Anal. Chem. 1993, 65, 1831-1634
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CORRESPONDENCE
A Simple Model for the Chromatographic Retentions of Polyhalogenated Biphenyls Paul G. Seybold' and Juli Bertrandf Departments of Chemistry and Biochemistry, Wright State University, Dayton, Ohio 45435 Polyhalogenated biphenyls (PHBs), especially polychlorinated biphenyls (PCBs),were for several decades extensively used in industry as transformer oils, flame retardants, and solvent extenders and in other applications.' The manufacture of these derivatives of biphenyl (BP) has now been largely discontinued due to concerns over their suspected toxicities2 and possible carcinogenicities.3~4 Nonetheless, PCBs and PHBs remain today as widespread environmental contaminants5-8 and have recently been a subject of some controversy.9-12 The most popular method of analysis for PHBs in environmental samples has traditionally been gas chromatography,l3 although liquid chromatography has also been employed. Mullin et al. reported the syntheses and gas chromatographicanalyses of all 209 PCB ~0ngeners.l~ More recently, HBfler et al.15 described the gas and high-pressure liquid chromatographic retentions of 57 mono- and polyhalogenated biphenyls. The usefulness of quantitative molecular structure-property relationships in revealing the mechanisms underlying chromatographic retention has been recognized for some time,'6 and a number of studies relating the structures of the PHBs to their chromatographic retentions have been reported. Hiifler et al.15 examined the relationship between molecular surface areas and chromatographic retentions of the PHBs. The GC retentions of the PCBs were analyzed by Hasan and J u r P using direct molecular descriptors and by Robbat et al.18 using both molecular connectivity indexes and other
* To whom correspondence should be addressed.
Present address: Martin Marietta Energy Systems, Oak Ridge National Laboratories, Oak Ridge, T N 37831. (I) Hutzinger, 0.;Safe, S.; Zitko, V. The Chemistry of PCBs;CRC Press: Cleveland, OH, 1974. (2) McConnell, E. E. Enuiron. Health Perspect. 1985, 60, 29-33. (3) Cairns, T.;Siegmund, E. G. Anal. Chem. 1981,53,1183A-l193A. (4) Falck, F., Jr.; Ricci, A.; Wolff, M. S.; Goldbold, J.;Deckers,P. Arch. Enuiron. Health 1992, 47, 143-146. (5) Murphy, T.J.; Formanski, L. J. Enuiron. Sci. Technol. 1985, 19, 942-946. (6) Safe, S.; Bandiera, S.; Sawyer, T.;et al. Enuiron. Health Perspect. 1985,60,47-56. (7) Consume? Reports, 1992, (Feb), 110-114. (8) O'Shea, T.J. Science 1991,253,1334. (9) Abelson, P. H. Science 1991,253, 361. (10) Meyers, J. P.; Colborn, T.Science 1991, 253, 1334. (11) Stone, R. Science 1992, 255, 798-799. (12) Hileman, B. Chem. Eng. News 1992, (Mar 9), 23-24. (13) Voyksner, R. D.; Bursey, J. T.; Pack, T.W.; Porch, R. L.'Anal. Chem. 1986,58, 621-626. (14) Mullin, M. D.; Pocbini, C. M.; McCrindle, S.; Romkes, M.; Safe, S. H.; Safe, L. M. Enuiron. Sci. Technol. 1984, 18, 468-476. (15) HBfler, F.; Melzer, H.; M6eke1, H. J.; Robertson, L. W.; Anklam, E. J. Agric. Food Chem. 1988,36, 961-965. (16) Kalizan, R. Quantitative Structure-Chromatographic Retention Relations; Wiley Interscience: New York, 1987. (17) Hasan, M. N.; Jura, P. C. Anal. Chem. 1988,60, 978-982. (18) Robbat, A., Jr.; Xyrafas, G.; Marshall, D. Anal. Chem. 1988,60, 982-985. +
0003-2700/93/0365-1631$04.00/0
molecular parameters. Rekker et al.19 examined structural influences on normal-phase PHB LC retentions. Hasan and Jurs20 obtained excellent regression models for both the GC and LC retentions of the PHBs based on structural descriptors. Their five-descriptor GC model, for example, utilized the third-order valence connectivity index, Kier's secondorder K shape index, and 'shadow area" projections on the xy, y z , and x z planes. Ong and HitesZ1adopted a fundamental approach based on molecular orbital calculations to analyze the GC retentions of the PCBs and other congeneric series. They employed Dewar's semiempiricalAM1 MO method to calculate molecular polarizabilities, ionization potentials, and dipole moments and then used these properties as regression parameters. The aforementioned structure-property studies have yielded important informationon the molecular structural features that influence retention in the halogenated biphenyls. Nonetheless, some of the studies have been criticized on the grounds that either (1)the descriptors employed could not be readily calculated by nonspecialists or (2) the physical interpretationsof the descriptors were obscure.21922 In earlier reports we examined relationships between the physical properties of alkane~,2~32~ alcohols,24-26 and halogenated hydrocarbonsZ7~28 and their molecular structures in order to find parsimonious regression models using readily calculated and easily interpreted descriptors of molecular structure. Here we show that this approach is useful in understanding the physical features responsible for the chromatographic retentions of the PHBs. As have earlier studies, we have taken the GC retention results of Hofler et al.15 for mono- and polyhalogenated PHBs on a DB-210-CB capillary column and the high-pressure LC results on ODS phases as experimental standards. The simplest reasonable molecular structure model for the chromatographic retention of these polyhalogenated compounds would include only the numbers of each type of halogen substituent present. However, it has long been recognized that halogens substituted at the ortho positions of PHBs are less effective in increasing retention than are those located (19) Rekker, R. F.;DeVries, G.; Bijloo,G. J. J. Liq. Chromatogr. 1989, 12,695-728. (20) Hasan, M. N.; Jurs, P. C. Anal. Chem. 1990,62, 2318-2323, (21) Ong, V. S.; Hites, R. A. A d . Chem. 1991,63,2829-2834. (22) Kaliszan, R. Anal. Chem. 1992,64, 619A-631A. (23) Needham, D. E.; Wei, 1.-C.; Seybold, P. G. J. Am. Chem. SOC. 1988, 110,4186-4194. (24) Seybold, P. G.; May, M.; Bagal, U. A. J. Chem. Educ. 1987,64, 575-581. (25) Hinckley, D. A.; Seybold, P. G.; Borris, D. P. Spectrochim. Acta 1986.42A. 747-754. (26) Hinckley, D. A.; Seybold, P. G. Spectrochim. Acta 1988, 44A, 1053-1059.
0 1993 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1903 500 y = 129.4 + 5.53x,
r = 0.998
I
200
T/
X I
-
i o
20
30
40
50
I
60
Atomic Number Flgure 1. Plot of regression coefficients for the halogens vs halogen atomic number.
2000
1000
3000
4000
Exp. Retention
Figure 2. Calculated vs experimental GC retentions for the polyhalogenated biphenyls.
at other p~itione.~5 Therefore, a discriminatingmodel should take into account not only the numbers and types of halogens present but also whether these halogens are or are not at ortho positions. In fact, such a model givesa very good account of the GC retentions. For example,
+ 230.7(*5.8)Nc, + 319.6(*6.5)NBr + 440.4(*31.0)N1- 169.0(*13.5)Nofl1175.3(f15.3)NorB, - 214.O(*51.4)Nor1 (I)
RI = 1631.6(*9.1)
1
n = 53, r2 = 0,990, s = 42, F = 736 0
where N X is the number of halogens of type X, NO,Xis the number of these halogens located at the ortho positions, n is the number of PHBs in the sample, r2 is the coefficient of determination (square of the correlation coefficient), s is the root-mean-squareerror, and F is the F-statistic of the model. Fluorine substitution causes only relatively small variations in the GC retentions of these compounds,and the parameters NFand NO,Fdid not meet the t-test (t > 4) for retention in the model. Note that two compounds (2,2’,4,4’,6,6’-Cl,J3P and 3,3’,4,4’-Br4BP)from the original experimental GC data set (55 measured compounds) of Hbfler et al.15 displayed exceptionally large residuals (>3a) and were deemed to be outliers. These compounds were omitted from the model. The final set consisted of 13 fluorinated, 21 chlorinated, 16 brominated, and 3 iodinated PHBs. Equation 1is of approximatelythe same statistical quality as the excellent five-parameter model (n = 53, r2 = 0.989,s = 45, F(5,47) = 866) obtained for the GC retentions of these compounds by Hasan and Jurs.20 It shows quite clearly that the influence of a halogen substituent on GC retention in the PHBs increases markedly as its atomic number increases (Figure 1). Likewise, the (negative)correction assigned to an ortho halogen also increases with atomic number and van der Waals radius. As noted, for statistical reasons eq 1 does not distinguish the small variations in GC retention caused by fluorine substitution. Some account of these variations can be achieved by relaxing the t-test requirement on the coefficients. If this is done, one finds that NFremains insignificant and only NO,Fenters the regression: RI = 1645.3(*10.3) + 227.5(f5.6)Nc1 + 315.1(f6.4)NBr + 426.7(f30.0)N1 20.5(f8.4)NorF - l68.8(fl2.8)NorcI 177.0(f14.5)NOrB,= 214.0(f48.8)Nor1 (2)
n = 53, r2 = 0.991, s = 40, F = 701
1000
0
2000
Exp. Retention Flgure 3. Calculated vs experimental LC retentions for the polyhalogenated biphenyls.
A plot of experimental retentions vs those calculated using eq 2 is shown in Figure 2. The LC retentions of the PHBs can also be modeled using these same descriptors, although with diminished statistical success:
+
+
+
RULC) = 381(*16) 144(18)Nc1 171(f7)NB, 270(*47)N1- 49(*13)No,, - 86(*17)No,cl 61(f20)NorB, - 168(f76)NorI (3)
n = 54, ? = 0.959, s = 62, F = 154 Two outliers, 4,4’-FzBP and 3,5-BrzBP, were identified and removed from the LC data set in this case. The results are plotted in Figure 3. In assessing the influence of ortho substituents on the chromatographic retentions of substituted biphenyls, considerable attention has been directed at the roles such substituents play in forcing the two phenyl rings into a nonplanar g e ~ m e t r y . ~It~is, ~generally ~ supposed that decreased retention is associated in some way with the loss of planarity of the biphenyl structure. Experimentalstudies,w7 (29)Sissons, D.; Welti, D. J. Chromatogr. 1971,60, 15-32. (30)Romming, C.;Seip, H. M.; Oymo, I.-M. A. Acta Chem. Scand. 1974,A B , 507-514. (31)Pederson, B. F. Acta Crystallogr. 1975,831,2931-2933. (32)Brock, C.P.; Kuo, M A ; Levy, H. A. Acta Crystallogr. 1978,B34, 981-985. (33)Singh, P.; McKinney, J. D. Acta Crystallogr. 1979,835,259-262. (34)Dynes, J. J.;Baudais, F. L.; Boyd, R. K. Can. J. Chem. 1985,63, 1292-1299. (35)Field, L. D.; Skelton, B. W.; Sternhell, S.;White, A. H. Aust. J. Chem. 1985,38,391-399. (36)Almenningen, A.; Bastiansen, 0.;Gunderson, S.;Samdal, S.; Skanke, A. J.Mol. Struct. 1985,128, 95-114.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993
Table 1. Calculated. Torsional Angles of the Polyhalogenated Biphenyls halonen none fluorine
chlorine
bromine
no. of ortho haloaens 0 0 (meta, para) 1 2 (same ring) 2 (opp rings) 3 4 0 (meta, para) 1 2 (same ring) 2 (opp rings) 3 4 0 (meta, para)
1 2 (same ring) 2 (opp rings)
3 4
iodine
0 1 2 (same ring) 2 (opp rings) 3 4
torsional angle (deg) calcd exDtl* 40 40 46 49 56 58 65 40 59 72 72 89 90 40 64 85 84 89 90 40 67 90 86 90
44c 44-45d.e 49e 60e 70e 42,f 45d 708 87,h 82 J 42-44' 81-9Oj 83 J
90
a M M X force field.43 Gas-phase electron diffraction or X-ray diffraction. Reference 44. Reference 36. e See ref 47. f Reference 32. Reference 30. References 3 1 and 33. Reference 37. J Reference
35.
as well as molecular mechanics,20*38-39 semiempirical,*1@and ab i n i t i ~ ~ lmolecular ,~* orbital calculations, confirm the increased nonplanarity of the rings that arises from ortho substitution. Table I shows the results of force field (molecular mechanics) calculations performed using the MMX force field.43 It is apparent (a) that the ring-ring torsional angle ( T ) in BP increases as the atomic number and van der Waals radius of the ortho halogen increases and (b) that this angle increases, at least to a point, with the number of ortho halogens. Despite the widespread acceptance of the idea that decreased chromatographic retention with ortho substitution is linked to the increase in the BP torsional angle there are some problems with this idea. First, no convincing mechanism has been given to explain how an increase in the torsional angle might reduce retention. Second, BP itself is nonplanar ( T = 44") in the gas phase44 and presumably in solution (although planar in the crystal due to packing forces45-47).It is not clear why an increase in T from, say,44"to some modestly larger angle should influence retention. We offer, therefore, an alternative explanation for the reduced effectiveness of ortho halogen substituents in prolonging retention in the PHBs: halogens at the ortho positions in the PHBs are less (37) Almenningen, A.; Bastiansen, 0.;Fernholt,L.;etal. J.Mol. Struct. 1985,128, 77-93. (38) Tsuzuki, S.; Tanabe, K.; Nagawa, Y.; Nakanishi, H.; Osawa, E. J. Mol. Struct. 1988, 178, 277-285. (39) Jaime, C.; Font, J. J. Mol. Struct. 1989, 195, 103-110. (40) de Bruijn, J.; Hermens, J. Quant. Struct.-Act. Relat. 1990,9,1121. (41) McKinney, J. D.; Gottachalk, K. E.; Pederson, L. J. Mol. Struct. 1983, 104, 445-450. (42) Tsuzuki, S.; Tanabe, K. J. Phys. Chem. 1991,95, 139-144. (43) PCMODEL, Serena Software, Box 3076, Bloomington,IN 474023076. Fernholt, L.;Cyvin, B. N.;Cyvin, (44) Almenningen, A.; Bastiansen, 0.; S. J.; Samdal, S. J. Mol. Struct. 1985, 128, 59-76. (45) Trotter, J. Acta Crystallogr. 1961, 14, 1135-1140. (46) Hargreaves, A.; Rizvi, S. H. Acta Crystallogr. 1962,15365-373. (47) Bastiansen, 0.;Samdal, S. J . Mol. Struct. 1985,128, 115-125.
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exterior and hence less exposed and less able to interact with thestationaryphase thanare thoseat the more exposed meta and para positions. In this view additional twisting of the biphenyl rings as a result of halogen substitution is an incidental result of this substitution and does not itself directly influence retention. (An indirect effect, in which halogens become somewhat more exposed as the torsional angle increases,might still take place. Moreover, the molecule may become more rigid as T increases.) If halogen exposure is a crucial factor influencingretention it might be expected that heavier halogens, because of their greater inherent exposure (due to their larger sizes and, possibly, greater torsional angles), should be relatively less 'penalized" (in terms of effect on retention) when placed at the ortho positions. This effect is apparent in the eq 2. One can define the relative effectivenessof an ortho halogen, Reff, compared to a "normal", i.e., meta or para, halogen, as R,ff = ( N x + No,x)/Nx. (Note that the ortho corrections NO,X are negative here.) For chlorine, bromine, and iodine, Reff= 0.26, 0.44, and 0.50, respectively. Thus, whereas ortho chlorines are only about a quarter as effective in prolonging retention as other chlorines, ortho iodines are about half as effective as other iodines. It may be possible to further test these ideas and to distinguish between them. Both experimentsand calculations (Table I) indicate that the increase in T is decidedly nonlinear with respect to the numbers of ortho halogens. For example, according to the calculations shown in Table I, the first ortho C1atom increases the torsional angle by about 19",the second by an additional 13", and the third by another 17",at which point the rings achieve an essentially perpendicular orientation; adding a fourth C1 atom should have no further effect on this angle. (AM1-MO calculation^^^ suggest that just two ortho chlorines suffice to cause a nearly perpendicular orientation of the rings.) The truncation is even more severe for Br and I substituents. Conversely, the retentions at first glance appear to be well described by a linear dependence on the number of ortho halogens of each type (eqs 1 and 2). Unfortunately, the present GC data set lacks sufficiently diverse substitution patterns, in particular PHBs with three or more heavy halogens, to test this. In fact, 2,2',4,4',6,6'CbBP, which has four ortho C1 atoms, was an outlier in both the present study and in the earlier PHB analysis of Hasan and Jurs and was therefore not used in the development of either model. This compound has a higher experimental RI (2784) than predicted by the linear model (eq 2, RI = 2336), a result at least consistent with the idea that the dependence is not linear. However, this single example is insufficient for a proper test of this question, and it would be beneficial to measure the retentions of additional, more highly substituted PHBs under the same conditions. In their analysis of the GC retention data of all 209 PCB congenerson an SE-54stationary phase, Hasan and Jurs17 found a satisfactory linear dependence on the number of ortho C1 atoms. Equations 1-3 show clearly that the principal intermolecular forces determining retention in the PHBs are dispersion forces rather than polar forces. The regression coefficients of the N X parameters increase with the atomic number (which in turn correlates with the atomic refraction, i.e., the atom's contribution to the molecular polarizability) of the halogen X (Figure 1). In contrast, the number of fluorine atoms, the most electronegative halogens, does not enter the equations as statistically significant. This result is not surprising, since Meyer and co-workers48 have shown that dispersion interactions dominate the cohesive energiesof even polar liquids such as acetonitrile (52% due to dispersion (48) Meyer, E. F.; Renner, T. A.; Stee, K. S.J. Phys. Chem. 1971,75, 642-648.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993
forces),chloromethane (73%), andacetone (70% ). Moreover, rudimentary calculations of interaction energies indicate the dominance of dispersion interactions over orientation and induction interaction energies for all but the most highly polar, small molecules.49 The importance of dispersion forces has been inferred in earlier studies, most notably in the examination of PCB retentions by Ong and who included molecular polarizability directly as a regression variable. Finally, it is of interest to relate the present results to those of previous studies of the PHBs. The relations observed by Hdfler et al.15 between the calculated total molecular surface areas of these compounds and their chromatographicretentions are logically indirect reflections of the dependences of the retentions on halogen numbers and halogen types demonstrated in eqs 1-3. Likewise, the dependencesof PHB retentions on three "shadow area" projections found by Hasan and Jurs2O indicate, as these authors have already noted, the importanceof the substituent halogen sizes and their relation (49) Levine, I. N. Physical Chemistry, 3rd ed.; McGraw-Hill: New York, 1988; p 805.
to dispersion forces. Presumably the path-3 valance connectivity index employed in the same study encodes information on the role of the ortho halogens. The specific contribution of the path-2 K shape index in describing positional and size effects in this analysis is less apparent. In general, the results of these studies show that a variety of different descriptions of molecular structure can be utilized advantageously in analyzing chromatographic retention. In order to be successful, a model for the PHB retentions must adequately describe both the different dispersion contributions of the halogens and their positional influences, which in this case are relatively simple (ortho or nonortho). Beyond this, the choice of a particular description will depend on the computational resources available to an individual investigator and preferences of interpretation.
RECEIVED for review October 1, 1992. Accepted March 11, 1993.
