STANFORD L. SMITHAND RICHARD H. Cox
198 ratios exist, namely, that the value of TDMC: CDMC depends on both the pressure of the system and the ratio of cis-butene-2: DM. This cautionary remark applies as well to much of the recent work in the literature on ordinary photolysis systems in which %H2 has
been found. Our previous finding of an increase with pressure of the ratio TDMC: CDMC, up to a plateau or maximum, now may have an explanation other than the previously alleged variation of the C H 2 :‘CH2 proportion with pressure.16
Solvent-Dependent H-H Couplings in Hexachlorobicyclo[2.2,1] heptenes’ by Stanford L. Smith and Richard H. Cox Depurtment of Chemistry, University of Kentucky, Lexington, Kentucky
40606 (Received June 89, 1967)
The solvent dependence of 2 J and ~ cis ~ and trans 3 J is ~investigated ~ in 1,2,3,4,7,7hexachlorobicyclo[2.2.1 Iheptenes substituted a t the 5 position with chloro, cyano, phenyl, hydroxy, carboxyl, or acetate groups. In all six compounds, ‘ J H Hdecreased (in the absolute sense) in solvents of increasing dielectric constant. The magnitude of the change ~ for all compounds except the hydroxyl- and carboxyl-substituted cases in 2 J is ~similar where strong association effects are evident. Changes in ’JHHwere 0.25 Hz or less in all cases. The general solvent effect (in the absence of specific association) is attributed to the solvent electric field which, in turn, is controlled by the net electric dipole of the solute molecule.
Introduction The solvent dependence of spin-spin coupling constants in many relatively rigid molecules is now an established phenomenon whose causes and characteristics are rapidly being e l u ~ i d a t e d . ~Previous ,~ investigations have focused on geminal H-H and H-F couplings in sp2 hybridized systems and on various single-bond c~uplings.~JSeveral examples of solventdependent vicinal couplings are also known, again in sp2 hybridized sy~tems.~*~+’OThe solvent dependence of J,,, across C2 in 4-methyl-l,3-dioxolane2 and the solvent dependence of J,,,,, in dl-dibromosuccinic anhydride” are the only known examples of solvent-dependent couplings in “rigid” spa hybridized systems. I n neither case are the results completely unambiguous. A number of investigators have noted an apparent solvent dependence of 2 J in~ flexible ~ systems such as 1,2-dichloro- and dibromopropanes,l2 various 1,1,2-trisubstituted ethanes,13 and 3,3-dimethylbutyl iodide.14 Unfortunately, it is not possible to determine whether these observed changes arise directly from some intrinsic solvent effect or indirectly from solvent-induced changes in conformer populations. In the latter event, changes in orientation of substituents with respect to the methylene group would be expected’s to produce small changes in the geminal H-H coupling. The existence of these examples The Journal of Physical Chemistry
raises potentially serious questions concerning the utilization of H -H (and H-F) couplings in the study of concentration and solvent effects on conformationally mobile systems. We therefore decided to examine and Hcis and trans 3 J ~ ~ the solvent dependence of ~ J H in a rigid spa hybridized system. (1) A summary of this work was presented at the Southeastern Regional Meeting of the American Chemical Society, Louisville, Ky., Oct 1966. (2) S. L. Smith and R. H. Cox, J . Chem. Phys., 45, 2848 (1966), and references therein. (3) 5. L. Smith and A. M. Ihrig, ibid., 46, 1181 (1967),and references therein. (4) V. S. Watts and J. H. Goldstein, J . Phys. Chem., 70, 3887 (1966). (5) H. M. Hutton, E. Bock, and T. Schaefer, Can. J . Chem., 44, 2772 (1966). (6) R. H. Cox and S. L. Smith, J . Mol. Spectry, 21, 232 (1966). (7) S. L. Smith and A. M. Ihrig, ibid., 22, 241 (1967). (8) H.M. Hutton and T. Schaefer, Can. J . Chem., 43, 3116 (1965). (9) H.M.Hutton and T. Schaefer, ibid., 45, 1111 (1967). (10) P. Laszlo and H. J. T. Bos, Tetrahedron Letters, 1325 (1965). (11) L. E.Erickson, J . A m . Chem. SOC.,87, 1867 (1965). (12) H. Finegold, J. Chem. Phys., 41, 1808 (1964). (13) E.I. Snyder, J . A m . Chem. Soc., 88, 1155 (1966). (14) G. M. Whitesides, J. P. Sevenair, and R. W. Goetz, ibid., 89, 1135 (1967). (15) A. A. Bothner-By in “Advances in Magnetic Resonance,” Vol. 1, J. S. Waugh, Ed., Academic Press Inc., New York, N. Y., 1965,p 197.
SOLVENT-DEPENDENT COUPLINGS I N
199
HEXACHLOROBICYCLOHEPTENES
The hexachlorobicyclo [2.2.1Iheptenes, I, obtained from the Diels-Alder reaction of hexachlorocyclopentadiene with various monosubstituted olefins are ideally suited to our purpose. They have been studied previously by Williamsonl6 in an investigation of sub-I N
I
I, X=CI,CN,CGHs,0Ac,OH,CO2H
.-C r r 7
N
stituent electronegativity effects on coupling constants and his results provide a check for our data. Williamson also discusses the evidence for the conformational rigidity of the bicyclo [2.2.l]heptene system. These compounds also offer unique opportunities to clarify further the solute structural factors (other than gross hybridization) required for the solvent dependence of couplings. With rare exception17 all examples of solvent-dependent geminal H-H couplings in nondouble-bonded systems occur in molecules having a heteroatom bonded directly to the methylene group in question. Only one such case has been examined in detail.6 This characteristic is absent in the bicycloheptenes. Similarily, fi substituents are known to ~ a~ J in~ affect the solvent dependence of 2 J and olefins. The bicycloheptenes offer the opportunity to investigate analogous effects for sp3 systems. From a somewhat different viewpoint, these compounds off er some information concerning the nature of the solvent-solute interactions involved. Orientation of the solvent dipole has been shown to be a significant factor in the solvent-solute interaction@) . similar leading to the solvent dependence of ~ J H F A dipole effect has been implicated in the solvent dependence of 'J". The six chlorines located on "one side" of the hexachlorobicyclo [2.2.1Iheptenes studied here provide an appreciable net dipole appropriately orientedLsto favor electron shifts away from hydrogen. If, in the absence of specific associations, the net dipole is important, then similar solvent variations should be observed for different X substituents. However, if local dipole effects are more important than the net dipole, then appreciable variations in the solvent de~ ~be expected as X is changed. pendence of 2 J might The results reported below yield, at least, some information about all of these factors.
-I
-I t
of S o l v e n t
us. solvent dielectric constant for Figure 1. Plot of Jgem substituted hexachlorobicyclo[2.2.1] heptenes. Vertical bars represent AO.1 Hz error limits.
and the remainder of the adducts were purified by vacuum sublimation. No solute impurities were observed in any of the nmr spectra. Spectro and reagent grade solvents were used without further purification. ~ Solvent-impurity peaks were not encountered in any of the experimental spectra. Samples were prepared as 5 mole % solutions with approximately 3y0 tetramethylsilane (TMS) being added as internal reference and lock signal. All samples were degassed by the freeze-thaw technique and sealed under vacuum. Spectra were obtained on a Varian HA-60-IL spectrometer operating in the frequency-sweep mode. Experimental line positions were obtained as the average of five or more independent scans and were calibrated by counting the difference in frequency of a peak from TMS with a Hewlett-Packard 521 CR frequency counter. The proton nmr spectra of these adducts represents a slightly perturbed ABX system Final parameters were calculated using the computer programs LAOC'N I11 and NMRIT. I n all cases, the final parameters are good to 0.1 Hz or better (typical probable errors were 0.020-0.005 Hz). Relative signs of the proton-coupling constants in the vinyl chloride adduct were determined using the technique of Freeman and Anderson.20 Details of this decoupling work have been reported elsewhereVz1
Experimental Section The styrene, vinyl chloride, acrylonitrile, acrylic acid, and vinyl acetate Diels-Alder adducts of hexachlorocyclopentadiene were a gift from Hooker Electrochemical Co. Acid hydrolysis of the vinyl acetate adduct yielded the vinyl alcohol adduct.1Q The vinyl acetate adduct was purified by vacuum distillation
(16) K.L.Williamson, J . Am. Chem. SOC.,85, 516 (1963). (17) H.M.Hutton and T. Schaefer, Can. J . Chem., 41, 684 (1963). (18) Careful examination of Drieding models shows the dipole to lie roughly in a plane passing through the bridgehead carbons and approximately perpendicular to the plane of the bridge (Cl, C7, C4). (19) E.K.Fields, J. Am. Chem. SOC.,78, 5821 (1956). Volume 76, Number 1 JanUaTy 1968
200
STANFORD L. SMITH AND RICHARD H. Cox
Table I : Nmr Parameters at 60 MHz for 5-Substituted 1,2,3,4,7,7-Hexachlorobicyclo[2.2.1] heptenes in Various Solventsa Sub-
stituent
JlZ
Solvent
c1
C6HlZ CHzClz MezCO DMSO
CN
C6H12 CHzClz MezCO MeNOz DMSO
CsH6
C6HlZ CHzCIz MezCO MeNOz DMSO
Acetate
C6HlZ CeH5C1 CeHsNOz DMSO
COzHb
cc14 CHzClz MezCO DMSO
OH*
C6HlZ CHzClz C&Noz DMSO
E
(gem)
1.99 9.08 20 46
-13.32 -13.49 -13.72 -13.85 Max change 0.53 1.99 -12.63 9.08 -12.85 20 -13.00 35 -13.05 46 -13.02 Max change 0.42 1.99 -12.85 9.08 -13.00 20 -13.05 35 -13.13 46 -13.27 Max change 0.42 1.99 -13.16 5.6 -13.33 32.2 -13.42 46 -13.54 Max change 0.38 1.99 -12.47 9.08 -12.71 20 -12.63 46 -12.51 Max change 0.24 -12.85 9.08 -12.98 3.52 -12.96 46 -12.87 Max change 0.13
a All values are given in hertzes from internal TMS. see text.
(cis)
3.16 3.09 3.03 2.97 0.19 4.10 4.06 4.02 3.99 4.02 0.11 4.27 4.29 4.19 4.37 4.38 0.19 2.47 2.49 2.41 2.48 0.08 4.11 4.12 4.00 3.95 0.17 2.51 2.48 2.36 2.38 0.15
8.20 8.16 8.14 8.19 0.06 9.15 9.21 9.11 9.23 9.10 0.12 9.16 9.10 9.12 9.12 9.16 0.06 7.58 7.63 7.63 7.68 0.10 9.00 9.07 8.96 8.82 0.25 7.70 7.72 7.58 7.56 0.16
Jaa
VI
VZ
VS
130.23 137.79 145.16 147.46
178.78 186.98 197.21 192.33
278.89 287.54 303.52 310.55
137.54 145.28 160.62 154.03 165.71
168.90 176.45 186.97 184.12 176.01
216.08 225.35 252.73 238.55 263.31
144.20 150.32 159.31 156.32 161.24
169.04 174.72 179.13 178.37 176.30
234.34 239.45 245.54 244.50 244.52
112.39 109.39 129.24 136.90
177.08 167.95 181.21 181,91
336.41 338.25 345.31 339.77
151.34 152.76 155.02 148 68
161.45 164.72 167.44 162.67
218.00 222.49 224.53 220.04
114.71 119.39 128.77 117.81
167.73 173.59 178.67 171,89
(278.5) (285.5) 301.45 279.70
’ Values for these substituents were obtained by an approximate solution;
Results and Discussion Final parameters for the nmr spectra of the adducts are given in Table I. With the exception of the vinyl alcohol and acrylic acid adducts (compounds where specific association is expected), there is a general correlation between Jgem and the solvent dielectric constant (Figure 1). In all cases, J,,, becomes more negative in solvents of increasing dielectric constant. A general correlation (with the exception of aromatic solvents) is observed between Jgemand the average of the chemical shifts of the two geminal protons for the styrene vinyl chloride, acrylonitrile, and vinyl acetate adducts. The changes in the vicinal coupling constants, although quite small, are probably real. No correlation is evident between the vicinal coupling constants and any solvent parameters. The vinyl alcohol and acrylic acid adducts present little useful information. Specific association is expected ( L e . , interaction of solvent with the OH in each case). In cyclohexane and methylene chloride, the X portion of the spectrum of the vinyl alcohol adduct The Journal of Physical Chemistry
Jia (/vans)
is broadened t o such an extent that the individual lines of the quartet could not be measured accurately. Data given in Table I for the vinyl alcohol adduct in these two solvents were obtained from an ABX approximationZ2using the AB portion of the spectrum. The chemical shift of the X proton in these cases is that estimated from the broad peak observed. The minor changes observed in the vicinal couplings (if real) may be due either to general solvent effects or to very slight conformational changes in the bicyclo[2.2.l]heptene skeleton. Thus, the generally accepted assumption that vicinal H-H couplings are not solvent dependent seems, for practical purposes, to be valid for the eclipsed conformation. There seems little reason to think the results would be significantly different for staggered conformations. (20) R. Freeman and W. A. Anderson, J. Chem. Phys., 37, 2053 (1963). (21) R. H. Cox and S. L. Smith, J. Phy8. Chem., 71, 1809 (1967). (22) J. W.Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Pergamon Press h c . , New York, N. Y., 1965,p 357.
SOLVENT-DEPENDENT COUPLINGS IN HEXACHLOROBICYCLOHEPTENES The measurable decrease (in the absolute sense) of for the ohloro-, cyano-, phenyl-, and acetatesubstituted compounds in solvents of increasing dielectric constant supports the previously noted correlation between the absolute sign and solvent-induced variations of geminal couplings. The magnitudes of the solvent-induced changes are about the same in these four compounds and are similar to effects observed previously. This suggests that hybridization of the carbon in the CH2 group is not a major factor in controlling It also suggests the solvent dependence of J,,,. that the solvent-dependent geminal couplings observed in flexible systems arise primarily from a direct solvent effect, not from conformational changes. The smaller changes observed in the hydroxyl and carboxyl conipounds undoubtedly reflect the effect of association at the /3 position, causing “appreciable” electronic and/or conformational changes in the molecule. Otherwise, the solvent-induced variation of Joe, across sp3 hybridized carbons seems to be relatively insensitive to substituent variations of the /3 position. The similarity in magnitude between the changes observed here where the CH2 group has no heteroatom bonded to it and those changes observed in previous examples where there has been a heteroatom bonded to the CH2 group is significant. Acidic (hydrogen-bonding) solvents are not likely to interact with an essentially aliphatic CH2 group. Similarly, an aliphatic CH, group should not hydrogen bond appreciably to basic solvents. Therefore, specijc donoracceptor interactions in the immediate vicinity of the CH2 group are not a fundamental requirement for the existence of solvent efects o n geminal coupling constants. This is not to say that specific interactions do not cause coupling-constant variations. There is abundant evidence that they do. Rather, we wish to emphasize the existence of a general bulk solvent effect. The similarity in magnitude of the changes in J,,, for different /3 substituents implies that the net solute dipole (or at least the over-all electrical asymmetry of
J,,,
201
the solute molecule) is more important than local dipoles or local electron distributions. This conclusion, in turn, implies that the bulk solvent effect is due to a solvent electric field whose magnitude and direction depend on the solute dipole orientation. Whether the solvent electric field arises from polarization of adjacent solvent molecules (reaction field effect), from momentary partial orientation of adjacent solvent molecules (dipole-dipole interactions, solvent stark effects), from an unequal distribution of solvent molecules in the solvation shell, or, as seems most likely, from some combination of such phenomena cannot now be specified. In summary, these results for the spa hybridized system essentially complete a tableau as far as a solute structural factors are concerned. Geminal H-H coupling constants are apparently all solvent dependent, regardless of the hybridization of the carbon involved. The only question remaining is the magnitude of the observed change which depends on the structure of the solute and on the particular solvents used.3 Vicinal H-H coupling constants may exhibit solvent dependence in a few cases. Neither the solute structural requirements nor the nature of the solvent-solute interactions necessary for solvent dependence of vicinal couplings is presently known, but the expected changes (if any) are likely to be smaller than those observed for geminal couplings. One fact, noted recently by Watts and Gold~tein,~ is becoming increasingly apparent. Solvent-dependent coupling constants offer an extremely sensitive and unique probe for the examination of both weak solvent-solute interactions and electron redistributions within molecules in solution. Utilization of this phenomenon for such investigations should lead to significant refinement of our present theories in these areas. Acknowledgments. The authors wish to thank the Hooker Chemical Co. for providing the compounds studied here. The financial assistance of the University of Kentucky computer is gratefully acknowledged.
vohme ‘78, Number 1
January 1968
