1838
PAUL
C. L.4UTERBUR
Vol. 83
bromide4 and for methyl benzenesulfonate and traced to this common source-the difference in isopropyl benzenesulfonate. The differences as the initial state solvation shells. well as similarities in the slopes of these plots are Derived Thermodynamic Parameters.-If the consistent with our assumptions. usual assumptions are made concerning equiThe slope for isopropyl bromide (1.21) indicates librium between the initial and transition states a larger temperature coeficient for k D t ~ / k ~ z and o further we assume that dC,*/dT = O,'? then than for the relaxation process in bulk water. This froni the empirical constants of equation 2, the result is consistent with a higher energy barrier derived parameters characterizing the activation to rotation in the initial state solvation shell process may be calculated. These are collected about the halide compared t o bulk water (4-5 in Table 111 along with the corresponding terms kcal.) andsthe implication of equation 6. Similar from the hydrolysis of isopropyl bromide in HaO. enhanced stability is indicated for the initial state TABLE I11 solvation shell about methyl bromideg and hence a similar temperature dependence for ~ D ~ O / ~ HA~ COMP.4RISOT O O F THERMODYSAMIC PARAMETERS FOR THE would be expected and is found. A4CTIVATION PROCESS I N THE HYDROLYSIS O F ISOPROPYL The slope for the benzenesulfonates is obviously BROMIDEI N HzO ASD D 2 0 less than unity. The difference between this value -In H10"I n DzOh--AS* AS*, and that for the halides is consistent both with the Tern], , AHx, cal /mole ATP, cal /mole cal /mole deg. cal /mole deg. absolute difference in the magnitude of the sol9.94 8 03 27,920 vent isotope effect already n ~ t e d and ~ . ~with the 0 27,180 24,670 - 1.03 50 24,360 -1 43 explanation advanced above. If the sulfonic 21,390 -10 43 100 21,540 -9 54 oxygens provide competing centers for hydrogen bonding and thus increase the rotational freedom a ~ c , * = - 56.4 cal./inole deg. * AC,* = -65.3 c J . / of contiguous water molecules, or, alternately, mole deg. if the adjacent water molecules are more strongly Since qualitative arguments lead to the conhydrogen bonded to these sulfonic oxygens in the initial state, the solvent isotope effect and the clusion that the major contribution to ACp* is temperature dependence thereof will be reduced. dFs/dl', it is gratifying to discover that there is a The latter hypothesis is probably more likely since significant difference between AC,,*(DzO) andAC,* the ability of sulfonic groups to confer greater (H20)and the values of both terms are more negasolubility on many organic molecules is well known. tive than that calculated by Conway30 and ourThe close parallel between the differences in the selves3*for the relaxation processes in bulk water. temperature dependence of the solvent isotope The differences between A H * and A S * for the two effect for halides and sulfonates noted above and media may likewise be interpreted as reflecting the corresponding differences in the heat capacity the greater stability of the initial state solvation of activation for hydrolysis, lCp*, may also be shell in DzO than in H20. 7 -
O C .
[ CONTRIBUTIOS
FROM h f E L L O N I S S T I T U T E , PITTSBURGH, P E S N S T L V A N I A ]
C13 Nuclear Magnetic Resonance Spectroscopy.
I. Aromatic Hydrocarbons
BY PAULC . LAUTERBUR' RECEIVED JULY 19, 1960 The CL3 nuclcar magnetic rcsonance spectra of benzene, toluene, the three xylenes, mesitylene, durene and hexanicthq 1benzene, and of biphenyl, naphthalene, phenanthrene, pl-rene, acenaphthylene and fluoranthene have been obtained and partially analyzed. Tentative assignments for the spectra of azulenr and 4,6,8-trirneth~lazulene are also presented. The ranges of Cr3chemical shifts in non-alternate hydrocarbons are much larger than those observed in alternates, suggesting that variations in 7-electron densities are primarily responsible for such shifts in aromatic rings.
Introduction The CI3 nuclear magnetic resonance (n.m.r.) spectra of several series of substituted benzenes and heterocyclic aromatics have been obtained and analyzed. The analyses of the complex spectra were usually accomplished by comparisons among the spectra of the methyl derivatives, a procedure made possible by the relatively small effects of methyl substitution on CI3 shifts and carbon-hydrogen spin-spin couplings in aromatic ring systems. The interpretation of the measured C13 shifts and carbon-hydrogen spin-spin couplings depends initially upon the establish(1) Multiple Fellowship on Silicones sustained by t h e Dow-Corning Corporation and Corning Glass Works.
ment of empirical correlations with more familiar chemical and physical parameters. This paper, the first of a series on the CI3 n.m.r. spectra of aromatic rings, therefore presents measurements and analyses of the spectra of benzene and seven methylbenzenes, and of biphenyl, three alternate and four non-alternate hydrocarbons. Deuteration of two of the latter, azulene and 4,6,8-trimethylazulene, in the 1,3 positions has made possible a tentative analysis of their spectra, from which it appears that the CI3 shifts may be determined largely by the 9-electron density distribution in such molecules. The spectrum of ferrocene has also been obtained. The CI3 resonance occurs a t much higher
K.M.R.SPECTRA OF AROMATICHYDROCARBONS
April 20, 1961
field than do those in other aromatic rings, and the spin-spin coupling is greater. The increase in the coupling may result from the increased s-character of the C-H bond expected for a carbon in a fivemembered ring, but its magnitude is rather less than might have been predicted.
Experimental N.m.r. Spectra.-All spectra were measured a t 8.50 hlc./s. in a magnetic field of about 7940 gauss, using a Varian 43000 High Resolution N.M.R. Spectrometer and a Varian 12-inch electromagnet with flux stabilizer. The dispersion mode was employed with radio-frequency fields ( H l )of three t o fifteen milligauss and sweep rates of about 0.4 gauss/min. The field inhomogeneity over the large samples was generally about 12 c./s. (11milligauss). Under these conditions, spectra were usually of the “rapid passage” type2-4 or intermediate between rapid and slow passage. Because of the asymmetry of such intermediate passage peaks, reliable measurements can only be made by measuring equivalent points in spectra taken with increasing and decreasing sweep fields and averaging the results. The inversion of rapid passage spectra with reversal of sweep direction and the often strong dependence of the appearance of a spectrum upon the sweep direction has made it desirable t o deviate somewhat from the usual conventions followed in the presentation of n.m.r. spectra. All C13 spectra in this and following papers will be presented in pairs; one with the peaks pointing up and high applied field t o the right and the other with peaks pointing down, as they are drawn by the recorder and high applied field t o the left. For both types, the direction of the sweep is from left to right, increasing in the first case and decreasing in the second. Another source of asymmetry and differences between spectra swept in the two directions is magnetization transfer: which causes the second doublet peak traversed t o be decreased in intensity by an amount dependent upon the rate of sweep and the relaxation times of both C13 and H1. Reversal of the sweep reverses the inequality, and the averaged heights are equal. Higher multiplets behave similarly. Meaningful intensity estimates thus require measurements on spectra swept in both directions. Peak widths are usually greater than the field inhomogeneity mentioned above. From the equations for pure rapid p a ~ s a g e ,the ~ . ~signal has the form
+ M [l
1839
The accuracy of the shift and coupling measurements is also difficult to assess. The variable asymmetries, signalt o noise ratios and overlaps all affect the measurements. Errors are likely t o be especially serious for peaks which are just barely resolved. Experiments a t other frequencies, a t which the pattern of interferences in spectra containing spin-spin multiplets will be completely different, will eventually provide more accurate data. The methyl quadruplets of the methylbenzenes provide a suitable set of resonances, free from the complications Of incomplete resolution, from which to estimate the precision of the measurements. As the multiplets are quite weak in several of the spectra, the precision estimate obtained will be conservative. The average differencebetween the methyl C13 shifts measured in two separately calibrated spectra of each of seven methylbenzenes was 0.4 p.p.m. For only two of the group, one of them the weak hexamethylbenzene spectrum, was the difference between two runs greater than 0.4 p.p.m. The average difference between the ClYH13spin couplings, measured in the same pairs of spectra, was 2 c./s. If it is assumed that the couplings are the Same in all of the methylbenzenes, the probable error or a single measurement is =t2 c./s. More information on the precision of measurement is given below in connection with the referencing of the spectra. The methylbenzene spectra and those of the polynuclear hydrocarbons were measured a t different times, and two different techniques were used. For the former compounds, a 3 mm. 0.d. glass tube was centered in the 15 mm. 0.d. sample cell. The reference tube contained a saturated aqueous solution of K2C03, enriched t o about 58% C 1 3 . The field inhomogeneity produced by the central tube8did not seriously affect the spectra under the conditions used. The shift of the K2C1303 reference relative t o the standard used in earlier measurements,gJo the carboxyl carbon in glacial acetic acid, was measured by the substitution techniquelo as 7.6 p.p.m. A new standard will be used in this and subsequent papers, however. The carboxyl C13 shift in acetic acid is concentration dependent, and the presence of small amounts of water can cause a measurable shift. The presence of the methyl resonance would also be undesirable if the acid was t o be used as an internal reference. All measurements reported previouslyOJO have been ultimately referred to the same sample of acetic acid, sealed under vacuum, and should be internally consistent. A much better choice for general use seems t o be carbon disulfide. It has a single strong sharp resonance a t even lower field than acetic acid and appears t o be almost immune t o solvent shifts, thus fulfilling the most important requirement for an internal reCS2 by volume in the ference. For example, solutions of solvents nitrobenzene, acetone, aniline, cyclohexane and bromoform gave shifts, after correction for bulk susceptibility differences,11of 0.5, 0, 0.2, 0.1 and -0.4 p.p.m., respectively, relative t o pure (2%. Each shift was based upon two measurements (eachan average between the twosweep directions) which hadamaximum difference of 0.4 p.p.m. The average difference was less than 0.2 p.p.m. Various other experiments have also failed t o show any solvent effects definitely larger than experimental error.I2 Carbon disulfide has, therefore, been adopted as the new primary reference standard for C13 spectra in this Laboratory. The symbol &, defined as
+
+ ($3]’/2
where M is nearly constant during resonance and A? is the distance, in c./s., from the center of the resonance. The sign depends, as mentioned above, upon the direction of approach t o resonance, and the width of the peak is proportional t o H I . I n practice, HI is adjusted empirically t o obtain the best compromise between signal strength and resolution in a given spectrum. Another source of broadening is unresolved spin-spin couplings with distant hydrogens. These have been resolved in a few C13 spectra6 and have been measured in proton spectra? They can be propagated through a t least three bonds and can be as large as 45 c./s. The peak widths in the absorption mode CI3 spectrum of benzenes are less than 20 c./s., requiring ortho- and meta-hydrogen couplings of less than 10 c./s. \-ariation of the distant couplings with structure will result in changes in the widths and heights of peaks and will affect intensity estimates, since the latter are based almost solely on peak heights. Because ot this and the low and variable signal-to-noise ratios, it is impossible t o give any good estimate of the reliability of the relative intensities reported, even when peaks are well separated. 12) F. Bloch, Phys. Ree., 70, 460 (1946). (3) E. R. Andrew, “Nuclear Magnetic Resonance,” Cambridge University Press, London, 1955,pp. 130-132, 222-223. (4) J. G. Powles, Proc. P h y s . Soc., 71,497 (1958). ( 5 ) Magnetization transfer by chemical reaction has been discussed by H. M. McConneII and D. D. Thompson, J . Chcm. Phys., 2 6 , 958 (19571,31, 85 (1959), and by A. Patterson, Jr., and R . Ettinger, Z. Elecfvochem., 64, 98 (1960). Transfer by relaxation of spin-coupled nuclei appears t o be quite similar, although a quantitative treatment of the rapid passage case has not been carried out. (6) P. C. Lauterbur, unpublished work. (7) G.J. Karabatsos, J . Org. Chem., 16,315 (1900).
+
+
+
will be used t o distinguish shifts on the C& scale from those referred to other standards. Shifts on the acetic acid scale will be called 6s and are related to the new scale by 6, = 6, $- 15.6 p.p.m. for external referencing. For the time being, it will be assumed that internal and external referencing with CS2 are equivalent, since the experimental uncertainties are (8) J. R. Zimmerman and M. R . Foster, J . P h y s . Chem., 61, 282 (1957). (9) P.C. Lauterhur, J . Chcm. Phys., 26, 217 (1957). (10) P. C. Lauterbur, Ann. N. Y.Acad. Sci., 70, (4),841 (1958). (11) J. A. Pople, W. G. Scbneider and H. J. Bernstein, “Highresolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., New York, N.Y.,1959,pp. 80-81 and Appendix C. (12) COi has been found to he similarly inert in a wide variety of environments; R. Ettinger and P.C. Lauterbur, to he puhlished.
1s40
PAULC. LAUTERBUR
Vol. s3
7
I cs*
I' CSZ
'
Figs. 1-16.--8.50 Mc./s. CI3 n.m.r. usually larger than corrections for bulk susceptibility differences or solvent effects. The tube of aqueous K&I3O3 described above is used as a secondary reference with 6 ( K2Cl3O3) = 23.2 p.p.in. Peak positions reported in frequency units (c./s.) will be symbolized byve. Calibration was accomplished, usually both before and after each pair of sample spectra, by recording the spectrum o f CSZ modulated at 720 c./s.I3 The oscillator frequency ( 1 3 ) J . T , Arniild and hl. E. Packard, J . Chem. P h y s . , 19, 1608 (1931).
Cli
I
x* 6
'
I
spectra of aromatic hydrocarbons. was monitored by an electronic frequency counter. During the work on the methylbenzenes, the acetic acid spectrum was used as a secondary calibration standard, with a shift difference between the methyl and carboxyl carbous of 157.7 p.p.m. The spectra of the deuterated azulenes were weak because only small quantities were available, and no one run gave a satisfactory picture of relative intensities. Tracings made from four or five superimposed spectra gave satisfactorily rcproduciblc shifts a n t 1 rclativc intciisities, ho~.c.vci..
April 20, 1961
N.M.R.SPECTRA OF AROMATIC HYDROCARBONS
Samples.-All samples were in 15 mm. 0.d. glass tubes filled t o depths of several cm. and contained C13 only in its natural abundance of 1.17,. The methylbenzenes and their solutions were deoxygenated by bubbling with N2 and were kept in tubes capped with standard taper ground glass joints. The (K2Cl3o3)reference tube was centered by a dimple in the flat bottom of the sample tube and a nipple on the cap. The other samples were sealed under vacuum in glass tubes, except for the deuterated azulenes, which were handled in the same way as the methylbenzenes, with the omission of the reference tube. Preparation of Deuterated Azulenes.-CF3COOD was prepared by mixing the theoretical amount of D20 with (CF3CO)z kept in a solid COz-trichloroethylene bath. Azulene or 4,6,8-trimethylazulene were added, a t room temperature, in the approximate ratio of one mole of hydrocarbon t o ten of acid. They dissolved a t once t o give yellow solutions whose proton spectra were immediately checked to confirm that exchange had taken ~ 1 a c e . l ~ The solutions were neutralized with saturated K 2 C 0 3in D20 and with anhydrous KzCO3 and the precipitated deuterated azulenes were filtered off, washed, dried and dissolved in CS2. Proton spectral4 showed that a t least iOY0 of the 1,3 protons had been replaced and that no exchange had taken place a t other positions. Sources of Samples.-Biphenyl, naphthalene, phenanthrene, benzene and the methylbenzenes were pure grades from standard commercial sources, except for durene, which was a gift from the Shell Development Company. Pyrene, acenaphthylene, fluoranthene and azulene were loaned by Drs. R . A . Friedel and G. Wood of the U . S. Bureau of Mines. 4,6&Trimethylazulene and an additional sample of azulene were purchased from the Columbia Organic Chemicals Company, Inc. Ferrocene was obtained from E. I. du Pont de Nemours & Company, Inc. A sample of o-xylene partially deuterated in the 4-position was furnished by Dr. G. H. Barnes of Mellon Institute.
Results and Discussion Benzene and Methy1benzene.s.-Figures 1 through 8 are the spectra of benzene and seven methylbenzenes. The upper spectrum of each pair was produced by an increasing sweep field and the lower by a decreasing sweep. The peak positions and measured heights are given in Table I, along with the assignments and the corresponding calculated peak heights, normalized to the total number of carbon atoms in each molecule. The frequency is defined to have the same sign as 6,) so that resonances a t higher field have larger vC values. Most of the assignments are obvious. The spectra consist of CH doublets, singlets for ring carbons carrying methyl groups and methyl quadruplets. The doublet splittings are about 150 c./s. and the single peak of the methyl-substituted carbons is usually almost coincident with the low-field components of the doublets a t an operating frequency of 8.50 Mc./s. I n the assignments, multiplet components are indicated by numbers in parentheses, counting from the lowest peak. The only arnbiguity in the assignments for resolved peaks occurs in the o-xylene spectrum. Peaks 2 and 3 are obviously the high-field components of two CH doublets. The assignment shown is based upon (14) Proton resonance spectra of azulene and of the azulene cation in C F C O O H have been reported by S. S. Danyluk and W. G. Schneider, J . A m . Chem. Soc., 84, 987 (1960). The proton spectrum of 4,6,8trimethylazulene has been reported b y E. G. Hoffmann, Z . Anal. Chem., 170, 177 (1959), A n n . , 644, 47 (1959). A detailed description of the proton spectra of the deuterated hydrocarbons and cations will be presented elsewhere. It should be noted now, however, t h a t an irreversible reaction takes place if azulene is left in trifluoroacetic acid solution for several hours a t room temperature. T h e blue-violet product dissolves in t h e acid without change in color and is much less soluble in organic solvents t h a n is azulene.
1841
the shift to high field characteristic of carbons para to a methyl group and upon the spectrum of Oxylene partially deuterated in the 4-position. THE 8.50
TABLE I SPECTRA O F BENZENEA S D
MC./S. cI3 N.M.R.
SOME METHYLBENZESES Compound
Benzene Toluene
Peak
1 2 1 2 3 4 5 6 1 2
YC, c./s.
473 632 475 631 1399 1522
--Height-Ohsd. Calcd.
2.97 3.03 3.38 2.43 0 0.48 0.38 0 4.21" 1.00 1.00 0.14 .64 .64 .25 3.98 1.80 0.21 .95 ,85 .21 2.12 2.00 1.84 0.20 .76 .72 .40 4.32 1.58 0.32 1.21 1.16 0.42
481 618 648 3 4 1295 5 1411 1539 6 1667 7 479 1 m-Xylene 2 640 1275 3 4 1404 3 1531 1652 6 1 473 +Xylene 2 493 623 3 4 1273 1400 5 1525 6 1649 7 479 1 Mesitylene 2 639 1273 3 4 1399 1527 5 1655 6 1 Durene' 0 1 455 1 . 0 7 2 507 3.35 597 1 . 1 6 3 4 1285 0.66 1412 1 . 5 9 5 1539 1 . 6 7 6 1658 0.50 7 -1 Hexamethyl- 0 benzeneb 1 513 5.43 2 1304 0.59 1424 2 . 6 3 3 1549 2 . 3 0 4 1679 0 . 8 5 5 a The actual measured height account the obvious broadening saturated solution in CSe.
o .Xylene
Assignments
3.00 3.00 3.50 2.50 0.12 .38 .38 .12 4.00 1.00 1.00 0.25 . 75 .75 .25 4.00 2.00 0.25
--
. ir? .75 .25 2.00 2.00 2.00 0.25 .75 .75 .25 4.50 1.50 0.38 1.12 1.12 0.38
..
1 .oo 4.00 1.00 0.50 1.50 1.50 0.50
..
was doubled to take into of the peak. * Run as a
Table I1 summarizes the parameters obtained by analysis of the data in Table I. The spinspin coupling constants are in good agreement with those measured in the proton spectra,I5 except (15) N. Muller and D. E. Pritchard, J . Chcm. Phys., 31, 768, 1471 (1959).
PAULC. LAUTERBUR
1842
where overlapping peaks obscure one component, as in mesitylene. The CH shifts differ little from that in benzene, except for small shifts to low field caused by o-methyls and to high field caused by a $-methyl. The data in Table I1 are neither complete nor reliable enough to make a quantitative discussion of the niethyl effects profitable, but the effects have been observed in the betterresolved spectra of methyl derivatives of a number of substituted benzenes and will be analyzed in detail in other papers of this series. TABLE I1 C13 CHEMICAL SHIFTSAXD SPIN-SPIN COUPLINGS IN BENZENE AND
METHYLBENZENES
----CH-Compound
6c,
p.p.m.
J. c./s.
C*-CHI dc, p.p.m.
-CH6c, p.p.m.
J
c./s.
Benzene 65.0 159" ... ... ... To1u en e 65.Zb 15gb 56.OC 171.8 12Zd o-Xylene (CH - 3 , 6 ) 63.6e (154)' 56.6c 173.6 128 (CH - 4, 5) 67.2' (154)' m-Xylene 65.80 1600 56.qC 172.6 127 p-Xylene 64.5 150 58.0 172.1 124 Mesitylene 66.1h (154)h 56.4" 172.2 12@ Durene 62.0 142 59.6 173.6 127 Hexamethylbenzene . , 60.4 174.9 124 Muller and Pritchard16 report 159 c./s. from the H1 spectrum. 'Average for all C H ; Muller and Pritchardl6 report an average coupling of 155 c./s. =Taken as the Muller and Pritchardls report 126 center of peak 1. c./s. e The measured separation of peaks 1 and 2 is 136 c./s. The 6, given was calculated from the position of peak 2 by assuming a coupling of 154 c./s., about the same as those in mesitylene and toluene. f The measured separation of peaks l and 3 is 166 c./s.; A coupling of 154 was assumed in the calculation of &. Average for all CH. h T h e measured separation of peaks 1 and 2 is 160 c./s., The coupling of 154 c./s. from the H1 spectrum was used in calculating &.
..
Some of the methyl C13 shifts were measured in an earlier investigation.10 After conversion of the data to the 6, scale and a slight adjustment of the calibration factor, l6 the two independent sets of data are in agreement within the stated experimental uncertainties, except for durene, for which the difference is 1.1 p . p m The methyl spin-spin couplings are all the same within experimental error, with an average value of 126 c./s., the same as those found by Muller and Pritchardlj for toluene and mesitylene. Biphenyl and Alternate Polynuclear Hydrocarbons.-The spectra of biphenyl, naphthalene, phenanthrene and pyrene are given in Figs. 9-12 and the peak positions, heights and assignments are listed in Table 111. The numbering for the latter three is that of the Ring Index and is shown.
The spectrum of biphenyl is very similar t o that of toluene, with no resolvable shifts between the (16) In reference 10 a value of 158.0 p.p.m. was used for the secondary calibration by the acetic acid spectrum, compared to the 157.7 p.p.m. used in this work.
Vol. 83
0 - , m- and p-carbons. The assignment of the substituted carbon resonance to a position nearly coincident with that of the low-field component of the CH doublet is based upon relative intensities.
TABLE 111 THE 8.50 MC./S. Cl3 N.M.R. SPECTRA OF BIPHENYL THREE ALTERNATEHYDROCARBONS CompoundG
YC,
Peak
c./s.
-Height,Obsd. Calcd.
AND
Assignments
+ +
468 6.24 7.00 C-1, 1' CH(1) 630 5.76 5.00 CH(2) Naphtha481 5.16 6.00 C-4a,8a CH(1) lene 638 4 . 8 4 4.00 CH(2) Phenan479 5.10 5.00 CH(1) threne 512 4 . 0 0 4.00 C-4a, 4b, 8a, 10a 637 4.90 5.00 CH(2) Pyrene 484 6.40 7.00 CH(1) +C-lob, 1Oc 515 3.84 4.00 C-3a, 5a, 8a, 10a 3 638 5.76 5.00 CH(2) Run as saturated solutions in C&. Biphenyl
1 2 1 2 1 2 3 1 2
The naphthalene spectrum was analyzed in the same way. The shift between the cy- and pcarbons must be quite small. In the phenanthrene spectrum, the peaks of the junction carbons have moved slightly toward high applied field, but there is still no resolvable difference between the doublets, except for a possible shoulder on the high field side. The spectrum of pyrene is slightly different, in that the relative peak heights suggest that two of the junction carbons give a peak a t lower field than the other four, combined with the low-field peaks of the doublets. A small highfield shoulder may also be present in this spectrum. The chemical shifts and spin-spin couplings are listed in Table IV. There appears to be some TABLE IV CIS CHEMICALSHIFTSA N D SPIN-SPIN COUPLINGS IN BIPHENYL AND THREE ALTERNATEHYDROCARBOSS
c-c
-CHp.p.rn.
J, c./s.
64.6 65.9 65.6 66.0
162 158 158 154
60,
Compound
60,
11. p. rn.
55.0" 56.6" 60.2 56. 9"vb 60.6' Taken as the center of peak 1. Based upon the tentative assignment in Table 111, the smaller shift would be that of C-lob, 1Oc and the larger that of C-3a,5a,8a,lOa. The existence of the resonance a t lower field is not certain, however. Biphenyl Naphthalene Phenanthrene Pyrene
tendency for shifts to higher field than benzene in the spectra of the condensed polynuclear hydrocarbons, but the effect is small. The average spin couplings are all about the same as in benzene. Thus, whatever factors may be responsible for variations in the magnetic shieldings of carbon nuclei in aromatic rings do not vary much in the alternate hydrocarbons that have been studied. The other property that best fits this description is the 1-electron density, qr, a t each carbon atom.17 In fact, the most significant feature of the classi(17)
c. A. Coulson and G. S. Rushbrooke, Proc. Combridgc P h t l . Soc ,
36, 193 (1940).
N.M.R.SPECTRA OF AROMATICHYDROCARBONS
April 20, 1961
fication into alternates and non-alternates is t h a t in the former all qr are almost exactly unity, but that large migrations of charge may occur in nonalternate molecules. Non-alternate Hydrocarbons.-The spectra of several non-alternate hydrocarbons have been obtained to test the hypothesis that the carbon shieldings are largely determined by the ?r-electron densities. Spectra of acenaphthylene and fluoranthrene are shown in Figs. 13 and 14, and the peak positions and heights are given in Table V. The numbering of the rings is shown.
4.6.8-TRIMETHYLAZULENE
1
TABLE V THE 8.50 Mc./s. C13 SPECTRA OF ACENAPHTHYLENE AND FLUORANTHRENE YO,
Compounda Peak c./s. Acenaphthylene 1 454 2 466
-Height---
Ohsd.
Calcd,
4 5
518 544
0.93 0.74
7
627 650
21 '. 4 92 } 3 . 9 1
400
500
600
4
700 c/s
vc
Assignments
2.42 2.60
1843
of deuteration on the 8.50 Mc./s. C13n.m.r. spectra of azulene and 4,6,8-trimethylazulene.
Fig.17.-Effects
(1). Peaks 5 and 6, nearly equal in height, must then be assigned as CH-1,3(2) and CH-5,7(2), although it cannot be determined a t this point Fluoranthene 1 470 6.56 CHk(1) f C1 which is which. Fig. 17 compares the average 2 524 3 . ~ ) " '''0° ~ ~ CH,(I) + Cn spectra of azulene, azulene-1,3-dz, 4,6,8-trimethyl3 628 4.00 6.00 CHk(2) azulene and 4,6,8-trimethylazulene-l,3-d~. It is 4 684 2.22}6'22 CHmW immediately apparent that peak 6 in each specRun as saturated solutions in C S . trum must be assigned as CH-1,3(2) and that peak There is a little more structure in the acenaph- 3 in the trimethylazulene spectrum is CH-1,3(1). thylene spectra than in those of the alternates, Peak 5 of azulene should therefore be assigned as but a complete assignment is not possible. The CH-5,7(2), as should peak 5 of trimethylazulene. spectrum is centered at about the same point as is The latter, if the assignment is correct, has been that of benzene. The fluoranthrene spectra show shifted considerably t o low field by the flanking considerably more structure, with a small doublet methyl groups. By elimination, peak 4 of trishifted to high field. Without more data i t is methylazulene must be CH-2(2), an assignment impossible t o say which set of carbons gives rise supported by its relatively small height, even to which peaks, but a tentative analysis into uni- though i t appears as a shoulder on peak 3. The dentified groups has been given in Table V on the assignments in the lower portion of the azulene spectrum present some difficulties. It seems unbases of couplings and relative peak heights. Azu1enes.-The best evidence for a relationship likely that peak 2 could be the low-field compobetween n-electron densities and shifts is found in nent of a doublet because the second component the spectra of azulene and its derivatives. The would be expected a t about v, = 600 c./s. Only if structure and numbering of azulene are shown. J were less than about 120 c./s. could the peak be concealed in peak 3. Consequently, peak 2 has been assigned as C-9,lO. This leaves peak 1 to be attributed to CH-4,8(1) and CH-6(1). CH-2(1) probably falls between peaks 1 and 2 , probably 9 ' 1 s closer to 1, in which it has been included in Table Its spectrum (Fig. 15 and Table VI) is complex and VI. The peak heights would be in better agreecannot be immediately interpreted. There appear ment with those observed if peak 1 were CH-6(1) C-9,lO and peak 2 were CH-4,S(1), to be a t least three and possibly four sets of doub- 4- CH-2(1) lets, with a number of peaks overlapping in the but this seems less likely on the other grounds cited center. A partial interpretation was made pos- above. The shifts and couplings in all four compounds, sible by a comparison with the spectra of 4,6,8trimethylazulene, shown in Fig. 16. If i t is as- assuming the assignments to be those of Table VI, sumed that the direct effect of methyl substitution are given in Table VII. is about the same as in the benzene series, the inBy making use of the best resolved peaks in the tensification of the low field part oi the spectrum four sets of spectra, along with the assumptions implies that one or both of peaks 1and 2 of azulene t h a t J(CH-1,3), J(CH-2) and 6, (CH-2) are not contain contributions from CH-4,8(1) and CH-6- changed by the methyl substitution, a set of best 4.00}CH(21
*a6 -
+
1844
Yol. 83
PAULC. LAUTERBUR
TABLE VI THE8 50 Mc./s. C13 K.M.R.SPECTRA OF AZULEVE AND
OF
TABLE VI1 C’3 CHEMICAL SHIFTSAND SPIS-SPIS COUPLINGS IS
METHYLA N D DEUTERIOAZULENES Compounds Azulene
YC,
Peak c./s. 1 396
2 3
4,0,8-Trimethyl. azulene
4,6,8-Trimethylazulene-l,3did
-Height,-obsd. calcd. 1.81 2.00
443
4
517 547
1.45 1.37 3.13
.5 6 1
673 709 407
1 .OO 1.24 3.28
1. O O 1 00 3.:0
476 560 581 624 723 1210 1232 1328 10 1358 11 1462 12 1480 13 1.576 1 4 1608
2.38 1.07 0.82 .90 .98 .12 .33 .41 .82 .41 .82 .I2
3 00 1.00 0.50 1.00 1.00 0.12 ,25 .38 .76 .38 .75 .12
2 3 4 5 6 7 8 9
2.00 1.00 3.00
.83
.25
1
396
1.79
2.00
2 3 4
439 512 550
1.34 1.57 2.10
2.00 1.00 2.00
5 6
E64 691
1.00 (0)
1
405
1.09 .521 3.34
2 3
477
3.00
4
589 627 660 721
2.55 r0.381 0.98 1.13 10.631 [0.30j
5 6 7
558
I
3.50
(0) 0.50 1.00
(0)
a
l
~
t
i
~
~
~
~
~
a J,
6c >
Assignmentsb CH-2(1) CH-4,8(1) CH-6(1) C-9,lO CH-5,7(1) CH-2(2) CH-4,8(2) f CH-6(2) CH-1,3(1) CH-5,7(2) CH-1,3(2)
+
+
+
+
+
+
CH-2(1) C-4,8 C6 CH-5,7(1) C-9.10 CH-1,3(1) CH-2(2) CH-5,7(2) CH-l,3(2) 6-CH3(1) 4,8-CHs(l) G-CHa(2) 4,8-CHa(2) B-CHa(3) 4,8-CH3(3) 6-CH3(4) 4,8-CHa(4) CH-2(1) CH-4,8(1) CH-F(1) C-9,IO CH-5,7(1) CH-2(2) CH-4,8(2) f CH-6(2) [CH-1,3(1)1 CH-5.7(2) [CH-1,3(2)1 CH-2(1) C-4,8 f C6 CH-5,7(1) C-9,lO [CH-l,3(1)1 CH-2(2) CH-5,7(2) ICD-1.31 [CH-l,3i2)]
+
+
+
+
+
+
+
Compound
Group
lzulene
p p.m.
c./s.
CH-1,3 CH-2 CH-4,8 CH-5,7 CH-6
73.9 162 55.4 152 35.4 152 70.0 156 Ti.5.4 152 52.1 , C-9,10 73.0 141 CH-1,3* .lzulene-1 ,3-d2 55.6 lt% CH-2 55.6 151 CH-4,8 69.2 152 CH-5,7 CH-6 55.6 154 C-9,lO
