ANALYTICAL CHEMISTRY. VOL. 50, NO. 9, AUGUST 1978
1337
LITERATURE CITED
to -1.3 (17) kcal/mol for AH and from 4 ( 8 ) to 5 (17) eu for AS, and these values are within the 95% confidence level uncertainties of values reported in Table V. Some checks for internal consistency can be made between the two d a t a sets run several months apart. The standard errors of estimate for the data in Tables I1 and IV reflect the maximum scatter in the data imposed by the total system. The standard errors at 20 "C are consistently lower than those a t 25 and 30 "C, and this may reflect better temperature control for the experiments nearer room temperature. When the rate constants in Table I are regressed against rate constants for the same conditions for the other data set a t 25 "C, the slope and intercept were 0.95 f 0.01 and 0.03 f 0.03, respectively, with a standard error of estimate of 0.04 s-' and a correlation coefficient of 0.997. T h e slope shows that t h e combined effects of chemical and instrumental variables contribute a difference of only 5% between the two data sets. We believe the results presented above represent convincing evidence t h a t the computer controlled stopped-flow system has much to offer for aspects of fundamental kinetic studies that involve large numbers of experiments under reasonably well defined conditions. Results presented elsewhere (18) described t h e performance for routine kinetic analyses.
R. J. Desa and 0.H. Gibson, Comput. Biomed. Res., 2, 494 (1969). B. G. Willis, J. A. Bittikofer, H. L. Pardue, and D. W. Margerum, Anal. Chem., 42, 1340 (1970). P. M. Beckwith and S. R. Crouch, Anal. Chem.. 44, 221 (1972). D.Sanderson, J. A. Bittikofer. and H. L. Pardue, Anal. Chem., 44, 1934 (19721. G.E. Mieling, R. W. Taylor, L. G. Hargis, J. English, and H. L. Pardue, Anal. Chem., 48, 1686 (1976). G. S. Lawrence, Trans. Faraday Soc., 52, 236 (1956). M. W. Lister and D. E. Rivington, Can. J . Chem., 33, 1572 (1955). V. E. Mironov and Yu. I. Rutkovskii, Zhur. Neorg. Khim., 10, 2670 (1965). J. F. Below. R. E. Connick. and C. P. Cotmel. . . J , A m . Chem. Soc.. 80. 2961 (1965). H. Wendt and H. Streklow, Z . Eiektrochem., 66, 228 (1962). S. Funakaski, S.Adachi, and M. Tanoka, Bull. Chem. SOC.Jpn., 46, 479 (19731. F P Cavasino and M Eigen, Ric S o , Part 2 Sez A , 4, 509 (1964) D M Goodall P W Harrison M J Hardv. and C J Kirk. J Chem Educ , 49, 675 (1972). H. E. Bent and C. L. French, J . Am. Chem. SOC., 63, 568 (1941). R. M.Milburn, J . Am. Chem. Soc., 7 9 , 537 (1957). R . Portanova et al.. Gazz. Chim. Ita/.. 98. 1290 (1968). "Critical Stability Constants, Vol. 4: Inorganic Complexes", R . M. Smith and A. E. Martell, Plenum Press, New York and London, 1976. 23, 1230 (1977). H. L. Pardue et al.. Clin. Chem. ( Winston-Salem, N.C.),
RECEIVED for review December 28, 1977. Accepted May 12, 1978. This work was supported by Grant No. CHE 75-1550 A01 from the National Science Foundation.
In-Situ Chemically-Modified Surfaces for Normal-Phase Liquid Chromatography R. K. Gilpin" and W. R. Sisco Research Division, McNea Laboratories, Camp Hi// Road, Fort Washington, Pennsylvania 79034
Another application for bonded phases is as adsorptive (Le., normal-phase mode) packings. A few workers have reported normal-phase studies on bonded materials (9-14). In this mode, even greater potential in selectivity may eventually be realized. T h e feasibility of forming bonded siloxane phases totally in-situ on porous layer materials and on completely porous small-particle silica has been demonstrated (15,16). These materials were evaluated for use in a reversed-phase mode. In this paper, three short-chain trichlorosilane modified silica gels for use as normal-phase high pressure liquid chromatographic packings have been prepared by a similar in-situ approach. Detailed investigations of efficiency and selectivity on these short-chain packings have been carried out using a series of 14 test solutes. All compound retentions have been examined on a relative basis using aniline as the reference compound.
I n this paper, the preparation of three short-chainlength trichlorosilane (n-butyl, 2-carbomethoxyethyl, and 3-cyanopropyl) modified silica gels by a totally in-situ process is reported. Detailed investigations of efficiency and selectivity of these modified materials along with unmodified silica have been carried out in a normal-phase mode using a series of 14 test solutes with water-saturated n-hexane as the mobile phase. All compound retentions have been examined on a relative basis using aniline as the reference solute. Columns have been found to vary in polarity in the order: silica, n-butyl, 2-carbomethoxyethyl, 3-cyanopropyl. I n addition, columnto-column reproducibility of these packings has been evaluated for multipreparations. Typical average deviations of relative retention for the 14 test solutes have ranged between 4 4 % .
In recent years t h e number of commercially available high-efficiency liquid chromatographic packings has grown a t a remarkable rate ( 1 , 2). This has been especially true for the chemically modified materials (3, 4 ) . In parallel growth has been the number of literature references of the application of these materials to analytical problems. In addition a number of reports have appeared describing the experimental preparation and characterization of various bonded-phase materials for liquid chromatographic application. These have been extensively reviewed in t h e literature (3-8). I n a majority of t h e reported studies and applications, bonded-phase materials have been used in the reversed-phase mode ( 4 ) ,with a combination of either water or an aqueous buffer and either methanol or acetonitrile as the mobile phase. 0003-2700/78/0350-1337$01.00/0
EXPERIMENTAL Column Preparation. The 2.4-mm i.d. by 25 cm stainless steel columns were slurry-packed with LiChrosorb SI 60 silica (av. dp -10 pm) as previously described (16). Following packing, each column was conditioned with 100 mL, of water, methanol, isopropanol, diethyl ether, 1,2-dichloroethane,and at least 500 mL of water-saturated n-hexane. After conditioning, all columns were characterized in terms of efficiencies losing nitrobenzene as the test solute and a mobile phase of water-saturated n-hexane. In addition, capacity factors for each test solute were calculated using benzene as the unretained peak. These data were obtained from duplicate injections using 6 to 8 different linear velocities covering a range from 0.11 to 1.1 cm/s (0.3 to 3.0 mL/min). Following packing and evaluation, suitable columns were bonded in-situ as described (16). In each case, 100 mL of C
1978 American Chemical Society
1338
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
water-saturated toluene followed by 200 mL of dry toluene were used to establish pre-reaction conditions. These solutions were pumped through at a rate of 5.0 mL/min. Immediately following this procedure, the pump was stopped and a reaction reservoir ( 3 / 8 in. 0.d. X 2 ft stainless steel tube) containing 30 mL of a 507' solution of trichlorosilane monomer in dry toluene was inserted into the system between the column and pump. The reaction solution was then pumped through the column at a rate of 1.0 mL/min, followed by 500 mL of dry toluene at the maximum attainable flow. The reaction reservoir was removed from the system, and 100 mL of water-saturated toluene, followed by 100 mL of watxacetonitrile (5:95) were pumped through the modified columns. Each column was subsequently conditioned using the same solvent series listed above for the uncoated silica. Equipment. Either a Waters Model 6000 pump, or an Altex Model 100 pump were used for the bonding reactions. Chromatographic experiments were performed on a Perkin-Elmer Model 1220 liquid chromatograph equipped with a Schoeffel Model 770 variable wavelength detector. All measurements were made at 254 nm. Retention measurements were made with the aid of a Hewlett-Packard Model 3352 data system. Injections were made on-column using Precision Sampling syringes. All column evaluations were carried out at ambient temperature (ca. 25-27 "C). Reagents. The n-hexane and toluene used were distilled in glass from Burdick and Jackson and analytical reagent grade from Mallinckrodt, respectively. The dry toluene was prepared hy refluxing over calcium hydride for at least 4 h and then stored over calcium hydride until used. Both the water-saturated nhexane and water-saturated toluene were prepared by stirring overnight at ambient temperature in the presence of distilled water. These solvents were then stored under static conditions prior to use. n-Butyltrichlorosilane, 2-carbomethoxyethyltrichlorosilane, and 3-cyanopropyltrichlorosilanewere obtained from Petrarch Systems and were used in the condition received.
-
???
oor-
: n
m II
-si Q,
-m
ln0d
ggg t
+I
222 ??? moo
h
ca
0
m I1
L rimoo
888 +I
+I
CY;
=
k'ithcomponent
k 'aniline
-
tr> tro
tr,
tra
- tro
tro
where (t,) was the retention time of the component of interest, (trJ the retention time of the unretained compound, benzene, and (Q the retention time of the reference compound, aniline. Aniline was chosen as the reference compound, since it was t h e parent structure of a majority of the test solutes studied. Additionally the retention of aniline was intermediate among t h e test compounds investigated. Reproducibility. 1,isted in Table 1 are mean relative retention data and resulting coefficients of variation in these d a t a for the series of 14 test compounds obtained on three
+
- 0 0
'99"
0 - 0
RESULTS AND DISCUSSION Columns modified with n-butyltrichlorosilane, 2-carbomethoxyethyltrichlorosilane, and 3-cyanopropyltrichlorosilane were prepared in-situ using optima pre-reaction and reaction conditions based on previous studies (16). After preparation, columns were conditioned with a solvent series of decreasing polarity ( I 7) and chromatographically characterized as normal-phase packings. In addition to selectivity and efficiency studies, reproducibility for multicolumn preparations was evaluated. T h e reproducibility studies were carried out on a relative basis using aniline as the reference compound. Changes in column efficiency between the modified and unmodified surfaces were examined using nitrobenzene as the test compound. I n each case, water-saturated n-hexane was used as t h e mobile phase. Selectivity was investigated using a series of 14 test solutes which are listed in Table I and water-saturated n-hexane as t h e mobile phase. Capacity factors for these test solutes ranged from -0.1 for biphenyl to -100 for phenol. From these data, relative retention (0;)data were calculated using aniline as the reference compound. The a: (ratio of capacity factor of each component to the capacity factor of aniline) was determined using the relationship
t
mom
e
Q
Q
0
0000
-
h
U
a
T
41 .r
h
m
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
10
20
30
40
60
50
1339
70
RETENTION TIME, min Figure 1. Representative chromatogram obtained on an unmodified silica column. Conditions: mobile phase, water-saturated n-hexane; flow rate, 1.6 mL/min; column temperature, ambient. Test solutes: (a) benzene, (b) biphenyl, (c) nftrobenzene, (d) o-chloroaniline, (e) N , Ndimethylaniline, (f) 2,6-dimethylphenol,(9)N-methylaniline, (h) rn-chloroaniline,(i) 2,6-dimethylaniline, (j) p-chloroaniline, (k) aniline
replicate unmodified micro-silica columns. Also listed in Table I are inter-column variations reported as percent relative average deviation (o:/a;) (100). All relative values for a given test solute are averages obtained from 5 to 10 experimental determinations per column. T h e inter-column variation for triplicate column preparation was typically less than 8% with a significant portion of these data falling between 1%and 4% ; in a few cases, higher values were obtained for those test solutes which eluted early. Also shown in Table I are mean relative retention (a:) data for the same test series obtained on three replicate column preparations for each of the column types: n-butyltrichlorosilane, 2-carbomethoxyethyltrichlorosilane, and 3cyanopropyltrichlorosilane. As in the case of the unmodified columns, overall inter-column variations for a particular type of modification were less than 8% with many test solutes showing smaller relative deviations. Absolute retention data for aniline reported as k’are listed in Table I. These data were determined from a representative column of each modification as shown in Figures 1 and 2. In terms of absolute retention, column-to-column variations were found to be similar to those reported in earlier studies ( 1 6 ) ; this was true when the batch of monomer and lot of silica remained constant. Changes in either resulted in higher coefficients of variation in reproducibility. When comparing the overall variation in a: data between column types, no statistically significant differences (p < 0.05 by ANOVA) were noted between modified and unmodified materials. These results indicate t h a t most inter-column variation within a particular modification type was the result of experimental factors other than those arising from the bonding process. One such factor is fluctuation in temperature of the column and mobile phase. Temperature has been found to affect significantly compound retention under the conditions studied (18);this has been attributed to changes in water content of the mobile phase and resulting adsorbed surface water. The three in-situ 3-cyanopropyltrichlorosilanemodified columns were compared to a similar commercially available column (ES Industries), using the 14 test compounds. Relative retention data are listed in Table 11. As in the case of the
10
20
10
20
10
20
RETENTION TIME, min
Figure 2. Representative chromatograms obtained on the various surface types: n-butyl, 3cyanopropy1, P-carbomethoxyethyl. Conditions: mobile phase, water-saturated n-hexane; flow rate, 2.0 mL/min; column temperature, ambient. Test solutes: same as Figure 1
in-situ modifications, the aai data on the commercially available column are mean values obtained from 5 to 10 experimental determinations for each solute. When comparing a i values, the columns were found to be similar in selectivity even though the capacity factor for each solute was approximately four times larger on the in-situ prepared surfaces. The smaller k ’values obtained on the commercially available surface are reconcilable in terms of differences in surface area and monomer loading between the packings (19). Of particular
1340
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
Table 11. Variation in Relative Retention for In-Situ and Commercially Available 3-Cyanopropyl Columns test compounds naphthalene biphenyl anthracene nitro benzene N, N-dimethylaniline N-me thylaniline 2,6-dimethylaniline aniline echloroaniline rn-chloroaniline p-chloroaniline 2,6-dimethylphenol 2,4-dimethylphenol phenol
relative retention,
a,i
Aa
BQ
8.2 x 10-3 1.1x 10-2 2.1 x 10-2 8.8 x 0.12 0.31 0.55
1.2 x 10-2 1.6 X
3.1
X
11.0 x 1 0 - 2
0.13 0.32 0.49
1.00 ( h ' = 35.8)
1.00 ( k ' = 9.21)
0.22 0.68 0.97 0.41 1.52 3.01
0.24 0.73 0.98 0.37
I 2
a4
Ob
01
IO
C l R R l t R V l l Q C l l V crn,~ec
1.19
2.53
a A, in-situ prepared columns. CY? results are mean values obtained from three replicate columns. B, commercially available column (ES Industries). a,' results are values obtained from one column.
Figure 3. Effect of mobile phase velocity on plate height for a typical micro-silica column before and after modification. Modified in-situ using n-butyltrichlorosilane. Chromatographic conditions: mobile phase, water-saturated n-hexane; test solute, nitrobenzene
(Table I) when the surfaces are ordered: silica, ri-butyl, 2-carbomethoxyethyl, and 3-cyanopropyl (A t o D, respectively). The observed discrepancies in the four earliest eluting interest is the excellent agreement between relative data, compounds (naphthalene, biphenyl, anthracene, and nitrowhich supports the argument that the observed selectivities benzene) on the silica and n-butyl surfaces are reconcilable are primarily the result of bonded functionality and not due in terms of their large coefficients of variation of .:yc These only to differences of bonded polymer bulk or matrix effects. consistent trends are explainable in terms of varying surface Thus relative retention has been used to examine surface polarity and the nature of t h e solute. selectivity and t o characterize t h e surface types in terms of The column types studied vary in polarity from t h e unvarying polarity. modified silica surface, which is t h e most positive due t o Efficiency. Tabulated in Table I11 are calculated mean surface hydroxyl groups, to 3-cyanopropyl which is the surface ratios of column efficiency before (Hbefore) to column efficiency with the greatest peripheral electron density. The n-butyl after (H*J chemical modification. These ratios (Hbefnre/Hafter) and 2-carbomethoxyethyl are intermediates of these extremes, are listed for each of the modification types for linear flow respectively. velocities of 0.38 and 0.76 cm/s. All Hbefoore and HaEter values The nonbonded electrons of t h e amine groups of aniline were determined from plots of efficiency vs. linear velocity and the substituted anilines studied have been found to of the mobile phase over an operating range of 0.11 to 1.1cm/s. contribute significantly to solute-surface interaction. This is consistent with other reported studies of the interaction of Data were collected using a mobile phase of water-saturated n-hexane and t h e test solute, nitrobenzene. Representative compounds containing -NH2 groups with silica gel (20-22). plots before and after modification are shown in Figure 3. Aniline and various mono-substituted aromatics have been These data were obtained on an rz-butyl modified surface but shown t o primarily interact with unmodified silica surfaces reflect t h e general trend for all modifications. Hbefore/Hdter through a one-point interaction (20-22). The solutes chosen ratios of approximately 0.8 (Table 111) were obtained for each in this work likewise exhibit one-point interaction with the modification type. surface of the packings. Alteration of the attachment site by A ratio of 1.0 a t both linear velocities would indicate the either electronic or steric means affects the degree of sosuperimposition of efficiency curves before and after modilutesurface interaction. These alterations included stationary fication over the 0.38 to 0.76 cm/s range. However, since the phase modifications as well as solute changes. The resulting effect from these changes was either increased or decreased Hbefnre/Hafter ratio was < L O , the modified columns were judged t o be slightly less efficient than the unmodified surfaces. retention relative to t h e parent compound as seen in Figure Although all evaluations were carried out using the same test 4. T h e chloroanilines when referenced to aniline showed solute and mobile phase, this argument neglects differences in h ' for nitrobenzene between the modified and unmodified greater affinity for increasingly electron-rich surfaces while N-methyl- and N,N-dimethylaniline showed greater affinity surfaces. for increasingly electron-deficient surfaces. These trends are Selectivity. Tabulated in Table I are a,' values as a function of column modification. Of particular interest are explainable in terms of t h e electronic nature of the solutes. changes in U I , ~as a function of surface modification and test Addition of a chlorine atom to the benzene ring of aniline effectively delocalizes the nonbonding pair of electrons of the solute type. All of t h e test solutes, within experimental variability, demonstrate consistent trends in relative retention nitrogen by virtue of its electron-withdrawing nature. This Table 111. Reproducibility of In-Situ Prepared Columnsa Based o n HETP Data relative av deviation, % Ab Bb
HbeforelHafter
modification n-butyl 2-carbomethoxyethyl 3-cyanopropyl
Ab 0.73
?
0.049
0.88 0.80
i
0.13 0.057
i
Bb 0.77 0.86 0.75
+_
i i
a Mobile phase: water-saturated n-hexane. Test solute: nitrobenzene. B, 0.76 cm/s.
0.0071 0.11
0.042
6.8
14.8 7.1
0.92 12.8 5.7
Linear flow velocity: A, 0.38 cm/s;
ANALYTICAL CHEMISTRY, VOL. 50, 1.5
1.0 p-CHLOROANILINE
.-IC(
0
m-CULOROANILINE
1.6-DIuErnvLAnILinE
0.5 1.6-DIYETHVLPHENOL
N - YETI(VLARIL1NE
1 0-CHLOROANILINE
0.0
ANTHRACENE IIPHENVL nwniHAtEnE
* A
B
C
D
COLUMN MODIFICATION
Flgure 4. Variations in average relative retention (a:)as a function of column modification: ( A ) unmodified, (E) n-butyl, (C) 2-carbomethoxyethyl, (D) 3-cyanopropyl
results in the nitrogen atom of the chloroaniline having more positive character than the nitrogen atom of aniline, thus favoring electron-rich surfaces which can interact more readily with the less basic nitrogen. This is shown in Figure 4 as an increase in relative retention for the column modifications A to D. The methyl substituent, on the other hand, when placed on the nitrogen atom of aniline affects a localization of charge about the nitrogen atom by virtue of the electron donating nature of the group. This situation causes the nitrogen atom to possess greater negative character than aniline, thus favoring electron-deficient surfaces which can interact more readily with the more basic nitrogen atom as illustrated by a decrease in aaifor the column modifications A to D. Within each column modification, the elution order of the test compounds depended on the degree of hindrance of the solute's site of interaction. This is seen in Table I as a decrease in a,' as the solute becomes increasingly hindered by substituents. This effect was seen on all columns investigated and found to be dominant over electronic considerations. From steric considerations, the availability of the nonbonded electrons of the methyl substituted anilines to interact with the surfaces is greatest in 2,6-dimethylaniline; the electronic availability decreases as the nitrogen atom is increasingly hindered by methyl groups. This situation is seen in Figure 4, where 2,6-dimethylaniline is retained longest on all columns investigated, while N,N-dimethylaniline is the least retained methylaniline on all columns. Although the electronic character of the methylanilines is opposite from the chloroanilines, as reflected in their reversed trends (Figure 41, the proximity of a bulky chlorine group to the solute's point. of interaction alters retention in the same way as the methyl
NO. 9,
AUGUST 1978
1341
substituent in the methylanilines. These steric considerations are illustrated as a large difference in retention between o-chloroaniline and p-chloroaniline (Table I). Since electronically these solutes should have similar CY: values, their wide separation is attributed to steric effects (23). The phenolic solutes' interactions with the various stationary phases are consistent with the above arguments. However, in order to better visualize the electronic and steric effects of the methyl substituents it i:; necessary to normalize the relative retention data to phenol. The mean relative retentions, upi,(where upiis defined analogously t o a i except relative to phenol instead of aniline) of 2,6-dimethylphenol are 0.18, 0.17, 0.16, and 0.14 for surfaces A to D respectively. Likewise, cy: values of 0.62, 0.59, 0.55, and 0.51 are obtained for 2,4-dimethylphenol. The electron donating effect of the methyl groups on the phenol ring effects an increased localization of the nonbonded electron about the oxygen atom. This results in the oxygen atom having greater negative character than phenol, thus favoring electron deficient surfaces which interact more readily with the more basic oxygen atom. Although this situation is similar to the case of methyl substituted anilines, it may not be apparent from Figure 4. However, plots of a< vs. column modification, would show similar trends for methyl substitution on phenol as seen for aniline. These results, in terms of steric arguments, are consistent with those of the substituted anilines. As the methyl groups block the site of interaction decreases. Since the 2,6-dimethylphenol has the most hindered oxygen atom, it shows the lowest affinity for the stationary phases studied, while the 2,4-dimethylphenol has a more available oxygen atom and exhibits greater retention. T h e four surface types examined form a useful set with varying polarity. High pressure liquid chromatography using columns packed with these surfaces can offer unique and selective separations; representative chromatograms obtained on the four surface types using selected test solutes are illustrated in Figures l and 2 for unmodified and modified packings, respectively. The changes in selectivity and resolution of solutes seen in these figures offer an alternative to modifying solvent composition.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
R. E. Majors, Am. Lab., 7 (lo), 13 (1975). R. E. Majors, J . Chromatogr. Sci., 15, 334 (1977). E. Grushka and E. J. Kikta, Jr., Anal. Chem., 49, 1004A (1977). C. Horvath and W. Meiander, J . Chromatogr. Sci., 15, 393 (1977). D. C. Locke, J . Chromatogr. Sci., 11, 120 (1973). A. Pryde, J . Chromatogr. Sci., 12, 486 (1974). V. Rehak and Smolkova, Chromatographia, 9, 219 (1976). I. Sebestian and I. Halasz, Adv. Chromatogr., 14, 75 (1976). E. Grushka and E. J. Kikta, Jr., Anal. Chem., 46, 1370 (1974). M. Novotny, S. L. Bektesh, K. B. Denson, K. Grohmann, and W. Parr, Anal. Chem., 45, 971 (1973). M. Novotny, S.L. Bektesh, and K. Grohmann, J . Chromatogr., 83, 25 (1973). K. K. Unger, N. Becker, and P. Roumeiiotis, J . Chromatogr., 125, 115 (1976). R. E. Majors and M. J. Hopper, J . Chrcmatogr. Sci., 12, 767 (1974). J. H. Knox and Pryde, J . Chromatogr., 112, 171 (1975). R. K. Giipin, J. A. Korpi, and C. A. Janicki, Anal. Chem., 47, 1498 (1975). R. K. Giipin, D. J. Carnillo, and C. A. Janicki, J . Chromatogr., 121, 13 ( 1976). L. R. Snyder and J. J. Kirkland, "Introduction to Modern Liquid Chromatography", J. Wiley and Sons, New York, N.Y., 1974, p 197. R. K. Giipin and W. R . Sisco, I V FACSS, Detroit, Mich., Nov. 1977. M. J. Telepchak. ES Industries, private communication, 1977. E. Soczewinski and W. Golkiewicz, Chromatographia, 5 , 431 (1972). E. Soczewinski, W. Golkiewicz, and W. Markowski, Chromatographia, 8, 13 (1975). E. Soczewinski, J . Chromatogr., 130, 23 (1977). H. C. Brown and A. Cahn, J . Am. Chem. Soc., 72, 2939 (1950).
RECEIVED for review January 9,1978. Accepted May 23, 1978.
