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J . Phys. Chem. 1985, 89, 4146-4150
recovered from five subsets of 21 data points from both set A and set B. In view of the finding in 6.2.3, the values of thermodynamic properties presented were obtained by direct fitting to the proposed equation. To avoid congestion in the table, only those representing satisfactory ('/15m) to very good ( ' / 5 0 0 0 ) experimental results are given. It is obvious that the great majority of the processed SR yield thermodynamic values which agree with the generating values to within the 90% confidence levels given in Table IX-providing five terms in the R In P ( m )equation are insisted on and providing
at least 16 experimental points are included in a temperature range of about 100 deg or more. Acknowledgment. L.E.S. is indebted for support, during the preparation of this paper, by a Research Corporation Cottrell Science grant and by two grants from the National Science Foundation (PRM-7908348; R 11-8305005). Acknowledgement is also made to the donors of the Petraleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Electron Spin Resonance Studies of Vanadyl Acetylacetonate in One- and Two-Component Solvents Bruce A. Kowert,* Oh-Wol Yoon, Charles H. Klestinske, Joseph F. Schmidt, Allen D. Baudendistel, and Michael J. Palazzolo Department of Chemistry, St. Louis University, St. Louis, Missouri 63103 (Received: April 25, 1985)
The vanadyl acetylacetonate (VOAA) radical has been studied by electron spin resonance (ESR) in several one- and two-component solvents. Analysis of the ESR line widths gave the reorientational correlation time, 78,for VOAA in these liquids. The one-component solvents were a series of ethers: dimethoxyethane, tetrahydrofuran, hnethyltetrahydrofuran, and tetrahydropyran. The 7@values were found to follow the modified Debye equation 7 8 = ( 4 ~ ? ? ~ ) / ( 3 kknis; the Boltzmann constant, Tis the absolute temperature, 4?r?/3 is a molecular volume, 7 is the solvent viscosity, and K is the anisotropic interaction parameter. Values of K were obtained for each of the four ethers. VOAA was also studied in naphthalene-benzene and naphthalene-toluene mixed solvents. The 78 for these systems, determined as a function of temperature and mixed solvent composition, also followed the modified Debye equation. Because of the near equality of the K values for benzene, toluene, and naphthalene, the relative values of 7#depended primarily on the factor 7 / T and conclusions concerning the short-range interactions of VOAA with the solvent could not be drawn solely from the 70 values. Consideration of the viscosity and thermodynamic properties of the naphthalene-benzene and naphthalene-toluene solutions, which are near ideal or regular, and comparison with earlier results for liquid-liquid mixed solvents indicated that the average K in the mixed solvents was likely to be a linear function of composition and the K values of the components of the mixed solvents (naphthalene and either benzene or toluene); Le., the solutions were dynamically "ideal".
I. Introduction The free radical vanadyl acetylacetonate (VOAA) has been used as an ESR (electron spin resonance) spin probe in a wide variety of The positions of the lines in the ESR spectra can be used to determine the isotropic hyperfine and Zeeman parameters. The line widths are determined by the anisotropic hyperfine and Zeeman parameters and the rate of the radical's reorientational motion. Analyses of the VOAA ESR spectra have been used to obtain the reorientational correlation time, 78, characteristic of the reorientation about an axis perpendicular to the unique V-0 b 0 n d . l ~The ~ axial symmetry of the anisotropic hyperfine and Zeeman parameters does not permit (1) Wilson, R.; Kivelson, D. J. Chem. Phys. 1966, 44, 154. (2) Wilson, R.; Kivelson, D. J . Chem. Phys. 1966, 44, 4440. (3) Walker, F. A.; Carlin, R. L.; Rieger, P. M. J. Chem. Phys. 1966,45, 4181. (4) Luckhurst, G. R.; Ockwell, J. N. Mol. Phys. 1969, 16, 165. (5) Angerman, N. S.; Jordan, R. B. J . Chem. Phys. 1971, 54, 837. (6) Hoel, D.; Kivelson, D. J. Chem. Phys. 1975, 62, 4535. (7) Hwang, J.; Kivelson, D.; Plachy, W. J . Chem. Phys. 1973, 58, 1753. (8) Kowert, B.; Kivelson, D. J . Chem. Phys. 1976, 64, 5206. (9) Bruno, G. V.;Harrington, J. K.; Eastman, M. P. J. Phys. Chem. 1977, 81, 1111. (10) Ahn, M.-K.; Ormond, D. E. J. Phys. Chem. 1978,82, 1635. (11) Ahn, M.-K.; Derlacki, 2.J. J. Phys. Chem. 1978, 82, 1930. (12) Campbell, R. F.; Freed, J. H. J . Phys. Chem. 1980, 84, 2668. (13) Patron, M.; Kivelson, D.; Schwartz, R. N. J. Phys. Chem. 1982, 86, 518.
0022-3654/85/2089-4146$01.50/0
a determination of the reorientation correlation time about the unique V-0 bond. The experimental values of 70 have been interpreted by using the modified Stokes-Einstein-Debye formula 1,2,7,14
where k is Boltzmann's constant, T i s the absolute temperature, 7 is the coefficient of shear viscosity, r is an effective radius determined from translational diffusion experiments, and K is a dimensionless parameter determined by the anisotropic interactions between VOAA and the solvent molecules. Several comments concerning the parameters r and K are in order. (a) The discussion in ref 7 establishes the equivalence between the radius obtained from the translational diffusion experiments and the radius required in eq 1. (b) The discussion in ref 4 and 7 also shows that eq 1, which seems to be written for a sphere, holds for the nonspherical VOAA molecule as well. (c) When K = 1, eq 1 is the well-known Debye expression. The ~ * ~ ~after introduction of K < 1 by Kivelson and c o - w o r k e r ~came a careful series of studies showed that the "volume", Y = ( 4 / 3 ) d , calculated from the experimental 7.9 and eq 1 with K = 1 was usually smaller than the value of Ycalculated with the value of (14) Kivelson, D.; Kivelson, M. G.; Oppenheim, I. J. Chem. Phys. 1970, 52, 1810.
(15) McClung, R.; Kivelson, D. J . Chem. Phys. 1968, 49, 3380.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4147
ESR Studies of Vanadyl Acetylacetonate TABLE I: Correlation Times and K Data solvent T, K 1 0 5 ~ / P~ K-1 ,
TABLE 11: Naphthalene-Toluene Data
MTHF
282 293 298 303 313
1.93 1.64 1.48 1.44 1.27
i011Ts, s 2.66 2.22 2.06 2.00 1.71
THF
283 293 298 303 313
1.92 1.66 1.55 1.45 1.27
2.7 1 2.36 2.19 2.04 1.74
0.846 0.852 0.847 0.843 0.821 av 0.84
283 293 298 303 313
3.48 2.82 2.58 2.34 1.96
4.41 3.71 3.40 3.19 2.57
0.759 0.788 0.790 0.8 17 0.786 av 0.79
28 1 293 298 303
1.94 1.60 1.48 1.38
2.57 2.12 1.85 1.71
0.794 0.794 0.749 0.743 av 0.77
THP
DME
K
0.826 0.811 0.834 0.832 0.807 av 0.82
r from the translational diffusion experiments. (d) When eq 1 holds, K for VOAA is a constant independent of temperature and pressure although it may vary from solvent to solvent; experimental values range from 0.48 to 1.00.7 (e) The anisotropic interaction constant K can be interpreted in terms of hydrodynamic models by using either “ s t i ~ k ”or~ lip"'^,^^ boundary conditions. For the “stick” case K = 1 while for “slip” one has K = E (0 5 Z 5 l), where is a factor which depends on the molecular geometry. Numerical calculations of E have been made by Hu and Zwanzig16 and Youngren and Acrivos.17 Boundary conditions which lie between the “stick” and “slip” as well as the existence of free spaces in the hydrodynamic continuum2’ have also been considered. A theoretical treatment of reorientations on the molecular level by Kivelson, Kivelson, and OppenheimI4 has shown that K is proportional to the ratio p/@,where T and F are intermolecular torques and forces, respectively. Although the T~ for VOAA have been determined as a function of temperature in several classes of organic liquids (alkanes, aromatic hydrocarbons, halocarbons, ketones, amines, and alcohols), only one ether, tetrahydrofuran, has been studied at only a single temperature (25 O C ) . I 0 We have now studied VOAA in four ethers, dimethoxyethane (DME), tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF), and tetrahydropyran (THP). Each solvent has been examined as a function of temperature, and eq 1 has been shown to be valid for VOAA in each ether. In solvent mixtures composed of two miscible liquids (A and B) Kowert and Kivelson8reasoned that the viscosity in eq 1 should be the bulk viscosity of the mixed solvent while K should be replaced by K
=
XAKA
+ XBKB
(2)
where xi is the mole fraction of liquid i in the mixed solvent and K~ is the anisotropic interaction parameter for VOAA in liquid i. The volume fractions of liquids A and B in the mixed solvent were also suggested as possible weighting factors in eq 2.s ESR studies of VOAA in mixed solvents, each component of which obeyed eq 1, gave values of T~ that were consistent with eq 1 and 2. The experiments were performed as a function of mixed solvent (16) Hu, C.-M.; Zwanzig, R. J . Chem. Phys. 1974, 60, 4354. (17) Youngren, G. K.; Acrivos, A. J. Chem. Phys. 1975, 63, 3846. (18) (a) Hoel, D.; Kivelson, D. J. Chem. Phys. 1975, 62, 1323. (b) Ahn, M.-K. Chem. Phys. Lett. 1977, 52, 135. (19) Kivelson, D. Faraday Symp. Chem. SOC.1977, No. 11, 7. (20) Kivelson, D.; Madden, P. Annu. Rev. Phys. Chem. 1980, 31, 523. (21) Dote, J. L.; Kivelson, D.; Schwartz, R. N. J . Phys. Chem. 1981, 85, 2169.
wt % Nu 0.0 21.9 29.3 21.9 29.3 0.0 12.2 21.9 0.0
XNQ
T, OC
0.000 0.167 0.230 0.167 0.230 0.000 0.0901 0.167 0.000
40.0 59.2 59.2 48.8 48.8 21.0 28.5 28.5 9.0
d % o P
1.oo 1.03 1.15 1.19 1.28 1.32 1.39 1.57 1.61
(vlT)/(v/%f 1.oo 0.98 1.06 1.12 1.21 1.27 1.40 1.54 1.56
a N = naphthalene; toluene is the solvent. ~ O I I T =~ ,1.64 ~ ~ s; ~ error for Tg/Tg,ref is f7%. c105(7/T)ref= 1.49 P K-I; error for ( v / T ) ( q / q r e f is f 4 % .
TABLE 111: Naphthalene-Benzene Data wt % Na 0.0 23.7 31.1 23.7 31.1
XNQ
T , OC
78/TB,reP
0.000 0.159 0.216 0.159 0.216
25.0 40.0 40.0 25.0 25.0
1.oo 1.02 1.15 1.31 1.41
(qInI(v/Oret 1.oo 0.99 1.07 1.28 1.40
“ N = naphthalene; benzene is the solvent. blO’lTs,ref = 1.99 s; error for ~ g / ~ g is, ~*7%. ~ f 10S(q/T)rcf= 2.02 P K-I; error for (v/T)(v/7‘)mf is f 4 % .
composition and temperature.s The solvent system n-butanoltoluene did not agree with eq 1 and 2, but n-butanol is not in agreement with eq 1 at the temperature used to study this mixed solvent.6 Mixed solvents can also be prepared by dissolving diamagnetic solid (solutes) in liquids (solvents). The approach developed for VOAA in liquid-liquid mixed solventss would seem to be appropriate for VOAA in these solute-solvent systems; the solute will affect the viscosity of the solution while the solute-radical pair will have its own particular K . To test eq 1 and 2, knowledge of the soluteradical K as well as the solvent-radical K is required. If the solute melts without decomposition, line width analyses of VOAA’s ESR spectra can be performed in the molten solute. The resulting K value can be used in the comparison of eq 1 and 2 with data for VOAA in the solutesolvent mixtures. If it is not possible to measure K in the solute melt, the T~ for one solutesolvent sample can be used to calculate K for the solute from the known K of the solvent and the solution’s composition; solutions of different composition can be checked for consistency with this solute K . As reported in this paper, we have determined T@ for VOAA in the two mixed solvents naphthalene-toluene and naphthalene-benzene. These systems were chosen because (a) naphthalene is reasonably soluble in both benzene and toluene, (b) previous studies of VOAA in toluene’q7and benzene2 have shown that eq 1 is obeyed, and (c) it was possible to determine K for VOAA in liquid naphthalene (mp 80 “C). Experiments were conducted as a function of naphthalene concentration in the temperature range 25 < T < 59 “C. Equations 1 and 2 were found to adequately describe the results.
11. Results and Discussion Ethers. The rg for VOAA as a function of temperature in the four ethers MTHF, THF, THP, and DME are given in Table I along with the corresponding values of K . Details of the experimental procedures used to obtain these quantities are given in section IV. Our results show that K for VOAA is a constant in each solvent for the temperatures we have studied. This is consistent with the linear dependence of T# on q / T found for VOAA in other organic solvents in this range of v/Tvalues. The values of K are not unusually large or small compared to those determined previously. As might be expected, there are no large variations from ether to ether; K(THP) = 0.79, K(DME) = 0.77, K(MTHF) = 0.82, K(THF) = 0.84. The uncertainty for these K values is *0.04. The K found for VOAA in T H F is somewhat larger than the value of K = 0.49 reported earlier;I0 a reevaluation of the
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The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Kowert et al. TABLE IV: Viscosity Data for Naphthalene-Benzene Mixtures at 25 OC
1.8
1
1.6
L
1.4
1
1.0
wt % N'
cp 0.605 0.657 0.726 0.770 0.826 0.876 0.917
Y,,ptbb
cp 0.605 0.660 0.729 0.777 0.828 0.879 0.915
Icalcd,c
" N = naphthalene. bFrom ref 24. CCalculatedby using eq 4 and nN = 1.975 cP,6 1 = -0.30 cP.
1
TABLE V: Viscosity Data for Naphthalene-Toluene Mixtures at 25 O C
0.8 0.8
1.0
1.2
1.4
1.6
1.8
Figure 1. Correlation times vs. Kq/Tfor VOAA in mixed solvents: (X), naphthalene-benzene mixed solvent; (a),naphthalene-toluene mixed solvent. VOAA-THF system by those workers, however, yielded a K in agreement with our value.22 Mixed Solvents. VOAA was studied in three naphthalenetoluene samples (12.2, 21.9, 29.3 wt % naphthalene) at 28.5, 48.8, and 59.2 O C while two naphthalene-benzene samples (23.7 and 3 1.1 wt % naphthalene) containing VOAA have been studied at 25.0 and 40.0 O C . We also determined 70 and K for VOAA in toluene, benzene, and naphthalene and compared our results with those in the literature. We found K = 0.64 f 0.04 for VOAA in toluene, which is in agreement with ref 7 and 8. For VOAA in benzene we obtained K = 0.59 at 25 OC,which agrees with the earlier value of K = 0.60.2*7 Line width studies in liquid naphthalene (discussed further in section IV) gave K = 0.59 for VOAA. The 70 values for the mixed solvent samples can be compared with those predicted by combining eq 1 and 2 To/Tg,ref
1O*XN 0.00 5.10 11.21 15.38 19.79 23.97 26.93
0.00 8.1 1 17.16 22.97 28.82 34.10 37.69
=
[ K / ~ r e f [l ( q /
T )/ (7 / V r e f l
(3)
where 7,9=f and 78 are the correlation times for VOAA in a pure solvent (benzene or toluene) and a mixed solvent containing naphthalene; ( q / n r e f , ( q / T ' ) , K,,f, and K (eq 2) have similar meanings. Because of the near equal values of K and compositions of our solutions, the factor K/Kref on the right-hand side of eq 3 will be virtually constant for each of the two sets of mixed solvent samples (naphthalene-benzene and naphthalene-toluene) and the changes in 78as the temperature is varied within each set should be due to changes in q / T , Le. 7o/7osref = ( V / T ) / ( q / T ) r e p Table 11 compares the experimental ratio 7g/70,reffor the naphthalenetoluene mixed solvents with the corresponding values of (q/T)/(q/T)Ef. We have used 70 and q / T for VOAA in toluene at 40 "C as our reference. Table 111 gives the same comparisons for the naphthalene-benzene mixtures with VOAA in benzene at 25 "C as the reference solution. The agreement between 70/Tg,=f and (q/T)/(s/T),, for both systems is seen to be good. The errors for ~ g / ~ o , , are ~ f f 7 % while those for (q/T)/(q/T')ref are f4%. When the benzene-naphthalene and toluenenaphthalene systems are compared (with toluene at 40 OC as the reference), the ratios ( q / T ) / ( q T ) r e for f the benzene and all but one of the benzenenaphthalene samples are some 8-12% larger than ~ g / ~ o , ~The ~ f . agreement is improved if the K values for the components are taken into account (although the deviations are not large considering the uncertainties in 7 0 , q / T , and K ) . Figure 1 shows the two systems to be mutually consistent when the factor is included. K,calculated from eq 2, is 0.59 for all of the benzenenaphthalene samples since K(benzene) = K(naphtha1ene) = 0.59 while the naphthalene-toluene samples have K = 0.64-0.63. The reference is still VOAA in toluene at 40 "C (Kref = 0.64). In ref 8 it was shown that eq 1 and 2 hold in ideal or near-ideal (22) Ahn, M.-K., personal communication
wt % N"
102XN
0.00 5.73 13.72 20.12 27.31
0.00 4.19 10.26 15.33 21.76
cp 0.552 0.584 0.639 0.687 0.747
I&d~c
cp
0.552 0.586 0.639 0.687 0.747
" N = naphthalene. *From ref 24. Calculated by using eq 4 and = 1.975 cP,bq = -0.64
T~
cP.
solutions and break down if the interactions between VOAA and the components of the mixed solvent become very different. It was also showns that mixed solvents with rTg in agreement with eq 1 and 2 had small excess viscosities. The agreement of our data with eq 1 and 2 indicates that the K value obtained for VOAA in liquid naphthalene (between 97 and 147 "C) is appropriate for naphthalene in our mixed solvents for 25-60 O C and, consequently, that the molecular interactions between VOAA and naphthalene, benzene, and toluene are reasonably similar. Unfortunately, the near equality of the K values for these three aromatic hydrocarbons as well as the limited solubility of naphthalene in benzene and toluene does not provide a stern test of eq 2. Conclusions concerning the ideal or xiear-ideal behavior (with respect to i ocannot ) be drawn with the same certainty as for the liquid-liquid solutions (where K for the two pure liquids were quite different and the entire range of liquid-liquid concentrations could be studied). We can, however, examine the viscosity data and thermodynamic properties for the naphthalene-benzene and naphthalene-toluene systems. As one might expect, they argue against any pronounced nonidealities and suggest, but certainly do not prove, that the dynamic interactions between VOAA and its solvent neighbors are relatively ideal. (While the thermodynamic properties are time independent equilibrium quantities, they reflect the nature of the intermolecular interactions that also determine the dynamic variables.) The excess viscosity is defined in terms of the modified Hill 4=
XAVA
+ XBVB + X A X B ~
(4)
where q is the solution viscosity, qA and qB are the visocisities of pure A and B, and x A and xBare the mole fractions of A and B; xAxB6qis the excess viscosity and is a measure of the deviation from ideal behavior. For comparison with the liquid-liquid results, the viscosity data for the naphthalene-benzene and naphthawere analyzed by using eq lene-toluene solutions at 25 0C24-26 4. The viscosity of naphthalene at 25 O C , qN, was taken to be 1.975 cP, a value obtained by extrapolation from viscosity data above its melting point (80 0C).26 As shown in Table IV and V, the viscosities of the mixtures can be calculated within 1% by using 6 1 = -0.30 CPfor the naphthalene-benzene solutions and 67 = -0.64 CPfor the naphthalene-toluene solutions. These values are roughly the same as those for the liquid-liquid solutions with 70 (23) Hill, N. E. Proc. Phys. SOC.,London, Sect. B 1954, 67, 149. (24) Washbum, E. W., Ed. "International Critical Tables"; McGraw-Hill: New York, 1928. (25) Kendall, J.; Monroe, J. P. J . Am. Chem. SOC.1917, 39, 1802. (26) Tamura, M.; Kurata, M.; Sata, S . Bull. Chem. SOC.Jpn. 1952, 25, 124.
ESR Studies of Vanadyl Acetylacetonate in agreement with eq 1 and 2 (chloroform-toluene, 67 = +0.092 cP; carbon tetrachloride-chloroform, 67 = -0.270 cP; carbon tetrachloride-methylene chloride, 6C = -0.394 c P ) ~and are much smaller than the 67 for the system whose T~ did not agree with eq 1 and 2 (n-butanol-toluene, 67 = -3.22 c P ) . ~Consequently, the excess viscosities, xAxe67, for our two sets of naphthalene solutions are relatively small and indicate that the deviations from ideality for these systems are not large. The thermodynamic properties of the naphthalene-benzene and naphthalene-toluene solutions also show near-ideal or regular b e h a ~ i o r . ~Examination ~.~~ of the density of the naphthalenetoluene and naphthalene-benzene solutions as a function of composition shows that the apparent molar volume of naphthalene, +VN, is essentially constant and equal to the molar volume of supercooled naphthalene (123 cm3 at 25 0C).27 At 12.87 "C, +VN is 123.4 f 0.2 cm3 in benzene solutions (2-9 wt % naphthalene) and 122.7 f 0.3 cm3 in toluene solutions (1-20 wt % naphthalene) while at 25 OC one finds 4VN = 124.5 f 0.2 cm3 in benzene (7-35 wt % n a ~ h t h a l e n e ) . Our ~ ~ own values in toluene-napthalene solutions range from +VN = 122 f 2 cm3 at 28.5 "C to +VN = 126 f 2 cm3 at 59.2 "C. Additionally, the solubility of naphthalene in an ideal solution at 20 OC is xN = 0.2612' where xN is the mole fraction of naphthalene. The measured value in benzene is xN = O.24lz7which, while smaller than the ideal value, is in good agreement with the value calculated from regular solution theory, xN = 0.240. For toluene solutions, the experimental solubility is x N = 0.224 which is also close to the regular solution calculation of xN = 0.228.27 111. Conclusions
The reorientational correlation times, T ~ of , vanadyl acetylacetonate (VOAA) have been shown to agree with the modified Stokes-Einstein-Debye expressions (eq 1-3) in several one- and two-component solvents. ESR line width studies and eq 1 have been used to obtain the following values of the anisotropic interaction parameter, K , for VOAA in a series of ethers in the temperature range 40 > T > 9 OC: dimethoxyethane, K = 0.77; tetrahydropyran, K = 0.79; 2-methyltetrahydrofuran, K = 0.82; tetrahydrofuran, K = 0.84. The experimental T~ for VOAA in naphthalene-benzene and naphthalenetoluene mixed solvents were found to follow eq 1-3. The changes in T~ as the temperature, T, and composition of the mixed solvents were varied were due primarily to changes in the factor 7 / T (7is the solution viscosity) because of the near equality of the K values for VOAA in naphthalene, benzene, and toluene. While the similarity of the K values of the mixed solvent components prevented a thorough test of the dynamic ideality of these solutions (eq 2), examination of their viscosities and thermodynamic properties suggested, as one might suppose, that the deviations from ideal behavior are relatively small. Our results also indicated that, within the experimental accuracy of the T,g determinations, the molecular interactions between naphthalene and VOAA in liquid naphthalene are the same as those between naphthalene and VOAA in naphthalene-benzene and naphthalene-toluene mixtures at temperatures well below the melting point of naphthalene.
IV. Experimental Section Solvents. Toluene (Fischer Scientific Co.) was shaken with cold concentrated H2S04,water, dilute NaOH, and again with water before being distilled twice from P205. The toluene was stored over CaH2 on the vacuum line. Benzene (Fisher Scientific Co.) was distilled twice from P,05 and stored over P,O, on the vacuum line. Naphthalene (Fisher Scientific Co., mp 79.5-80.0 "C)was used without further purification. The ethers (THF, MTHF, THP, DME) were also from Fisher Scientific Co. They were distilled quickly with the fraction within (27) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. "Regular and Related Solutions": Van Nostrand Reinhold: New York, 1970. (28) Lewis, G. N.; Randall, M. "Thermodynamics", 2nd ed.: McGrawHill: New York, 1961.
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4149
0
X
x
I
0.0 0.0
1.o
2.0
3.0
Figure 2. Line width parameter 7,in units of gauss, vs. q / T for VOAA in toluene: ( X ) , this work; (e),data from ref 1.
f 2 "C of the boiling point being collected. This first distillate was refluxed with potassium metal for several hours; collection of the fraction within f l OC of the boiling point was made over lithium aluminum hydride (LAH). The second distillate was refluxed over LAH for several hours followed by collection (over LAH) of the fraction within f0.2 "C of the boiling point. This solvent was carefully degassed and distilled into a storage vessel containing LAH on the vacuum line. Sample Preparation. ESR samples were prepared by using the vacuum line procedures and sample tubes previously employed for liquid-liquid mixed solvents.8 Slight modifications were necessary for the samples containing naphthalene because naphthalene is a solid; weighed amounts of naphthalene were placed directly into the sample tube with VOAA. The naphthalene and VOAA were then placed under vacuum for several hours before measured volumes of solvents (benzene or toluene) were distilled into the sample tube (which was sealed under vacuum). The naphthalene-VOAA sample was also sealed under vacuum after several hours of pumping on the VOAA and naphthalene. Viscosity Data. Viscosity data for THF,29qMMTHF,30,31THP,30 DME,31 liquid naphthalene (discussed further below), and the naphthalene-benzene mixtures2&%were taken from the literature. The kinematic viscosities for toluene as well as the naphthalene-toluene mixtures were measured with an Ubbelohde viscometer while the corresponding densities were measured with a 10-cm3 pycnometer. Both the density and viscosity determinations were made in a temperature-controlled water bath (f0.5 "C). The viscosities for toluene were in agreement with those reported by Barlow et al.32 The error for the ratio (7/7)/(7/nref is 1 4 % and reflects uncertainties in temperature, composition, density, and kinematic viscosity. ESR Line Width, T ~ and , K Determinations. A Varian Model V-4502-15 ESR spectrometer was employed for all of our line width studies. The temperature was controlled (f0.5 "C) by a Varian E-257 variable-temperature unit. The actual temperature was measured with a chromel-alumel thermocouple placed in the ESR cavity. The eight-line ESR spectrum of VOAA was recorded on a 1-kG sweep, and the relative intensities of the individual lines were determined. The width of the M = line was measured with the tetracyanoethylene (TCNE) anion radical (prepared in dimethoxyethane by reduction with potassium, AN = 1S75 G) as a standard.33 The widths of the remaining seven lines were then calculated from that of the M = line and the relative (29) Carvajal, C.; Tolle, K. J.; Smid, J.; Szwarc, M.J . Am. Chem. SOC. 1965,87, 5548. (30) Nicholls, D.; Sutphen, C.; Snvarc, M. J. Phys. Chem. 1968,72,1021. (31) Slates, R. V.;Szwarc, M.J . Phys. Chem. 1965, 69, 4124. (32) Barlow, A. J.; Lamb, J.; Matheson, A. J. Proc. R . Soc. London, A 1966, 292, 322. (33) Goldman, S. A,; Bruno, G. V.; Polnaszek, C. F.; Freed,J. H. J. Chem. Phys. 1972, 56, 716.
J . Phys. Chem. 1985, 89. 4150-4155
4150
intensities. The line widths were analyzed by’,’ AH(M)=a+PM+y@+6M3
(5)
Our results for VOAA-toluene (no added naphthalene) in the temperature range 3 13 > T > 282 K were compared with those of Wilson and Kivelson* and Hwang et a].;’ these authors used the M = -3/2 line as their standard and calibrated the magnetic field using a gauss meter and frequency counter. Our values of the line width parameters were found to be in good agreement with those from ref 1 and 7, justifying the use of the TCNE anion as the line width standard in the studies reported here. Figure 2 shows our y values for VOAA in toluene as well as several values from ref 1. Our value of y = 0.308 G for VOAA in benzene at 25 O C is also consistent with that of Wilson and Kivelson (y = 0.305 G at 24 oC).2 The reorientational correlation times, io, for all of our samples were obtained from y by using eq 7-9 of ref 6. The combination of anisotropic hyperfine parameters, Pa = a, - (1/2)(a, + a,), is needed to calculate io from y,and the value determined for VOAA in toluene was used. This is not a source of uncertainty in the analysis of the line width data for VOAA in the ethers; measurements of the isotropic hyperfine splitting constant, a. = (l/3)(ax a, + aJ, in M T H F fluid solution combined with the value of a, found in an M T H F glass showed that Pa = (3/2)(a, - ao) for VOAA in M T H F was within 1% of that for VOAA in toluene. The line width parameter y (rather than @)was used to determine r@because fl is subject to larger experimental uncertainties than y5J7and because P is a strong function of the nonsecular spectral densities.’V7 y , which depends weakly on the nonsecular spectral densities, is almost directly proportional to r0. There are still questions concerning the functional form of the nonsecular spectral densities; recent studies of VOAAI3 used i g determined from y to check models for the nonsecular spectral densities appearing in the theoretical expression for fl. In a related study of a series of vanadyl P-diketonate complexes, Eagles and M ~ C l u n gfound ~ ~ that the value of ~r~obtained from /3 and y differed slightly but that the value of ~r~derived from the total M-dependent line width, W(M)= PM + r@ + 6M3,was in close agreement with the value found from y. Our interest in this paper
+
(34) Eagles, T. E.; McClung, R. E. D. Can.J . Phys. 1975, 53, 1492.
is the dependence of ig i.e. K, , on solvent. Consequently, y has been used to calculate io. K was determined from eq 1 . We have used r3 = 5 5 A3 for all of our samples since the molecular radius, r, for VOAA in benzene,’ toluene,]’ and THF” is the same within experimental error. In the temperature range 9-40 ‘C we determined K = 0.64 f 0.04 for VOAA in toluene; both the value of K (0.64) and its characteristic uncertainty (f0.04) are the same as in ref 7. The uncertainty for the K values of the four ethers is also f0.04. As noted in section 11, K for VOAA in benzene at 25 OC also agrees with a previous determination.*S7 We studied VOAA in liquid naphthalene at 100, 120, and 140 OC while Wilson and Kivelson2 performed measurements at 97 and 147 C . Using all five sets of data, we find K = 0.59 f 0.06. The viscosity data of ref 35 have been used for liquid naphthalene; this data is consistent with other Although examination of all data for liquid naphthalene does indicate some ~ncertainty,~’ several determinations are in close agreement at 80 and 100 “C (see data from ref 830 and 930 in LandoltB e r n ~ t e i nin~ addition ~ to our ref 24-26, 35, 36, and 38). If only the VOAA line width data in liquid naphthalene at 97 and 100 “ C are considered, we still find K = 0.59. Acknowledgment. The variable-temperature unit used in this work was purchased with funds provided by the St. Louis University Beaumont Faculty Development Fund and the Department of Chemistry, St. Louis University. B.A.K. also received a summer stipend from the Beaumont Faculty Development Fund and 0.-W.Y. was partially supported by a grant (to B.A.K.) from the Research Corporation. The final draft of this paper was written while B.A.K. was on sabbatical leave at the Institute of Materials Science (IMS) of the University of Connecticut; the assistance of the IMS staff with its preparation is gratefully acknowledged. Registry No. Dimethoxyethane, 110-71-4; tetrahydrafuran, 109-99-9; 2-methyltetrahydrofuran,96-47-9;tetrahydropyran, 142-68-7;benzene, 7 1-43-2; toluene, 108-88-3;naphthalene, 91 -20-3. (35) Bingham, E. C.; Hatfield, J. E. J . Appl. Phys. 1935, 6, 64. (36) Weast, R. C., Ed. “Handbook of Chemistry and Physics”, 56th ed.; Chemical Rubber Publishing Co.: Cleveland, OH, 1975. (37) Andrussow, L.; Schramm, B. “Landolt-Bernstein Zahlenwerte and Funktionen”, 6th ed.; Springer-Verlag: New York, 1969. (38) Saji, K. Busseiron Kenkyu 1956, No. 96, 8 3 . See: Chem. Absfr. 1957, 51, 5489b.
A Theoretical Model for Methanol Formation from CO and H, on Zinc Oxide Surfaces R. C . Baetzold Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (Received: October 29, 1984; I n Final Form: May I O . 1985)
Models are developed for the polar (0001) and nonpolar (1010) surfaces of ZnO in order to consider methanol formation from adsorbed carbon monoxide and hydrogen atoms. The heats of adsorption of H,CO and OH,CO ( x = 0-3) species involved in methanol formation are computed to determine the enthalpy changes of reaction. Reaction sequences involving formyl or formate intermediates are considered. We propose that the reaction mechanism is catalyzed by the Cu+to proceed through a methoxy intermediate on Cu+/ZnO with a lowering of the energy pathway. The ZnO surfaces are poor donors and function primarily as acceptors of electron density from CO. The donor role of Cu+ is demonstrated on the polar surface by increasing the heat of adsorption of acceptor adspecies and decreasing the heat of adsorption of donor adspecies.
Introduction Zinc oxide is a material of significant catalytic importance. It is best known as one of the components of a commercial catalyst that is nearly fully selective to methanol in CO/H2 gas feeds.’ ( 1 ) Kung, H. H. Carol. Rev. Sci. Eng. 1980, 22, 235.
0022-3654/85/2089-4150$01.50/0
In addition, this rather basic oxide has been used as a support for a number of catalysts. As is typical for many of the oxide surfaces, OH groups are present unless the surface has h e n treated to specifically dehydroxylate it. Likewise, oxygen vacancies on the surface of this oxide can cause an adjacent metal ion to have reduced coordination number. Thus the surface composition of 0 1985 American Chemical Society
