Environ. Sci. Technol. 1992, 26, 959-964
Matsui, M.; Takase, Y. Senryo to Yakutuin (Dyestuffs and
C.; Cass, G. R. Enuiron. Sci. Technol. 1990, 24, 1004. (42) Grosjean, D.; Williams, E. L., 11; Druzik, J. R. Removal of air pollutants by sorbents used in museum HVAC-chemical
Chemicals) 1982, 27, 10. Desai, C. M.; Vaidya, B. K. J. Indian Chem. SOC.1954,31,
filtration equipment. Int. J. Museum Manage. Curatorship, submitted.
261.
Iwamoto, K. Bull SOC.Chem. Jpn. 1935,10, 420. Porter, J. J.; Spears, S. B., Jr. Text. Chem. Color. 1970,2, 191.
Bachman, G. B.; Connon, N. W. J. Org. Chem. 1969,34, 4121.
Salmon, L. G.; Nazaroff, W. W.; Ligocki, M. P.; Jones, M.
Receiued for review September 19, 1991. Revised manuscript received January 27,1992. Accepted January 28,1992. This work has been supported by a contract from The Getty Conseruation Institute, Marina del Rey, CA.
NMR Spectroscopic Investigation of Hydrogen Bonding in Atrazine Gereon J. Welhouse and Wllllam F. Bleam" Soil Science Department, University of Wisconsin, Madison, Wisconsin 53706
The solution properties of atrazine are investigated using NMR spectroscopy. In aprotic solvents, atrazine exists as a mixture of four conformational isomers related by rotation of the alkylamino side chains. The partial double-bond character responsible for this restricted rotation implies that there is a separation of charge within the atrazine molecule. As a result of this separation of charge, atrazine is able to both donate and accept hydrogen bonds. One consequence of this cooperative H-bonding is that atrazine dimerizes in aprotic solvents. In CC14solvent, the dimerization constant is 49 M-l. Introduction
Atrazine (l),a herbicide frequently used by corn and sorghum growers has been detected in many surface streams through the Midwest (1).Recent surveys have also detected low levels of atrazine in some Wisconsin groundwaters (2). In spite of the widespread use of atrazine and its persistence in the environment, very little is known about the chemistry of the interactions between atrazine and the soil or between atrazine and any agents that may carry it through the soil.
H
H
1
An illustration of this situation is provided by recent experience in the Lower Wisconsin River (LWR) and Central Sands (CS) agricultural regions in Wisconsin. In both these regions corn is grown under similar irrigation practices and atrazine application rates, yet atrazine is detected in LWR groundwater at levels far above those observed in the CS groundwater. This despite the fact that both these areas have predominantly sandy soils that are similar in organic matter content, mineralogy, surface area, and pH-factors known to affect atrazine adsorption. We are investigating the chemistry of atrazine interactions through the use of nuclear magnetic resonance (NMR), a spectroscopic technique that provides details about interactions at the molecular level. These molecular interactions control the solvation of atrazine, its adsorption onto soil surfaces, and iB degradation into nonphytotoxic hydroxyatrazine. These same interactions govern the important adsorption parameters Kd and KO,, as well as the octanol/water partitioning coefficient, Km. Understanding 0013-936X/92/0926-0959$03.00/0
the chemistry behind these phenomena will increase the ability to predict and control the environmental impacts of atrazine use. In this paper we present and discuss the NMR spectra of atrazine in five aprotic solvents and show that these spectra provide important clues about the solution behavior of atrazine. Two brief NMR investigations of atrazine have been published. The proton NMR spectrum of atrazine in chloroform (CDC1,) and trifluoroacetic acid (TFA) solvents has been reported (3). Atrazine in TFA was found to have its NH proton signal shifted downfield and split into two signals when compared to the spectrum of atrazine in CDC1,. This was interpreted as showing that atrazine is protonated by the strong acid TFA, and it was suggested that protonation takes place on the triazine ring nitrogens. Subsequently, the 13CNMR spectrum of atrazine was reported in these same solvents (4). It was found that TFA rapidly converted atrazine into hydroxyatrazine, instead of the protonated atrazine reported earlier. The hydroxyatrazine was then protonated by TFA. From the published results it is not possible to determine whether the proton NMR spectrum of atrazine in TFA shows a protonated form of atrazine, an atrazine spectrum shifted due to a solvent effect, or the spectrum of hydroxyatrazine. Recently, high-resolution lH and I3C NMR spectral data for atrazine in dimethyl sulfoxide (DMSO) solvent were published (5). On the basis of the NMR data and accompanying variable temperature experiments, it was proposed that atrazine exists as a mixture of four conformational isomers, related by restricted rotation of the side-chain alkylamino groups. We present the results of high-resolution lH NMR experiments on atrazine in aprotic solvents of different polarity. The results confirm that atrazine exists as a mixture of four conformational isomers, related by restricted rotation about the alkylamino side chains. The NMR data imply that there is significant development of charge polarity in the atrazine molecule. Dilution experiments show that atrazine dimerizes in these inert solvents, and the formation constant is calculated for CC14 solvent. Materials and Methods
Chemicals. Technical grade atrazine (98% pure) was supplied by the Ciba-Geigy Corp. and was recrystallized twice from hexane. NMR solvents were obtained from the Aldrich Chemical Co. and stored over 4-A molecular sieves. NMR. High-resolution proton spectra were obtained on a 400-MHz Bruker spectrometer at the National Magnetic Resonance Facility in Madison, WI. Spectra were acquired by applying a 5.6-ps (goo) pulse over a
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 5, 1992 959
Table I. Summary of Proton NMR Data Showing Chemical Shifts and Isomer Distributions for Atrazine in Five Aprotic Solvents a t 293 K" proton chemical shift CH2(e)
solvent
CH&)
CH&
COD,,
1.15; 3; t, 6.5
1.17; 4; d, 6.5
3.42; 2; dq, 6.8
4.21; 4; dsp, 6.9
CH(i)
"(e)
CCl,
1.21; 2; t, 6.1
1.25; 3; d, 6.4
3.45; 2; dq, 6.3
4.18; 3; dsp, 6.7
CsDG
0.70; 2; t, 7.1
0.75; 3; d, 6.5
3.11; 2; dq, 7.0
4.02; 3; dsp, 6.8
C7Hs
0.81; 3; t, 7.2
0.80; 3; d, 6.6
3.06; 3; dq, 6.6
3.94; 3; dsp, 6.8
CDC1,
1.19; 1; t, 7.7
1.22; 1;d, 6.8
3.43; 2; dq, 6.4
4.15; 3; dsp, 6.7
NH(i)
isomerb
f(i)'
5.25 4.70 5.10 4.60 5.24 4.83 5.11 4.66 5.06 4.59 4.99 4.63 4.77 4.52 4.70 4.54 5.15 5.06 5.03 4.90
I I1 I11 IV I I1 I11 IV I I1 I11 IV I I1 I11 IV I I1 I11 IV
0.43 0.31 0.18 0.08 0.47 0.29 0.17 0.07 0.42 0.28 0.20 0.10 0.42 0.25 0.23 0.10 0.47 0.23 0.23 0.08
6.65 6.30 4.80 4.90 6.50 6.17 4.95 4.95 6.00 5.83 4.50 4.71 5.47 5.31 4.43 4.56 5.49 5.29 5.15 5.04
(e) and (i) refer to ethylamino and isopropylamino protons, respectively (see 1). The data for the carbon-associated protons are shown in the following order. Average chemical shift of the composite signal (in ppm from TMS). Number of overlapping multiplets that make up the composite. Multiplicity of the splitting, and coupling constant (Hz):t, triplet; d, doublet; dq, doublet of quartets; dsp, doublet of septets. "Atrazine isomers are labeled as in Figure 2. 'Mole fraction of each atrazine isomer.
spectral width of 4000 Hz. Typically 16K data points, accumulated by summing 64 transients with a relaxation delay of 3 s, were zero filled to 32K before Fourier transformation. The chemical shifts are referenced directly to internal TMS (0.0 ppm). CH proton signals were assigned based on known chemical shifts and coupling constants (6). Peak areas were obtained by integration. NH protons were assigned to either the ethylamino or the isopropylamino side chains using saturation-transfer experiments (7, 8). In these experiments either the ethylamino CH, or isopropylamino CH proton signal was saturated by radio frequency irradiation prior to data acquisition. This presaturation decreased the signal intensity for those NH protons adjacent to the irradiated protons. The concentration of atrazine varied due to its low solubility in some of the solvents used. Concentrations ranged from 4.2 mM for cyclohexane-dI2(C6D12) to 6.8 mM for tetrachloromethane (CCl,), 13.8 mM for toluene-d, (C,D,) and benzene-d6 (C6D6), and 27.8 mM for chloroform-d (CDC1,). Variable-temperature experiments were conducted using the standard Bruker probe heating coil to adjust the NMR solution temperature. The probe is accurate to within 2 "C. Dilution experiments were conducted by adding solvent (CCl, with 10% C6D12for lock signal) to the atrazine solution. Each addition diluted the concentration of atrazine by two-thirds from an initial concentration of 6.8 mM.
Results and Discussion The N M R spectra of atrazine in five aprotic solvents are summarized in Table I. Reported in this table are the chemical shifts, multiplicity, and coupling constants for each specific resonance. It was discovered during this investigation that the chemical shifts for atrazine are very sensitive to concentration and temperature. Consequently, the chemical shifts reported in Table I are valid only for these specific concentrations at 293 K. An interesting feature of the data is that instead of observing a single multiplet for each proton resonance, we 960
Environ. Sci. Technoi., Vol. 26, No. 5, 1992
6 5
6 0
5 5
5 0
4 5
4 0
3 5
3 0
PPH
Figure 1. Part of the proton NMR spectrum of atrazine in benzene-d, at 288 K. The chemical shifts are reported in parts per million (ppm) from the internal standard TMS and are slightly dlfferent from those reported in Table I, which were acquired at 293 K. NH signals are found between 4.3 and 6.8 ppm and are identified by isomer (I-IV of Figure 2) and by side chain (e, ethylamino; and i, isopropylamino). CH, signals are found at 3.0 ppm and CH signals at 4.0 ppm. The insert is an expansion of the CH, signal showing that it is composed of overlapping multiplets for the four atrazine isomers. Each multiplet is a doublet of quartets with relative intensities 1:4:6:4:1 shown above isomer 111.
found as many as four overlapping multiplets. An example of this is seen in the partial spectrum of atrazine shown in Figure 1. The inset shows an expansion of the signal for the CH, protons of atrazine. This signal is actually composed of four distinct overlapping multiplets, each of which is a five-peak pattern that results from the splitting of the CH, signal first into a quartet by the CH, protons of the adjacent methyl group and then into a doublet by the adjacent NH proton (6). Since the coupling constants are similar, the multiplet is a doublet of quartets with relative intensities 1:4:6:4:1. Each multiplet is the signal for a distinct isomer and the labels I-IV identify these isomers in Figure 2. A similar situation is seen in the CH signal at 3.8-4.2 ppm in Figure 1. In this case each of the four multiplets
N
K,
N
K,
iR
I
I1
CI
c1 N
N
iP'\NANAN.H H.NANAN, H
I Et
H
111
I Et
I iR IV
Figure 2. Four conformational isomers of atrazine. The isomers are related by restricted rotation of the alkylamino side chains and interconvert at room temperature. Interconversion is slow enough that dlstlnct NMR signals are observed for each isomer.
is a doublet of septets due to the splitting of the CH signal into seven peaks by the six protons on the adjacent isopropyl methyl groups and then splitting of this pattern into a doublet by the NH proton. Isomers. These overlapping, duplicated peak patterns suggest that atrazine exists as a mixture of as many as four distinct conformational isomers. Supporting this claim are the many distinct NH proton signals observed when only two are expected based on structure 1. Figure 1,taken at 288 K, shows eight distinct NH signals corresponding to two NH signals for each of the four isomers. These signals are labeled according to isomer (I, 11,111,IV)and side chain (e = ethylamino, i = isopropylamino) of atrazine. The four atrazine isomers are shown in Figure 2. The NMR signals for these isomers have different intensities, showing that each isomer is present in a different amount. The relative proportion of each isomer was obtained from the integrated peak areas of the NH signals. These proportions are given in Table I. The existence of these isomers is confirmed by the results of variable-temperature NMR experiments in C6D6 solvent. The variable-temperature experiment takes advantage of the interconversion of the isomers. As the rate of interconversion increases at higher temperatures, signals for interconverting isomers move closer together until at the coalescence temperature only a single average signal is observed. This process is illustrated by the spectra in Figure 3. As the temperature is increased, signals I-e and 11-e, related by rotation of the isopropylamino group, move together and coalesce at 335 K. Signals for I-i and 111-i, related by rotation of the ethylamino group, move together and coalesce at about 325 K. (Isomerizations involving simultaneous rotations of both side chains, e.g., I1 isomerizing to 111, show coalescence at even higher temperatures and are not considered here.) The rate of isomerization can be estimated using eq 1
for exchange between unequally populated isomers (7), where pais the population of isomer a, P b is the population of isomer b, X is 2m,(Av), T , is the lifetime of the major isomer in seconds (=X/2xAv), and Av is the difference in chemical shift of the exchanging isomers. For example, 76 Hz separates I-e and 11-e. Once X is found by use of eq 1, T, is calculated from it and used to determine k,, the isomerization rate, using the relation kc = l / T c . In this way
I
I
I
6.0
I
I
5.0
PPM
I
1
4.0
3.0
Figure 3. Variable-temperature spectra of atrazine In benzene-d,. NH signals for isomers I-i and 111-1coalesce at about 325 K. Signals for I-e and II-e coalesce at about 335 K. (Labels correspond wlth those in Figure 2.)
Table 11. Free Energy of Activation (AG*) for Rotation of Alkylamino Side Chains in Atrazine and Several Structurally Related Molecules CH3'fJ"qCH3
CH3
I
"flyo o
2.1
0
H
a
N
q
C
CH3 IH 3
0
"2
2.2
2.3
compd
solvent
AG*,kJ/mol
ref
2.1 2.2 2.3 2.3 atrazine atrazine (i)" atrazine (e)n
DMSO DMSO DMSO benzene DMSO benzene benzene
69.5 74.4 61.5 61.1 69.1 67.4c 67.OC
22 23 24 24 5 b b
(e) and (i) refer to atrazine ethylamino and isopropylamino side chains, respectively. bThis work. fl.1 kJ/mol, estimated from an uncertainty in the coalescence temperature of &5 K.
the rate of NH-e isomerization at 335 K (due to isopropylamino rotation) was calculated as 252 (fll)s-I and the rate of N H i isomerization at 325 K (due to ethylamino rotation) as 98 (f5) S-I, The free energy of activation, AG* (kJ/mol), for isomerization is estimated using the isomerization rate (kc), coalescence temperature (T,),and eq 2, a modified form of the Eyring equation (7). Free energies of activation for AG* = 0.01914T,[10.319 + log (Tc/kc)l (2) both ethylamino and isopropylamino isomerizations are reported in Table I1 along with those reported for isomerizations in three other structurally related amidines. The atrazine values are consistent with these reported values. All the molecules in Table I1 isomerize by rotation of alEnviron. Sci. Technol., Vol. 26, No. 5, 1992
961
c1
CI
I
4a
4b
t
t
c1
CI
H
H
4c
\L 1.3
H 4d
Flgure 4. Atrazine resonance structures showing the development of positlve charge on the ethylamino nltrogen due to delocalization of its lonapair electrons into the triazine ring. Similar resonance structures can be drawn involving the isopropylamino nitrogen and for both sideGhain nitrogens in each of the three other conformational isomers.
kylamino groups that are adjacent to an unsaturated carbon (sp2)atom. Delocalization. The restricted rotation that is responsible for the existence of distinct isomers is caused by delocalization of the side-chain nitrogen lone-pair electrons into the triazine ring as shown in the resonance structures in Figure 4. The delocalization of nitrogen lone-pair electrons into an adjacent a-electron system is a fairly common and well-understood phenomenon (9,10). When observed, it indicates that partial double-bond character (sp2hybridization) has developed in the C-N bond that undergoes rotation. Delocalization also leads to separation of charge within the atrazine molecule with positive charge developing on the side-chain nitrogens while negative charge is delocalized around the triazine ring. This separation of charge is likely to be more pronounced in polar solvents and aqueous solution (11). The actual structure of atrazine is not represented by any one particular resonance structure shown in Figure 4 but by a weighted average of all resonance structures. These resonance structures serve only as a means of mentally keeping track of the electronic chargee distribution within the molecule. They show that electron density is shifted from the side-chain alkylamino nitrogens onto the triazine ring nitrogens. In work related to that discussed here, we have used 15N-labeledatrazine to find that the nitrogen para to the chloride substituent carries most of this redistributed electron density and is the site where atrazine is protonated. These interpretations are based on changes in the observed 15Nchemical shifts and TIrelaxation rates (12). Separation of charge involving nitrogen lone-pair electrons and protons suggests the possibility of hydrogenbonded interactions. Both hydrogen-bond donating (from side-chain NH) and hydrogen-bond accepting (to triazine N) can take place in the atrazine molecule presenting the possibility of self-association or dimerization. Dimerization. To check for atrazine dimerization, a series of dilution experiments was performed in which a solution of atrazine in CCll was diluted by the sequential addition of pure solvent. By recording the proton NMR spectrum at each atrazine concentration, we found that the extent of dimerization was reflected by changes in the proton chemical shifts, as shown by the spectra plotted in Figure 5. The change in chemical shifts was then used 962
h
I
I
H
0.88 mM
I
Envlron. Sci. Technol., Vol. 26, No. 5, 1992
I
I
I
6.0
I
5 .0
I
4.0
I
I
3.0
PPM Flgure 5. Dilution experiments on atrazine in CCI, at 298 K. Decreasing the solution concentration shifts the NH atrazine signals I-e, 11-e, and I-i due to decreased hydrogen-bonded dimerization. The chemical shift change is used to calculate the dimerizatlon constant (see text).
to calculate the formation constant, Kf, for the atrazine dimer, as described below. Since the atrazine dimer is held together by hydrogen bonds which typically have bond energies of about 20-30 kJ/mol, complexes break and re-form rapidly at room temperature and an average NMR signal is observed for the protons involved in the bonding. The observed signal represents the average of the signals for atrazine monomer and dimer weighted by the fraction of time spent in each form. The dimerization of atrazine is represented by eq 3 and the formation constant by eq 4. From eq 4 it can be seen that as the concentration of atrazine [A] changes, the concentratioin of dimer [D] is expected to change by Kf[A12. A+A=D (3)
Kf = [Dl/[A12 (4) The observed chemical shift is an average of the chemical shifts for [A] and [D] weighted by their relative populations. This averaged chemical shift changes with [A] in a way that allows Kf to be calculated by using an iterative procedure (13). The mole fractions of atrazine monomer (f,) and dimer ( f d ) are expressed in terms of the observed chemical shift (6,) and the unknown chemical shifts for the monomer (6,) and dimer (6d) using eqs 5 and 6. Equations 5 and 6 are substituted into the equilibrium fm fd
- 6o)/(6d - 6,)
(5)
= (60 = 6m)/(6d - 6,)
(6)
=
(6d
quotient 4 to derive eq 7 , which is used for linear regression of 6, against [(a, - 6m)/[A]o]1/z.The first regression is made using an estimate for 6,. K is found from the intercept
and slope of the regression, and from K , the fraction of dimer is calculated using eq 8. Regression of 6, against f d
c1
Table 111. Formation Constants (Kf)a n d Chemical Shifts for Atrazine Dimerization signala I-e I-i 11-e 111-i IV totale
Kf, M-' 66 (*2)d 5 (5) 51 (1) 6 (5) 49 (5)
mole fract 0.48 0.48 0.27 0.18 0.08 0.99
amb
6dc
1986 (*3) 1940 (1) 1938 (2) 1885 (5)
3457 (k15) 3041 (648) 3365 (14) 2765 (699)
Labels refer to the NH signal used to monitor dimerization and are the same as those used in Figures 1and 2. Monomer chemical shift. CDimerchemical shift. dAverage and 95% CI found by regression of the 7-point curve. In practice atrazine chemical shifts are accurate only to f 3 Hz due to small variations in solution temperature and concentration. e Weighted average for atrazine calculated from the K pand mole fraction for each isomer.
6-1
CI
CI
CI 6-2
N "\NANAN/E'
according to eq 9 returns a new estimate of 6, as the intercept. This new value for 6, is taken back to eq 7 for
H + 1
H
/ I
H
H
(8K[A]O + l)li2- 1 fd
= (8K[A]" 60 = 6,
+ 1)lI2 + 1
+ fd(8d - 6,)
NYN
(9)
the next cycle of regression. This process is continued until the new value of 6, returned by eq 9 is close enough to the previous value [d,(new) - 6,(old) < 0.2 Hz]. In practice, convergence was achieved within five iterations for the isopropyl signals. The ethyl signals exhibited some instability, making it necessary to use linear regression instead of the recommended 2' polynomial regression. Convergence was achieved within six iterations. Results of this analysis are summarized in Table 111. The dimerization is stronger for the ethylamino (NH-e) protons than for isopropylamino (NH-i) protons, shown by the larger formation constants and greater chemical shift changes for formation of the dimer. Formation constants for NH-e in isomers I and I1 are 66 and 51 M-l, compared to 5 and 6 M-l for NH-i in isomers I and 111. The chemical shift difference between monomer and dimer for NH-e (about 1400 Hz or 3.5 ppm) is greater than for NH-i (about 1000 Hz or 2.5 ppm). These observations together suggest that hydrogen bonding involving ethylamino NH is stronger than those involving isopropylamino NH. This may be due to greater steric interference from the isopropylamino group. The hydrogen-bonding interactions that we believe are responsible for dimerization are shown in Figure 6. Atrazine acts as both hydrogen-bond donor from side-chain NH and hydrogen-bond acceptor at the triazine ring nitrogen para to the chloride substituent. This cooperativity strengthens the interaction and gives rise to the cyclic structure shown. Dimerization is strongest for isomer I, where both NH-e and NH-i protons have the proper orientation for cooperative H-bonding. Hydrogen bonding lowers the free energy of the atrazine isomers that dimerize and increases their relative population. This interpretation is supported by the mole fraction data presented in Table 111, which show that isomer I1 (Kf = 51) is more abundant than isomer I11 (Kf = 6). Isomer I (Kf= 71) is much more abundant than isomer IV (Kf = 0) due to a combination of effects. Isomer I is stabilized by hydrogen-bonded dimerization while isomer IV is destabilized by steric overlap of the ethyl and isopropyl side chains, both of which are oriented antiperiplanar to chloride. An alternative explanation of atrazine isomer populations based on molecular dipoles has been presczlted (5).
CI
6-3 Figure 6. Hydrogen-bonded dimers of atrazine. These cyclic dimers form by the cooperative interaction of two molecules, each acting as both hydrogen-bond donor and hydrogen-bond acceptor. The triazine nitrogen atom para to chloride is the acceptor ske and the side-chain NH protons are the donors. Rotation of the side chains shown In 6-1 and 6-2 give rise to isomers that are less effective in dimerization, probably due to steric hinderance.
Table IV. Formation Constants ( K , ) for Dimerization of Atrazine a n d Several Related Compounds
0
I
H 4.1
I
H 4.2
I
I
H
H
4.3
4.4
compd
name
Kf, M-'
ref
4.1 4.2 4.3 4.4 1
maleimide 2-pyrrolidinone pyrazole N-methylimidazolidinone atrazine
22 197 47 130 49
25 25 25 26 a
This work.
An overall dimerization constant was calculated for atrazine from the dimerization constants and mole fractions of each isomer. The value of this constant, 49 M-l, is similar to that reported for other molecules with both hydrogen-bond donating and accepting abilities, four of which are summarized in Table IV. An overall Kf of 49 M-l means that about 30% of atrazine exists in dimerized form in these aprotic solvents at room temperature when the total atrazine concentration is 5 mM. The formation constants presented in Table I11 were calculated under the assumption that only dimerization affected the atrazine chemical shifts. Higher order complexes were not considered important at these low atrazine concentrations. I t is also assumed that atrazine accepts a hydrogen bond only at the ring nitrogen para to the chloride substituent. This assumption is based on NMR experiments we conducted using 15Nring-labeled atrazine which led to the conclusion, based on chemical shift and relaxation data, that this nitrogen has the greatest electron Environ. Sci. Technol., Voi. 26, No. 5, 1992
Q63
density and strongest interaction with protons (12). The specific interaction between this para nitrogen and the side-chain NH protons shown in Figure 6 enforces a synperiplanar geometry and favors the formation of a cyclic dimer over extended-chain higher order complexes. Hydrogen-bond formation is known to cause a downfield shift of NH proton NMR signals, reflecting the reduced electron density around the protons involved. For example, NH signals of adenosine and cytidine (nucleic acid base pairs similar in structure to atrazine) are shifted downfield by 1.5 ppm (24) and the NH signal of histidine is shifted by 3.0 ppm (15) upon H-bond complex formation in aqueous solution. Similarly, a downfield shift of 2.5 ppm is observed for the NH signal of peptides (16) and for the CH signal of dimedone (17) when hydrogen-bonded complexes are formed. Conclusion
Atrazine in aprotic solvents exists as a mixture of four isomers. These isomers are related by restricted rotation about the side-chain alkylamino group and interconvert fairly rapidly at room temperature. Since they interconvert, it is not important to know which isomer more closely represents atrazine in a given situation but instead to recognize that all conformations will be presented to a potential adsorbing surface or interacting molecule. The existence of these isomers says much about the potential reactivity of atrazine. For example, the restricted rotation due to delocalization of the nitrogen lone-pair electrons into the triazine ring creates partial double-bond character and a substantial separation of charge within the atrazine molecule. This creates conditions favorable for hydrogen-bonding interactions involving either the acidic NH protons of the alkylamino side chains or the basic lone-pair electrons on the triazine ring, or both. The dimerization of atrazine is an example of cooperative bonding involving simultaneous H-bond donating and accepting. Cooperative hydrogen bonding may provide an effective mechanism for adsorption of atrazine onto soil organic matter (18,19) and mineral surfaces (20). It may also stabilize the tetrahedral intermediate leading to the hydrolytic degradation of atrazine (15, 21) catalyzed by these surfaces. Atrazine dimerization is probably not an important consideration in aqueous solution due to the low solubility of atrazine in water (0.14 mM) and to competing hyrogen-bonding interactions between atrazine and the solvent. Both these factors reduce dimerization. We are conducting further experiments to evaluate the strength of other atrazine hydrogen-bondinginteractions, particularly those involving cooperative donating and accepting where there is suitable geometry for strong complex formation. It is possible that strong hydrogen-bonding interactions will affect the aqueous solution behavior of atrazine. Acknowledgments
We thank the staff at the National Magnetic Resonance Facility at Madison (NMRFAM) for their technical assistance.
964
Environ. Sci. Technol., Vol. 26, No. 5, 1992
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Received for review July 10, 1991. Revised manuscript received November 26,1991. Accepted January 16,1992. W e thank the Ciba-Geigy Corp. for their financial support and for providing samples of atrazine. W e also gratefully acknowledge financial support from the Federal Hatch Project No. 3289 and the Wisconsin Graduate School Project No. 920284.
