2430
GEORGE B. SAVITSKY AND KEISHINAMIIIAWA
Vol. 67
THE ADDITIVITY OF CARBON-13 CHEMICAL SHIFTS IN THE CHBX, CZH6X, i-CSH,X, t-C,H,X SERIES BY GEORGE B. SAVITSKY AXD KEISHISAMIKAWA Department of Chemistry, University of California, Davis,California Received M a y lY, 196s Previous results on C13chemical shifts on a- and p-carbons in the series (CH,),CH,-,X for various X indicated that the effects of replacement of the hydrogen atoms on the a-carbon by methyl groups were nearly additive. The C13chemical shifts toward lower field per methyl substitution depend very little on the nature of S in case of p-carbon but vary considerably in case of a-carbon. These literature results supplemented by further experimental measurements for X = COOH, CsHj are discussed in terms of possible correlation between the absolute magnitude of the relative shifts on a-carbon and C-X bond lengths.
Introduction part of his extensive studies on the 1i.m.r. CI3 chemical shifts, Lauterbur' investigated the series (CHJ.CH,-.X (X = C1, Br, I, OH, NOJ. His plots of chemical shifts on the a- and @-carbonsfor each X vs. n showed a fairly linear relationship, indicating that the effects of replacement of the hydrogen atoms on the a-carbon by methyl groups were nearly additive, with few exceptions. More recently, Spiesecke and Schneider2 determined chemical shifts in various CH3X and CzHsX compounds, using spherical sample and reference containers, and thus obviating bulk susceptibility effects. If these more recent results are used in the RBr and RC1 series, more nearly linear plots and hence better additivity relationships result for these series. Whereas @-carbon plots consist of nearly parallel straight lines, the most striking feature of the a-carbon plots is the great variation in the slopes of best-fitting straight lines with the nature of X. The relative chemical shifts in both plots are toward decreasing field with increasing methyl substitution and follow the normally accepted order of positive inductive effects of the alkyl groups: t-C4H9> i-C3H7 > CzHs > CHs. This order appears to be in line with the qualitative picture of gradual electron density withdrawal from the alkyl groups toward the electronegative X and hence to progressively lower shielding on the alkyl carbons. In compounds where possible contributions of nonbonded resonance structures (hyperconjugation) may take place, the over-all differences in the electron releasing effects of the alkyl groups for each methyl substitution could become somewhat moderated by hyperconjugation, the order of hyperconjugation effects in the alkyl series in question usually being assumed to be opposite to that of the inductive effects, although both mechanisms of electron density release lie in the same direction. Since among the RX compounds which have been investigated only in the RSO, series may hypercoiijugation be operative, we have determined the Cl3 chemical shifts for two other series in which hyperconjugation is also supposed to take place, namely RCOOH andjRC6H5. This was done with the view of comparing the a-carbon plots of two groups of compounds with and without possible hyperconjugation effects in the light of any correlation betwen a-carbon slopes aiid the nature of X. Before attempting to make any correlation of the P.C. Lauterbur, A n n . A' Y . Acad. Sci., 70, 841 (1958). (2) €1. Spieseoke and W. G. Sclineider, J. Chem. Phgs., 36, 723 (1901). (1)
a-carbon slopes with X we also decided to check the chemical shifts for a few points in Lauterbur's plots by using essentially the same sample cell arrangement as in the work of Spiesecke and Schneiderj2 in view of some discrepancies between their results and those of Lauterbur.' Experimental Our measurements were made with respect to aqueous 2-CI3sodium acetate as standard reference, but all chemical shifts subsequently were referred to benzene. 811 experimental conditions and instrumental procedures were identical with those described by Spiesecke and Schneider,2 except that no provision for spinning of the sample was made. All determinations were made on neat liquids, which were commercially available reagents and which were used without purification. Only pivalic acid, which is a solid, was measured by making a concentrated solution of it in CCla.
Results and Discussion Our measurements are summarized in Table I. These results together with results previously obtained' are plotted in Fig. 1 (@-carbon chemical TABLE I a- AND p-Cl3 CHEMICAL SHIFTSOF (CH,)&H,IN
nX COMPOUXDS
P . P x REFERRED TO BEXZENE
Compound
a-C
CH3COOH CzHjCOOH i-CJIiCOOH t-GH QCOOH CHaC& C&C& i-CaH7CciH~ L-CrHgCaHb CHaOH C,H,OH i-CaHiOH I-CaHoOH z-C~H~C~ t-CdHgCl t-C4HgBr
109.2 102.9 95.1 90.1 108.8 101.1 95.6 94.2 80.7 71.8 66.1 61.3 76.8 63.3 6i.6
P-C
121.7 111.1 102.4
114.6 105.9 98.1 111.9 104.7 99.2 102.6 95.2 92.4
shifts) and Fig. 2 (a-carbon chemical shifts). It is seen from Fig. 2 that in the case of RBr and RC1 series, the experimental points determined in the cell with concentric spheres (ref, 2 and present work) result in better linearity. I n general, points determined in cylindrical arrangement by Lauterbur' tend t o be displaced somewhat toward lower field except in the case of CH3Br. The curvature in the ROH series obtained by Lauterbur, however, has been substantiated by our results except that our shifts are about 2 p.p.m. to higher field. This may be a t least partially due to cumulative experimental errors resulting from referring shifts to different standards.
ADDITIVITY OF
Nov., 1963
243 1
CARBON-13 CHEMICAL S H I F T S
It can be seen from Fig. 1 that the additivity of chemical shifts on p-carbon for each methyl substituent is very good. All plots are nearly parallel and the series R?\'Oz, RCOOI-I, RCsH6 do not show any exceptional behavior. Actually, oiily the ROH series seems to be somewhat out of line indicating deviation from additivity. The plots shown in Fig. 2, on the other hand, present several distinctive features. The halogen series show the best linearity if one assumes that the two points for C€IaCl and CH3Br determined in spherical cell are more nearly correct. I n the series in which hyperconjugation may be operative (X = KOZ,COOH, C&6) there exists deviation from additivity in shifts from isopropyl to t-butyl group. This deviation is the most pronounced in the case of alkylbenzenes. Thus, the difference between the CU-C'~ chemical shifts of isopropylbenzene and k-butylbenzene is only 1.4 p.p.m., whereas the difference between methylbenzene and ethylbenzene is 7.7 p.p.m. One could argue that, if the normal linear trend in a - 0 3 shifts is related to electronreleasing ability of the alkyl groups, this deviation from linearity may indeed be due to hyperconjugation effects superimposed on the inductive effects. However, among the compounds studied, the alcohol series presents the greatest deviation from linearity in both plots. On the other hand, it, is conceivable that in case of alcohols the difference in degree of intermolecular association via hydrogen bonding might account for the observedldeviatioii from linearity in both a- and p-carbon plots. At the same time, the possibility of increased strain on the t-butyl group by bulky substituents such as phenyl group cannot be disregarded entirely.
120
110
4a
"100 d
90
c
-
a S p i e s e c k e and Schneider b- Lauterbur
-
3
2
1
n.
Fig. l.--p-CIa chemical shifts in the series ( CH3)nCHb .X.
I
a -Spiesecke and Schneider
120
fi100 a .-'
80
TABLE I1
x
dx Reference 1.38 U 3.9 1.78 a,b 13.5 1.94 b RBr 17.2 2.12 b RI 20.6 1.43 a ROH 6.5 1.47 U RNO, 7.8 1 50 U RCOOEF 6.3 1.50 U RCe& 6.6 a David R. Lide, Jr., Tetrahedron, 17, 125 (1962). J. W. Simmons, et ul., Phys. Rev., (2) 74, 1246 (1948). Series
ItP RCl
As to the possible correlation between the various slopes in Fig. 2 and the nature of X, it becomes immediately apparent that the slopes tend to be greater the bulkier the atom of the X group which is immediately attached to a-carbon. If for each series R X we define the quantity AX which is the average chemical shift of cu-C13 to lower field per methyl substitution, A , will represent the slope of the straight line of best fit for halogen series and an average of three differences in shifts (GMe - EiEt), (GEt -. Gi-Pr), (si-Pr - Gt-Bu) for the remaining plots which cannot be adequately represented by a straight line. Table I1 lists the AX thus obtained and the lengths of the bonds dx between the a-carbon and 1,he atom of the X group immediately attached to it. The big has been determined only from two points available in the literature2 which are also plotted in Fig. 2. Figure 3 shows the plot of AX vs. d X which reveals an iiicrease of A x with increasing bond leiigth dx. This treiid suggests that the a-carboii-13 cheinical
GO
3
2
0
1 n.
Fig.
Z.--cr-C13
chemical shifts in the series (CH,),,CH,- "&X.
1.4
dx in
Fig. 3.-The
I 1.8
1.6
A.
I
2.0
plot of A, us. d,.
shifts in these systems are related to the relative polarizability or deformability of the electron density in the C-X bond by induction. .As one goes froin R = CH3 to
2432
WAYNE
E. BELLA S D M. T A G A M I
R=C2Hs in each series, the extent of the shift of electron density away from the or-carbon or the actual enhancement of charge transfer toward the electronegative X which is due to the electropositive inductive effect of the additional methyl group seems t o be more important in a longer than in a shorter bond. Thus, for the RI series, the relative change in electron shielding on the a-carbon per substitution of hydrogen by a methyl group is of greater magnitude than it is in the RF series, the relative changes in all other groups being intermediate. That the relative spatial displacement of the center of gravity of the negative end of the dipole moment or additional increment in charge transfer toward X in R X series with the increase in the inductive effect of R is more significant for longer C-X bonds, in going from CHaX to C2HSX, for example, seems to be borne out by the dipole moment measurements of the methyl and ethyl halides listed in Table 111. TABLE I11 DIPOLEMOMENTS OF METHYL AND ETHYL HALIDES X
PCHsX
PCZESX
PCHaX
- PCaPHsX
Reference
F
1.81 1.92 0.11 a c1 1.83 2.00 .17 b Br 1.75 1.99 .24 b I 1.60 1.93 .33 b C. P. Smyth and K. B. McAlpine, J. Chem. Phys., 2, 499 (1934). P. C. Mahanti, Phil. Mag., (7) 20, 274 (1935).
We see that the increments in dipole moments in going from CH3Xto C2HsXfollow the order of increas-
VOI.
ti7
ing C-X bond lengths, which in this case is opposite to the order of electronegativities. It is difficult to explain the relative constancy of the slopes in &C13 shifts. The small differences in these slopes, however, do exist and may still be significant. Thus in the halogen series (X = GI, Br, I) the slopes still follow the order I > Br > C1, but then the slope for X = COOH is even greater than that for X = I. It seems, therefore, that the induced polarity of the C-X bond may have only a second-order effect on these slopes and that some other factors predominantly account for the rather significant chemical shifts of /&carbon resonance toward lower field per methyl substitution. We do not feel prepared to advance any plausible explanation for these effects. Also, the evidence presented for hyperconjugation effects cannot be regarded as conclusive. On the other hand, if our tentative explanation of the large variation in the slopes of a-carbon plots is correct, the bond lengths between the carbon in question and substituent groups should be considered together with other factors such as electronegativities, types of hybridization, and magnetic anisotropies of various groups in future theoretical appraisals of chemical shifts. Acknowledgments.-The authors wish to thank Professor C. P. Nash for his helpful suggestions and Professor W. E. Thiessen for proofreading the nianuscript.
HIGH-TEMPERATURE CHEMISTRY OF THE RUTHENIUM-OXYGEN SYSTEM' BY WAYNEE. BELLAKD M. TAGAMI General Atomic Division of General Dynamics Corporation, John Jag Hopkins Laboratory f o r Pure and Applied Science, S a n Diego, California Received M a y 28, 1963 The ruthenium-oxygen system has been studied over the temperature range 800 to 1500" and over the oxygen pressure range 0.01 t o 1.0 atm. Results show that solid RuOz is the only stable condensed oxide under the conditions of the study. The dissociation pressure of the oxide reaches 1 atm. a t 1540'. The effect of oxygen pressure on vapor pressure indicates that the important vapor species are RuOa and RuOi. From the pressure data, the following heats of formation, AHozss,and standard entropies, X"Z~S,were obtained: -72.2 f 2.0 kcal./mole and 12.5 f 2.0 e.u. for RuOz(s), - 18.0 f 4.0 kcal./mole and 63.7 f 4.0 e.u. for RuOt(g), and -46.7 f 5.0 kcal./mole and 65.5 f 5.0 e.u. for Ru04(g).
Introduction As part of a systematic investigation of the high-temperature chemistry of transition element compounds, the ruthenium-oxygen syste 1 was studied. Condensed phases and vapor species were identified, dissociation and vapor pressures were measured, and therniodynamic data were calculated from the pressure data. Prior to the start of this work, literature on the behavior of the ruthenium-oxygen system a t high temperature was scant and to some extent discordant. Remy and Kohnza reported a few dissociation pressure data for RuOz(s). Alcock and Hooper2bstudied the volatility of ruthenium in oxygen as a function of temperature in the range 1200 to 1400' and as a function of oxygen pressure a t 1280'. They found the vapor pressure t o be propor-
tional t o p ~ and, ~ assuming ~ / ~the solid phase to be ruthenium metal, deduced the vapor species to be RuzO. Schafer, Gerhardt, and Tebben3studied theeffect of oxygen pressure on vapor pressure a t 800' and in the range 1465 t o 2090' and found evidence for the vapor species Ru04and Ru03. They observed the dissociation pressure of RuOz to be much less than that reported by Remy and Kohn. Recently, Schafer, et a1.,4,5have completed an exhaustive study of the ruthenium-oxygen system at high temperature. We mere pleased to receive preprints of publications covering their work and find good agreement between their results and the results of our study. Experimental Dissociation Pressure Studies.-Dissociation
pressures were
(1) This work w a s suppoited in part by the U. S . Atomic Energ> Coni-
mission under Contract AT(04-31-164. ( 2 ) (a) H. Remy and M. Kohn, Z. anorg. allgem. Chem., 137, 365 (1924); (b) C. B. Alcock and G. W. Hooper, Proc. Roy. Sue. (London), 6 2 5 4 , 551 (1960).
(3) H. Schafer, W. Gerhardt, and A. Tebben. Angew. Chem., 73,27 (1961). (4) H. Schafer, G. Schneidereit, and W. Gerhardt. 2. anorg. allgem. Ckem.. 319, 327 (1963). (5) H. Sohafer, A. Tebben, and W. Gerhardt, abid., 321,41 (1963).
