupling constaiits arc tlicii reported and discussed for a series of ten 2-substitutcd propenes in dilute tetraIneth~lsilarlesolutiuns.
In earlier work we have studied the effects of chemical substitution oii the proton magnetic resonance shifts and couplings in the propargylie system H-C-C-CH2X, and the allenic system H2C=C=CHX. I n these studies, the cheniical shifts were measured in dilute cyclohexane (CM) or tetrainethylsilane (T1IS) solutions in order to minimize differences in environmental “medium effects,” and the results were standardized in terms of the former solvent with respect to an external H,O (liq.) reference.’ The present work extends these methods to the substituted p r o p i y l system, beginning with the 2-substituted (I I I . 13, LVhiyyle, J . H C;olilstein, I.ei,n 3landel1, (;. S. I i e d d y and I< M c C l u r e , THISJ U U K N A L , 81. 1 3 2 1 (I959), ( 2 ) E. I3 W h i p p l e , J. H. Goldstein and I x u n IIandell, J . ( I i t , i n Z’/Z\,A , 30, 11O!J (1959). (:3) I > J a ~ i ) , the arialysis may proceed ( 4 ) .I.A. Poplc, Xi-. C;. Sclincirlcr and €1. J . Uernstein, “ H i g h R e s d w tiun Suc1e;rr l l a p n c t i c Resonance,” IIcGra~v-HilI Book Company, Iric, N e w Virrk, S Y , , lU.j!,, Cii 1 1 ( a ) Ti. V,‘. I’eshrnrlen and I S. \VauKh. J . ( h c , i z . PIIS:., 30, 011 tI!M!l).
PROTON hf AGNE TIC
June 20, 1960
STATIONARY STATES AND ALLOWED State Spin eigenfunction
(coseHap
r M-Y I'). cy r ) M- - y r Y yr ) r
'/2(~.4 '/?(wA
+ +
+
Ex -
'/'~(wA '/*(WA
+
SYSTEhZS, Energy"
'/2)
WB)
WU)
+
Relative intensity
1 zk sin 2 8 ~ 1 1 =k sin 28r
I/~M(JAx
'/Z(JAX
1 V
JUS)
-v
LOX
r )M
iV
w x * (CH+I - C M ) wx * (CM+l+ C M ) ' / 2 { [(WA - W B ) h f ( J . 4 ~- JBX)]' M = .WX CM sin 2eM = ' / ? J A B 3 for A1 transitions involving M = 4 for A , transitions involving M = 2 for E transitions involving =
+
CM
=
In units of h.
+
+
ll
IM~
directly by simple multiplet rules6 with due allowances for overlap in the AB patterns. This can be seen by expanding the general expression for CM in Table I and neglecting terms higher than first order, Most of the systems reported here approach this limit. h more complex situation occurs when the AB shifts are nearly degenerate and due to the relatively small couplings the spectrum tends to be poorly resolved. This is illustrated in the 2chloropropene (TMS) spectrum shown in Fig. 1.' Since the X group lines are still symmetrical
TYPEABX3
+ +
+ '/JAB + + JDS)-CM - ' / ~ J A+ B ' / Y N ( J A x+ JBX)*CM + + - ' / ~ ( J A+ S JUS) WX
301 1
4-' I I J A B ' / s I ~ J ( J A4x IBX) EX - '/JABCM $- W B ) ' / I J A B- ' / 2 1 ! f ( J P l X $. J B X ) WB)
Frequency
-yr)ll+l--l(r)ht
a
Ex
+ sin@x@)(r)~
PP( r I s1
Transition
\I+
TABLE I TRANSITIONS FOR &SPIN ( I =
cua(r)H
Y r )M
yriN -I( r
o( r
RESONANCE IN 2-SUBSTITUTED PROPENES
3(coSew+lcoseM sin8M+Isin6M)2 N(cosehr+Isin8M - sin83i+lcos8M)2 JAB?}"'
'/2
only only
group spectrum, the value IJAX+JBXi = 2.03 agrees within the error of the measurements with the previous value, supporting the assumption that the couplings do not change. From the splitting of the inner pair of methyl group lines, the value I J A x - J B x ~ = 0.73 gives J A X = 1.38 and J B X = 0.65, the A proton having been labelled by the stronger methyl group coupling. From the five
ili
'k
' yR/ ___
'-_i-j_._~-_
Fig. 1.-Spectrum
._--.-
---.-
-.!
1-
of 2-chloropropene in tetramethylsilane solution.
about wX, the center of gravity of the methyl group spectrum gives directly its shift, and the quantity ~ J * X + J B= ~ ~2.05 c.p.s. is determined directly from the splitting of the two most intense lines. It is difficult to assign precise values to the remaining variables, however. Advantage can be taken of the fact that chemical shifts are as a rule much more sensitive t o the environment than are nuclear spin couplings, which may allow one to obtain an independent measurement of the latter in solvents which through intermolecular effects largely remove the AB degeneracie~.~Thus the 2-chloropropene spectrum in benzene solution has the appearance in Fig. 2, which approaches the simple multiplet limit where the analysis is straightforward. From the outer lines in t h e methyl (6) H. S. Gutowsky, D. W. hIcCall and C. P . Slichter, P h y s . Rev., 84. 589 (1951). E. L. Hahn and U . E . Maxwell, ibid., 84, 1286 (1951). (7) Spectra were measured a t .40 megacycles on the Varian Model 4300B High-Resolution System. w rcfers to the chemical shifts in c.p.s. a t the operating frequency and 8 to the shifts in k.Ixm.
Fig. 2.-Spectrum
of 2-cliloropropene in benzene solution. (Vinyl group is a t left.)
high-field vinyl lines i t is apparent that JAI( J A B , and this also accounts for the qualitative appearance of the B group spectrum to lower field. The best over-all fit occurs with J A B = 1.3 C.P.S. Using these values for the couplings, the spectrum in TMS can then be treated in terms of the single variable AW = WA - W B , the indicated value being Aw = 1.2 c.P.s., which gives the results shown in Fig. 3. Experimental values far the shifts and couplings in a variety of 2-substituted propenes are summarized in Table 11. In most cases the spectra could be analyzed directly by a procedure similar to t h a t for 2-chloropropene (B). The methallyl halide spectra, Fig. 4, were further complicated by methylene group interactions but the patterns were otherwise straightforward. The .A and B assignments in Table I1 are made on the basis of [JAXI>[JBXI rather than on the basis of chemical shifts. The uncertainties in the J A B couplings are generally greater than for the long-range interactions. J A Xand J s ~ .
E. B. WHIPPLE,J. H. GOLDSTEIN AND LEONMANDELL
3012
Fig. 4.-Spectrum
Fig. 3 . 4 b s e r v e d and calculated spectra for 2-chloropropene. (Upper figure is methyl group.)
Discussion of Results The first question t h a t arises concerning the data in Table I1 is the geometrical assignment of the vinyl protons. It is assumed for this purpose that the relative magnitudes of the couplings will be consistent for the series. This is suggested by the rather. constant difference of 0.6 C.P.S. between the JAX and J B X couplings over the range of Y substituents. TABLE I1 CHEMICAL
COUPLING CONSTANTS PROPENES,H*C=CY-CH:
SHIFTSa A N D STITUTED
IN
%SUB-
Y Subst.
6~~
hern
6xm
JAX
JBX
JAB
-Br
-0.42 -0.24 2.82 1 . 4 0 . 8 2 . 0 -c1 .03 .OO 2 . 9 9 1 . 3 8 .65 1 . 2 5 -0COCHa .53 .45 3.19 1 . 2 .6 1.0 -0cH1 1.3 1 . 3 3.31 .. .. .. -CHO -1.08 -0.78 3.27 1 . 6 1.0 1 . 1 -CN -0.58 - .47 3 . 1 5 1 . 7 1.15 1 . 0 -CHI .42 .42 3 . 3 6 1 . 2 5 1 . 2 5 .. -CH,C1 .20 .05 3 . 2 4 1 . 5 0.85 1 . 6 -CH2Br .20 .01 3 . 2 1 1 . 5 .8 1 . 5 .8 1.5 -CHzI .24 - .09 3 . 1 7 1 . 5 Relative t o H20 (liq.) external reference for dilute soluParameters listed according t o ABXI tion in cyclohexane. notation with A defined by 1 J A XI > I JBX 1 .
From the chemical shifts of 2-chloropropene in T M S and benzene solutions, the “specific medium effects,” can be approximated* for the three proton types, the results being summarized in Table 111. Similar effects in acetone solutions are also recorded, the small values in this solvent indicating t h a t association shifts are relatively unimportant. The medium effects in benzene solution are thus by inference primarily a n i ~ o t r o p i c . ~The comparable effects in the later solvent on the vinyl proton “A” and the methyl protons ,suggest that these lie on the same side of the solute molecule. The differences probably are due to a tendency on
on1,
(8) More precise values are obtained by extrapolation to infinite dilution in the various solvents. (9) A. A. Bothner-By a n d R. E. Glick, J . Chent. P h y s . , 26, 1651 (1957).
VOl. 82
of rnethallyl iodide in tetramethylsilane solution.
the part of the solute to be oriented with its dipole moment pointing away from the region of high a-electron density above and below the plane of the solvent molecule (Fig. 5). This reasoning would assign the greater long-range coupling to the vinyl hydrogen cis to the methyl group. T o confirm the assignment, similar measurements were performed on vinyl chloride solutions, where the assignments are known,1° and the data recorded in Table 111 compares favorably with the assigned results in 2-chloropropene. A similar trend was observed, but not measured, in 2-bromopropene solutions. The assigned order of couplings, cis > trans, is consistent with results reported by Alexander1’ in butene-1, and by Pople, Schneider and Bernstein12 in a-methylstyrene. It is also interesting to note a t this point t h a t the “A” shifts in methallyl halides are affected less by the allylic substituent than are the corresponding “B” shifts, which could be explained on the basis of direct interactions of the type indicated by I. H>c=c
