J . Phys. Chem. 1990, 94, 5230-5233
5230
Hydration of 18-Crown-6 in Carbon Tetrachloride. Infrared Spectral Evidence for an Equilibrium between Monodentate and. Bidentate Forms of Bound Water in the 1:l Crown-Water Adduct Samuel A. Bryan,l* Richard R. Willis,lb and Bruce A. Moyer* Chemistry Division, Oak Ridge N a t i o n a l L a b o r a t o r y , P.O. Box 2008, Oak R i d g e , Tennessee 37831 -61 19 (Received: December 18, 1 9 8 9 )
The interaction of water with 18-crown-6 (18C6) in carbon tetrachloride has been investigated by FTIR techniques. As shown by least-squares analysis, the functional dependence of the absorbance of OH stretching bands (3800-3200-cm-’ region) on the H20and 18C6 concentrations indicates the formation of 1 :1 1 8C6-H20 adducts, giving the value 15.6 f 1.2 M-’ for the molar equilibrium constant at 25.0 O C . The number, positions, and widths of the OH stretching bands of the 1 : l 18C6-H20 adduct indicate the presence of two forms of bound water, one in which the H,O molecule dangles from an ether oxygen atom by a single hydrogen bond and one in which H 2 0 bridges two ether oxygen atoms.
Introduction An important consideration in understanding chemical recognition phenomena concerns the initial conformation of the host and its interactions with the surrounding environment. In this connection, hydrogen bonding between crown ethers and water has been of particular interest.2-8 Structural features of crown-water hydrogen bonding in the solid state have become well established primarily through X-ray crystallography, although the structures are often complicated by the presence of a third component.6*8 In most cases, both of the hydrogen atoms of the water molecule are engaged in hydrogen bonding, and the oxygen atoms receiving the hydrogen bonds may or may not both belong to the polyether. In the former case, water is bound by a single hydrogen bond to an ether oxygen atom; the other water hydrogen is usually found to interact with a nearby hydrogen-bond acceptor. ng Scientist. Current address: Department of Chemistry, n University, Statesboro, GA 30460. (b) Undergraduate Research Participant. (2) (a) Golovkova, L. P.; Telyatnik, A. 1.; Bidzilya, V. A. Theor. Exp. Chem. (Engl. Trans/.) 1984, 20, 219-222; see also references cited within. (b) Yakshin, V. V.; Abashkin, V. M.; Laskorin, B. N. Dokl. Akad. Nauk SSSR (Engl. Trans/.)1979, 244,27-29. (c) Yatsimirskii, K. B.; Budarin L. 1.; Telyatnik, A. 1.; Gavrilova, Z . A. Dokl. Akad. Nauk SSSR (Engl. Trans/.) 1979, 246, 469-471. (d) De Jong, F.; Rheinhoudt, D. N.; Smith, C. J. Tetrahedron Lett. 1976, 1371-1374. (3) Interest on the hydration of crown ethers also stems from work on recognition of neutral molecules as summarized in reviews: (a) Vogtle, F.; Sieger, H.; Muller, W. M. Top. Curr. Chem. 1981, 98, 107-161. (b) Vogtle, F.; Muller, W. M.; Watson, W. H. Top. Curr. Chem. 1984, 125, 131-164. (4) Ranghino, G.: Romano, S.;Lehn, J. M.; Wipff, G. J . Chem. Soc. 1985, 107,7873-7877. ( 5 ) Fukushima, K.; Ito, M.; Sakurada, K.; Shiraishi, S. Chem. Left. 1988, 323-326. (6) Water-crown hydrogen bonding has been demonstrated by X-ray crystallography. See, for example: (a) Bradshaw, J. S.;Chamberlin, D. A,; Harrison, P. E.; Wilson, B. E.; Area, G.; Dalley, N. K.; Lamb, J. D.; Izatt, R. M. J . Org. Chem. 1985,50, 3065-3069. (b) Fuller, S.E.; Stoddart, J. F.; Williams, D.J. Terruhedron Left. 1982,23, 1835-1836. (c) Goldberg, I. Acta Crystallogr. 1978, B34.3387-3390. (d) Newkome, G. R.; Taylor, H. C. R.; Fronczek, F. R.; Delord, T. J.; Kohli, D. K. J . Am. Chem. SOC.1981, 103, 7376-7378. (e) Newkome, G. R.; Fronczek, F. R.; Kohli, D. K. Acta Crystallogr. 1981. 837, 21 14-21 17. ( f ) Grootenhuis, P. D. J.; Uiterwijk, J. W. H. M.; Reinhoudt, D. N.; Van Staveren, C. J.; Sudholter, E. J. R.; Bos, M.; Van Ecrden, J.; Klooster, W. T.; Kruise, L.; Harkema, S. J. Am. Chem. SOC. 1986, 108,780-788. (g) Gokel, G. W.; Garcia, B. J. Tetrahedron Left.1977, 317-320. (h) Grossie, D. A.; Watson, W. H.; Vogtle, F.; Muller, W. M. Acta Crysfallogr. 1982,838,3157-3159. (i) Elbasyouny, A,; Brugge, H. J.; Von Deuten, K.; Dickel, M.; Knochel, A.; Koch, K. U.; Kopf, J.; Melzer, D.; Rudolph, G. J . Am. Chem. SOC.1983, 105, 6568-6576. (j)Caira, M. R.; Watson, W. H.; Vogtle, F.; Muller, W. Acta Crystallogr. 1984, C40,491-493. (k) Watson, W. H.; Galloy, J.; Grossie, D. A,; Vogtle, F.; Muller, W. M. J . Org. Chem. 1984, 49, 347-353. ( I ) Cusack, P.A,; Patel, B. N.; Smith, P. J.; Allen, D. W.; Nowell. 1. W. J . Chem. Soc., D d f o n Trans. 1984, 1239. (m) Suh, 1.-H.; Namgung, H.; Yoon, Y. K.; Saenger, W.; Vogtle, F. J . Inclusion Phenom. .. .. . 1985. ..,..3. 21-26. _. (7) Bryan, S . A.; McDowell, W. J.; Moyer, B. A,; Baes. C. F.; Case, G. N. Soluent Extr. Ion Exch. 1987, 5, 71 7-738. (8) Burns, J. H.; Nordlander. E. H. Inorg. Chim. Acta 1986, 115. 31-36. ~
~
~
0022-3654/90/2094-5230$02.50/0
In the latter case, a water molecule bridges two ether oxygens. Monte Carlo calculations4 have shown that both bonding modes would be expected in aqueous solution, depending on the conformation of the crown ether, and Raman evidenceS for the accommodation of water by crown ethers in aqueous solution has been reported. Whereas each crown ether molecule is likely to be hydrogen bonded to several water molecules in this environment, crown ethers in wet nonaqueous solvents form 1:l crown-water adducts,2 where the number of possible hydrogen bonds is limited to only two. The structural nature of water binding by crown ethers in nonaqueous solvents, however, has not yet been studied. We have recently been addressing this question as part of our of the crown ethers as selective extractants for certain metal cations. As reported herein, we have found it possible using FTIR spectroscopy to distinguish for the first time between monoand bidentate forms of the bound H20molecule in the 1:l crown-water adduct of 18-crown-6 (18C6) in wet carbon tetrachloride.
Experimental Section All IR spectra were obtained on a Digilab FTS-60 Fourier transform infrared spectrophotometer. The constant-temperature spectra were obtained with use of a thermostated demountable liquid cell holder equipped with silver chloride or silver bromide windows separated by a 0.35-cm Teflon spacer. The variabletemperature IR data were taken with use of a 5.00-cm path length infrasil quartz cell equipped with a Teflon cap and Luer-type connectors for syringe filling. The 5-cm IR cell was enclosed by a modified UV-vis thermostated sample holder. The filling syringe was left attached to the IR cell via a flexible Teflon tube during the variable-temperature studies to allow for expansion and contraction of the solvent. The temperature of each of the cells was monitored with a thermistor attached directly to the sample cell wall. Carbon tetrachloride (Burdick and Jackson high-purity solvent) was used as received. When dry CCI, was needed, the solvent was refluxed and distilled from P20s. The complete removal of (9) (a) Kinard, W. F.; McDowell, W. J.; Shown, R. R. Sep. Sci. Techno/. 1980, 15, 1013-1024. (b) Kinard, W. F.; McDowell, W. J. J. Inorg. Nucl. Chem. 1981, 43, 2947-2953. (c) McDowell, W. J.; Case, G.N.; Aldrup, D. W. Sep. Sei. Techno/. 1983, 18, 1483-1507. (d) Burns, J. H. Inorg. Chim. Acta 1985, 102, 15-21. (e) McDowell, W. J.; Moyer, B. A,; Case, G. N.; Case, F. 1. Solvent Extr. Ion Exch. 1986, 4, 217-236. (f) Moyer, B. A,; McDowell, W. J.; Ontko, R. J.; Bryan, S. A,; Case, G.N. Soluent Extr. Ion Exch. 1986. 4 , 83-93. (g) Burns, J. H.; Kessler, R. M. Inorg. Chem. 1987, 26, 1370-1375. ( h ) McDowell, W. J.; Moyer, B. A,; Bryan, S.A,; Chadwick, R. B.; Case, G. N. Proc. Int. Solvent Extr. Conf. (ISEC ‘86) Munchen, FRG 1986, 1, 477-482. (i) Burns, J. H.; Bryan, S. A. Acta Crystallogr. 1988, C44, 1742-1746. 0 ) McDowell, W . J. Sep. Sei. Technol. 1988, 23, 1251-1268. (k) Chadwick, R. B.; McDowell, W . J.; Baes, C. F., Jr. Sep. Sci. Technol. 1988. 23. . . ..., ~1311-1324 .. . .
( I O ) Baes. C. F., Jr.; McDowell, W. J.; Bryan, S. A. Soloent E x f r . Ion E.tch. 1987. 5 . 1-28.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5231
Hydration of 18-Crown-6 in Carbon Tetrachloride
w
0
’
z
3
015-
z$
010-
a:
005-
000-
I
1
t 0 00
I
0 05
0 10
0 15
0 20
TOTAL [18C6]
Figure 2. Plot of absorbance of O H stretching bands vs concentration of added 18C6 at 25.0 OC. (Data taken from Figure I.) The solid curves represent the best fit assuming only formation of the 1:l 18C6-H20 adduct according to eq I . Nine parameters were varied in the fit, including the molar equilibrium constant and the molar absorptivities for free H 2 0 and the adduct at each of four frequencies (see supplementary material).
the H 2 0 molecule at 3707 and 3616 cm-l are visible in the O H stretching region (Figure IA). As 18C6 is added, these bands are gradually replaced by bands with maxima at 3687, 3601, and 3535 cm-’. An approximate isosbestic point occurs at 3698 cm-l, suggesting the simple formation of a 1:l adduct, according to the equation 3800
3600
3400
WAVE N UMBERS
Figure I . Spectral overlay of varying concentrations of 18C6 in a CCI, solution containing constant H 2 0 concentration. The spectrum of dry CCI, has been subtracted out in each case. Path length = 0.35 cm (thermostated AgBr cell). Temperature = 25.0 OC. [H2OItotal= 8.7 X IO-’ M. [18C6],,,,, = 0.00 (A), 0 . 0 0 5 0 4 (B), 0.0101 (C), 0.0202 (D), 0.0504 (E), 0.101 (F), and 0.202 M (G).
water from the solvent was confirmed by the absence of water bands in the IR spectrum. To prepare CCI, solutions containing known amounts of water, aliquots of CCI, saturated with excess water at 25.0 f 0.1 OC were diluted with dry CCI,. Solutions were prepared with freshly cleaned and dried volumetric glassware, and all solvents were manipulated through septa by using syringe techniques. All molarities were corrected for density changes of the solvent as a function of temperature. The crown ether 18crown-6 (Aldrich) was recrystallized from hexanes and dried in a vacuum desiccator prior to use.
Results and Discussion The IR experiment summarized in Figures 1 and 2 shows that 18C6 at concentrations less than ca. 0.2 M in CCl, interacts with dissolved H 2 0 to form the 1:1 18C6-H20 adduct at 25 OC. In the experiment, the total H 2 0 molarity ([H20],,,,,) in CCI, was M while the total 18C6 molarity held constant at 8.7 X ([ 18C6],,,,,) was increased from 0.0 to 0.2 M. With no added 18C6, only the asymmetric and symmetric O H stretchesll-20 of (1 I ) Jacob, J.; Leclerc, J.; Vincent-Geisse, J. J. Chim. Phys. 1969, 66, 970-976. (12) (a) Schioberg, D.; Luck, W. Specrrosc. Lerr. 1977, 10,613-618. (b) Burneau, A.; Corset, J. J . Chim. Phys. 1972, 69, 142-152. (13) (a) Magnusson, L. B. J . Phys. Chem. 1970, 74, 4221-4228. (b) Shippey, T. A.; Symons, M. C. R.; Brivati, J. A. Mol. Phys. 1979, 38, 1693-1698. (c) Christian, S.D.; Johnson, J. R.; Affsprung, H.E.; Kilpatrick, P. J. J. Phys. Chem. 1966, 70, 3376-3377. (14) McTigue, P.; Renowden, P. V. J . Chem. Soc., Faraday Tram. I 1975, 71. -.1784-1789. (15) Kirchner, H . H . Z . Phys. Chem. 1970, 73, 169-183. (16) Greinacher, Von E.; Luttke, W.; Mecke, R. Z. Elekrrochem. 1955, -TP- , -21-11 - - .. (17) Mohr, S. C.; Wilk, W. D.; Barrow, G . M. J . Am. Chem. SOC.1965, 87, 3048-3052. (18) Josien, M. L. Discuss, Faraday SOC.1967, 43, 142-144. (19) Bellamy, L. J.; Pace, R. J. Spectrochim. Acra 1972, 28A. 1869-1876.
18C6 + H 2 0
18C6.H20
(1)
By use of least-squares procedures,10,21plots of absorbance at 3709,3687,3601, and 3535 cm-I vs [18C6],, (Figure 2) confirm the formation of a 1:l adduct according to eq 1 (see supplementary materal; a paragraph at the end of the paper discusses the availability of supplementary material). The molar equilibrium constant (concentration quotient) at 25.0 OC was calculated to be 15.6 f 1.2 M-I, within the range 4-70 M-’ observed in other studies2 of macrocyclic ethers under different conditions of solvent and temperature. For the peak maxima 3687, 3601, and 3535 cm-’ assigned to the 1:l 18C6-H20 adduct, the molar absorptivities ( e . M-l cm-I) were found to be 45 f 2, 101 f 3, and 114 f 3, respectively. For the peak maximum of 3709 cm-l assigned to free water, t was found to be 32 f 1, in excellent agreement with the reported values of 3114 and 34.20 In the analysis, 18C6 and H 2 0 were treated as being monomeric in CCI, (eq 1). Previous results show that H 2 0 dissolved in CCI, has little aggregation tendency (less than 2-3% dimer under our condit i o n ~ ) . ~ Good ~ * ~ evidence ~ - ~ ~ that 18C6 is also monomeric in CCI, is furnished by (a) the independence of carbon-I3 NMR chemical shifts vs 18C6 concentration in CC14,25 (b) the monomeric equilibrium behavior of 18C6 in benzene, in which 18C6 gives the same IR spectrum as in CC14,2sand (c) the monomeric nature of dicyclohexano-l8-crown-6(at concentrations comparable with those used here) in wet CCI, as shown by vapor-phase osmometry.’ The formation of 1:l crown-water complexes as observed here and elsewhere2 can be insightfully compared to the hydration of simple ethers and other monofunctional bases in CCI,. A reasonably large body of work employing IR spectrophotometry,12-20-26-28 N M R ~ p e c t r o m e t r y , ~and * , ~thermodynamic ~ mea(20) Clew, D. N.; Rath, N. S. Can. J. Chem. 1971, 49, 837-856. (21) Busing, W. R.; Levy, H. A. “OR GLS, A General FORTRAN Least Squares Program”; Report ORNL-TM-271; Oak Ridge National Laboratory: Oak Ridge, TN, 1962. (22) Muller, N.; Simon, P. J . Phys. Chem. 1967, 71, 568-572. (23) (a) Masterton, W. L.; Seiler, H. K. J . Phys. Chem. 1968, 72, 4257-4262. (b) Johnson, J. R.; Christian, S . D.; Affsprung, H. E. J. Chem. SOC.1965, 1-6. (c) Goldman, S. Can. J . Chem. 1974, 58, 1668-1680. (24) Johnson, J. R.; Christian, S. D.; Affsprung, H. E. J . Chem. SOC.A 1966, 77-78. (25) Mosier-Boss, P. A.; Popov, A. 1. J . Am. Chem. SOC.1985, 107, 61 68-61 74.
5232 The Journal of Physical Chemistry, Vol. 94. N o . 13, 1990
Bryan et al.
TABLE I: Infrared OH Stretching Frequencies of Ether Hydrates in CCI," type I A 1 : 1 complex baseb
tempc
THF 30 25
p-diox
30 25
Et20
baseb 18C6
temo' 4.4 25.0 55.3
type IB 2:l complex
unbonded O H 3687 3683 ( e = 60) 3690 ( t = 142)
bonded OH 3483 3500 3520 ( t = 115)
3690 3683 3689 3684 3687 3687 3689
3520 3510 3513 (e
= 86)
3490 3490 3469
(e
asym HOH 3573 3572 (E = 128)
sym H O H 3508 3499 ( e = 1 1 1 )
rmsd 3541 3536
3584 3585 3583 3579 3580
3518 3525 3512 3516 3509
355 1 3555 3548 3548 3545
3600 3555
3525 3520
3563 3538
= 83)
type IIA 1 : l complex unbonded OH bonded O H 3684 ( w = 13) 3687 (w = 16) 3688 (w = 18) 3534
type IIB 1 : l complex asvm HOH svm HOH 3597 ( w = 29) 3534 (0= 34) 3601 ( u = 29) 3608
rmsd 3566
ref 19 20 14 16 17 18 19
20 14 17 19 ref this work this work this work
Hydrogen-bond acceptor: a Frequencies and full widths ( w ) at half-height are given in cm-'. Molar absorptivities (e) are given as M'I cm-'. THF, tetrahydrofuran; p-diox, p-dioxane; E t 2 0 , diethyl ether; and 18C6, 18-crown-6. 'Cell temperature ("C) if specified. dRoot mean square of the frequencies of the asymmetric and symmetric HOH stretches for type 1B and IIB adducts.
~ u r e m e n t s ~has ~ *shown ~ ~ - ~that ~ these simpler bases form 1:l and 2:l base-water adducts in nonpolar solvents as shown below for the case of ethers:
i
R IA
I
H
R
as evidenced by two broad, shifted HOH stretches lying in the ranges 3572-3600 and 3499-3525 cm-1.15-20926 Thus, in view of the presence of the narrow band at 3687 cm-' in the spectra of the I8C6-water adduct (Figure 1 and Table I), one of the isomers is plainly a species in which the H 2 0 molecule is attached by only a single hydrogen bond in monodentate fashion (IIA), as shown below:
IB
The 1:l adduct (IA) is the predominant hydrate at ambient 0 0 6b temperature provided the ether concentration is below ca. 1 M in CCI4. Thus, in our experiments at [18C6],,,,, 5 0.2 M, the IIA IIB 2: 1 18C6-H20 adduct is expected to be unimportant, as supported Assuming hydrogen bond strengths comparable to the other ethers by the observed equilibrium behavior (Figure 2). In addition, we in Table I, the expected broad bonded OH stretch of IIA would note that the OH stretching bands in H20-saturated CC14 solutions lie in the range 3490-3520 cm-I, close to the position of the broad of 18C6 obey Beer's law up to at least 0.1 M. This behavior peak observed in Figure 1 at 3535 cm-'. We assign the broad follows since water saturation (unit water activity) assures that band at 3601 cm-I to the asymmetric OH stretch of a second 1:l [ H 2 0 ] = 8.7 X M.24Thus, for the formation of only the 1 : l 18C6-Hz0 species in which the H20molecule is attached by two adduct the ratio [18C6.H20]/[18C6] is constant, and with the hydrogen bonds in bidentate fashion (IIB), analogous to IB except mass-balance expression [ 18C6],,,,, = [ 18C6.H20] + [ 18C61, it that the ether oxygen atoms both belong to the same molecule. is apparent that [ 1 8C6.H20] (together with its corresponding The expected (Table I) position of the asymmetric O H stretch absorbance) is proportional to [ 1 8C6],,l. Deviations from Beer's of IIB would be 3570-3600 cm-I. Since the corresponding symlaw would occur if the 2:1 18C6-H20 adduct formed significantly. metric OH stretch of IIB would be expected in the range Since the equilibrium analysis showed the presence of only the 3499-3525 cm-I (Table I), it must be completely superimposed 1:l l8C6-H20 adduct, the presence of three major IR bands upon the bonded OH stretch of the monodentate form IIA. (3687, 3601, and 3535 cm-I) attributable to O H stretches beReferring to the peak positions shown in Figure 1, we conclude longing to bound H 2 0 implies the coexistence of more than one that the hydrogen bonding in IIA and IIB is, in the spectral sense, isomeric form; we attribute a weak band at approximately 3220 of near normal strength for ether-water interaction by comparison cm-' to the expected'2-m*2b28 overtone of the bending OH vibration. with the peak positions reported for IA and IB (Table I).12-20,262s We base our assignments of the three OH stretching bands upon Close comparison of the positions of the bonded OH stretching extensive studies of the vibrational spectra of the hydrates of ethers bands of IIA with those of IA indicates a slightly weaker hydrogen and other weak bases in nonpolar solvents.12-20~26-28 Table I bond in IIA, as follows from its slightly higher stretching frequency summarizes the reported bands and assignments corresponding (Le., smaller shift from the value of free water). A similar conto the water stretching vibrations of the I : ] (IA) and 2:l (IB) clusion is possible for IIB by comparing the positions of the base-water adducts of tetrahydrofuran, p-dioxane, and diethyl asymmetric and symmetric OH stretching bands of IIB and IB. ether. Briefly, IA complexes are characterized by C, local symmetry in which only one of the water hydrogens is b ~ n d e d . ' ~ - ' ~ - Table ~ ~ I lists the root mean square (rms) of the asymmetric and symmetric OH rms as a measure of the strength of the OH bonds The unbonded OH stretching band is sharp and lies in the range in IIB, and we conclude from the higher frequency of the rms 3683-3690 cm-', whereas the bonded OH stretching band is compared to the frequency of the bonded OH stretching band of broadened and shifted markedly to lower energy (3469-3520 cm-l) 11A that the hydrogen bonds in IIB are individually weaker than relative to free On the other hand, IIA complexes the single hydrogen bond in IIA. We also note that, with regard involve a doubly hydrogen-bonded water in Cz,)local symmetry, to the molar absorptivities determined for the 1:l adduct (Figure 2), the values 45 i 2, 101 f 3, and 114 f 3 M-l cm-I corre(26) Narvor, A. L.; Gentric, E.; Saumagne, P. Can. J . Chem. 1971. 49. sponding to the peak maxima 3687,3601, and 3535 cm-' represent 1933-1 939. (27) Moyer, B. A.; Caley, C. E.; Baes, C. F., Jr. Solvent Exrr. Ion Exch. reasonable values for a combination of IIA and IIB based on 1988.6. 785-81 7. published molar absorptivitites for ether hydrates IA and IB (Table (28) Narvor, A . L.; Gentric, E.; Lauransan, J.; Saumagne, P. J . Chem. 1). Published spectral' of mixtures of IA and IB, in fact, show SOC.,Faraday Trans. I 1916, 72, 1329-1 332. a remarkable resemblance to the spectra in Figure 1, except that (29) Takahashi, F.; Li, N. C . J . A m . Chem. SOC.1966. 88. I 117-1 120.
Hydration of 18-Crown-6 in Carbon Tetrachloride
3800
3700
3600
3500
cm-
'
3400
3300
M at 25.0 Figure 3. FTIR spectra of a solution of 18C6 (1.53 X "C) and H 2 0 ( 4 X IO-' M at 25.0 "C) in CCI, taken at various temperatures in CCI,. For each spectrum shown, the spectrum of unbound (free) water at the indicated temperature has been subtracted out by use of the software supplied with the Digilab FTS-60; thus, only the bands of bound water are visible.
the relative peak heights in our system exhibit no dependence on the base and water molarities. Since the relative concentrations of IIA and IIB could not be varied by changing the 18C6 and H 2 0 concentrations at 25 "C, it was desirable to determine the effect of temperature on the OH stretching spectra to find evidence for reversible interconversion of the isomers. As shown in Figure 3, we collected the FTIR spectra of 1 8C6-H20 solutions in CCI, over the range 4.4-55.3 OC. Spectral parameters are given in Table I. The data indeed indicate a gradual shift from predominance of spectral features assigned to IIB at the lower temperature extreme to predominance of spectral features assigned to IIA at the upper extreme. The shift was found to be fully reversible, giving reproducible spectra regardless of the direction of temperature cycling. Both species apparently involve negative enthalpies of formation since the absolute absorbances of all bound-water bands decrease as the temperature is raised. Because of the expected dependence of molar absorptivities on temperature (possibly as much as 30% change over the whole range),'* a full thermodynamic analysis including appropriate correction for molar-absorptivity change by peak-shape analysis is nontrivial and will be reported at a later date together with data on the hydration of other crown ethers. The existence of two modes of water binding by 18C6 in CCI, represents an example of a host-guest interaction in which the host does not completely accommodate the bonding sites of the
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5233 guest despite being conformationally able to do so. In the present case, the bonding interactions are limited to two, and they are expected to be weakly favorable; the enthalpy of hydrogen bonding of water to simple ethers lies in the range -3 to -4 kcal/mol per bond based on either expermental value^'^,*^ or EC parameter^.^^ Precedence for the bidentate form of water bound by 18C6 may be found in the reported X-ray structures6-*and Monte Carlo calculation^,^ and thus, it does not seem surprising to see evidence for this binding mode in our solution FTIR experiments. In fact, given the favorable bonding conditions for the bidentate form, it may be asked why the monodentate form is as important as it is. Supporting this point, a statistical mechanical estimate3' of the fraction of bound water that would be bidentate if all of the 18C6 oxygen atoms were ideally placed for bidentate water binding is 99% or greater at 25 OC (taking the enthalpy of hydrogen bond formation to be -3 to -4 kcal/mol). Thus, many of the macrocycle oxygen atoms in the various populated conformations of 18C6 in CCI4 are probably not ideally placed for bidentate bonding, and some unfavorable (by enthalpy or entropy) conformational adjustments may be needed to accommodate the second hydrogen bond. Some support for this conclusion comes from Monte Carlo calculations$ which show that the stable Ciconformation of 18C6 does not promote bidentate water binding. In summary, evidence has been presented for the formation of a 1 : I crown-water complex that consists of two isomeric forms wherein the bound water may either be mono- or bidentate. The evidence is harmonious with, and extends naturally from, what has been previously learned of the hydration of both simple and macrocyclic ethers in solution and in the solid state.
Acknowledgment. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US. Department of Energy, under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. This work was also made possible by appointments to the Faculty Research Participation Program and Postgraduate Research Training Program sponsored by the U S . Department of Energy and administered by Oak Ridge Associated Universities (S.A.B.) and to the Oak Ridge Science Semester Program at Oak Ridge National Laboratory under contract with the Great Lakes Colleges Association/Associated Colleges of the Midwest (R.R.W.). We thank G.M. Begun, J. H. Burns, E. Johnson, and J. M. Simonson for helpful comments in preparing the manuscript. Supplementary Material Available: Text describing the derivation of equilibrium model, FORTRAN code, and table listing the least-squares output (includes input experimental data) for fitting the data in Figures 1 and 2 (6 pages). Ordering information is given on any current masthead page. (30) Drago, R. S. S t r u t . Bonding 1973, 15, 73-139. (31) Johnson, E. Oak Ridge National Laboratory, personal communication, 1989.
