8881
J. Phys. Chem. 1991, 95, 8881-8886
for 6 min a t only 298 K). A preoxidized surface containing appreciable adsorbed CO did not provide a band a t 2250 cm-l,] which indicates that CO displaces N2 from Rho sites on a partially oxidized Rh/AI2O3 surface. The other three bands noted above were observed on the preoxidized surface even in the presence of heavy CO contamination. It should be noted that the 2305-cm-l band in Figure 4 decreased in intensity with a concomitant increase in intensity of the 2250-cm-I band as the pressure of N2 was raised. This indicates that N 2 may cause some conversion of Rh6+sites to Rho sites, although we do not understand the mechanism for this process because N2 is not generally considered to be a reducing agent. Although our early work’ seemed to show that the N2 causing the 2250-cm-’ band was present on the same surface sites as those giving rise to the “gem-dicarbonyl” species, which are known* to correspond to Rh+, it is now clear from Figure 3 that the 2250cm-l band must correspond to N2adsorbed on a metallic Rh site. This conclusion was reached earlier by Wang and Yates2 in their elegant low-pressure, low-temperature studies. (8) Rice, c. A.; Worlev, S. D.; Curtis, C. W.; Guin, J. A.; Tarter, A. R. J . Chem. Phys. 1981, 74, 6487 and references cited therein.
Conclusions All IR-detectable traces of CO and C 0 2 can be removed from high-pressure N 2 (99.999%) if it is passed through a catalytic converter containing Rh/A1203 powder maintained at 373 K and if trapping at 158 K is employed. The resulting pure N2provides the same Rh/AI2O3 surface species a t 8000 Torr and 298 K as does pure N2 at low temperatures and pressures. An infrared band at ca. 2250 cm-’ can be assigned to the adsorption of N2 on metallic Rh. A band at 2301 cm-’ observed for preoxidized Rh/A1203 or prereduced Rh/A1203 when substantial impurity CO is present can be assigned to adsorption of N 2 on supported Rh*+, with CO serving as the oxidizing agent when it is present as an impurity on a prereduced surface. Acknowledgment. We thank the Strategic Defense Initiative Organization’s Office of Innovative Science and Technology through Contract N60921-86-C-A226 with the Naval Surface Warfare Center and the U S . Army Advanced Concepts and Technology Committee through Contract DAAAl5-88-K-0001 (J.P.W. and W.C.N.) for support of this work. Registry No. Rh, 7440-16-6; N2. 7727-37-9; CO, 630-08-0; C02, 124-38-9.
Infrared Spectroscopy at High Pressure: Interaction of H, and D, with Rh/Ai,O, J. P. Wey, W. C. Neely, and S. D. Worley* Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: May 6, 1991)
A novel high-pressure-infrared-cell reactor has been employed to study the interaction of ultrapure, high-pressure H2 and D2 with Rh/A1203films. The frequencies for the Rh-H and Rh-D stretching modes were measured to be 2013 and 1441 cm-I, respectively. In addition, a band at 1618 cm-’ for water on the A1203support was observed to develop concomitantly with the 201 3-cm-’ band. It was postulated that H2 dissociates on the Rh sites to produce a weakly bound Rh-H surface species, with the remaining H spilling over to the support where it reacted with surface hydroxyl groups to produce H20. The new Rh-H species was easily removed by reduction of H2 pressure above the catalyst. This weakly bound species is probably relevant to the catalytic chemistry for reactions involving H2 as a reactant over supported Rh. It was further suggested that the heretofore elusive Rh-H stretching mode for the previously postulated Rh carbonyl hydride surface species, observed spectroscopically during catalytic methanation, is present as a weak low-frequency shoulder on the band due to the C-O stretch for that species.
Introduction Infrared spectroscopy has become one of the most, if not the most, important analytical probes for identifying and monitoring surface species on supported transition-metal catalysts. While the vast majority of work reported in this area has concerned surface species produced at low pressures (generally 1200 Torr), recent studies in these laboratories have focused on those species generated at higher pressures (up to 104 Torr) such as commonly exist in industrial reactors. For these investigations a novel high-pressure-infrared-cell reactor was designed and constructed. This reactor, which is capable of operation in the 104-104-Torr pressure regime and a t temperatures ranging from 100 to 600 K, has been described in detail recently; it was employed in a study of the interaction of high-pressure N 2 with Rh/AI20, at ambient temperature.’ Detection of an infrared band corresponding to Rh-H for supported Rh has proved elusive. When CO is hydrogenated over supported Rh catalysts at ca. 100 Torr or less total pressure, infrared bands for the usual CO/Rh species (“gem-dicarbonyl”, ‘linear”, and “bridged”)2 are not detected; rather, only a new band centered between 2020 and 2045 cm-I dependent upon coverage is ob~erved.~The same band is detected upon hydrogenation of Author to whom correspondence should be addressed.
0022-365419112095-8881$02.50/0
C 0 2 over supported Rh, its frequency dependent upon the nature of the support (2020-2025 cm-l for Rh/A1203, 2028-2032 cm-l for Rh/Si02, 2038-2042 cm-’ for Rh/Ti02).4 Solymosi and co-workers have attributed this band to the C-O stretching mode of a ‘rhodium carbonyl hydride” species.s*6 U ‘ I \
-0
/”
Rh
Work in these laboratories has supported this assignment. Deuterium substitution caused a red shift of ca. 5-10 cm-l which is close to the expected shift for D-isotopic substitution two bonds removed from the C O oscillator (high quality a b initio computations for Rh(C0)H vs Rh(C0)D in the gas phase predict a red shift of ca.3 cm-’).’,* However, a puzzling point has always been (1) Wey, J. P.;Burkett, H. D.; Neely, W. C.; Worley, S.D. J . Am. Chem. Soc. 1991, 113, 2919. (2) Yang, A. C.; Garland, C. W. J . Phys. Chem. 1957,61, 1504. (3) Worley, S.D.; Mattson, G. A.; Caudill. R. J . Phys. Chem. 1983, 87, 1671. (4) Henderson, M. A.; Worley, S. D. J . Phys. Chem. 1985. 89, 1417. (5) Solymosi, F.; Erdohelyi, A.; Kocsis, M. J . Cural. 1980, 65, 428. (6) Solymosi, F.; Erdohelyi, A. J . Curul. 1981, 70, 451. (7) Henderson. M. A.; Worley, S.D. J . Phys. Chem. 1985.89, 392. (8) McKee, M. L.; Dai, C. H.; Worley, S. D. J . Phys. Chem. 1988, 92, 1056.
0 1991 American Chemical Society
8882 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 that if the carbonyl hydride species is indeed present during the hydrogenation of CO and C 0 2 over supported Rh, then where is the expected infrared band corresponding to the Rh-H stretching mode? This question was addressed at length without a final definitive answer being provided in our earlier work.* Efforts to detect the Rh-H stretching band for catalytic surfaces exposed to low-pressure (e100 Torr) H2 alone have not been encouraging either. Early studies of the interaction of H2 at pressures 1 5 Torr with a Rh film (“mirror”) using a multiple reflection IR cell claimed to have been successful in detecting 23 infrared absorption bands in the 1500-2200-~m-~ region attributable to adsorption of hydrogen on the Rh s ~ r f a c e .One ~ of the authors in a later paper suggested that the many bands were due to multiple crystallographic sites available on the Rh polycrystalline films.I0 We are unable to offer an alternative explanation for these observations, but in any case, such bands do not exist for H2 interacting with supported Rh. There has been a published report of the infrared spectra for the H2/Rh/AI2O3catalyst system at low pressure (1 -1 W5Torr) and temperatures ranging from 173 to 303 K.” In that work infrared bands were observed in the 1890-1 930-cm-’ region which did not disappear upon evacuation. Although the authors attributed their bands to Rh-H species, it is likely that since the bands were not removed upon evacuation, they corresponded to the bridged Rh carbonyl species produced by a small amount of CO impurity in the H2 which was employed. Thus, to our knowledge the IR band or bands for the Rh-H stretching mode(s) for supported Rh have not yet been reported even though it is well-known that H2 does interact with supported Rh catalysts.12 On the other hand, the metal-hydrogen stretching band has been observed for other supported transition metals such as Pt,’*16 Ir,l7 and Ni.’” In the case of Pt/MgO a reversible interaction of hydrogen with the transition metal has been reported to give rise to IR bands at 21 20 and 2060 cm-’ attributable to terminal Pt-H species, whereas an irreversible form of hydrogen corresponding to strongly adsorbed hydrogen gave a band at 950 cm-’ assigned to a hydrogen atom interacting with more than one Pt atom.I3 The 2120- and 2060-cm-’ bands had been observed previously and assigned to reversibly bound Pt-H,I4J5 although some believe that the 2060-cm-’ band is actually due to adsorbed impurity C 0 . l 6 An infrared band at 1880 cm-’ that shifted to 1360 cm-‘ upon deuterium substitution was reported for the Ni-H stretch for Ni/A1203 at 293 K and 200 Torr of hydrogen pressure.I7 The deuterium labeling experiment established that the 1880-cm-’ band for this system was not due to impurity CO. It was conceivable before the current study that all of the adsorbed hydrogen for supported Rh was strongly bound in an irreversible fashion which was either infrared inactiveI9 or in a region of the spectrum (below 1000 cm-I) masked by strong continuous absorption by the support. In should be noted that the IR band for the Rh-H stretch in organometallic Rh hydride complexes generally falls in the 2000-2 150-cm-’ region,sx which, (9) Pickering, H. L.; Eckstrom, H. C. J . Phys. Chem. 1959, 63, 512. (IO) Smith, W. H.; Eckstrom, H. C. J . Phys. Chem. 1968, 72, 369. ( I I ) Kavtaradze, N. N.; Sokolova, N. P. Russ. J . Phys. Chem. 1970,44, 1485.
( I 2 ) For example, see the NMR studies in: Sanz, J.; Rojo, J. M. J . Phys.
Chem. 1985,89, 4974. (13) Candy, J. P.; Fouilloux, P.; Primet, M. Surf Sci. 1978, 72, 167. (14) Dixon, L. T.; Barth, R.; Gryder, J. W. J. Catal. 1975, 37, 368. ( I 5 ) Pliskin, W. A.; Eischens, R. P. Z . Phys. Chem. 1960, 24, 1 I . (16) See: Szilagi, T. Infrared Spectroscopy of Adsorbed Hydrogen. In
Hydrogen Effects in Cafalysis;Paal, Z . , Menon, P. G., Eds.; Dekker: New York, 1988; p 183 and references quoted therein. ( I 7) Bozon-Verduraz, F.; Contour, J.; Pannetier, G. C.R. Acad. Sci. (Paris) 1969, 269, 1436. (18) Nakata, T. J. Chem. Phys. 1976, 65,487. (19) Delgass, W. N.; Haller, G. L.; Kellerman, R.; Lunsford, J. H. Spectroscopy in Heterogeneous Catalysis; Academic Press: New York, 1979 pp 34, 60. (20) Takesada, M.;Yamazaki, H.; Hagihara, N. Bull. Chem. SOC.Jpn. 1968, 41, 270. (21) Ito, T.; Kitazume, S.;Yamamoto, A,; Ikeda, S.J . Am. Chem. SOC. 1970, 92, 301 1. (22) Dewhirst, K. C.; Keim, W.; Reilly, C. A. Inorg. Chem. 1968, 7, 546.
Wey et al. of course, is the region in which the C-0 stretching modes for CO/Rh species occur. Thus, it is possible that the Rh-H bands for supported Rh have not been detected and/or assigned because of interference caused by strongly absorbing C O or impurity CO species. However, recent sophisticated ab initio calculations have predicted the frequency for gas-phase Rh-H to lie in the extensive range of 1150-2030 cm-I, depending upon the basis set employed,8*25-28 so an accurate experimental determination of the Rh-H stretching frequency for supported Rh is certainly needed. The current work in which high-pressure, ultrapure H2 and D2 were employed for Rh/AI2O3at ambient temperature was successful in detecting an IR band at 201 3 cm-’ for weakly bound, reversible hydrogen on Rh. This work should be relevant to the many catalytic processes in which hydrogen is a participating reactant. Experimental Section The supported Rh/A1203samples (2.2 wt 76 Rh) were prepared as IR-transparent films by spraying a slurry of RhC13.3H20 (Johnson Matthey), A1203(Degussa Aluminumoxid C, 100 m2 g-I), spectroscopic grade acetone, and distilled-deionized water onto a 25-mm CaF2 IR window held at 353 K. The solvent evaporated, leaving a film of RhC13.3H20/A1203containing 4.4 mg cm-2. The window containing the film was then mounted in the high-pressure-infrared-cell reactor which has been described in detail.’ In the reactor the film was evacuated at 373 K to 10” Torr for 1 h, then reduced to 2.2% Rh/AI2O3 by exposure to 100 Torr of H2 at 473 K in exposure/evacuation cycles of 10, 5, 10, and 20 min, and finally evacuated to 10” Torr at 298 K for 1 h. The H2 used for the reduction cycles and later for the highpressure exposures was obtained from Air Products at a stated purity of 99.999% and subjected to a catalytic converter to remove all CO impurity with subsequent trapping at 77 K. The catalytic converter, which has been demonstrated to remove all impurity CO from high-pressure N 2 through adsorption and oxidation to C 0 2 by impurity oxygen, was constructed from stainless steel tubing and contained 2 g of dry 5% RhCI3-3H2O/Al2O3powder prepared similarly to the catalytic films.29 The powder in the converter was heated at 373 K at 10” Torr for 1 h, reduced in 400 Torr of H2 at 423 K for 10 h, and evacuated at 10” Torr at 298 K before introduction of the high-pressure H2 (or D2) samples needed in the experiments. The converter was held at 373 K during the introduction of all high-pressure H2 or D2. The 77 K trap removed any C02produced by oxidation of impurity CO by impurity oxygen. The IR spectra to be presented in this study were obtained by using an IBM 32 Fourier transform spectrometer operated at 2-em-’ resolution. Generally, IO00 scans were accumulated over a period of 15 min for each spectrum, although this number was necessarily less for the isotopic exchange experiments to be described. Results and Discussion Figure 1 shows the results obtained when successively increasing doses of ultrapure H2 were added to the prereduced 2.2% Rh/ A1203catalyst film at 298 K. A weak broad band at ca. 2013 cm-’ began to develop at a pressure of 100 Torr (Figure 1a), and this band intensified as the pressure was increased to a level of 8003 Torr (Figure le). A concomitant increase was observed for a band which developed at 1618 cm-I. Neither of these bands was produced for high-pressure H2 over AI2O3alone. The band at 201 3 cm-l can be assigned to the Rh-H stretching mode for (23) Saceo, A.; Ugo, R.;Moles, A. J . Chem. SOC.A 1966. 1670. (24) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem. SOC.A 1966, 1711. (25) Balasubramanian, K.; Liao, D. W. J . Chem. Phys. 1988, 88, 317. (26) Paniagua, J. C.; Illas, F. Chem. Phys. Lerr. 1990, 170, 561. (27) Rochefort, A.; Andzelm, J.; Russa, N.; Salahub, D. R. J . Am. Chem. Soc. 1990, 112, 8239. (28) Mains, G . J.; White, J. M. J . Phys. Chem. 1991, 95. 112. (29) Wey, J. P.; Neely, W. C.; Worley, S. D. J . Phys. Chem. 1991, in
press.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8883
Interaction of H2 and D2 with Rh/A120,
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at 298 K and increasing pressures of (a) Figure 1. Infrared spectra for the interaction of H2with a prereduced 2.2%Rh/A1203film (4.4 mg 100, (b) 1033, (c) 2109, (d) 4063, and (e) 8003 Torr. The equilibration time was 10 min at each new pressure.
Figure 2. Infrared spectra (differencespectra) for the interaction of H2 with a prereduced 2.2%Rh/AI2O3film (4.4 mg of (a) 302, (b) 903, and (c) 8003 Torr. The equilibration time was 10 min at each new pressure.
a Rh-H surface species. It is definitely not due to a RhCO species from C O impurity because it has been demonstrated that the catalytic converter used in this study removes all measurable CO. In any case, a low coverrIge of impurity C O would give rise to the three RhCO species normally observed (gem-dicarbonyl at 2100 and 2030 cm-I, linear at 2040-2080 cm-l, and bridged at 1800-1 950 cm-I), but none of which were observed. The band at 1618 cm-' can probably be attributed to H 2 0 adsorbed on the AI2O3 s ~ p p o r t . ~ * ~ ' Cavanagh and Yates have reported the infrared spectra for 100 Torr of D2 interacting with 2.2%Rh/AI2O3 at 300 K.32 Although they did not apparently resolve an IR band attributable to Rh-D at low pressure, they did observe deuterium exchange with surface hydroxyl groups and postulated rapid D2 dissociation on Rh sites, (30) Pliskin, W.A.; Eischens, R. P.J . Phys. Chem. 1955, 59, 1156. (31) Peri. J. B.; Hannan, R. B. J. Phys. Chem. 1960,64, 1526. (32) Cavanagh, R. R.; Yates, J. T. J . Coral. 1981, 68, 22.
at 298 K and pressures
followed by fast spillover to A1203,slow migration across A1203, and then fast exchange with surface hydroxyls.32 This process was greatly hindered by blocking Rh sites with adsorbed C0.32 In a similar interpretation, we would propose rapid dissociation of H2 on Rh sites to produce Rh-H, which we have observed spectroscopically, with the remaining H atoms spilling over to the support where they react with surface hydroxyl groups to produce H20on the support. In support of this interpretation, Figure 2 shows the concomitanr loss of intensity in the surface hydroxyl region of the IR as the 2013- and 1618-cm-I bands grow in intensity. Figure 3 illustrates that as H2 is expanded out of the cell reactor at 298 K the 2013-cm-' band decreases in intensity until it vanishes upon evacuation at lo4 Torr. On the other hand, the 1618-cm-l band does not begin to disappear until prolonged evacuation is effected. It is notable that the bands at 3741 and 3688 cm-l in Figure 2 do partially reintensify as the 201 3-cm-' band vanishes. Presumably a portion of the H atoms removed during evacuation
Wey et al.
8884 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
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2000
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1800
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HAVENUMBERS
Figure 4. lnfrared spectra for the interaction of D2 with a prereduced (in H2)2.2% Rh/AI2O3film (4.4 mg cm-2) at 298 K and increasing pressures of (a) 710, (b) 2120, (c) 4100, (d) 8010, and (e) 9382 Torr. The equilibration time was 10 min at each new pressure.
recombine with support 0 atoms to partially replenish the surface hydroxyls. Thus, the interaction of H2 with Rh/A1203causing the 2013-cm-l Rh-H band is rather weak. This is likely the ‘weakly bound, reversibly adsorbed hydrogen” for supported Rh which is analogous to the similar species suggested for supported Pt.13-16.19 In fact, the Rh-H bond would appear to be considerably weaker than the Pt-H bond given that high pressures of H2 are necessary to maintain the Rh-H surface species at ambient temperature. The H 2 0 surface species on Alto3 is much more strongly held. Figure 4 shows the infrared spectra corresponding to the interaction of ultrapure D2 with a 2.2% Rh/A1203 film. As can be seen, there is no band structure attributable to RhCO surface species, confirming our contention that the catalytic converter removes all CO from the high-pressure gas streams. The band centered near 1441 cm-l increases in intensity with D2 pressure in a manner analogous to that of the 2013-cm-I band for Rh-H.
The average isotope shift for the Rh-H mode obtained from published values for those organometallic Rh complexes for which data are available for both hydrides and the corresponding deuterides is 572 6 cm-I.*% When this average value is used, the predicted band center for the Rh-D mode is 1441 6 cm-l. Thus, there can be little doubt that our assignments for the Rh-H and Rh-D species are correct. The expected isotopically shifted bands for HDO and D 2 0 on the A1203 support could not be clearly resolved. The HDO band may be represented by the shoulder on the low-frequency edge of the 1441cm-’ band in Figure 4. The D 2 0 band would be expected near 1 150 cm-l; this band, if present, is obscured by the absorption cutoff due to the CaF2 windows and
*
*
A1203.
Figure 5 shows the infrared spectra in the 2000-4000-~m-~ region for the sample discussed for Figure 4. As the pressure of D2 was increased, isotopic exchange occurred on the A1203 support following D2 dissociation on the Rh sites, as well as probable
Interaction of H2 and D2 with Rh/AI2O3
"
i
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8885
I-'."
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Figure 6. Infrared spectra for H/D exchange over a prereduced 2.2% Rh/AI20, film (4.4 mg cm-2) at 298 K (a) in the presence of 48 Torr of Dz, (b) 25 s following the addition of 6520 Torr of Hz,(c) after 134 s, (d) after 25 min, and (e) after 171 min.
reaction with the hydroxyl groups to produce HDO and/or D20. The bands near 3727,3661, and 3563 cm-' corresponding to O H absorption decline in intensity, while those near 2742 and 2576 cm-l corresponding to OD absorption are enhanced. The spectra in Figure 5 bear close resemblance to those obtained by Cavanagh and Yates in their low-pressure D2/Rh/AI2O3 study.32 Figure 6 presents the results of an isotopic exchange experiment. The D2 from the sample film used for the data in Figures 4 and 5 was expanded from the cell reactor until the pressure reached 48 Torr. The weak broad band centered at 1441 cm-' in Figure 6a resulted. Then 6520 Torr of H2 was added to the cell, and the 1R spectra were monitored as a function of time. After 25 s of elapsed time, the data for spectrum 6b were accumulated. Already the 1441-cm-I band had almost vanished with the concomitant appearance of the 201 3-cm-' band for Rh-H and the 1618-cm-' band for H 2 0 on A1203. The exchange process was complete by 25 min of elapsed time (spectrum 6d). The kinetics
of the exchange process should be observable by FTIR if instrumentation utilizing superior rapid-scan, enhanced-sensitivity capabilities is available. Finally, with reference to the Rh carbonyl hydride species mentioned under Introduction, we can now propose a solution to the dilemma of the frequency of the Rh-H stretch in that species. We have often observed a weak low-frequency shoulder on the 2020-2040-cm-~ band which has been assigned to the C-O stretch of that species. Previously we thought that the weak low-frequency shoulder was due to a second carbonyl hydride species that contained two H atoms, because the presence of a second hydride group would be expected to lower the frequency of the C-0 stretching mode further by increased donation of electron density into the ll* orbital of the CO ligand. Now we can suggest that the weak low-frequency shoulder on the RhCO band could be due to the Rh-H stretching mode of the Rh carbonyl hydride species. It is conceivable that adsorbed CO might promote the bonding
'
J . Phys. Chem. 1991, 95, 8886-8891
8886
of H to Rh to form the carbonyl hydride species in the presence of low-pressure H2,33although our earlier a b initio calculations predict almost the same bond energy for Rh-H in the hydride and in the carbonyl hydride species.8 Conclusions
It can be concluded from this work that our high-pressure-infrared-cell reactor, in conjunction with a catalytic converter for purification of high-pressure gas streams, is a versatile piece of equipment that can be utilized to spectroscopically investigate problems of interest and importance to catalytic chemists. In this work we have detected, to our knowledge for the first time, a Rh-H (33) We thank a reviewer for this suggestion.
surface species on a supported Rh catalyst. Its frequency is 2013 cm-’ which, of course, is in the same general spectral region as C-O stretching frequencies for RhCO species such as the proposed Rh carbonyl hydride. It can also be concluded that the Rh-H species contains a rather weak Rh-H bond, which may well be a relevant point in explaining the catalytic chemistry involving hydrogen as a reactant over supported Rh.
Acknowledgment. We thank Professors G. L. Haller, J. T. Yates, and W. K. Hall for illuminating discussions relevant to this work. We acknowledge the support of the Office of Naval Research and the Strategic Defense Initiative Organization’s Office of Innovative Science and Technology. Registry No. Rh, 7440-16-6; H2,1333-74-0; D2,7782-39-0.
Linear Solvation Energy Relationships. Correlation and Prediction of the Distribution of Organic Solutes between Water and Immiscible Organic Solvents Yizhak Marcus Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: December 3, 1990; In Final Form: March 12, 1991)
Distribution data of monofunctional aliphatic and mono- and bifunctional aromatic solutes (species 2), having up to 10 carbon atoms, between water and 25 water-immiscible essentially dry solvents (species 1) have been correlated with properties of the solvents and solutes. The expression log Kwo = A,V2AbHI2+ A,p,,,& + A@dAaIyields essentially the same coefficients A,, A,, and A, for all the solvents, including aliphatic and aromatic hydrocarbons, halogenated hydrocarbons, ethers, and esters. These expressions are rationalized in terms of the solutesolvent interactions that take place in the two liquid phases. The A, and A, coefficients for the hydrogen-bonding terms are used to estimate the basicity and acidity of anions and cations on the same scale that is generally applicable to nonelectrolytic organic substances.
Introduction
In a previous paper by Kamlet et al.’ it was shown that the distribution of nonelectrolyte solutes between 1-octanol and water is well correlated by equations that are linear combinations of dependences on up to 5 solute parameters. The equations include a cavity term, depending on the volume of the solute, polarity/ polarizability terms, and hydrogen-bonding terms, pertaining to the hydrogen bond donation (HBD) and acceptance (HBA) properties of the solutes. Specifically, the logarithm of the distribution ratio of the solute between 1-octanol and water at infinite dilution, log Kwo was shown to obey the expression: log Kwo = 0.32 5.35Vi;/lOo - 1 . 0 4 ( ~ *- 0.366) - 3.848, O.IOa,, n = 245, r = 0.9959, u = 0.131 (1)
+
+
of the solvents and of water. In this context “dry” means that the water-saturated solvent has substantially the same properties as the neat solvent. Various modifications of eq 1 were tested, in order to obtain the simplest expression that is statistically compatible with the distribution data for the entire set of solvents. The solute property data set (subscript 2) included the molar volumes (at 25 “C), U2, intrinsic ~ o l u m e s ,VI, ~ - ~or Vfl,and the solvatochromic parameters 7r*2,62, a,2, and ,9,,2: The solvent properties data set (subscript 1) included the cohesive energy density (square of the Hildebrand solubility parameter), aHI2,the polarity/polarizability parameters a*’and 6’, and the HBD and HBA properties a I and 0,. The expression that is expected to describe the distribution data is, in analogy with eq 1
log Kwo = A b
+ A$V2 +
- A i s 2 ) + A’,am2 + A)BBmZ
In this expression Vi is the intrinsic volume of the solute,2 a* is its solvatochromic polarity parameter, 6 is a polarizability correction, 6, is the HBA ability and amis the HBD ability, both of the (monomeric) solute, n is the number of solutes in the correlation, r is the correlation coefficient, and u is the standard deviation of the fit. The term in a, is barely significant (the standard deviation of its coefficient of 0.10 is 0.04).’ In the present investigation, the distribution of organic solutes between each of 25 essentially ‘dry” water-immiscible solvents and water is correlated by means of expressions similar to eq 1, except that they take explicitly into account certain properties
where the A’ coefficients include the properties of the solvents relative to those of water and V2is either U2, Vi2,or Vx2.Also, the “absolute“ values of a*,Le., those relative to vacuum rather than to cyclohexane, were tried. These were obtained4 by the addition of 1.06 to the commonly used a* values, to account for the polarizability of cyclohexane itself. The initial statistical analysis of the data in the present study , am2,and Bm2 was made with respect to the properties V2,T * ~ 62, of the solutes and with the coefficients A !, A i , A $, A 2, and A $,
( 1 ) Kamlet, M . J.; Doherty, R. M.; Abraham, M. H.; Marcus, Y . ;Taft, R. W. J . Phys. Chem. 1988, 92, 5244. (2) Leahy, D. J . Pharm. Sci. 1986,75,629. Pear1man:R. S.In Partition Coefficient Determination and Estimation; Dunn, W. J.; Block, J. H.; Pearlman, R. S.,Eds.;Pergamon: New York, 1986; p 3.
(3) McGowan, J. C. J . Appl. Chem. Biotechnol. 1978,28,599. Ibid. 1984, 34A, 38. Abraham, M. H.; McGowan, J. C. Chromatographia 1987,23,243. (4) Abboud, J.-L.M.; Guiheneuf, G.; Essfar, M.; Taft, R. W.; Kamlet, M. J. J . Phys. Chem. 1984,88,4414.
0022-3654/91/2095-8886$02.50/0
(2)
0 1991 American Chemical Society
