320
R. 0.Bates, B. E. Wagner, and E. H. Poindexter
Dynamic Fluorine-I 9 Polarization in Fluorinated Strained Cyclic Alkanes and Alkenes Richard
D. Bates, Jr.,*
Chemistry Department, Georgetown University, Washington, D.C. 20057
Burkhard E. Wagner, and Edward H. Polndexter U.S. Army Electronics Technologyand Devices Laboratory (ECOM), Fort Monmouth, New Jersey 07703 (Received September 2, 1975) Publication costs assisted by the Petroleum Research Fund and Army Research Office
Studies of dynamic polarization of 19Fnuclei in a series of perfluorocycloalkanes and perfluorocycloalkenes and their partially chlorinated analogues are reported. The experiments test effects that changes in the internal structure of the molecule have on the behavior of the peripheral fluorine atoms in nonaromatic species. With galvinoxyl interactions are dominated by dipolar coupling of the spins, with ultimate enhancements in the range -237 to -270. For the perfluorocycloalkanes enhancements are predominantly dipolar (-171 to -213 for bis(dipheny1ene)phenylallyl (BDPA), -182 to -227 for 2,2-diphenyl-l-picrylhydrazyl (DPPH)), with little variation for the four-, five-, and six-member ring species studied. The fluorines on the saturated carbons of the 1,2-dichloroperfluorocycloalkenes show more scalar coupling than the perfluorocycloalkanes (-143 to -158 for BDPA, -164 to -179 for DPPH), but the highest degree of scalar coupling is obtained for the perfluorocycloalkenes. The enhancements of the fluorines attached to unsaturated carbons are separated from the total observed enhancement and are shown to produce couplings significantly more scalar than others previously observed (+2 to -52 for BDPA, +256 to +223 for DPPH.) No evidence of correlation of DNP results for polarization of peripheral fluorine atoms on molecules when compared with high field NMR results is found.
Introduction Ultimate NMR signal enhancements (V,) obtained in solution dynamic nuclear polarization (DNP) studies have been used to draw conclusions about intermolecular coupling, complexation, and preferential site These enhancements are sensitive to the major changes in molecular electronic structure that affect the nature and chemical environment of the polarized nucleus, and to the availability of the polarizing free-radical unpaired electron.l-* However, the dominant dependence is on the molecular dynamics of the two interacting species. The demonstration that the observed DNP enhancements do not depend significantly on the minor steric and electronic differences that govern high resolution NMR chemical shifts and spin-spin couplings is crucial to allow general application of the technique without the need to consider each coupling on a case by case basis. When compared with DNP investigations of saturated fluorocarbons, studies of aromatic fluorocarbons have indicated a significant increase in scalar coupling resulting from the polariza~,~ changes in substitution of the ring K s t r ~ c t u r e .However ent position of difluorobenzenes produced very small changes in DNP result^.^ Investigations of fluorine and phosphorus polarizations in phosphonitrilic fluorides and chlorides demonstrated similar behavior among the members of each series and a lack of conjugation of the 19FnuThis paper reports the study of clei to the ring K sy~tems.8*~ a series of perfluorocycloalkanes and perfluorocycloalkenes and their partially chlorinated analogues, for which there is considerable variation in the high-field NMR behavior.1°-12 The goal is to test effects that changes in the internal structure of the molecule have on the behavior of the peripheral fluorine atoms in nonaromatic species. Additionally, one of the major disadvantages of DNP to date has been the inability to distinguish between rhemiThe Journal of Physical Chemistry, Vol. 80, No. 3, 1976
cally different 19F atoms on the same receptor, as the chemical shifts at the low magnetic field strengths required for DNP experiments are too small to resolve signals. By comparison of the perfluorocycloalkenes with the corresponding species in which the carbons adjacent to the double bond are blocked by chlorine atoms, resolution of the contribution to the enhancements by the two chemically different nuclei may be possible. Finally, examining the relative amounts of dipolar and scalar coupling for samples in this series will elucidate (1)the influence of free-radical polarizers on the electronic structures of these species, and (2) the effect on intermolecular spin coupling of coplanarity of the fluorines and the carbon ring system.
Theory Detailed accounts of the theory of DNP in free-radical solutions are a ~ a i l a b l e . Equation ~ , ~ ~ ~ ~1~defines the observed enhancement, G(P):
u,N
= 'ye r - s + c YN 2q + r + s
+c
(2)
A(P) and A(0) are the NMR signal amplitudes of nucleus N when the ESR line of the free radical is pumped or unpumped, respectively. The ultimate enhancement of nucleus N, U r n Nis, the observed low-field enhancement extrapolated to complete saturation of the radical ESR line (limit of the saturation function, SJP), going to one), and complete domination of the nuclear relaxation times by coupling with the radical (limit of the leakage factor, f ~going , to one). As shown in Figure 1,the terms r , s, and q , are the coupled electron-nuclear dipolar relaxation transition probabilities, and c is the corresponding relaxation induced
19F Polarization in
321
Fluorinated Cyclic Alkanes and Alkenes
Experimental Section
-+ Figure 1. Combined energy states and relaxation probabilities for a system of unpaired electrons coupled weakly to spin I = 112 nuclei.
by a scalar mechanism. The electron and nuclear gyromagnetic ratios are ye and YN,respectively. Values for UrnF are commonly obtained by the ratio method in which the ratios of the observed enhancement of the fluorine nucleus of interest to that of a proton in the same sample at the same applied radio-frequency power are averaged for several applied powers, as shown in
Provided the radical concentration is high enough that proton and fluorine leakage factors are approximately one, the values for UmFcan be obtained from the ratio and -329.5, the ultimate enhancement obtained for low-field proton samples, to give sound results. The dipolar and scalar components can be resolved by using eq 2, as q, r, and s are in a ratio of 3:2:12 in the low-field limit, allowing a relative value of c to be obtained from UrnF. One difficulty encountered with some of the samples in the present study is that they contain fluorine atoms of more than one type. As chemical shifts a t the low magnetic fields used in the experiments are too small to resolve the different fluorine NMR signals, the enhancements obtained, and hence the values of lJrnF,are composites of all contributing nuclei. The least sophisticated, and presently the only practicable, means to estimate the contribution of a given type nucleus to the total observed enhancement is to assume that the contribution of each type nucleus is simply proportional to the number of that type nucleus per molecule. This assumes that unpumped signal intensities are proportional solely to the number of atoms per molecule of a given type, that the leakage factors for all nuclei in different chemical environments are near one, and that the chemical shifts are so small that the two peaks are additive, both when polarized and when unpolarized. Then the ultimate enhancement of one type nucleus can be determined using eq 4 if the ultimate enhancement of the other nuclei contributing to the same signal can be estimated. N F ( ~is’ ) the number of fluorine atoms of type i per molecule; Urnp(;) is the ultimate fluorine enhancement of that type of atom; UmF(obsd) is the composite ultimate fluorine enhancement observed experimentally; and the sum over i is the sum over all types of fluorine atoms.
UrnFO’)=
Samples were prepared by dissolving a weighed portion of the free radical (for 0.02 m solutions) in a three-component mixture of (1) cyclic fluorocarbon, (2) benzene, and (3) carbon tetrachloride in a volume ratio of 1:1:4. This solvent combination, which has been used previously? was necessary to ensure that (1)all four components are miscible a t the conditions of the experiment, and (2) all samples are run in the same solvent system for reliable comparison. That the choice of solvent system may affect the relative degree of scalar coupling has been previously demow strated? and will be examined in depth in a forthcoming paper.15 The variation is most significant for solvents with markedly different values of the cohesive energy density 6. For benzene, 6 = 9.2, and for carbon tetrachloride, 8.6; typical values of this function for the perfluorocycloalkanes are between 5.7 and 6.1.l68l7Thus the dissimilarity among the three solvent components may have some effect on the results obtained; however, as all the samples were run in the same solvent system, consistent results within the set should be obtainable. The cyclic fluorocarbons were commercial products with stated minimum purity of 98%. The free radicals used in the study, the structures of which are shown in Figure 2, were chosen to provide a wide range of typical scalar to dipolar polarization ratios. Galvinoxyl (GALV) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) were commercial products with stated purity of 98%. Bis(dipheny1ene)phenylallyl (BDPA) was synthesized by Professor C. F. Koelsch. Samples were degassed by several freeze-pump-thaw cycles before sealing. Samples of C6F6 and C4F8, both gases at room temperature, were prepared by trapping the gas a t 77 K in a degassed mixture of radical, C6H6, and CC14, then sealed. Samples were run at 25°C and 7 5 G (7.5 mT) as described previ~usly.~ Results a n d Discussion Experimental values of UrnF,calculated from observed enhancements by using eq 1 and 3, are given in Table I for each fluorocarbon with the three free radicals. Observed fluorine enhancements for all the samples studied are highly negative, indicating that the coupling is dominated by the intermolecular electron-nuclear dipolar interaction rather than the scalar mechanism. The ultimate fluorine enhancements obtained with the radical GALV are among the most negative ever observed, nearly to the dipolar limit. Typically, ultimate fluorine enhancements with GALV have been in the range from -200 to -250, with the most negative value of -270 obtained with l,l,l-trifluorotrichloroethane,lt4 in which the fluorines are on a saturated carbon atom. These highly dipolar enhancements with GALV reflect the inaccessibility of the free-radical electron in the GALV ~ t r u c t u r e . ~ All polarizations produced by BDPA and D P P H are more positive (greater scalar contribution) than those produced by GALV. For BDPA and DPPH, the enhancements shown in Table I are most negative with cycloalkanes. The D P P H I9F enhancements are slightly more dipolar than the BDPA values for all but one of the molecules in which the fluorines are bound to saturated carbons, including those cycloalkenes with the two unsaturated positions occupied by chlorine atoms. These results agree both in range and radical order with previous aliphatic samples for which comparisons can be drawn.2J Two types of chlorine-substituted species have been inThe Journal of fbysical Chemistry, Vol. 80,No. 3, 1976
322
R. D.Bates, 13. E. Wagner, and E. H. Poindexter
fluorocycloalkenes hove two fluorines attached to nominally sp2 carbons, which may exhibit markedly different enhancements from those of the sp3 carbon fluorines on the Nsame molecule. As a first approximation eq 3 may be used to resolve the two contributions to the enhancements obtained. The fluorine enhancements on these chloro compounds can be used as an approximation of the enhanceGALV ments of the fluorines bound to the saturated carbons on the perfluorocycloalkenes, an approximation supported by the observation that partially chlorinated perfluorocyclobutanes behave analogously with the perfluorocyclobuC(CH313 C(CH313 tanes themselves. The results of this calculation for the four-, five-, and six-member ring perfluorocycloalkenes are Figure 2. Structures of free radicals used in the investigation. shown in Table 11, along with the percent scalar coupling calculated by using eq 2. Despite the large uncertainties inherent in this method of resolving the two contributions to cluded in this study. Results with all three radicals for the the observed enhancements, the percent scalar coupling for two partially chlorinated perfluorocyclobutanes show little the BDPA and DPPH with the 1,2 fluorines is consistent difference from the enhancements obtained for the perfluofor all three species investigated. The BDPA value of aprocyclobutane itself. Thus, for these saturated cyclobuproximately 32% scalar coupling is nearly identical with the tanes, the effect of partial chlorine substitution on the fluoscalar contribution to the polarization of hexafluorobenrine DNP is, if any, below the detectable limit in the curzene, long the standard for comparison for unsaturated flurent set of samples. High-field magnetic resonance studies orine enhancement^.^ The planar BDPA with delocalized have shown considerable differences in the chemical shifts electron has been very versatile in inducing scalar polarizaof the fluorine nuclei of these species. Perfluorocyclobution in aromatic fluorocarbons for which the possibility of tane has a shift of +135.2 ppm relative to CFCl3; 1,1,2,2plane-plane collisions e ~ i s t s . For ~ , ~the ~ ~same aromatics, tetrachlorotetrafluorocyclobutane has a shift of +114.0 ppm relative to CFC13; and 1,2-dichlorohexafluorocyclobu- enhancements with DPPH are typically slightly more dipolar than are BDPA enhancements, as, for example hexaflutane exhibits intermediate values for the different fluorine orobenzene is -2 with DPPH and f 2 0 with BDPA.3 Quite atoms.1° Evidently, the intramolecular mechanisms giving the contrary is observed for the sp2 fluorines on the perrise to the differences in the chemical shifts have little effluorocycloalkenes. Here the DPPH polarizations are more fect on the intermolecular polarization of the fluorine nuthan 50% scalar, and the enhancements which average clei by free radicals. For these fluorinated cyclobutanes, the around +230 are much more positive than any previously fluorines are unaware of the nature of the rest of the moleobserved for 19Fnuclei with DPPH. The perfluorocycloalkcule. ene observed enhancements with DPPH are actually slightIn contrast, the fluorines bound to the saturated carbons ly more scalar than those with BDPA, in contrast to other on the cycloalkenes which have the positions adjacent to fluorine enhancements for those homologous series which the double bond blocked by chlorine atoms have enhancehave been studied: except for the phosphonitrilic fluoments with BDPA and DPPH which are significantly less r i d e ~However, .~ the resolved enhancements due to the sp2 dipolar than found for the corresponding perfluorocycloalfluorines are much more positive than for BDPA. DPPH, kanes. For these chlorocycloalkenes the fluorine atoms one with its localized unpaired electron on the nitrogen atom, is carbon removed from the double bond also show a downfar superior, when compared with BDPA, in polarizing by field shift compared to the perfluorocycloalkane analogues. the contact route the fluorines attached to double-bonded For example, the near fluorines on 1,2-dichlorooctafluorocarbons than it is in polarizing fluorines on aromatic fluocyclohexene-1 lie at +110.6 ppm, and the far at +134.1 rocarbons. The high-field NMR chemical shifts for the ppm, while CsFlz gives +133.0 ppm for all fluorines, all fluorines on the unsaturated carbons show some variation with respect to CFC13.l1 In these chlorocycloalkenes the (CdF6, +130.4 ppm; C5F8, f150.2 ppm; and CsFlo, +151.9 shifts are also comparable to those obtained with the chloppm; all with respect to CFCb), but all are shifted to highrocycloalkanes, but definite differences are observed in the er field than the other fluorine atoms on the same moleDNP results. Previously, an increase in scalar coupling for cule, approaching hexafluorobenzene a t $162.6 ppm.12 molecules containing chlorine atoms has been observed for Again, the intermolecular coupling observed in DNP experseveral types of molecules (31Pin Poc13, l3C in cc14,31Pin iments does not parallel the intramolecular-based chemical phosphonitrilic chlorides), but in these cases, the nucleus shift data. exhibiting the greater degree of scalar coupling has been attached directly to the ~ h l o r i n e . ~ *Increased ~ ~ , ' ~ IH scalar coupling in CHC13 has been observed with the di-tert- butyl Conclusions nitroxide radical, but this has been attributed to the tendency of the proton to hydrogen bond to the nitroxide, The interactions of all cyclic fluorocarbons studied with rather than the effect of the chlorine atom.20~21 In the galvinoxyl free radical are dominated by the dipolar coupling of the fluorine nuclei with the free-radical unpaired present study, the polarized fluorines observed are several electron. atoms removed from the chlorines, yet the fluorine enhancements are markedly more scalar for BDPA and Mrith BDPA and DPPH, fluorine enhancements vary DPPH than in the corresponding perfluorocycloalkanes. considerably with the nature of the fluorocarbon. For the perfluorocycloalkanes, enhancements are predominantly The BDPA and DPPH enhancements are more positive dipolar, with little variation evident for the four-, five-, and for the perfluorocycloalkenes than for the corresponding six-member ring species studied. Partial chlorination of 1,2-dichloroperfluorocycloalkene-1compounds. These perBDPA
DPPH
The Journal of Physical Chemistry, Vol. 80, No. 3, 1976
I9F
Polarization in Fluorinated Cyclic Alkanes and Alkenes
323
TABLE I: Ultimate Fluorine Enhancements from DNP Measurements for Cyclic Fluoroalkanes and -alkenes with the Free Radicals GALV, BDPA, and DPPH _____l_____l___.
_ _ _ _ _ I I _ I _ _ _
___I___
U, F
Formula
Compound ___Perfluorocyclobutane 1,2-Dichlorohexafluorocyclobutane
GALV
BDPA
DPPH
-__l________l__ I _ -
C4Cl4F4 "laZF4
SC1
6'
1,1,2,2-Tetrachlorotetrafluorocyclobutane
Perfluorocyclobu tene 1,2-Dichlorotetrafluorocyclobutene-1 Perfluorocyclopentene
C4F6
2 F6
1,2-Dichlorohexafluorocyclopentene-1
Perfluorocyclohexene
F, 0
C6C12F8
1,2-Dichlorooctafluorocyclohexene-1
C6F1'2
Perfluorocyclohexane
-264 i -270i -251 i -237i -2522 -261 i -266 i -262i -240 i -265i:
7 5 18 5 6 5 4 5 4 14
-193 t -171 .t -1771 -96t -1435 -118 t -157 F -1371 -158 F -213 i
7 2 3 1 9 2 9 4 9 7
-182i 7 -193 i 9 -220i 8 -35t 1 - 1 6 4 ~6 -711 3 -179 + 8 -86i- 3 -165i 6 -227 i 11
TABLE 11: Ultimate Enhancements and Percent Scalar Coupling of "F Nuclei Bound to Unsaturated Carbons in Perfluorocycloalkenes by Correcting Observed Ultimate Enhancement for 19F Nuclei Bound to Saturated Carbons on the Molecule Species measd
Species used for cor
C4F6
C5F8 C6F10
5C1,F6
c6c12
U,F(BDPA)
% C-BDPA
U,F (DPPH)
% C-DPPH
-2+ 21 -1 + 34 - 5 3 ~56
33.2 33.6 28.1
+223 i 1 5 +253 + 14 +230i 39
56.1 59.2 55.9
perfluorocyclobutanes gives no change in fluorine enhancements. Perfluorocycloalkenes show marked increase in I9F scalar coupling when compared with the saturated analogues. Resolution of the signal component resulting from the fluorines bound to the unsaturated carbons shows that the enhancements for these fluorines are more positive with DPPH than for other 19F nuclei that have been examined previously. This reversal of positions of BDPA and DPPH with respect to largest scalar contribution in polarizing 19F nuclei has also been observed for the phosphonitrilic fluorides, another series of predominantly nonplanar, unsaturated ring corn pound^.^^^^^^^ The fluorines out of the plane of the double bond in the 1,2-dichloro-substitutedperfluorocycloalkene-1 compounds show increased fluorine scalar coupling as well with both BDPA and D P P H when compared to the corresponding perfluorocycloalkanes. The trends observed in the polarization of fluorine nuclei by intermolecular coupling with the unpaired electron of the free radicals do not correlate with observed chemical shifts from high-resolution NMR experiments. Moreover, studies of the radical-induced fluorine relaxation showed no unusual behavior for these samples. The intermolecular interactions studied by DNP are a sensitive probe of the environment of the nucleus which is exposed to other molecules in the system, and are not necessarily reflections of the inner electronic structures of the molecule itself. Thus, DNP can serve as a useful means of investigating the intermolecular dynamics without the necessity of detailed concern with minor variations in the nature of the individual molecule. Acknowledgment. The authors gratefully acknowledge the help of Anthony J. Montedoro in sample preparation and in operation of instrumentation, and the aid of Ruthann I. Bates for critically reading the manuscript. One of the authors (R.D.B., Jr.) acknowledges the Do-
nors of The Petroleum Research Fund, administered by the American Chemical Society, and AROD, under Grant No. DAHC04-75-G-0042, for partial support of this work.
References and Notes (1) E. H. Poindexter, J. 8. Stewart, and P. J. Caplan. J. Chem. Phys.. 47,
2862 (1967). (2)J. R. Stewart, E. H. Poindexter, and J. A. Potenza, J. Am. Chem. SOC., 89, 6017 (1967). (3)J. A. Potenza and E. H. Poindexter, J. Am. Chem. Soc., 90, 6309 (1968). (4)J. Potenza, Adv. Mol. Relaxafion Process, 4, 229 (1972). (5) B. E. Wagner, J. N. Helbert, R. D. Bates, Jr., and E. H. Poindexter, Chem. Commun., 748 (1973). (6)B. E. Wagner, R . D. Bates. Jr., and E. H. Poindexter. lnorg. Chem., 14, 256 (1975). (7)W. Muller-Warmuth and A. Yalciner, Ber. Bunsenges. Phys. Chem., 75, 763 (1971). (8)R. A. Dwek. N. L. Paddock, J. A. Potenza, and E. H. Poindexter, J. Am. Chem. SOC.,91, 5436 (1969). (9)E. H. Poindexter, R. D. Bates, Jr., N. L. Paddock, and J. A. Potenza, J. Am. Chem. SOC.,95,1714 (1973). (10) J. Feeney, L. H. Sutcliffe, and S. M. Walker, Mol. Phys., 11, 117 (1966), and references cited therein.
(11)J. Feeney, L. H. Sutcliffe, and S.M. Walker, Mol. Phys., 11, 137 (1966), and references cited therein. (12)V. W. Gash and D. J. Bauer, J. Qrg. Chem., 31,3602 (1966). (13)E. H. Poindexter, P. J. Caplan, 8. E. Wagner, and R. D. Bates, Jr., J. Chem. Phys., 61,3831 (1974). (14)R. D. Bates, Jr., E. H. Poindexter, and B. E. Wagner, J. Chem. Phys.. 59,
3031 (1973). (15)J. A. Potenza and J. W. Linowski, personal communication. (16)J. Hildebrand, J. M. Prausnitz. and R. C. Scott, "Regular and Related Solutions", Van Nostrand-Reinhold, New York, N.Y.. 1970,p 213. (17)R. L. Scott, J. Am. Chem. SOC.,70, 4090 (1948). (18)R. D. Bates, Jr.. B. E. Wagner, and E. H. Poindexter. Chem. Phys. Lett., 17, 328 (1972). (19)J. A. Potenza. E. H. Poindexter, P. J. Caplan, and R . A. Dwek, J. Am. Chem. SOC.,91, 4356 (1969). (20)R. D. Bates, Jr., E. H. Poindexter, and B. E. Wagner, 166th National Meeting of the American Chemical Society, Chicago, Ill., Aug 1973,No. Phvs. 3. (21)K. Endo, B. Knuettel, I. Morishima, T. Inubushi, and T. Yonezawa. Chem. Phys. Lett., 31, 387 (1975). (22)P. J. McQuillin, "Alicyclic Chemistry", Cambridge University Press, London 1972 74.. -, n (23)C. H. Chang, R. F. Porter, and S. H. Bauer, J. Mol. Struct., 7, 89 (1971).
The Journal of Physical Chemistry, Vol. 80, No. 3, 1976
