GIAO NMR calculations for atrazine and atrazine dimers: comparison of theoretical and experimental 1H and 13C chemical shifts

Journal of Molecular Structure (Theochem) 588 (2002) 45±53

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GIAO NMR calculations for atrazine and atrazine dimers: comparison of theoretical and experimental 1H and 13 C chemical shifts Zhizhong Meng, W. Robert Carper* Department of Chemistry, Wichita State University, Wichita, KS 67260-0051 USA Received 12 November 2001; revised 25 February 2002; accepted 12 March 2002

Abstract The gauge-including atomic orbital method for calculating 1H and 13C nuclear magnetic shielding tensors at both the Hartree±Fock (HF) and density functional levels of theory is applied to atrazine (2-chloro-4-ethylamino-6-isopropyl-amines-triazine) and atrazine dimers. Atrazine calculations include the HF/6-31G p, HF/6-31G pp, and HF/6-3111G(d,p) basis sets. Atrazine density functional calculations (DFT) use the B3LYP and B3PW91 methods and the 6-31G p, 6-31G pp, and 63111G(d,p) basis sets. Hartree±Fock atrazine dimer calculations include the 6-31G p and 6-31G pp basis sets. Atrazine dimer B3LYP calculations are reported for B3LYP/6-31G p and B3LYP/6-31G p//B3LYP/6-31G pp. The 1H and 13C NMR chemical shifts compare favorably with those reported for atrazine monomer±dimer equilibria in solution. The multiple atrazine N±H 1H chemical shifts correlate with N±H 1H chemical shifts calculated for monomeric atrazine at both the HF and density functional levels of theory. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Atrazine dimers; ab initio NMR spectra; Atrazine; 1H atrazine chemical shifts; 13C atrazine chemical shifts

1. Introduction Atrazine (2-chloro-4-ethylamino-6-isopropyl-amines-triazine) is one of the most widely used herbicides in the world and is shown in Fig. 1 [1]. Its degradation half-life in soil ranges from 1.5 months to 5 years [2,3]. This intensive use in agriculture has led to accumulation of atrazine in the environment, with residual subsurface and surface water concentration levels of up to 54 mg l 21 [4]. Atrazine has been previously the subject of a semi-empirical conformational study [5], an NMR study of its proposed dimer formation in * Corresponding author. Tel.: 11-316-978-3120; fax: 11-316978-3431. E-mail address: [email protected] (W.R. Carper).

aprotic solvents [6], a PM3 study of the effects of hydration on metal ion±atrazine dimer complexes [7] and a supermolecule study of the total hydration of atrazine dimers at the semi-empirical (PM3) level [8]. The reported formation of atrazine dimers based on NMR spectra of atrazine in aprotic solvents suggested a theoretical chemical shift ( 1H and 13C) analysis at the ab initio level. This type of study would help to verify the existence of atrazine dimers in solution and to possibly determine the degree of atrazine dimer formation in solution. The ab initio methods that were considered include the gauge-including atomic orbital (GIAO) and continuous set of gauge transformations (CSGT) methods [9±13]. The GIAO method uses basis functions that have a speci®c ®eld

0166-1280/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0166-128 0(02)00116-1

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Z. Meng, W.R. Carper / Journal of Molecular Structure (Theochem) 588 (2002) 45±53

Fig. 1. Molecular structure of atrazine (B3LYP/6-311 1 G(d,p)).

dependence [11]. In a comparison study of the GIAO and CSGT methods, it was found that the GIAO method converged more rapidly with large basis sets than did the CSGT method [14]. Consequently, this study applied the GIAO method to atrazine and the atrazine dimer. As pointed out by a number of investigators, the GIAO method using large basis sets is quite successful in reproducing chemical shifts for a number of nuclei including 1H, 13C, 17O, 31P, and 33S [13±18]. 2. Theoretical section The ab initio calculations (Hartree±Fock (HF) and density functional calculations (DFT)) reported herein were obtained using gaussian 98 [19] on either a Compaq DP264 parallel workstation or on a SGI Origin 2000 at the University High Performance Computer Center. All structures were fully optimized with the exception of the B3LYP/6-31G p//B3LYP/631G pp atrazine dimer structures in which the fully optimized B3LYP/6-31G p geometry was used for a single point calculation at the 6-31G pp level. Vibrational analyses were performed on all fully optimized structures to insure the absence of negative vibrational frequencies and to verify the existence of a true minimum in every case. Hartree±Fock and DFT were applied to the atrazine dimer structure in order to compare relative accuracies by these two methods of analysis. In order to compare isotropic shieldings with experimental chemical shifts, the NMR parameters

for TMS (tetramethylsilane) were calculated for each basis set and used as the reference molecule. The numbering systems for atrazine and atrazine dimers are shown in Figs. 1 and 2. 3. Experimental section An analytical standard sample of atrazine (2chloro-4-ethylamino-6-isopropyl-amine-s-triazine) was supplied by Ultra Scienti®c and triply recrystallized from cyclohexane. The compounds 2-chloro4,6-bis(ethylamino)-s-triazine and 2-chloro-4-ethyl6-isopropyl-amine-s-triazine often exist as impurities in atrazine synthesis and can be detected by 13C NMR using DEPT [20]. All NMR spectra were obtained using a Varian Unity Inova spectrometer operating at 399.742 MHz ( 1H) and 100.515 MHz ( 13C). All samples were triply degassed and sealed with a torch. 4. Results and discussion 4.1. Atrazine The fully optimized B3LYP/6-3111G(d,p) atrazine and the single point B3LYP/6-31G p//B3LYP/631G pp atrazine dimer structures are shown in Figs. 1 and 2. The ab initio B3LYP/6-3111G(d,p) atrazine structure (Fig. 1) is virtually identical to that reported using a semi-empirical method (PM3) [21] in a previous study [7]. All nine ab initio structures of

Z. Meng, W.R. Carper / Journal of Molecular Structure (Theochem) 588 (2002) 45±53

47

Fig. 2. Molecular structure of atrazine dimer (B3LYP/6-31G p//B3LYP/6-31G pp).

atrazine (HF/6-31G p, HF/6-31G pp, HF/6-3111G(d,p), B3LYP/6-31G p, B3LYP/6-31G pp, B3LYP/63111G(d,p), B3PW91/6-31G p, B3PW91/6-31G pp and B3PW91/6-3111G(d,p)) are similar and all ab initio atrazine dimer structures (HF/6-31G p, HF/631G pp, B3LYP/6-31G p and B3LYP/6-31G p// B3LYP/6-31G pp) are essentially identical. Molecular structures determined with DFT methods are generally considered to be as or more accurate than HF structures. In this case, both DFT and HF methods yield the same molecular structures for atrazine and the atrazine dimer. All of the ab initio DFT and the HF atrazine dimer structures are similar but not identical with the lowest (PM3) energy atrazine dimer structure reported previously [7]. In the ab initio versions reported herein, the two triazine rings are still stacked, however, the triazine ring has been ¯ipped over and rotated slightly so that the Cls are on opposite sides from each other. The N-ethyl and N-isopropyl sidechains are opposite to each other in both the semiempirical (PM3) and ab initio forms (Fig. 2) reported herein. 4.2. Planarity of side chain NH and triazine ring The iso-propyl C10±H±N1±triazine ring±H±N8± ethyl C13 (Fig. 1) system is planar in all calculated structures (atrazine monomers and dimers) reported herein. A similar result was observed for the PM3 study of atrazine and atrazine dimers [7]. The terminal

methyl groups in all calculated atrazine structures have individual Hs that are van der Waals distances from the off-ring Ns, N1 (i-propyl) and N8 (ethyl). The H23´ ´ ´N1 and H18´ ´ ´N8 distances in Fig. 1 are Ê . These distances are comparable to 2.785 and 2.718 A Ê ) and N the sum of van der Waals radii for H (1.17 A Ê ) of 2.72 A Ê [22]. In the atrazine dimer structure (1.55 A (B3LYP/6-31G p//B3LYP/6-31G pp) shown in Fig. 2, the H23´ ´´N1 (i-propyl) and H18´ ´ ´N8 (ethyl) distances are somewhat shortened to 2.731 and Ê (i-propyl N´ ´ ´H) and 2.688 and 2.674 A Ê 2.726 A (ethyl N´ ´ ´H). 4.3. Intramolecular and intermolecular hydrogen bonding The ethyl or isopropyl N±H´ ´ ´ring N distances are Ê in the fully optimized DFT struc2.401 and 2.402 A ture (B3LYP/6-3111G(d,p)) shown in Fig. 1. These distances are less than the sum of van der Waals radii Ê ) and N (1.55 A Ê ) of 2.72 A Ê [22]. In the for H (1.17 A atrazine dimer (B3LYP/6-31G p//B3LYP/6-31G pp) shown in Fig. 2, these distances have lengthened by Ê to 2.386 and 2.384 (i-propyl) A Ê and more than 0.3 A Ê 2.391 and 2.400 A (ethyl). Similar results are obtained for the other atrazine and atrazine dimers, regardless of basis set used or method of calculation (HF or DFT). We failed to observe any intermolecular hydrogen bonds in the dimeric atrazine structures calculated by either HF or DFT methods.

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Table 1 13 C chemical shifts (ppm) of atrazine Fig. 1C

6-31G p

6-31G pp 6-3111G(d,p)

Experimental

HF (C) 3 164.101 165.671 172.419 4 164.605 166.229 173.109 5 175.111 176.457 182.161 10 37.370 38.126 39.230 13 33.082 33.440 34.060 16 16.877 17.031 17.405 20 21.772 21.975 22.430 22 22.386 22.667 23.101 RMS (ppm) 3.39 3.57 6.42

164.897 165.671 168.123 42.940 35.843 14.703 22.422 22.422

B3LYP (C) 3 156.171 158.528 170.402 4 156.678 159.084 171.209 5 172.681 174.887 185.417 10 43.060 44.151 46.971 13 36.776 37.419 39.153 16 16.369 16.670 17.240 20 22.338 22.773 23.395 22 22.502 22.832 23.924 RMS (ppm) 5.07 4.15 7.03

164.897 165.671 168.123 42.940 35.843 14.703 22.422 22.422

B3PW91 (C) 3 4 5 10 13 16 20 22 RMS (ppm)

164.897 165.671 168.123 42.940 35.843 14.703 22.422 22.422

155.902 154.737 167.895 156.582 155.425 168.647 171.684 170.515 181.746 41.922 39.787 44.624 36.088 33.381 37.432 16.570 13.329 16.549 22.445 19.427 22.665 22.810 19.793 23.244 4.73 5.57 5.15

4.4. NMR studies of atrazine dimers An NMR study of atrazine in aprotic solvents reported multiple sets of N±H proton signals whose inter-conversion would have to be slow on the NMR time-scale [6] to ®t the proposed dimeric model of atrazine at room temperature. Chemical shift studies as a function of atrazine concentration were used to calculate atrazine dimer formation constants [6]. Formation of atrazine dimers was attributed to hydrogen bonding including cyclic dimeric forms in which hydrogen bonds were formed between ring nitrogens and protons attached to either the ±NHCH2CH5 or ± NHCH(CH3)2 sidechains. However, hydrophobic bonding is often enhanced in aprotic solvents and it seemed logical that ring hydrophobic interactions

might play a role in atrazine dimer formation. The molecular structures shown in Figs. 1 and 2 indicate intra-molecular hydrogen bonding and stacking of the triazine rings and sidechains. There is no evidence of hydrogen bonds between the atrazine monomers in Fig. 2. The shortest distance between C and N on Ê. opposite triazine rings in the atrazine dimer is 3.4 A Ê ) between triazine rings is typical This distance (3.4 A of stacked aromatic ring systems where weak p±p p Ê ) [23]. This distance interactions occur (3.2±3.4 A Ê ) in addition to the obvious stacking of triazine (3.4 A rings in Fig. 2 suggests that hydrophobic forces play a role in atrazine dimer formation in the gas phase and possibly in the liquid phase. 4.5. Comparison of theoretical and experimental 13C chemical shifts of atrazine The 1H decoupled 13C spectrum of atrazine revealed multiple 13C signals for the CH (42.940, 42.682 ppm) and CH2 (35.843, 35.668 ppm) carbons with the larger signals (peak areas . 4:1) located at higher ®elds. There are also separate 13C peaks for the two methyl Cs from the i-propyl group with the down®eld peak (22.422 ppm) area being larger than the up®eld (22.923 ppm) peak by approximately 4:1. These peaks must come from separate atrazine species (monomers or dimers) in solution, as indicated by their differences in peak area. Table 1 contains the GIAO 13C NMR chemical shifts d (in ppm) of atrazine relative to TMS. The experimental results were obtained using a 0.2 M atrazine solution (CDCl3) at 100.515 MHz ( 13C), 399.742 MHz ( 1H) and 25 8C. Assignments for 13C and 1H chemical shifts (d in ppm) were made using H,H±COSY, H,C±COSY (HETCOR), and DEPT [20,24,25]. Attempts to use H,H-NOESY did not produce distinguishable signals as the exchange processes are not always slow and thus magnetization transfers take place and obscure the H,H-NOESY spectrum [26]. The GIAO 13C chemical shifts in Tables 1 and 2 provide a reasonable reproduction of the experimental data obtained using 0.2 M atrazine in CDCl3 at 25 8C. We have assigned the more intense 13C peaks to the atrazine monomer and the weaker peaks to the atrazine dimer, consistent with the 1H assignments in the following sections. It is also possible that the more

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Table 2 Atrazine dimer 13C chemical shifts Fig. 2C

C Type

HF/631g p

HF/631g pp

B3LYP/631G p

B3LYP/6-31G p// B3LYP/6-31G pp

Experimental

2 5 6 10 11 16 17 18 30 33 37 41 42 46 48 50 RMS (ppm)

NyC±Cl NyC±N (ethyl) NyC±N (i-propyl) CH± CH2 ± CH3 ±(ethyl) CH3 ±(i-propyl) CH3 ±(i-propyl) CH3 ±(ethyl) CH2 ± NyC±N (ethyl) NyC±Cl NyC±N(i-propyl) CH± CH3 ± (i-propyl) CH3 ± (i-propyl)

173.851 163.353 164.069 37.470 32.414 14.341 21.583 23.012 14.612 32.396 164.238 174.575 163.294 37.226 22.309 21.636 3.26

175.339 165.022 165.697 38.225 33.038 14.480 21.891 23.325 14.818 32.878 165.916 176.011 164.893 38.076 22.525 21.864 3.33

171.174 155.569 156.336 43.352 36.634 14.249 22.514 23.415 15.215 35.634 156.612 171.762 155.404 43.157 22.307 22.256 4.84

166.658 152.406 153.121 41.856 34.898 12.317 20.705 21.488 12.677 34.738 153.139 167.352 152.056 41.787 20.491 20.820 6.46

168.123 164.897 165.671 42.940 35.843 14.703 22.422 22.923 14.703 35.668 164.897 168.123 165.671 42.682 22.422 22.923

intense 13C peaks contain a contribution from both the atrazine monomers and dimers. In view of this possibility, both strong and weak peaks are listed in Table 2 that contains the GIAO 13C chemical shifts for atrazine dimers using HF/6-31G p, HF/6-31G pp, B3LYP/631G p and B3LYP/6-31G p//B3LYP/6-31G pp methods. There are two very similar GIAO 13C chemical shifts for each type of 13C and these reproduce the experimental data in a reasonable manner. 4.6. Comparison of theoretical and experimental 1H chemical shifts of atrazine The calculated (GIAO) and experimental 1H NMR chemical shifts are contained in Table 3. The experimental results are similar to those reported previously [27]. The N±H Hs undergo considerable changes in chemical shift as a function of concentration, solvent, and temperature. In CDCl3, one observes multiple N± H 1H peaks that include two intense peaks at 6.563 and 5.527 ppm. There are also two less intense N±H 1 H peaks at 6.247 and 5.212 ppm. The peaks at 5.527 and 5.211 ppm are doublets and can be assigned to N± Hs directly connected to i-propyl groups with JH±N±C±H ˆ 6:8 Hz: The peaks at 6.563 and 6.247 ppm are singlets and are assigned to the N±Hs connected to the ethyl groups.

The ratio of peak areas for the 6.563 and 6.247 ppm H peaks is 2.91 for a 0.2 M atrazine solution at 25 8C. This ratio decreases to 2.83 for a 0.5 M atrazine solution and increases to 3.16 for a 0.02 M atrazine solution at 25 8C, indicating some type of atrazine monomer±dimer equilibrium. Assuming that atrazine dimer formation is an exothermic process, it is suggested that the more intense peaks (6.563 and 5.527 ppm) can be assigned to the monomer and the lesser peaks (6.247 and 5.211 ppm) to the atrazine dimer. In CDCl3, we also observe multiple 1H chemical shifts for the CH and CH2 groups, consistent with the existence of more than one atrazine specie in solution. The ethyl CH2 group chemical shift consists of two groups of overlapping multiplets …JH±C±C±H ˆ 7:2 Hz† that separate out as two pairs of quartets. The most intense pair of quartets have centers at 3.471 and 3.454 ppm and the second pair of quartets have centers at 3.384 and 3.367 ppm. The more intense pair of quartets (3.471 and 3.454 ppm) has been assigned to the atrazine monomer and the second pair of weaker quartets (3.384 and 3.367) to the dimer. In a similar manner, we also observe C±H multiplets that can be separated (not the only solution) as two pairs of overlapping septuplets. The most intense pair of overlapping septuplets have centers at 4.212 1

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Table 3 1 H chemical shifts (ppm) of atrazine monomer (d ˆ doublet; t ˆ triplet; q ˆ quartet; s ˆ septuplet) H type

HF 6-31G p

HF 6-31G pp

HF 6311 1G(d,p)

B3LYP 6-31G p

B3LYP 6-31G pp

Experimental

11 12 14 15 14,15 17 18 19 17±19 21 23 24 25 26 27 28 23±28

N±H (i-propyl) N±H (ethyl) CH2 (ethyl) CH2 (ethyl) Avg. CH3 (ethyl) CH3 (ethyl) CH3 (ethyl) Avg. CH (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) Avg.

3.284 3.565 3.759 2.587 3.173 0.825 0.965 1.606 1.132 3.922 0.914 1.343 1.187 0.765 0.885 1.681 1.129

3.594 3.893 3.712 2.490 3.101 0.802 0.906 1.578 1.095 3.892 0.850 1.287 1.106 0.740 0.826 1.657 1.078

3.643 3.998 2.489 3.729 3.109 1.516 0.882 0.877 1.091 3.866 1.284 0.834 1.085 0.904 0.703 1.576 1.064

3.526 3.764 3.977 2.658 3.318 0.835 0.874 1.438 1.049 4.269 0.878 1.302 1.147 0.744 0.956 1.622 1.108

3.900 4.141 4.073 2.680 3.377 0.870 0.859 1.458 1.063 4.362 0.873 1.301 1.146 0.721 0.988 1.620 1.108

5.527(d) 6.563 3.454(q) 3.471(q) 3.463 1.213(t) 1.213(t) 1.213(t) 1.213(t) 4.212(s),4.170(s) 1.232(d) 1.232(d) 1.232(d) 1.232(d) 1.232(d) 1.232(d) 1.232(d)

Fig. 1H

H type

B3LYP 6-3111G(d,p)

B3PW91 6-31G p

B3PW91 6-31G pp

B3PW91 6-3111G(d,p)

Experimental

11 12 14 15 14,15 17 18 19 17±19 21 23 24 25 26 27 28 23±28

N±H (i-propyl) N±H (ethyl) CH2 (ethyl) CH2 (ethyl) Avg. CH3 (ethyl) CH3 (ethyl) CH3(ethyl) Avg. CH (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) CH3 (i-propyl) Avg.

4.050 4.428 2.758 4.139 3.448 0.944 1.444 0.835 1.074 4.382 0.807 1.179 1.293 0.641 1.582 1.038 1.090

3.565 3.775 4.035 2.651 3.343 0.839 0.864 1.375 1.026 4.322 0.873 1.308 1.145 0.755 0.969 1.545 1.099

3.957 4.208 4.174 2.731 3.452 1.366 0.887 0.861 1.038 4.498 1.307 1.169 0.897 0.991 0.787 1.562 1.119

4.026 4.331 4.170 2.709 3.439 1.348 0.922 0.802 1.024 4.433 0.800 1.258 1.125 1.531 0.619 0.998 1.055

5.527(d) 6.563 3.454(q) 3.471(q) 3.463 1.213(t) 1.213(t) 1.213(t) 1.213(t) 4.212(s),4.170(s) 1.232(d) 1.232(d) 1.232(d) 1.232(d) 1.232(d) 1.232(d) 1.232(d)

Z. Meng, W.R. Carper / Journal of Molecular Structure (Theochem) 588 (2002) 45±53

Fig. 1H

Z. Meng, W.R. Carper / Journal of Molecular Structure (Theochem) 588 (2002) 45±53

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Table 4 Atrazine dimer 1H NMR chemical shifts (ppm) (d ˆ doublet, t ˆ triplet, q ˆ quartet, s ˆ septuplet) Fig. 2H

H type

HF 631G p

HF 631G pp

B3LYP 631G p

B3LYP/6-31G p// B3LYP/6-31G pp

Experimental

12 13 14 15 19 20 21 22 20±23 23 24 25 26 27 28 23±28 29 31 32 29,31,32 35 36 38 47 49 51 52 53 54 55 56 51±56

N±H (ethyl) N±H (i-propyl) CH2 (ethyl) CH2 (ethyl) CH (i-propyl) CH3 ±(ethyl) CH3 ±(ethyl) CH3 ±(ethyl) Avg. CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) Avg. CH3 ±(ethyl) CH3 ±(ethyl) CH3 ±(ethyl) Avg. CH2 (ethyl) CH2 (ethyl) N±H(ethyl) N±H(i-propyl) CH(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) CH3(i-propyl) Avg.

2.996 3.672 3.043 2.920 3.897 1.218 1.088 0.988 1.219 1.027 1.829 1.149 0.916 0.908 1.607 1.239 1.169 1.253 1.237 1.219 2.989 3.147 3.226 3.245 3.742 0.899 1.162 1.323 0.886 0.672 1.895 1.140

3.362 3.950 2.993 2.880 3.909 1.164 1.025 0.951 1.163 0.969 1.771 1.087 0.869 0.845 1.535 1.179 1.191 1.169 1.130 1.163 2.956 3.115 3.570 3.591 3.731 0.853 1.098 1.251 0.862 0.631 1.858 1.092

3.208 4.094 3.281 3.142 4.318 1.164 1.090 0.969 1.127 1.020 1.984 1.098 0.933 0.978 1.429 1.240 1.141 1.104 1.137 1.127 3.384 3.583 3.536 3.547 4.096 0.873 1.158 1.296 0.972 0.643 1.855 1.133

3.703 4.586 3.359 3.167 4.587 1.178 1.086 0.969 1.131 1.023 2.037 1.036 0.923 1.022 1.360 1.233 1.112 1.134 1.147 1.131 3.583 3.617 3.960 3.949 4.096 0.873 1.158 1.296 0.972 0.643 1.855 1.133

6.247 5.211(d) 3.384(q) 3.367(q) 4.073(s) 1.185(t) 1.185(t) 1.185(t) 1.185 1.204(d) 1.204(d) 1.204(d) 1.204(d) 1.204(d) 1.204(d) 1.204 1.185(t) 1.185(t) 1.185(t) 1.185 3.384(q) 3.367(q) 6.247 5.211(d) 4.035(s) 1.185(t) 1.185(t) 1.185(t) 1.185(t) 1.185(t) 1.185(t) 1.185

and 4.170 ppm …JH±C±C±H ˆ 6:4 Hz† and the weaker pair of overlapping septuplets have centers at 4.073 and 4.035 ppm. As is the case throughout this report, we assign the more intense peaks to the atrazine monomer and the weaker peaks to the atrazine dimer. The ethyl CH3 Hs appear as a strong triplet centered at 1.213 ppm …JH±C±C±H ˆ 7:2 Hz† and a weaker triplet centered at 1.185 ppm. Similarly, the i-propyl CH3 Hs exist as a strong doublet centered at 1.232 ppm …JH±C±C±H ˆ 6:4 Hz† and a weaker doublet centered at 1.204 ppm. The stronger CH3 H peaks are assigned to the atrazine monomer (1.213 and 1.232 ppm) and the weaker peaks (1.185 and 1.204 ppm) to the atrazine dimer. The calculated (GIAO) and experimental 1H NMR

chemical shifts for HF and DFT versions of the atrazine dimer are given in Table 4. The N±H (i-propyl) Hs undergo a considerable increase in chemical shift from the monomer to the dimer in all cases. This increase in chemical shift is considerably greater than the increase in chemical shift for the N±H (ethyl) Hs, resulting in a reversal of the order indicated for the atrazine monomers. A possible explanation is that solvation is more extensive in atrazine monomers and that the i-propyl±N±Hs are somewhat protected from the bulk solvent. Calculated chemical shifts of the remaining Hs in Table 4 match reasonably well with the experimental results. The methyl Hs are averaged out assuming a rapid rotation about the C±C axis in each case. There

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Z. Meng, W.R. Carper / Journal of Molecular Structure (Theochem) 588 (2002) 45±53

Table 5 Total energies (Hartrees) (d ˆ doublet, t ˆ triplet, q ˆ quartet, s ˆ septuplet) Method

Atrazine p

HF/6-31G HF/6-31G pp B3LYP/6-31G p B3LYP/6-31G pp

21042.876951 21042.903605 21047.286520 21047.309921

(Atrazine)2 22085.758756 22085.812065 22094.578289 22094.624553 DH (kcal/mol)

HF/6-31G p HF/6-31G pp B3LYP/6-31G p B3LYP/6-31G pp

23.05 23.05 23.29 22.96

are at least two C±H (i-propyl) values that can be explained by the existence of atrazine monomers and atrazine dimers in solution. 4.7. Comparison of methods and basis sets used in the GIAO NMR calculations The RMS (root mean square) deviations given in Table 1 indicate that the HF method provides a somewhat better ®t for 13C chemical shifts than the DFT (B3LYP) method. The exception is the B3PW91/63111G(d,p) calculation that is better than the HF/63111G(d,p) calculation of the atrazine dimer. In Table 2, the HF method provides the best ®t with the experimental 13C chemical shifts for the atrazine dimer. With the exception of the N±H Hs, both the HF and DFT methods are in reasonable agreement with the 1H spectrum of atrazine given in Tables 3 and 4. 4.8. Comparison of total energies for the atrazine and atrazine dimers Table 5 contains the total energies of the four atrazine dimers and the separate atrazine monomers. The enthalpy of reaction for each atrazine dimer is approximately 23.0 kcal. This result is suggestive of weak dimer formation in the gas phase and is consistent with the NMR results in the liquid phase. 5. Conclusions The gas phase ab initio models of atrazine and

atrazine dimers calculated with the HF and DFT methods produce structures that support the concept of hydrophobic interactions between atrazine molecules. Application of the GIAO method yields 13C and 1H chemical shifts that are in agreement with experimental spectra with the exception of multiple N±H Hs. Both previous and present NMR data support the formation of multiple atrazine species including monomers and dimers in solution. Finally, total energy calculations support the concept of weak atrazine dimer formation in the gas phase. In general, it appears likely that atrazine dimers exist in the liquid state as well as in the gas phase.

Acknowledgements The authors wish to thank Professors P.G. Wahlbeck and C.K. Larive for numerous helpful discussions. Financial support was provided by EPA Grant R 827589-01-0 to W.R.C.

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