Anal. Chem. 1003, 65, 1969-1979
1989
Development of a Gas Chromatographic Scale of Solute Hydrogen Bond Acceptor Basicity and Characterization of Some Hydrogen Bond Donor Phases by Use of Linear Solvation Energy Relationships Jianjun Li,+Yunke ZhangJ and Peter W. Carr* Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455
Although there are hundreds of stationary phases available for gas chromatography, very few are even moderate hydrogen bond donors. In a search for strong hydrogen bond donor phases,we studied retention in some unusual liquids including two highly fluorinated alcohols, stearic acid, a fluorinated amide, and a phenolic phase. Retentiondata for more than 80 solutes spanning a very wide range in size, dipolarity, and hydrogen bond acceptor and hydrogen bond donor strength were examined. The results generally validate our previously developedmethod for measuringsolute hydrogen bond basicity parameters. The retention data are well correlated by using solvatochromic linear solvation energy relationships based on a new set of GC-based solute parameters. Several strong hydrogen bond donor phases were identified, and their chromatographic properties were compared to solvatochromic properties measured by direct spectroscopic studies.
INTRODUCTION Recently, we investigated the use of solvatochromiclinear solvation energyrelationships (LSERa)to characterizesolutesolvent interactions in gas chromatography.'-2 The following equation is able to correlate a large body of retention data on a considerable number of stationary phases: SP = SP,
+ 1 log L16 +
SA?
+ d6, + ami + bo:
(1)
SP may be the logarithm of a capacityfactor, specific retention volume, or a partition coefficient, but it cannot be a retention index. SPOis a solute-independentconstant characteristicof the specific columnunder study,Lleis the partition coefficient of the solute from the gas phase to n-hexadecane at 298 K, AZ**C is a new GC-based solute dipolarity-polarizability parameter, and 62 is the Kamlet-Taft polarizability correction parameter. ai and 6 are solute hydrogen bond donor acidity and solute hydrogen acceptor basicity parameters, respectively, which are also based on GC retention data.'" Most of the cavity formation and dispersion interaction energy are To whom correspondence should be addreeaed. Current address: The Procter & Gamble Co., Miami Valley Laboratories, P.O. Box 398707, Cincinnati, OH 45239. 8 Current address: Department of Chemistry,Baylor University,Waco, t
TX 76798.
(1) Li, J.; Zhang, Y.; Carr, P. W. Anal. Chem. 1992, 64, 210. (2) Li, J.;Zhang, Y.;Dallae,A. J.; Carr,P. W. J. Chromatogr.1991,550, 101. (3) Li, J.; Zhang, Y.; Ouyang, H.; Carr, P. W. J. Am. Chem. SOC. 1992, 114,9813. 0003-2700/S3/0365-1969$04.00/0
incorporated into the 1 log LIS term. However, a single term cannot fully account for all of these interactions so to some extent dispersive and polarizability interactions also show up in the sr;fand d62 terms. The term su~**Cprimarilyreflects contributions of the dipole-dipole and solute dipole-stationary phase-induced dipole interactions to retention. It is likely that this term also includes some dispersion energy. For aromatic and polyhalogenated compounds, which have different polarizabilities relative to aliphatic solutes, a minor correction term (d62) is often required. The 6 parameter is conveniently defined to be 0 for aliphatics, 0.5 for polyhalogenated species, and 1.0 for aromatic species, whether it acts as a solvent or solute. Finally, aai and b 6 represent the contributionsto retention resulting from solute-to-solute and solvent-to-solute hydrogen bond formation, respectively. A very similar approach, but which differs in some minor detail, was developed and used extensively by Abraham and his co-workers."lO In their work, Abraham replaced the Kamlet-Taft 62 term with a new excess molar reflection parameter (Rz), defined as the molar refraction of the solute less the molar refraction of an alkane of the same van der Waals volume. A comparison of the present approach to that of Abraham and co-workers and Poole's thermodynamic solvation model in predicting retention in gas chromatography and stationary-phase characterization has been reviewed by Poole et al.11J2 Using eq 1we obtained excellent fits to retention data (the average standard deviation in log k' was 0.05, and r > 0.995) for over 100 stationary phases.' For almost all phases, the I , s,and a coefficients are the only coefficients that were found to be significant. This formed the basis for a new scheme for the classification of GC stationary phases.' The d and b coefficients were always small or insignificant. These results clearly show that there are no GC phases that are really strong hydrogen bond donors (acids), but there are many polar and strong hydrogen bond acceptor (basic) phases. Therefore, our previously developed approach of using gas chromatographic retention data for estimating solute LSER parameters (4) Abraham, M. H.; Whiting, G. S.;Doherty, R. M.; Shuely, W. J. J. Chem. SOC.,Perkin Trans. 2, 1990,1451. (5) Abraham, M. H.; Whiting, G. S.;Doherty, R. M.; Shuely, W. J. J. Chem. Soc., Perkin Trans. 2 1990,1851. (6) Abraham, M. H.; Whiting, G. S.; Doherty, R. M.; Shuely, W. J. J. Chromatogr. 1990,618, 329. (7) Abraham, M. H.; Whiting, G. S.;Doherty, R. M.; Shuely, W. J. J. Chromatogr. 1991,587, 213. (8) Abraham, M. H.; Whiting, G. S.; Doherty, R. M.; Shuely, W. J. J. Chromatogr. 1991,587, 229. (9) Abraham, M. H.; Hamerton, 1.; Rose, J. B.; Grate, J. W. J . Chem. SOC.,Perkin Trans. 2 1991,1417. (10) Abraham, M. H.; Whiting, G. S . J. Chromatogr. 1992,594, 229. (11) Poole, C. F.; Kollie, T. 0.;Poole, S. K. Chromatographia 1992, 34,281. (12) Kollie, T. 0.;Poole, C. F.; Abraham, M. H.; Whiting, G. S . Anal. Chim. Acta 1992, 259, 1.
0 IS93 Ametlcan Chemlcal Soclety
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ANALYTICAL
CHEMISTRY, VOL. 85, NO. 15, AUGUST I , 1993
could not be applied to the development of a method for solute hydrogen bond acceptor basicity. The lack of strong hydrogen bond donor phases has also been pointed out by Abraham6 and Poole.11-13 Since then AbrahamQhas studied a number of nonvolatile liquids based on 4,4‘-isopropyldienediphenol (Bisphenol A) and other bisphenols as GC stationary phases and candidate coatings for piezoelectric sorption detectors. A few very strong hydrogen bond donor phases have been identified. However, all the phases they studied are phenolic, and the number of solutes is small (ranged from 20 to 32 for different phases). Recently we synthesized several new very strong hydrogen bond donor phases. This has led to the development of a new hydrogen bond basicity scale (A) based on the use of relative retention on two gas chromatographic phases.3 We used a very powerful hydrogen bond acid [4-dodecyl-a,a-bis(trifluoromethy1)benzyl alcohol] as an active hydrogen bond donor stationary phase and a related ether [4-dodecyl-a,abis(trifluoromethy1)benzyl methyl ether] as a chemically similar but hydrogen bond inert reference stationary phase. In comparison to Abraham’s solute hydrogen bond acceptor parameter (#),1‘ which is based on equilibrium constants for formation of 1:l hydrogen-bonded complexes,the pi scale should be more useful in explaining retention in gas chromatography where the donor is in excess and when the solute has several hydrogen bond acceptor sites.3 A major advantage of the new approach is that relative retention on the two phases depends only on solute basicity (A) and not on the other solute characteristics (log L16, ?r2*pc, and CY;). This leads to better precision and perhaps better accuracy in values of /3i established by this method. In this work, in an attempt to identify strong hydrogen bond donor phases and examine the applicability of the 6 parameter to the characterization of those phases, we measured the retention of 87 compounds spanning an extremely wide range in chemical characteristics on five unusual liquids. These phases include both OH and NH donors. We included both types of donors because others have noted a family dependence to solvent 6 measurement when OH and NH donors were used as test probes.16J6 The retention data were then correlated using eq 1. An evaluation of A as a solute hydrogen bond acceptor basicity scale was made in light of the correlation results. Several very strong hydrogen bond donor phases were identified. EXPERIMENTAL SECTION
Preparation of Stationary Phases and Columns. The chemical structures of the five stationary phases studied in this work are shown in Figure 1. Inspection shows that these phases were chosen so as to “match”dispersive interactions as much as possible and at the same time incorporate a powerful hydrogen bond donor into the structure. The procedures for purifying or synthesizing the phases and column preparation are described below. Column temperatures were chosen to facilitate solute elution but not so high as to rapidly volatilize the stationary phases. Stearic acid (SA) was obtained from Aldrich in 99+ 5% purity. It was used as received. A 13.9%(w/w) stationary-phasepacking was prepared by dissolving stearic acid in pentane and mixing it with Chromosorb W-HP(Supelco,60/80 mesh). The solvent was evaporated slowly in a Rotovap. A l/,-in. (i-d.) stainless column 14 in. in length was used. The column was conditioned (13) Poole, S. K.;Poole, C. F. J. Chromatogr. 1990,500, 329. (14) Abraham, M. H.; Grellier, P. L.;Prior,D. V.;Morris, J. J.; Taylor, P. J. J. Chem. SOC., Perkin Trans 2 1990,521. (15) Laurence,C.; Nicolet,P.; Helbert, M. J. Chem.Soc.,PerkinTram. 2 1986,1081. (16) Maria, P.-C.; Gal, J.-F.; Franceschi, J.; Fargin, E. J. Am. Chem. SOC.1987,109, 483.
.HJC(CH~), & O -:H CF3
a,a-Bis(trifluoromcthyl)-pdodccyl benzyl alcohol @IFOH) H?
3-Pentadecylphenol (PDP) 0 cH3(cHZ)78A~
Stearic acid (SA)
Flgure 1. Chemical structures of the statlonary phases.
at 80 O C overnight and all retention measurements were made at this temperature. 3-Pentadecylphenol(PDP)was obtained from Aldrich in 90% purity as an unctuous, lumpy solid. It cannot be wed directly for our purposes. To purify the phenol, 15g was dissolved in 60 mL of diisopropyl ether and the resultant mixture was twice decolorized with activated carbon. The solvent was removed by Rotovap distillation. The purified material was then dissolved in hexane in a Erlenmeyer flask covered with a sheet of filter paper, which allowed slowevaporationof the hexane. After sitting at room temperature for 2 days, the recrystallized solid was filtered, washed with cold hexane, dried by suction for 30 min, and then left in a Petri dish to dry at room temperature. Fluffy white fine crystals (3g) were obtained. Its purity was not further characterized. A 20% (w/w) packing material was prepared as above. The column was conditioned at 110 O C overnight,and all retention measurements were made at this temperature. N-Tetradecyl-l,l,l-trifluoroacetamide(TFAMIDE)was prepared by reaction of tetradecylaminewith trifluoroacetic anhydride in dichloromethane. Both reagents were from Aldrich. The reaction was carried out by dissolving 3 g of tetradecylamine in a 100-mL flask in 8 mL of dichloromethane. Trifluoroacetic anhydride (6 g) was added via an addition funnel, which allowed introduction of the reactant while the flask was capped and the solution stirred with a magneticstirring bar. The flask was kept capped while stirringfor about 3 hat room temperature. A closed reaction flask is needed to prevent loss of trifluoroacetic anhydride. The high heat of reaction can causerapid evaporation of the anhydride if the flask is not capped. Thin-layer chromatography of the reaction mixture was performed to ensure complete conversion of the amine. The solvent and the trifluoroacetic acid formed during the reaction were removed by Rotovap distillation. The raw product was recrystallized from isopropyl ether. The filtered crystals were washed with cold isopropyl ether, dried by suction for 30 min, and then left in a Petri dish to dry at 40 O C overnight. The Fial product was 3 g and the melting point is 63-64 O C . The structure of the amide was confirmed by FT-IR and proton NMR spectroscopy. A packing of 11.1% of the amide stationary phase was prepared in the same way as above, conditioned at 100 O C for 4 h, and then left at 80 O C overnight. All retention measurements were made at 80 O C . a-Trifluoromethyl alcohols (TFOH) cannot be prepared directly by reaction of trifluoroacetone with a Grignard reagent” when there are @-hydrogensin the Grignard reagent. After this was recognized, the literature indicated that Grignard reagents react with trifluoroacetic acid to give a mixture of a ketone and a secondary alcohol (not a tertiary alcohol, which might be expected from a double-addition reaction).ls l,l,l-Trifluoroeicosanol was prepared by this reaction, and then the ketone so formed was reduced with LiA& (17) McBee, E. T.; Piece, 0. R.; Higgine, J. F. The reducing action of Grignard reagents on fluorinated carbonyl compounds. J. Am. Chem. SOC.1952, 74, 1736. (18) Dishart, K. T.; Levine, R. A new syntheeia of ketones containing one perfluoroalkyl group. J. Am. Chem. SOC.1966, 78, 2268.
ANALYTICAL CHEMISTRY, VOL. 85, NO. 15, A M S T 1, 1993
To prepare the Grignard reagent, 2.4 g of magnesium turnings was added to a 250-mL flask. A bifurcated connector was used to connect a condenser above the flask, and a drying tube was connected over the condenser toprevent exposure to atmospheric moisture. An addition funnel containing 20.88 g of l-bromooctadecane in 50 mL of anhydrous ether was connected to the other arm of the connector. After about 10 mL of the bromide was added to the flask, the mixture was warmed in a water bath to initiate the reaction. The start of the reaction is observed when the solution becomes cloudy. The remaining bromide was added slowly at room temperature. After all the bromide had been added, 100 mL of ether was added to help dissolve the reaction product. The mixture was gently refluxed in a water bath for 3 h. The reaction was considered complete when only a small amount of magnesium was left in the bottom of the flask. TheGrignardreagentwasdecanted toa2WmLflask. Adrying tube was connected to the top of the flask while it was cooled in an ice-butanol bath to about -10 OC. Trifluoroacetic anhydride (4.55g) wasthen added dropwisewith stirring. The milky mixture formed by the reactionwasthen left to warm to room temperature. The reaction mixture was then slowly poured into a 500-mL separatory funnelcontaining 20 g of ice and 20 mL of 10%sulfuric acid while the ice mixture was swirled. The organic layer in the top was separated, washed with sodium bicarbonate, dried with saturated sodium chloride, and then dried over Drierite. The solution was then filtered through a rapid speed filter, and the solvent was removed by Rotovap distillation. The mixture of the ketone and alcohol prepared as above was dissolved in about 50 mL of anhydrous ether. Lithium aluminum hydride (2 g) was carefullyadded to the solution very slowly. The reaction was continued at room temperature for 3 h, and 20 mL of 10% sulfuric acid was added to dissolve the inorganic sludge formed at the bottom of the flask. The organic layer was separated,washed, and dried as above. The solvent was removed by Rotovap distillation. The raw product was recrystallized from hexane. Thin-layer chromatography indicated that there were still two components in the product. The recrystallized product was again dissolved in hexane, and the second recrystallizedcomponent was collected confirmed anddried. The structure of l,l,l-trifluoroeicosanolwas by proton NMR and mass spectrometry. The melting point of the alcohol is 55-57 O C . A packing containing 13.2% of the l,l,ltrifluoroeicosanolphase was prepared in the same way as above, conditioned at 90 O C for 4 h, and then left overnight at 70 O C . All retention measurements were made at 70 OC. A precolumn of the same length was used to saturate the carrier gas. The syntheses and column preparation for a,a-bis(triflu0romethy1)-p-dodecylbenzylalcohol (denoted as HFOH, hexafluor0 alcohol) and a,a-bis(trifluoromethy1)-p-dodecylbenzyl methyl ether (denoted as HFOMe) have been presented el~ewhere.~ Retention measurements on these two columns were made at 80 OC.
Apparatus. All retention measurements were performed on an F&M Scientific Model 5750 chromatograph from HewlettPackard. The temperature controller was replaced with a YSI Model 72 proportional temperature controller. The heating elementa of the chromatograph were replaced so that precise control could be achieved (f0.1 OC). Chromatograms and retention times were recorded with a Hewlett-Packard 3390 reporting integrator. In general retention times were precise to about 1% from day to day and capacity factors could be reproduced to within 0.5%. In general the flow rate was about 60 mL/min throughout the work. Procedure. Most of the samples were injected as the headspace vapor above the pure liquid or above a mixture of solutes. Generally about 2-5 r L of sample was injected. No attempt was made to deconvolvepartitioning and interfacial adsorption effects. We have avoided reporting data for badly tailed peaks and for peaks whose retention varied with the amout injected. Furthermore, because our interest focuseson the effect of solute structure on change in tetention from solute to solute rather than on absolute thermodynamic functions such as the freeenergy of retention, we did not attempt to accurately measure the amount of stationary phase. The SPoterm will vary with the amount of stationary phase but we anticipate only minor effect
1971
on the other fitting coefficients(1, s , a, d , b). In prior worklOwith a very strong hydrogen bond base phase, we studied the effect of the amount of stationary on the fitting coefficients. There was no significant effect on any coefficient other than SPo. Solvatochromic Measurements. All spectroscopic measurements were made using a Varian DMS 200 spectrophotometer with a slit width of 0.2 nm, 20 nm/min scan rate, a smoothing constant of 5 8, and 0-1-cm-path length quartz cells. The wavelength of the spectrophotometer was calibrated daily by using a holmium oxide filter, and the stability of the instrument throughout the measurements was indicated by no more than a 0.10-nm change in any of the six holmium oxide peaks monitored. Peak maxima were determined using the "9/10n method prescribed by Kamlet and Taft in order to minimize the effect of changes in band shape with so1vent.m Because all the stationary phases measured in this work are solids at room temperature, they were f i t put into a small vial and heated in an oven until melted and then the specific indicator was added to the melted solvent. While it was still a liquid, the solution was placed in a cuvette and thermostated to the same temperature as in the retention studies. Each of the measured scales is discussed below. Dipolarity-Polarizability (**). Two ** indicators @mitroanisole and p-nitro-NJV-dimethylaniline) were used. The equations of Kamlet and T a f P were used to calculate the ?y* values. **@-nitroanisole) = (34.17 - u)/2.41 **@-nitro-NJV-dimethylaniline) = (28.1 - u)/3.436 where u is the frequency of maximum absorbance of the indicator in the solvent in units of kilokaysers (kK). Hydrogen Bond Acceptor Basicity (8). Two pairs of indicators @-nitrophenol/p-nitroanisoleand p-nitroaniliie/p-nitro-N,Ndimethylaniline)lsJ*@were usedtomeasure the fl of the stationary phases. The equationsof Laurence and co-workers15@were used to calculate the 8 values. @@-nitrophenol/p-nitroanisole) = -[u(phenol) - (1.0434u(anisole)- 0.57)1/2.075 8@-nitro-aniline/p-nitro-N,N-dimethylaniline)= +(aniline) - (0.984lv(NJV-dimethylaniline)+ 3.49)]/2.83
where u(phenol), u(anisole), daniline), and u(NJV-dimethylaniline) are the frequencies of maximum absorption of the indicators p-nitrophenol, p-nitroanisole, p-nitroaniline, and p-nitro-NJV-dimethylaniline in the given solvent (in units of kK), respectively. As pointed out by Laurance and Berthlet, the NH and OH indicators exhibit family-dependent behavior.16 That is they often give quite different results for oxygen and nitrogen bases. The basicity of water as measured by these two sets of indicators differs by nearly 0.3 unit. Hydrogen Bond Donor Acidity (a). We used one a indicator [Dimroth and Reichardt's betaine dye, &(30)]25-26to measure the hydrogen bond donor acidity of the stationary phases. The equation of Marcus%was used to calculate the u values. a = 0.06923,(30)
- 2.09 - 0.1478 + 0.196 -0.9**
Where E~(30)(kcal/mol) = 2.859 X lV/X (nm),13X is the A, of the longest wavelength band of the spectrum of the betaine indicator in the solvent.
RESULTS AND DISCUSSION Fitting the log Y Data. The solute parameters and the measured log k' values used in this work are given in Table (19) Carr, P. W.; Zhang,Y.; Li, J., unpublished data (20) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. SOC.1977, 99, 6027. (21) K e e t , M. J.; Taft, R. W. J. Am. Chem. SOC.1976,98,377. (22) Nicolet, P.; Laurence, C. J . Chem. SOC.,Perkin Tram. 2 1986, 1071. (23) Reichardt, C. Solvents and Soluent Effects in Organic Chembtry, 2nd ed.; VCH: Weinheim, Germany, 1988. (24) Taft, R. W.; Kamlet, M. J. J. Am. Chem. SOC.1976,98,2888. (26) Marcus, Y. J. Solution Chem. 1991,20, 929.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Table I. Solute Parameters and log k' Values no.
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
62 63 64 65 66 67 68 69 70 71 72 73 74 75
compd
log L'6
pentane hexane 2-methylpentane heptane octane nonane decane undecane dodecane tridecane tetradecane 1-hexene cyclopentane cyclohexane cycloheptane chlorobutane 1,2-dichloroethane dichloromethane chloroform carbon tetrachloride ethyl ether propyl ether butyl ether isopropyl ether propylaldehyde methyl acetate ethyl acetate propyl acetate acetone 2-butanone 2-pentanone cyclopentanone cyclohexanone acetonitrile propionitrile nitromethane nitroethane nitropropane triethylamine dimethylformamide dimethylacetamide dimethyl sulfoxide pyridine dioxane tetrahydrofuran benzene toluene ethylbenzene propylbenzene butylbenzene o-xylene m-xylene p-xylene fluorobenzene chlorobenzene bromobenzene iodobenzene o-dichlorobenzene p-dichlorobenzene nitrobenzene anisole acetophenone benzonitrile benzaldehyde N-methylaniline Nfl-dimethylaniline aniline methanol ethanol 1-propanol butanol isobutyl alcohol pentanol hexanol heptanol
2.163 2.668 2.507 3.173 3.677 4.176 4.685 5.191 5.696 6.200 6.705 2.571 2.426 2.906 3.543 2.716 2.572 1.997 2.478 2.822 2.066 2.971 3.954 2.561 1.770 1.946 2.359 2.861 1.766 2.269 2.726 3.093 3.580 1.537 1.978 1.839 2.313 2.773 3.008 2.922 3.357 3.110 2.969 2.788 2.521 2.792 3.343 3.785 4.239 4.714 3.947 3.868 3.867 2.785 3.630 4.022 4.505 4.453 4.404 4.433 3.916 4.458 3.913 3.935 4.492 4.753 3.934 0.916 1.462 1.975 2.539 2.381 3.057 3.550 4.067
a
rz*,C a
a; (I
-0.18 -0.16 -0.14 -0.14 -0.12 -0.12 -0.11
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.06 0.16 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.05 0.00 0.06 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.14 0.00 0.20 0.35 0.29 0.32 0.31 0.31 0.32 0.34 0.33
-0.10 -0.09 -0.08 -0.07 -0.07 -0.06 0.00 -0.01 0.19 0.39 0.34 0.27 0.16 0.03 0.03 0.04 0.03 0.35 0.33 0.31 0.32 0.38 0.39 0.40 0.58 0.59 0.62 0.64 0.67 0.66 0.65 0.02 0.81 0.80 1.00 0.60 0.45 0.27 0.29 0.29 0.30 0.30 0.30 0.31 0.29 0.28 0.36 0.44 0.51 0.59 0.56 0.53 0.91 0.52 0.80 0.85 0.75 0.70 0.57 0.76 0.35 0.27 0.30 0.30 0.28 0.32 0.33 0.35
gb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.08 0.08 0.06 0.04 0.04 0.40 0.30 0.29 0.25 0.37 0.47 0.49 0.48 0.52 0.48 0.48 0.57 0.56 0.37 0.41 0.16 0.17 0.18 0.64 0.97 1.06 1.54 0.90 0.79 0.61 0.10 0.11 0.11 0.11 0.11 0.12 0.12 0.12 0.10 0.09 0.10 0.09 0.09 0.10 0.21 0.22 0.49 0.40 0.42 0.31 0.26 0.42 0.62 0.52 0.52 0.52 0.48 0.52 0.51 0.51
SAc
TFAMIDEd
PDPe
TFOHf
HFOHa
-0.013 0.355 0.241 0.718 1.075 1.427 1.778 2.125 2.476 2.839 3.194 0.326 0.254 0.616 1.105 0.563 0.584 0.137 0.485 0.628 0.110 0.719 1.425 0.422 0.049 0.178 0.471 0.824 0.154 0.493 0.816 1.241 1.615 0.032 0.334 0.268 0.569 0.881 ndh 1.746 2.161 2.578 1.656 0.975 0.718 0.663 1.054 1.366 1.672 2.021 1.514 1.439 1.427 0.678 1.334 1.647 2.030 1.981 1.928 2.140 1.580 2.182 1.811 1.790 2.198 2.240 1.935 -0.045 0.271 0.666 1.045 0.888 1.424 1.786 2.138
-0.319 0.035 -0.067 0.385 0.726 1.068 1.404 1.744 2.078 2.396 2.756 0.013 -0.073 0.278 0.740 0.305 0.317 -0.119 0.209 0.311 -0.083 0.449 1.119 0.156 0.078 0.203 0.506 0.849 0.278 0.620 0.929 1.335 1.729 0.225 0.570 0.290 0.595 0.891 0.431 2.004 2.454 2.574 1.274 0.776 0.606 0.381 0.751 1.053 1.360 1.691 1.202 1.123 1.116 0.412 1.030 1.323 1.677 1.681 1.573 2.066 1.318 2.210 1.881 1.737 1.943 1.941 1.734 -0.057 0.163 0.532 0.892 0.718 1.258 1.611 1.964
-0.115 0.203 0.112 0.519 0.824 1.128 1.428 1.725 2.020 2.316 2.609 0.206 0.164 0.484 0.921 0.499 0.560 0.119 0.398 0.512 0.455 0.899 1.489 0.661 0.467 0.638 0.919 1.227 0.749 1.031 1.308 1.804 2.174 0.583 0.856 0.452 0.735 1.018 ndh 2.652 3.166 3.393 2.080 1.449 1.188 0.593 0.932 1.202 1.463 1.762 1.340 1.264 1.254 0.58 1 1.176 1.468 1.831 1.758 1.693 2.116 1.506 2.490 2.046 2.012 2.157 2.148 2.019 0.305 0.581 0.898 1.239 1.054 1.571 1.889 2.194
0.009
-0.086
0.398 0.285 0.781 1.168 1.532 1.905 2.276 2.661 3.038 3.416 0.377 0.263 0.644 1.151 0.645 0.615 0.122 0.459 0.643 0.559 1.098 1.824 0.823 0.518 0.771 1.120 1.497
0.284 0.183
0.864 1.197 1.535 2.027 2.451 0.661 0.999 0.508 0.867 1.198 1.820 2.846 3.386 ndh 2.309 1.665 1.370 0.728 1.151 1.475 1.798 2.167 1.639 1.563 1.555 0.734 1.402 1.720 2.108 2.071 1.999 2.433 1.789 2.841 2.359 2.266 2.486 2.529 2.309 0.407 0.723 1.089 1.495 1.288 1.894 2.279 2.653
0.648 1.006 1.361 1.714 2.068 2.419 2.769 3.116 0.295 0.162 0.519
0.996 0.609 0.585 0.079 0.370 0.652 0.819 1.262 1.944 0.867 0.797 1.120 1.458 1.804 1.195 1.462 1.785 2.333 2.701 0.882 1.232 0.620 0.981 1.297 2.066 3.247 3.762 ndh 2.899 2.430 1.803 0.644 1.050 1.361 1.669 2.019 1.520 1.448 1.438 0.686 1.290 1.588 1.946 1.934 1.849 2.427 1.810 2.999 2.496 2.440 2.500 2.523 2.386 0.522 0.858 1.204 1.593 1.396 1.972 2.329 2.686
ANALYTICAL CHEMISTRY, VOL. 65,
NO. 15, AUGUST 1, 1993 1873
Table I. (Continued) no.
compd
log L'6
76 77 78 79 SO 81 82 83 84 85 86 87
2-propanol 2-butan01 tert-butyl alcohol isopentano1 cyclohexanol trifluoroethanol HFIP benzyl alcohol phenol acetic acid propionic acid butanoic acid
a
1.750 2.322 1.994 2.885 3.594 1.315 1.370 4.162 3.641 1.750 2.290 2.830
rz*.c a 0.21 0.22 0.19 0.28 0.37 0.37 0.47 0.71 0.77 0.54 0.52 0.54
I%:a
flZb
SAc
TFAMIDEd
0.29 0.28 0.25 0.34 0.31 0.66 1.11 0.43 0.69 0.72 0.54 0.57
0.53 0.50 0.53 0.51 0.53 0.15 0.02 0.51 0.23 0.50 0.50 0.48
0.470 0.836 0.611 1.298 1.891 4.003 0.323 2.889 2.038 1.515 1.889 2.223
0.303 0.621 0.366 1.133 1.649 0.132 0.575 2.210 2.057 0.770 1.198 1.540
log k' PDPI
TFOHf
0.729 1.038 0.827 1.447
HFOHg
0.905 1.256 1.033 1.756 2.360 0.162 0.321 2.781 2.230 1.513 1.938 2.263
ndh 4.108 4.178 2.382 1.814 1.019 1.326 1.633
1.052 1.387 1.210 1.851 2.025
-0.080 -0.259 2.742 1.799 1.183 1.522 1.838
0 From ref 1. From ref 3. log capacity factors on the SA column at 80 "C. log capacity factors on the TFAMIDE column at 80 "C. * log capacity factors on the PDP column at 110 "C. flog capacity factors on the TFOH column at 70 "C.8 log capacity factors on the HFOH column at 80 "C.h No data.
3.5
I
3.5
I
I
I
I
I
3.0 3.0
2.5 2.5
6
2.0
Li 7
d
(d
&
lS5
h
1.0
M 0 H
M
0
d
2.0
V
V
0.5
0.0
1 i8 0.0
-0.5
-
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1 .s
I
I
O/
e
1 I
I
C
4
0.0 1.0' 0.0
0.5
1.0
1.5
log Flguro2. Plot of calculated log k'vs experimental values for the HFOH phase (see eq 2).
I. The a", parameters were obtained from the relative retention of the solutes on the hexafluoro alcohol (HFOH) and its methylated ether phases (HFOMe) as described elsewhere.3 The results of fitting of log k' values to eq 1are presented below. For the hexafluoro alcohol column, an excellent fit was obtained (eq 2).
+
log W'(HF0H) = (-1.47 f 0.01) (0.693 f 0.003) log L" (0.67 f 0 . 0 2 ) a p (-0.26 f 0.01)6,
+
+ (-0.09 f 0.02)CYi + (2.22
0.02)Oi (2)
SD = 0.027, r = 0.9995, n = 85 Because the a", parameter was measured on the basis of the relative retention on this phase and its methyl ether, this result is not surprising. The goodness of fit does not provide any real support for the accuracy or general applicability of our 9 ,: values. The data calculated from eq 2 are compared to the experimental data in Figure 2. The dotted line is the line of perfect agreement between the experimental and
2.5
3.0
3.5
"expt.
Flgure 3. plot of caIcubtBcI tog k v s experk"el values for the TFOH phase (see eq 3).
calculated values. Cyclohexanol and pyridine are obvious outliers (see Figure 2). They were excluded from the regression given in eq 2. The sign and magnitude of the coefficients for this fit and others will be discussed below. The final regression results for the l,l,l-trifluoro-3eicosanol (TFOH) phase are given in eq 3. Again this is a very good regression, but several outliers, as identified in Figure 3, were removed. The outliers were the three carboxylic arirla
+
..
2.0
nMP n M A anrl dinrana
+
log W'(TF0H) = (-1.51 f 0.02) (0.742 f 0.005) log L" (0.56 f 0.03)a;pc + (-0.18 f 0.02)6,
+
+
+
(0.48 f 0.03)~~: (1.58 f 0.03)G (3)
SD = 0.046, r = 0.998, n = 80 Even though six solutes were excluded we feel that the overall goodness of fit is quite good given the very great chemical differences in the 80 solutes that were included in the fit. Very acceptable regression results were obtained for the phenol phase (PDP) (see eq 4 and Figure 4). The obvious
1074
ANALYTICAL
3.01
CHEMISTRY,VOL. 65, NO. 15, AUGUST 1, 1993
1 2.5
t
2.0
-
PDP
2.5
TFAMIDE
76 1.5 0
41
-
M 0
0" OS5/
1.0
-
0.5
-
OQ
0.0 .d
-.--0.5
'0
0.0
0.5
1.0
1.5
Figuro 4. Plot of calculated log k'vs phase (see eq 4).
2.0
2.5
3.0
-0.5 -0.5
3.5
experimental values for the PDP
Figure 5. TFAMIDE
+
+
+
3.0
+ +
I
I
I
I
I
0.5
1.0
1.5
2.0
2.5
I
Plot of calculated log k'vs experimental values for the phase (see eq 5).
outliers (see Figure 4) DMF, DMA, dioxane, 2,2,2-trifluoroethanol (TFE), and hexafluoro-2-propanol (HFIPA) were excluded from the final regression. Note that the carboxylic acids were well fitted. Despite the considerable chemical differences between the fluoro alcohols and phenol, a very acceptable fit is obtained. log k'(PDP) = (-1.32 f 0.02) (0.602 f 0.005) log L16 (0.66 f 0.03)~2.'~(-0.13 f 0.02)S2 (0.29 f 0.03)~~; (1.46 f 0.03)Oi (4)
I
0.0
ci
1
0'
S t e a r i c Acid
0
I
LA
SD = 0.044, r = 0.998, n = 80 Note that the large coefficient of 0; indicates that the phenol phase is a very strong hydrogen bond donor. It is almost as strong as the TFOH phase. The regression results for the fluorinated amide and the stearic acid phases are given in eq 5 and 6 and are plotted in
+ + +
log k'(TFAM1DE) = (-1.66 f 0.02) (0.67 f 0.006)log L" (0.90 f 0 . 0 3 ) ~ ; ~ ~(-0.19 f 0.02)6, (0.78 f 0.03)~~; (0.66 f 0.04)g ( 5 )
+
+
+
log k'(SA) = (-1.46 f 0.02) (0.702 f 0.005) log L" (0.43 f 0 . 0 2 ) ~ ~ * (1.23 * ~ f 0.05)~~: (0.39 f 0.03)/3;(6)
+
+
0.5
1.0
1.5
Figwe 6. Plot of calculated log k'vs phase (see eq 6).
SD = 0.053, r = 0.997, n = 80
+
0.0
SD = 0.040, r = 0.999, n = 75 Figures 5 and 6, respectively. DMF, DMA, DMSO, triethylamine (TEA), dioxane, aniline, and N-methylaniline were excluded from the fit on the fluorinated amide column. For the stearic acid column, DMF, DMA, DMSO, pyridine, the three carboxylic acids, benzyl alcohol, phenol, TFE, and HFIPA were excluded. Again, we note that these are really remarkably good fits and on the whole the solute parameters (log LIS, T ~ * ? C , ai,g) appear to be transferable to these new phases.
2.0
2.5
3.0
3.5
experimental values for the SA
We note from the above regressionsthat the most frequent outliers are either very p o h t r o n g hydrogen bond acceptors (DMF, DMA, DMSO, dioxane, pyridine)-or very strong hydrogen bond donor solutes (TFE, HFIPA, phenol, carboxylic acids). In some cases the lack of fit for these solutes could be due to the fact that these speciesare either extremely strongly retained (DMSO, DMA, DMF) or very weakly retained (TFE, HFIPA) and thus subject to some experimental imprecision and perhaps systematic error. Alternatively, the lack of fit for these solutes could be due to the use of incorrect g"z parameters for these solutes. The latter point will be examined in detail below. The fact that sometimes the carboxylic acids fit well and sometimes not seems to indicate that this problem is not due to dimerization of the acids in either the gas or liquid phase.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Table 11. Solvatochromic Parameters for the Stationary Phases parameters HFOH TFOH TFAMIDE SA PDPj 0.96 0.63 0.79 0.17
-0.09 0.04
0.39 0.73 0.56
0.47 0.62 0.54 0.41 0.87 0.64 0.69 0.48 0.58
1.24 0.62 0.74 0.41 0.57
0.18 0.16 0.17 0.40 0.15 0.28
0.003
A-
0.93
0.61
0.31
-
-
-
T* value using indicatorp-nitroanisole. b u* value using indicator p-nitro-NJV-dimethylaniline.c x* average of r*(l)and u*(2). d 6 value using p-nitrophenollp-nitroanisole. e 6 value using p-nitroanilinelp-nitro-NJV-dimethylaniline. f9 , average of B(1) and 8(2).8 a value using indicator E~(30)and u*(l) and B(1). a value using indicatorE~(30) and u*(2) and b(2). a average of a(1) and 4 2 ) / No data due to severe overlap of ita spectra with that of the indicators. No data due to the occurrenceof a proton-transferreactionbetween stearic acid and the indicator [ET(30)1. @
A ready explanation for the lack of fit of DMA and DMF is adsorption at either the gas-liquid or gas-solid interface. It is most confusing to note that the carboxylic acids are "over" retained even on the stearic acid column. Stearic acid should be a very effective adsorption site blocker as far as the carboxylic acids are concerned. Despite the fact that we had to eliminate several obvious outliers on each column, the quality of fits is very satisfactory. Clearly the overall precision of the data sets is quite high despite the fact that it took a number of days to acquire each set and in several cases we had to use both a short column and a long column to measure the wide range in k' values. Chemical Interpretation of the Coefficients. As required by the fundamental concepts underlying the use of linear solvation energy relationships, the coefficients defined by the regression analysis characterize the chemistry of the stationary phase and thus they should be related to the chemical nature of the phase. They are not merely arbitrary (freely adjustable) fitting coefficients. We note that the 1 coefficients on all five phases are quite similar, they range from 0.600 for the phenol phase at 110 OC to 0.742 for the hexafluoro alcohol phase at 80 "C. The 1 coefficient for a nonpolar stationary phase such as squalane is 0.742 at 80 OC1 and 0.674 at 100° C.* We point out here that when we compared the 1 coefficients, we compared them to those of nonpolar phases at the same temperature because these coefficients are temperature dependent.12 Therefore, on the basis of their 1 coefficients all five phases are similar to a nonpolar stationary phase with respect to their ability to separate members of a homologous series. This results because all five stationary phases have long alkyl chains. The trifluoro alcohol and stearic acid phases have longer alkyl chains and thus larger 1 coefficients. All five stationary phases are dipolar and therefore have significant s coefficients. Previously we showedl92 that a phase's s coefficientcorrelatesquite well with measured values of the solvatochromic a* parameter of the liquid. Based on the observeds coefficient, this predicts that the a* values for all five phases should be fairly small. We measured the u*, a,and j3 of the stationary phases at the column temperatures using the original spectroscopic methodology as described above and the regression equations of Kamlet-Taft,ZO Laur e n ~ e , ~and ~ , Marcus.% *~ The solvatochromic data are shown in Table 11. We now compare the spectroscopically measured a* of the stationary phases with the s coefficients. We used two a* indicators (p-nitroanisoleand p-nitro-N,N-dimethylaniline).
107s
We note that agreement in the measured u* values between the two indicators is extremely poor for the three fluorinated stationary phases, particularly for the fluorinated amide. In addition, the a* values for these three stationary phases are higher than that can be estimated on the basis of the observed chromatographics coefficients.' While the difference in r* as measured with these two indicators is larger than we would like there is considerable precedence. u* values of protic liquids are difficult to measure and generally show considerable variance between indicators.% In addition, in a recent study of a series of liquids as simple as the alkylbenzenes we observed systematic differences as large as 0.10 unit in the u* values obtained with five pr0bes.n Nonetheless it is not clear why these phases behave so differently from previously examined stationary phases.' They are fluorinated and this may differentially influence the spectroscopic and chromatographic retention processes. Despite our uneasiness with the dispersion in u* values obtained with the two indicators, both the average u* values and the s coefficients predict the same order of dipolarity for the three phases, that is, fluorinated amide > hexafluoro alcohol > trifluoro alcohol. Both u* indicators give very similar u* values (0.16 and 0.18) for stearic acid. This value is surprisingly low compared to values reported for other carboxylicacids (for example, 1-heptanoicacid was estimated to have a T* value of 0.50 %). However, a u* of 0.17 for stearic acid, when used with our general equation from ref 1, s coefficient = (0.01 f 0.07) + (2.49 f 0.l2)ahlwnt,predicts exactly the s coefficient obtained for stearic acid (0.42). The solvatochromic parameters for the phenol phase could not be measured due to severe spectroscopic overlap of the solvent with both a* indicators. The comparison between the a coefficients and the spectroscopically measured j3 values is more complicated and less satisfactory. First, we note the agreement between the two j3 indicator pairs @-nitrophenollp-nitroanisole and p-nitroanilinelp-nitro-N,N-dimethylaniline) is not good. This could be due to both fluorination and the existence of the well-known family-dependent relationshipls such that OH donors @-nitrophenol) and NH donors @-nitroaniline1act differently toward the same acceptor. However, the measured j3 values still give some indication as to the relative hydrogen bond acceptor strengths of the stationary phases. The hexafluoro alcohol phase has no measurable hydrogen bond acceptor strength (a is virtually zero) due to the presence of the two CF3 groups. Its j3 is very small (average j3 between the two indicators is 0.04). The trifluoro alcohol and fluorinated amide phases only have one CFSgroup adjacent to the acceptor group and should have some hydrogen bond acceptor strength. Both the a coefficients obtained from the LSERs and the j3 values obtained spectroscopically for these two phases indicate that they are hydrogen bond acceptors although their measured j3 values suggest that they are much stronger hydrogen bond acceptors than we infer from the chromatographicallyobserved a values. For stearic acid, the indicator pair p-nitrophenollp-nitroanisolegives a j3 value of 0.4 whereas p-nitroanilinelp-nitro-N,N-dimethylaniline gives a j3 value of 0.15. The j3 value of 0.4 for stearic acid actually is in good agreement with the 0 values for other carboxylic homologs, which are estimated to be 0.45.26 Moreover, a value of 0.4, when used with our general equation from ref 2 [ a = (0.09 f 0.16) (2.96 A 0.24)j3,1,,J predicts an a coefficient close to that obtained in this work (1.23). As pointed out above, it was our purpose to prepare strong hydrogen bond donor phases; thus, as expected, very signif-
+
~~~
(26) Chawla,B.; Pollack, S.K.; Lebrilla,C. B.;Kamlet, M.J.; Taft, R. W. J. Am. Chem. Sac. 1981,103,6924. (27) Ulrich, E. T.; Carr, P. W. J. Phys. Chem. 1991,95, 10197.
1976
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
icant b coefficientswere observed on all five phases, especially for the two fluoro alcohols and the phenol. However, the b coefficients of the fluorinated amide and stearic acid phases were considerably smaller than expected. In contrast, in all of our studies of nearly 100 other GC phases, the largest b coefficient was only about 0.8 for Zonyl E 7.’ We note that not only is the b coefficient statistically significant but for the two fluoro alcohols and the phenol it is the largest coefficient relative to the s, a, and b coefficients. This is important in that the ranges of the corresponding solute parameters ( T ~ P C , ai, G) are approximately 0-1. Based on our previouslydeveloped approach, a method for the accurate and precise measurement of solute j3i values requires that the b coefficientbe quite large relative to the other Coefficients. The b coefficient should be related to the stationary-phase hydrogen bond donor strength measured directly by the solvatochromic comparison method. We used Richardt’s betaine dye E ~ ( 3 0 )as the stationary phase a indicator. Because the measured E ~ ( 3 0value ) depends not only on the a but also on the A* and 0 of the stationary phase, different a values are obtained when different values of A* and j3 are used. From the averaged a values for the stationary phases we see that the fluoronated amide is a weaker hydrogen bond donor than either the hexafluoro alcohol or the trifluoro alcohol. The a value of stearic acid cannot be measured using E ~ ( 3 0due ) to complete proton transfer between carboxylic acids and the betaine dye.23 The hexafluoro alcohol phase has the largest b coefficient ( b = 2.22) whereas the trifluoro alcohol and the phenol phases have smaller b coefficients ( b = 1.58 for the trifluoro alcohol and 1.46 for the phenol). The relative hydrogen bond donor strength (i.e., the relative value of the b coefficient) of the stationary phases, as deduced from chromatographic retention, is in good agreement with other studies of the relative hydrogen bond donor strength of simple analogs. IR studies28sB of the hydrogen bond donor strength of hexafluoro2-propanol, trifluoroethanol, and phenol show that HFIPA is a stronger hydrogen bond donor than phenol, and phenol is a stronger donor than trifluoroethanol. The solvatochromic a values of these analogs show the same trend as the chromatographic results, HFIPA (a= 1.96) > TFE (a= 1.49) > 3-methylphenol (a = 1.13).26 Even though there is no general relationship between Bransted acidity and hydrogen bond acidity across different families of acids,30 there are good relationships within a family.31 Thus, we were surprised to see the extremely low hydrogen bond acidity of the stearic acid phase ( b = 0.39). We had thought that stearic acid would have a solvent a value similar to that of other carboxylic acids (for example, l-heptanoic acid a = 1.20 25) and thus it should have a much higher b coefficient. The weak hydrogen bond donor acidity of the stearic acid phase could be due to dimerization in the GC column since carboxylicacids dimerize extensively even in the gas phase.32 The d coefficient was significant and negative for all phases except the stearic acid phase. This certainly agrees with what we observed before.lg Since stearic acid is aliphatic and has a zero 6, it is not surprising that it has no d coefficient. The other phases have significant d coefficients, probably because they are either aromatic or fluorinated. Comparison of Solute Hydrogen Bond Basicities As Measured with the Various Phases. In following our (28) Purcell, K. F.; Stikeleather,J. A.; Brunk, S. D. J.Am. Chem. SOC. 1969,91,4019. (29) Sherry, A. D. Spectroscopicand CalorimetricStudies of Hydrogen Bonding. In The Hydrogen Bond; Schuster,P., Zundel, G., Sandorfy, C., Eds.;North-Holland Publishing Co.: New York, 1976; p 1217. (30)Amett, E.M.; Mitchell, E.J. J.Am. Chem. SOC.,1971,93,4052. (31) Abraham, M. H.; Duce, P. P.; Morris, J. J.; Taylor, P. J. J.Chem. SOC.Faraday Tram 1, 1987,83, 2867. Markovits, G. Y.; Perry, I. J.Phys. Chem. 1975,79,239. (32) Levy, 0.;
1.4
1
/ i
1.2.
1.0
.
0.6
.
0.6
.
0.4
.
0.2
.
0.0
.
n
x
0 W
0
Q
-0.2
I/ v1
-0.2
I
I
I
I
I
I
I
1
1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
pg (rel. ret.)
Figure 7. Plot of the calculated 6 from the HFOH phase in this work vs the 6 values based on the relative retention.
previous methodology in developing chromatographically based scales of solute hydrogen bond acidity and dipolaritypolarizability, we computed a solute hydrogen bond basicity parameter (Table 111)using the measured retention data and the regression equation for each phase. Our purpose was to see whether the previously developed j3i scale could be applied to these new donor phases. Equation 1was put in the form of eq 7 to allow the calculation of pi.
Y = bj3i = log k’ - (SP,+ 1 log L“
+ SA^'^^ + db, + aai)
(7)
It should be evident that measurement of in this fashion will be sensitive to errors in the capacity factor and any of the other parameters on the right-hand side of eq 7. In ~ could be contrast, our previously developed ~ 2 . ’ values estimated without knowing ai. Although measurement of ai did require knowledge of log Lla and 7r2*vC, it did not require knowledge of pi. In our previous method3 for the measurement of &, use of a closely matched reference phase circumvented the need to know log L16, a2*sC, and ai. The approach based on eq 7 leaves considerable latitude for propagation of both determinate and random errors. The standard deviation in the derived value of G was estimated via eqs 8 and 9. a2(Y) = &log k’)
+ a2(SPo)+ a2(Z) + a2(s)+ u2(d)+ &a) (8)
[a2(17/p+ ~ ~ ( b ) / b ~ I ” ~ (9) Equation 9 clearly shows that use of phases with very large b coefficients relative to the other parameters should give more precise and probably more accurate values for 0;. The coefficients SPo, 1, s, d , a, and b were obtained by regressing log k’ against the solute parameters using eq 1. The standard deviations [a(log k’), u(SPo), a(l), a(s), a@), a@), and a(b)] were obtained from the same regression. These considerations lead us to believe that the capacity factor data for the fluorinated amide and the stearic acid columns will not give reliable estimates of 0; because the average standard deviation from these two phases are very large, 0.07 and 0.11 (shown in the last row of Table 11). However, we decided to u(a”,)/a”, =
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
1977
Table 111. 0: Calculated from Individual Phases.
compdno. relret HFOH TFOH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.08 0.08 0.06 0.04 0.04 0.40 0.30 0.29 0.25 0.37 0.47 0.49 0.48 0.52 0.48 0.48 0.57 0.56 0.37 0.41 0.16 0.17 0.18 0.64 0.97 1.06 1.54 0.90 0.79 0.61
0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00
0.00 -0.01 0.01 0.00 -0.01 0.01 0.09 0.06 0.03 0.04 0.04 0.37 0.29 0.29 0.24 0.36 0.46 0.48 0.48 0.53 0.49 0.49 0.57 0.58 0.39 0.41 0.17 0.19 0.19 0.64 0.96 1.06
0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.02 0.01 0.00 0.03 0.09 0.05 0.02 0.01 0.05 0.33 0.24 0.24 0.26 0.32 0.40 0.43 0.43 0.52 0.49 0.49 0.56 0.59 0.40 0.41 0.15 0.17 0.17 0.67 1.06b 1.196
ndc
ndc
0.966
0.91 0.52b 0.53
0.74 0.60
0: PDP 0.01 0.01 0.00 0.01 0.01 0.01 0.00 0.00 -0.01 -0.02 -0.03 0.01 0.04 0.03 0.08 0.08 0.08 0.03 0.03 0.05 0.35 0.29 0.28 0.29 0.34 0.39 0.43 0.43 0.52 0.51 0.51 0.62 0.67 0.39 0.39 0.14 0.16 0.17
ndc 1.196 1.37b 1.54 0.93 0.56b 0.57
TFAMIDE
SA
0.05 0.05 0.04 0.05 0.03 0.05 0.03 0.03 0.01 -0.02 0.00 0.01 0.01 -0.02 0.04 0.11 -0.03 -0.05 -0.05 0.06 0.21 0.12 0.13 0.09 0.32 0.35 0.41 0.41 0.55 0.55 0.55 0.56 0.64 0.36 0.45 0.11 0.16 0.16 0.07b 1.34b 1.57b 1.72b 0.85 0.23b 0.45
0.00 0.02 -0.01 0.03 0.02 0.03 0.01 -0.02 -0.03 -0.01 -0.01 0.01 0.08 0.08 0.22 0.06 -0.03 -0.14 -0.35 0.08 0.26 0.20 0.25 0.17 0.24 0.29 0.33 0.32 0.47 0.46 0.46 0.70 0.79 0.11 0.25 0.08 0.23 0.22 ndc 2.16b 2.4gb 3.8gb 2.13b 0.72 0.76
compd no. relret HFOH TFOH 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 av SDd
0.10 0.11 0.11 0.11 0.11 0.12 0.12 0.12 0.10 0.09 0.10 0.09 0.09 0.10 0.21 0.22 0.49 0.40 0.42 0.31 0.26 0.42 0.52 0.52 0.52 0.52 0.48 0.52 0.51 0.51 0.53 0.50 0.53 0.51 0.53 0.15 0.02 0.51 0.23 0.50 0.50 0.48 0.02
0.10 0.11 0.11 0.11 0.12 0.13 0.13 0.13 0.10 0.09 0.08 0.07 0.09 0.07 0.21 0.21 0.49 0.42 0.42 0.29 0.25 0.40 0.51 0.52 0.50 0.50 0.47 0.51 0.51 0.51 0.53 0.50 0.53 0.52 0.35b 0.13 0.02 0.51 0.24 0.51 0.49 0.46 0.02
fl: PDP
0.13 0.10 0.14 0.11 0.13 0.11 0.12 0.10 0.13 0.11 0.15 0.13 0.15 0.12 0.15 0.12 0.11 0.07 0.10 0.09 0.10 0.10 0.08 0.12 0.10 0.10 0.08 0.09 0.20 0.20 0.18 0.17 0.48 0.51 0.41 0.40 0.38 0.41 0.25 0.27 0.24 0.25 0.36 0.37 0.56 0.51 0.54 0.52 0.52 0.51 0.51 0.52 0.46 0.46 0.51 0.52 0.51 0.53 0.50 0.52 0.55 0.53 0.50 0.51 0.53 0.52 0.51 0.52 0.53 ndc 0.14 -0.03b 0.06 -0.24b 0.51 0.50 0.32 0.24 0.6gb 0.49 0.76b 0.53 0.70b 0.51 0.04 0.04
TFAMIDE 0.17 0.17 0.17 0.18 0.20 0.22 0.20 0.21 0.13 0.12 0.08 0.03 0.11 0.05 0.24 0.15 0.54 0.53 0.43 0.14b 0.18 0.23b 0.57 0.53 0.50 0.49 0.42 0.49 0.49 0.49 0.53 0.44 0.46 0.50 0.48 0.15 0.16 0.51 0.45 0.37 0.65 0.58 0.11
SA 0.08 0.10 0.10 0.07 0.13 0.19 0.16 0.14 0.04 0.13 0.15 0.17 0.18 0.17 0.20 0.15 0.41 0.37 0.40 0.04 0.31 0.11 0.44 0.57 0.54 0.54 0.43 0.52 0.50 0.48 0.65 0.58 0.74 0.49 0.76 -1.33b -2.216 1.63b -0.746 1.67b 2.326 2.076 0.07
fl: calculated from log k’ of differentphases using eq 7. Outliers identified in the plots of log k ’ d c vs log kleXptin respective phases. No data. d Average standard deviation of fl: calculated using eq 9. process the data and compare the results to those for the results od the other columns. In order to minimize the effect of gross outliers in influencing the coefficients, we used a zero-lag Kalman filter procedure33 to determine the coefficients used in eqs 7-9. In general, the 4; values calculated from the capacity factors measured with the hexafluoro alcohol,trifluoro alcohol, and phenol phases agree acceptably well with the initial a”, values. Those that do not agree with the initial a”, value are evident in plots of calculated /3; values against the initial 8; (Figures 7-10). The solid lines shown in each figure are those which would occur if agreement were perfect. The bars denote the standard deviation calculated via eq 9. The solutes which fall off the ideal lines in Figures 7-10 are the same compounds identified as outliers in Figures 2-5. For compounds whose calculated values differ from phase to phase, we believe that the /3; values obtained using the hexafluoro alcohol and hexafluoro ether are correct. Agreement of calculated on the basis of retention on the fluorinated amide phase with the initial values is not
a;
(33) S. C. Rutan, P. W. Carr, Anal. Chim. Acta, 1988,215, 131.
nearly as good as that observed with the hexafluoro alcohol, trifluoro alcohol, and phenol phases (see Figure 10). To some extent we expected poorer agreement because solvatochromic studies15t22 clearly show a “family dependence”. That is OH donors and NH donors behave differently toward spectroscopic probes of solvent hydrogen bond basicity. Upon detailed inspection of Figure 10, we see that different classes of compounds form different lines. For example, the alkyl ethers (21-24 as shown in Table 111) all have much lower /3: values on the fluorinated amide phase. The anilines (65, 67), alkylbenzenes (46-531, and halogenated benzenes (5459) all form family lines in this plot. We also observe a monotonic trend in the a”, (TFAMIDE) (see Table 111)among the n-alkanes, all of which should have a”, values equal to zero (see the points falling on the vertical axis in Figure 10).These trends suggest a fundamental difference in the behavior of the fluorinated amide phase and the OH donor phases described above. Finally, we wish to comment on the relationship between the hydrogen bond donor acidity and Bransted acidity of the donor phases and the effect of proton transfer on our parameters. We did not measure the pK of the donor phases
1078
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993 1.6
,
I
I
,
l
I
1.4 0.5 1.2
n
1 .o
w
n
n
0.4
1
I
OM
3 0.8
W
0.4
0.0 o.2
-02
j
,,+
2b
1
W
i
VN
Q.
I2 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
(rel. ret)
6
Flguro 8. Plot of the calculated from the TFOH phase in this work vs the values based on the relative retention.
6
1.4
1.0 n
a CI
Flguro 10. Plot of the calculated from the TFAMIDE phase (an NH donor) in this work vs the values based on the relative retentlon (an OH donor).
6
Table IV. pK. and Hydrogen Bond Donor Acidity
t
stat phase
model compd
HFOH
HFIPA TFE 3-methylphenol acetic acid 2,2,2-trifluoroacetamide
TFOH
-
i
J
0.8 -
PDP SA TFAMIDE a
b
a
2.26 1.61 1.46 0.41 0.73
0.56 0.58 ndaa nda 0.31
PK~
9.30 12.39 10.09 4.16 nda
nda, no data available.
Table V. Results of Principal Components Analysis
i 0.0
1
1 VI -0.2
f
I
I
I
I
I
I
I
iI
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
@; Flguro 0. Plot of the calcuted
(rel. ret.)
6 from the POP phase In this work vs
the @: values based on the relative retention.
used in this work. Instead we use the pK, of model compounds (Table IV) for comparison. From Table IV, we can see that stearic acid is the strongest Bransted acid yet the weakest hydrogen bond donor. There was only one compound (triethylamine)that did not elute from the stearic acid column, probably due to proton transfer. As we did not observe proton transfer for any other compound on the stearic acid phase, we conclude that proton transfer for all other phases is at most a minor effect. PCA of the log K Data and a; Values. We performed principal component analysis (PCA) of the log k' data and A data calculated from all five phases. The results are presented in Table V. Only one principal component is
eigenvector 1 2
3 4
eigenvalue
For log K' Data on the Five Phases 2.9200 95.11 0.1291 4.20 0.0151 0.49 0.0031 0.12 For
1 2 3
% var
% cumvar
95.11 99.32 99.81 99.93
fii Calculated from the Five Phases 0.6310 0.0194 0.0071
81.64 11.03 0.98
81.64 98.67 99.66
needed to explain 95 % of the variance in the five data sets. This stands in distinct contrast to our previous PCA results, which used a very wide variety of polar, nonpolar, and hydrogen bond donor and nondonor phases.19 In that study three factors were needed to explain 96 % of the variance in the data set. The present result is due to the fact that the set of five phases used in this work ie really quite similar. That is the balance of chemical and physical interactions does not vary much from phase to phase. We stress the fact that the high percentage of the variance due to a single component (factor) does not mean that any single term in eq 1can explain 95% of the data. Even the cavity dispersion term 1 log ,516, which is the most important term governing retention in gas-liquid chromatography, only explains 43 % of the variance in the data set for the hexafluoro alcohol phase and 75% of the variance in the data set for the stearic acid phase.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
i
4
0.0
~
a
: .-I
CONCLUSIONS
-
The solute hydrogen bond acceptor basicity parameter
1: WOH
-0.2
-
2 WOH 9: W)ymP
-
4 PDP
-0.4
0 4
component is needed to explain 99% of the variance. Examination of the loadings plot shows that the fluorinated amide phase, that is, the NH donor, can be distinguished from the three OH donor phases.
lo':"
0.4: 0.2
N
1979
-0.8 I
0.2
(6) established previously can be applied to the character-
5: SA
1
I
,
I
I
I
I
1
0.3
0.4
0.5
0.8
0.7
0.8
0.9
Loading 1 Flgure 11. Principal component plots of loading 1 against loadlng 2. (Loadlngs 1 and 2 are the coefflclents conespondlng to the prlncipal components 1 and 2 of the five scales.)
In the spirit of Maria and Gal's analysis of basicitydependent properties,lB we also carried out a PCA of the five 6 scales developed in this work. We found that one needs two principal components to describe 99% of the variance in BT; (see Table V). This suggests that the five 6 scales are distinct. However, when the factor loadings, that is, the coefficients of the principal components for each scale, are plotted (see Figure 11)against one another we clearly observe that the stearic acid a", scale is unique. When based on stearic acid is deleted, only a single factor is needed to explain 96.4% of the variance in the remaining four scales. A second
ization of strong hydrogen bond donor phases by using linear solvation energy relationships. Three stationary phases includingtwo fluorinated alcohols were found to be very strong hydrogen bond donors. Stearic acid, on the other hand, was found to be a very weak hydrogen bond donor. Generally good agreement was observed between the chromatographically determined stationary-phase properties and spectroscopically determined quantities. However, discrepancies between the two independent approaches were found for the fluorinated hydrogen bond donor phases.
ACKNOWLEDGMENT We thank Mr. Grant Anderson for the measurement of the solvatochromic data. This work was supported in part by grants to the University of Minnesota from the National Science Foundation and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. J.L.gratefullyacknowledgesreceipt of an industrial fellowship sponsored by the 3M Co. RECEIVED for review September 29, 1992. Accepted May 5, 1993.
