J. Phys. Chem. 1991, 95,6701-6706 6701

for H202 removal, the actual rate constant is immaterial because H202 will come to a steady-state concentration after a few pulses. Since its yield is 21% as large as the reducing radical pool, it will contribute 21% of the total reaction rate. From the above reactions

k3h = kl4[NH2'] + 2klS[H'l + kI8[%<1

which is slightly dependent on [H']/[e,-1, but the average of 2kls and k18 is 2.1 X 1O'O M-' s-'. Most of the work reported here was done at a total radical concentration of 7 X 10-8 M, and from the yields given earlier [NH,'] was 3.2 X 10-8 M and [H'] + [%-I was 3.8 X 10-8 M, so k3h is expected to be 1500 s-I. The ex- perimental value was about 1600 s-l at 25 OC (see text). The calculated value of k3, is more complex because of the H 2 0 2 reaction. The portion of the rate due to reactions 16-18, k16- [NH,'] + 2k17[ea<] + k18[H'], is 1500 s-l for the case in which [e -1 = [H']. This is the same value as calculated above for k3h, SO^,,^ should be 1 500/0.79 or 1900 s-l when corrected for re- action 19. The 400-s-' contribution from H 2 0 2 is all attributed

expected, indicating reaction with impurity. If a rate constant of 2 x 1O'O M-' s-l is assumed, then the impurity concentration was 2 X lo-' M.

The two other reactions which might be considered are e - + NzH4 (k = 2.3 X lo6 M-' s - ' ) ~ and H' + NH3 - H2 + N%;, which has not been measured in water but which should be about 25 M-' s-' at 25 OC in the gas phase.22 The latter is not likely to be important at 0.1 M NH3 and N2H4 does not rise above 3 X lo4 M, so the first reaction cannot contribute measurably to the rate.

In 0.05 M NH3 with no added NH4+, H' is rapidly converted to e,- and e!; disappearance is mainly second order after about 20 pulses with a first-order component made up of impurity reactions (about 700 s-I) and the H202 component. The ratio of NH2' to ea; is 0.85, so the second-order rate constant, based on the amount of eW formed, is 2k17 + 0.85k16. kl7 should be about 4 X lo9 M-' s-l at the ionic strength of this solution, 0.001 M. On this basis kI6 was found to be 2.5 X 10'O M-' s-I.

to the kk portion of kah, so kk is expected to be 2300 s-' for the [e,;] = [H'] case. The observed k3, was 2-5 times larger than (22) KO, T.; Marshall, P.; Fontijn, A. J . Phys. Chem. 1990, 94, 1401.

Influence of Alcohol Addition on the y-CD:Pyrene Complex

Jonathan B. Zung? Arsenio Muiioz de la Pes&* Thilivhali T. Ndou, and Isiah M. Warner* Department of Chemistry, Emory University, Atlanta, Georgia 30322 (Received: January 4, 1991)

The interaction of y-cyclodextrin (CD) with pyrene in the presence of different linear, branched, and cyclic alcohols is reported. In the presence of y-CD, the fluorescence emission spectra of pyrene showed only the monomer emission. The 1 :1 stoichiometry between y-CD and pyrene has been confirmed in the presence of the alcohols examined. Apparent formation constants were calculated by using the variation in the I/III vibronic band ratio of pyrene and nonlinear regression analysis. The apparent formation constants increased dramatically in the presence of the alcohols. Of all the alcohols examined, cyclohexanol produced the largest enhancement in the apparent formation constant (a 37-fold increase). The present work suggests that the role of the alcohol in the yCD:pyrene complex is somewhat different than recently reported for the 8-CD:pyrene complex.

Introduction Cyclodextrins (CDs) are torus-shaped molecules formed from

the a-l,4 linkages of glucopyranose units. The interior of the CD is fairly nonpolar due to the glycosidic oxygen and hydrogens lining the walls of the cavity. The CDs are of particular interest due to their ability to form host-guest complexes with different guest species.'J Owing to its unique photophysical properties, pyrene is an excellent fluorescent probe for examining the complexing ability of the CDS."~ In the presence of solvents of different polarity, the fluorescence emission spectrum of pyrene changes dramatically. An assessment of the microenvironment surrounding the pyrene can be made using the changes in the vibronic band ratio. The changes in the ratio can then be used to examine and comment on the interactions between the CD and pyrene and to estimate formation constant^.^+'

It is well documented that both b- and y C D form complexes with pyrene; however, the exact nature of the complex and stoichiometry are not so clear.+-" Numerous, often conflicting, reports on the stoichiometry of the complexes have appeared in the literature. For y-CD, a 1:l complex has been observed below the aqueous solubility of pyrene,*J' while both 1:2 and 2:2 y- CD:pyrene complexes have been observed at higher pyrene con- centrations due to the formation of the pyrene e~cimer.~'- '~ Most of the differences in the results can therefore be attributed to the low aqueous solubility of pyrene. Recently, using steady-state

fluorescence measurements and nonlinear regression analysis, we reported the existence of a 2:l 8-CD:pyrene complex and a 1:l y-CD:pyrene complex. This study was based upon the variation of the I/III vibronic band ratio of pyrene with increasing CD concentration while maintaining the pyrene concentration below the aqueous solubility limit.8

It has been shown that the addition of select third components such as alcohols and surfactants can affect the formation of CD complexes.'h2' NakajimaIs demonstrated that addition of ethanol

(1 ) Szejtli. J . Cyclodextrin Technology; Kluwer Academic: Dordrecht,

(2) Sacnger, W. Angew. Chem., Int. Ed. Engl. 190, 19, 344-362. (3) Nakajima, A. Bull. Chem. Soc. Jpn. 1981,44, 3272-3277. (4) Dong, D. C.; Winnik, M. A. Can. J . Chem. 1984, 62, 256b2565. ( 5 ) Edwards, H. E.; Thomas, J. K. Carbohydr. Res. 1978,65, 173-182. (6) Nakajima, A. Sjxctrochim. Acfo 1983, 39A, 913-915. (71 Kusumoto. Y. Chem. Phvs. Lett. 1987. 136. 535-538.

1988.

(8) Muiioz deia Peiia, A.; Nhou, T.; Zung,'J. B.'; Warner, J. M. J . Phys. Chem. 1991. 95. 3330-3334.

(9) Almgren,' M.; Griesser, F.; Thomas, J. K. J . Am. Chem. Soc. 1979,

(10) Patonay, G.; Shapka, A.; Diamond, P.; Warner, I. M. J . Phys. Chem.

( 1 1 ) Hamai. S. J . Phys. Chem. 1989, 93, 6527-6529. (12) Kobayashi, N.; Saito, R.; Hino, H.; Hino, Y.; Ueno, A.; Osa, T. J .

(13) Yorozu, T.; Hoshino, M.; Imamura, M. J . Phys. Chem. 1982, 86,

(14) Kano, K.; Takenoshita, I.; Ogawa, T. Chem. k i f . 1982. 321-324. (15) Nakajima. A. Bull. Chem. Soc. Jpn. 1984, 57, 1143-1 144.

101, 219-29 1.

1986, 90, 1963-1966.

Chcm. Soc., Perkin Trans. 2 1983, 1031-1035.

4426-4429.

0022-3654/91/2095-6701$02.50/0 Q 1991 American Chemical Society

6702 The Journal of Physical Chemistry, Vol. 95, No. 17, 1991

to the 8-CD:pyrene complex caused further association and suggested the possibility of a ternary association. Although he did not comment on the origin of the effect, it was apparent that ethanol favored enhanced complex formation. Kano et al.I4 re- ported the formation of a 1 :2 y-CD:pyrene complex above the aqueous solubility of pyrene. They noted that the monomer emission increased at the expense of the excimer emission upon addition of I-butanol to the yCD:pyrene complex, thereby sug- gesting a 1 : 1 y-CD:pyrene complex and not a 1 :2 y-CD:pyrene stoichiometry. Using fluorescence measurements, Ueno et aL2I observed a marked enhancement in fluorescence upon addition of cyclohexanol to a y-CD:a-naphthyloxyacetic acid complex. Their results suggested that the marked enhancement in fluorescence is a result of the cyclohexanol acting as a space regulator molecule. This phenomenon allows for a better tit of the guest molecule inside the CD cavity. More recently, Hamai16 reported on the interaction between 8-CD and different alcohols. It was suggested that a ternary CD:pyrene:alcohol complex was formed upon addition of the alcohol.

Using fluorescence lifetime measurements, we have previously demonstrated a dramatic enhancement in the lifetimes and for- mation constants of the y-CD:pyrene complex in the presence of select alcohol^.'^ Of all the alcohols examined, tert-butyl alcohol (2-methyl-2-propanol) provided the greatest overall enhancement in the formation constant (over 2 orders of magnitude). More recently, we reported on the effects of alcohol size and volume in forming ternary complexes with pyrene and @-CD.20

In this article, we report on the effect of addition of different alcohols (linear, branched, and cyclic) on the y-CD:pyrene com- plex and on the relationship between the alcohol and CD using the variation in the I/III vibronic band ratio. The 1:l stoi- chiometry between pyrene and y-CD has been confirmed in the presence of the alcohols examined. Formation constants are also computed by using nonlinear regression analysis.

Experimental Section ycyclodextrin was obtained from American Maize Products

(Hammond, IN) and pyrene (99+%) from Aldrich. All alcohols used in this work were of high purity (Aldrich) and used as received. A stock solution of pyrene was prepared in cyclohexane. An aliquot of the stock solution was transferred to a 100-mL volumetric flask and the cyclohexane was evaporated off by using dry nitrogen.* The appropriate solutions were then made from the stock solution. The pyrene concentration was maintained at 1 .O X 1 0-7 M in all experiments. The resulting solution was then mechanically shaken and allowed to equilibrate for 18 h prior to analysis. Fluorescence emission spectra were acquired on a modified Perkin-Elmer 650-1 Os fluorometer2z using an excitation wavelength of 337 nm. Excitation and emission slits with bandwidths of 2 and 1.8 nm were used, respectively. All mea- surements were acquired at 21 f 0.1 OC. Three individual spectra were acquired for each sample. An average I/III vibronic band ratio is computed by use of all three measurements. The resulting I / I I I values are accurate to f0.02.

Results and Discussion Pyrene is widely used as a fluorescence probe for investigating

environmental changes due to the unique ability of the pyrene monomer emission fine structure to change in the presence of different solvents. As the environment around the pyrene is

Zung et al.

(16) Hamai, S . J. Phys. Chcm. 1989, 93, 2074-2078. (17) Nelson, G.; Patonay, G.; Warner, 1. M. Anal. Chem. 1988, 60,

(18) Buvari. A.; Szejtli, J.; Barcza, L. J . Inclusion Phenom. 1983, I ,

(19) MatPui. Y.; Mochida. K. Bull. Chcm. Soc. Jpn. 1979,52,2808-2814. (20) Mufioz de la Pefia, A,; Ndou, T.; Zung, J. B.; Greene, K.; Live, D.;

(21) Ueno, A.; Takahashi, K.; Hino, Y. ; Osa, T. J . Chcm. Sot. , Chem.

(22) Zung, J. E.; Nelson, G.; Warner, I. M. Lob. Microcomputer 1989,

274-279.

151-1 57.

Warner, 1. M . J. Am. Chem. Sot. 1991, 113, 1572-1577.

Commun. 1981, 194-195.

20, 145-150.

a," I

0.54 . I ' I . 1 . I 0 4 8 12 16

Y-CD Conccnwtion (mM)

2.0,

1.7

1.4

1.1

0.8

0.54 . , . , . , . 0 4 0 12 16

Y -CD Concenwtion (mM)

Figure 1. (A, top) Influence of y-CD concentration on the I/III vibronic band ratio of pyrene in the presence of different straight-chain alcohols, in the absence of alcohol (D), ethanol (A), I-propanol (0), I-butanol (A), and I-pentanol (m). [Pyrene] = 1.0 X M. (B, bottom) Influence of y-CD concentration on the I/III vibronic band ratio of pyrene in the presence of different branched and cyclic alcohols, in the absence of alcohol (D), 2-propanol (A), ?err-butyl alcohol (0), cyclopentanol (A), and cyclohexanol (m). [Pyrene] = 1.0 X IO-' M and [y-CD] = 1.0 X lo-) M.

altered, a concomitant change in the I/III vibronic band ratio is noted. The observed changes in the band ratio can then be correlated with the polarity of the environment surrounding the pyrene. It has been demonstrated that the complexation equi- librium between pyrene and CDs can be easily monitored by following the changes in the observed I/III ratio of pyrene.s8,z0 This parameter is the weighted average of the I/III ratio of the uncomplexed (free) pyrene and that of the complexed pyrene. We have already reported on the advantages of using this parameter for establishing the stoichiometry and for calculating the formation constants of CD:pyrene complexes in pure water.8 Due to the problems that arise from the low aqueous solubility of pyrene, the I/III ratio is a more reliable parameter than the overall fluorescence intensity.'O This is also a more convenient method for examining the complexation of pyrene in alcoho1:water mix- tures than monitoring the overall fluorescence intensity. The fluorescence intensity may be influenced by the CD complexation and/or media effects, which may ultimately lead to an ambiguous interpretation of the results. Unlike the overall fluorescence intensity, the I/III ratio does not change significantly unless the environment surrounding the pyrene is altered. Therefore, from the changes in the I/III ratio, we can follow the degree of com- plexation between the guest molecule (pyrene) and the CD host.

In pure water, pyrene has a I/III ratio of approximately 1.87, characteristic of a fairly polar en~ironment .~ Upon addition of y-CD, the I/III ratio steadily decreases until approximately 10 mM, a t which point the I/III ratio begins to level off a t a p proximately 0.81. This suggests that the pyrene is in a more nonpolar environment than in just a pure aqueous system. This change in the ratio is attributed to the formation of an inclusion complex with the CD.

Influence of Alcohol. The effect of increasing concentrations of y-CD on the I/III ratio in the presence of different alcohols at a constant alcohol volume of 1.0% v/v is shown in Figure 1,

M and [y-CD] = 1.0 X

Addition of Alcohol to the y-CD:Pyrene Complex The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6103

A and B. In the presence of the alcohol, the I/III ratio decreases more rapidly upon addition of CD than in a pure aqueous solution. The extent of the decrease is dependent upon the particular alcohol present and suggests a change in the formation constant for the complex. It should be noted that, in the absence of CD, the alcohols do not have any significant influence on the I/III ratio. For example, the addition of 1 .0% v/v cyclopentanol to an aqueous pyrene solution does not alter the I/III ratio. The addition of 5.0% v/v cyclopentanol causes the I/III ratio to decrease slightly from 1.87 to 1.85, which suggests that there is little or no association between the pyrene and the alcohol. It further suggests that the effect of the medium (alcohol) on the I/III parameter is negligible. Likewise, for the other alcohols examined, there was little or no change in the I/III ratio upon addition of alcohol to the aqueous pyrene solution in the absence of CD.

The apparent formation constant of the CD:pyrene:alcohol complex depends upon the alcohol concentration. We chose to use 1 .O% v/v alcohol solutions in order to have enough alcohol to be in excess over the most concentrated CD solution used. In addition, at this concentration of alcohol, there are no apparent media effects. It is seen from Figure 1A that the change in the slope and the change in the I/III ratio with increasing y-CD concentration is more pronounced in the presence of 1-pentanol than in ethanol. The smaller, more polar alcohols do not appear to have as much an effect on the slope as do the larger more nonpolar alcohols. The greatest change in the slope and lowest I/III ratio for the straight-chain alcohols was observed for 1- pentanol, while the highest I/III value and smallest change in the slope was observed for ethanol. Likewise, for the branched and cyclic alcohols examined, it is apparent that the most nonpolar species (cyclohexanol) causes the greatest change in the slope, while a less pronounced change in the slope is observed for the most polar species, 2-propanol, as shown in Figure 1B. The more nonpolar alcohols again appear to be making the environment surrounding the pyrene inside the CD cavity more hydrophobic.

In the plateau region where the I/III ratio is leveling off, it is reasonable to assume that the pyrene is totally complexed. The I/III ratio in this region steadily decreases with a decrease in the alcohol polarity. The observed I/III values in the presence of 10 mM y-CD for the linear alcohols (1 -0% v/v) are shown below:

water > ethanol > 1-propanol > I-butanol > I-pentanol 0.81 > 0.79 > 0.74 > 0.65 > 0.61 For the branched and cyclic alcohols, there is also a correlation

between the polarity of the alcohols and the observed I/III ratio as shown below:

2-propanol > 2-methyl-

0.75 > 0.72 > 0.65 > 0.63

> cyclopentanol > cyclohexanol 2-propanol

The trend between the alcohol polarity and observed I/III ratio can also be correlated with the apparent formation constants of the y-CD:pyrene:alcohol complex, as described in more detail in the following section.

In the case of the 8-CD:pyrene complex,20 we observed a similar trend in the presence of the linear alcohols; however, the variation of the I/III ratio for the branched and cyclic alcohols did not follow the trend observed with y-CD. For 8-CD, the I/III ratio varied among the branched and cyclic alcohols with no clear correlation between the size and polarity of the alcohol. We previously reported a 2: 1:2 stoichiometry for the 8-CD:pyrene:alcohol com- plex.m In that complex, the pyrene is proposed to be encapsulated by two CD molecules and the two alcohols are positioned at the open ends of the CD cavity. From this configuration, the different observed I/III ratios in the presence of 8-CD can be rationalized in terms of minimization of the pyrene interactions with the bulk water at the open ends of the cavity. In the present case, the stoichiometry of the y-CD:pyrene complex is known to be 1 : 1 ,* However, above the aqueous solubility of ~y rene ,2~ Kano et al.

(23) Pearlman, R. S.; Yalkowsky, S. H.; Banerjee, S. J. Phys. Chem. Ref. Data 19%4, 13, 555-562.

have reported a 1:2 y-CDpyrene ~omp1ex.l~ They demonstrated that a change in the stoichiometry of the 1:2 y-CDpyrene complex occurs above the aqueous solubility of pyrene upon addition of 1-butanol. In the presence of 1-butanol, the 1:l monomer y- CD-pyrene complex was enhanced at the expense of the degra- dation of the 1:2 y-CD:pyrene excimer. The existence of a 1:2 y-CD:pyrene complex at higher pyrene concentrations suggests that there is still enough room in the interior of the CD to ac- commodate a second guest molecule of appropriate size. It further suggests the formation of a ternary y-CDpyrene:butanol complex in the presence of the alcohol. This does not, however, suggest the number of alcohols which may be involved in the complex. In the present case, the alcohol can enter further into the cavity instead of being positioned on the rim of the cavity as was sug- gested for the 8-CD complex.20 This is in sharp contrast to the case with 8-CD in which the size constraint of the 8-CD limits the cavity to accompanying only one pyrene.

The equilibrium expression for a 1 : 1 complex between the CD and pyrene (P) can be represented as follows:

P + CD * PCD (1)

Kl = [PCDl/[PI[CDl (11) where [PCD] is the equilibrium concentration of the CD:pyrene complex at a given CD concentration; [PI and [CD] are the equilibrium concentrations of pyrene and cyclodextrin, respectively.

In the presence of an alcohol (A), the equilibrium expression for the ternary complex is represented by the following expression:

PCD + nA PCDA, (111)

K2 = [PCDA,]/[PCD][A]" (IV) An overall formation constant can then be represented by 4, where

(VI In the presence of 1 .O% v/v alcohol, the alcohol is in large excess compared to both the pyrene and CD over the range of CD concentrations used and is free of medium effects. Therefore, the apparent equilibrium constant is given by

6 = [PCDAnI /[PI [CDI [AI"

K' = [PCDA,] / [PI [CD] (VI) Assuming the observed I/III value (R) is the weighted average of the complexed and free pyrene,*v9 the fraction of pyrene com- plexed can be expressed as

where [PIo is the initial pyrene concentration and Ro and RI are the I/III values of pyrene in water-alcohol medium and the complex, respectively. Taking into account that [CD] >> [PCDA,], we can assert that [CD] r [CD],. Rearranging and combining eq VI with eq VII, the following expression is derived:

(VIII)

Nonlinear least-squares regression analysis can thus be used to calculate the apparent formation constants for the ternary complex in the presence of different alcohols. Initial parameters used for the regression analysis were determined from the classical double reciprocal plots as described in ref 8. From the double reciprocal plots, the 1 : 1 stoichiometry for the yCD:pyrene complexes was confirmed in the presence of all the alcohols examined. A double reciprocal plot for the y-CD:pyrene:cyclopentanol complex is shown in Figure 2. The apparent formation constants for these complexes along with the volumes of the respective alcohols are given in Table I. For the linear alcohols, the apparent formation constants increase with increasing alkyl chain length. The tendency for the more nonpolar alcohols to form stronger com-

(24) Still, W. C., et al. Macromodel Version 2.5, Department of Chem- istry, Columbia University, NY, 1989.

6704 The Journal of Physical Chemistry, Vol. 95, No.

30 I 17, 1991

0 12500 25ooo 37Mo 5oooO

11 [CDI

Figure 2. Double reciprocal plot for the y-CD:pyrene:cyclopentanol complex proving the 1:1 stoichiometry (9 = 0.997). [Pyrene] = 1.0 X 10-7 M.

TABLE I: Apparent Formrtioa Coastrats for y-CbPyrene:Alcohol’ Comdtxes in the Presence of 1.0% v/v Alcohol

volume’ of alcohol Kf, M-’ alcohol, A3

water 192 20.1 ethanol 288 54.3 1 -propanol 322 72.0 2-propanol 683 72.1 I-butanol 886 88.6 tert-butyl alcohol 1857 88.2 I-pentanol 2522 105.7 cyclopentanol 2950 95.0 cyclohexanol 7140 111.8

mVolume of pyrcnc is 186 A’. bEstimated volume as calculated by using the program MACRO MODEL.^'

plexes with the CD is reasonable since they prefer the nonpolar CD cavity more than the more polar alcohols. The largest ob- served formation constant is seen for 1-pentanol, while the smallest apparent formation constant is estimated for ethanol. The for- mation constants for the branched and cyclic alcohols also increase as the size and polarity of the alcohols increase. The largest apparent formation constant is seen with cyclohexanol, which is also the largest and most nonpolar of all the branched and cyclic alcohols examined.

The trend in these data suggests that the larger alcohols form stronger ternary complexes than the smaller more polar alcohols. Judging from space filling CPK models, it is seen that the 1:l y-CD:pyrene complex still has significant open space inside the CD cavity. It has been shown that, if the concentration of pyrene is above its aqueous solubility, then two pyrene molecules can fit into the cavity, although the exact nature of the complex is unclear. Both 1:l and 2:2 stoichiometries have been reported.I3J4 Since in the present case, only one pyrene molecule is included in the cavity, it is reasonable to expect that this extra space can readily be filled by the alcohols, particularly since the alcohols are smaller in size as compared to the pyrene molecule. The present data suggest that larger alcohols occupy more space inside the cavity and are more conducive to a better fit, and hence the larger apparent formation constants.

It should be noted that the actual size, or more appropriately the volume, of the alcohol is not the sole factor in determining the overall effect of the alcohol on the y-CD:pyrene complex. Branching in the alcohol chain appears to lead to a stronger overall ternary complex than the corresponding straight-chain alcohol. This is evident by the higher formation constants observed for the branched alcohols as compared with their respective straightchain counterparts. For example, 1 -propanol and 2-propanol have virtually the same volume (approximately 72.0 A)); however, the apparent formation constants for the ternary complex in the presence of the respective alcohols are very different. The apparent formation constant for the complex with 2-propanol is 2 times greater than the complex with 1-propanol. Similarly, 1-butanol and rert-butyl alcohol which have the same volume, have different

Zung et al.

effects on the y-CD:pyrene complex. The presence of tert-butyl alcohol causes a larger increase in the apparent formation constant than with 1-butanol.

The more interesting case is for 1-pentanol and cyclopentanol. Although, 1-pentanol has a larger volume than cyclopentanol it does not have as great an effect on the formation constant as docs cyclopentanol. One possible explanation for this observation is that due to its fairly long carbon chain, 1-pentanol is not entering completely inside the cavity. An alternative or more plausible explanation is that the cyclopentanol occupies more space inside the cavity than does the straight chain, 1-pentanol. The structures for 1 -pentan01 and cyclopentanol are

U I

cyclcpenlanol 1 gentanol

Clearly, 1-pentanol is longer than cyclopentanol, while the cy- clopentanol is bulkier than 1-pentanol. This bulkiness is most probably what causes the larger enhancement in the apparent formation constant in the presence of cyclopentanol. Thus cy- clopentanol can better act as a regulator inside the cavity. The most dramatic enhancement in the apparent formation constant was observed for cyclohexanol. In lieu of the enhancements observed with the other alcohols, it is reasonable to attribute this further enhancement to the increased bulkiness of cyclohexanol. This type of space filling or ‘space regulating” arrangement by a third component has previously been reported for the y-CD: a-naphthyloxyacetic acid:cyclohexanol complex.*’ In that case, the cyclohexanol was postulated to fill a void in the CD, which leads to a stronger overall complex. No other alcohols were examined in that work.

The effect of the size of the alcohol on the apparent formation constant for the 8-CD:pyrene complex is different from what has been observed in the present case with y-CD. For &CD, it was observed that the formation constant of the ternary complex increases with increasing alcohol size and reaches a maximum value in the presence of 1-butanol and then the value begins to decrease with increasing alcohol size.*O A similar trend, where a maximum in the formation constant was followed by a decrease, was observed with the branched and cyclic alcohols. These trends suggest that there is limited space for the alcohol on the open ends of the CD, which is in agreement with the CPK models. The CPK models suggest that the portion of the pyrene which is included inside the 8-CD cavity effectively fills the cavity and leaves little void space inside the CD. Thus, the alcohols which are of the appropriate size must interact with or ‘cap” the primary side of the CD.

Recently, the continuous variation method (or Job’s method) has been applied to three-component systems, for determining the stoichiometry between 8-CD and different alcohols.I6qm In their studies, both HamaiI6 and Muiloz de la Peila et alSm reported a 1:l 8-CD:alcohol stoichiometry. In the present work the same approach was applied to determine the stoichiometry between y C D and the different alcohols. The results were inconclusive. In view of our findings, this approach is probably not suitable for determining the stoichiometry between the alcohol and y-CD, due to the low alcohol concentration. In the present case, the alcohol and CD concentrations are maintained at 2.0 X lo-’ M. This concentration was chosen since it is the concentration at which the I/III ratio still is changing and not on the plateau region of the curve as shown in Figure 1. At this concentration, however, there is an insufficient amount of alcohol to affect the y-CD:pyrene complex. This is clearly seen by the dramatic difference in the apparent formation constants for the ternary complex in the presence of 1.W v/v (0.1091 M) and 0.025% v/v (2.7 X lW3 M) cyclopentanol. The respective apparent formation constants are 2950 and 21 5 Mu’. The latter value is similar to the value obtained in the absence of alcohol. This suggests that at 2.0 X M

Addition of Alcohol to the y-CD:Pyrene Complex The Journal of Physical Chemistry, Vol. 95, No. 17, 1991

1.6 5 I

0 1 2 3 4 5

% Alcohol (vh) Figure 3. Effect of adding varying amounts of 1-butanol (.) and cy- clopentanol (0) on the y-CD:pyrene:ethanol complex. [Pyrene] = 1 .O X IO-’ M, [CD] = 1.0 X IO-) M, and 1.0% v/v ethanol.

cyclopentanol the alcohol has no influence on the stability constant of the complex. A higher concentration of alcohol is therefore needed in order to observe the effect of the alcohol on the y- CD:pyrene complex. Studies at total concentrations above 2.0 X lV3 M were not informative since the I/III ratio is approaching the plateau region of the curve. Additionally, the dramatic effect of the alcohol on the y-CD:pyrene complex is only observed when the alcohol is in excess over the CD. This is not the case when equal concentrations of alcohol and CD are used. More con- centrated solutions were investigated; however, the same results were obtained.

In spite of not being able to determine the CD:alcohol stoi- chiometry using the Job’s method, it is reasonable to assume that at least one alcohol is present in the y-CD:pyrene complex, es- pecially for the larger alcohols. The larger, more bulky alcohols such as cyclopentanol and cyclohexanol clearly fill more of the void inside the CD. For the smaller alcohols, such as ethanol and 1-propanol the possibility exists that more than one alcohol may be involved in the complex. The stoichiometry is being investigated further by using computer-based molecular modeling experiments.

Competition between Mixed Alcohols. The effect of adding a different alcohol to the ternary y-CD:pyrene:alcohol complex was examined. In the presence of 1.0% v/v ethanol, the I/III ratio of the y-CD:pyrene complex is 1.58. Upon addition of increasing amounts of cyclopentanol to the complex, the I/III ratio dra- matically decreases as shown in Figure 3. This decrease suggests that the cyclopentanol is displacing the ethanol from the ternary complex and is itself entering inside the CD. The addition of 1 .O% v/v cyclopentanol to the y-CD:pyrene:ethanol complex causes the I / I I I ratio to decrease to the same value as would be observed if no ethanol were present. Likewise, the addition of 1 .O% v/v 1-butanol to the y-CD:pyrene:ethanol complex causes the I/III ratio to decrease to 1.23, which is the same value observed in the presence of only I-butanol. The displacement of the ethanol by the more nonpolar alcohols can mainly be attributed to the higher affinity of the larger, more nonpolar alcohols for the CD cavity. This is also evident from the differences in the apparent formation constants for the ternary complexes. Clearly, both cyclopentanol and 1-butanol form stronger ternary complexes than does ethanol. Similar results were also observed for mixed alcohol systems containing terr-butyl alcohol and cyclopentanol. Although the I / I I I ratio decreased upon addition of cyclopentanol, the ratio did not decrease to the I / I I I value in the presence of only cy- clopentanol. This suggests that there is more competition between these two alcohols and that some of the tert-butyl alcohol may still be included in the cavity or simply that some complexes contain tert-butyl alcohol and some cyclopentanol.

It is interesting to note that in the case of adding ethanol to the y-CD:pyrene complex containing 1 .O% v/v cyclopentanol, the I / I I I ratio increases slightly after addition of 1.0% v/v ethanol (Figure 4). This suggests that the ethanol is affecting the mi- croenvironment surrounding the pyrene. Presumably, in the presence of cyclopentanol, there is still some void space inside the cavity which the ethanol may fit into. This would account for

0 1 2 3 4 5 6

%Alcohol (vh) Figure 4. Effect of addition of second alcohol to ternary y-CD:pyr- ene:alcohol complex. (0) Constant 1.0% v/v ethanol while adding cy- clopentanol and (.) constant 1.0% v/v cyclopentanol while adding eth- anol. [Pyrene] = 1.0 X IO-’ M, [CD] = 1.0 X IO-’ M.

the slight change in the I/III ratio. At higher amounts of ethanol added, it is possible that media effects are responsible for the increase in the I/III ratio. It is unlikely, however, that the ethanol is displacing the cyclopentanol from the ternary complex since the cyclopentanol has a much higher apparent formation constant than does the ethanol when in the ternary complex. It is possible, however, that ethanol, due to its small size, is able to fit into the remaining void in the CD. This then suggests the possibility that more than one alcohol may be participating in the complex.

Role of tbe Alcohd in tbe 8- vs y C D Ternary Pyrene Complex. The role of the alcohol in the formation of the ternary y-CD: pyrene:alcohol complex appears to be different than in the ternary 0-CD complex. We have recently shown that, for the 8-CDpyrene complex, the effect of the alcohol size and volume are the dominant factors in determining the alcohols overall effect on the complex. In that work, we noted that proper size matching between the pyrene, CD, and alcohol is crucial in determining the overall effect of the alcohol on the formation constants. The branched and cyclic alcohols which were too large and bulky to fit in the open end of the CD did not tend to form strong ternary complexes with 0-CD, while their smaller counterparts were able to form stronger complexes. For y-CD, there does not appear to be a correlation between the size and volume of the alcohol with the pyrene and CD as with the 8-CD complex for the alcohols examined. Owing to the limited solubility of the larger alcohols, we were unable to investigate their role in the ternary complex. The larger, more bulky alcohols appear to form stronger complexes. The major difference between the two complexes is the extra available space inside the y-CD cavity. Owing to the larger y-CD cavity, a second molecule of appropriate size may easily fit inside the cavity, while the smaller cavity size of 0-CD is only able to accommodate one pyrene.

Conclusion The present study suggests that the alcohols may be acting as

“space regulator” molecules in the formation of the ternary complex. Presumably, the alcohols are facilitating the formation of a stronger complex by filling the void inside the cavity, with cyclohexanol having the greatest effect on the y-CD:pyrene complex. In light of the fact that there is extra space inside the cavity, it is reasonable to expect the alcohols to coinclude with the guest inside the CD. Additionally, the nonpolar alcohols would much prefer to be inside the hydrophobic cavity than in the hydrophilic region outside the CD. We are currently investigating the role of the alcohol in the complexation process using molecular modeling. The molecular modeling will hopefully provide some insight into the possible geometries and orientations of both the pyrene and alcohol inside the y C D cavity.

The dramatic effect of alcohol addition on the y-CD:pyrene complex has far ranging implications in other areas of chemistry. For instance, in reverse-phase HPLC separations, the addition of small amounts of alcohol to a CD mobile phase can be used to achieve a reasonable retention time for polynuclear aromatic

6706

hydrocarbons (PAHs) and hence shorter analysis time. In ad- dition, this should lead to better selectivity in the separation. The presence of the alcohols will also reduce the competition between the solvent and stationary phase for the polynuclear aromatics.

Acknowledgment. This work was Supported in Part by grants from the National Science Foundation (CHE-900 141 2 ) and the National Institutes of Health (GM 39844). A.M.P. acknowledges support from D.G.I.C.Y.T. of the Ministry of Education and

J. Phys. Chem. 1991, 95,6706-6709

Science of Spain for the grant that made possible his research in Professor Warner's laboratory. We are also grateful to G. A. Reed of American Maize Products for providing the CDs used in this work.

Registry No. y-CD-pyrene, 87832-35-7; y-CD.H20, 134970-99-3; y - ~ ~ . ~ 3 ~ ~ ~ , ~ ~ , I 34971-00-9 y-~D.H3CCH2CH20H, 134971-01-0; y - ~ ~ . ( ~ , ~ ) , ~ ~ ~ ~ , 13497 1-02-1; y - ~ ~ . ~ 3 ~ ( ~ ~ 2 ) , 0 ~ , 134971-03-2; y-CD.(H,C),COH, 134971-04-3; y-CD.H3C(CH2),0H, 134971-05-4; y-CD-cyclopentanol, 134971-06-5; y-CD-cyclohexanol, 134971-07-6.

Conformatlonal Adaptlon of Poly(ethy1ene oxide). A 13C NMR Study

Mikael Bjorling,* Gunnar Karlstriim,+ and Per Linse Physical Chemistry 1, Chemical Center, P.O.B. 124. S-221 00 Lund, Sweden (Received: November 21, 1990; In Final Form: March 26, 1991)

The "C NMR chemical shift was used as a probe for the average partition of ratational conformers around the C-C bond in the O-CH2-CH2-0 segments of poly(ethy1ene oxide). Dividing the conformers into a large group of trans and a smaller group of gauche conformers, we concluded that the trans conformers have a higher (downfield) average chemical shift than the gauche conformers. The shift of the main PEO I3C line with changing environment was interpreted as an adaption in the partition between the two groups of conformers. Furthermore, the trans conformers had nonpolar character and were favored at high temperatures, whereas the gauche conformers had polar character. The measurements were compared to the predictions of a model proposed by Karlstrom, and a semiquantitative agreement was found.

Introduction Poly(ethy1ene oxide) (PEO), pure or in solution, has been

extensively studied as a component of nonionic surfactants and block copolymers but also in its own right. Specifically, Raman,'+ NMR,5-8 and other methods"' have been used to study the conformational changes induced by changing the solvent or the temperature. Analysis of the Raman spectra'+ suggests that the largest variation of the conformation with changing environment is due to shifting the ensemble average of rotational conformers around the C-C bond in the O-CH2-CH2-0 segments. It is empirically found that decreasing the temperature or increasing the polarity (or hydrogen bonding capacity) of the solvent favors the gauche conformers. These observations are corroborated by NMR coupling constants5-* and in some respect by theoretical calculations.'OJ1 Furthermore, the free energy of interaction between PEO and a polar solvent goes through a maximum with increasing temperature. This is manifested in the contraction of PEO coils with increasing temperature'* and by the phase sepa- ration at a lower consolute pointI3 in water and formamide.

KarlstrBm" has proposed a model where the conformational adaption to the environment provides the mechanism of the changing free energy of interaction, thus connecting the above observations. The gist of the model is the division of the rotational conformers into two groups. Members of the smaller group (gauche) interact favorably with polar moieties, whereas members of the larger group (trans) have less favorable interaction with polar groups. The partitioning of conformers between the two groups therefore depends on the temperature and the local en- vironment. With this extra degree of freedom extending the Flory-Huggins theory, KarlstrBm succeeded in choosing a set of parameters that gave a reasonable reproduction of the PEO-water binary phase diagram. The simple but powerful model has been successfully incorporated in a number of calc~1at ions '~J~ and supported by phase behavior studies," but its predictions of the conformational changes have never been tested directly. This is our aim.

'Theoretical Chemistry, Chemical Center, P.O.B. 124, S-221 00 Lund, Sweden.

We argue that the I3C NMR chemical shift of the methylene carbons in PEO provides a measure of the ensemble average of conformers. Such an interpretation has been attempted previously by Ahlnas et a1.'* The basic assumption is that the gauche group gives rise to a lower (upfield) average chemical shift than does the trans group. Rapid interconversion between the conformers produce one single peak whose shift is indicative of the partition between the groups. A number of observations support this as- sumption. Dioxane, which obviously is constrained in an extreme gauche conformer, has a I3C chemical shift (66.5 ppm, relative to TMS)19 which is considerably lower than that of the uncon- strained dimethoxyethane (72.3 ppm).20 Analysis of the "C shifts

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0022-3654/91/2095-6706$02.50/0 0 1991 American Chemical Society