Environ. Sci. Technol. W93, 27, 494-500
Reineke, W.; Knackmuss, H.-J. Appl. Environ. Microbiol. 1984,47, 395-402.
Oltmanns, R. H.; Rast, H. G.; Reineke, W. Appl. Microbiol. BiotechnoL 1988, 28, 609-616. Evans, W. C.; Smith, B. S. W.; Moss, P.; Fernley, H. N. Biochem. J. 1971,122, 509-517. Gaunt, J. K.; Evans, W. C. Biochem. J. 1971,122,519-526. Ornston, L. N. J. Biol. Chem. 1966,241, 3800-3810.
(32) Kemp, M. B.; Hegeman, G. D. J. Bacteriol. 1968, 96, 1488-1499. (33) Nyholm, N.; Lindgaard-Jargensen,P.; Hansen, N. Ecotoxicol. Enuiron. Saf. 1984, 8, 451-470.
Received for review May 29,1992. Revised manuscript received October 29, 1992. Accepted November 2, 1992.
Atrazine Hydrogen-Bonding Potentials Gereon J. Welhouse and Wllllam F. Bleam"
Department of Soil Science, University of Wisconsin-Madison, Madison, Wisconsin 53706 Formation constants for complexation between atrazine and four hydrogen-bond donors and four acceptors are obtained by linear regression of nuclear magnetic resonance chemical shift data. Since the donors and acceptors were selected from among the compounds used to establish relative hydrogen-bonding scales, these formation constants provide a measure of atrazine's relative donating (a,) and accepting (0), parameters. The calculated parameters (a, = 0.42; @, = 0.49) show that atrazine has intermediate reactivity toward donating and accepting hydrogen bonds with monofunctional complexing agents.
Introduction Atrazine is an important agricultural herbicide used in the United States-over 36 million kg was applied nationwide in 1990 (1). It degrades slowly with a half-life measured in months or years (2,3) depending on soil conditions. Degradation (by dealkylation) is fastest in the soil A horizon due to biological activity (4)but slows dramatically in the subsoil where chemical hydrolysis to hydroxyatrazine is the dominant mechanism. To assist in developing management practices that keep atrazine in the soil A horizon, we are investigating the mechanisms of atrazine adsorption to soil surfaces. At least seven mechanisms have been proposed for atrazine adsorption (5),yet even after 20 years there is little agreement on their relative importance (6, 7). Nuclear magnetic resonance (NMR) experiments show that atrazine has a distribution of electronic charge represented by the resonance structures in Figure 1 (8). Basicity is localized on the triazine ring nitrogens, leaving the side-chain alkylamino groups to serve as sources of Yacidic"protons for dipolar and hydrogen-bonding interactions. This distribution of electronic charge is similar to that found in the amide functional group, and we conclude that, like amides, atrazine is reluctant to undergo chemical adsorption (7), electron-transfer (9, IO),cationexchange (11),and metal complexation (12)reactions with soil surfaces in aqueous solution. While the chemical adsorption and electron-transfer mechanisms may be important in the slow degradation of atrazine and its irreversible binding to soil organic matter (7, 9),we are focusing on reversible binding mechanisms that can be used to lower solution concentrations while maintaining herbicide activity. We are investigating whether three of the adsorption mechanisms-hydrophobic partitioning, dipolar interaction, and hydrogen bonding-act together in the adsorption of atrazine to soil organic matter, the soil component most strongly correlated with atrazine adsorption (13,14). 494
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One of these mechanisms, hydrogen bonding, offers the promise of strong specific adsorption that can be used to control atrazine migration in the soil. In this report, we evaluate the hydrogen-bonding potential of atrazine by measuring formation constants for hydrogen-bond complexation with hydrogen-bond donors (HBD) and hydrogen-bond acceptors (HBA). We use donors and acceptors of known hydrogen-bondingpotential so that atrazine activity is rated on the hydrogen-bond activity scales developed by Abraham et al. (15,16).This atrazine potential can be used to estimate formation constants for atrazine complexes with any compound on these scales. Our procedure makes use of a change in the NMR chemical shift of atrazine NH signals when a hydrogenbond complex is formed. Titration experiments are performed by mixing atrazine with various concentrations of complexing agents and measuring the change in chemical shift positions. Formation constants are obtained by linear regression of the chemical shift data against the concentration of complexing agent. Proton NMR chemical shifts are sensitive indicators of hydrogen-bond complex formation, and formation constants for complexation of enols have been calculated from chemical shifts using linear regression as a front end for nonlinear optimization (17). Complexation constants have also been measured for hydrogen bonding of phenols (18, 19) and CH protons (20,21)using linear regression.
Experimental Methods Materials. Technical grade atrazine (98% pure) was supplied by the Ciba-Geigy Co. and recrystallized twice from hexane. Reagents were obtained from the Aldrich Chemical Co. and used as received, except that liquids were stored over 4-A molecular sieves. CC14solvent (Aldrich) was purified by distillation and stored over molecular seives. High-resolution proton spectra were obtained on a 400-MHz Bruker spectrometer at the National Magnetic Resonance Facility at Madison, WI.Spectra were acquired by applying a 5.6-ms (goo) pulse over a spectral width of 4000 Hz. Typically 8K data points were accumulated as the sum of 64 transients, separated by a relaxation delay of 3 s. Total acquisition time for each spectrum was -4 min, and the data were zero filled to 16K before Fourier transformation. All experiments were conducted in CC4, a noninteracting solvent convenient for observing hydrogen-bond complexation. The solvent contained 10% C6DI2to p:ovide the NMR lock signal. Probe temperature was maintained at 298 K with the standard Bruker heating coil,
0013-930X/93/0927-0494$04.00/0
0 1993 American Chemical Society
c1
I
R3
Isomer - R1 1 2 3 4
H H Et Et
Rl
R2
R3
R4
Et Et H H
H iPr H iPr
iPr H iPr H
Et = ethyl; iPr = isopropyl
c1
c1
I
I
H
H
H
H
atrazine resonance structure
Figwe 1. Four conformatbnal lsomers of atrazine created by restricted rotation of the alkylamino side chains. The restricted rotation Is attributed to delocallzatlon of the nitrogen lone pair electrons into the trlarlne rlng which creates the partial doublabond character shown. The resonance structure suggests that atrazine reactivity is like that of amides, which have similar resonance structures.
accurate to within 2 K. All chemical shifts are referenced directly to internal TMS (0.0 ppm). Methods. About 2 mL of a 0.007 mol/L atrazine solution was prepared in CC1, and 0.3 mL added to an NMR tube. The atrazine NMR spectrum is first acquired and then an appropriate amount (3-10 gL) of complexing agent is added directly to the NMR tube by syringe, and the spectrum is acquired again. The atrazine/complexing agent solution in the NMR tube is then diluted by adding fresh portions of the atrazine/CC14 solution followed by acquisition of the NMR spectrum to determine the chemical shift at each dilution. In this way, the complexing agent is diluted by a constant proportion (generally 2:3) while the concentration of atrazine is held constant to minimize the effects of dimerization on the chemical shifts. Granot (22) has determined the optimum conditions for collecting data on complexation from NMR titration curves. The optimum conditions depend on the formation constant ( K ) being measured; in the case of atrazine (A) reacting with hydrogen-bond donors or acceptors (B) to form a complex (C) (eq l),the optimum concentration of
A +B =C
K = [C]/[A][B] (1) A is at 1/2K and the concentration of the titrant B is varied from 0.4[A] to 11[A]. The optimum number of titration points, 15, is collected by increasing [B] by a constant factor between points. In practice, the portion of the titration curve sampled by the data is restricted on one end by low atrazine solubility and on the other by systematic errors introduced by high concentrations of complexing agent. For example, a formation constant of 10 L/mol calls for an optimum atrazine concentration of 0.05 mol/L, yet the maximum atrazine solubility in CCl, is only 0.007 mol/L. Futhermore, increasing the [B] above -0.5 mol/L adds secondary dipolar and solvent permitivity effects to the atrazine
chemical shifts, complicating the interpretation of the changes in chemical shift with a simple 1:l complexation model (23, 24). The [B] is also limited by the dynamic range of the NMR detector. In some cases, large peaks from a 1mol/L solution of complexing agent obscure the peaks for the 0.007 mol/L atrazine. The combination of secondary solvent effects and NMR dynamic range considerations restrict [B] to a maximum of -0.5 mol/L. The NH signals of isomer 1 (Figure 1)are the strongest of the eight atrazine NH signals (there are two NH signals for each of the four atrazine isomers). Of these two, the isopropylsmino NH signal was selected for linear regression because it is less affected by atrazine dimerization than the ethylamino NH signal (8). Dimerization is a competing reaction that causes the chemical shifts to move in response to atrazine concentration and introduces curvature to the data (25,26). Although the dimerization reaction has an effect on the chemical shift of the NH-i signal, this effect is small compared to the effect introduced by the complexing agents. We assume, therefore, that the dimerization reaction does not significantly affect the complexation data and that the formation constants calculated by linear regression of the isopropylamino NH signal represent overall atrazine hydrogen-bond activity. Data Analysis. The chemical shift titration data are fit to a linear model using the Higuchi iterative procedure (18)to find the formation constants and chemical shift for atrazine in a 1:l complex. The observed chemical shift is the weighted average of chemical shifts for uncomplexed and complexed atrazine species shown in eq 2, where 6,
is the observed chemical shift and aA is the atrazine chemical shift in the absence of complexing agent. 6A is estimated by extrapolating the data to [BO] = 0 using polynomial regression. JC is the chemical shift of the complex and [Ao] is the total atrazine concentration. The relative amounts of the uncomplexed and complexed species (A/Ao and C/Ao, respectively) depend on the formation constant relating these two species. Mass balance relations for A and B (eqs 3 and 4) are substituted into the equilibrium expression (eq 1)to obtain eq 5, which expresses the concentration of C in terms of a formation constant K and the initial concentrations Ao and BO. Ao=A+C
(3)
Bo=B+C [AoIIBol- [CI([Aol + [BO1 - [Cl) = (1/K)[Cl
(4) (5)
Equation 5 is used to derive the regression equation. First the mass balance of A is substituted into eq 1to get an expression for C in terms of A" and the observed chemical shift (eq 6). Substituting eq 6 into eq 5 gives 7 used for iterative linear regression (18). [C] = [Ao](
p) C
- &A
The formation constant ( K ) and complex chemical shift
(ac) are found by regression of [Bo]/(6, - 6 ~ against ) [A'] + [BO] - [C]. In the initial regression [C] is set equal to
0, but in subsequent iterations C is obtained from the Environ. Scl. Technol., Vol. 27, No. 3, 1993 495
previous regression slope [1/(6c - SA)] and eq 6. The iteration is repeated until the new estimate of C agrees with the old value to within a set tolerance. Convergence is usually reached in three iterations. Atrazine forms a cyclic dimer in C C 4with a formation constant of 49 L/mol (8). Because of its cyclic arrangement, the atrazine dimer is less active than the monomer toward complexation with other donors and acceptors in solution. Since dimerization reduces the concentration of monomers involved in complexation, it affects the formation constants for this reaction. We attempted to correct for the effects of dimerization by including the dimer in the mass balance used to derive eq 7, but the corrected regression equation did not significantly improve the goodness of fit of our data, as measured by r2. Dimerization of complexing agents affects the complexation reaction in a similar manner. Phenols (27) and alcohols (28)dimerize in CC4,but these dimers are slightly more reactive than the monomers toward hydrogen bonding. The slight increase in reactivity offsets the slight decrease in monomer concentration, and for this reason, dimerization of complexing agents is not considered in these results. Experimental Errors. There are two contributions to the error in calculating formation constants from titration curves. One is due to the random scatter of the data, and the other is related to the portion of the titration curve sampled by the data. Error due to experimental scatter can be reduced by increasing the number of data points, but error due to incomplete curve sampling can only be improved by sampling over a larger segment of the curve or repeating the entire experiment. Experimental scatter is estimated by a2 from the linear regression while errors due to incomplete curve sampling are calculated from information theory. Deranleau (29) estimated that the rate of accumulation of information is at a maximum when 76% of an NMR titration curve has been sampled. At least this much of the curve must be sampled for the data to “prove” a particular model. Sampling less that 76% of the curve introduces additional uncertainty to the data analysis. Sample “completeness” (I,)is calculated with eq 8 and
I I, = - = c - c2 ln(c) + (1 - c ) ln(1 ~ - c)
(8)
IT
the experimental ratio c (=C/Ao), which is a measure of the extent of complexation. I, is used with eq 9 to estimate S$ = (0.83- 1,)’ (9) the relative variance in K due to incomplete sampling, s t . The constant 0.83 in eq 9 is the sample completeness when 76% of the curve is sampled. The random error of the regression, s,2, is estimated from the relative errors in the slope (m)and intercept ( b ) according to eq 10. Together, se2 = ( A b / b ) 2
+ (Am/mI2
(10)
s,2 and :s are used to calculate the confidence intervals with eq 11and appropriate values for Student’s t , N p (the number of data points), and Ne (the number of experiments). 1
Errors due to experimental scatter are generally -5% for all experiments. Confidence intervals of formation constants greater than 20 L/mol are due to experimental 406
Envlron. Scl. Technol., Vol. 27, No. 3, 1993
3200
I
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-
I
J
0
0
data
T model
-
2800 chem. 2600 -
o
DMS 0 0
2000 f 0.0025
I
I
0.025
0.25
I
I
2.5
I
25
Conc. Complexing Agent (mol/L) Flgure 2. Plots of atrazine Isopropyl NH chemlcal shifts vs the concentration of acetone (ACE) and dimethyl sulfoxlde (DMSO). The chemical shifts (In hertz relative to Internal TMS) vary over concentrations of complexing agents ranging from 0.0 (pure CCI,) to 14 mol/L (pure complexing agent). The curves represent a 1:l complexatlon model and are calculated from the formation constants and chemical shlfts found by linear regresslon of the filled data points. Chemlcal shifts represented by the hollow data points drift off the regression llne due to secondary solvent effects when the concentration of complexlng agents rlses above 0.5 mol/L (solid line). Dashed vertical lines indlcate the point at whlch 76% of atrazine Is complexed.
scatter only and generally run about 10-15%. Confidence intervals of formation constants less than 20 L/mol have additional error from incomplete curve sampling and are larger .
Results All of the compounds tested formed hydrogen-bond complexes with atrazine, as revealed by changes in chemical shifts of the atrazine NH protons. Figure 2 is a “titration curve” plot of atrazine chemical shifts vs concentrations of acetone (ACE) and dimethyl sulfoxide (DMSO) extending from 0.0 (pure CClJ to -14 mol/L (pure complexing agent). These titration curves were calculated with the formation constants and chemical shifts estimated by linear regression of the filled data points. The titration curves in Figure 2 show that chemical shifts measured at high concentrations of ACE and DMSO do not fit the simple 1:l complexation model. The data reveal two major factors affecting atrazine chemical shifts-hydrogen-bonding effects are dominant at low concentrations of complexing agent with solvent effects becoming significant at high concentrations. The drift in the chemical shifts at high concentrations of DMSO and ACE is likely due to changes in the solvent permitivity (24) and/or dipolar effects (23). These plots were obtained with the use of fully deuterated acetone and DMSO; other experiments with nondeuterated complexing agents are restricted to concentrations below 0.5 mol/L by the dynamic range of the NMR spectrometer, Experimental data used to find the formation constants in this report were collected at concentrations of B below 0.5 mol/L (solid vertical line) to reduce solvent effect distortions by sampling only the portion of the curve representing hydrogen-bond complexation. The short dashed lines mark the point at which 76% of the atrazine is in the complexed form, “proving” that the data fit a complexation model (29). It is apparent from
7.5 I
I
I
I
7.5
7.0
6.5
6.0
I
5.5
I
5.0
7.0
6.5
I
4.5
PPM Flgure 5. Stacked plot of atrazine NH proton chemical shlfts at concentratbns of the hydrogen-bond donor HFIP ranging from 0.0to 0.040 mol/L. The change in chemical shifts is consistent wlth hydrogen-bond compiexatlon. Symbols l-e and 2-e refer to the ethylamlno NH protons of atrazine isomers 1 and 2 (seeFigure 1); 14 refers to the lsopropylamino NH proton of isomer 1 that Is used In the ilnear regression. All spectra were recorded at 298 K and at a constant atrazine concentration of 0.007 mol/L. The HFIP proton signal is marked by X and moves downfieid at lower concentrations. Spikes at 6.45 and 7.25 ppm are from solvent impurities.
Figure 2 that the filled data points "prove" atrazine complexation with DMSO but only suggest complexation with acetone. Data collected for compounds with weak formation constants have an additional uncertainty from incomplete curve sampling. Figure 3 shows how the atrazine NH chemical shifts change in response to complexation at various concentrations of the HBD hexafluoro-2-propanol(HFIP). Each signal is a time-weighted average of the chemical shifts for atrazine in its monomeric and complexed species, and the exact position depends on the extent of hydrogen-bond complexation. The change in chemical shifts is fairly uniform for all the NH signals, supporting our selection of the isopropylamino NH proton of isomer 1 (1-i) to represent overall atrazine activity. Figure 4 shows a similar migration of chemical shifts when the HBA dimethyl sulfoxide (DMSO) is added. Signals for both NH-i (14)and NH-e (1-e) of isomer 1 shift downfield at higher DMSO concentrations. The two plots presented in Figures 3 and 4 are typical of those observed for interaction of atrazine with the other HBDs and HBAs used in this study. The formation constants for hydrogen-bond complexation with HBD donors are presented in Table I. Atrazine accepts a hydrogen bond from active protons on HFIP, phenol (PHEN), trifluoroethanol (TFE), and pyrrole (PYRR). Strongest complexation is observed between atrazine and the donor HFIP (Kf = 32.7 L/mol). Complexation with PHEN (15.9 L/mol) and TFE (9.8 L/mol)
6.0
5.5
5.0
4.5
PPM Flgure 4. Stacked plot of atrazine NH proton chemical shifts at concentrationsof the hydrogen-bond acceptor DMSO ranging from 0.0 to 0.884 moi/L. The change In chemical shifts Is consistent with hydrogen-bond complexation. Labels l-e, 2-e, and 1-1 have the same meanlng as In Figure 3. Ail spectra were recorded at 298 K and at a constant atrazine concentration of 0.007 mol/L. Spikes at 6.45 and 7.25 ppm are from solvent impurlties.
Table I. Formation Constants and Chemical Shifts for Complexes Formed between Atrazine and Hydrogen-Bond Donors and Acceptors compound
abbr
hexafluoro-2-propanol HFIP phenol PHEN TFE trifluoroethanol pyrrole PYRR dimethyl sulfoxide DMSO N-methylpyrrolidinone NMP pyridine PYR acetone ACE
KP
6Cb
CIA"
32.7 (14.4) 15.9 (14.4) 9.8 (13.3) 2.6 (12.5) 26.7 (12.9) 13.8 (14.7) 3.3 (13.0) 2.3 (f1.4)
2153 (111) 2252 (i.49) 2263 (163) 2241 (1158) 2923 (196) 2535 (1158) 2531 (1415) 2390 (1190)
0.87 0.80 0.77 0.31 0.96 0.77 0.38 0.67
a Formation constants (in L/mol) for hydrogen-bonded complexes of atrazine measured in CCl., solvent at 298 K. Numbers in parentheses are the 95% confidence intervals. bAtrazine NH chemical shift when fully complexed (in Hz). The chemical shift of uncomplexed atrazine is 2075 Hz. cPortion of the titration curve samded bv the data.
is intermediate, and the weakest complexation is found with PYRR (2.6 L/mol). Atrazine also acts as a hydrogen-bond donor from its side-chain NH protons. Formation constants for complexes formed between atrazine and the hydrogen-bond acceptors DMSO, ACE, N-methylpyrollidinone (NMP), and pyridine (PYR) are also presented in Table I. Stronger hydrogen-bond complexes are formed between atrazine and DMSO (Kf = 26.7 L/mol) and NMP (13.8 L/mol). Weaker complexes are formed with PYR (3.3 L/mol) and ACE (2.3 L/mol). The formation constants reported in Table I were calculated from measurements made on the NH-i proton of atrazine only. However, there are two active NH protons Envlron. Sci. Technoi., Voi. 27, No. 3, 1993
407
Table 11. Atrazine Scaled Formation Constants (log KH) and Hydrogen-Bond Donating (a)and Accepting (8) Parameters compd" log Kfb HFIP PHEN TFE PYRR DMSO NMP ACE PYR
1.515 1.201 0.991 0.415 1.427 1.140 0.362 0.519
L'
D'
log KHd
a or p'
1.224 0.946 0.910 0.643 1.2399 1.2145 0.7758 1.0151
0.250 -0.057 -0.098 -0.391 0.2656 0.2359 -0.2420 0.0139
1.034 1.330 1.197 1.235 0.936 0.744 0.778 0.497
0 = 0.460 (10.013) 0 = 0.520(f0.024)
0 = 0.495 (*0.029) (3 =
0.507 (10.323) a = 0.439 (10.015) a = 0.400 (10.022) a = 0.405 (10.060) a = 0.345 (10.193)
dH
6 TFE
HFIP
PYRR
PHEN
9.8
32.7
2.6
15.9
Et \
"Abbreviations are taken from Table I. bKffrom Table I. c L and D are the Abraham "activity" parameters for each compound (15, 16). dScaled hydrogen-bond formation constants. e Atrazine hydrogen-bond donating (a) and accepting (0) potentials. Weighted-average values are 0.42 (10.03)for a and 0.49 (h0.02)for 0. Numbers in parentheses are the 95% confidence intervals.
on atrazine, NH-i and "-e, both of which react to form complexes. For this reason a statistical correction (multiplication by a factor of 2) must be applied to the formation constants in Table I to describe atrazine activity in adsorption and partitioning experimentswhere both NH protons are active (30). Statistical corrections to atrazine complexes with hydrogen-bond donors are not needed since these complexes form at only one site. Atrazine has a different formation constants for each of its hydrogen-bond complexes, yet each constant is an expression of the same inherent hydrogen-bonding potential of atrazine. This atrazine inherent or scaled potential (KH) is calculated from each formation constant (Kf)using eqs 12 and 13 and slope (L) and intercept (0)parameters that reflect the inherent hydrogen-bonding potential of each complexing agent (15, 16, 32). log K o b s
L log KH + D
(12)
log KH = (log Kobs - D)/L The L and D parameters for each complexing agent are taken from published tables (15,16,31)and are presented in Table 11,along with the corrected formation constants. In theory, these parameters should correct all the observed constants to a single value of the scaled formation constant. In practice, the L and D parameters are successful in grouping the raw formation constants more closely together. The complexing agents used in this study are all reference compounds used to derive a universal hydrogenbonding scale. Formation constants measured with these compounds rank atrazine's reactivity in relation to all other monofunctional hydrogen-bond donors and acceptors. The relative hydrogen-bond donating (a)and hydrogen-bond accepting (0) abilities of atrazine, calculated from log KH values using eqs 14 and 15, are presented in Table 11.
+ PZH= (log KOH+ 1.1)/4.636
aZH= (log KaH 1.1)/4.636
(14) (15)
Atrazine a parameters determined against the hydrogen-bond acceptors are grouped around 0.41, except for PYR which is lower at 0.35. The 0 parameters determined against hydrogen-bond acids are grouped closely around 0.49. Average values and 95% confidence intervals are 0.42 (i0.03) for a and 0.49 (3t0.02) for 0. These were obtained by weighting each individual parameter in Table I1 for its relative uncertainty, giving more weight to the more precise values. 498
Environ. Scl. Technol., Vol. 27, No. 3, 1993
iR
/"-"
atrazine-HFIP complex
Flgure 5. Formation constants (In L/moi from Table I) from complexes formed between atrazine and hydrogen-bond donors. Complexation between the proton donor HFIP and the filled electron orbital on the para nitrogen of atrazine Is shown.
Discussion In a previous report we identified the triazine ring nitrogen para to the chloride substituent as the site where atrazine accepts a hydrogen bond (8). We used 15N-ringlabeled atrazine in NMR experiments to show that the Tl relaxation time of the para nitrogen decreases more than the ortho nitrogens when the complexing agent acetic acid is added to a benzene solution of atrazine. Since relaxation of 15Nis due primarily to dipolar interactions with protons (321, the increased relaxation rate (decreased TI) is consistent with our suggestion that the para nitrogen associates with the proton during hydrogen bonding. The 15Nresults identify only that complexation at the para nitrogen is stronger than complexation involving the ortho nitrogens. Weaker complexation to the ortho nitrogens may contribute to the overall formation constants reported in Table I, which were calculated from changes in the isopropyl NH chemical shift. But this chemical shift is an average signal representing the composite effects of all hydrogen-bonding interactions, and the magnitude of the formation constant is not affected by the location (ortho or para nitrogen) of the interaction. The proposed interaction of atrazine with HBDs is pictured in Figure 5. The para nitrogen of isomer 1 (Figure 1) is freely accessible to complexing agents, but the other isomers have steric interference from the sidechain alkylamino groups which may reduce complexation. Access to the para nitrogen in isomer 4 is hindered by both side-chain alkyl groups (see Figure l), and this isomer probably does not form the complex shown in Figure 5. Since less than 10% of atrazine is in the form of isomer 4, the reduced reactivity of this isomer has little effect on the overall activity measured by isomer 1 (8). The interaction of atrazine with HBAs is shown in Figure 6. Although hydrogen bonding from only the isopropylamino NH of isomer 1 to DMSO is shown, both side-chain NH protons can be donated to form complexes with HBAs. The formation constants were calculated under the assumption that the reactivity of isopropylamino NH (NH-i) is the same as the reactivity of ethylamino NH (NH-e). However, dimerization experiments show that NH-e is more active than NH-i toward hydrogen bonding
0
0
ACH3
CH3’
II
S ‘CH,
CH3
ACE
DMSO
2.3
26.7
iPr
0’ 0 CH3
I
\T
NMP
PYR
13.8
3.3
/” -H
atrazine-DMSO complex Flgure 6. Formation constants (in Llmol from Table I) for complexes formed between atrazine and hydrogen-bond acceptors.
(8) so the formation constants may underestimate the true atrazine activity of isomer 1 slightly. Atrazine’s a and 0hydrogen-bonding parameters suggest an intermediate position on the universal logarithmic scales which run from 0 to 1. These parameters quantify atrazine activity and can be used to calculate formation constants with the great variety of compounds for which a and 0 parameters are known (15,16,31). The internal consistency of this approach is demonstrated by estimating formation constants for the HBDs and HBAs used in this study. The a and 0parameters for atrazine are matched against the complimentary parameters for the complexing agents and the formation constants calculated with eq 16 (31).The predicted constanh are presented in Table 111. log K = 7 . 3 5 4 ~ -~ 1.094 ~0~ (16) The results in Table I11 show that the strongest complexation is calculated for those compounds having the largest a and p parameters. The good agreement between the experimentally observed formation constants and those calculated with eq 16 is expected since these formation constants were used to obtain the atrazine parameters, but the results do show the internal consistency of the parameters. Soil organic matter contains a great variety of structures, including carboxylic acid and ester, quinonic and phenolic, ether, ketone, and hydroxyl functional groups (33,341.The a and 0parameters of these functional groups can be used with eq 16 and the atrazine values established in this work to predict that hydrogen-bond complexes of intermediate strength will form with atrazine. These results suggest that hydrogen bonding is an important mechanism for atrazine adsorption in soil organic matter. Hydrogen bonding is the result of both dipolar and orbital-overlap interactions (35,36).Both of these interactions are stronger in hydrophobic solvents which reduce the effect of solvent permittivity on the dipolar interaction and the competition from water for orbital interactions. We propose that hydrogen bonding is strongest within hydrophobic domains of soil organic matter and promotes partitioning of atrazine into these domains.
Summary Eight hydrogen-bonded complexes of atrazine were observed by NMR spectroscopy, and their formation con-
Table 111. Hydrogen-Bonding Parameters and Predicted Formation Constants for the Complexing Agents in This Study Kf
compda
ab
Pb
HFIP PHEN TFE PYRR DMSO NMP PYR ACE atrazine
0.77 0.60 0.57 0.41 0
0.03 0.22 0.18
0 0
0.77 0.63 0.50
0.4 0.42
0 0.78
predc
obsd
55
33 16 10 2.6 27 14 3.3 2.3
12 10 2.6 21 19 7.1 2.8
0.50
a Abbreviations are from Table I. * a and 0 taken from published values (15,16). ‘Formation constant calculated with eq 7 or 8 and the appropriate a and /3 parameter for each complexing agent matched against the complimentary parameter for atrazine. Observed formation constant.
stank have been calculated by linear regression. These formation constants rank atrazine on universal scales of hydrogen-bond donating and accepting parameters. The intermediate size of the parameters (a = 0.42; 0 = 0.49) indicates that atrazine forms moderately strong complexes with many of monofunctional hydrogen-bond donors and acceptors found in soil organic matter. Complexation is strong enough to influence adsorption, especially within hydrophobic domains in soil organic matter. The procedure outlined in this report to determine atrazine hydrogen-bonding potentials is general and can be applied to any organic compound. This procedure can also be used to determine hydrogen-bonding potentials in other solvents, and in mixed solvents (31).The interactions of organic molecules like atrazine can be used to probe the active functional groups on soil surfaces and in soil organic matter. We are now experimenting with a fluorine-19 label on atrazine for use as a probe in combination with the NMR procedure outlined here. Acknowledgments We thank the staff at the National Magnetic Resonance Facility a t Madison (NMRFAM) for their technical assistance. Registry numbers supplied by the authors: atrazine, 1912-24-9;hexafluoro-2-propanol, 920-66-1;phenol, 10895-2; trifluoroethanol, 75-89-8; pyrrole, 109-97-1;dimethyl sulfoxide, 67-68-1; acetone, 67-64-1; N-methylpyrrolidinone, 872-50-4; pyridine, 110-86-1. Literature Cited Periera, W. E.; Rostad, C. E. Environ. Sei. Technol. 1990, 24, 1400-1406. Burkhard, N.; Guth, J. A, Pestic. Sei. 1981, 12, 45-52. Armstrong, D. E.; Chesters, G.; Harris, R. F. Soil Sei. Soc. Am. Proc. 1967, 31, 61-66. Adams, C. C.; Thurman, E. M. J.Environ. Qual. 1991,20, 542-547. Hamaker, J. W.; Thompson, J. M. Organic Chemicals in the Soil Environment; Goring, C. A. I., Hamaker, J. W., Eds.; Dekker: New York, 1972; pp 49-143. Calvet, R. Enuiron. Health Perspect. 1989, 83, 145-177. Senesi, N.; Chen, Y. Toxic Organic Chemicals in Porous Media; Gerstl, Z., Ed.; Springer-Verlag: New York, 1989; Chapter 3. Welhouse, G. J.; Bleam, W. F. Environ. Sci. Technol. 1992, 26,959-964. Senesi, N.; Testini, C. Geoderma 1982,28, 129-146. Senesi, N.; Testini, C.; Miano, T. M. Org. Geochem. 1987, 11, 25-30. Environ. Scl. Technol., Vol. 27,
No. 3, 1993 499
Envlron. Sci. Technol. 1993, 27, 500-505
(11) Russel, J. D.; Cruz, M.; White, J. L.; Bailey, G. W.; Payne, W. R.; Pope, J. D.; Teasley, J. I. Science 1968, 160, 1340-1342. (12) Terce, M.; Calvet,R. Z. Pflanzenkrankh. Pflanzenschutz, Sonderheft 1977,8, 237-243. (13) Talbert, R. E.; Fletchall, 0. H. Weeds 1964, 13, 46-52. (14) Borggard, 0. K.; Streibig, J. C. Acta Agric. Scand. 1988, 38, 293-301. (15) Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem. Soc., Perkin Trans. 2 1989,699-711. (16) Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Morris, J. J.; Taylor,P. J. J. Chem. Soc., Perkin Trans.2 1990,521-529. (17) Nadler, E. B.; Rappoport, Z. J. Am. Chem. SOC.1989, I l l , 213-223. (18) Nagano, M.; Nagano, N. 1.;Higuchi, T. J. Phys. Chem. 1967, 71, 3954-3959. (19) Gurka, D.; Taft, R. W. J. Am. Chem. SOC.1969, 91, 4794-4801. (20) Slasinsky, F. M.; Tustin, J. M.; Sweeney, F. J.; Armstrong, A. M.; Ahmed, Q. A.; Lorand, J. P. J. Org. Chem. 1976,41, 2693-2699. (21) Lorand, J. P.; Nelson, J. P.; Gilman, R. D.; Staley, K. L.;
4. (33) Wilson, M. A.; Vassallo, A. M.; Perdue, E. M.; Reuter, J. H. Anal. Chem. 1987.59. 551-558. (34) Schnitzer, M.; Preston, C. M. Soil Sei. SOC.Am. J. 1987, 51. 639-646. (35) Moelwyn-Hughs,E. A. The Chemical Statics and Kinetics of Solutions; Academic Press; New York, 1971; p 35. (36) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899-926.
SOC.1965,87, 3620-3626. (24) Slejko, F. L.; Drago, R. S. Inorg. Chem. 1973,12,176-183. (25) Jackel, H.; Stamm, H. Arch. Pharm. (Weinheim,Ger.) 1988, 321, 213-219. (26) Chudek, J. A.; Foster, R.; Mackay, R. L.; Page, F. M.;
Received for review June 1,1992. Revised manuscript received November 4,1992. Accepted November 10,1992. We thank the Ceiba-Giegy Corp. for their financial support and for samples of atrazine. We gratefully acknowledge financial support from the Federal Hatch Project 3289, the Wisconsin Graduate School Project 920284, and the NRI Competitive Grants ProgramlUSDA Grant 91-37012-6795.
Chambers, J. R.; Kirk, H. D.; Moeggenborg, K. J.; Farlow, D. L. J. Phys. Org. Chem. 1990,3,659-669. (22) Granot, J. J. Magn. Reson. 1983, 55, 216-224. (23) Taft, R. W.; Klingensmith, G. B.; Ehrenson, S. J.Am. Chem.
Twiselton, D. R. J. Chem. SOC.,Faraday Trans. 1 1988,84, 1145-1152. (27) Huyskens, P. L. J. Am. Chem. Soc. 1977,99,2578-2582. (28) Frange, B.; Abboud, J.-L. M.; Benamou, C.; Bellon, L. J. Org. Chem. 1982,47,4553-4557. (29) Deranleau, D. A. J . Am. Chem. SOC.1969,91,4044-4049. (30) Bailey, W. F.; Monahan, A. S. J. Chem. Educ. 1978, 55, 489-493. (31) Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Taft, R. W.;
Morris, J. J.; Taylor, P. J., Laurence, C.; Berthelot, M.; Doherty, R. M.; Kamlet, M. J.; Abboud, J.-L. M.; Sraidi, K.; Guiheneuf, G. J. Am. Chem. Soc. 1988,110,8534-8536. (32) Levy, G. C.; Lichter, R. L. Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy;Wiley: New York, 1981;Chapter
Cooperative Hydrogen Bonding of Atrazine Gereon J. Welhouse and Willlam F. Bleam"
Department of Soil Science, University of Wlsconsln-Madison, Madison, Wisconsin 53706 Formation constants are determined for hydrogen-bond complexes between atrazine and compounds commonly found in soil organic matter. Weak to moderately strong complexes (Kf = 2-30 L/mol) are formed with amine, hydroxyl, and carbonyl functional groups. Strong complexation is observed with the carboxylic acid (Kf = 212 L/mol) and amide (Kf = 276 L/mol) functional groups, which interact cooperatively with atrazine by simultaneously donating and accepting a hydrogen bond. These results confirm that hydrogen bonding provides a mechanism for atrazine adsorption to soil surfaces and identifies functional groups that have high affinity for atrazine.
C1
H
I
R3
Atrazine Isomer - R1 1
Introduction We recently reported that atrazine, 1, forms complexes with hydrogen bond donors and acceptors in CCll solvent (I). The formation constants for hydrogen-bond complexes range from weak to moderate (Kf = 2-30 L/mol) for these compounds, which interact with atrazine by forming a single donor-acceptor hydrogen bond. The compounds used in our previous study are reference compounds that establish universal hydrogen-bond activity (2, 3). In this paper we report formation constants for complexation between atrazine and functional groups found in soil organic matter. Two of these compounds, pyrrolidinone (NHP) and acetic acid (HOAC), have the potential to form strong cooperative hydrogen bonds with atrazine (4). Atrazine hydrogen-bond activity is calibrated with the a and 0 parameters than rank atrazine reactivity on 500 Envlron. Scl. Technol., Voi. 27, No. 3, 1993
2
3 4
H H Et Et
Et Et H H
R3
114
H iPr H iPr
iPr H iPr H
Et = ethyl; iPr = isopropyl
universal hydrogen-bonding scales ( 2 , 3 ) . Atrazine's parameters (a = 0.42; p = 0.50) demonstrate that it can donate and accept hydrogen bonds. We now test the predictive power of the a and @ parameters by estimating complexation constants from the a and p parameters of atrazine and the compounds used in this study and comparing these with the experimental results. As in our previous study, we measure the change in NH chemical shifts of atrazine as it is titrated with a complexing agent. The resulting titration curve data are fit to a 1:l complexation model using linear regression to obtain the formation constants.
0013-936X/93/0927-0500$04.00/0
0 1993 American Chemlcal Society
