DAVIDF. KOSTERAND ALFREDDANTI
486
Proton Magnetic Resonance Studies of Ten Diolefins'"
by David F. Kosterlb and Alfred Danti Department of Chemistry, Texas A . & M. University, College Station, Terns
(Received August 20,1964)
The proton magnetic resonance spectra of ten diolefins, with six to eight interacting spins and more complicated than any studied heretofore, have been measured with care and the complete theoretical spectra calculated for four of them. Chemical shifts and coupling constants are tabulated as completely as possible. The ken compounds are 1,2-butadiene, 1,2-pentadiene, 3-methyl-l,2-butadienej 2,3-pentadiene, 2-methy1-ly3-butadiene, 2,3-dimethy1-lj3-butadiene, 1,cis-3-pentadieneI 1,trans-a-pentadiene, 1,4-pentadiene, and 1,5hexadiene. Symmetry factoring of the energy levels was used in a number of instances and in some cases the molecule was broken up into lesser interacting parts before running the entire eight-spin problem. An interesting additive effect of substituents in reducing both the four-bond and five-bond coupling constants in allenic systems has been found. A noticeable coupling through six bonds was evident in two of the conjugated compounds.
Introduction The proton magnetic resonance spectra of several compounds containing cumulative and conjugated double bonds have been analyzed previously.2-7 I n most of these cases only three to five interacting protons were involved. I n this study, more complicated members containing six to eight interacting protons have been examined. Where there were no more than eight interacting nuclei and the spectrum was well resolved, the theoretical spectrum was calculated completely using the computer program of Reilly and Swalen.8 In the allenic type systems, an additive effect on the four- and five-bond coupling constants appears to exist. Experimental The eight compounds listed in Tables I and 11,along with 1,4-pentadiene (IX) and 1,5-hexadiene (X), were examined. The Roman numerals appearing beneath the semistructural formulas in Tables I and I1 will be used when referring to these compounds. All samples were extremely pure API Research samples available on loan from the API Research Projects 44 and 58B.9 Spectra were obtained on the pure liquids and 50% and 10% solutions (by volume) in CC1,. Results reported for I through IV are for the pure liquid, the remainder being for the 5Ooj, solutions. For these hydrocarbons, spectra a t different concentrations were very much alike. Spectra on which detailed The Journal o j Physical Chemistry
calculations were performed were run three times and peak positions averaged. A trace of tetramethylsilane was used as an internal reference. Improved resolution was obtained by degassing the samples by the freeze-thaw technique a t liquid nitrogen temperature and sealing them under vacuum in precision Varian A-60 tubes. Spectra of all of the compounds were recorded on a Varian Model A-60 spectrometer and for two of the compounds spectra were also obtained a t 100 Mc. through the courtesy of Varian Associates. Sample temperature was approximately 37'. Line positions measured from TMS as (1) (a) T h e studies were supported by T h e Robert A. Welch Foundation, Houston, Texas. Support for one summer for the Predoctoral Fellow was received from the American Petroleum Institute Research Project 44. (b) Predoctoral Fellow of T h e Robert A. Welch Foundation. (2) E. B. Whipple, J. H. Goldstein, and W. E . Stewart, J. A m . Chem. SOC.,81, 4761 (1959). (3) E. B. Whipple, J. H . Goldstein, and L. Mandell, J. Chem. Phys., 30, 1109 (1959). (4) E . I. Snyder and J. D. Roberts, J. A m . Chem. SOC.,84, 1582 (1962). (5) S. L. M a n a t t and D. D. Elleman, ibid., 84, 1579 (1962). (6) J. A. Elvidge and L. M. Jackman, Proc. Chem. SOC.,89 (1959). (7) E. 0. Bishop and J. I. Musher, Mol. Phys., 6 , 621 (1963). (8) C. A. Reilly and J. D. Swalen, J. Chem. Phys., 37, 21 (1962). (9) American Petroleum Institute Research Project 44, Chemical Thermodynamic Properties Center, Department of Chemistry, Texas A. & M. University, College Station, Texas, and A P I Research Project 58B, Petroleum Research Laboratory, Department of Chemistry, Carnegie Institute of Technology, Pittsburgh 13, Pa.
PROTON MAGNETIC RESONANCE OF DIOLEFINS
487
Table I : Chemical Shifts and Coupling Constants for Alkyl Allenes" 1,%Butadiene
H (1)
\ '
H
1,2-Pentadiene (3) (4)
(3)
c=c=c
/'
(1) "(2)
I
H
\
/
c=c=c
3-Methyl-l,2-butadiene
\ PHZCH3 c=c=c H
(1)
\H(2)
' H
I1
/
(3) \CB*
111
2,3-Pentadiene (4) CHI
g)
/
\
/c=c=c
(1)H
\ CHA 3)
IV
6, p.p.m.
1 2 3 4
4.498 4.943 1.587 ...
4.547 5.034 1.951 0.993
4.402
... 1.620
4.890
... 1.563
J , 0.p.s.
12 13 23 34
-6.67 3.45 7.10 ...
6.77 3.50 6.23 7.51
3.15b
...
-6.35 3.20 6.80
...
a Except for 111, where. a firsborder spectrum yields to immediate interpretation, data were obtained by calculation of the theoretical spectra. All data refer to the pure liquid. * Snyder and Roberts' report a value of 3.03 f 0.06 C.P.S.
1 2 3 4 5 6
4.87 f 0.05 4.87 f 0.05 4.94 f 0 . 0 5 5.05 f 0 . 0 5 6.35 f 0.05 1.79 f 0 . 0 3
4.86 f 0.05 4.96 f 0.05
4.99 f 0.10 5.07 f 0.10 6.58 f 0.05 5.92 f 0.05 5.41 f 0.05 1.70 f 0.03
...
..
1.86 f 0.03
4.83 f 0.10 4.92 f 0.10 ? ? ? 1.72 f 0 . 0 3
J , c.p.8.
12 13 14 16 23 24 26 34
a
2.2 10.5 4 . 6 1.2
1.0
--0.6(?) 16.6
-0.6
1.2 1.5
35
10.5
45 46 56
17.4
4.6(?)
10.5 11.o
1.5 6.8
5.8
These data were obtained from first-order interpretation of the spectra and refer to samples diluted to 50% by volume with CC1,.
Volume 60, Number I Februaru 1966
488
zero are accurate to within k0.3 cycle. Differences in peak positions could be measured to k0.05 C.P.S. on expanded scans. Calibration of the chart for linearity was checked against CHC13 and panisaldehyde. Calculations were performed on the IBM 709 computer of the Data Processing Center a t Texas A. & 31. University. The spevtra in full have been contributed to the “Catalog of NMR Spectra” of the API Research Project 44 and may be found there.” The calculated spectra are also shown. Where two calculated spectra appear, thtl bottom one with fewer lines pertains to an initial calculation with certain lesser interacting protons left out. The upper calculated spectrum then refers to the full eight-spin problem. Interpretation of the Spectra. Where the spectra were sufficiently resolved, the theoretical spectrum was calculated. This was the case for I, 11, IV, and IX. Case I11 gives a first-order spectrum and the values given in Table I came directly from our experimental measurements. The remaining spectra were analyzed by first-order interpretation to give approximate chemical shifts and coupling constants for most of the protons. The general procedure for arriving a t the calculated spectrum can be illustrated for I. Because the allenic structure is DZd, one would expect the methyl proton resonance to be approximately two triplets. The high-field quintet that is obtained can be explained as two overlapping triplets. To give such a symmetrical quintet, Jzamust be very close to twice J13, J H being very nearly equal to the spacing of the lines in the quintet. From approximate values of the chemical shifts and J I 2 ,a calculated spectrum closely resembling the experimental was obtained. The calculated transitions were then assigned observed frequencies, the assignment being facilitated by symmetry factoring the energy levels.12l 3 The assigned transitions between these levels, when put into an energy level program (XMRES2),*gave a set of experimental energy levels. Iteration on these energy levels gave the calculated spectrum and the final set of chemical shifts and coupling constants used to calculate it. (See Table I.) All coupling constants of the allenic compounds were assumed to be positive except the four-bond Jlz constant which has been shown to be of opposite sign.45 However, it was found that a change of sign here did not affect the calculated spectrum noticeably. Due to the large number of transitions and length of computer time, it was desirable to simplify the compounds containing eight protons. In the case of 11, the previously mentioned procedure was followed The Journal o.f Physical Chemiatry
DAVIDF. KOSTERAND ALFREDDANTI
by ignoring the methyl protons, which are only coupled to the methylene protons. This gave all the coupling constants except J34. The complete spectrum was then calculated including the methyl protons. For TI, one obtains 256 energy levels that can be symmetry factored into eight groups, four of which are doubly degenerate; 869 transitions were assigned and the complete calculated spectrum with one iteration ran 198 min. on the IBM 709. Similarly, IX (lP-pentadiene) was first treated as
to obtain a good fit to the vinyl part of the spectrum. The final spectrum with all eight protons was then calculated. The agreement was good and no iterations were performed to arrive a t the final spectrum. The results are given in a ,later discussion of this compound. The spectrum of IV caused some difficulties in that there is no easy way to treat it without considering all eight protons (A3A3*XX*). A fairly good fit for the methyl proton resonance was obtained by removing one proton from each methyl and treating it as
H
/
c=c=c
\
(2)
H
This allows coupling constants to be determined with fair accuracy. The final calculated spectrum including all eight protons was then calculated. The fit was good and no iterations were performed.
Results and Discussion In discussing the compounds studied, it will be convenient to separate them into three groups; those containing cumulative, conjugated, and isolated double bonds. (10) Accurate measurements on panisaldehyde made available by N. F. Chamberlain, Humble Oil and Refining Co., Baytown, Texas. (11) “Catalogs of N M R Spectra,” American Petroleum Institute Research Project 44, Chemical Thermodynamic Properties Center, Department of Chemistry, Texas A. & M. University, College Station, Texas, B. J. Zwolinski, Director; Serial No. 465-475. (12) H.M. McConnell, A. D. McLean, and C. A Reilly, J . Chem. Phys., 23, 1152 (1955). (13) E.B. Wilson, Jr., ibid., 27, 60 (1957).
PROTON MAGNETIC RESONANCE OF DIOLEFINS
489
Table I11 : Experimental and Predicted Coupling Constants in Allenic Systems Compound
H
\
Ref.
--Four-bond JHH couplingExptl. Predicted
,
Exptl.
-Five-bond JH.CH* coupling--Predicted
3.45 f 0 . 0 5
(No entry, parent molecule)
This work
6.67 f 0.05
This work
6.35 f 0.10
6.34
3.20
0.10
3.12
4
5.80 f 0.10
5.77
2.40 f 0.10
2.22
This work
3.15 f 0 . 0 5
3.12
4
2 . 1 4 f 0.10
2.22
c=c=c I zk
IV
H \
dCWC4 H ' H
\ H
/
c=c=c
/CH*
\ CHa
I11
Cumulative Double Bonds. Several previous studies2-5 on compounds containing the allenic structure have all shown two distinct characteristics; a shift to high field of the allenic proton resonahce relative to a vinyl proton, and relatively large four- and five-bond couplings between protons separated by this a-electronic structure. Both effects have been treated theoretically. l4t15 Very good predictions on the magnitude of the coupling constants (including relative sign) have been made. l 4 The chemical shifts and coupling constants for I, 11, 111, and IV are summarized in Table I. Utilizing these data, along with coupling constants reported for other substituted a l l e n e ~ , ~it- ~appears that an additive effect exists in the four-bond J H H and fivebond J H c H s coupling constants. If allene, with the four-bond J H Hequal to 7.0 cycles12 is considered the parent compound, the effect of substituting one proton in allene by C1, Br, I, CH,, or a CH2CH3group is found to be a decrease in the fourbond J" coupling constant by 0.9, 0.7, 0.7, 0.33, and 0.23 cycle, respectively. If each substituent has an additive (or subtractive) effect, a C1 and a CH3 on the allenic structure might be expected to reduce J" by 1.23 cycles, a C1 and Br to reduce it by 1.6 cycles,
etc. Unfortunately, only a few of these compounds are available to check this. As shown in Table 111, those that are known do check very closely. It was thought that this same additive effect might also exist in the five-bond J H , c H a coupling, and it is here that some strength is added to the argument. If now I is considered the parent compound, and the five-bond J H , c H a coupling is examined in substituted compounds, it is found that the C1 and CH3groups have about the same effect here as on the J" four-bond coupling. The predicted and known four- and fivebond coupling constants are summarized in Table 111. A reasonable explanation of this appears available. Theoretical calculation^^^ of these long-range couplings involve a u- and a-electronic term, the u-term making only a small contribution. It is evidently the aelectronic structure that allows this relatively large coupling, and these mobile electrons are easily affected by substituents. I t is not readily apparent why C1 and CH3 should both reduce the magnitude of this long-range coupling. (14) M. Karplus, J. Am. Chem. Soc., 8 2 , 4431 (1960). (15) J. A. Pople, J . Chem. Phya., 24, 1111 (1956).
Volume 69, Number 2
February 1966
DAVIDF. KOSTERAND ALFREDDANTI
490
Correlations of proton coupling with substituent have been made for substituted ethanes. 16-18 These have been of the form, J = A - B Z E , where A and B are constants and E is the electronegativity of the substituent,(s) based on Huggins electronegativities. One might hope to find a similar correlation for allenic systems, if enough compounds of the type XHC= C=CHY were available. However, from the standpoint of electronegativities, this does not appear too promising for the limited number of compounds for which data are now available. Information cited previously shows that bromoallene and iodoallene have the same coupling. The effect of substituents on the one-bond 13C-H coupling has also been studied extensively. 1 9 - 2 1 Conjugated Double Bonds. Previous studies on substituted 1,&butadienes have been reported.‘jr7 A relatively small trans-vinyl coupling and a large 2,3coupling (protons on carbons 2 and 3 of 1,3-butadiene) were reported for the trans-trans-muconic ester.‘j Recent calculations7 on the trans-trans-muconic acid are more consistent and it has been suggested that several J’s may have been interchanged in the ester. Approximate coupling constants obtained in this study are in agreement with the latter author^.^ The spectra of V, VI, VII, and VI11 were analyzed by inspection where possible. The 100-Mc. spectrum of V I P greatly facilitated the assignment of the overlapping lines. The large number of overlapping resonances discouraged any detailed calculation. Our results are summarized in Table 11. The lack of information on VI11 is due to its extremely complex spectrum. The resonances of protons 3, 4, and 5 cannot be distinguished from an array of lines from -333 to 385 cycles. Approximate coupling constants for V have been, reported elsewhere. 2a It is interesting to note that there appears to be a detectable six-bond coupling between either proton@) 1 and (or) 2 and the methyl protons in VI1 and VIII. Inspection of the resonance lines of protons 1 and 2 in VI1 and VI11 suggests that it is the proton cis to the methyl group that is more strongly coupled, Le. H
C=C
/ \ c=c CH3 H \ / c=c
\
C=C
\
(3)
CH3
The vinyl resonance of VI showed two broad lines. The line at higher field was assigned to proton 1 as is the case in VI1 and VIII. Attempts to analyze the spectrum of 1,3-butadiene (not listed in any of the T h e Journal of Physical Chemistry
Tables and not mentioned heretofore) at 60 and 100 M C .proved ~ ~ fruitless. Isolated Double Bonds. Only two compounds were studied in this group, I X and X. The spectrum of I X (1,4-pentadiene) was analyzed as described earlier (eq. l ) , with the following values of J and chemical shifts giving a good calculated spectrum 61 = 4.920 (p.p.m.) S2 = 4.950
83 =
J i z = 2.20
-1.50
513
=
10.3
J14
= J24
=
5.710 84 = 2.723 Jz3
-1.30
= J34
16.9 = 6.30
The negative sign given to J 1 4 and J 2 4 is on the basis of earlier work.6~24-27 One would expect that the inclusion of one more methylene group between the two vinyl groups would have very little effect on the vinyl proton resonance. This is not the case in going from IX to X (1,5-hexadiene). The twelve lines expected for X are clearly evident, but they are superimposed on a much more complex group of lines. Likewise, the remaining terminal vinyl and methylene proton spectrum in X is not clearly resolved as in IX. At present, this cannot be easily explained, but it may be that the nonterminal vinyl protons in X are also coupled to the methylene protons in the position p to them. This would require a coupling through four single bonds. Acknowledgments. We are grateful to The Robert A. Welch Foundation, Houston, Texas, for support of these studies. The API Research Project 44 also contributed support for one summer. We express thanks to the Data Processing Center of Texas A. & M. University for computer calculations. We acknowledge the help from F. S. Mortimer, C. A. Reilly, and J. D. Swalen of Shell Development, Emeryville, Calif., (16) R.C.Glick and A. A. Bothner-By, J.Chem. Phys., 25,362 (1956). (17) c. N. Banwell and N . Sheppard, Discusswne Faraday SOC., 34, 115 (1962). (18) R. J. Abraham and K. G. R. Pachler, Mol. Phys., 7 , 165 (1963). (19) H. S. Gutowsky and C. 8. Juan, J. Am. Chem. Soc., 84, 306 (1962). (20) C. Juan and H. 5. Gutowsky, J. Chem. Phys., 37, 2198 (1962). (21) A. W.D o u g h , ibid., 40, 2413 (1964). (22) Spectrum obtained by Varian Associates on a 100-Mc. instrument. Courtesy of James Shoolery and Norman Bhacca. (23) J. A. Pople, W. G. Schneider, and H . J. Bernstein, “HighResolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N . Y., 1959,pp. 244,245. (24) S.Alexander, J . Cham. Phys., 28, 358 (1958). (25) F.S. Mortimer, J. Mol. Spectry., 3, 335 (1959). (26) A. D.Cohen and N. Sheppard, Proc. R o y . Soc. (London), A252, 488 (1959). (27) C. N. Bsnwell, A . D. Cohen, N. Sheppard, and J. J. Turner, Proc. Chem. Soc., 266 (1959).
GENERATION OF CATALYTIC ACTIVITY IN SILICAGEL BY IONIZING RADIATIOX
in making available the complete computer programs and the guidance which they gave. We acknowledge discussions with Dr. R. AI. Hedges and the encourage-
49 1
ment of Dr. B. J. Zwolinski. The research samples were made available through the API Research Projects 44 and 5SB.
Generation of Catalytic Activity in Silica Gel by Ionizing Radiation
by C. Barter and C. D. Wagner Shell Development Company, Emeryiille, Calijornia
(Received August $0, 1964)
In a previous paper it was reported that acid centers are generated in silica gel by the action of ionizing radiation, in vacuo. These centers persist in the gel, in the absence of radiation, but are thermally sensitive. At 25" they disappear with a half-time of a few hours, as shown by the loss of acid titer and activity for isobutylene polymerization. Further study of irradiated silica gel has disclosed that radiation generates at least two other types of chemically active centers. These are stable a t more elevated temperatures and are active for butene interconversions and for the conversion of cyclopropane to propylene.
Introduction That irradiation of silica gel with X-rays results in surface changes other than the generation of acid centers has been suggested by spectroscopic evidence. Following butylamine neutralization of the gel or thermal decay of its generated acid centers, the absorption spectrum of adsorbed p-dimethylaminoazobenzene is not at all characteristic of this indicator in its neutral form, in solution or adsorbed on unirradiated gel. The surface changes responsible for the altered spectrum are reversible; they may be due to atomic displacements, for the heating of irradiated silica gel at 500" for 10 hr. prior to adsorption of the indicator results in an absorption spectrum identical with that obtained with unirradiat ed ge I. The active centers studied in this work niay be similar to those observed by Mikovsky and Weisz2 as a result of neutron irradiation ( l o w n.v.t.) of silica gel; they attributed the centers to atomic displacement.
Experimental Davison 950 silica gel, 60-200 mesh, surface area 625 m.2/g., aluminum content
