9524 DOI: 10.1021/la9049132 Langmuir 2010, 26(12), 9524–9532Published on Web 02/25/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Sulfonamide Antibiotics Embedded in High Silica Zeolite Y:

A Combined Experimental and Theoretical Study of Host-Guest

and Guest-Guest Interactions

Ilaria Braschi,†,‡,* Giorgio Gatti,‡ Geo Paul,‡ Carlo E. Gessa,† Maurizio Cossi,*,‡ andLeonardo Marchese‡

†Dipartimento di Scienze e Tecnologie Agroambientali, Universit�a di Bologna, Viale G. Fanin 44, 40127Bologna, Italy, and ‡Dipartimento di Scienze e Tecnologie Avanzate, Centro Interdisciplinare Nano-SiSTeMI,

Universit�a del Piemonte Orientale “Amedeo Avogadro”, Viale T. Michel 11, 15121 Alessandria, Italy

Received January 7, 2010. Revised Manuscript Received February 12, 2010

A combined experimental and computational study of the interactions of three sulfonamides;sulfadiazine,sulfamethazine, and sulfachloropyridazine;embedded into the cages of high silica zeolite Y is here proposed. Forall host-guest systems, the close vicinity of aromatic rings with zeolite framework was evidenced by multidimensionalandmultinuclear (1H, 13C, 29Si) SS-NMRmeasurements. Host-guest and guest-guest interactionswere also elucidatedby in situ FTIR spectroscopy and confirmed by ab initio computational modeling. Single molecules of sulfamethazineand sulfachloropyridazine were stabilized inside the zeolite cage by the vicinity of methyl and amino groups,respectively. Sulfadiazine is present in both monomeric and dimeric forms. Multiple weak H-bonds and van der Waalstype interactions between organic molecules and zeolite are responsible for the irreversible extraction from water of allthe examined sulfa drugs.

Introduction

Sulfonamide antibiotics (sulfa drugs), a class of synthetic sulfa-nilamide derivatives discovered and tested in the 1930s by theNobel laureate G. Domagk,1 are nowadays widely used for thetreatment of infections in human therapy,2 livestock production,and aquaculture.3 These antibiotics are active as competitiveinhibitors of p-aminobenzoate in the folate synthesis4 and areknown to induce high level of resistance through a bypassmechanism.5 Growing resistance means that once effective andcheap antibiotic treatments for infection have been lost.6

In soil and water compartments, sulfa drugs exist mainly inanionic form due to the pKa value of sulfonamide group (pKa

5.0-7.5).7,8 The use of large amounts of nonbiodegradablesulfonamides, which directly reach water bodies through hospitaland fish farming waste waters,9 ensures that they remain in theaquatic bodies, exerting their selective pressure for long periods oftime. Bacterial flora in the environment surrounding aquaculturesites and sewage sludges contain an increased number of antibiotic-resistant bacteria.10,11 Nevertheless, no sorbents with specific adsorp-tion potential and favorable kinetics have been identified to date.

Zeolites, characterized by three-dimensional networks of chan-nels and cavities ofmolecular dimensions, represent a valid solution

for these purposes: they act as sorbents whose selectivity and acti-vity depend on their structure and chemical composition.

A highly dealuminated zeolite Y has been proposed to irrever-sibly remove high amount of sulfonamide antibiotics (up to 25 gfor 100 g of zeolite) with very favorable kinetics (adsorptionhigher than 90% in less than 1 min) from highly polluted water.12

The embedding of sulfonamides inside zeolite pores was revealedby unit cell parameter variations and structural modificationsobtained by X-ray structure analysis carried out using the Rietveldmethod on the exhausted zeolite.12

The present paper is focused on the study of guest-guest andhost-guest interactions between three sulfonamides (sulfadiazine,sulfamethazine, and sulfachloropyridazine) and dealuminatedzeolite Y by a multidisciplinary experimental approach includingSS-NMR and FTIR spectroscopy and ab initio computationalmodeling. The key factors driving the adsorption process will beenlightened.

Methodologies

A. Materials. Sulfadiazine (4-amino-2-N-pyrimidinylbenze-nesulfonamide, SD), sulfamethazine (4-amino-N-(4,6-dimethyl-2-pyrimidinyl)benzenesulfonamide, SM), and sulfachloropyrida-zine (4-amino-N-(6-Cl-3-pyridazinyl)benzenesulfonamide, SC) werepurchased as analytical standards by Dr. Ehrenstorfer GmbH(Germany) with a purity of 99.5%, 99.5%, and 98.0%, respec-tively. Sulfonamides chemical structures are shown in Table 1.

Highly dealuminated zeolite Y with 200 SiO2/Al2O3 molarratio and specific surface area of 750 m2 g-1 was purchased inits protonated form (code HSZ-390HUA) from the Tosoh Corp.(Japan). The zeolite was charged at themaximal amount of sulfo-namides exposing the sample to subsequent adsorption cycles30 min each at room temperature (RT) in the presence of a sulfo-namideaqueous solutions atmaximal solubility (72, 136, and174μM

*Corresponding author: e-mail: ilaria.braschi@unibo.it; maurizio.cossi@mfn.unipmn.it.(1) Domagk, G. Dtsch. Med. Wochenschr. 1935, 61, 250.(2) Hemstreet, B. A. Pharmacotherapy 2006, 26, 551.(3) Sarmah, A. K.; Meyer, M. T.; Boxall, A. B. Chemosphere 2006, 65, 725.(4) Bayly, A. M.; Macreadie, I. G. Trends Parasitol. 2002, 18, 49.(5) Acar, J.; Rostel, B. Rev. Sci. Tech. 2001, 20, 797.(6) Livermore, D. M. Lancet Infect. Dis. 2005, 5, 450.(7) Qiang, Z.; Adams, C. Water Res. 2004, 38, 2874.(8) Ishihama, Y.; Nakamura, M.; Miwa, T.; Kaijma, T.; Asakawa, N. J. Pharm.

Sci. 2002, 91, 933.(9) Halling-Sorensen, B.; Nors Nielsen, S.; Lanzky, P. F.; Ingerslev, F.; Holten

Lutzhoft, H. C.; Jorgensen, S. E. Chemosphere 1998, 36, 357.(10) S�eveno, N. A.; Kallifidas, D.; Smalla, K.; van Elsas, J. D.; Collard, J. M.;

Karagouni, A. D.; Wellington, E. M. H. Rev. Med. Microbiol. 2002, 13, 15.(11) Cabello, F. C. Envir. Microbiol. 2006, 8, 1137.

(12) Braschi, I.; Blasioli, S.; Gigli, L.; Gessa, C. E.; Alberti, A.; Martucci, A.J. Hazard. Mater. 2010, DOI 10.1016/j.jhazmat.2010.01.006.

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for SD, SM, and SC, respectively) with a zeolite/antibiotic solu-tion ratio of 1 mg/2 mL.12 The maximal adsorption capacity wasreached after eight cycles and accounted for 16%, 20%, and 25%of SD, SM, and SC, respectively.12 Sulfonamide-zeolite werefinally air-dried and used for spectroscopic analysis.

The amount ofwater adsorbed and the concentration of silanolgroups into bare zeolite was measured by TGA analysis perfor-med on a Setaram SETSYS Evolution instrument under oxygen(gas flow 100 mL min-1), heating the samples up to 1273 K with5 K min-1 rate.

B. Nuclear Magnetic Resonance Spectroscopy. Solidstate NMR (SS-NMR) spectra were acquired on a BrukerAVANCE III 500 spectrometer and a wide bore 11.7 T magnetwith operational frequencies for 1H, 29Si, and 13Cof 500.13, 99.35,and 125.77 MHz, respectively. A 4 mm triple-resonance probewithMASwas employed in all the experiments. The samples werepacked on a Zirconia rotor and spun at a MAS rate between 10and 15 kHz. The magnitudes of radio-frequency fields, υrf, were100 and 42 kHz for 1H and 29Si, respectively. The relaxationdelays, d1, between accumulations were between 1 and 120 s for1H and 29Si MAS NMR, respectively. For the 13C{1H} CPMASexperiments, the magnetic fields υrf

H of 55 and 28 kHz were usedfor initial excitation and decoupling, respectively. During the CPperiod the 1H RF field υrf

H was ramped using 100 increments,whereas the 13C RF field υrf

C was maintained at a constant level.During the acquisition, the protons are decoupled from thecarbons by using a TPPM decoupling scheme. A moderateramped RF field υrf

H of 62 kHz was used for spin locking, whilethe carbonRF fieldυrf

Cwasmatched toobtain optimal signal andthe CP contact time of 2 ms was used. For the FSLG HETCORexperiment,13-15 the samplewas packed in a ZrO2HRMAS rotorand the volumewas restricted to themiddle of the rotor.A protonυrf of 100 kHz was used for FSLG decoupling and duringacquisition (tppm15 decoupling). A moderate ramped RF fieldυrf

H of 62 kHzwas used for spin locking, while the siliconRF fieldυrf

Si was matched to obtain optimal signal. For the FSLGHETCOR, experiments were recorded with 4K scans, 32 rows,MAS of 12.24 kHz, and a CP contact time of 0.2 ms. The sample

temperature was controlled by the BrukerBVT 3000 digital varia-ble temperature unit. Temperature calibration was performedusing Pb(NO3)2 as a 207Pb MAS NMR chemical shift thermo-meter. All chemical shifts are reported using δ scale and areexternally referenced to adamantane at 38.48 ppm.

Sulfonamide-zeolite samples were outgassed at 423 K (5 Kmin-1 heating rate) for 3 h in order to remove adsorbedwater andtransferred into the NMR rotor in a glovebox to limit the sampleremoistening.

C. FTIR Spectroscopy. Infrared spectra were collected on aThermoElectronCorp. FTNicolet 5700 spectrometer with 4 cm-1

resolution. Self-supporting pellets of zeolite Y and sulfonamide-zeolite sampleswere obtainedwith amechanical press at ca. 7 tonscm-2 and placed into an IR cell equipped with KBr windowspermanently attached to a vacuum line (residual pressure: e1 �10-4 Torr; 1 Torr= 133.33 Pa), allowing all treatments and D2Oadsorption experiments to be carried out in situ. Air-driedsulfonamide-zeolite sampleswere outgassed at 423K (5Kmin-1

heating rate) for 3h inorder to removeadsorbedwater beforeFTIRspectroscopic analysis. FTIR spectra of pure sulfonamides inCH2Cl2 solutionwere performed in aNaCl standard cell for liquids.

D. Models and ab InitioCalculations.All calculationshavebeen performed at the density functional level (DFT) withTURBOMOLE V5.10 package.16 The GGA functional BLYP17,18

and the hybrid functional B3LYP19 were used, along with aGaussian-type atomic orbital basis formed by Hay and Wadtbasis set20-22 for Si valence shell, and 6-31G basis set23 pluspolarization functions24 (resulting in the so-called 6-31G(d,p) set)for all the other elements. Hay and Wadt pseudopotentials20-22

were used to account for Si core electrons in order to reduce thecomputational burden.

The geometry optimizations were performed by running a firstcalculation with the pure GGA functional, exploiting the highly

Table 1. Structures and Chemical Characteristics of Sulfonamide Antibiotics under Investigation (DFT: Density Functional Theory)

(13) Vega, A. J. J. Am. Chem. Soc. 1988, 110, 1049.(14) Fyfe, C. A.; Zhang, Y.; Aroca, P. J. Am. Chem. Soc. 1992, 114, 3252.(15) vanRossum, E. J.; Forster, H.; deGroot, H. J. M. J. Magn. Reson. 1997,

124, 516.

(16) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys. Lett.1989, 162, 165.

(17) Becke, A. D. Phys. Rev. A 1988, 38, 3098.(18) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.(19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.(20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.(21) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.(22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.(23) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.(24) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007.

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efficient RI (resolution of the identity)25 procedure, and thenrefining the optimization with the hybrid B3LYP functional.Harmonic vibrational spectra were calculated at the same levelfor the minimum-energy conformations.

Results and Discussion

The structure of crystalline faujasite Y was obtained fromRietveld refinement12 (CIF file available as Supporting Infor-mation). The unit cell contains eight cages of approximatelytetrahedral shape, as illustrated in Figure 1. Each cage has fourlarge access windows, formed by 12-membered T rings, while thecage walls are formed by 4- and 6-membered T rings. Thediameter of such a cavity is∼16 A, safely allowing the embeddingof at least one sulfonamide molecule.

Thermogravimetric analysis (TGA) of bare zeolite was per-formed in order to define the amount of adsorbed water andsilanol groups per zeolite unit cell (Figure 1S in SupportingInformation). The weight loss from RT to 473 K in the zeolitedue to adsorbed water accounts for ca. 1% of the initial weight,whereas the weight loss of ca. 2% in the range 473-1273K is dueto water formed by condensation of silanol groups located eitheron the external surface of the zeolite crystal or in internal frame-work defects.26 Taking into account both silanol and sulfo-namide concentration, the following unit cell compositions canbe defined:

½Si186O360ðOHÞ24�SD7:71

½Si186O360ðOHÞ24�SM8:26

½Si186O360ðOHÞ24�SC10:09

In the composition for simplicity, (i) the content of adsorbedwater is not reported because it is dependent on temperature andhumidity, (ii) a single silicon vacancy has been considered togenerate four hydroxyl groups, and (iii) aluminum atoms are notconsidered, their concentration being negligible with respect tosilicon atoms (200 SiO2/Al2O3 molar ratio). Since the unit cellcontains eight cages, the compositions reported above indicatethat one drug molecule is embedded in each cage, on average.

The nature of the interactions between sulfa drugs and zeolitehas been further investigated by SS-NMR, FTIR, and ab initiocalculations.

A. SS-NMR. Standard CPMAS conditions were used toacquire the 13C CPMAS NMR spectra (Figure 2) on puresulfonamides and zeolite-drug host-guest systems. Two of theadsorbed samples, namelyY-SC andY-SM, present considerablybroader peaks compared to pure sulfa drugs. This anisotropicbroadening is consistent either with a distribution of environ-ments for adsorbed sulfa drugs or with a reduced mobility of themolecules in the cage. Surprisingly, on the other hand, the Y-SDsystem shows very good resolution with sharp lines which arerarely observed in such solid systems: conversely to what is statedfor the other drugs, this can be attributed either to a greatermobility or to amore regular packing of the adsorbed sulfadiazinemolecules.Clearly thepackingorders inside the cages are differentfrom the crystalline state, as shown by the different NMR signalsof SD and Y-SD in Figure 2.

Since by simple CPMASmeasurements it is difficult to enlight-en the interactions present in such systems, 2D heteronuclearcorrelation (HETCOR) experiments were performed. Such experi-ments make use of the dipolar interactions present betweenabundant nuclei (1H) and nonabundant ones (29Si) to generateinformation about the proximity between these nuclei. Figure 3shows the 1H-29Si HETCOR data for different drug loadedzeolite systems. A very short cross-polarization contact time(0.2 ms) has been used in all experiments to map the correlationsbetween the framework silicon sites and drug protons.

The 29SiMASNMRofpure zeolite reported in Figure 3a alongthe 29Si dimension shows a single sharp peak characteristic of alone Q4 Si site,27 indicating high crystallinity and ordering.Whereas the broadening andupfield shift of the silicon resonancesin the zeolite after drug adsorption are due to host-guestinteractions between the sulfa drugs and zeolite framework asshown in Figure 3b.

In the 1HMASNMR (Figure 3d, 1H dimension), the signal at4.9 ppm is assigned to water, partly readsorbed during themeasurements. Very weak signals of silanol groups at 1.8 ppmare also found, testifying their presence in limited amount inagreement with TG and FTIR measurements.

It is interesting to note that in Y-SD system the 1H signal ofsulfonamide group (Figure 3d) falls at significantly downfield(11 ppm) than in the other drug-zeolite systems, where this reso-nance is expected at 9.4 ppm,28 but it is masked by the aromaticrings signals. Sucha shift could bedue either to a greater acidity or

Figure 1. (a)Unit cell of dealuminated zeoliteY structure, (b) single cage extracted from the unit cell, and (c) cagewith Si valencies saturatedby OH groups, used in the computational models.

(25) Eichkorn, K.; Treutler, O.; Ohm, H.; Haser, M.; Ahlrichs, R. Chem. Phys.Lett. 1995, 242, 652.(26) (a) Zecchina, A.; Bordiga, S.; Spoto,G.;Marchese, L.; Petrini, G.; Leofanti,

G.; Padovan, M. J. Phys. Chem. 1992, 96, 4985. (b) J. Phys. Chem. 1992, 96, 4991.

(27) Klinowskji, J.; Thomas, J. M.; Audier, M.; Vasudevan, S.; Fyfe, C.;Hartman, J. S. J. Chem. Soc., Chem. Commun. 1981, 570.

(28) Hunger, M. Catal. Rev.;Sci. Eng. 1997, 39, 345.

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to the existence ofH-bonds: since sulfadiazinehas an intermediateacidity among the sulfa drugs (Table 1), it is likely that in adsor-bed form amide proton is involved in H-bonding. This result is inagreement with FTIR data (vide infra), showing that Y-SD amideproton absorbs at a frequency (3141 cm-1) significantly lowerthan that observed for Y-SM and Y-SC (3300-3400 cm-1).

In the 1H-29Si HETCORNMR experiments, the most intensecross-peaks observed for all the three systems (Figure 3) are ascri-bed to a correlation between aromatic protons and framework Siatoms (in the range from 6 to 10 ppm in 1H and from -107 to-108 ppm in 29Si dimension). In addition, a correlation betweensulfamethazine methyl protons and Si atoms is observed (crosspeak at 2.6/-107 ppm). In the Y-SC system a weak correlation isobserved at ca. 4 and -108 ppm in the 1H and 29Si dimensions,respectively, which is attributed to amine protons. Interestingly,all cross peaks inY-SC system are upfield shifted in the 29Si dimen-sion as compared to the other two systems, possibly depending ona more pronounced aromatic ring current effects. Different rea-sons can explain the weakest correlation observed in the Y-SDsystem: (i) the lowest loading of sulfadiazine (16 g in 100 g ofzeolite compared to 20 and 25 g for sulfamethazine and sulfa-chloropyridazine, respectively),12 (ii) a highermobility of themole-cules inside the cages, or (iii) a greater distance from the cage walls.

Since in the above discussion the mobility of adsorbed drugsinside the cages has been invoked to explain some spectral fea-tures, this pointwas further investigatedby temperature-dependent13CCPMASNMR.As the temperature rises, the chemical inequi-valencies arising from the various molecular conformations areexpected to be averaged by the increased molecular motions.Figure 4 shows variable temperature 13C CPMASNMRofY-SDand Y-SC recorded at 223 K, RT, and 323 K.

In Y-SC spectra the spinning sidebands disappear by increas-ing the temperature, as molecular motions disrupt the molecular

orientation and average the chemical shift anisotropy. On thecontrary, in the Y-SD system the spinning sidebands remainalmost unaffected even at 323 K, showing that no chemical shiftanisotropy averaging occurs: this can be ascribed to the pre-sence of specieswith reducedmobility inside the cages. Besides thetemperature-dependent spectra, we note that the behavior ofadsorbed sulfadiazine appears different from the other drugs indiverse experiments: (i) better resolved signals in 13CMASNMRmeasurements and (ii) the occurrence of intermolecular H-bondscausing the downfield shift of Y-SD amide proton in the 1HMASNMR ascribed to intermolecular H-bonds. All these features canbe explained by assuming that sulfadiazine forms dimers insidethe zeolite cage: this hypothesis is supported by the results dis-cussed in the following IR and computational sections. As amatter of fact, the bulky sulfadiazine dimers would present a verylow mobility in the cage, and also a reduced number of possibleorientations, resulting in a greater order.

On the other hand, the spectrum recorded for the Y-SD systemat 223 K shows the presence of broader components underneaththe sharp peaks, revealing a fraction of molecules whose motionsare limited at low temperature. This indicates that a part of theadsorbed sulfadiazine is still in the monomeric form and behaveslike other sulfa drugs.B. FTIR. Sulfonamides in CH2Cl2 Solution. The inter-

pretation of IR spectra was facilitated by comparing the observedabsorptions with the theoretical frequencies computed for the threeisolated molecules: the simulated harmonic spectra are shown

Figure 2. 13C CPMAS-NMR spectra of pure and adsorbed sulfa-diazine (SD and Y-SD, respectively), sulfamethazine (SM andY-SM), and sulfachloropyridazine (SC and Y-SC).

Figure 3. 2D 1H-29Si HETCOR NMR spectra of sulfadiazine(SD), sulfamethazine (SM), and sulfachloropyridazine (SC) adsor-bed into zeolite Y. (a) 29Si MAS NMR spectrum of pure zeolite,(b) 29SiMASNMRspectrumof sulfonamide loaded zeolite, (c) 2Dprojection of sulfonamide loaded zeolite in Si dimension, and(d) 1HMASNMRof sulfonamide loaded zeolite in 1H dimension.

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along with their experimental counterparts in Figure 5. Thecomputedharmonic frequencies are systematically overestimated,but it can safely be assumed that the spectral pattern is reproducedaccurately enough to allow the interpretation, especially whenanalogous molecules can be compared as in the present case. InFigure 5 the visual comparison is helped by downshifting thetheoretical frequencies in order to make the first peak for sulfa-chloropyridazine to coincidewith the corresponding experimentalabsorption, and this leads to a shift of 180 and 70 cm-1 in thehigh- and low-frequency region, respectively. The main featuresof the experimental spectra are well reproduced by the calcula-tions, especially in the highwavenumber regionwhere amineNH2

and sulfonamide NH stretching vibrations absorb. The mostimportant vibrational frequencies are listed in Table 2 for thethree sulfa drugs, with the assignments obtained by the compa-rison of theoretical and experimental results.

Noteworthy, the experimental asymmetric (νasym) and sym-metric (νsym) NH2 stretching vibrations are very similar for thethree molecules and are respectively found at 3501 cm-1 and inthe range at 3408-3406 cm-1. The corresponding harmonicvibrations in the calculated spectra are found at 3681-3668 and

3578-3567 cm-1, respectively. The stretching frequency of thesulfonamideNHgroup is significantly different for the three sulfadrugs falling at 3331, 3379, and 3383 cm-1 for sulfachloropyri-dazine, sulfadiazine, and sulfamethazine, respectively, and thisfollows the acidic character of the molecules (Table 1), with thelowest frequency for the most acidic group. The computedharmonic frequencies are respectively found at 3529, 3583, and3583 cm-1 for sulfachloropyridazine, sulfadiazine, and sulfa-methazine.

The in-plane amine bending mode (δNH2) for the three sulfadrugs is found at very similar frequencies (1624-1623 cm-1) andiswell reproduced by the computationalmodel (1696-1695 cm-1).As for the in-planeδNHmode of the sulfonamide group, only forsulfadiazine the calculations show an almost pure mode absorb-ing at 1499 cm-1: experimental absorption at 1454 cm-1. Thisvibration is coupled with heterocycle quadrant modes in sulfa-methazine (computed at 1638 cm-1, experimental at 1560 cm-1)and sulfachloropyridazine (computed at 1597 cm-1, experimentalnot clearly identified).

Sulfonamides Adsorbed into Zeolite Y. The IR spectrum ofpure zeolite Y was recorded after outgassing at 423 K (dottedcurve in Figure 6): it shows a narrow band at 3738 cm-1 and abroad absorption in the 3750-3000 cm-1 range due to free andH-bonded silanols, respectively, located either at external or atinternal defects of the zeolite framework.26 According to SS-NMR and TGA results, a limited number of defects (ca. 3 silanolgroups per cage on average) are present in this zeolite, with apossible effect on the adsorption process. In the spectrum of drugloaded zeolite (continuous black curve inFigure 6), however, onlya limited fraction of silanol groups seem to interact with theadsorbed drugs, as testified by the broad band shown by all thesamples in the 3750-3000 cm-1 range, overlapped to the stretch-ing bands of amine and sulfonamide groups (3500-3000 cm-1).

The NH2 stretching bands fall in the 3493-3463 cm-1 (νasym)and 3390-3359 cm-1 (νsym) ranges, with a more complex patternthan the molecules in CH2Cl2 (Figure 6 and Table 2). Thepresence of several components suggests that the amino groupexperiences the influence of different parts of the zeolite cage;all

Figure 4. 13C CPMAS-NMR spectra of Y-SD (A) and Y-SC (B)recorded at different temperatures (asterisk denotes spinning side-bands).

Figure 5. Infrared spectra of (a) sulfadiazine, (b) sulfamethazine,and (c) sulfachloropyridazine in CH2Cl2 solution (black line)compared to those computed in vacuum (red line). The wavenum-ber axis for the calculated spectra has been downshifted by 180 and70 cm-1 in the high- and low-frequency region, respectively (asteriskindicates bands whose shape and intensity cannot be recordedwith precision because strongly overlapped to the most intenseCH2Cl2 band at 1430 cm-1).

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having similar but low polarizing effects;and may account fordifferent orientations of the molecules in agreement with 13CCPMAS NMR findings. The NH2 stretching bands are slightlyshifted to lowerwavenumbers in comparison to the freemolecules(Δν e 45 cm-1) because of the formation of very weak H-bondswith oxygen atoms of the zeolite framework. Correspondingly,the bending frequency of the amino group (δNH2), 1629-1627cm-1, shows a very limited upward shift (Δδ e 6 cm-1) uponadsorption: this result is in full agreement with the H-bondingtheory, which predicts shifts to higher frequencies of the bendingmodes much more limited than the downward shift of thestretching vibrations.29

Similarly towhat is found for sulfonamides inCH2Cl2 solution,the sulfonamide νNH mode of all adsorbed molecules stronglyoverlaps to the amine νsymNH2, and this makes difficult its preciseidentification. The presence of both νsymNH2 and νNH in the3430-3330 cm-1 range was clearly identified by H/D isotopicexchange experiments (spectra not shown) because of the diffe-rent downward shift of the two vibrations. The deuterated sulfo-namides showed in fact distinct absorptions in the 2535-2490 and2490-2420 cm-1 ranges assigned to νND and νsymND2 vibra-tions, respectively. Table 3 reports on the experimental andcomputational results obtained for sulfadiazine, where it can beclearly seen that the νsymNH2 bands, due to sulfadiazine mole-cules oriented in different fashion inside the zeolite cages, have alarger shift to lower wavenumber than the νNH absorption(νNH/νND of 1.38 and 1.36, respectively).

According to the limitedmodifications of the νNH frequenciesupon adsorption, also the δNH frequencies are not significantlyshifted (see Table 2 and Figure 6). Moreover, for both sulfa-methazine and sulfachloropyridazine adsorbed into zeolite, theselow-frequency modes have components related to motions of theheterocycle ring at 1560 and 1552 cm-1, respectively.

The band observed at 3248 cm-1 in the spectra of all threeantibiotic-zeolite systems can be ascribed to the overtone of theNH2 bending vibration (2δNH2) whose fundamental is found at1629-1627 cm-1.30 This assignment is fully supported by the H/Dexchange experiments.

Table 2. Infrared Wavenumbers (cm-1) for Sulfonamides Computed in Vacuo, Experimentally Obtained in CH2Cl2 and Adsorbed into Zeolite Y

sulfadiazine sulfamethazine sulfachloropyridazine

vibrational modesc computed in CH2Cl2

adsorbedinto zeolite computed in CH2Cl2

adsorbedinto zeolite computed in CH2Cl2

adsorbedinto zeolite

νasymNH2 3670 3501 3493, 3469 3668 3501 3463 3681 3501 3490νsymNH2 3569 3406 3390, 3362 3567 3406 3385 3578 3408 3385νNH 3583 3379 3408

3178a 3145a 3583 3383 3400-3300 3529 3331 3400-33002δNH2 3248 3248 3248δNH2 1695 1623 1629 1696 1623 1628 1695 1624 1627νPh quadrant þ δNH2 1659 1597 1597 1659 1597 1597 1659 1596 1596νPh and het quadrant 1626 1568 1571ν het quadrant þ δNH 1638 1560 1560 1597 n.d.b 1552ν het quadrant 1646, 1620 1578 1580 1616 1550 1550δPh in-plane CH 1549 1504 1501 1549 1504 1502 1549 1504 1502def CH3 out-of-phase 1514 1469 1470δNH in-plane 1499 1454 1477a

δNH in-plane þ δ het CHin-plane

1467 1430 1428

δNH in-plane þ ν het CN 1418 1383 1385ν het C-amide N 1486 1437 1442 1470 1432 1432def CH3 in-phase 1436 1383 1383ν Ph sextant 1383 1345 1348ν Ph and het sextant 1382 1348 1347 1387 1360 1358

aFrequencies attributed to sulfadiazine dimer (see text and Table 3). bThis vibration has not been clearly identified in the experimental spectrum ofsulfachloropyridazine. cFor definition of ring vibrational modes see ref 30, pp 261-266.

Figure 6. Infrared spectra of sulfonamides in CH2Cl2 (blue lines)and adsorbed into zeoliteY (black lines): (a) sulfadiazine, (b) sulfa-methazine, and (c) sulfachloropyridazine. In gray dotted line theinfrared spectrum of pure zeolite. Dotted lines are traced in corres-pondence of νasym and νsymNH2 vibrations of sulfonamides inCH2Cl2 solution, which are very similar for the three molecules.

Table 3. Experimental and Calculated Wavenumbers of

Sulfadiazine in the Monomer Forma

νNH νND νNH/νND

modes exp calc exp calc exp calc

νasymNH2 3493 3670 2612 2713 1.34 1.353469 2593 1.34

νsymNH2 3390 3569 2462 2579 1.38 1.383362 2445 1.38

νNH monomer 3408 3583 2506 2623 1.36 1.37aThe experimental values are related to the molecules inside the

zeolite cage, whereas the calculated values are related to the moleculesoptimized in vacuo with the B3LYP functional.

(29) Novak, A. In Structure and Bonding (Berlin); Dunitz, J. D., Hemmerrich, P.,Holm, K. R. H., Ibers, J. A., Joergensen, C. K., Neiland, J. B., Reinen, D., Williams,R. J. R., Eds.; Springer-Verlag: Berlin, 1974; Vol. 18, pp 177-216.

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It is of note that the IR spectrum of Y-SD shows a specificfeature in the NH stretching region at 3145 cm-1, which is absentin the other two adsorbed sulfonamides. A band in this positioncan only be due to the sulfonamideNHgroup forming anH-bondof medium strength: the downward shift with respect to thefrequency of the sulfadiazine in CH2Cl2 solution (3379 cm-1) is234 cm-1. In parallel, an upward shift of δNH of 23 cm-1 (from1454 to 1477 cm-1) was found for adsorbed sulfadiazine. How-ever, in the optimized computed structures described in the nextsection (Figure 7) the distance between cage oxygen atoms andsulfadiazine amide nitrogen is too large (ca. 4 A) to explain suchan interaction. In fact, the νNH frequency at 3141 cm-1 isexpected for a H-bond with N 3 3 3X (O, N) distance lower than3 A.29,31 In addition, the oxygen atoms electron density of thenetwork of the high silica zeolite used in this work is too low toallow medium strength H-bonds. Therefore, it is proposed thatsulfadiazine forms dimers inside the zeolite cage where the amidehydrogenof amolecule is bound to an electrondonor of the other.The structures of possible dimers in vacuo and inside the zeolitecage are described in the computational section.

Other spectral changes as a consequence of drug adsorption aremainly related to an increased intensity of bands assigned to thearomatic ring vibrations (ν benzene quadrant and sextant respec-tively at 1597 and 1348 cm-1 for sulfadiazine and benzenequadrant at 1596 cm-1 and ν benzene and heterocycle sextantat 1358 cm-1 for sulfachloropyridazine). These spectral features

consist of a perturbation of ring dipole moments, and theyindicate an interaction of aromatic rings with the zeolite frame-work. In the same spectral region, a new band at 1353 cm-1 isvisible for Y-SM: the corresponding band in the pure antibioticspectrum is assigned to benzene and heterocycle sextant ν. Verylikely, the interaction of one or both the aromatic rings with thesorbentmakes the two vibrations uncoupled and the formation ofa new band occurs.C. Computational Modeling.As anticipated in the previous

section, the geometries of the three isolated sulfa drugs wereoptimized (Table 1) and the corresponding harmonic frequenciescomputed for comparison with the IR spectra recorded in CH2Cl2(Table 2). Then the three molecules were optimized inside thezeolite cage: the cage was modeled by a single cavity extractedfrom the crystalline structure,12 with the free valencies on siliconatoms saturated by outward pointing -OH groups (as shown inFigure 1c), so that the cage model was comprised by 48 Si atoms,72 O atoms in the cage wall, and 48 OH groups. During theoptimizations the cage atoms were kept fixed in their crystallinepositions. Figure 7 shows some pictures of the optimized drug/zeolite systems, also reporting the smallest distances between thedrug atoms and cage walls. The corresponding Cartesian coordi-nates are available as Supporting Information. Atomic partialcharges were computed with Mulliken partition scheme for iso-lated and adsorbed sulfonamides: very small variations werefound (below 0.1 au) upon adsorption, and no significant chargetransfer was detected between drugmoieties nor between drug andzeolite. All the organic heavy atoms are farther than 3 A from thecage, in agreement with the findings of XRPD patterns recordedfor the same systems;12 the aromatic rings are always arranged

Figure 7. Different perspectives of DFT optimized structure of sulfonamides embedded in zeolite Y cage (oxygen: red; hydrogen: white;nitrogen: blue; carbon: green; chlorine: purple; sulfur: bright yellow; silicon: orange). The distances (A) of the drug atoms closest to cagewallsare also reported.

(30) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. In Introduction to Infrared andRaman Spectroscopy, 3rd ed.; Academic Press: New York, 1990; p 340.(31) Gieck, C.; Bisio, C.; Marchese, L.; Filinchuk, Y.; da Silva, C. E.; Pastore,

H. O. Angew. Chem., Int. Ed. 2007, 46, 8895.

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parallel to the walls, and the distances are compatible with thepolarization transfer observed in the 2D NMR experiments andwith the perturbation of ring dipole moments observed in FTIR.

For all the adsorbed molecules, the optimized distances bet-ween amine hydrogen atoms and cage oxygen atoms are largerthan 2.5 A, indicating very weak H-bonding, in agreement withthe small red shift observed for theNH2 stretching with respect tothe free molecules. Sulfonamide proton is found in all systems atlarge distance from the cage oxygen atoms (about 3 A), and theNH bond is oriented toward the cage access window (mainly inY-SCandY-SM): these geometrical features allowonly extremelyweak, if any, H-bonding. This finding is consistent with the IRspectra of Y-SC and Y-SM, where the NH stretching frequenciesare practically unchanged upon adsorption. On the other hand, asdiscussed above, the Y-SD IR spectrum shows a marked red shiftfor this frequency, which can only be explained with a mediumstrength H-bond involving the sulfonamide hydrogen, so that wepropose that a dimer is formed inside the cage.

Three sulfadiazine dimer conformations were optimized in thegas phase to account for all the possible H-bonds involving thesulfonamide proton (sketched in Chart 1): in the structure labeledNHN each proton is bound to the partner pyrimidine nitrogen, inNHO to the partner sulfonic oxygen, in MIX one NH 3 3 3O andoneNH 3 3 3Nbonds are formed.Themost favorable dimerizationenergywas computed forNHN:-15.7 kcalmol-1, with respect tothe separate monomers, while NHO and MIX energies are -8.0

and -11.7 kcal mol-1, respectively. Then the harmonic spectrawere simulated for the three dimers: the sulfonamideNHstretchingfrequency resulted red-shifted by 405 cm-1 for NHN and160 cm-1 for NHO. The experimental shift is 234 cm-1, ifcompared with the monomer in CH2Cl2 solution, or 253 cm-1,if compared with the monomer isolated into the zeolite cage(Table 2). The computed harmonic vibration of NHN dimer iscloser to the experimental value (3178 compared to 3145 cm-1)than that of themonomer (3583 compared to 3408 cm-1), and thisaccounts for the different weight of the anharmonic corrections inthe two cases, as usual when a H-bonded proton is compared to afree one. In the MIX conformation, on the other hand, the NHstretching was separated in two distinct absorptions, red-shiftedby 148 and 413 cm-1 with respect to the monomer: this featureis incompatible with the observed spectra, so that the MIX con-formation was rejected.

The next step was to embed NHN and NHO dimers into thezeolite cage already used for studying the monomer adsorptionand rerun the optimizations: while the starting points (with thedimers in their gas phase conformation)were clearly too crowded,with several near-contacts between atoms, during the optimiza-tions the molecules were able to rearrange in order to fit the cagequite smoothly (Figure 8). The embedded NHN dimer resultedsomehow distorted with respect to the isolated structure, but theintermolecularH-bondmoiety was almost unchanged, so that theNH stretching frequency is expected to be close to that computed

Figure 8. Different perspectives ofDFToptimized structure of sulfadiazine dimer embedded in zeoliteY cage (oxygen: red; hydrogen: white;nitrogen: blue; carbon: green; sulfur: bright yellow; silicon: orange).

Chart 1. Possible H-Bondings in Sulfadiazine Dimers

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in the gas phase.On the other hand, during the optimization in thecage the NHO dimer was broken with a considerable loss of theintermolecular H-bonds. These results led us to conclude thatsulfadiazine can dimerize in the NHN structure inside the zeolite,leading to a satisfactory interpretation of the IR spectra. Recallthat, as anticipated above, the presence of dimers can also explainsome of the SS-NMR features observed for Y-SD and also justifythe reduced loading of this drug in zeolite,12 apparently in contrastwith its lower steric hindrance, compared to sulfamethazine andsulfachloropyridazine: in fact, the formation of dimers could blockthe access of further molecules inside the zeolite crystals.

Conclusions

Highly dealuminated zeolite Y has proven to adsorb veryefficiently from water three sulfonamide molecules largely usedin human and animal therapy (sulfadiazine, sulfamethazine,sulfachloropyridazine): the structure of drug/zeolite adsorbatesand the interactions ruling the adsorption process were elucidatedwith a combination of SS-NMR, FTIR, and computationalmethods. Previous X-ray powder diffraction (XRPD) studiesdemonstrated that the sulfonamide molecules are actuallyembedded inside the zeolite cages and gave some insights aboutthe distances between organic atoms and inorganic framework.Here it is proposed for the first time a detailed study of thesulfonamides conformation inside the zeolite cage and of thepossible host-guest interactions.

2D HETCOR NMR spectra show that polarization is trans-ferred from all the hydrogen atoms to the zeolite silicon atoms: inparticular, the aromatic rings are expected to be arranged parallelto the cage walls at a distance compatible with previous XRPDresults. Embedded sulfadiazine exhibits a peculiar behavior:CPMAS resonances are more resolved, and HETCOR polariza-tion transfer is lower than in the other systems, either due to ahighermobility or to a greater orientation order inside the zeolite.The latter alternative is favored by temperature-dependent CPMASNMR, showing that the spin anisotropies are not averaged bymolecular motions even at high temperature, except for a minorfraction of the sample. In addition, the signal of embedded sulfa-diazine sulfonamide proton is downfield shifted, suggesting theoccurrence of intermolecular H-bonding.

FTIR spectra havebeen completely resolved for the three drugsin CH2Cl2, also thanks to the comparison with theoreticalharmonic frequencies. The interpretation of vibrational spectrafor the adsorbed drugs, supported by theoretical calculations

and by isotopic substitution experiments, shows that very weakH-bonds establish between amine protons and zeolite oxygenatoms and that the aromatic ring dipole moments are deeplyperturbed by the interaction with the zeolite framework.

The FTIR spectrum of adsorbed sulfadiazine presents aspecific feature in the sulfonamide stretching region, confirmingthat the sulfonamide proton is engaged in a medium-strengthH-bond: the presence of sulfadiazine dimers inside the zeolite isproposed to explain this feature along with the NMR findings.Three different dimer conformations were simulated and theirharmonic frequencies computed, and then the structures werereoptimized inside a zeolite model cage. The computationalresults, compared to the recordedFTIR spectrum, led to concludethat sulfadiazine forms dimers inside the zeolite, where eachpartner sulfonamide proton is bound to the other’s pyrimidinenitrogen.

The formation of dimers, whichmay limit pore access, explainswhy sulfadiazine is adsorbed in lower amount in high silica zeoliteY, especially if compared with sulfachloropyridazine. It wastherefore inferred that both host-host and host-guest interac-tions should be taken into account in rationalizing the adsorptioncapacity of microporous systems toward complex organic mole-cules.

The most relevant finding of this study is that the adsorptionprocess is driven by multiple interactions due to weak H-bondingbetween sulfa drug amine protons and lattice oxygen atoms andto hydrophobic interactions between the aromatic rings andzeolite cage walls. These interactions are strong enough to ensurethe irreversible extraction from water of all the examined sulfadrugs. The present multidisciplinary study paves the way forfurther investigations on this topic, which is relevant for environ-mental, health, and chemical issues.

Acknowledgment. The authors thank Dr. Annalisa Martucciand Prof. Alberto Alberti for fruitful discussions on structuraldetails. G.G. and G.P. acknowledge Regione Piemonte for apostdoc fellowship. The financial support from “FondazioneCompagnia di San Paolo” for the acquisition of Bruker AVANCEIII 500 SS-NMR spectrometer is gratefully acknowledged.

Supporting Information Available: Cartesian coordinatesfor sulfadiazine, sulfamethazine, and sulfachloropyridazine;thermogravimetric analysis of pure zeolite Y. This material isavailable free of charge via the Internet at http://pubs.acs.org.