3887
SOLVENT EFFECTS ON 13C-H COUPLING PARAMETERS
purity content on the oxygen reduction at a silver electrode in an alkaline electrolyte was recently found5 which is due to similar causes. Work on a more definite identification of the exposed crystal planes is now in progress.
impurity additions with topographic changes described in the literature. With face-centered-cubic structures the f 111 and f 1001 facets are stabilized by treatment with oxygen, and the rounded edges between the facets become sharper as the oxygen pressure increase^.'^-^^ The changes in shape with pressures are reversible, as are the observed desorption characteristics with oxygen pretreatment. Similarly, the presence of impurities is knownI3 to affect the morphology of the surface. A marked effect of thermal pretreatment and im-
1
Acknowledgment. The authors are indebted to W. &I.Hickam and his group for carrying out the isotopic dilution experiments. (14) R. Y.Meelheim, et al., Actes Congr. Intern. Catalyse, 9e, Paris, 1960,2, 2005 (1961).
Solvent Effects on W-H Coupling Parameters and Chemical Shifts of Some Halomethanes
by V. S. Watts and J. H. Goldstein Department of Chemistry, Emory University, Atlanta, Georgia 30322
(Received June 17, 1966)
Medium effects on the chemical shift and 13C-H coupling of bromoform have been determined in a series of 30 solvents representing a variety of functional groups. Similar observations have been carried out for 13 substituted methanes as the neat liquids and as solutions in cyclohexane, carbon tetrachloride, and dimethylformamide. The observed behavior can in general be correlated with the structure of the solvents and solutes studied. The results are adequately explained in terms of specific molecular interactions, in particular, hydrogen bonding. The advantages of using 13C-H couplings as a criterion for molecular interactions are pointed out.
Introduction The effect of solvent media on nmr coupling parameters has been the subject of considerable recent interest. Variability with solvent and/or concentration has been established for the cases of geminal H-H, 1--5 directly bonded l3C---H69’ geminal P-H (PCH),* and vicinal H-F couplings through C--C and C=C bonds.g At the present time it is not entirely clear whether these changes are primarily produced by specific interactions6 or whether they arise from more general effects such as the reaction fields induced by solutes in the dielectric solvent medium.lOsll
In an effort to clarify this problem further we have carried out solvent-effect studies of two types: (1) the effect of an extended series of both saturated and (1) V. S. Watts, G. S. Reddy, and J. H. Goldstein. J . Mol. Spectry., 11, 325 (1963). (2) B.L.Shapiro, R. M. Kopchik, and S. J. Ebersole. ibid., 11, 326 (1963). (3) B. L. Shapiro, R. M. Kopchik, and S. J. Ebersole, J. Chem. Phys., 39, 3154 (1963). (4) P.Bates, S. Cawley, and S. 5. Dsnyluk, ibid., 40, 2415 (1964). (5) V. S. Watts and J. H. Goldstein, ibid., 42, 228 (1965). (6) D.F. Evans, J. Chem. Soc., 5575 (1963).
Volume 70. Number 18 December 1966
3888
unsaturated organic solvents on the W-H coupling of a single solute, CHBr3, and (2) the effect of certain selected saturated organic solvents on the I3C-H coupling in a number of halogen- and cyano-substituted methanes. A study similar to (1) above was carried out by Evans on CHCls but with a less extensive series of solvents.6 In the first phase of the investigation correlations were sought between observed changes in J(CH) and various properties of the solvent such as the dielectric constant, dipole moment, structural features, etc. For the 30 solvents employed there is no indication of a significant correlation with the dielectric constant or dipole moment, but the magnitudes of the effects observed do tend to group together according to the solvent type (alcohol, amine, etc.). These results suggest specific interactions as the predominant or, perhaps, sole factor involved. The second phase of this work was designed to reveal the influence of the substituents on the sensitivity of J(CH) to the medium. Previously, infrared studies have indicated that a single electronegative substituent does not render an CY proton appreciably hydrogen bonding.l2 Our results, however, show that even a single iodine is sufficient to produce noticeable protondonor activity.
V. S.WATTSAND J. H. GOLDSTEIN
Table I: Solvent Effects on Nmr Parameters of CHBra" Mole %
Experimental Section The solutes and solvents used were commercially available products. Where their nmr spectra indicated the presence of impurities these were removed by distillation using a spinning-band column. Each of the solutes was observed as the neat liquid and at 20-25 mole yo in cyclohexane (CH), carbon tetrachloride (CCl,) , and dimethylformamide (DMF). Bromoform was also observed at 20-25 mole yo in each of the solvents listed in Table I. Samples of the liquid solutes were prepared by weighing the solute and solvent into an nmr tube and adding a few drops of TMS to serve as the internal reference. Samples of the gaseous solutes were prepared by using a vacuum system to condense the solute into a weighed amount of solvent and TMS in an nmr tube and then reweighing. All spectra were taken on a Varian Associates Model A-60 spectrometer and were calibrated by the usual side-band technique using an audio oscillator continuously monitored by a frequency counter. Normal proton frequencies were obtained as the average of three forward and three reverse sweeps. For the 13C-H satellite spectra at least four forward and four reverse sweeps were used. The average deviation for each peak was approximately 0.04 cps. In some cases both the upfield and downfield 13C-H The Journal of Physical Chemistry
Solvent
CHBrs
Cyclohexane Carbon tetrachloride Bromoform Chloroform Methyl alcohol Ethyl alcohol Isopropyl alcohol &Butyl alcohol Diethyl ether Diisopropyl ether Isobutyraldehyde n-Heptaldehyde Acetyl chloride Acetone 4Heptanone Cyclohexanone %-Hexylamine Cyclohexylamine &Butylamine Diisopropylamine Triethylamine Tri-n-propylamine Tributylamine Dime thylf ormamide Benzene Thiophene Bromobenzene Chlorobenzene Benzonitrile
25.19 23.28
406.10 409.72 410.55 410.64 428.82 433.18 433.18 432.31 434.44 433.58 419.52 425.68 416.55 443.39 444.13 442.99 471.32 474.43 478.68 464.78 458.38 432.52 431.77 458.97 362.73 373.88 392.48 386.66 419.00
... 21.70 22.18 22.67 22.39 23.63 23.61 25.17 23.15 23.16 24.43 22.13 23.33 23.57 22.71 23.16 22.60 23.40 24.06 22.76 25.39 22.12 23.25 22.71 23.12 22.48 22.55
"Parameters are given in cps; TMS.
JCH
w
w
204.31 204.60 205.40 205.25 208.12 208.36 208.3g 208.34 208.89 208.91 206.83 207.68 206.79 209.89 209.65 209.98 210.92 211.19 211.38 210.00 209.19 207.33 207.15 211.60 206.17 205.81 205.66 205.38 208.04
is referenced to internal
satellite patterns could not be observed owing to the solvent protons. In such cases the value of J was obtained from that half which could be observed plus or minus approximately 0.4 cps to correct for the asymmetry of the proton peak with respect to the two 13C-H patterns.
Results The chemical shifts ( w ) and W - H coupling parameters (J)of CHBr3were determined in 24 aliphatic and 6 aromatic solvents and are listed in Table I. As shown in Figure 1 there is a fair correlation between the sol~
~~~
~~
~
~~~~~
~~
(7) V. S. Watts, J. Loemker, and J. H. Goldstein, J . Mol. Spectry., 17, 348 (1965). (8) M.Gordon and C. E. Griffin, J . C h m . Phgs., 41, 2570 (1964). (9) S. Ng, J. Tang, and C. H. Sederholm, ibid., 42, 79 (1965). (10) A. D. Buckingham, T. Schaefer, and W. G. Schneider, ibid.. 32, 1227 (1960). (11) A. D.Buckingham, Can. J . Chem., 38, 300 (1960). (12) A. Allerhand and P. von R. Schleyer, J . Am. Chem. Soc., 85, 1715 (1963).
SOLVENT EFFECTS ON '3C-H COUPLING PARAMETERS
3889
-4;
L
;r
2 10
Figure 1. Plot of w us. J ( W - H ) for bromoform in nonaromatic solvents.
pi
I
I
2
vent effects on J and w for the aliphatic solvents. It is possible to group the magnitudes of the solvent effect on J and w according to the type of solvent molecule. However, because of the limited number of solvents of each type, such a classification must be regarded as tentative. When considered in terms of the functional groups in the aliphatic solvent molecules, the increases in IJl and /wI, relative to the corresponding values in CH, occur in the following order. J : halogen < CHO = NR2< OH < O R < C 4 = NHR < NH2; w : halogen < CHO < OH = NR2 < OR < C=O < NHR < NH2. The aromatic solvents produced an upfield displacement in the chemical shifts and a slight increase in J with respect to the cyclohexane values. I n these solvents there appears to be no correlation between the effects on J and w . In the second phase of this study eight halomethanes, acetonitrile, chloroacetonitrile, and methylchloroform were observed as neat liquids and as solutions with mole ratios of 1: 4-1 :5 in cyclohexane (CH), carbon tetrachloride (CC1,) , and dimethylformamide (DMF). The values of w and J so obtained are listed in Table 11, from which it can be seen that in each case W D M F < W C H and J D ~ > ~ FJcH. The observed changes in w and J on going from CH to CC1, are relatively small. The considerably larger differences between the values in DMF and those in CH represent the effect experienced on replacing an inert medium, CH, by the highly polar, proton-acceptor solvent, DRIF. Figure 2 shows that there is a reasonably good linear correlation between Aw = / W D M F - W C H ~and AJ = J D M F- J C H . There is also a fair correlation between AJ and J
AJ
6
Figure 2. Plot of differential solvent effects on w and J for the substituted methanes: circles, values of A@ and AJ (see text); diamonds, Values Of WDYF WCCld and J D U F JCCld.
-
(although this is not shown graphically here), the points tending to cluster somewhat according to the number of substituents present. The value of AJ increases with the degree of substitution, increasing roughly twofold for each substitution of hydrogen by a particular halogen. Insofar as the values for the halomethanes in Table I1 permit generalization, there appears to be a relationship between the solvent effects on J and both the structural type of the solute and the nature of its substituent. In terms of AJ (the difference between J in DRIF and CH) the solvent effect decreases in the order CHX3 > CHzX2or CH2XY > C H Z , and the effect of the substituents follows the order C1 > Br > I. The differences Jneat - J C H are rather small (0.81.6 cps) for all the halomethanes, but in addition are clearly largest for the group of dihalomethanes. For acetonitrile the solvent effects on J are very small (as is also true in the case of methylchloroform). When C1 is replaced by CN in both CHsC1 and CHzClz the parallelism of the solvent effects on w and J no longer holds. The result of this substitution is to increase the effect on w but decrease it for J .
Discussion The effect of the aliphatic solvents of Table I on the chemical shift and C-H coupling of bromoform (a proton donor) tends to increase with the proton-acceptVolume 70, Number I d
December 1966
V. S. WATTSAND J. H. GOLDSTEIN
3890
Table II: Solvent Effects on the Nmr Parameters of Some Halogenated Methanesa Solute
Solvent
DMF CHCL CClr CH DMF CHBra CCh CH DMF CHzClz ccl4 CH DMF CHtBrCl CCL CH DMF CHZBrz cc14 CH DMF CH2BrI CCl4 CH DMF CHzIz CClr CH DMF CHzClCN Cclr CHc DMF CHaCl ccl4 CH DMF CHsBr
cc4 CH DMF
CHaI
cc14 CH DMF CHsCN CCh CHc DMF
Mole % compd
0
J'$C-H
Av dev
24.36 100.00 21.46 22.53 22.12 100.00 23.28 25.19 22.35 100.00 21.48 24.02
-492.32 -434.17 -434.77 -429.17 -458.97 -410.55 -409.72 -406.10 -341.99 -319.56 -317.22 -310.74
23.66 100.00 22.83 24.04 21.17 100.00 22.92 23.61 22.26 100.00 21.71 23.67 21.25 100.00 22.46 24.28 23.42 100.00 23.06
-333.96 -312.78 -309.60 -303.17 -321.16 -300.06 -295.99 -290.90 -292.68 -276.90 -274.12 -269.79 -246.98 -235.00 -234.10 -231.02 -280.81 -251.18 -246.87
216.46 208.91 208.26 208.11 211.60 205.40 204.60 204.31 180.55 178.11 176.75 176.48 181.33 178.96 177.70 177.38 181.63 179.22 177.98 177.74 177.54 176.20 175.28 174.73 173.80 172.92 172.15 171.93 162.16 161.22 159.69
0.04 0.03 0.07 0.05 0.05 0.04 0.06 0.04 0.05 0.06 0.04 0.05 0.02 0.04 0.07 0.05 0.07 0.05 0.05 0.05 0.06 0.02 0.03 0.06 0.05 0.03 0.03 0.03 0.03 0 03 0.04
...
...
21.68 100.00 25.18 19.22 23.20 100.00 27.74 26.07 22.38 100.00 21.72 23.42 22.66 100.00 22.18
-184.28 - 179.09 -179.06 172.05 - 163.46 -158.58 157.73 -151.08 - 132.79 -130.97 -129.49 -123.18 - 125.89 - 117.70 -118.17
150.40 149.64 149.18 148.58 152.14 151.44 150.98 150.54 151.59 151.09 150.65 150.31 135.99 136.15 135.66
0.05 0.03 0.03 0.04 0.04 0.03 0.05 0.05 0.03 0.06 0.05 0.05 0.05 0.04 0.05
...
...
24.16
CHsCCL Cc14 CH a Parameters are given in cps; because of limited solubility.
The Journal of Phyeical Chmistrv
w
24.50 22.37
is referenced to internal TMS.
...
-
..*
...
-167.08
133.77 133.46 133.31 133.25
- 163.71 -164.03 -159.05
' Average deviation of the calibrated values of
... 0.04
0.04 0.04 0.04 W.
Not observed
3891
SOLVENT EFFECTS ON 13C-H COUPLING PARAMETERS
ing ability of the solvent. The values of Aw and AJ (Table 11) provide a measure of the hydrogen-bonding effect of DRlF relative to the inert medium CH. These quantities on the whole tend to increase with the expected proton-donating ability of the solutes. On the other hand, in the highly polar CH3CN J is only slightly affected by the transition from CC14 to DMF, although the shift is affected appreciably. These observations can be more plausibly accounted for in terms of the specific interactions between the solvent and solute, in particular hydrogen bonding, than by any more general mechanism. In comparing the situation in any solvent with that in the neat solute, consideration must also be given to association in the latter state. I n the hydrogen-bonded complex the electron-rich proton acceptor repels the charge from the vicinity of the bonded proton toward the carbon atom.6 This increases the s character of the carbon orbital bonded to the proton with a corresponding increase in the value of J . The parallelism between the solvent effects on J and w (see Figure 1) is qualitatively in accord with such a mechanism. The points in Figure 1 fall on two distinct curves. The values for all of the amines studied fall on the upper curve while those for all the solvents containing oxygen are on the lower curve. It is impossible to decide whether the values for those solvents containing only halogen substituents fall on one of these two curves or define a third, owing to the small magnitudes of their effects. This separation into groups must arise in the differences in the relative importance of the factors responsible for the observed solvent effects. The probable source of these differences is the anisotropy contribution of the acceptor atom in the hydrogen-bonded complex, which has been discussed in this connection e l ~ e w h e r e . ' ~ ~The ' ~ correlation between A J and Aw is linear for both solvent groups but has a greater slope for the amines. This appears reasonable since as the strength of the hydrogen bond increases, the proton would be expected to approach more closely to the seat of anisotropy. There is no apparent correlation between the solvent effects and the basicities of the amines used.15 However, these amines do not differ greatly in their pK, values (:-10.5-11.0). In this situation steric effects might be expected to be important in hydrogen bonding to the bulky bromoform molecule. That such is the case is supported by the relative order of solvent effects: primary > secondary > tertiary amines. Finally it is interesting to note that DMF falls on the curve for oxygen compounds indicating that it bonds through its oxygen atom.
On the basis of the above mechanism of interaction it is to be expected that the solvent effect in a strong acceptor such as DMF would increase with the protondonor ability of the solute. The observed effect, as measured by AJ, does indeed increase in the order CHaX < CH2X2 < CHX3 in the approximate ratio 1:2:4. (A somewhat different order is found for the less polar solvents, CH and CCL, namely CHzX2 > CHX3 CH&. The differences in J observed for all these structures are not very large.) These results may reasonably be attributed to the breakup of selfassociated complexes of the solute on dilution with CH or CC14. McClellan and Nicksic have recently reported studies of the chemical shifts of a series of halomethanes and haloethanes as pure liquids and as solutions in DMSO, CH, and CCL.lS From the results they concluded that all these compounds are weakly associated to about the same extent. In these loose complexes it is possible that statistical factors can become important. Thus there are six ways to form hydrogen-bonded dimers in C H Z or CHX3 and eight in the case of CH2X2, which might explain the observed order of the solvent effect in CH or CCL for these structures. In general the solvent effect observed here for CH3,X, follows the order C1 > Br > I. For the haloforms Creswell and Allred have found the enthalpies of hydrogen-bond association t o fall in the same order: CHC13 > CHBrs > CHI3." Infrared methods have also been used to investigate the hydrogen bonding of these compounds to bases using dilute solutions in CCL. In this way Allerhand and Schleyer found the order of changes in the C-H stretching frequencies to be CHBr3 > CHI3 > CHC13.12 They also found that the replacement of C1 by cc13 and CN as well as by Br and I resulted in the enhancement of the spectral shifts produced by a common base. The use of very dilute solutions to study 13C-H couplings would likewise be desirable, but it is not practical at the natural abundance of this isotope with the usually available techniques. Both Aw and AJ correlate reasonably well with the Taft inductive parameters of the halogens, l8 as is shown for AJ in Figure 3. The correlation for AJ ~
~~~~
~
~
~
(13) B. B. Howard, C. F. Jumper, and M. T. Emerson, J. Mol. Spectry., 10, 117 (1963). (14) D.P. Eyman and R. S. Drago, J . A m . Chem. SOC.,8 8 , 1617 (1966). (15) H. K.Hall, Jr., ibid., 7 9 , 5441 (1957). (16) A. L. McClellan and S. W. Nicksic, J . Phys. Chem., 69, 446 (1965). (17) C. J. Creswell and A. L. Allred, J . Am. Chem. Soc., 8 5 , 1723 11963). , (1.8)R. W. Taft, Jr., and I. C. Lewis, Tetrahedron, 5 , 210 (1959).
.~..
Volume 70, Number 12 December 1966
3892
V. S. WATTSAND J. H. GOLDSTEIN
I
0'
,
~r*L-xj
7
Figure 3. Plot of AJ us. the sum of the Taft substituent parameters for the substituted methanes.
(but not for Aw) is poorer for the CN substituent, and it has been pointed out above that the solvent effect on J for acetonitrile is small. The effect of aromatic solvents on J is interesting. I n benzene J(CHBr3) is -2 cps larger than in CH and -0.8 cps greater than the value for the pure compound. This difference, though small, is significant and suggests the existence of a characteristic aromatic effect. Such an effect could arise, for example, from the complexing of the proton with the T charge of the aromatic ring as has been postulated elsewhere.15 The values of J and w in each substituted aromatic solvent are intermediate between those in benzene and those in aliphatic solvents with the same substituent. This suggests a competition between the complexes of the
The Journal of Physical Chemistry
bromoform proton with the aromatic ?r cloudlg and with the aromatic substituent. Rapid equilibration between the two types of complexes must obviously be assumed to account for the observation of only one spectrum. We should like finally to discuss some of the more general aspects of the method employed here to study solute-solvent interactions. In similar investigations nmr spectroscopists have usually relied upon changes in chemical shifts as an index of such int,eractions. In those instances where investigators noted the importance of anisotropy effects it has been found necessary to attempt an estimation of these contributions on the basis of an assumed model and suitable theoretical calculations. l a ,l4 In this study we have perferred instead to place primary reliance on the 13C-H coupling parameter of the solutes while at the same time comparing its behavior with that of the chemical shifts. The use of this parameter ( J ) is advantageous because of its insensitivity to anisotropy effects which are a major source of difficulty in the use of chemical shifts. Since the factors influencing AJ and Aw are likely to be different, their simultaneous use offers obvious advantages.
Acknowledgments. This work was supported in part by grants from the National Science Foundation and the National Institutes of Health. We are indebted to Lee H. Mtmayer and K. M. Pryse, both National Science Foundation Undergraduate R e search Participants, for their assistance in various phases of this study. (19)
w.G
Schneider, J . Phys. Chem., 66, 2653 (1962).
