200
R. H.Cox, L. W. Harrison, and W. K. Austin
drated by a monolayer, whereas the lower field line, which is orientation independent, is due to Cu(II) hydrated by two or more 'layers of water. It was possible, however, to investigate the nature of the hydrated copper(1l) ions when the interlamellar regions of Cu(I1) hectorite were fully expanded by soaking the silicate in water for 48 hr. Under these conditions the 001 X-ray reflection corresponded to an interlamellar thickness of about 10 A and the esr spectrum of an oriented film sample consisted of a single isotropic line with g =
2.192, independent of its position with respect to H. Thus when several layers of water are present the Cu(l1) ion tumbles rapidly, averaging the g , a n d g l components. Acknowledgment. The support of the National Science Foundation through Grant No. GP-33878 is gratefully acknowledged. The authors wish to thank Dr. Roger V. Lloyd for many helpful discussions and for his assistance in obtaining the Q-band spectrum of the Libby Cu(II) vermiculite.
on A s ~ ~ c ~ffects a t on ~ ~the~ Nuclear Magnetic Resonance Parameters of Aromatic ions. 111. Cyc oiionatetraenyl Anion, Cyclooctatetraene Dianion, and Tropyliium Cation' ~i~~~~~ H. Cox,* Lester W. Harrison, and Walter K. Austin, Jr. Repnrtment of Chemistry, University ofGeorgia, Athens, Georgia 30601 (Received August 21, 1972) Publica'jon costs assrsted by the Petroleum Research Fund
The effect of counterion, solvent, and temperature on the proton nmr spectrum of the cyclononatetraenyl anion, cyclooctatetraene dianion, and tropylium cation have been investigated. In DME and THF, the lithium and sodium salts of the cyclononatetraenyl anion exist as an equilibrium mixture of contact and solvent-separated ion pairs whereas the potassium, rubidium, and cesium salts exist only as contact ion pairs. Salts of cyclooctatetrene dianion exist as contact ion pairs in all solvents examined. Lithium-7 nmr shifts support this view. The nmr spectrum of several tropylium salts shows the chemical shift of the tropylium cation to be independent of counterion but solvent dependent. These results suggest that the tropylium salts exist in solution as free solvated ions and solvent-separated ion pairs.
Introduction Several recent investigations have led to a better understanding of the physical and chemical properties of organoalkali metal salts in ether solution.2-10 Of particular importance has been the finding that some carbanions exist in solution ab ia rapidly equilibrating mixture of two types of ion pairs (contact and solvent separated), whose ratio depends upon the counterion, solvent, and temperat ~ r e This . ~ concept has been firmly established experimentally and has bean used to explain a wide variety of previous data on carbanion solution^.^^-^^ Further work bas shown that this ion pairing scheme must be expanded to include at least two different types of contact ion pairs in equilibrium with the solvent separated ion pair.sJO These two types of contact ion pairs are thought to exist in different solvataon states. Other things being equal, the major factor determining the fraction of solvent-separated ion pairs formed in a series of aromatic anions appears to be the area over which the negative charge may be delocalized. Since it has been previously reported that the cyclopentadienyl anion forms only contact ion pairs with alkali metal cations in ether solvents,14 it appeared of interest to determine the ring size necessi3ry in an annulene anion in order to have formation of solven1,-separated ion pairs. Furthermore, from previous investigations, it appeared that the effect of solvent1 and concentration1* on the chemical shifts of ion The Journai of Physical Chemistry, Vol. 77, No. 2, 1973
pairs might be different for contact and solvent-separated ion pairs. Since all protons are equivalent in the annulene anions, investigation of these ions permits a larger concentration range to be examined than with previous examples. We report here the results of our proton and lithium-7 nmr investigation of the ion pairing of the cyclononatetraenyl anion (CNT-) and the'dianion of cyclooctatetraene (COT2-) in various ether solvents. The results suggest that it is possible to have both contact and solvent-separated ion pairs present in solutions of CNTwith the proper choice of solvent and metal ion. Salts of (1) For part I i of this series see R. H. Cox, Can. J. Chem., 49, 1377 (1971). (2) T. E. Hogen-Esch and J. Srnid, J. Amer. Chem. Soc., 88, 307, 316 (1966). (3) L. I. Chan and J. Srnid, J. Amer. Chem. Soc., 90, 4654 (1966). (4) T. Eilingsen and J. Smid, J. Phys. Chem., 73,2712 (1969). (5) T. E. Hogen-Esch andJ. Smid, J. Amer. Chern. Soc., 87, 669 (1965). (6) R. V. Slates and M. Szwarc, J. Amer. Chem. SOC., 89, 6043 (1967). (7) D. Nlchoils, C. Sutphen, and M. Szwarc, J . Phys. Chem., 72, 1021 (1968). (8) N. Hirota, J. Amer. Chem. SOC., 90, 3603 (1966). (9) N. Hirota, R. Carraway, and W. Schook, J. Amer. Chem. SOC., 90, 3611 (1968). (10) J. W. Burieyand R. N. Y0ung.J. Chem. Soc. €3, 1016 (1971). (11) M. Szwarc, "Carbanions, Living Polymers and Electron Transfer Processes," Interscience, New York, N. Y., 1968. '(12) M. Sswarc, Accounts Chem. Res., 2, 87 (1989). (13) J. F. Garst, "Solute-Solvent interactions," J. F, Coetzee and C. D. Ritchie, Ed., Marcel Deckker, New York, N. Y.. 1969, p 539. (14) J. E. Grutzner, J. M. Lawlor, and L. M. Jackrnan, J. Amer. Chem. SOC.,94, 2306 (1972).
Ion Associatior. Effects of Aromatic Ions
COT2- form only contact ion pairs in all solvents examined. Secondly, as a complement to the above studies, we report the effect of solvent and anion on the proton nmr spectrum of the trcpylium cation (C7H7+). Experirnenital Section Lithium salts of CNT- were prepared by allowing lithium metal to react with 9-chlorobicyclo[6.1.O]nonatriene according to the method of Katz and Garratt.15 Likewise, the remainder of the alkali metal salts of CNT- were prepared from 9-methoxybicyclo[6.1.0]nonatriene.15Salts of COT2- were prepared by the two-electron reduction of freshly distilled cyclooctatetraene using the appropriate alkali metaLL6Tropylium hexafluorophosphate and hexachloroantimonate were commercial samples (Cationics, Inc.) and were used without further purification. Tropylium fluorobocate aind perchlorate were prepared from cycloheptatriime using standard literature pr0~edures.l~ The remainder of the tropylium salts were prepared from tropylium fluoroborate using exchange reactions.18 Physical properties of these salts were identical in all respects with those previously reported. All anion samples were prepared using high-vacuum techniques. Ether solvents were stored under vacuum with sodium benzophenone ketyl and were distilled on the vacuum line i m K ~ e ~ i ~ prior ~ e l y to use. Solutions were prepared in the previously described "onion dome" nmr celIslg to a cmcentration of 0.1 F , TMS was added as an internal reference and lock signal source, and the nmr cells were finally sealed under vacuum. The samples tulles were inverted and the solutions were allowed to remain in contact with the alkali metal mirror until formation of the respective ions, as evidenced by the disappearance of the nmr signals for the starting materials and the appearance of a new singlet characteristic of the respective ion. A t this point the nmr spectra were consistent with the formation of only one species. All spectra were obtained shortky after formation of the ion. Samples of the tropylium salts were prepared gravimetrically to ithe appropriate concentration (0.05 F ) using freshly distilled spectrograde solvents. TMS (ea. 3%) was added as an internal reference and lock signal source for the nonaqueous solutions. External TMS was used for the aqueous soiutions and the chemical shifts were corrected for diamagnetic susceptibility differences. Proton spectra were recorded on a Varian Associates -100 spectrometer operating in the field-sweep mode with a probe temperature of 29". Line positions were obtained by utilizing- the frequency difference network with spectra run oar a 5 0 - w ~sweep width with a sweep time of 1000 sec. Low temperature spectra were recorded on a Hitachi R-20 spectrometer operating at 60 MHz. Line positions were caiibrated by the side-band technique. Lithobtained on the HA-100 spectrometer ETR mode. The reported shifts are the average of four alternate upfield and downfield scans calibrated by the side. band technique using an external reference of 1.0 FA ~ ~ U ~ Qlithium U S chloride. Results and ~ ~ $ ~ ~ s ~ ~ ~ o Cation Effects. The chemical shift of CNT- obtained with several alkali metal cations and two solvents i s given in Table 1. It 1s clear from the data that the shifts are dependent on both the cation and solvent. For CNT- in THF solution, the protons are shielded in the order Rb > Cs > I,i > N a 3 K. In DME the order is Li > Na > Cs >
201
TABLE I: Chemical Shifts of Cyclononatetraenyl Anion as a Function of Solvent and Cation Cation
THFapb
DME
Li Na K Rb
6.931 6.971 7.027 6.923 6.924
6.773 6.904 7.042 7.029
CS
nThe formal concentration is 0.1 M. TMS.
6.945
* In ppm downfield from internal
Rb > K. Although the over-all variation in chemical shift with cation is small, the changes are nevertheless outside the range of experimental error. We have previously shown that varying the cation with a given anion and solvent results in perturbations of the electron density on the anion such that the larger the cation, the larger the effective negative charge on the anion.lJO If contact ion pairs are formed, the chemical shifts of the anion will be at highest field with cesium and at lowest field with lithium. Deviations from this order are observed if a significant fraction of solvent-separated ion pairs is formed. This method has recently been applied by others to study the ion pairing in a variety of anion systems.14,21,22 Thus, on the basis of the data in Table I for CNT-, it appears that this system forms both solvent-separated and contact ion pairs.23The data suggest2* that predominantly contact ion pairs are formed in THF solution with cesium, rubidium, and potassium as the cation and that a significant fraction of solvent-separated ion pairs are formed with lithium and sodium as the cation. A larger fraction of solvent-separated ion pairs i s formed with lithium than with sodium consistent with previous investigaThe data in DME are similar to those in THF with the exception that a larger fraction of solvent-separated ion pairs are formed with lithium and sodium in DME. Similar results have been observed in previous investigations and are thought to be related to the cation solvating ability of THE' and DMES2y5 Variable temperature experiments support the above conclusions. On lowering the temperature to -48", there is essentially no change in the chemical shift of the Iithium salt of CNT- in DME. However, the proton absorption of the lithium salt of CNT- in THF moves upfield upon lowering the temperature to -48" and becomes almost identical (6.776 ppm) with that in DME at 29" (Table I). The proton shift of the sodium salt of CNTalso moves upfield in both THP (6.927 ppm) and DME (6.774 ppm) upon lowering the tem erature to -48". The (15) T. J. Katz and P. J. Garratt, J. Amer. Chem. SOC.,86, 5194 (1964). (16) T.J. Katz, J. Amer. Chem. SOC., 82, 3784,3785 ('1960)., (17) K. Conrow, Ofg. Syfl., 43, 101 (1963). (18) H. J. Dauben, L. R. Honner, and K. M. Harmon, J. Org. Chem., 25, 1442 (1980). , (19) R. H. Cox, E. G . Janzen, and J. L. Gerlock, J , Amer. Chem. Soc,, 90, 5906 11968). (20) R. H. Cox, j . Phys. Chem., 73, 2649 (1969) (21) ~ J. W. Burlev. R. Ife, and R N. Youna, Chem. Commun.. 1256 (1970). (22) V. R. Sandel and F. J. Kronzer, Abstracts of papers presented at the 162nd National Meeting of the American Chemical Society, Washington, D. C., Sept 12-17, 1971, No. O R G N 81 123) I t is interesting to note that the uv maxima are essentialiy the same for both lithium and potassium cyclononatetraenide in THF,15 indicating that it would not be possible to examine the ion pairing in this system using uv spectroscopy since the maxima are identical for the contact and solvent-separated ion pairs. -
\
The Journal of Physical Chemistry, Val. 77, No. 2, 7973
R. H. Cox, L. W. Harrison, and W . K. Austin
202 TABLE I I: Variation of the Chemical Shifts of Cyclooctatetraene Dianion with S801ventand Cationa Cation I _
Solvcn?
Ether Dioxane %Methyltetrahydrofuran Tetrahydrofuran Bimiethoxyethane Diglyme
VHsc
VHnC
Li
Na
cp"!
Rb
@'Lid
5.694
4-10.07 +9.42
5.695
5.841 5.812 5.785
5.702 5.726 5.797 5.710 5.773 5.844
5.754 5.773
5.700 5.747
4-8.55 4-9.21
5.716
5.756
UH'
5.697 5.914 5.885 5.720 5 744
U H , ~ K
aThe formal concentration is 0.1 M. In ppm downfield from internal cal shift of cyclocictatetraene (0.1 M ) in ppm downfieid from internal TMS. Chemical shift of cyclooctatetraene dianion in ppm downfield from internal TMS. Lllhium-7 chemical shift of 2Li+, COT2- in pprn upfieid from external aqueous 1.0 M LiCI.
fact that the chemical shift of the lithium salt of CNT- in DME exhibits no change on lowering the temperature suggests that 1 he fraction of solvent-separated ion pairs present at rooni temperature is essentially 100%. At room temperature, the fraction of solvent-separated ion pairs decreases in the order Li+ CNT- (DME) > Na+ CNT(DME) > LI+ CNT- (THF) > Na+ CNT- (THF). These low-temperature data indicate that the fraction of solvent separated ion pairs present at -48" approaches 100% for N a + CNT- in. DME and Li+ CNT- in THF. Since the chemical shift for Na+ CNT- in THF a t -48" is only slightly upf ielcl. from the shift at room temperature, it must be cortcluded that this system still contains an appreciable fraction of contact ion pairs at -48". The results for the effect of cation on the nmr spectrum of CNT- clearly show that solvent-separated ion pairs are formed. Thiv [!)]annulene anion however must be taken as an upper limit for the ring size needed for the formation of solvent separated ion pairs since the cycloheptatrienyl anionz4 is not stable enough to permit investigation. The fact that CNT - forms solvent-separated ion pairs whereas eyclopentadren yl anion forms only contact ion pairs14 is consistent with previous experiments concerning the relative stability O~ these two ions. Equilibrium studies of the reaction of lii+ CNT- with cyclopentadiene suggests that CNT- is more thermodynamically stable than the cyclopentadienyl ~ K ~ O I ISince . ~ ~ the type of ion pairs formed in solution depends upon the results of two opposing forces,ll Coulonnbic attractions between cation and anion us. solvation of the alkali metal cation, it is quite reasonable that with a more stable anion, the Coulombic attractions would be rciduced such that solvation of the cation becomes the dam nant factor. The data are given in Table I1 for the effect of cation and solvent 0x1 the nmr spectrum' of COT2-. In diethyl ether, the protons are shielded with cation in the order Rb M :* Na LB. Assuming that shift correlations obtained from rrkonovalent anionsz0 will also apply for dianions, these data indicate that only contact ion pairs are formed in diethyl ether. However, in both THF and DME, the relative order of proton shielding is Rb > Li > K > Na. On the basis of previous trends,l4Yz0this would indicate that contact ion pairs are formed with Rb, IC, and Na and that a srriall fraction of solvent-separated ion pairs are formed with Li i ~ both i THF and DME. The results for the lithium salts of COT2- are not entirely consistent
-
>#
The Journal o f Phyricai Chemistry, Vol. 77,
No. 2, 7973
in that the chemical shift is further downfield in DME than in THF. Previous investigations of monovalent ion pairs where there is rapid equilibrium between contact and soIvent-separated ion paics have consistently shown that a larger fraction of solvent-separated ion pairs are formed in DME than in THF.2,3 This has usually been attributed to the bidentate coordination o f DME.ll Therefore, if solvent-separated ion pairs are formed with the dilithium salts of COT2-, it would be expected that the chemical shift of COT2- should be further upfield in DME than in THF, similar to the data for CNT-. Of the several types of ion pairs possible for the COT2system, the following are considered here as being the most important: (a) both cations bound to the cation as tight or contact ion pairs, (b) one cation bound tightly and the other bound loosely with a layer of solvent separating it from the dianion, (c) both cations separated from the dianion by a layer of solvent (solvent-separated ion pair), and (d) free ions. Clearly, the data on COT2- for the dirubidium, dipotassium, and disodium salts in THF and DME as well as all the salts in diethyl ether indicate ions of type a. Since the data for the dilithium salts in THF and DME do not allow a clear distinction to be made as to the types of ion pairs formed, we have obtained the lithium-7 chemical shifts for further inforrnation (Table 11). The large upfield shift observed for the lithium-7 chemical shift of the dilithium salt of COT2- suggests that the lithium cations are located above and below the T cloud of the dianion in a sandwich arrangement similar to that found for other aromatic Free ions (d) can immediately be ruled out on the basis of the large upfield shifts.28 Furthermore, contact ion pairs (a) would appear to be ruled out on the basis of the range of lithium-7 shifts with solvent (1.52 ppm). In previous investigations of monovalent contact ion pairs (cyclopentadienyl and 1phenylallyl systems) the variations in the lithium-7 chemical shift with solvent has been within 0.3 ppm.14 While the shifts with solvents are in the expectfed order, the range is not large enough to indicate solvent-separated ion pairs (c) in THF and DME. A reasonable explanation is that we are observing a situation closely resembling b or that we are observing an equilibrium between two types of contact ion pairs.8J0 Although we are not able to distinguish between these two possibilities, we prefer the latter since previous investigations of the electronic spectra of the lithium salts of some aromatic dianions in MeTHF have been interpreted in terms of contact ion pairs? Solvent Effects. Previous investigations have shown that cation solvation plays a dominant role in the physical and chemical properties of organoalkali systems in ether s o l ~ t i o n s . l ~It- ~is~ thought that the average interionic 'distance in both solvent-separated and contact ion pairs changes with the nature of the solvating medium. It has been suggested that, with a given contact ion pair, changing to a more polar solvent should result in spectral changes similar to those produced by changing to a larger cation.ll A more polar solvent should solvate the cation H. J. Dauben, Jr., and M. R . Rifi, J. Arner Cherrr. Soc., 85, 3041 (1963). E. A. LaLancette and R. E. Benson, J. Amer. Cherrr. Soc., 87, 1941 (1965). R. H. Cox, H. W. Terry, Jr., and L. W.Harrison, J. Arner. Chem. SOC.,93, 3297 (1971). J. A. Dixon, P. A. Gwinner, and D. C. i i n i , J. Amer. Chenr. SOC., 87, 1379 (1965). The lithium-7 shift of a solvated free lithium ion is close to that of aqueous lithium chloride.
Ion Association Effects of Aromatic Ions
better, resulting in an increase in the average interionic distance due to a weakening of the Coulombic attractions between cation and anion. Thus, changing to a more polar solvent should produce an upfield shift in the nmr spectrum of a contact ion pair or, in the case of visible and uv spectra, should result in a shift of Amax to longer wavelengths. This prediction has been verified in the case of visible and uv spectra.2-6,loConsideration similar to the above when applied to a system where there is rapid equilibrium betweren contact and solvent-separated ion pairs also suggests that the proton chemical shifts should move upfield with increasing solvent polarity due to an increase in the fraction of solvent-separated ion pairs.20 This has also been adequately borne out by experiment.1,14J5q20 All previous investigations11,13 have suggested the following order for the solvation of an alkali metal cation: DG > DME 3 THF > MeTHF > D O > DE.29 However, it is interesting that the effect of solvent on the chemicail shift of a given salt of COT2- does not follow the expected trends. The dipotassium salt of COT2forms only contact ion pairs and the proton chemical shifts of this salt in several ether solvents are given in Table 11. The chemical shift appears to higher field with solvent in the order 7°F > DG > DME > MeTHF > DO > DE. This same relative order is also observed for the disodium, dirubidiunn, and dilithium salts of COT2- (Table 11).Thus, thLe order of the shift in DG and DME appear to be exceptions oased 5n previous investigations.loJ2 However, several 01 her examples have been reported where the proton chemical shift of an anion which forms only contact ion pairs are downfield in DME with respect to THF. These include che potassium and rubidium 4,5-methyleneand 9,10-dihy(ir0-4,5-methylenephenanthrenides,~ cesium l,riphenylmethanide, rubidium and cesium fluorenide, potassium, rubidium and cesium indenide, lithium cyclopentadienide,14 and potassium, rubidium, and cesium cyclononatetramide (Table I). Similarly, in lithium-7 nmr studiesz6 of the cyclopentadienyl and indenyl anions, this same trend is observed One possible explanation for the observed solvent order is that it is due to the existence of two types of contact ion pairs which differ by the degree of cation solvationsJO and thus, the interionic distance. This is not entirely satisfactory, however, since the interionic distance in DME would be expected to be larger than that in THF. Another possibility is that the ion pairs exist as higher aggregates rather than as discrete ion pairs. The close proximity of the aromatic aiions in the aggregated ion pairs could affect the nrnr paramecer observed for such systems due to the ring currents in the aromatic anion. Aggregation has been suggested to explain the downfield shifts of other aromatic aniondd with decreasing concentrations. Changing the concentration by a factor of 10 ( 5 X to 5 X 10-I) of 2LI+, C O ' P - in TWF and DME does produce small (-3 Hz) changies in the chemical shift toward lower fields with deci*easing concentration. Although this offers a seasonable explanation, a firm decision must await the examination of a carbanion system which is known to be monomeric in 1°F and DME solutions. Probably a better explanation is that two TWF niolecules are able to disperse the positive charge on the cation better than one DME molecule in the contact ion pair. In the first case the positive charge is stabilized by two molecules whereas in the latter case the positive charge on the cation is stabilized by two :itoras within the same molecule. Energetically, this would lead to a larger interionic distance in the ~
203
TABLE I l l : Chemical Shiftsa of the Tropylium Cation as a Function of Solvent and Anionb Anion Solvent
Br
CHzClz CH3C(O)CH3 CH3CN CH3N02
9.162 9.541 9.286 9.398 9.340
DMSO D20 DMF
9.553
I
9.272 9.361 9.209 9.577
Cion
BF4
9.560 9.256 9.360 9.353 9.226 9.548
a In ppm downfield from internal TMS. salt was 0.05 F.
IJ
9.381 9.338 9.338 9.523
SbC16 PFs I . 9.601 9.250 9.364 9.341
9.552 9.252 9.348 9.332
The concentration of tropylium
contact ion pair in THP compared with DME consistent with the results.
/7 +OhMdO+
Tropylium Cation. The investigation of the nmr spectrum of the tropylium cation was frustrated by both the solubility and stability of several of the salts. Nevertheless, meaningful data were obtained for the fluoroborate, perchlorate, bromide, iodide, hexachloroantimonate, and hexafluorophosphate. The proton shift of the tropylium cation as a function of the above anions and several solvents is given in Table 111. The shift of the fluoroborate as a function of solvent has been reported p r e v i ~ u s l ywhile ~~ this work was in progress and agrees with the results reported here. In contrast to the results obtained previously for carbanions,20for a given solvent, the chemical shift of the tropylium cation is not dependent to any significant extent upon the counterion. However, the shift for a given counterion exhibits a relatively large solvent dependence (-0.3 ppm). Previous results with carbanion^^^^^^^^ ,20 and ammonium salts32 have shown the chemical shift of contact ion pairs to be a function of counterion. Therefore, the fact that the shift of the tropylium cation is independent of anion suggests that these tropylium salts are either in the form of solvent-separated ion pairs and/or dissociated into free solvated ions. Previous conductance studies support this view. Tropylium hexachlorantiornonate has been shown to be largely dissociated in methylene chloride solution.33 Similar results34 were obtained for tropylium bromide in sulfur dioxide. Using these dissociation constants with the concentration used in this investigation, a large degree of dissociation is expected. However, it is not possible to determine the per cent dissociation from the present nmr data. The solvent dependence of the shift (Table 111) is probably due to cation solvation. Varying the concentration of the fluoroborate from 0.2 to 0.01 M in water and nitromethane results in only small changes (--I Hz) suggesting (29) DG = diglyme, D M E = dimethoxyethane, THF = tetrahydrofuran, MeTHF = 2-methyltetrahydrofuran, DO = :,ri-dioxane, and DE = diethyl ether. (30) T. Schaefer and W. G. Schneider, Can. J. Chem., 41,966 (1963). (31) T. G. Beaumont and K. M. C. Davis, J. Chem. Soc. 5, 592 (1970). (32) R. P. Taylor and I. D. Kuntz, Jr., J . Amer. Chem. Soc., 92. 4813 (1970), and reference therein. (33) P. M. Bowyer, A. Ledwith, and D. C. Sherringtan, J . Chem. Soc. 6, 1511 (1971). (34) N. N. Lichtin and P. Pappas, Trans. N. Y . Acad. Sci., 143 (1958).
The Journal of Physical Chemistry, Vol. 77, No. 2, 7973
V. Vitagliano, L. Costantino, and A. Zagari
204
that the shifta are pot due to ion association. Furthermore, the fact that there is no correlation of the shifts with dielectric constant of the solvent tends to rule out reaction field effects as the causative factor.35 Each of the solvents used contains an atom (oxygen or nitrogen) with unshared electrons which could help stabilize the positive charge thru coordination of the polar end of the solvent with the T cloud of the tropylium cation. Such solvation could influence the proton shift thru charge density and electric field effects.
ConcludingRelrnairks The results of this investigation clearly show that both contact and solvent. separated ion pairs are possible in ether solution of the cyclononatetraenyl anion. The percentage of each type of ion is a function of cation, solvent, and temperature. Only contact ion pairs are indicated for ether solutions of the cyclooctatetraene dianion. The data obtained for the tropylium cation suggest that salts of this ion exist in solution as either solvent-separated ion pairs and/or dissocialed, solvate free ions.
Finally, the need to exercise caution in using proton nmr chemical shifts to determine electron densities should be stressed. For example, with cyclononatetraenyl anion in DME, the shift varies by -0.3 ppni with cation. This dependence leads to a 2.7 ppm variation in the shift per electron used in determining electron densities.30 A similar variation of 0.3 ppm is observed for the tropylium salts with solvent leading to a range of 2.1 ppm for the shift per electron. These observations tend to rule out the earlier suggestionz0 that the dependence of the chemical shifts of an aromatic ion with counterion is a function of electron density effects and lends further support to the suggestion that these shifts are due to electric field effects.14
Acknowledgment. Support of this work by a grant from the Petroleum Research Fund administered by the American Chemical Society is gratefully acknowledged. (35)
A. D. Buckingham, T. Schaefer, and W.G . Schneider, J. Chern. Phys., 32, 1227 (1960); A. D.Buckingham, Can. J. Chem., 38, 300 (1960).
etween Acridine Orange and Poly(styrenesu1fonic acid) M. Yitagliano,” L. Costantino, and A. Zagari is:ituh? Chimico, Universita degli Studi di Napoli, Naples, ltaiy (Received March 7, 7972)
The interaction between Acridine Orange and poly(styrenesu1fonic acid) has been studied spectrophotometrically. The experimental results can be interpreted by an “Ising-type” model which accounts for the adsorption of dye molecules along an infinite linear chain of fixed adsorption sites. The effect of temperature and added salt on the Acridine Orange-poly( styrenesulfonic acid) has been also investigated.
Introduction The peculiar behavior of some ionic dyes when bound to a polyelectrolyte matrix has long been recognized.2-* Bradley and Wolfs interpreted the adsorption spectra changes of these dyes in terms of concentration variations along a polymeric chain having fixed adsorption sites. These authors studied the interaction between Acridine Orange (AO) and a number of biological polyelectrolytes including DNA. They suggested an expression describing the dye distributiori along the polyelectrolyte chain which takes into account the tendency of the bound dye to aggregate, th,at is7 its “stacking tendency,” in which a first neighbor interaction parameter was introduced. Several papers have been subsequently published on the interaction between biological polymers and acridine dyes.fi The interest in these dyes has been promoted by their mutagenic effect on nucleic acids,? so that several attempts have been made to interpret their binding to them. Papers on the interaction between dyes and synthetic polyelectrolytes can also be found in literature.6J+16 Synthetic polyelectrolytes generally exhibit a very high stacking tendency for the dye, so that dilution of the dye on the polymer matrix appears only at extremely high The Jour@ 01 Physical Chemistry, Vol. 77, No. 2, 1973
values of the ratio between the available binding sites concentration and the dye concentration (i. e., PID ratio).8JO In this respect the behavior of polyjstyrenesulfonic (1) This research has been carried on with the financial support of Italian C.N.R. (2) E. Rabinowitch and L. F. Epstein, J. Amer. Chern. SOC.. 63, 69 (1941). (3) L. Michaelis and S. Granick, J. Arner. Chem. Soc., 57, 1212 (1945), (4) L. Michaelis, J. Phys. Chern., 54, 1 (1950). ( 5 ) D.F. Bradley and M. K. Wolf, R o c . Nan. Acad. Sci. U. S., 45, 944 (1959). (6) For a wider literature see ref 16 and 24. (7) S. Brenner, L. Barnett, F. H. C. Crick, and A. Qrgei, J. Moi. Bioi., 3, 121 (1961). (8) G. Barone, R. Caramazza, and V. Vitagliano. Ric. Sci., 32 (11-A), Vol. 2. No. 6. 554 (1962). (9) R . Caramazza, L. Costantino. and V. Vitagliano, Ric. Sci., 34 (11-A), Vol. 4, No. 1, 67 (1964). (10) G. Barone, V. Crescenzi, F. Quadrifoglio, and V. Vitagliano, Ric. Sci., 36, 503 (1966). (11) V. Crescenzi, F. Quadrifoglio, and V. Yitagiiano, J. Macrornoi. Sci., Chem., 1,917 (1967). (12) W. H. J. Stork, Thesis, University of Leiden, 1970. (13) G. Schwarz, S. Klose, and W. Balthasar, Eur. J. Biochern.. 12, 454 (1970). (14) G. Schwarz and W. Balthaser, fur. J. Biochern., 12, 461 (1970). (15) B. C. Myhr, Biopolymers, 10, 425 (1971). (16) V. Vitagliano and L. Costantino, J. Phys. Clrem., 74, 197 (4970).
